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Combing oxygen vacancies on TiO2 nanorod arrays with g-C3N4 nanosheets for enhancing photoelectrochemical degradation of phenol
Materials Science in Semiconductor Processing 109 (2020) 104954
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Combing oxygen vacancies on TiO2 nanorod arrays with g-C3N4 nanosheets for enhancing photoelectrochemical degradation of phenol Fan Qi , Weijia An , Huan Wang , Jinshan Hu , Hongxia Guo , Li Liu , Wenquan Cui * College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic Materials, North China University of Science and Technology, Tangshan, Hebei, 063210, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: g-C3N4 nanosheets Oxygen vacancies TiO2 nanorods Z-Scheme heterojunction
We reported a novel TiO2-x/g-C3N4 nanorod arrays photoelectrode by urea drop-calcined and NaBH4 reduction. Due to synergistic effect of the oxygen vacancies and the g-C3N4 nanosheets, the TiO2-x/g-C3N4 photoelectrode exhibited excellent photoelectrochemical property and photoelectrocatalytic activity. The photocurrent density of TiO2-x/g-C3N4 was 6-fold and 1.5-fold higher than TiO2 and TiO2/g-C3N4, respectively. Meanwhile, the phenol degradation rate of TiO2-x/g-C3N4 photoelectrodes was as high as 83% under the stimulated solar light. Besides, the oxygen vacancies could promote the formation of the Z-scheme heterojunction between the TiO2 nanorods and g-C3N4 nanosheets.
1. Introduction Photoelectrocatalytic (PEC) materials are widely researched in water splitting [1], CO2 reduction [2], solar energy storage [3], and organic pollutant degradation [4]. Since 1972, Fujishima et al. came up with TiO2 electrode could be used to split water under ultraviolet light [5]. TiO2 was the common catalyst, and has attracted much attention. However, it is not widely utilized that limited by the lower visible light efficiency and the charges recombination [6,7]. For a long time, how to improve the catalytic performance of TiO2, and the adjustment of microstructure has been the research focus. One-dimensional nano materials own advantages in optical absorption and carrier migration, due to their unique structure and high specific surface areas [8]. TiO2 nanorod, TiO2 nanotube and TiO2 nanofiber are conducive to improving the directional transfer of charge and enhancing photoelectrocatalytic performance [9]. TiO2 mostly responds in the ultraviolet region due to the relatively narrow absorption range in sunlight, how to improve the response range is the focus of research. Many ways have been studied to effectively reduce photo-generated charges recombination, including doping [10], loading [11], modifying [12], and sensitizing [13]. Many studies have modified TiO2 with other materials to form heterostructure. The band structure between TiO2 and other materials could promote the separa tion of charges. g-C3N4 is a π-conjugated material. The van der Waals force is weak between layers in bulk g-C3N4 [14], and easily destroyed to
form nanosheets, thus g-C3N4 has been studied widely. Xu et al. [15] reported that the H2 evolution performance of g-C3N4 photocatalyst enhanced due to the coexistence of Pt0 and Pt2þ. The adjustment of band structure between heterogeneous structures is beneficial for charge separation, and defect engineering is the com mon strategy. Introduction of the defects properly can promote light absorption and improve charge separation. Carbon defect [16], oxygen vacancy [17] and nitrogen defect [18] have been reported widely. In metal oxides, oxygen vacancy is the most common and widely reported. The introduction of the oxygen vacancies on metal oxide semi conductors can decrease the band gap and broaden the absorption of sunlight. Meanwhile, the oxygen vacancies not only act as active centers which capture charges to block recombination, but also reduce the activation energy of the molecule to enhance the water splitting and organics degradation [19,20]. Zheng reported that due to the oxygen on the surface of the TiO2 lattice was taken away, the electron density increased, and part of Ti4þ in the crystal lattice was reduced to Ti3þ [21]. It would generate a donor level that was beneficial to the ground state electrons under the conduction band, improving light absorption and charge transport. Common methods for introducing the oxygen va cancies include reduction in hydrogen or anoxic (oxygen-free) atmo sphere [22], chemical vapor deposition [23], high-energy particle bombardment [24], NaBH4 reduction [25], and metal replacement [26]. Based on the above, the scant literatures reported oxygen vacancies and g-C3N4 nanosheets synergistic effect to improve the overall
* Corresponding author. E-mail address: [email protected] (W. Cui). https://doi.org/10.1016/j.mssp.2020.104954 Received 1 October 2019; Received in revised form 23 December 2019; Accepted 17 January 2020 Available online 24 January 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Scheme 1. Schematic diagram of synthetic route over TiO2-x/g-C3N4 photoelectrode.
