Semiconductor Heterojunctions for Enhanced Photocatalytic Hydrogen production

Transcription

1 Semiconductor Heterojunctions for Enhanced Photocatalytic Hydrogen production Abdou Lachgar Center for Energy, Environment, and Sustainability Department of Chemistry NCEAC/University of Sindh, Jamshoro, Pakistan, Feb , 2017

2 Introducing Wake Forest University Location

3 Data Founded in 1834 Enrollment Undergraduate: ~5,000 Graduate and professional schools: ~2,500 Total enrollment: ~7,500 Faculty: ~400 Faculty/Student ratio: 1/10 Rankings: 27 th among 300 national Cost ( ) Tuition: ~ $49,000 Room and Board and other fees: ~$15,000 Total: ~$64,000 Financial aid: 75% Endowment: $2.25 billion

4 Research is Performed at Three Campuses Reynolda Campus Innovation Quarters Biomedical Sciences Campus

5 16 tenured/tenure track faculty 3 lecturers People 3 full-time instrumentation managers 1 Instructional Technology Specialist ~32 full-time graduate students Postdoctoral associates/research assistant professors/visiting scholars Undergraduate researchers

6 Research Projects in my lab Molecular Building Block Approach Catalysts for Wasteto-Fuel Conversion Heterogeneous Photocatalysis NSF and WFU NC Biofuel Center NAS and USAID Center for Energy, Environment, and Sustainability

7 Outline Background: Hydrogen as fuel Photocatalysis Challenges and Potential Solutions Visible-light-active heterojunctions as photocatalysts Case study: g-c 3 N 4 /Sr 2 Ta 2 O 6 Synthesis Characterization Photocatalytic activity Proposed mechanism Summary

8 Hydrogen as Fuel Largest mass-specific energy content: 2 H 2 (g) + O 2 (g) 2 H 2 O (g) DG rxn =-285kJ/mol MJ/kg, compared to 44.5 MJ/kg for gasoline. Coal 18 % Elect rolysi s 4 % ~8 kg of H 2 to drive a range of 400 kms Clean H 2 O is the only product Most common fuel for fuel cells However, 96% of hydrogen is produced from fossil fuels. Low energy density: 8 kg H 2 occupy 90 m 3 at 1 atm Oil 30 % Natu ral Gas 48 %

9 Photosynthesis vs. Photocatalysis Photosynthesis 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Photocatalytic water splitting Both systems are uphill processes 2H 2 O 2H 2 + O 2 A Kudo and Y Miseki, Chemical society reviews 2009, 38, 1,

10 Semiconductor photocatalysis? Semiconductors absorb energy to generate electrons and holes in CB and VB. The photo-generated carriers can be used in electrochemical reactions (i) Excitation (ii) Migration (iii) Surface chemical rxn. A. Kudo and Y. Miseki, Chemical Society Reviews 2009, 38, 1,

11 Semiconductor photocatalysis - History Photocatalytic water slitting was first reported by Fujishima and Honda in 1972 o Electron hole generation) TiO 2 + h e - + h + o Oxidation at the TiO 2 electrode 2H 2 O + 4h + O 2 + 4H + o Reduction at the Pt electrode 2H + + 2e - H 2 Overall reaction 2H 2 O 2H 2 + O 2 A. Fujishima, K. Honda, Nature 1972, 238, 37 38

12 Semiconductor photocatalysis - History After 1972, lots of photocatalytic materials have been discovered. H + /H 2 O 2 /H 2 O Large band gap (>3.0 ev) Unstable Unsuitable band positions. B. Adeli, F. Taghipour, ECS J Solid state sci technol 2013;2:Q118-Q126

13 Two Major Challenges Absorption range Most photocatalysts so far studied are active only in the UV which represents about 5% of solar spectrum. Lifetime of photogenerated carriers Recombination of photogenerated carriers is thermodynamically favored. Low Efficiency

14 Potential Solutions 1. Cocatalyst loading 2. Band gap engineering Metal ion doping Anion doping 3. Sensitization Dye sensitization Semiconductor Heterojunctions S. Patnaik et al, RSC Adv., 2016,6, M Ni et. al, Ren Sus En Rev 2007, 11,

15 Semiconductors heterojunctions as photocatalyst Two semiconductors to form heterojunction. p-n or non p-n junction Three Different possible combinations

16 Visible-light-active Semiconductor Heterojunctions Three types of visible light active semiconductor heterojunctions Type 1: Visible light active and UV active components Type 2: Two visible light active components Type 3: Z-type of mechanism S. Adhikari, A. Lachgar, Renewable & Sustainable Energy Reviews, Submitted.

