Shape Tailored TiO 2 Nanostructures and Its Hybrids for Advanced Energy and Environmental Applications: A Review

Transcription

1 Shape Tailored TiO 2 Nanostructures and Its Hybrids for Advanced Energy and Environmental Applications: A Review Kamal Kumar Paul and P. K. Giri, 1 Department of Physics, Indian Institute of Technology Guwahati, Guwahati , India Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati , India Abstract Shape tailored TiO 2 nanostructures with various dimensionality (zero to three dimension) have unique physicochemical and functional properties that facilitates its efficient energy and environment applications, e.g., solar light driven photocatalytic hydrogen generation and decontamination of organic/inorganic toxic pollutants, CO 2 reduction into the hydrocarbon fuels, solar cells, supercapacitors and lithium-ion batteries etc. However, the wide band gap nature and the fast recombination of the photogenerated charge carriers in TiO 2 usually limit its overall performance under solar light illumination. In this review, we present a state of the art on the fabrication techniques of shape tailored TiO 2 nanostructures and the strategies employed to make the system catalytically more efficient. Though shape tailored TiO 2 nanostructures with large specific surface area and highly energetic (001) facet exposed TiO 2 nanostructures (2D and 3D) can enhance the photocatalytic efficiency to a reasonable extent, further surface engineering is needed for the modification of the electronic band arrangement, visible light sensitization and efficient charge separation. Herein, TiO 2 heterostructures (HSs) with metal/non-metal doping, surface fluorination, plasmonic noble metal nanoparticles (NPs) and coupling with the narrow band gap suitable semiconductor (type-ii) are discussed in details covering from zero dimensional to three dimensional heterostructures. The synthesis strategies, charge transfer mechanism and their participation in the photocatalysis are elaborated. Though one dimensional TiO 2 HSs have been widely studied, we present the recent development of critical surface engineering strategies of two and three dimensional systems, which give rise to the excellent properties including the enlargement of surface area, light absorption capability and efficient separation of electrons/holes resulting in the superior performance in advanced applications. Based on recent breakthroughs in the field, future directions and outlook of the field are presented at the end. Keywords: Shape tailored TiO2 nanostructures; 0D TiO2; 1D TiO2; 2D TiO2; 3D TiO2; Plasmonic heterostructure; Type-II heterostructure; Visible light photocatalysis; Hydrogen production. Date of Submission: 30 September, 2017 Date of Acceptance: 9 October, Address: giri@iitg.ernet.in 1 Corresponding author, giri@iitg.ernet.in

2 Graphical Abstract Comprehensive review on the shape tailored TiO 2 nanostructures and its heterostructures for energy and environmental applications. Growth of various dimensional (0D-3D) TiO 2 nanostructures and their heterostructures for the application in solar light driven photocatalysis. Focus on the visible light photocatalysis for environmental cleaning and hydrogen evolution by multi-dimensional TiO 2 heterostructures and hierarchical structures. Noble metal nanoparticles (plasmonic heterostructures) and semiconductorsemiconductor (type-ii) heterostructures for efficient photocatalysis emphasized. Latest development in the field and open challenges for the improvement of efficiency and stability of the catalysts discussed.

3 Contents 1. Introduction 2. Crystal Structures of Various Phases of TiO 2 3. Synthesis of Shape Tailored TiO 2 Nanostructures 3.1. Synthesis of 0D TiO 2 Nanostructure 3.2. Synthesis of 1D TiO 2 Nanostructures Sol-gel and Template Assisted Method Hydrothermal Method Solvothermal Method Electrochemical Anodization Method Electrospinning Method 3.3. Synthesis of 2D TiO 2 Nanostructures Hydrothermal Method Chemical Vapor Deposition Method 3.4. Synthesis of 3D TiO 2 Nanostructures by Hydrothermal/solvothermal Method (001) Faceted Nanocubic Structures (001) Faceted Nanoflower Structures D Microspheres D Hierarchical Microstructures 4. Surface Engineering of TiO 2 Nanostructure and Its Application in Catalysis D TiO 2 Quantum Dots and Its HSs D TiO 2 HSs D TiO 2 HSs D TiO 2 HSs 5. Conclusion and Outlook 6. References Doping: Metal and Non-metal Elements Plasmonic 1D TiO 2 HSs With Noble Metals TiO 2 Type-II HSs

4 1. Introduction The rapid industrialization, continuous growth of global economy and population in the past few decades have steadily raised the awareness of potential global crisis of energy because of our rapacious consumption of fossil fuels at an extremely fast rate. Thus, the global warming and climate change becomes the inevitable consequence of various pollution in the environment resulting from the extensive use of fossil fuels. 1-3 For the sustainable development of our society, the advancement of technologies towards the renewable green energy systems as well as the environmental decontamination becomes the most urgent task. Parallel to the ongoing various renewable energy projects, such as supercapacitors, solar cells and lithium ion (Li-ion) batteries 46, optical photon induced catalysis or photocatalysis by a suitable semiconductor has attracted great attention and research fascination due to its diverse potential application in the field of energy and environment involving the decomposition of organic and inorganic pollutants 7, CO 2 reduction into hydrocarbon fuels 8, 9 and photocatalytic water splitting for hydrogen production 10. After the remarkable discovery of photocatalytic water splitting in TiO 2 electrode under the ultraviolet (UV) irradiation by Fujishima and Honda 10, notable progress has been made in designing the efficient semiconductor photocatalysts. Thus, the semiconductor TiO 2 has been paid great attention, established to be most advantageous and extensively used for the various catalytic and energy applications such as, photocatalytic degradation of industrial pollutants 11, 12, CO 2 reduction 13, 14, water splitting for H 2 evolution 15, 16, solar cells 17, 18, supercapacitors 19, 20, lithium ion batteries 21, 22 and biomedical devices 23, 24. TiO 2 satisfies the prerequisite to be an efficient photocatalyst keeping the redox potential of highly reactive oxygenated species (hydroxyl and superoxide radicals, hydrogen peroxide etc.) in forbidden energy gap. Initially the

5 investigations were concentrated mainly on the TiO 2 nanoparticles (NPs) and an excellent performance was observed only under the UV light irradiation. This is because of the large band gap of TiO 2 (~ 3.2 ev for anatase and brookite, (~ 3.0 ev for rutile) and thus the threshold of excitation wavelength falls in the UV region of the solar spectrum. 25 Since, only (~ 5% of the incident radiation on the earth s surface lies in the UV zone and the rest 95% in the visible ((~ 43%) and infrared zone ((~ 52%), the performance of TiO 2 nanostructures as a natural solar light photocatalyst is usually not satisfactory and it needs further investigation for lowering the activation energy of TiO Another unavoidable difficulties with the TiO 2 NPs is the high recycling cost after each reaction due to its small size and high dispersion in the aqueous solution. As the photocatalysis is fully a surface property of a semiconductor, enhancement in the specific surface area may lead to the higher efficiency by increasing the catalytically active sites. 27 Thus, shape tailored TiO 2 nanostructures with various dimensionality such as (i) zero dimensional (0D) quantum dots (QDs), (ii) one dimensional (1D) nanorods (NRs), nanowires (NWs), nanobelts (NBs), nanotubes (NTs), nanoribbons (NRbs), (iii) two dimensional (2D) nanosheets (NSs) and three dimensional (3D) superstructures, nanoflowers (NFs), nanospheres etc. have been studied extensively in the last decade resulting in the significant advancement in the field of photocatalysis and energy applications. In bare homogeneous TiO 2 nanostructure, the recombination process of photogenerated excitons is observed to be random and thus recombination is highly probable before reaching the surface to initiate redox reactions causing low efficiency of photocatalysis. Fig. 1 shows a schematic of the photocatalysis process. Additionally, TiO 2 nanostructure can utilize only (~ 5% of the incident solar light and shows slow charge carrier transfer, making it inefficient in the industrial scale.

6 Fig. 1: A schematic illustration of various steps of photocatalysis by TiO 2 : (I) formation of free excitons, (II) photoinduced charge recombination, (III) photoinduced charge transfer to the oxygen or hydroxyl ion (IV,V) oxidation and reduction caused by valence band hole and conduction band electron, respectively. Reproduced with permission from 28, Y. Wang, et al., Nanoscale 5 (18), 8326 (2013). 2013, RSC. To overcome these limitations, modification of physicochemical and optical properties of the shape tailored titania and its heterostructures (HSs) is indispensable. Therefore, enhancement in the efficiency of the photocatalytic process by tuning the electronic band structure and light harvesting range of TiO 2 to the visible and NIR region has been achieved through various strategies, such as coupling with a narrow band gap semiconductor (type-ii HSs) 29, 30, doping with metal and non-metal ions 31-36, co-doping with multiple foreign elements 37, depositing noble metal NPs to form plasmonic HSs, surface sensitization with organic dyes 41, 42, surface fluorination 43, 44, etc. In the last decade, an appreciable development has been made on the fabrication of shape tailored nanoscale TiO 2 HSs with various dimensionality. In this review, we first provide a summary of the crystal structures of TiO 2 in distinct phases followed by the detailed methodologies for the fabrication of shape tailored TiO 2 nanostructures, including 0D, 1D, 2D and 3D nanostructures. Next, we focus on the tuning of electronic structure, the range of optical absorption and charge separation efficiency in the TiO 2 HSs fabricated by depositing plasmonic noble metal NPs, coupling with various appropriate narrow band gap semiconductors, doping