Fig. 1. XRD patterns of (a) TiO2, TiO2/g–C3N4–20, TiO2/g–C3N4–40, TiO2/g–C3N4–60, (b) TiO2, TiO2/g-C3N4, TiO2-x/g-C3N4.
photoelectrocatalytic performance. Li [27] reported a novel photo catalyst via calcining NaBH4 and TiO2/g-C3N4 mixture to introduce Ti3þ in TiO2. However, Wen et al. reported that the defects could be intro duced in the g-C3N4 during it was calcined with NaBH4 [28]. For TiO2/g-C3N4, the method of blending calcination is easy to cause defects in both, and calcination has a certain risk and consumes a lot of energy. In our work, the TiO2-x/g-C3N4 nanorod arrays photoanode was pre pared by hydrothermal, urea drop-calcined and NaBH4 reduction at normal temperature. According to EPR and XPS results, we discussed that the defects were on TiO2 instead of g-C3N4. The oxygen vacancies
can serve as the “bridge” that attracts electrons and holes on different semiconductors to conduct directional migration. Meanwhile, the g-C3N4 nanosheets and the oxygen vacancies can synergistically improve the transfer of charge and the degradation activity of phenol. 2. Experimental section The characterization, photoelectrochemical tests, the degradation of phenol, and the detailed prepared process can be obtained in supporting information.
Fig. 2. The top-view SEM image of (a) TiO2 and (b) TiO2-x/g-C3N4, the inside is the sectional view of TiO2-x/g-C3N4. 2
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Fig. 3. TEM of (a, b) TiO2 nanorod and (c, d) TiO2-x/g-C3N4, (e) the mapping imagines of TiO2-x/g-C3N4.
Fig. 4. (a) UV–Vis DRS images and (b) PL images.
3. Results and discussion
[30]. The peaks corresponding to g-C3N4 in the composite were not obvious in TiO2/g–C3N4–20, it was likely the g-C3N4 was little. In TiO2/g–C3N4–60, the characteristic peak appearing at 27.4� corre sponded to g-C3N4 (002) crystal planes. The peaks corresponding to TiO2 and g-C3N4 appeared simultaneously, and no other peaks were detected, indicating that the material was made of TiO2 and g-C3N4. After treated with NaBH4, the diffraction peaks of TiO2-x/g-C3N4 were lower than TiO2/g-C3N4 (Fig. 1b), indicating that oxygen vacancies could cause TiO2 surface lattice disorder [31]. In Fig. 2a, the top view shows that the nanorods densely covered on the surface of FTO. The length of the nanorod was 2.0 μm, and its orientated growth. TiO2-x/g-C3N4 was rough than TiO2, it is possible g-
The TiO2-x/g-C3N4 nanorod arrays photoanode was prepared by a series of reactions. As described in Scheme 1, the TiO2 nanorod was synthesized by hydrothermal method [29]. TiO2-x/g-C3N4 photoanode was synthesized by urea drop-calcined calcination and NaBH4 reduc tion, the detail process can be obtained in supporting information. In Fig. 1a, all diffraction peaks matched the rutile phase TiO2 (JCPDS No. 79–1640) by excluding the affection of FTO. The peaks centered at 36.3� and 65.8� were indexed to the TiO2 (101) and (112) crystal planes. The main characteristic peak 62.2� corresponded to the (002) crystal plane, indicating that the TiO2 nanorod was highly grown orientation 3
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Fig. 5. (a) EPR images of TiO2 and TiO2-x/g-C3N4, (b) the full XPS element scanning. The compare (c) Ti 2p and (e) O 1s of different photoelectrodes. (d) Ti 2p, (f) O 1s.
C3N4 loaded on TiO2 (Fig. 2b). In Fig. 3c and d, the thickness of the gC3N4 nanosheet was 10 nm, and there was a clear boundary between the TiO2 and g-C3N4. The TiO2 nanorod diameter was 60 nm, and the lattice fringes were clearly seen (Fig. 3a and b), proving its crystallinity is good. Fig. 3e shows the Ti, O, C, and N elements were distributed evenly, affirming the g-C3N4 nanosheets were loaded on TiO2 nanorods.
In Fig. 4a, the absorption band edge of TiO2 was approximately 410 nm, corresponding to the band gap of 3.02 eV. The introduction of the gC3N4 and oxygen vacancies caused the shift to the longer wave. The band gaps can be calculated via the Kubelka-Munk function [32]. The TiO2/g-C3N4 and TiO2-x/g-C3N4 were 2.90 and 2.83 eV, respectively, proving the introduction of oxygen vacancies on TiO2 nanorods could 4
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Fig. 6. EIS plots of the samples under (a) light on and (b) off, the different bias potentials on EIS plots of (c) TiO2-x/g-C3N4 and (d) TiO2.