17 Bi 2 O 3 /WO 3 and Bi 2 O 3 /Ta 3 N 5 Heterojunctions Example of Type II Bi 2 O 3 /WO 3 Pseudo first order rate constants for photocatalytic degradation process of RhB or 4-NA under visible light ( 420 nm). Example of Type III Z-Scheme Bi 2 O 3 /TaON and Bi 2 O 3 /Ta 3 N 5 Amount of hydrogen gas evolved for different samples in 4 hours (50 mg of catalyst in 50 ml of 20 % aqueous methanol solution irradiated with visible light ( 420 nm) Scheme for electron-hole separation at the Bi 2 O 3 /WO 3 heterojunction S. Adhikari et al., RSC Adv., 2015, 5, S. Adhikari et al., RSC Adv., 2015, 5,

18 g-c 3 N 4 /Sr 2 Nb 2 O 7 (CN/SNO) Heterojunction g-c 3 N 4 / SrTa 2 O 6 Heterojunction - Rationale R Hydrogen CN/SNO (per mole of CN ) = 11 X pristine CN S. Adhikari et al., ChemSusChem, 2016, 9,

19 g-c 3 N 4 / SrTa 2 O 6 Heterojunction - Rationale In-situ: g-c 3 N 4 /SrTa 2 O 6 heterojunction, in which one component is metastable oxide Heterojunction: Relatively low temperature by Chemie Douce (soft chemistry) method Photocatalytic study of K 2 SrTa 2 O 7, H 2 SrTa 2 O 7, and metastable, SrTa 2 O 6. No previous report: Photocatalysis of metastable oxide P. J. Ollivier and T. E. Mallouk, Chem. Mater., 1998, 10 (10), pp

20 g-c 3 N 4 / SrTa 2 O 6 Heterojunction - Synthesis Synthesis Key Steps g-c 3 N 4 / SrTa 2 O 6 Heterojunction 550 o C 4 hr Synthesis of K 2 SrTa 2 O 7 by SSR Proton exchange to obtain H 2 SrTa 2 O 7 Hydrothermal treatment Melamine Grinding followed by sonication 200 o C 24 hr Hydrothermal treatment S. Adhikari et al., Applied Catalysis B: Environmental, Submitted. Calcination to obtain CN/STO heterojunction

21 g-c 3 N 4 / SrTa 2 O 6 Heterojunction Crystal structure parameters K 2 SrTa 2 O 7 H 2 SrTa 2 O 7 SrTa 2 O 6 g-c 3 N 4 Tetragonal Tetragonal Cubic Hexagonal S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.

22 (224) (110) Intensity, a.u. (110) Intensity, a. u. Intensity, a. u. (210) (320) (001) (310) (410) (211) (311) (321) (620) (540) g-c 3 N 4 /SrTa 2 O 6 Heterojunction - PXRD PXRD patterns of KSTO, hydrated KSTO and proton exchanged form HSTO PXRD patterns of HSTO heated at different temperatures PXRD patterns of CN, STO and CN/STO heterojunction (002) (001) (001) (002) (002) (100) (004) (101) (101) (105) (110) (105) (200) (200) (200) (0010) (107) KSTO hydrated (215) (1110) HSTO (2010) KSTO degree STO TTB phase degree (210) (211) degree Upon heating, HSTO converts to metastable cubic phase of SrTa 2 O 6 (STO) at ~500 o C STO converts to Tetragonal Tungsten bronze phase of SrTa 2 O 6 at ~ 900 o C. The Heterojunction CN/STO is made of metastable SrTa 2 O 6 and CN. S. Adhikari et al., Applied Catalysis B: Environmental, Submitted. (001) HSTO (002) (100) (110) (200) 900 o C 850 o C 750 o C 550 o C 450 o C 350 o C 25 o C (100) (100) (110) (111) (200) CN (220) STO CN/STO