7 etc. Finally, we look into their influence on the photocatalytic hydrogen evolution and environmental decontamination properties. 2. Crystal Structures of Various Phases of TiO 2 Titania (TiO 2 ) generally exists in three distinct polymorphs (phases): rutile, anatase, and brookite, where anatase and rutile phases have tetragonal crystal structure with space groups I41/amd and P42/mnm, respectively and brookite has orthorhombic crystal structure with space group Pbca. 45 In each polymorph, titanium cations are found to be six-fold coordinated to oxygen anions, which forms distorted TiO 6 octahedra. The crystal structure of TiO 2 differs with phase due to the distinct spatial arrangement of the TiO 6 octahedra building blocks (see Fig. 2). While anatase has corner sharing TiO 6 octahedra, in rutile phase edge sharing is observed and the brookite shows the both. Among the different crystal phases, rutile is considered to be the most thermodynamically stable bulk phase. At the nanoscale, anatase and brookite are generally regarded as more stable due to their low surface energy, although there exists some debate on this issue. 46, 47 Additionally, there exists another exotic crystal phase of pure TiO 2 (space group C2/m), known as TiO 2 (B) (Fig. 2d). This phase is generally produced by solution based process, like hydrothermal method. It has a very open framework structure despite having the identical chemical formula. Thus, it possesses a lower density and a larger specific capacity than other phases, which are extremely advantageous in the applications of photocatalysis, solar cells and energy storage, such as lithium-ion batteries The detailed growth studies have revealed that the crystal phases are determined by the experimental conditions and also post growth treatments (synthesis method, ph of the medium, annealing duration and annealing temperature). Zhang et al. 52 have thermodynamically analyzed the limiting crystal size to be stable under a certain crystal phase. Anatase can have the size up to 11 nm, for brookite nm and for the bulk phase, rutile, it should be more than 35 nm. Yang et al. 53 obtained different

8 crystal phases (rutile, anatase, and brookite) by a hydrothermal method using different ph values of peroxide titanic acid solution. With ph value lower than 8, only rutile phase was found. When the ph was varied in the range 8-10, brookite and rutile phases were obtained in the mixed phases. However, with the ph value higher than 10, only anatase phase was obtained. Santara et al. 54 discussed the effect of annealing temperature and environment on the crystal phases and surface feature of the TiO 2 nanostructures. Prior to the annealing, no peak was observed in the XRD pattern of the TiO 2 nanoribbon. The growth of TiO 2 (B) phase from hydrogen titanate was observed at annealing temperature 500 C. When the temperature was increases to 700 C, the B-phase started converting into anatase and a mixture of these two was obtained. When annealed at 900 C, the TiO 2 nanoribbons were found in anatase and rutile mixed phase, indicating a partial transformation of anatase into rutile phase. Beyond 900 C only phase detected was rutile in bulk phase. Tay et al. 55 synthesized a mixed phase anatase/brookite TiO 2 nanostructure by a simple hydrothermal method, which shows enhanced hydrogen production activity compared to the highly crystalline pure brookite and P25. Fig. 2: A schematic representation of various TiO 2 polymorphs: (a) Rutile; (b) Anatase; (c) Brookite; and (d) TiO 2 (B). Each purple sphere represents Ti atom, and the blue octahedra represent TiO 6 blocks. Oxygen atoms at the corner of the octahedra are omitted for clarity. Reproduced with permission from 45, Y. Zhang, et al., RSC Advances 5 (97), (2015). ). 2015, RSC.

9 3. Synthesis of Shape Tailored TiO 2 Nanostructures Distinct morphological feature of semiconductor nanostructures acts as extremely determining factor for the growth of high performance photocatalysts for hydrogen evolution as well as environmental cleaning, Li-ion batteries, solar cells, etc. Thus, shape tailored various surface morphology of TiO 2 including zero dimension (quantum dots), one dimension (nanorods, nanowires, nanoribbons, etc.), two dimension (nanosheets, nanoplates, etc.) and three dimension (flowers, hierarchical architectures) are being extensively investigated to address the challenges of energy crises, environmental issues, etc. of the modern era Synthesis of 0D TiO 2 Nanostructure 0D TiO 2 quantum dots (QDs) based nanostructures are comparatively least studied system among various dimension nano-heterostructures (1D, 2D and 3D) for the energy and environmental applications. To grow the shape tailored nanostructures, hydrothermal method is one of the most widely used methods for its versatility with controllability, repeatability, relatively lower cost and industry based large scale production. Recently, few approaches have been explored to prepare the 0D TiO 2 QDs based various HSs after the breakthrough work by Pan et al. 56 on the hydrothermal synthesis of [001] facet dominated TiO 2 nanosheet with selfdecorated TiO 2 QDs. Their long time hydrothermal strategy and subsequent defect curing by the hydrothermal washing are the key factors to obtain defect-free nanosheets decorated with TiO 2 quantum dots with sizes nm, as shown in Fig. 3. The band gap enlargement of TiO 2 due to the formation of QDs and its HSs with TiO 2 nanosheet increases photoactivity significantly which is highly promising for photocatalytic performances.

10 Fig. 3: HRTEM images of [001] faceted TiO 2 nanosheet with TiO 2 QDs decoration after the hydrothermal reaction of 72 hrs (a-d) and 168 hrs (f-i). (b,g) the corresponding lattice d-spacing and the inset of (c,d) exhibit the size distribution of TiO 2 QDs. HRTEM images of [001] faceted TiO 2 nanosheet with TiO 2 QDs decoration. (e,j) the formation of pure anatase TiO 2 by XRD and Raman spectra, respectively. Reproduced with permission from 56, L. Pan, et al., Chem. Commun. 49 (59), 6593 (2013). 2013, RSC. Xu et al. 57 designed TiO 2 QDs on Cu 2 O octadecahedra (Cu 2 O O) single crystals with [110] and [100] exposed facets synthesized by a sol gel method. Fig. 2(a-d) shows the details of the structural and morphological properties of the TiO 2 QDs and Cu 2 O HSs. The Fig. 4(d) demonstrates the synthesis pathways of the Cu 2 O octadecahedra, TiO 2 QDs and their HSs. Special technique of coupling the TiO 2 QDs with the Cu 2 O facets to form numerous stable p-n heterojunctions is used here for the extremely efficient charge migration through the interface of the heterostructures. Sood et al. 58 synthesized TiO 2 QDs by a simple template free sol-gel method with size distribution 5-8 nm. Besides the broad emission spectrum in the range nm due to the oxygen vacancy defects, a prominent emission peak at 379 nm clearly suggested the direct band gap nature of the as synthesized TiO 2 QDs, see Fig. 4.

11 Fig. 4: (a) The XRD patterns of TiO 2 QDs, Cu 2 O octadecahedra and HSs. (b-d) their TEM images. (e) Schematic pathways of synthesis of TiO 2 QDs, Cu 2 O octadecahedra and HSs. (f-h) XRD, PL and HRTEM of TiO 2 QDs, respectively. Reproduced with permission from 57, X. Xu, et al., ACS Appl. Mater. Interfaces 8 (1), 91 (2016). 2016, ACS, (a-e). Reproduced with permission from 58, S. Sood, et al., J Alloys Compd. 650 (Supplement C), 193 (2015). 2016, Elsevier Ltd. (f-h) Synthesis of 1D TiO 2 Nanostructures Sol-gel and Template Assisted Method One of the key synthesis processes of wet chemistry method is the sol gel method with low temperature and easy control in surface morphology is generally used to synthesize 1D TiO 2 nanostructures. In general, the TiO 2 sol-gel is prepared by mixing titanium isopropoxide or titanium butoxide in acetic acid followed by a heating to high temperature with vigorous magnetic stirring which allows hydrolysis and condensation reactions. Thus shape tailored TiO 2 nanostructures are obtained. Joo et al. 59 have prepared 3.4 nm (diameter) 38 nm (length) sized TiO 2 nanocrystals from sol-gel reaction between titanium(iv) isopropoxide and oleic acid at 270 C. They have successfully controlled the length of the as-grown NRs between nm by adding 1-hexadecylamine to the reaction mixture as a co-surfactant. The template-assisted

12 method is also extensively used to fabricate the 1D TiO 2 nanostructures. An anodic aluminum oxide (AAO) nanoporous membrane consisting of parallel straight array of nanopores with well controllable dimension (diameter and length), is usually considered as a template. Finally, the template is removed by chemical etching and 1D TiO 2 nanostructures remain on the substrate. Sander et al. 60 have fabricated dense, thermally stable and well-aligned TiO 2 nanotubes by sequential atomic layer deposition (ALD) process with TiCl 4 as the precursor material using template assisted method. A combination of sol-gel and template assisted method produces better control over the growth of 1D TiO 2 nanostructures and high performance in the real life applications. For example, Attar 61, Qiu 62 and Lin et al. 63 synthesized 1D TiO 2 nanorods, nanotube arrays and nanowires, respectively by a combined template-assisted sol gel method as shown in Fig. 5. Fig. 5: (a) TEM image of as-synthesized TiO 2 nanocrystals. (b) HRTEM image of TiO 2 nanorods. (c-d) top and side view of titania nanotube array on Si substrate with outer diameter 40 nm and length 1.5 m. TEM image of TiO 2 nanotubes oriented in cross section (e) and a single nanotube (f). FESEM images of anodic alumina membranes: (g) morphology and cross-section of the template with pore size nm and (i) top view of the template with pore sizes nm. (h) FESEM image of TiO 2 NRs array with diameter nm and (j) shows the TEM image. (k-l) FESEM images of well aligned TiO 2 and ZnO

13 nanotube arrays. (m-n) XRD and SEM image of TiO 2 nanowires (NWs), respectively. Reproduced with permission from 59, J. Joo, et al., J. Phys. Chem. B 109 (32), (2005). 2005, ACS, (a-b). Reproduced with permission from 60, M.S. Sander, et al., Adv. Mater. 16 (22), 2052 (2004). 2004, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, (c-f). Reproduced with permission from 61, A. S. Attar, et al., Mater. Chem. Phys. 113 (2), 856 (2009). 2009, Elsevier Ltd, (g-j). Reproduced with permission from 62, J. Qiu, et al., Nanotechnology 17 (18), 4695 (2006). 2006, IOP Science, (k-l). Reproduced with permission from 64, Y. Lin, et al., J. Phys.: Condens. Matter 15 (17), 2917 (2003). 2003, IOP Science, (m-n) Hydrothermal Method Hydrothermal method is the most used chemical method for the fabrication of shape tailored TiO 2 nanostructures including various 1D architectures. Usually the reaction is conducted in a stainless steel chamber with a Teflon liner under high temperature with autogenous pressure. It is probably the simplest method for tuning the surface morphology at the lowest production cost. Kasuga et al. 65, 66 discovered a template free new route to fabricate TiO 2 nanotubes with diameter ~ 8 nm and length ~ 100 nm considering amorphous TiO 2 as a precursor material treating at high temperatures with concentrated NaOH solvent. Afterwards, many research articles have reported on the growth of TiO 2 nanotubes 67, 68, nanowires 69, 70, nanorods 47, 71, 72, nanobelts 73, 74, nanosheets by the hydrothermal method. Depending upon the solvent used, the synthesis procedure can generally be divided into two methods: first, acid hydrothermal method where the reactants are titanium salts and concentrated hydrochloric acid (HCl) and second, alkali hydrothermal method where the reactants are TiO 2 nanoparticles and concentrated sodium hydroxide (NaOH) solution. These two methods have different reaction mechanism which produces different 1D morphological features of TiO 2.