enhance the absorption of sunlight and promote the excitation of charges. In Fig. 4b, the fluorescence intensity of TiO2-x/g-C3N4 was higher than that of TiO2/g-C3N4 and TiO2, and there was a red shift compared to TiO2. Due to fluorescence intensity related to the recom bination of photogenerated charges, thus it is likely that the oxygen vacancies on TiO2 nanorods surface might become the recombination centers for the electrons on the TiO2 conduction band and the holes on the valence band of g-C3N4 [33]. Many studies reported that oxygen vacancies could promote the formation of Z-scheme, leading to the enhancement of fluorescence intensity [34]. In Fig. 5a, the signal with g ¼ 2.003 was enhanced in TiO2-x/g-C3N4, corresponding to the unpaired electrons which belongs to the single electron-trapped oxygen vacancies [35,36], confirming the oxygen va cancies were on TiO2 surface Between 3440 and 3530 G, there were not the signal with g ¼ 2.0021, which corresponded to the unpaired elec trons of sp2-C within π-conjugated aromatic rings [37,38], confirming no nitrogen vacancies. It proved there were no the nitrogen vacancies in g-C3N4 through NaBH4 reduction at normal temperature. In the result of XPS, the full scan elemental spectrum g-C3N4 loaded on TiO2 (Fig. 5b). Compared with TiO2, the binding energy of the Ti 2p and O 1s of TiO2-x/g-C3N4 was negatively shifted (Fig. 5c and e), it is possible that the oxygen vacancies on TiO2 surface led to the electron density changed. In Fig. 5d, the peaks centers at 458.6 and 464.6 eV correspond to the Ti4þ species. Meanwhile, the peaks at 458.2 and 463.7 eV correspond to Ti3þ [39], proving that the oxygen on the surface of TiO2 lattice was taken away, leading to the electron density increased and the valence state of Ti changed [40]. In Fig. 5f, the peaks at 529.7 and 531.8 eV were from the –OH and Ti–O–Ti [41]. The C 1s spectrum could be
divided into 284.6, 285.1 and 288.3 eV (Fig. S1a.), corresponding to the C–C, sp2C–N, and sp3C–N in the g-C3N4 structure [42]. In Fig. S1b, the three resolved peaks centered at 399.06, 399.5 and 400.13 eV corre sponded to the N–H, the N-(C)3 and C–N–C band [43], respectively. The peaks in C 1s and N 1s spectrums met g-C3N4 structure and remained unchanged, indicating that the defects were in TiO2 instead of g-C3N4. We compared the N 1s between TiO2/g-C3N4 and TiO2-x/g-C3N4, and found the change was hardly any (Fig. S1c), indicating that the defects were on TiO2 instead of g-C3N4. Meanwhile, by comparing the N 1s in g-C3N4 and g–C3N4–NaBH4 reduction, we found that the peaks were almost unchanged (Fig. S1d), demonstrating that TiO2/g-C3N4 electrode was immersed in NaBH4 solution could not cause g-C3N4 structural change, and it needed the more energy to form the nitrogen vacancies, such as calcining the mixture of NaBH4 and g-C3N4 [28], or treating g-C3N4 under hydrogen atmosphere in the high temperature [37]. In Fig. 6a and b, the arc radius of the nyquist plots of TiO2, TiO2/gC3N4 and TiO2-x/g-C3N4 electrodes reduced in turn. It is possible that the g-C3N4 and oxygen vacancies could reduce the interface resistance be tween electrode and electrolyte solution synergistically, and improve the separation efficiency of carrier. Along with the increasing applied bias, the arc radius of the nyquist plots reduced, suggesting that the voltage increased was of benefit to the transfer of photogenerated charges (Fig. 6c and d). Meanwhile, the arc radius of the nyquist plots enhanced after adding the phenol. This phenomenon can be explained by the adsorbed phenol on the surface of the electrode can impede the charge migration in interface between solid electrode and solution. The adsorption of phenol on the electrode surface is favorable for its degradation [44] (Figs. S2a and b). The photocurrent density of 5
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Fig. 7. (a) The transient photocurrent responses (i–t) of the samples, (b) the different bias potential of transient photocurrent responses.
Fig. 8. (a) The linear sweep voltammetry curves of the samples, (b) the photocurrent stability of TiO2 and TiO2-x/g-C3N4, Mott-Schottky plots of (c) TiO2, (d) TiO2/gC3N4 and TiO2-x/g-C3N4.
TiO2-x/g-C3N4 was 6-fold and 1.5-fold higher than that of TiO2 and TiO2/g-C3N4 (Fig. 7a), indicating the transfer of charge was obviously enhanced after introducing the oxygen vacancies. On one hand, the g-C3N4 nanosheets and the oxygen vacancies synergistically affect the
absorption of visible light. On the other hand, oxygen vacancies could be used as trap to restrain the recombination of charge, enhancing the separation efficiency. Meanwhile, with the bias voltages increased, the transient photocurrent responses enhanced (Fig. 7b). 6
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Fig. 9. (a) The degradation of phenol over different photoelectrodes, (b) the EC, PC and PEC processes, (c) the various bias voltages, (d) the different light sources.