23 g-c 3 N 4 /SrTa 2 O 6 Heterojunction: Microscopy SEM images High-resolution STEM images SEM images for (a) KSTO, (b) HSTO, (c) STO, (d) CN, (e, and f) CN/STO heterojunction. (g, h and i) are the elemental mappings for Sr, Ta and N in image (f). STEM images for (a) CN, (b) STO and (c, and d) CN/STO heterojunction. Color codes for EDS mapping in (d): N (purple),ta (yellow), Sr (pink)

24 g-c 3 N 4 / SrTa 2 O 6 Heterojunction DRS and TGA DRS for different catalysts TGA/DSC CN and STO 1:1 mass ratio KSTO (E g = 3.92 ev), HSTO (3.96 ev), STO (3.94 ev) : UV light region Heterojunction : Extended absorption range up to 450 nm S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.

25 g-c 3 N 4 / SrTa 2 O 6 Heterojunction - XPS XPS Study C1s N1s Binding energy, ev Binding energy, ev Survey spectra: Hetrojunction CN and STO only Binding energy, ev C1s and N1s peaks in heterojunction: shifted towards lower binding energy

26 g-c 3 N 4 / SrTa 2 O 6 Heterojunction - Activity Photocatalytic hydrogen production for different catalysts. Amounts of hydrogen per g of catalyst Sample UV light, µmol/h Visible light, µmol/h KSTO HSTO STO CN CN/STO Intimate contact matters!!! Blend sample does not show any enhancement Heterojunction = 137 mmol/h/mole of CN = 9 X pristine CN [AQY = 0.35 % for CN, 2.62 % for CN/STO heterojunction]

27 g-c 3 N 4 / SrTa 2 O 6 Heterojunction - Mechanism VB of STO is lower than that of CN, and CB of CN is higher than that of STO. STO (E g = 3.94 ev) cannot be excited upon visible light irradiation. Photocatalytic hydrogen production by CN/STO: photogenerated electrons in CN.

28 g-c 3 N 4 / SrTa 2 O 6 Heterojunction - Mechanism PL EIS Nyquist plot D CN = ~ 1200 Ω D CN/STO = ~ 400 Ω The suppressed PL intensity: less recombination Small resistance: easy migration of photogenerated electrons from CN to STO S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.

29 Summary Semiconductor heterojunction: To address two major problems in Semiconductor photocatalysis: Absorption range and recombination rate The H 2 evolution for CN/STO heterojunction (per mole of CN) = 9 x than that of pristine CN (under visible light irradiation) Enhanced activity of CN/STO: Efficient charge separation A well designed heterojunction: three major factors Selection of materials: nature and stability Suitable band positions: band alignment Proper synthetic methods: intimate contact

30 Acknowledgements Shiba Adhikari, Cynthia Day, Marcus Wright, Wake Forest University Zili Wu, Rui Peng, Karren More, Ilia Ivanov, Oak Ridge National Lab (ORNL) Carrie L. Donley, for XPS at UNC Chapel Hill Zachary D. Hood, Vincent Chen, for TRPL at Georgia Institute of Technology

31 Thank you

32 Additional slides

33 Activity of CN/SNO vs other heterojunctions Amount of Hydrogen evolved from photocatalytic water reduction with different g-c 3 N 4 based photocatalysts under visible light irradiation. Catalyst Reaction Conditions Co-catalyst loading Amount of Hydrogen per mol of C 3 N 4 Enhancement factor Reference g-c 3 N 4 /Sr 2 Nb 2 O 7 aqueous methanol (10 % vol) solution 2.5 wt. % Pt 107 mmol/h 11 times than the pristine g-c 3 N 4 This work g-c 3 N 4 /Sr 2 Nb 2 O 7 blend sample aqueous methanol (10 % vol) solution 2.5 wt. % Pt 13.8 mmol/h 1.3 times than the pristine g-c 3 N 4 This work g-c 3 N 4 /TiO 2 aqueous methanol (12.5 % vol) solution 0.5 wt. % Pt 14 mmol/h 2 times than the pure g-c 3 N 4 J. Alloys Compd. 2011, 509, L26 L29. g-c 3 N 4 /In 2 O M L-ascorbic solution 0.5 wt, % Pt 20 mmol/h 5 times than the pure g-c 3 N 4 Appl. Catal. B Environ. 2014, 147, The amount of hydrogen produced per mole of g-c 3 N 4 in the CN/SNO is much higher than the reported values for similar heterojunctions.