14 During alkali-hydrothermal process, layered sodium titanate (Na 2 Ti 4 O 9 ) is produced in an intermediate reaction stage. As the Na + ions are residing between the edge-shared TiO 6 octahedral layers, it can be easily replaced by protons (H + ion) in a dilute acidic medium. Thus, hydrogen titanate (H 2 Ti 5 O 11.H 2 O) is formed and the interlayer distance becomes enlarged because of the larger intercalated size effect of H 2 O molecules causing weaker static interaction between neighboring TiO 6 octahedral sheets. Thus, hydrogen titanate can easily be exfoliated to form nanosheets which in turn curl up from the edges to form TiO 2 nanotubes to minimize the high surface tension. 79, 80 Additional hydrothermal conditions like type of precursor, its size, concentration of NaOH solvent, temperature and reaction duration may affect the structure and morphology of the nanostructures. Morgan et al. 81 showed the effect of concentration of NaOH and the reaction temperature on the formation of various nanostructure from P25 (TiO 2 ) through alkaline hydrothermal treatment (see Fig. 6). Similar results have been reported by Tanaka 82 and Peng et al. 83, an investigation of the effect of concentration of NaOH, temperature and reaction time duration on the nanostructure formation. Fig. 6: Morphological shape evolution of P25 (TiO 2 ) to (a) nanotube and (b) nanoribbon after hydrothermal treatment at 160 C and 220 C, respectively. Reproduced with permission from 81, D. L. Morgan, et al., Chem. Mater. 20 (12), 3800 (2008). 2008, ACS.

15 Fig. 7: (a) FESEM image of TiO 2 nanotubes obtained at 500 rpm. (b) TEM image of (a), the arrow shows nanotubular structure. (c-f) FESEM images of TiO 2 nanotubes obtained from the hydrothermal reaction with temperature maintained at 130 C for 24 hrs at stirring rates of 0, 300, 400, 500 and 1000 rpm, respectively. (g) XRD patterns of TiO2 nanotubes at various stirring speed. Reproduced with permission from 84, Y. Tang, et al., Angew. Chem. Int. Ed. 53 (49), (2014). 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. For further modification and improvement in the nanostructures, several approaches like microwave assisted heating process, ultrasonication and magnetic rotation of the reactants during the hydrothermal treatment were adopted. Stirring based hydrothermal method has been proved to be an extremely beneficial for the growth of much longer nanotube (up to tens of micrometers) (see Fig. 7). The optimized nanotubes with very high aspect ratio exhibit ultrafast rechargeable 84, 85 Li-ion battery materials with high stability.

16 Solvothermal Method Similar to the hydrothermal process, the solvothermal method is also a common growth process for various TiO 2 nanostructures. The only difference between the hydrothermal and the solvothermal method is the use of organic solvent (ethanol, ethylene glycol, n-hexane, etc.) in solvothermal method, while only water is used as reaction solution in hydrothermal method Wang 91, Santara et al. 53, 92 have grown nanowires, nanotubes and nanoribbons by a solvothermal method considering ethanol, glycerol and ethylene glycol as solvents, respectively, see Fig. 8 (a,b,e-h). The as-grown nanowires and nanotubes are suitable for the high performance Li-ion battery materials, whereas the nanoribbons with modified effective band gap and optical absorption window are promising for the photocatalysis under solar light. Nitrogen and fluorine co-doped TiO 2 nanobelts (Fig. 8(c)) and vertically aligned TiO 2 nanorods (Fig. 8(d)) have also been reported for high performance photocatalysis and dye sensitized solar cells, respectively. 91, 93 Selection of appropriate solvent is the key factor for the growth and various surface modifications of the nanostructures.

17 Fig. 8: SEM micrographs of different TiO 2 nanostructures synthesized by the solvothermal method: (a) nanowires, (b) nanotubes, (c) nanobelts, and (d) nanorods. Reproduced with permission from 91, Q. Wang, et al., Inorg. Chem. 45 (17), 6944 (2006). 2006, ACS (a, b). Reproduced with permission from 93, Z. He, et al., ACS Appl. Mater. Interfaces 4 (12), 6816 (2012). 2012, ACS, (c). Reproduced with permission from 94, J. Zhao, et al., J. Power Sources 255 (Supplement C), 16 (2014). 2014, Elsevier Ltd. (d). (e-f) FESEM images of TiO 2 (B) nanoribbons synthesized at 180 C for 16 hrs and 500 C post growth treatment. The inset of (e) shows the EDX spectrum. (g) its TEM image. Reproduced with permission from 53, B. Santara, et al., Nanoscale 5 (12), 5476 (2013). 2013, RSC. (h) FESEM images of TiO 2 nanoribbons calcined at various temperatures: 500 C, 700 C and 900 C, respectively. Reproduced with permission from 92, B. Santara, et al., J. Phys. Chem. C 117 (44), (2013). 2013, ACS. Despite its versatility and advantages of the hydrothermal and solvothermal method over the other growth processes, it has still some disadvantages. First, the nanostructures grown by this method are not always uniform in size and shape. Second, the as-grown 1D nanostructures are generally too small for the better performance in the real life applications. Third, the reaction rate is very slow and it takes long time to grow desired materials. Fourth, the growth of monolayer 2D materials (MoS 2, MoSe 2, WS 2, etc.) is not well controllable by this method Electrochemical Anodization Method In 1984, Assefpour-Dezfuly et al. 95 reported the growth of porous, well aligned 1D TiO 2 nanotube arrays by two step process: etching in alkaline peroxide and then anodization in chromic acid. Presently, anodization process is commonly used for the synthesis of 1D TiO 2 nanostructured materials on Ti foil substrate. The TiO 2 nanotubes synthesized by this method are very promising for the photoanode material due to its high performance, stability and easy synthesis procedure. Afterwards, Zwilling et al. 96 made a breakthrough by growing nanoporous

18 titania nanotube in fluoride containing electrolytes on Ti foil. Since then, a lot of attention has been paid to optimize the controlled growth of the 1D TiO 2 nanostructures. Fig. 9: SEM images of the TiO 2 nanotubes categorized under (a) 2 nd generation in the Na 2 SO 4 /NaF electrolyte, (b) 4 th generation in the F - free electrolyte, (c) 3 rd generation in ethylene glycol/fluoride electrolytes, (d) shows a inclined view of (c) and (e) is the TEM image of a TiO 2 nanotube corresponding to (c), (f,g) top and cross section view, respectively of 5 th generation with multi step anodization in ethylene glycol/fluoride electrolytes, (h,i) top and cross section view, respectively of 5 th generation with two-step anodization process with ethylene glycol and fluoride electrolytes. Reproduced with permission from 97, J. M. Macak, et al., Electrochimica Acta 50 (18), 3679 (2005) 50 (18), 3679 (2005). 2005, Elsevier Ltd. (a). Reproduced with permission from 98, R. Hahn, et al., Electrochemistry Communications 9 (5), 947 (2007). 2007, Elsevier Ltd. (b). Reproduced with permission from 99, M. Paulose, et al., J. Phys. Chem. C 111 (41), (2007). 2007, ACS, (c-e). Reproduced with permission from 100, M. Ye, et al., J. Am. Chem. Soc. 134 (38), (2012). 2012, ACS, (f-g). Reproduced with permission from 101, M. Ge, et al., J. Mater. Chem. A 3 (7), 3491 (2015). 2015, RSC, (h-i). It is now well understood that the nature of electrolyte, it s ph, applied voltage, reaction duration and temperature of the reaction solution determine the morphology and the dimension of the TiO 2 nanostructures. The as-grown TiO 2 nanotubes of various generations are shown in Fig. 9. Even though, a remarkable progress on the optimized growth of 1D TiO 2 nanostructure has been achieved, development is still going on for the further improvement of the length of the nanotubes in F - based organic-inorganic neutral electrolytes.

19 Electrospinning Method In 2003, Li et al. 102 synthesized TiO 2 nanofibers having diameter ~ 55 nm by a electrospinning method for the first time. In this process, a polymer solution is injected from a needle of a syringe like chamber under the influence of strong electric field. Due to the continuous gathering electric charges, the polymer solution starts stretching to form ultralong and Fig. 10: (a) Schematic of an electrospinning setup. (b) SEM image of TiO 2 /PVP nanofibers grown from an ethanol solution. (c) Change in the surface morphology of the nanofibers after thermal treatment. (dg) FESEM images of TiO 2 nanofibers calcined at 500, 600, 700, 800 C, respectively. Reproduced with permission from 103, Y. Yang, et al., Nanoscale 5 (21), (2013). 2013, RSC. (a,c-g). Reproduced with permission from 102, D. Li, et al., Nano Lett., 3 (4), 555 (2003). 2003, ACS (b). hollow nanofibers. More and more attention are being paid to this method due to the easy formation of single-phase and highly crystalline TiO 2 nanofibers. Fig. 10(a) shows a schematic of the electrospinning process and the (b) is the FESEM images of TiO 2 nanofibers grown by it. Zhu et al. 104 reported the growth of porous-shaped anatase TiO 2, cluster-shaped anatase TiO 2, hierarchical-shaped rutile TiO 2 and nano-necklace rutile TiO 2 by the electrospinning method followed by the annealing at temperatures 500, 600, 700 and 800 C, respectively. It is observed that the diameter of the nanofibers shrink from 200 nm to < 100 nm due the effect of annealing at high temperatures though the axial continuity is unaltered. Ag, CuO, ZnO, carbon, etc. can be introduced into the nanofibers to modify the surface features for the improvement in many application like photocatalysis