In Fig. 8a, before 1.3 V (vs. SCE), the photocurrent density rapidly reduced to zero when the lamp turned off. And over 1.3 V, it might be that the strong voltage caused the material changed. The photocurrent of TiO2-x/g-C3N4 remained unchanged for a long time (Fig. 8b), proving that its property was stable under the simulated sunlight. The flat band potential of TiO2 was 0.27 V [45]. The flat band potentials of TiO2/g-C3N4 and TiO2-x/g-C3N4 photoelectrodes were 0.45 V and 0.56 V (Fig. 8c and d). The negatively shift of the flat band potential
indicated that the photogenerated charges had stronger reduction abil ity after NaBH4. In Fig. 9a, there was almost no change of phenol concentration without catalyst, indicating that the phenol did not self-degrade. The phenol degradation of TiO2-x/g-C3N4 photoelectrode was as high as 83% within 5 h, higher than TiO2 and TiO2/g-C3N4. Different g-C3N4 nano sheets loads and NaBH4 concentrations were measured (Fig. S3), the TiO2/g–C3N4-40 and TiO2-x-5.0/g-C3N4 showed a higher activity,
Fig. 10. (a) The stability experiment over TiO2-x/g-C3N4 photoelectrode, (b) the XRD patterns of before and after reaction. 7
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Fig. 11. (a) The free radical capture experiment, (b) the fluorescent hydroxyl capture experiment.
and the oxygen vacancies could promote the recombination of charges with the lower redox ability, thus generating the more hydroxyl radicals. According to the data and results above, a possible reaction mech anism was put forward (Fig. 12). The TiO2 and g-C3N4 could generate charges under sunlight, and the oxygen vacancies served as the recom bination centers for the electrons on TiO2 conduction band and holes on g-C3N4. The electrons on g-C3N4 transferred to the Pt electrode with electrochemical workstation, and reacted with oxygen to form O2�-. Some hþ on TiO2 valence band reacted directly with organic pollutants, the others reacted with the surface adsorbed OH /H2O to form �OH. The transfer of charge followed the Z-scheme reaction system. The entire redox ability and PEC activity were effectively improved by sacrificing a pair of electron-hole pairs with weak redox.
Fig. 12. Schematic diagram of PEC degradation.
4. Conclusions
indicating that the g-C3N4 nanosheet and the oxygen vacancies had the best number, respectively. The PEC showed an enhanced activity than that of PC and EC, proving the improvement of TiO2-x/g-C3N4 activity was synergistic between photocatalysis and electrocatalysis (Fig. 9b). In Fig. 9c, the activity increased with the applied voltage, and the voltage was more than 1.0 V, the activity reduced. It is possible that electro catalytic predominates and the surface of the photoanode would be covered with contaminants, leading to the reduction of conductivity [46]. In Fig. 9d, the PEC efficiency of TiO2-x/g-C3N4 photoelectrode was higher than TiO2 and TiO2/g-C3N4 in the UV, visible light (�420 nm) and the simulated sunlight, proving the TiO2-x/g-C3N4 photoelectrode had the excellent catalytic performance. In Fig. 10a, it is possible the degraded pollutants or intermediates adsorbed on the surface and hindered the active sites, the activity was slightly decreased. However, the degradation rate was maintained 80% after cycle tests. In Fig. 10b, the peaks were still sharp and no new peaks were found, proving that the TiO2-x/g-C3N4 has good stability and could be recycled used. The capture experiment of free radical is important to explore the reaction mechanism of experiments. Tert-butanol (TBA), EDTA-2Na and p-benzoquinone are common used to capture �OH, hþ, and O2�-, respectively [47,48]. In Fig. 11a, the reactivity decreased from 83% to 52%, 37%, and 13%, upon the trapping agent of TBA, EDTA-2Na and p-benzoquinone, proving O2�- was the main free radical in experiments, and �OH and hþ also played the important roles. In ESR spectra, the O2�and �OH were proved to play a vital role in catalytic process (Fig. S4). In fluorescent hydroxyl capture experiments (Fig. 11b), TiO2-x/g-C3N4 produced more hydroxyl radicals than TiO2. Due to the valence band of TiO2 is more positive, more conducive to generate hydroxyl radicals,
We successfully synthesized TiO2-x/g-C3N4 nanorod arrays photo electrode by urea drop-calcined and NaBH4 reduction. The g-C3N4 nanosheets and the oxygen vacancies could synergistically promote the separation of photo-generated charges and enhance the overall stability. The oxygen vacancies served as the recombination centers to improve the overall catalytic performance, and the entire redox ability and PEC activity were effectively improved by sacrificing a pair of electron-hole pairs with weak redox. The charge transfer of TiO2-x/g-C3N4 provides a new perspective in the study of photoelectrochemical degradation. CRediT authorship contribution statement Fan Qi: Formal analysis, Investigation, Writing - original draft. Weijia An: Project administration, Data curation. Huan Wang: Meth odology. Jinshan Hu: Resources, Funding acquisition. Hongxia Guo: Resources, Funding acquisition. Li Liu: Validation. Wenquan Cui: Conceptualization, Writing - review & editing, Supervision. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51672081), the support program for one hundred excellent talents of innovation in Hebei provincial universities (III) (No. SLRC2017049), Youth Fund Project of Hebei Province Department of Education (QN2018056).
