Microwave modification of surface hydroxyl density for g-c 3. with enhanced photocatalytic activity

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1 Materials Research Express PAPER Microwave modification of surface hydroxyl density for g-c 3 N 4 with enhanced photocatalytic activity To cite this article: Na An et al 2018 Mater. Res. Express View the article online for updates and enhancements. This content was downloaded from IP address on 07/05/2018 at 19:41

2 RECEIVED 18 January 2018 REVISED 10 February 2018 ACCEPTED FOR PUBLICATION 20 February 2018 PUBLISHED 7 March 2018 PAPER Microwave modification of surface hydroxyl density for g-c 3 N 4 with enhanced photocatalytic activity Na An 1, Yang Zhao 2, Zhiyong Mao 1,4, Dinesh Kumar Agrawal 3 and Dajian Wang 1,4 1 Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin , People s Republic of China 2 China academy of civil aviation science and technology, Beijing , People s Republic of China 3 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, United States of America 4 Authors to whom any correspondence should be addressed. mzhy1984@163.com and djwang@tjut.edu.cn Keywords: photocatalyst, suface hydroxyl group, photodegradation activity, g-c 3 N 4 Abstract Microwave modification was performed on graphitic carbon nitride (g-c 3 N 4 ) photocatalysts to tail the surface hydroxyl content for enhanced photocatalytic activity in this work. The influence of microwave heating on the surface hydroxyl density was investigated by a suite of characterization methods. The microwave treated g-c 3 N 4 (MT-g-C 3 N 4 ) delivered a higher photocatalytic activity in degradation of Rhodamine B (RhB) under visible light irradiation than pristine g-c 3 N 4 due to its improved separation efficiency of photogenerated charge carries and promoted absorption capacity of RhB reactants on surface, which resulted from the increased surface hydroxyl density induced by microwave treatment. This study provides a simple and convenient method to modify g-c 3 N 4 materials with enhanced photocatalytic activity for the potential application in photocatalytic elimination of environmental pollutants. Introduction With the rapid development of global industrialization, chemical and textile industries discharged severe organic pollutants including phenols, dyes, phthalates, benzoic acid etc are introduced into our environment, arousing increasing global concern on environmental pollution [1 4]. Advanced oxidative processes, which are activated through the generation of high reactive species such as hydroxyl radicals, is one of the most effective technologies suitable for the elimination of organic pollutants to harmless end products. In the past decades, significant progress has been made in utilization of photocatalytic degradation of organic pollutants over semiconductors by harvesting of solar energy, and reveals promising prospects to alleviate this environmental problem [5 7]. Among various semiconductors, TiO 2 and ZnO were the most widely used photocatalysts in environmental purification. However, the large band gap (E g 3.2 ev) of traditional TiO 2 and ZnO-based catalysts results in a low solar energy utilization efficiency in the visible-light range, which enormously restricts its practical application. In view of the fact that visible light contributes to about 43% of solar radiation energy, many attempts have been paid to exploit visible-light-driven photocatalysts with high efficiency [8 10]. Graphitic carbon nitride (g-c 3 N 4 ), with a graphitic π-conjugated stacked structure consisting of tri-striazine repeating units, has been vastly studied in recent years due to its merits in good thermal and chemical stability, as well as cost effectiveness [11 13]. Especially, the suitable band gap (E g 2.7 ev) of g-c 3 N 4 enables its widespread acceptability as a visible light response photocatalyst for various photocatalysis applications, including the hydrogen generation from water splitting [14],CO 2 photoreduction into hydrocarbon fuels [15], and degradation of organic pollutants [16]. In a typical photodegradation system, the superoxide radical ( O - 2 ) and hydroxyl radical ( OH) are proposed to be the main active species involved in the oxidation processes. Among of them, OH radicals generate from the reaction of photogenerated holes in valence band over a semiconductor with surface hydroxyl groups or adsorbed water molecules IOP Publishing Ltd