20 3.3. Synthesis of 2D TiO 2 Nanostructures 2D faceted TiO 2 nanosheets with various surface energies are at the focus of intense research in the field of energy and environmental remediation. The key factor on tuning the exposed crystallographic facets of TiO 2 as well as its facet percentage without disturbing its stability is the nature of solvents used, capping and shape controlling agents and impurities introduced into the crystals The capping agents used to be adsorbed selectively and interact differently with the different crystalline facets. This strategy is used to design nanocrystals with various dominant reactive facets. 109 The capping agents play the important role to reduce the surface free energy of the facets which in turn guide the nanocrystal growth along the specific facets. Additionally, the solvent used for the reaction is also very significant and it determines the facet orientation of the crystal during the growth process. This is due to the fact that the atomic arrangement near the surface and the surface affinity of the solvent vary considerably with orientation of the facet and can influence the growth rates in a favorable direction. 1, 106 Ong et al. 110 succinctly explained the role of solvent and impurities or additive in controlling the shape and facet orientation. Several fabrication techniques have been developed for the growth of 2D TiO 2 nanocrystals dominated with (001) facets. Among all, most explored and easy method is the surface fluorination to stabilize the highly reactive (001) facets Hydrothermal Method Recently, researches are focused on the development of 2D anatase single crystals with large percentage of reactive (001) facets due to its extremely high reactivity using a simple facile hydrothermal method Various techniques are used to calculate the exposed facets like,

21 FESEM, TEM and XRD simply by assuming the whole surfaces are occupied by the (101) and (001) facets. 72, 110, 117 Yang et al. 78 reported for the first time the hydrothermal synthesis of anatase TiO 2 single crystals with 47% exposed (001) facets considering TiF 4 as a precursor material and hydrofluoric acid (HF) as a shape controlling agent. Each single truncated bipyramid shaped anatase TiO 2 nanocrystal is composed of two flat square surfaces corresponding to (001) facets while the four isosceles trapezoidal surfaces are ascribed to the (101) facets. The average interfacial angle between these two facets is estimated to be ~ 68.3 which is consistent with the theoretical value. 118 Following this work, synthesis of (001) facet dominated 2D TiO 2 nanosheets have been extensively studied with various surface modification for photocatalytic applications. Wen et al. 119 synthesized large area highly reactive and purely anatase TiO 2 nanosheets fully dominated by (001) and (100) facets with relative amount of 98.7% and 1.3%, respectively which are generally thermodynamically unfavourable due to their high surface energy. The as-grown TiO 2 nanosheets are very large in the length ~ 4.1 m and (001) facet dominated surfaces are showing appreciably high photocatalytic activity (see Fig. 11). Yu 120 and Wang et al. 121 synthesized (001) facet dominated anatase TiO 2 nanosheets by the hydrothermal method, which are highly beneficial for the photoelectric conversion efficiency in dye-sensitized solar cells as compared to the P25. Han 112, Yan 122, Tian 123 and Liu et al. 124 also synthesized (001) faceted TiO 2 nanosheets which behave as

22 Fig. 11: (a) SEM image of (001) faceted TiO 2 single crystal. Inset shows the SAED pattern. (b,c) FESEM images of TiO 2 nanosheets synthesized at 210 C for 24 hrs. Inset of (c) shows the corresponding TEM image. (d-h) TEM images of various TiO 2 nanosheets efficient for the catalytic application. Reproduced with permission from 111, H. G. Yang, et al., Nature 453 (7195), 638 (2008). 2008, Nature Publishing. Group (a). Reproduced with permission from 119, C. Z. Wen, et al., Chem. Commun. 47 (15), 4400 (2011). 2011, RSC, (b,c). Reproduced with permission from 120, J. Yu, et al., Nanoscale 2 (10), 2144 (2010). 2011, RSC, (d). Reproduced with permission from 112, X. Han, et al., J. am. chem. soc. 131 (9), 3152 (2009). 2009, ACS, (e). Reproduced with permission from 123, F. Tian, et al., J. Phys. Chem. C 116 (13), 7515 (2012). 2012, ACS, (f-g). Reproduced with permission from 122, B. Yan, et al., RSC Adv. 6 (8), 6133 (2016). 2016, RSC, (h). good photocatalysts. Wu and co-workers reported a facile non-aqueous synthesis route for the preparation of rhombic-shaped anatase TiO 2 nanocrystals with exposed (001) for the first time. 125 Following this work, Gordon et al. 117 synthesized TiO 2 NCs having size nm by a controlled non-aqueous seeded growth considering the precursors TiF 4 and TiCl 4 where the TiF 4 enables the exposure of (001) facets (see Fig. 12). Dinh et al. 126 demonstrated the TiO 2 nanocrystal growth with various shapes: rhombic, truncated rhombic, spherical, dog-bone,

23 truncated and elongated rhombic, etc. by using a modified hydrothermal route employing both as capping agents (see Fig. 12). In the water vapor environment a simple variation of oleic acid and oleylamine molar ratio, precursor or the reaction temperature make extremely fine tuning over the shape evolution of the TiO 2 nanocrystals, similar to the works discussed in the reference. 127 Fig. 12: TEM images of 2D TiO 2 NCs synthesized using precursor TiF 4 (a,c), a mixed precursor of TiF 4 and TiCl 4 (b,d). (a,b) and (c,d) are the images of TiO 2 synthesized in the presence of oleylamine and 1octadecanol, respectively. TEM images of TiO 2 having (e) the rhombic shape (f) truncated rhombic shape, (i) spherical shape, (g) dog-bone-shape, (h) truncated and elongated rhombic shape, (j) elongated spherical shape and (k) dumbbell shape. Insets show high-magnification images of the corresponding images. Reproduced with permission from 117, T. R. Gordon, et al., J. Am. Chem. Soc. 134 (15), 6751 (2012). 2012, ACS, (a-d). Reproduced with permission from 126, C. Dinh, et al., ACS Nano 3 (11), 3737 (2009). 2009, ACS, (e-k) Chemical Vapor Deposition Method Though the chemical based growth mechanism of TiO 2 is now firmly established, the synthesis of pure TiO 2 nanostructure by physical method is not yet well explored. Lee et al. 128 designed high density (001) facet dominated TiO 2 nanosheets both in anatase and rutile phases on silicon and silicon coated substrates via a chemical vapor deposition method, as shown in Fig. 13. They have shown that the H 2 autoignition induced below its autoignition temperature (536 C) could prevent the phase transformation from anatase to rutile.

24 Fig. 13: (A,B) Schematic representation along with the FESEM micrographs of (001) facet dominated anatase and rutile TiO 2 nanosheets by CVD method with H 2 flow. Reproduced with permission from 128, W. Lee, et al., Cryst. Growth Des. 12 (11), 5792 (2012). 2012, ACS Synthesis of 3D TiO 2 Nanostructures by Hydrothermal/solvothermal Method (001) Faceted Nanocubic Structures Numerous techniques have been explored for the fabrication of 3D TiO 2 architectures with different shapes. Notable progress has been reported for the preparation of (001) facet dominated 3D nanostructures by hydrothermal and modified hydrothermal methods. As the average surface formation energy is maximum for (001) facet (0.90 J m -2 ) which is almost double than that of the (101) facet (0.44 J m -2 ) these facets are highly catalytically active and are very promising for the future generation of energy and environmental cleaning. 75 3D TiO 2 single crystal architectures have been synthesized by various precursor materials like TiF 4, TiCl 4, Tetrabutyl titanate, titanium isopropoxide, TiOSO 4, etc. 32, 129, 130 Yang and co-workers demonstrated a novel solvothermal route for the shape tailored synthesis of 3D single crystals of anatase TiO 2 with dominating percentage of reactive facets prepared by considering a water-2-

25 propanol as solvent (see Fig. 14). 78 Wen et al. 131 demonstrated the solvothermal synthesis of single crystalline TiOF 2 nanocube and its transformation to anatase TiO 2 for the first time by the calcination under various atmospheres such as pure argon, moist argon and pure hydrogen sulfide (H 2 S). After this breakthrough work, similar reports have been published on the solvothermal synthesis of 3D TiO 2 single crystals with cubic morphology for the efficient photocatalysis as well as the energy applications (see Fig. 14).43, 132, 133 Fig. 14: (a-f) FESEM micrographs of (001) facet dominated anatase TiO 2 nanocubic structures. Reproduced with permission from 78, H. G. Yang, et al., J. am. chem. soc. 131 (11), 4078 (2009). 2009, ACS (a). Reproduced with permission from 131, C. Wen, et al., Chem. Commun. 47 (21), 6138 (2011). 2011, RSC, (b-c). Reproduced with permission from 133, L. Wu, et al., CrystEngComm 15 (17), 3252 (2013). 2013, ACS, (d). Reproduced with permission from 129, Z. Lai, et al., J. Mater. Chem. 22 (45), (2012). 2012, RSC, (e,f) (001) Faceted Nanoflower Structures Chen et al. 21 reported the growth of 3D hierarchical nanoporous spheres of lateral size a few hundred nm composed of ultrathin anatase TiO 2 nanosheets (thickness ~ 3 nm) for the first time with nearly 100% exposed (001) facets, as shown in Fig. 15. The nanostructures with high

26 specific surface area (~ 170 m 2 g -1 ) is reported to be extremely beneficial for the reversible and ultrafast Li-ion battery application. Besides, Liu et al. 75 reported a novel growth process via a simple hydrothermal route of 3D anatase TiO 2 nanoflowers composed by truncated tetragonal pyramidal TiO 2 nanocrystals. Ti powder was used as precursor material and the hydrofluoric acid (HF) as the shape controlling agent. The size of the nanoflowers are reported to be nm while the truncated tetragonal pyramidal TiO 2 nanocrystals have size nm. Following this work, several reports have been published with tuned (001) facet percentage with various precursor material. Ma et al. 134 considered the shape controlling agents as disodium ethylene diamine tetraacetate (EDTA) along with the HF solution while TiF 4 as the precursor material and they observed much larger nanoflowers with length up to 1.2 m. Numerous studies have been performed thereafter to enhance the photocatalytic performance, dye sensitized solar cell and Li-ion battery application simply by controlling the (001) facet percentage Fig. 15: (a,b) FESEM and TEM images of nearly 100% (001) facet dominated anatase TiO 2 nanosphere, respectively. (c) FESEM image of (001) faceted TiO 2 nanoflowers. (d) FESEM image of etched (001) faceted TiO 2 nanoflowers with size m. Reproduced with permission from 21, J. S. Chen, et al., J. am. chem. soc. 132 (17), 6124 (2010). 2010, ACS, (a,b). Reproduced with permission from 75, M. Liu, et al., Nanoscale 2 (7), 1115 (2010). 2010, RSC, (c). Reproduced with permission from 135, Q. Xiang, et al., Chem. Commun. 47 (15), 4532 (2011). 2011, RSC, (d).