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Appendix A. Supplementary data
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Zhai, Distinctive defects engineering in graphitic carbon nitride for greatly extended visible light photocatalytic hydrogen evolution, Nano Energy 44 (2018) 73–81. [19] T. Soltani, A. Tayyebi, H. Hong, M.H. Mirfasih, B.K. Lee, A novel growth control of nanoplates WO3 photoanodes with dual oxygen and tungsten vacancies for efficient photoelectrochemical water splitting performance, Sol. Energy Mater. Sol. Cells 191 (2019) 39–49. [20] S.V.P. Vattikuti, A.K.R. Police, J. Shim, C. Byon, In situ fabrication of the Bi2O3V2O5 hybrid embedded with graphitic carbon nitride nanosheets: oxygen vacancies mediated enhanced visible-light-driven photocatalytic degradation of organic pollutants and hydrogen evolution, Appl. Surf. Sci. 447 (2018) 740–756. [21] J.Y. Zheng, Y.H. Lyu, C. Xie, R.L. Wang, L. Tao, H.B. Wu, H.J. Zhou, S.P. Jiang, S. Y. Wang, Defect-Enhanced charge separation and transfer within protection layer/ semiconductor structure of photoanodes, Adv. Mater. 30 (2018) 1801773. 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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Combing oxygen vacancies on TiO2 nanorod arrays with g-C3N4 nanosheets for enhancing photoelectrochemical degradation of phenol Fan Qi , Weijia An , Huan Wang , Jinshan Hu , Hongxia Guo , Li Liu , Wenquan Cui * College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic Materials, North China University of Science and Technology, Tangshan, Hebei, 063210, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: g-C3N4 nanosheets Oxygen vacancies TiO2 nanorods Z-Scheme heterojunction
We reported a novel TiO2-x/g-C3N4 nanorod arrays photoelectrode by urea drop-calcined and NaBH4 reduction. Due to synergistic effect of the oxygen vacancies and the g-C3N4 nanosheets, the TiO2-x/g-C3N4 photoelectrode exhibited excellent photoelectrochemical property and photoelectrocatalytic activity. The photocurrent density of TiO2-x/g-C3N4 was 6-fold and 1.5-fold higher than TiO2 and TiO2/g-C3N4, respectively. Meanwhile, the phenol degradation rate of TiO2-x/g-C3N4 photoelectrodes was as high as 83% under the stimulated solar light. Besides, the oxygen vacancies could promote the formation of the Z-scheme heterojunction between the TiO2 nanorods and g-C3N4 nanosheets.
1. Introduction Photoelectrocatalytic (PEC) materials are widely researched in water splitting [1], CO2 reduction [2], solar energy storage [3], and organic pollutant degradation [4]. Since 1972, Fujishima et al. came up with TiO2 electrode could be used to split water under ultraviolet light [5]. TiO2 was the common catalyst, and has attracted much attention. However, it is not widely utilized that limited by the lower visible light efficiency and the charges recombination [6,7]. For a long time, how to improve the catalytic performance of TiO2, and the adjustment of microstructure has been the research focus. One-dimensional nano materials own advantages in optical absorption and carrier migration, due to their unique structure and high specific surface areas [8]. TiO2 nanorod, TiO2 nanotube and TiO2 nanofiber are conducive to improving the directional transfer of charge and enhancing photoelectrocatalytic performance [9]. TiO2 mostly responds in the ultraviolet region due to the relatively narrow absorption range in sunlight, how to improve the response range is the focus of research. Many ways have been studied to effectively reduce photo-generated charges recombination, including doping [10], loading [11], modifying [12], and sensitizing [13]. Many studies have modified TiO2 with other materials to form heterostructure. The band structure between TiO2 and other materials could promote the separa tion of charges. g-C3N4 is a π-conjugated material. The van der Waals force is weak between layers in bulk g-C3N4 [14], and easily destroyed to
form nanosheets, thus g-C3N4 has been studied widely. Xu et al. [15] reported that the H2 evolution performance of g-C3N4 photocatalyst enhanced due to the coexistence of Pt0 and Pt2þ. The adjustment of band structure between heterogeneous structures is beneficial for charge separation, and defect engineering is the com mon strategy. Introduction of the defects properly can promote light absorption and improve charge separation. Carbon defect [16], oxygen vacancy [17] and nitrogen defect [18] have been reported widely. In metal oxides, oxygen vacancy is the most common and widely reported. The introduction of the oxygen vacancies on metal oxide semi conductors can decrease the band gap and broaden the absorption of sunlight. Meanwhile, the oxygen vacancies not only act as active centers which capture charges to block recombination, but also reduce the activation energy of the molecule to enhance the water splitting and organics degradation [19,20]. Zheng reported that due to the oxygen on the surface of the TiO2 lattice was taken away, the electron density increased, and part of Ti4þ in the crystal lattice was reduced to Ti3þ [21]. It would generate a donor level that was beneficial to the ground state electrons under the conduction band, improving light absorption and charge transport. Common methods for introducing the oxygen va cancies include reduction in hydrogen or anoxic (oxygen-free) atmo sphere [22], chemical vapor deposition [23], high-energy particle bombardment [24], NaBH4 reduction [25], and metal replacement [26]. Based on the above, the scant literatures reported oxygen vacancies and g-C3N4 nanosheets synergistic effect to improve the overall
* Corresponding author. E-mail address: [email protected] (W. Cui). https://doi.org/10.1016/j.mssp.2020.104954 Received 1 October 2019; Received in revised form 23 December 2019; Accepted 17 January 2020 Available online 24 January 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Scheme 1. Schematic diagram of synthetic route over TiO2-x/g-C3N4 photoelectrode.