34 Semiconductors Heterojunctions Use two or more semiconductors to form composites. Generally divided into p-n and non p-n junction Three possible combinations of two semiconductors to form heterojunctions Type 2 was chosen to explore the visible light active semiconductor heterojunctions. S. Adhikari, A. Lachgar, Renewable & Sustainable Energy Reviews, Submitted 2016.

35 g-c 3 N 4 /Sr 2 Nb 2 O 7 composite: XPS High resolution XPS spectra Shiba Adhikari et al., ChemSusChem, 2015, Submitted

36 AQY, TON, TOF

37 AQY, TON, TOF Using the activity of 100 mg of sample for 1 hour of visible light irradiation ( = 420 nm), the apparent quantum yields (AQY) were calculated to be 0.38 % for CN and 2.45 % for the CN/SNO heterojunction. The amount of hydrogen produced after 15 hours of visible light irradiation (975 µmol by 100 mg of CN/SNO) was used to determine the turnover number (TON) for CN/SNO, which was found to reach 1800 in 15 hours with a turnover frequency (TOF) of 120. The details of each equation used for the calculation of AQY, TON and TOF are provided in the experimental section. AQY = 2.45 % for CN/SNO TON = 1.8 and TOF = 1.2 in 15 hrs

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39 Hydrogen amount,mmol/h % (by mass) of CN in composite CN/SNO Figure S5. Amounts of hydrogen generated by different composite (CN/SNO) samples varying the mass percentage of CN in final composite. Conditions: 50 mg catalyst, 50 ml 10 % vol methanol aqueous solution, 300 W Xe-lamp with filter for visible light irradiation ( 420 nm).

40 Table S2. Photocatalytic overall water splitting on Sr 2Nb 2O 7 (SNO) or g-c 3N 4 (CN) powder sample under UV or visible ( 420 nm) light irradiation. The reaction was performed on 100 mg of catalyst in 50 ml of pure water (without any hole scavengers like methanol) with or without 2.5 % (by weight) Pt cocatalyst. Surface Co-catalyst Rate of gas evolution µmol/h area 2.5% loading Catalyst Light source m 2 /g (by weight) H 2 O 2 SNO UV 64.7 none 5.2 trace SNO UV 64.7 Pt SNO Visible 64.7 Pt or none 0 0 CN Visible or UV 5.40 none 0 0 CN Visible 5.40 Pt CN UV 5.40 Pt CN/SNO visible 47.2 none 0 0 CN/SNO UV 47.2 none CN/SNO UV 47.2 Pt

41 Intensity, a.u. Hydrogen evolution, mol Before After st 2nd 3rd Irradiation time, hr , degree Figure 10. Recyclability test for the CN/SNO heterojunction: photocatalytic hydrogen generation for different cycles. The photocatalytic setup was degassed with Ar for 30 minutes after each cycle. Figure S6. PXRD patterns of CN/SNO composite photocatalyst before and after the photocatalytic test.

42 Empirical equation used to calculate band positions E CB = X 0.5 E g + E 0 E VB = E CB + E g E g is the band gap energy of the semiconductor, E 0 is a scale factor relating the reference electrode s redox level to absolute vacuum scale (E 0 = 4.5 ev for NHE), Х is the electronegativity of the semiconductor, which can be expressed as the mean of the absolute electronegativities of the constituent atoms. Example: X for Sr, Nb, O are 2.0, 4.40 and 7.54 ev respectively. Electronegativity of Sr 2 Nb 2 O 7 is {(2.0)*2 + (4.40)*2 + (7.54)*7}1/11 = 5.36 ev. Eg = 3.58 ev E CB (Sr 2 Nb 2 O 7 ) = /2* = ev E VB (Sr 2 Nb 2 O 7 ) = = ev