27 D Microspheres Liu et al. 138 demonstrated a fluoride mediated self-transformation based fabrication of hollow TiO2 microspheres of diameter 1-2 m, which is composed of anatase polyhedral, as shown in Fig. 16. Yu and coworkers 139 also have shown a novel one-step hydrothermal strategy for the growth of surface-fluorinated TiO 2 hollow microspheres and tabular-shaped anatase single crystals (001) facets exposed considering ammonium bi-fluoride (NH 4 HF 2 ) as a shape controlling agent. Recently, Liu et al. 140 showed a new facile method consisting of electrospray Fig. 16: (a,b) FESEM images of 3D microsphere in purely anatase phase. (c) FESEM image of electrosprayed TiO2 spheres and (d) shows its magnified view clearly showing the core-shell structure. Reproduced with permission from138, S. Liu, et al., J. am. chem. soc. 132 (34), (2010). 2010, ACS, (a). Reproduced with permission from139, J. Yu, et al., CrystEngComm 12 (3), 872 (2010). 2010, RSC, (b). Reproduced with permission from140, B. Liu, et al., Langmuir 27 (13), 8500 (2011). 2011, ACS, (c,d). and hydrothermal treatment to grow mesoporous core-shell TiO 2 microspheres composed of tiny anatase TiO 2 nanocrystals and dominated by high-energy facets. It is proposed that the amorphous TiO 2 network formed by the electrospray plays a crucial role for the formation of mesoporous TiO 2 spheres. The core-shell structure with very specific surface area as well as

28 exposed high energy facets makes the system a very promising candidate for application in photocatalytic hydrogen evolution and DSSCs D Hierarchical Microstructures Fabrication of 3D hierarchical flower-like nanostructure has attracted enormous attention due to their large density of active sites, high surface area and greater pore volume, as these properties are extremely beneficial for the adsorption and catalytic application. A glycerol mediated solvothermal technique followed by calcinations under various preselected conditions has been demonstrated by Tian et al. 141 (see Fig. 17). The 3D morphology evolution by dissolution and recrystallization is fully determined by the reaction species, temperature, and duration of nucleation. Further, H 2 O 2 was also employed as a structure-tuning agent to grow purely anatase TiO 2 microflowers with (001) facets exposed. As HCl has a tendency to assist the growth in the rutile phase, an interplay between the HCl and H 2 O 2 finally determines the crystal structure, phase, morphology, and facet orientation. The phase evolution of TiO 2 has been observed to vary gradually from pure rutile to bi-crystalline phases of anatase and rutile and finally to pure anatase with increasing H 2 O 2 concentration in the range M. 142 Recently, titanium glycerolate precursor has been used by Kim et al. 143 as both the TiO 2 precursor as well as the sacrificial templates which discards the requirement of additional surfactants or ionic additives. Similar growth mechanism i.e.; nucleation-aggregation-recrystallization of 3D hierarchical anatase superstructures with high percentage of exposed (001) facets has been successfully demonstrated via a simple one-pot solvothermal strategy. 130

29 Fig. 17: (a,b) FESEM images of 3D microstructures at different magnifications obtained from 24 h solvothermal reaction. (c,d) FESEM and TEM images of TiO 2 microstructures, respectively using 1.0 M H 2 O 2 and 1.6 M HCl. (e,f) FESEM images at different magnifications of (001) facet exposed 3D TiO 2 superstructures synthesized without any kind of additives or capping agents. Reproduced with permission from 141, G. Tian, et al., CrystEngComm 13 (8), 2994 (2011). 2011, RSC, (a,b). Reproduced with permission from 142, T. Li, et al., Ind. Eng. Chem. Res. 52 (20), 6704 (2013). 2013, ACS, (c,d). Reproduced with permission from 130, G. Li, et al., CrystEngComm 16 (46), (2014). 2014, RSC, (e,f). 4. Surface Engineering of TiO 2 Nanostructure and Its Application in Catalysis In the last few decades, various photocatalysts have been explored, such as TiO 2, ZnO, WO 3, Bi 2 WO 6, ZnWO 4, etc. Among them, TiO 2 nanostructures have proved its extremely high potential as a photocatalyst and a positive sign can be observed in its commercialization as a photocatalyst. The coupling of different semiconductors results in improved photocatalytic activity. TiO 2 shows appreciably good photocatalytic activity irrespective of the nature of the medium of catalytic reaction: in aqueous pollutants, air pollutants or the CO 2 reduction from the hydrocarbon fuels. Energy and environmental problems gradually become more serious with the fast development of technology and economy. Specially, huge amount of waste water and heavily toxic gases from the various factories are destroying the diversity of water life by contaminating it and introduces non-curable disease in the human life. Thus, an urgent as well as effective solution for these problems was needed to maintain the growth of economy and sustainable development of technology. TiO 2 has been established to be an excellent

30 photocatalyst and it can be commercialized due to its abundant availability in the nature, nontoxicity, strong oxidizing as well as reducing power and long-term stability towards the photo and chemical corrosion. The limitations of the bare TiO 2 nanostructures is its UV illumination sensitivity (wide band gap, anatase: 3.2 ev, rutile: 3.0 ev) and weak separation efficiency of the photogenerated charge carriers leading to the efficient recombination which results in low photocatalytic activity under the solar irradiation. 144 After the photo-excitation, electrons at the conduction band may attack the oxygen molecules to produce superoxide radicals, and the valence band holes can react with water to generate hydroxyl radicals simultaneously which in turn degrades the pollutants, see Fig. 1, showing a possible simple reaction mechanism of the photocatalysis. In comparison to the photocatalysis, photoelectrocatalysis has been shown to have higher efficiency of catalysis i.e., the degradation pollutants as well as hydrogen generation. In this case, a low bias voltage applied to the TiO 2 electrode can transfer the photocarriers efficiently through the external circuit inhibiting their fast recombination. Thus, plenty of active species (hydroxyl, superoxide radicals) present on the TiO 2 surface accelerates the catalytic efficiency. Being a wide band gap semiconductor, the optical absorption of TiO 2 fully falls in the UV region (~ 380 nm). Besides, the excited state average life time of the photogenerated electrons and holes is too short to participate in the redox reaction for bare TiO 2 nanoparticles. However shape tailored 1D, 2D and 3D TiO 2 nanostructures exhibit superior charge transport due to the high specific surface area, exposed high energy facets, low axial resistivity (for 1D structures), etc. Though the shape tailored TiO 2 architectures are showing better results in degradation as well as hydrogen evolution it is still necessary to modify the electronic arrangement in the nanostructure by incorporating several techniques to make it industry level efficient. Several modifications have been explored to prevent recombination of electrons/holes and to achieve

31 higher photocatalytic activity such as: (1) doping with metal and non-metal elements, (2) loading noble metal nanoparticles to drive surface plasmon resonance (SPR) and to create hot electrons and (3) fabrication of staggered type (type-ii, in general) heterostructure with another semiconductor for synergic absorption and efficient interfacial charge separation for better utilization of solar energy D TiO 2 Quantum Dots and Its HSs The TiO 2 QDs can have even larger band gap than its bulk counterpart making it disadvantageous for the utilization of solar light. As the separation of the photocatalyst from the pollute solution after photocatalysis is an important and inevitable part, use of only TiO 2 QDs for photocatalysis is not recommended. But, a proper heterostructure with different phase or microstructure can be beneficial to create longer life time of charge carriers through the interfacial charge transfer, making the system promising. Pan et al. 56 demonstrated a TiO 2 QD HSs with 2D (001) faceted TiO 2 nanosheet. The band position of narrow band gap 2D TiO 2 and large band gap 0D QDs are suitable enough for the efficient transfer of photogenerated electrons from the 2D to 0D surface and vice versa for the holes. Thus, a uniformly coated TiO 2 nanosheet with TiO 2 QDs (synthesized after 168 hrs of hydrothermal treatment) shows superior degradation efficiency under UV light, see Fig. 18(a-c). Besides, a highly stable and recyclable p-n heterojunction photocatalyst composed of Cu 2 O octadecahedra and TiO 2 QDs has been studied by Xu and coworkers. 57 Incorporation of TiO 2 QDs enhances the visible light sensitization and the charge transfer from the p-type Cu 2 O to n-type TiO 2 is thermodynamically favorable leading to high efficiency (97% in 60 min under visible light), see the Fig. 18 (d-g). Sood et al. 58 studied UV light irradiated dye degradation by TiO 2 QDs and observed that several factors like ph of the aqueous solution, catalyst dose, reaction temperature and initial dye concentration affected the degradation rate. They found a maximum degradation in acidic ph

32 with optimum catalyst dose 0.75 g/l as 95 % after 60 min of irradiation at ambient reaction condition of 25 C, see Fig. 18(h-k). Fig. 18: Photocatalytic degradation rate constants in unit surface area of as-grown samples and P-25 (a), scheme of interfacial charge transfer between TiO 2 QDs and nanosheets (b) and for clean nanosheets (c). UV-visible absorption spectra of TiO 2 QDs, Cu 2 O octadecahedra and their HSs. The same for P25 has been recorded for comparison (d), degradation curves of methyl orange (MO) in presence of various catalysts with irradiation (visible) time (t) (e), stability of Cu 2 O/TiO 2 QDs as photocatalysts (f) and interfacial charge transfer mechanism for enhanced photocatalytic performance (g). Impact of (h) catalyst amount, (i) ph, (j) initial dye concentration and (k) reaction temperature on the photocatalytic performance under UV irradiation. Reproduced with permission from 56, L. Pan, et al., Chem. Commun. 49 (59), 6593 (2013). 2013, RSC, (a-c). Reproduced with permission from 57, X. Xu, et al., ACS Appl. Mater. Interfaces 8 (1), 91 (2016). 2016, ACS, (d-g). Reproduced with permission from 58, S. Sood, et al., J Alloys Compd. 650, 193 (2015). 2015, Elsevier Ltd. (h-k).