3 Generally, an increase of surface hydroxyl content will enhance the photocatalytic activity in degradation of organic pollutions. And this relationship was well demonstrated by researchers for TiO 2 photocatalysts [17, 18]. For example, Sclafani et al reported that the high activities of rutile TiO 2 is attributed to its large amount of hydroxyl group on the surface and the surface hydroxyl group might not only trap the photogenerated holes in the valence band but also enhance the chemisorption of O 2 molecules in the conduction band [19]; Simonsen et al disclosed that the amount of surface hydroxyl group highly influenced the photocatalytic activity of TiO 2 films tested for the degradation of stearic acid [20, 21]; In situ electron spin resonance measurements performed by researchers and revealed that larger amount of surface hydroxyl group is beneficial for the oxidation of acetic acid by the produced OH radicals [17];Du et al declared that the surface hydroxyl group density was the most important factors that governed the selectivity of the photocatalytic oxidation of cyclohexane to cyclohexanone over TiO 2 materials [22]; Similar correlation was also found between the hydroxyl group population of rare earth metals doped TiO 2 and the degradation rate of methylene blue [23]. However, the influence of surface hydroxyl group on the photocatalytic activity for g-c 3 N 4 photocatalysts is seldom reported even though the surface hydroxyl group has been recognized to play a key role in the photocatalytic processes. On the contrary, the research attention for g-c 3 N 4 photocatalysts was mainly focused on the specific surface area, morphology, defects and doping as well as the heteroconjuction fabrication [24 27]. Very recently, our research work demonstrated that atmosphere plasma treatment could introduce hydrophilic OH and COOH groups on the surface of g-c 3 N 4, thereby enhancing the photodegradation efficiency of RhB [28]. The enhancement of photocatalytic activity for g-c 3 N 4 induced by oxygen plasma surface modification was also demonstrated by Ji et al through introducing oxygen functional groups [29]. Various strategies such as UV irradiation, acid treatment, thermal treatment and synthesis condition control were carried out to tail the surface hydroxyl group density of TiO 2 photocatalysts [19 21, 23]. Microwave energy was widely used in materials synthesis and surface modification in view of the advantage that microwave heating can considerable reduce the treatment time with a reduction in the energy consumption [30 32]. As for our concerned g-c 3 N 4 materials, rapid microwave-assisted synthesis technology was well demonstrated in references [33 35]. However, there are no reports about the surface modification of g-c 3 N 4 materials using microwave treatment by far to the best of our knowledge, even though microwave treatment has been reported as an effective strategy to modify the surface chemical functional groups of carbon materials [36], and zeolite catalysts [37]. Herein, microwave modification is employed to tail the surface hydroxyl content of g-c 3 N 4 photocatalysts for an enhanced photodegradation rate of organic pollutions. The influence of microwave treatment on the phase structure, morphology, chemical composition, optical property, especially the surface hydroxyl content and the photocatalytic activity in eliminating RhB as well as the enhancement mechanism are investigated in detail. Experimental Synthesis Pristine graphitic carbon nitride (g-c 3 N 4 ) was prepared by using urea as the starter material through a direct thermal condensation method. Briefly, urea (10 g in powder) in a covered crucible was calcined in a muffle furnace at 550 C for 3 h using a heating rate of 10 C min 1. After the sample was naturally cooled to room temperature and well ground, pristine g-c 3 N 4 powder with a light-yellow was obtained with a yield of 10%. Microwave treatment A household microwave oven with a power of 600 W was used to modify the surface hydroxyl content of g-c 3 N 4. Typically, A