Fig. 1. XRD patterns of (a) TiO2, TiO2/g–C3N4–20, TiO2/g–C3N4–40, TiO2/g–C3N4–60, (b) TiO2, TiO2/g-C3N4, TiO2-x/g-C3N4.
photoelectrocatalytic performance. Li [27] reported a novel photo catalyst via calcining NaBH4 and TiO2/g-C3N4 mixture to introduce Ti3þ in TiO2. However, Wen et al. reported that the defects could be intro duced in the g-C3N4 during it was calcined with NaBH4 [28]. For TiO2/g-C3N4, the method of blending calcination is easy to cause defects in both, and calcination has a certain risk and consumes a lot of energy. In our work, the TiO2-x/g-C3N4 nanorod arrays photoanode was pre pared by hydrothermal, urea drop-calcined and NaBH4 reduction at normal temperature. According to EPR and XPS results, we discussed that the defects were on TiO2 instead of g-C3N4. The oxygen vacancies
can serve as the “bridge” that attracts electrons and holes on different semiconductors to conduct directional migration. Meanwhile, the g-C3N4 nanosheets and the oxygen vacancies can synergistically improve the transfer of charge and the degradation activity of phenol. 2. Experimental section The characterization, photoelectrochemical tests, the degradation of phenol, and the detailed prepared process can be obtained in supporting information.
Fig. 2. The top-view SEM image of (a) TiO2 and (b) TiO2-x/g-C3N4, the inside is the sectional view of TiO2-x/g-C3N4. 2
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Fig. 3. TEM of (a, b) TiO2 nanorod and (c, d) TiO2-x/g-C3N4, (e) the mapping imagines of TiO2-x/g-C3N4.
Fig. 4. (a) UV–Vis DRS images and (b) PL images.
3. Results and discussion
[30]. The peaks corresponding to g-C3N4 in the composite were not obvious in TiO2/g–C3N4–20, it was likely the g-C3N4 was little. In TiO2/g–C3N4–60, the characteristic peak appearing at 27.4� corre sponded to g-C3N4 (002) crystal planes. The peaks corresponding to TiO2 and g-C3N4 appeared simultaneously, and no other peaks were detected, indicating that the material was made of TiO2 and g-C3N4. After treated with NaBH4, the diffraction peaks of TiO2-x/g-C3N4 were lower than TiO2/g-C3N4 (Fig. 1b), indicating that oxygen vacancies could cause TiO2 surface lattice disorder [31]. In Fig. 2a, the top view shows that the nanorods densely covered on the surface of FTO. The length of the nanorod was 2.0 μm, and its orientated growth. TiO2-x/g-C3N4 was rough than TiO2, it is possible g-
The TiO2-x/g-C3N4 nanorod arrays photoanode was prepared by a series of reactions. As described in Scheme 1, the TiO2 nanorod was synthesized by hydrothermal method [29]. TiO2-x/g-C3N4 photoanode was synthesized by urea drop-calcined calcination and NaBH4 reduc tion, the detail process can be obtained in supporting information. In Fig. 1a, all diffraction peaks matched the rutile phase TiO2 (JCPDS No. 79–1640) by excluding the affection of FTO. The peaks centered at 36.3� and 65.8� were indexed to the TiO2 (101) and (112) crystal planes. The main characteristic peak 62.2� corresponded to the (002) crystal plane, indicating that the TiO2 nanorod was highly grown orientation 3
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Fig. 5. (a) EPR images of TiO2 and TiO2-x/g-C3N4, (b) the full XPS element scanning. The compare (c) Ti 2p and (e) O 1s of different photoelectrodes. (d) Ti 2p, (f) O 1s.
C3N4 loaded on TiO2 (Fig. 2b). In Fig. 3c and d, the thickness of the gC3N4 nanosheet was 10 nm, and there was a clear boundary between the TiO2 and g-C3N4. The TiO2 nanorod diameter was 60 nm, and the lattice fringes were clearly seen (Fig. 3a and b), proving its crystallinity is good. Fig. 3e shows the Ti, O, C, and N elements were distributed evenly, affirming the g-C3N4 nanosheets were loaded on TiO2 nanorods.
In Fig. 4a, the absorption band edge of TiO2 was approximately 410 nm, corresponding to the band gap of 3.02 eV. The introduction of the gC3N4 and oxygen vacancies caused the shift to the longer wave. The band gaps can be calculated via the Kubelka-Munk function [32]. The TiO2/g-C3N4 and TiO2-x/g-C3N4 were 2.90 and 2.83 eV, respectively, proving the introduction of oxygen vacancies on TiO2 nanorods could 4
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Fig. 6. EIS plots of the samples under (a) light on and (b) off, the different bias potentials on EIS plots of (c) TiO2-x/g-C3N4 and (d) TiO2.