33 4.2. 1D TiO 2 HSs Doping: Metal and Non-metal Elements A controlled incorporation of a suitable secondary element into the TiO 2 lattice may introduce additional energy levels in between the forbidden region of TiO 2 nanostructured materials and thus can sensitize the enhanced visible light absorption and suppressed recombination of the photogenerated electrons and holes. The first report on the successful doping of nitrogen into TiO 2 by sputtering method in a nitrogen rich environment showed extended optical absorption band from UV to visible region, resulting to the superior visible light photodegradation of methyl orange (MO). 31 Following this work, several reports have been published on the doping with various non-metal ions such as nitrogen (N) 145, carbon (C) 146, sulfur (S) 147, 148 and fluorine (F) 149, 150 fabricated by various methods including hydrothermal, sputtering and thermal treatment method (see Fig. 19). Introduction of doping element causes the narrowing of the effective band gap of the TiO 2 which facilitates the interfacial transfer of photogenerated carries and thus, dramatically enhanced photocatalytic activity under visible light irradiation. Besides the individual doping, the existing literature is mainly concentrated on the codoping with N and F, or N and S (see Fig. 19). 93, 151 These results mainly demonstrate the incorporation of additional energy band, enhanced visible light absorption, lower recombination rates, making the TiO 2 nanostructures suitable to be commercially used. sp 2 -hybridized single layer graphene has been widely used for the surface modification of TiO 2 nanostructures due to its unique mechanical, electrical, and thermal properties. 146, 147, 152 Recently, Ge et al. 153 showed improved photocatalytic dye degradation and hydrogen evolution by a reduced graphene oxide/tio 2 nanotube composite fabricated by electrodeposition followed by carbonation technique. 101

34 Fig. 19: (a,b) SEM images of anodized TiO 2 nanotubes immersed in water and in aqueous ammonia solution with the concentration of 1:1 (water:ammonia), respectively. The bar scale is 200 nm. (c) The absorption spectra of TiO 2 nanotube arrays immersed in water and different concentration nitrogen doped TiO 2, inset shows the corresponding Tauc plot to estimate the effective band gap. (d) Photocatalytic degradation of MO under visible light irradiation. (e) Schematic diagram showing the band structure of pure TiO 2 and N-doped TiO 2 nanotubes. A and DI represent ammonia solution and deionized water, respectively. (e,f) Morphological features of N F co-doped TiO 2 nanobelts with different magnifications. (h) Pore diameter distribution and inset shows the BET surface area analysis of bare TiO 2 nanobelt ((represented as B ).) and N-F co-doped nanobelts (represented as A ). (i) UV-visible absorption spectra of nanobelts and corresponding Tauc plot. (j) Degradation of dye solution with irradiation time in presence of various catalysts. Reproduced with permission from 145, X. Hou, et al., solid state sci. 29, 27 (2014). 2014, Elsevier Ltd. (a-e). Reproduced with permission from 93, Z. He, et al., ACS Appl. Mater. Interfaces 4 (12), 6816 (2012). 2012, ACS, (f-j). Doping with the ions of transition metal such as Fe, V and Cu has also been explored for the improvement of the photoelectron conversion efficiency under the solar light illumination suppressing the recombination of photogenerated charge carriers and stability Though the doping is beneficial for the improvement of charge separation and photocatalysis, it is observed that the performance of TiO 2 is fully dependent on the doping amount, energy state, electron configuration of the doped element and also on the percentage of doping. Excessively

35 high amount of doping may act as the recombination center rather than the trap centre, resulting in low photocatalytic activity Plasmonic 1D TiO 2 HSs with Noble Metals The most promising approach for the enhancement of visible light absorption, high charge separation at the interface and hence the photocatalytic activity is to decorate the TiO 2 nanostructures with noble metal nanoparticles such as Au, Ag, Pd, Pt, Cu, etc In general, Fermi level position of Au, Ag, Pt, etc. noble metals lies just below the conduction band of TiO 2. Thus, the metal NPs can act as trap centers for the photogenerated electrons from valence band to conduction band upon visible light illumination. 165 Besides, a properly designed TiO 2 HS with noble metals involves a coupling interaction between the plasmonic oscillations and the electromagnetic radiation incident on the nanostructures with the same frequency. Due to this strong coupling, excitation of extremely intense and highly localized surface plasmon resonance (LSPR) is observed. These plasmonic nanostructures with TiO 2 may be advantageous for the Fig. 20: A schematic of generation of hot electrons, their transfer and creation of Schottky junction at the interface of both systems. efficient light trapping component integrated in photocatalytic devices. The plasmonic nanostructures drive the LSPR excitation following the light absorption and eventually the plasmons may decay non-radiatively in a femtosecond time scale, giving its energy to the

36 electrons in the conduction band of TiO 2. Thus, highly energetic electrons (1-4 ev for the Ag or Au NPs) are produced in thermal non- equilibrium with the atoms of the TiO 2 known as hot electrons (see Fig. 20). After the migration of these electrons from the plasmonic material to the TiO 2 in contact forms a metal-semiconductor Schottky junction, which may accelerates the 71, 166, 167 rate of photocatalysis. Among the various existing strategies of fabrication of plasmonic noble metal NPs (photoreduction method, plasma sputtering, electrodeposition, electrospinning and hydrothermal method) on TiO 2 nanostructures, the hydrothermal method has been established to be the most easy and versatile method in controlling the shape, size and dispersion over the TiO 2 template. Ye et al. 100 sensitized TiO 2 nanotube arrays with Pd quantum dots (of size ~ 3.3 nm) by a facile hydrothermal method (see Fig. 21), which showed a superior photoelectrocatalytic water splitting under UV irradiation. Lin et al. 168 reported the deposition of Pt NPs of size nm titanate nanotubes by a wet impregnation method. Ghaffari et al. 169 adopted a UV photo chemical reduction method to deposit monodisperse Au NPs with size 8-12 nm on TiO 2 NRs. Nam et al. 170 decorated Ag and Au NPs separately with size 5-10 nm on the TiO 2 nanofibers via a one-step electrospinning process which enables the specific capacity to increase up to 20% in Li-ion battery application (see Fig. 21). Recently, our group has reported on the ultrahigh photodegradation efficiency of Ag NPs (size ~ 17 nm) decorated anatase TiO 2 NRs HSs synthesized by a solvothermal method followed by a UV light induced photoreduction procedure. Almost 100% of degradation efficiency with two sequential rate constants is observed, which was explained on the basis of plasmonic effect induced enhanced visible light absorption, hot electrons generated charge transfer through the interface of the HSs and N-deethylation process of RhB. 71 Yang et al. 171 reported 100% photodegradation of RhB by Ag nanocrystal (size 3.8 nm) decorated TiO 2 nanotubes under visible light. Cheng et al. 159

37 demonstrated a novel synthesis strategy of Ag@TiO 2 core-shell nanostructure by a vaporthermal method and showed the superiority of the system as a photocatalyst over the commercial P25. Fig. 21: (a,b) SEM and TEM images of Pd/TiO 2 nanotube HSs. (c) amount H 2 evolved in photocatalytic water splitting. (d,e) FESEM and TEM micrographs of Ag/TiO 2 nanofibers, (f) enhanced degradation efficiency of organic dye. (g) TEM image of Au NRs encapsulated by TiO 2 layers (h) extinction spectra of Au NRs display two SPR bands at 516 and 655 nm, corresponding to their transverse and longitudinal SPR modes, respectively. (i) Shows the normalized degradation curves with irradiation time. (j) FESEM image of Ag@TiO 2 core-shell structure and inset shows the degradation efficiency. (k) TEM image of Ag/TiO 2 NRs with 1.5:1 weight ratio and (l) sequential decay kinetics of HSs. (m-o) TEM images of TiO 2 nanofibers decorated with Au, Pt and both, respectively. Inset shows the HRTEM lattice fringe patterns. (p) Shows improved H 2 evolution by the bi-metal decoration. (q) Schematic diagram showing the LSPR absorption, generation of hot electrons and their transfer to accelerate the photocatalytic reaction. Reproduced with permission from 100, M. Ye, et al., J. Am. Chem. Soc. 134 (38), (2012). 2012, ACS, (a-c). Reproduced with permission from 169, M. Ghaffari, et al., Electrochimica Acta 76, 446 (2012). 2012, Elsevier Ltd. (d-f). Reproduced with permission from 172, N. Zhou, et al., Nanoscale 5 (10), 4236 (2013). 2013, RSC, (g-i). Reproduced with permission from 159, B. Cheng, et al., J. Hazard. Mater. 177 (1), 971 (2010). 2010, Elsevier Ltd. (j). Reproduced with permission from 71, K. K. Paul, et al., J. Phys. Chem. C 121 (36), (2017). 2017, ACS, (k,l,q). Reproduced with permission from 173, Z. Zhang, et al., J. Phys. Chem. C 117 (49), (2013). 2013, ACS, (m-p).

38 Wang et al. 37 showed a hydrothermally grown TiO 2 NWs with a diameter of nm anchored with 1 At% of Pt NPs with the average size ~ 5 nm exhibit very high photodegradation of MB under UV light. Thus, plasmonic metal NPs modified TiO 2 nanostructures can effectively enhance visible light absorption, suppress the recombination of photogenerated electron/hole pairs, and facilitate their transfer through in the interfaces improving the photocatalytic hydrogen production activity, solar cells, supercapacitors and 171, lithium batteries TiO 2 Type-II HSs The discovery of semiconductor heterostructure by Herbert Kroemer and Zhores I. Alferov opened up a wide research window in the field of high speed optoelectronics, photocatalysis and energy storage applications. The band position and their alignment at near the interface solely determine the catalytic behavior of a semiconductor heterojunction. Among the various semiconductor heterojunctions, a type-ii (staggered type) band arrangement of TiO 2 with a suitable semiconductor develops a band bending at the heterojunction resulting to a built-in electric field, Fig. 22: Schematic band alignment of staggered type semiconductor heterojunction showing charge separation at the interface. which actually drives the photogenerated electrons and holes in two opposite directions (see Fig. 22). The formation of heterojunction by coupling an n-type semiconductor (say, TiO 2 ) with a p- type semiconductor (say, Cu 2 O, Ag 2 O) 47, 175, 176 develops an electric field from n-type to p-type semiconductor, which increases the carrier separation efficiency at the interface.