certain amount of above obtained pristine g-c 3 N 4 was dispersed in hydrous ethanol by ultrasound and then treated in the microwave oven for a certain time (3, 4, 5 min) to obtain microwave treated g-c 3 N 4, denoted as MT-g-C 3 N 4. The temperature of the ethanol solution during the treating process was measured to be about 80 C by an infrared thermometer. Characterization The phase structure of prepared samples was analyzed by an x-ray diffractometer (XRD)(Rigaku, RINT Ultima- III, Japan) with Cu-Kα radiation. Scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS)(Hitachi, S-4800) and transmission electron microscope (TEM, JEM-6700F, Japan) were used to characterize the morphology and component. The x-ray photoelectron spectroscopy (XPS) measurements was performed by using a Thermal Fisher Scientific 250 XI spectrometer. Fourier transform infrared (FTIR) spectra were collected on a Bruker WQE-410 FTIR spectroscopy. UV visible spectrometer (TU- 1901, China) with a 60 mm diameter integration sphere and fluorescence spectrometer (Hitachi F-4600, Japan) 2

4 Figure 1. XRD patterns for as-prepared pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. was used to measure the Ultraviolet visible (UV vis) absorption spectra and the photoluminescence spectra (PL), respectively. The Brunauer Emmett Teller (BET) surface area was calculated from the nitrogen adsorption-desorption isotherms using AUTOSORB-1 analyzer. The surface hydrophilicity was measured by contact-angle (CA) measurements (Drop Shape Analyzer, DSA100). Photoelectric current (PC) response measurements was performed on an electrochemical workstation (CHI600E, China) as reported in our previous work [28]. Photocatalytic activity testing Photocatalytic activities of pristine g-c 3 N 4 and MT-g-C 3 N 4 samples were tested by photodegradation of RhB. Photocatalyst powder (0.1 g) was suspended in an RhB aqueous solution (100 ml, 20 mg L 1 ) within a Pyrex photocatalytic reactor, which was continuously cooled by a circulating water system. Before exposure to the light, the suspensions were magnetically stirred for 30 min in the dark to ensure that RhB could reach the absorption-desorption equilibrium on the photocatalysts surface. Then, the suspensions were irradiated by a 300 W Xenon lamp with an optical filter (>420 nm). The suspension was withdrawn regularly at time intervals of 5 min to measure the UV vis absorption spectra. The photodegradation ratio of RhB was evaluated by monitoring the intensity of the absorption band maximum. Results and discussion The crystal structure of prepared pristine g-c 3 N 4 and MT-g-C 3 N 4 samples was identified by x-ray diffraction (XRD), as shown in figure 1. Both the pristine g-c 3 N 4 and MT-g-C 3 N 4 samples show two diffraction peaks centered at 13.1 (100) and 27.3 (002), which are assigned to the in-plane structural packing motif and the interplanar stacking of conjugated aromatic rings, respectively [38]. The diffraction patterns behave no obvious variation, indicating the microwave treatment did not change the crystal structure of the pristine g-c 3 N 4. The morphologies of pristine g-c 3 N 4 and MT-g-C 3 N 4 samples were investigated by SEM and TEM, as shown in figure 2. The pristine g-c 3 N 4 sample is composed of irregular thick sheets, as depicted in the the lowmagnification SEM image (figure 2(a)). High-magnification SEM image (inset in figure 2(a)) exhibits that the thickness of pristine g-c 3 N 4 nanosheets is about nm. TEM image (figure 2(c)) of pristine g-c 3 N 4 sample shows that the nanosheets have a large number of in-plane pores with diameters of nm. As compared to pristine g-c 3 N 4, MT-g-C 3 N 4 sample (figures 2(b) and (d)) features same nanosheets structure with abundant of pores. These results reveal that microwave treatment process shows less influence on the microcosmic morphology of g-c 3 N 4. From the EDX elemental analysis results, we found that the C/N molar ratio (1.40) of pristine g-c 3 N 4 is lower than that of MT-g-C 3 N 4 (1.51), indicating the possible formation of nitrogen-vacancy defects in the PT-g-C 3 N 4 framework induced by microwave treatment. In addition, and O content for pristine g-c 3 N 4 (1.40 at%) is also lower than that of MT-g-C 3 N 4 (3.25 at%). This reveals the increase of oxygen containing species for the MT-g-C 3 N 4 after microwave treatment. Later results (FTIR and DRS) would verify that these composition changes results from the transformation of terminal C-NH 2 groups to C-OH groups induced by microwave treatment. 3