enhance the absorption of sunlight and promote the excitation of charges. In Fig. 4b, the fluorescence intensity of TiO2-x/g-C3N4 was higher than that of TiO2/g-C3N4 and TiO2, and there was a red shift compared to TiO2. Due to fluorescence intensity related to the recom bination of photogenerated charges, thus it is likely that the oxygen vacancies on TiO2 nanorods surface might become the recombination centers for the electrons on the TiO2 conduction band and the holes on the valence band of g-C3N4 [33]. Many studies reported that oxygen vacancies could promote the formation of Z-scheme, leading to the enhancement of fluorescence intensity [34]. In Fig. 5a, the signal with g ¼ 2.003 was enhanced in TiO2-x/g-C3N4, corresponding to the unpaired electrons which belongs to the single electron-trapped oxygen vacancies [35,36], confirming the oxygen va cancies were on TiO2 surface Between 3440 and 3530 G, there were not the signal with g ¼ 2.0021, which corresponded to the unpaired elec trons of sp2-C within π-conjugated aromatic rings [37,38], confirming no nitrogen vacancies. It proved there were no the nitrogen vacancies in g-C3N4 through NaBH4 reduction at normal temperature. In the result of XPS, the full scan elemental spectrum g-C3N4 loaded on TiO2 (Fig. 5b). Compared with TiO2, the binding energy of the Ti 2p and O 1s of TiO2-x/g-C3N4 was negatively shifted (Fig. 5c and e), it is possible that the oxygen vacancies on TiO2 surface led to the electron density changed. In Fig. 5d, the peaks centers at 458.6 and 464.6 eV correspond to the Ti4þ species. Meanwhile, the peaks at 458.2 and 463.7 eV correspond to Ti3þ [39], proving that the oxygen on the surface of TiO2 lattice was taken away, leading to the electron density increased and the valence state of Ti changed [40]. In Fig. 5f, the peaks at 529.7 and 531.8 eV were from the –OH and Ti–O–Ti [41]. The C 1s spectrum could be
divided into 284.6, 285.1 and 288.3 eV (Fig. S1a.), corresponding to the C–C, sp2C–N, and sp3C–N in the g-C3N4 structure [42]. In Fig. S1b, the three resolved peaks centered at 399.06, 399.5 and 400.13 eV corre sponded to the N–H, the N-(C)3 and C–N–C band [43], respectively. The peaks in C 1s and N 1s spectrums met g-C3N4 structure and remained unchanged, indicating that the defects were in TiO2 instead of g-C3N4. We compared the N 1s between TiO2/g-C3N4 and TiO2-x/g-C3N4, and found the change was hardly any (Fig. S1c), indicating that the defects were on TiO2 instead of g-C3N4. Meanwhile, by comparing the N 1s in g-C3N4 and g–C3N4–NaBH4 reduction, we found that the peaks were almost unchanged (Fig. S1d), demonstrating that TiO2/g-C3N4 electrode was immersed in NaBH4 solution could not cause g-C3N4 structural change, and it needed the more energy to form the nitrogen vacancies, such as calcining the mixture of NaBH4 and g-C3N4 [28], or treating g-C3N4 under hydrogen atmosphere in the high temperature [37]. In Fig. 6a and b, the arc radius of the nyquist plots of TiO2, TiO2/gC3N4 and TiO2-x/g-C3N4 electrodes reduced in turn. It is possible that the g-C3N4 and oxygen vacancies could reduce the interface resistance be tween electrode and electrolyte solution synergistically, and improve the separation efficiency of carrier. Along with the increasing applied bias, the arc radius of the nyquist plots reduced, suggesting that the voltage increased was of benefit to the transfer of photogenerated charges (Fig. 6c and d). Meanwhile, the arc radius of the nyquist plots enhanced after adding the phenol. This phenomenon can be explained by the adsorbed phenol on the surface of the electrode can impede the charge migration in interface between solid electrode and solution. The adsorption of phenol on the electrode surface is favorable for its degradation [44] (Figs. S2a and b). The photocurrent density of 5
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Fig. 7. (a) The transient photocurrent responses (i–t) of the samples, (b) the different bias potential of transient photocurrent responses.
Fig. 8. (a) The linear sweep voltammetry curves of the samples, (b) the photocurrent stability of TiO2 and TiO2-x/g-C3N4, Mott-Schottky plots of (c) TiO2, (d) TiO2/gC3N4 and TiO2-x/g-C3N4.
TiO2-x/g-C3N4 was 6-fold and 1.5-fold higher than that of TiO2 and TiO2/g-C3N4 (Fig. 7a), indicating the transfer of charge was obviously enhanced after introducing the oxygen vacancies. On one hand, the g-C3N4 nanosheets and the oxygen vacancies synergistically affect the
absorption of visible light. On the other hand, oxygen vacancies could be used as trap to restrain the recombination of charge, enhancing the separation efficiency. Meanwhile, with the bias voltages increased, the transient photocurrent responses enhanced (Fig. 7b). 6
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Fig. 9. (a) The degradation of phenol over different photoelectrodes, (b) the EC, PC and PEC processes, (c) the various bias voltages, (d) the different light sources.