39 Though numerous fabrication techniques have been developed for the synthesis of semiconductor HSs including physical (sputtering, physical vapor deposition) and chemical (hydrothermal, sol gel, sonochemical, electrodeposition, microwave and chemical vapor deposition) approaches, hydrothermal technique is extremely popular due to its low cost, versatility against the shape tuning and multipurpose advantages over the other processes. Liu et al. 177 synthesized a type-ii heterojunction composed of TiO 2 (B)/anatase core-shell nanowire (see Fig. 23). As the conduction as well as the valence band of anatase lies above the respective bands of TiO 2 (B), an efficient charge transfer may happen at the junction, which is described to be responsible for the enhanced photocatalytic activity towards the decomposition of organic dye, methyl orange (MO). Zhou and co-workers demonstrated the high photocatalytic activity by anatase TiO 2 nanobelt designed with Ag 2 O NPs in the UV irradiation. 178 Following this work several reports have been published focusing on the mechanism of photocatalytic degradation of dye with various ionic nature. 47, 176 Zhou et al. 179 reported a novel synthesis route of TiO 2 nanobelt wrapped with few layer MoS 2 for the first time via the hydrothermal method (see Fig. 23). Rough surface of TiO 2 nanobelt was used as the template for the growth of few layer MoS 2 film. The 50 wt% of MoS 2 loaded on TiO 2 showed maximum photocatalytic hydrogen production activity at rate of 1.6 mmol h -1 g -1. Following this work, several unique approaches have been reported for the further improvement with MoS 2 layered structure. 180, 181 Wang et al. 182 incorporated wide band gap SnO 2 nanocrystals on the TiO 2 nanofibers via electrospinning method followed by a facile hydrothermal method. Cadmium sulfide (CdS) is also a commonly used candidate to form a type-ii HSs. It has the band gap of ~ 2.4 ev which is able to absorb visible light up to 520 nm. Shao et al. 183 developed a novel route to deposit CdS NPs uniformly on the TiO 2 nanotubes via a constant current

40 Fig. 23: (a,b) SEM and TEM images of TiO 2 (B)/anatase core-shell nanowires, (c) Photocatalytic performance in decomposition of dye. (d) TEM micrograph of Ag 2 O/TiO2(B) NRs, (e) UV-visible absorption spectra of HSSs along with the bare TiO 2, (f) enhanced degradation efficiency of organic dye MO under visible light. (g) FESEM image of few layer MoS 2 coated TiO 2 nanobelts (h,i) corresponding photocatalytic activity in dye degradation and H 2 evolution, respectively. (j,k) FESEM images of Copper(II) Phthalocyanine decorated TiO 2 nanofibers, (l) corresponding UV-visible absorption and (m) degradation efficiency under visible light. (n) A schematic diagram explaining the charge separation and their participation in the redox reaction to initiate the decomposition process. Reproduced with permission from 177, B. Liu, et al., ACS Appl. Mater. Interfaces 3 (11), 4444 (2011). 2011, ACS, (a-c). Reproduced with permission from 47, K. K. Paul, et al., 27 (31), (2016). 2016, IOP Science, (df,n). Reproduced with permission from 179, W. Zhou, et al., Small 9 (1), 140 (2013). 2013, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (g-i). Reproduced with permission from 184, M. Zhang, et al., ACS Appl. Mater. Interfaces 3 (2), 369 (2011). 2011, ACS, (j-m). electrochemical deposition process. Compared to the bare TiO 2 nanotube arrays the CdS/TiO 2 HSs showed around 11-fold enhancement in photoelectrochemical activity. Similar reports with semiconductors CdS QDs 185, CdSe 186, CdS/CdSe 187, C 3 N 4 188, Sn 3 O 4 189, RuO 2 190,

41 Bi 2 MoO 6 191, Copper(II) phthalocyanine 184, ZnO 192, ZnIn 2 S have also explored the development of type-ii HSs towards the commercialization of the photocatalysts D TiO 2 HSs Noble metal NPs loaded highly energetic (001) faceted 2D TiO 2 HSs have also been explored extensively. As discussed earlier, the (001) facets have high average surface energy resulting to the highly active sites for the photocatalytic application towards environmental remediation as well as energy applications. Thus, several strategies have been developed to increase the surface area of exposed (001) facets. The plasmonic noble metal NPs (Ag, Au, Pd, Pt, etc.) anchored with the 2D TiO 2 nanosheets can influence the spectral response, generation of photoinduced carriers, their separation and participation in the redox reactions which actually control the kinetics of photocatalysis, as discussed for the 1D TiO 2 HSs case (section 4.2.2). Till date, various fabrication techniques have been developed to prepare plasmonic noble metal decorated 2D TiO 2 nanostructures such as, impregnation, UV light induced photodeposition, coprecipitation and sol-gel methods As, the photocatalytic activity in a plasmonic semiconductor nanocomposites is strongly influenced by the size, shape and the spatial distribution of the NPs, it is important to develop an easily controllable methodology to decorate uniform and well-dispersed metal NPs on the TiO 2 nanosheets. Recently, Zhu et al. 197 demonstrated a novel facile route to synthesize (001) facet dominated TiO 2 nanosheets uniformly decorated with Au NPs (average size ~ 5 nm) by a polyol reduction process. The LSPR induced absorption in Au NPs makes the HSs visible light sensitive having the characteristic absorption peak at ~ 546 nm (see Fig. 24). The degradation of RhB under visible light has been studied and obtained complete degradation after 60 min of irradiation. Wang et al. 198 proposed the growth of plasmonic coupling photocatalyst, Au/TiO 2 /Au nanostructure

42 directly on a Ti foil. The enhancement in the near-field amplitude of LSPR in the vicinity of metal NPs boosts the kinetics of photocatalysis of the neighboring semiconductor. Here, authors have optimized the thickness (~ 5 nm) of the middle layer TiO 2 nanosheets to be satisfied the condition of required for the coupling effect. Thus, this system advantageous as a plasmonic coupling photocatalyst compared to the bare TiO 2 nanosheet. Besides the Au NPs, Pt NPs loaded TiO 2 nanosheets also reported by several groups for the enhanced H 2 production by the photocatalytic water splitting. Yu et al. 158 reported that the 2 wt% of Pt NPs decorated (001) facet exposed anatase TiO 2 nanosheets exhibit highest H 2 evolution of mol h -1. Following this work, Qi et al. 199 fabricated CdS sensitized Pt/TiO 2 nanosheet HSs with exposed (001) facets via a hydrothermal treatment followed by a photochemical reduction (see Fig. 24). It is observed that the H 2 evolved under the visible and UV irradiation are 265 and 590 mol h -1, respectively.

43 Fig. 24: (a,b) TEM images of (001) faceted TiO 2 sheet before and after Au deposition, respectively, (c) Kubelka-Munk function reflecting the absorption of the HSs and inset shows a magnified view of the visible region, (d) corresponding photocatalytic performance in decomposition of RhB under visible region. (e,f) TEM micrographs of TiO 2 nanosheets before and after Pt NPs deposition, respectively, (g) UV-visible absorption spectra of HSs along with the bare TiO 2, (h) enhanced photocatalytic H 2 evolution by the HSs. (i,j) TEM micrographs of 2D TiO 2 nanosheets prepared by C 3 N 4 template method, before and after Pt loading, respectively, (k) degradation of C 2 H 4 by the various catalysts, (l) a schematic showing the generation of hot electrons and their migration to the TiO 2 surface to facilitate the enhanced degradation. Reproduced with permission from 197, S. Zhu, et al., Appl. Catal. B: Environ. 119, 146 (2012). 2012, Elsevier Ltd. (a-d,l). Reproduced with permission from 158, J. Yu, et al., J. Phys. Chem. C 114 (30), (2010). 2010, ACS, (e-h). Reproduced with permission from 200, X. Pan, et al., ACS Appl. Mater. Interfaces 8 (16), (2016). 2016, ACS, (i-k). Jiang and co-workers developed a hybrid nanostructure of TiO 2 nanosheets designed with Au nanocubes and Au nanocages for the sensitization of broad solar spectrum from visible to NIR region and plasmon-mediated photocatalytic hydrogen production. 201 Additionally Pd nanocubes increase the catalytically active sites enormously to enhance the hydrogen production rate. Doping with metal and non-metal elements in (001) facet exposed TiO 2 nanosheet can improve the optical absorption simply by modifying the band arrangement of the nanostructure and shifting the threshold of the optical response value from UV to the visible region. Introduction of a new element may substitute the lattice oxygen and form interstitial species in the vicinity of near surface regions. Thus, visible light sensitization can effectively be incorporated by narrowing the band gap due to the formation of mid band gap energy levels of doping element. Doping of TiO 2 (001) facets with non-metal elements such as nitrogen was demonstrated by Xiang et al. 202 for the first time (see Fig. 25). They observed superior visible light photocatalytic

44 H 2 -production activity by the self-doped TiO 2 nanosheets (exposed facet 67%) with nitrogen. To put the step forward, it was expected that employment of other non-metal element into the (001) faceted TiO 2 may also boost the optical absorption. This was indeed realized by Liu and coworkers 203 who demonstrated a synthesis procedure of sulfur (S) doped TiO 2 sheets with highly reactive (001) facets via HF hydrolysis. Though the intrinsic band gap of the TiO 2 nanosheet is unaltered after the doping (3.17 ev), an additional absorption band is detected to fall in the visible region (400 and 550 nm) due to the localized states associated with the S dopant, as shown in Fig. 25. Analytical study on density of states (DOS) revealed that the localized dopant state lying in between the band gap of (001) facets is closer to the valence band maximum facilitating the stronger oxidative holes responsible for the high photocatalysis. Rather than the single element doping as discussed above, doping by multiple elements in 2D TiO 2 nanosheets may give synergistic effect resulting to the enhanced photocatalytic performance. 204 Xiang et al. 151 reported the hydrothermal preparation of N and S co-doped (001) facet exposed TiO 2 nanosheets considering thiourea as the doping precursor. Compared to the bulk TiO 2, the N-S co-doped TiO 2 nanosheet exhibits the appearance of a new absorption shoulder at ~ nm due to the introduction of localized states near the valence band edge and the improved absorption in the range of nm as a result of the combined effect of N and S co-doping, further confirmed by the DFT calculation.