5 Figure 2. (a), (b) SEM images; (c), (d) TEM images and EDX elemental analysis results for pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. Figure 3. (a) FTIR spectra; (b) XPS survey spectra; and (c) Photos of contact angle measurements for pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. The chemical composition and surface functional groups of pristine g-c 3 N 4 and MT-g-C 3 N 4 samples were furter investigated by FTIR spectra and x-ray photoelectron spectroscopy (XPS). From the FTIR spectra (figure 3(a)), it was found that both g-c 3 N 4 and MT-g-C 3 N 4 samples show similar characteristic stretching modes of CN heterocycles from cm 1, and typical breathing modes of the triazine units at 810 cm 1, revealing their substantially similar chemical structures. Additionally, two broad peaks around between 4

6 Figure 4. (a) The photocatalytic degradation activity of RhB for pristine g-c 3 N 4 and MT-g-C 3 N 4 with variable microwave treatment time (3, 4, 5 min); (b) Kinetic fit for the degradation of RhB for pristine g-c 3 N 4 and MT-g-C 3 N 4 samples; (c) Changes in absorption spectra of RhB solution with variable photodegradation reaction time for MT-g-C 3 N 4 (4 min) photocatalysts; (d) Recyclability of MT-g-C 3 N 4 photocatalysts for the degradation of RhB cm 1 and cm 1 assigned to surface -NH groups and -OH groups were recorded. It was worth noting that the absorbance of -OH group for MT-g-C 3 N 4 sample is higher than that of pristine g-c 3 N 4. This reveals that the content of surface -OH groups was increased for MT-g-C 3 N 4 after microwave treatment. Three sharp peaks at 286, 396, and 529 ev associated with the C 1s, N 1s, and O 1s signals were recorded on the XPS survey spectra (figure 3(b)) [39]. The observation of oxygen in the samples is mainly attributed to the surface hydroxyl groups. Similar to the EDX elemental analysis results, quantitative analysis from XPS spectra reveals that the O content of MT-g-C 3 N 4 (3.57 at%) is larger than that of pristine g-c 3 N 4 (1.18 at%), suggesting there is a higher density of oxygen-containing species (-OH) on the MT-g-C 3 N 4 surface. The increase of surface -OH content was further confirmed by the enhanced hydrophilicity determined by contact angle measurements using deionized water as test liquids, as shown in figure 3(c). After microwave treatment, the contact angle decreases from 26.6 to The increased hydrophilic surface -OH groups induced by microwave treatment was responsible for this decrease of contact angle. The photodegradation activity of pristine g-c 3 N 4 and MT-g-C 3 N 4 samples was tested using RhB as a model pollutant, as shown in figure 4. It was found that the absorption-desorption reach equilibrium after 20 min in dark for all the g-c 3 N 4 and MT-g-C 3 N 4 samples (figure 4(a)). Control experiment result indicates that the RhB degradation ratio could be ignored in the absence of irradiation, indicating that RhB is degradated via photocatalytic process other than the adsorption effect. Compared with the pristine g-c 3 N 4, enhanced photocatalytic activity is detected for the MT-g-C 3 N 4 samples. With increasing the microwave treatment time, their photocatalytic activity increases first and then decreases, the optimal microwave treatment time was recorded to be 4 min. The photocatalytic degradation kinetic rate constants of these photocatalysts are obtained by plotting ln(c/c 0 ) with the irradiation time (min) using a simplified Langmuir-Hinshelwood model ( ln(c/c 0 ) = kt) [32], as shown in figure 4(b). It is obviously that the kinetic rate constants of MT-g-C 3 N 4 are higher than that of pristine g-c 3 N 4, indicating the better photocatalytic activity. The highest kinetic rate constant for MT-g-C 3 N 4 (4 min) is evaluated to be min 1, which is near 2 times higher than that of pristine g-c 3 N 4 (0.098 min 1 ). The characteristic absorption peak of RhB solution at 554 nm shifts to shorter wavelengths and the intensity decreases and disappeares eventually under visible light irradiation with the prescence of MT-g-C 3 N 4 photocatalyst (figure 4(c)). These changes indicate the remove of an ethyl group and the destroy of benzene ring from RhB. The stability of catalytic performance for MT-g-C 3 N 4 photocatalysts were evaluated by a 4-run cycling (figure 4(d)), which displays that the photodegradation activity of MT-g-C 3 N 4 shows no obvious decrease. 5