In Fig. 8a, before 1.3 V (vs. SCE), the photocurrent density rapidly reduced to zero when the lamp turned off. And over 1.3 V, it might be that the strong voltage caused the material changed. The photocurrent of TiO2-x/g-C3N4 remained unchanged for a long time (Fig. 8b), proving that its property was stable under the simulated sunlight. The flat band potential of TiO2 was 0.27 V [45]. The flat band potentials of TiO2/g-C3N4 and TiO2-x/g-C3N4 photoelectrodes were 0.45 V and 0.56 V (Fig. 8c and d). The negatively shift of the flat band potential
indicated that the photogenerated charges had stronger reduction abil ity after NaBH4. In Fig. 9a, there was almost no change of phenol concentration without catalyst, indicating that the phenol did not self-degrade. The phenol degradation of TiO2-x/g-C3N4 photoelectrode was as high as 83% within 5 h, higher than TiO2 and TiO2/g-C3N4. Different g-C3N4 nano sheets loads and NaBH4 concentrations were measured (Fig. S3), the TiO2/g–C3N4-40 and TiO2-x-5.0/g-C3N4 showed a higher activity,
Fig. 10. (a) The stability experiment over TiO2-x/g-C3N4 photoelectrode, (b) the XRD patterns of before and after reaction. 7
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Fig. 11. (a) The free radical capture experiment, (b) the fluorescent hydroxyl capture experiment.
and the oxygen vacancies could promote the recombination of charges with the lower redox ability, thus generating the more hydroxyl radicals. According to the data and results above, a possible reaction mech anism was put forward (Fig. 12). The TiO2 and g-C3N4 could generate charges under sunlight, and the oxygen vacancies served as the recom bination centers for the electrons on TiO2 conduction band and holes on g-C3N4. The electrons on g-C3N4 transferred to the Pt electrode with electrochemical workstation, and reacted with oxygen to form O2�-. Some hþ on TiO2 valence band reacted directly with organic pollutants, the others reacted with the surface adsorbed OH /H2O to form �OH. The transfer of charge followed the Z-scheme reaction system. The entire redox ability and PEC activity were effectively improved by sacrificing a pair of electron-hole pairs with weak redox.
Fig. 12. Schematic diagram of PEC degradation.
4. Conclusions
indicating that the g-C3N4 nanosheet and the oxygen vacancies had the best number, respectively. The PEC showed an enhanced activity than that of PC and EC, proving the improvement of TiO2-x/g-C3N4 activity was synergistic between photocatalysis and electrocatalysis (Fig. 9b). In Fig. 9c, the activity increased with the applied voltage, and the voltage was more than 1.0 V, the activity reduced. It is possible that electro catalytic predominates and the surface of the photoanode would be covered with contaminants, leading to the reduction of conductivity [46]. In Fig. 9d, the PEC efficiency of TiO2-x/g-C3N4 photoelectrode was higher than TiO2 and TiO2/g-C3N4 in the UV, visible light (�420 nm) and the simulated sunlight, proving the TiO2-x/g-C3N4 photoelectrode had the excellent catalytic performance. In Fig. 10a, it is possible the degraded pollutants or intermediates adsorbed on the surface and hindered the active sites, the activity was slightly decreased. However, the degradation rate was maintained 80% after cycle tests. In Fig. 10b, the peaks were still sharp and no new peaks were found, proving that the TiO2-x/g-C3N4 has good stability and could be recycled used. The capture experiment of free radical is important to explore the reaction mechanism of experiments. Tert-butanol (TBA), EDTA-2Na and p-benzoquinone are common used to capture �OH, hþ, and O2�-, respectively [47,48]. In Fig. 11a, the reactivity decreased from 83% to 52%, 37%, and 13%, upon the trapping agent of TBA, EDTA-2Na and p-benzoquinone, proving O2�- was the main free radical in experiments, and �OH and hþ also played the important roles. In ESR spectra, the O2�and �OH were proved to play a vital role in catalytic process (Fig. S4). In fluorescent hydroxyl capture experiments (Fig. 11b), TiO2-x/g-C3N4 produced more hydroxyl radicals than TiO2. Due to the valence band of TiO2 is more positive, more conducive to generate hydroxyl radicals,
We successfully synthesized TiO2-x/g-C3N4 nanorod arrays photo electrode by urea drop-calcined and NaBH4 reduction. The g-C3N4 nanosheets and the oxygen vacancies could synergistically promote the separation of photo-generated charges and enhance the overall stability. The oxygen vacancies served as the recombination centers to improve the overall catalytic performance, and the entire redox ability and PEC activity were effectively improved by sacrificing a pair of electron-hole pairs with weak redox. The charge transfer of TiO2-x/g-C3N4 provides a new perspective in the study of photoelectrochemical degradation. CRediT authorship contribution statement Fan Qi: Formal analysis, Investigation, Writing - original draft. Weijia An: Project administration, Data curation. Huan Wang: Meth odology. Jinshan Hu: Resources, Funding acquisition. Hongxia Guo: Resources, Funding acquisition. Li Liu: Validation. Wenquan Cui: Conceptualization, Writing - review & editing, Supervision. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51672081), the support program for one hundred excellent talents of innovation in Hebei provincial universities (III) (No. SLRC2017049), Youth Fund Project of Hebei Province Department of Education (QN2018056).
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Appendix A. Supplementary data
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