45 Fig. 25: (a) TEM image of N-doped (001) faceted TiO 2 sheet, (b) UV-visible absorption spectra along with the P25, inset shows the digital photographs of the samples. (c) Structural models showing (001) faceted TiO 2 nanosheet with S atoms substituting lattice oxygen. Oxygen, titanium and sulfur atoms are represented by red, gray and yellow spheres, respectively. Calculated electronic structures of (001) facets of S-doped TiO 2 (d), a comparison of photodecomposition of RhB under the irradiation in the range nm. (f) HRTEM image of N-S co-doped TiO 2 nanosheet, (g) UV-visible absorption spectra of as grown and doped TiO 2. The absorption band in the visible region is increasing with the doping percentage. Band structure plots (h,j) and total density of states (DOS) (i,k) for pure TiO 2 and N-S codoped TiO 2, respectively, (l) shows the corresponding photocatalytic dye degradation of RhB. Reproduced with permission from 202, Q. Xiang, et al., Chem. Commun. 47 (24), 6906 (2011). 2011, RSC, (a-b). Reproduced with permission from 203, G. Liu, et al., J. Colloid Interf. Sci. 349 (2), 477 (2010). 2010, Elsevier Ltd. (c-e). Reproduced with permission from 151, Q. Xiang, Phys. Chem. Chem. Phys. 13 (11), 4853 (2011). 2011, RSC, (f-l). An exceptionally advantageous hybrid structure by directly growing 2D anatase TiO 2 nanosheets with (001) exposed facets on the graphene support is useful for the enhanced photocatalytic hydrogen evolution, fast lithium storage, etc D TiO 2 HSs Among the various shaped TiO 2 nanostructures, the 3D superstructures and its composites are least studied system in the field of energy and environmental applications. In

46 spite of having several advantageous properties such as high specific surface area, large number of catalytically active sites, easy charge transfer through the interfaces, the 3D nanostructure is not well explored may be because of the tedious synthesis process. But, in 2009 Wu et al. 210 reported the growth of truncated wedge-shaped TiO 2 nanostructures as shell over the Au NPs as core by a simple and flexible hydrothermal route (see Fig. 26). To stabilize the structure, the immediate layer on the Au NPs is of (101) faceted TiO 2 which minimizes the average surface energy of the whole system. Then, epitaxially segmented orientation growth of (001) faceted TiO 2 was observed on the TiO 2 layer. The F - ions hydrolyzed TiF 4 precursor acts as the shape controlling agent to form well defined wedge-like TiO 2 shells and also contributes to the exposed (001) facets. The superior photocatalytic activity of the Au@TiO 2 core-shell HSs was investigated by decomposition of acetaldehyde in gaseous phase. Wu et al. 211 has developed an exceptional strategy for the bottom up synthesis of AuNR/TiO 2 nano-dumbbell like structures with spatially separated Au and TiO 2 regions, by wet-chemistry technique (see Fig. 26). The as prepared Au NRs are exposed in their side wall which acts as the optical window for the incident light as well as it offers a fast lane transfer of the photogenerated hot electrons to the TiO 2 side, making the AuNR/TiO 2 HSs extremely high efficient for the H 2 generation under visible/nir light. Separation of photocatalysts after the reaction still remains a tedious job for the researchers. Shen and coworkers 212 developed a new magnetically separable plasmonic 3D sandwich-structured Fe 3 O 4 /SiO 2 /Au/TiO 2 photocatalyst, which shows easy magnetic separability, higher absorption in the visible region and thus the enhanced photocatalytic activity towards the decomposition of organic compounds under the illumination of simulated sunlight (see Fig. 26). Following these works, similar reports with other plasmonic NPs like Ag and graphene have also discussed on the enhancement strategy of photocatalysis

47 Fig. 26: (a,b) TEM images of the as-grown core-shell 2 HSs, (c) Schematic diagram showing the proposed pathways of formation of core-shell 2 HSs with truncated wedge-shaped morphology, (d) UV-visible absorption spectra along with the P25, (e) Comparative diagram of photocatalytic decomposition of acetaldehyde in the gas phase. (f) Schematic illustration of the synthesis process of the sandwiched magnetic TiO 2 HSs, (g) The detailed illustration of the sandwich-structure, (h) Typical SEM image of the composite, (i) TEM image of a single magnetic TiO 2 HS, the enlarged sections show the truncated facets of TiO 2. (j,k) Enhanced UV-visible absorption and so as the photocatalysis by the magnetic TiO 2 HSs, as shown respectively. (l) TEM image and (m) STEM elemental mapping of AuNR/TiO 2 HSs, (n,o) comparison of H 2 evolution rate and degradation of methylene blue by various catalysts under visible illumination, (p) structure and mechanism of enhanced photocatalysis under visible light by a AuNR/TiO 2 dumbbell over a core/shell AuNR@TiO 2. Reproduced with permission from 210, X. Wu, et al., Langmuir 25 (11), 6438 (2009). 2009, ACS, (a-e). Reproduced with permission from 212, J. Shen, et al., J. Mater. Chem 22 (26), (2012). 2012, RSC, (f-k). Reproduced with permission from 211, B. Wu, et al., J. Am. Chem. Soc. 138 (4), 1114 (2016). 2016, ACS, (l-p).

48 The formation of type-ii HS composed of 3D TiO 2 microstructure grown by hydrothermal method and an another semiconductor (Ag 2 O NPs) having suitable band position was discussed by Sarkar et al. 176 Uniform decoration of p-type Ag 2 O NPs on the n-type TiO 2 forms numerous number of type-ii p n heterojunction which promotes an efficient charge separation due to built-in electric field at the junction as shown in Fig. 27. Fig. 27: (a) FESEM image of Ag 2 O/TiO 2 HSs where (b) shows the enlarged view, (c) TEM image of the above mentioned HSs, (d) exhibits the enhanced UV light induced dye degradation. (e) FESEM micrograph of TiO 2 /CdS porous hollow microspheres, (f) the corresponding TEM image, (h) UV-visible absorption spectra of CdS, TiO 2, and TiO 2 /CdS HSs. (i) shows the average rate of hydrogen evolution and TiO 2 /CdS porous hollow microspheres HS gives the maximum rate, labelled as C. Reproduced with permission from 176, D. Sarkar, et al., ACS Appl. Mater. Interfaces 5 (2), 331 (2013). 2013, ACS, (a-d). Reproduced with permission from 217, Y. Huang, et al., Dalton Trans. 45 (3), 1160 (2016). 2016, RSC, (e-i). Thus, an enhanced UV light induced photocatalytic activity was demonstrated by the optimized HSs with apparent first-order rate constant of min 1. Different staggered type HSs of 3D TiO 2 demonstrating superior photocatalytic activity was reported with CdS 217, Cu 2 O 218, Fe 3 O and discussed in details in ref 28, 55.

49 5. Conclusion and Outlook Since 1972, intense research efforts have been devoted to the fabrication, properties and surface engineering of TiO 2 nanostructures and their influence in various applications has given rise to a rich database for the future developments. The latest breakthrough works on the synthesis and modifications of shape controlled TiO 2 and its composites certainly brought fascinating as well as advantageous properties for the enhanced photocatalytic performance. This review article provided a state of the art on the advancement of strategies in the novel fabrication of shape controlled TiO 2 with various dimensionalities followed by their applications in environment and energy issues. A proper understanding of the surface chemistry in controlling the morphology of TiO 2 and its HSs with distinct facets exposed is necessary for the designing of high quality and atmosphere stable devices. Most of the frequently used fabrication techniques from 0D to 3D nanostructures including hydrothermal/solvothermal method, electrochemical anodization method, sol-gel method, template-assisted method, electrospinning method and chemical vapor deposition method have been critically reviewed. Among them, the hydrothermal route is found to be the most popular for the fabrication of shape controlled different dimensional TiO 2 and its nanoheterostructures. This is because of the easy control over the size, shape and for the low cost. Enhancement of the specific surface area as well as the catalytically active sites, surface modification with the metal/non-metal doping, formation of type-ii and plasmonic HSs can accelerate the kinetics of photocatalysis, solar cells, supercapacitors and lithium-ion battery performances. Even though the shape tailored TiO 2 based photocatalysts have evolved to be most promising material and capable enough to solve the future needs for diverse applications,

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59 Biographies: Kamal Kumar Paul graduated (Bachelor of Science (with Honors in Physics) from the University of Kalyani in 2011 and obtained his Masters of Science degree in Physics from Indian Institute of Technology Delhi in Then he joined the Department of Physics, Indian Institute of Technology Guwahati for his PhD in 2013 and his research areas of interest are TiO 2 and 2D materials (e.g. MoS2, g-c3n4, WS 2 etc.) hybrids for energy and environment applications. Prof. P. K. Giri obtained his Ph.D. in Physics from Indian Institute of Technology Kanpur in 1998 and then moved to Italy for postdoctoral research with ICTP TRIL fellow-ship. During , he worked in Indira Gandhi Centre for Atomic Research, Kalpakkam as a Scientist. Later he moved to Indian Institute of Technology Guwahati as an Assistant Professor of Physics. Presently he is a full Professor of Physics and Center for Nanotechnology at the same institute. For his outstanding research contributions, he received several national/international awards/fellowships including ICTP TRIL fellowship (1998), DAE Young Scientist Award (2000), DAAD Exchange visit Fellowship (2010), JSPS Invitation Fellowship for log-term research in Japan(2012) etc. He has published 120 Research articles including 5 review articles in high profile international journals and holds one Patent to his credit. He is an Executive Council Member of Electron Microscopy Society of India and a life member of Materials Research Society of India. His research areas of interests are semiconductor nanostructures, optoelectronics, 2D Materials based hybrid nanostructures for energy and environmental applications, nano biosensors, etc.