7 Figure 5. Nitrogen adsorption-desorption isotherms of pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. Figure 6. UV vis diffuse reflectance spectroscopy (DRS) and the plots of the (αhv) 2 versus hv photon energy for pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. It was well recognized that the photocatalytic activity of a photocatalyst is strongly determined by its physical and chemical properties, such as the specific surface area, light absorbing capacity and photogenerated charges seperation effeciency [40, 41]. Generaly, a larger specific surface area enables more active sites and absorbed more reactants on its surface, then leading to a higher photocatalytic activity. The N 2 adorption-desorption isotherms were measured firstly to demonstrate the influence of microwate treatment on the BET specific surface areas, as exhibited in figure 5. It could be seen that the BET specific surface areas of pristine g-c 3 N 4 and MT-g-C 3 N 4 is 66.7 m 2 g 1 and 72.5 m 2 g 1 respectively. In comparision with pristine g-c 3 N 4, the only 10% increase from 66.7 m 2 g 1 to 72.5 m 2 g 1 for MT-g-C 3 N 4 reveals that the microwave treatment shows no obvious effect on the specific surface area. In other words, the slight increase of specific surface area induced by the microwave treatment is not the dominant reason for the enhanced photocatalytic activity. The optical absorption properties were investigated by the UV vis spectroscopy to clarify the enhanced photocatalytic mechanism. Figure 6 depicts the UV vis diffuse reflectance spectroscopy (DRS) of pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. It was found that pristine g-c 3 N 4 shows an absorption edge at about 450 nm, corresponding to the intrinsic absorption from valence to conduction band [11]. After microwave treatment, the absorption threshold shifts to a shorter wavelength. Correspondingly, the band gap energy of g-c 3 N 4 and MT-g-C 3 N 4 was evaluated to be 2.85 ev and 2.88 ev respectively via the Tauc plot calculated from the DRS data (inset of figure 6). This slight increased optical band gap of MT-g-C 3 N 4 might be attributed to the higher surface -OH density induced by microwave treatment. Thus, the change of optical absorption induced by microwave treatment would show a negative influence on the photocatalytic activity, from the perspective of light absorbing 6

8 Figure 7. (a) Photoluminescence (PL) spectra (λ ex = 375 nm); and (b) transient photocurrents for pristine g-c 3 N 4 and MT-g-C 3 N 4 samples. capacity. It was well recognized that the band gap of g-c 3 N 4 photocatalysts is strongly dependent on the formation of nitrogen vacancies in the framework. Niu et al reported that the occurrence of nitrogen vacancies would narrow the band gap of g-c 3 N 4 remarkably [39]. However, DRS depicts that MT-g-C 3 N 4 behaves a slight increased band gap. This confirms the above discussed increase of C/N ratio is attributed to the transformation of terminal C-NH 2 groups to C-OH rather than the formation of nitrogen vacancies. After the light being absorbed by photocatalysts, the separation efficiency of the photogenerated carries is crucial for the photocatalytic reaction. In order to evaluate the separation efficiency of photogenerated carries, photoluminescence (PL) and photocurrent measurements were demonstrated, as shown in figure 7. Generally speaking, a lower PL intensity indicates a high separation rate of photoexcited electron-hole pairs. Figure 7(a) displays the PL spectra of pristine g-c 3 N 4 and MT-g-C 3 N 4 excited by 375 nm. Obviously, the PL intensity of MT-g-C 3 N 4 is lower than that of pristine g-c 3 N 4, revealing that the separation efficiency of photogenerated electron-hole paris was enhanced by the microwave treatment [28]. In addition, the enhanced separation efficiency of photogenerated carries was further demonstrated by the comparision about the transient photocurrent responses for pristine g-c 3 N 4 and MT-g-C 3 N 4 under visible light irradiation in an on-and-off cycle mode, as depicted in figure 7(b) It is obviously seen than the photocurrent of MT-g-C 3 N 4 much higher than that of pristine g-c 3 N 4, indicating a higher separation efficiency of photogenerated charge carriers [29]. Both the PL and photocurrent analysis showed that the separation effeciency of photoexcited electro-hole pairs for MT-g-C 3 N 4 is effectively enhanced afther microwave treatment, then resulting in a enhanced photocatalytic activity in degradation of RhB. Figure 8 describes the possible enhanced mechanism of photocatalytic activity over MT-g-C 3 N 4 photocatalysts. For a typical photodegradation system, the superoxide radical ( O - 2 ) and hydroxyl radical ( OH) are proposed to be the main active species involved in the oxidation processes of organic pollutants. The OH radicals are dominantly produced by the reaction of photogenerated holes in valence band over a semiconductor with surface hydroxyl groups or adsorbed water molecules. As aboved anlysised that the microwave treatment would increase the surface -OH groups denisity. Thus, the enhancement of photocatalytic activity in 7

9 Figure 8. Possible enhanced mechanism of photocatalytic activity for MT-g-C 3 N 4 photocatalysts induced by microwave treatment. degradation of RhB for MT-g-C 3 N 4 is mainly attributed to the presence of larger amount of surface -OH groups, which would trap the photogenerated holes by formaiton of surface OH. The effective formaion of surface OH resulted from higher surface -OH content might not noly inhibit the recombination of photogenerated electronhole paris but also supply more OH active species for oxidation reaction [38]. In addition, the increased hydrophilic surface -OH groups induced by microwave treatment would enhance the hydrophilicity for MT-g-C 3 N 4. The higher hydrophilicity is beneficial to adsorb more organic molecues on the surface of catalysts, thereby facilitating the photocatalytic activity [20]. Conclusion In summary, microwave modification was employed to increase the surface -OH content of g-c 3 N 4 photocatalysts. As a result, the photodegradation efficiency in remove of RhB for microwave treatment g-c 3 N 4 (MT-g-C 3 N 4 ) under visible light irradiation is near 2.0 times higher than that of the pristine g-c 3 N 4. The enhanced photocatalytic activity is attributed to the improved separation efficiency of photogenerated charge carries and promoted absorption capacity of RhB reactants on surface, resulting from the increased surface hydroxyl density induced by microwave treatment. Acknowledgments We gratefully acknowledge the financial support by the National Natural Science Foundation of China (nos and ) and Foreign Experts Thousand Talents Program (Tianjin, China). ORCID ids Zhiyong Mao References [1] Hoffmann M R, Martin S T, Choi W and Bahnemann D W 1995 Environmental applications of semiconductor photocatalysis Chemical Reviews [2] Chen H, Nanayakkara C E and Grassian V H 2012 Titanium dioxide photocatalysis in atmospheric chemistry Chemical Reviews [3] Kanakaraju D, Glass B D and Oelgemoeller M 2014 Titanium dioxide photocatalysis for pharmaceutical wastewater treatment Environmental Chemistry Letters [4] Palmisano L et al 2011 Titania photocatalysts for selective oxidations in water Chemsuschem [5] Cui Y J, Wang Y X, Wang H and Chen F Y 2016 Graphitic carbon nitrides: modifications and applications in environmental purification Progress in Chemistry [6] Luo B, Liu G and Wang L 2016 Recent advances in 2D materials for photocatalysis Nanoscale [7] Addamo M et al 2008 Environmentally friendly photocatalytic oxidation of aromatic alcohol to aldehyde in aqueous suspension of brookite TiO(2) Catalysis Letters [8] Liu G, Niu P and Cheng H-M 2013 Visible-light-active elemental photocatalysts Chemphyschem [9] Tong Z et al 2015 Three-dimensional porous aerogel constructed by g-c 3 N 4 and graphene oxide nanosheets with excellent visible-light photocatalytic performance Acs Applied Materials & Interfaces

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