English

• 1. INTRODUCTION

  Economic development and industrialization led to a rapid improvement of living standards, but it also exposed energy crisis, which hindered sustainable development^[1-3]. As an advanced environmental protection method, semiconductor photocatalysis technology has become a hot spot in hydrogen production^[4-7]. Major breakthroughs in photocatalytic degradation have been achieved in catalytic desulphurization, photocatalytic water decomposition, photocatalytic reduction of organic compound synthesis, antibacterial effects, and so on.

  In related areas, photocatalytic semiconductors such as ZnO and TiO₂ are widely used for many virtues^[8-10]. First, TiO₂ is safe, non-toxic and harmless to humans^{[10, 11]}. Second, TiO₂ is widely found in the earth's crust, cheap and low production costs. Unfortunately, the actual photocatalytic performance of TiO₂ is much lower than urgent requirements^[12-15]. For example, TiO₂ can absorb most ultraviolet light, but only a small part of visible light can be absorbed, and photo-induced electrons can be easily recombined. Researchers propose many strategies to overcome these deficiencies. First, doping TiO₂ with other substances (metal or non-metal elements and other semiconductors) can extend the lifetime of photogenerated electrons. Second, increasing the specific surface area of TiO₂ makes more catalytic site proud exposure. The integration of above methods may be a desired strategy for improving photocatalytic performance.

  In general, the performance of a photocatalyst mainly depends on its chemical and physical structures, which is crucial for photon adsorption, charge carrier transfer, and catalytic surface reaction during photocatalysis. Until now, the deposition of precious metals such as Au, Pt, and Ag on the TiO₂ surface can reduce the recombination rate of h⁺/e^-, which extends the existence of electron holes and raises photocatalytic activity of TiO₂^[16-18]. Au is stable and not easy to accumulate in photocatalyst catalytic reactions. Under light irradiation, Au nanoparticles can extend the spectral response range of TiO₂ from the ultraviolet region to the visible region, which further synergizes photocatalytic activity. The doping of high surface area TiO₂ (HSTiO₂) with Au is a preferable alternatives^[19-22].

  In this study, Au-HSTiO₂ was first used as a cocatalyst for photocatalytic hydrogen production. We prepared TiO₂ with high specific surface area by sol-gel method, which has greater bandgap and photocatalytic activity based on quantum size effect. Au nanoparticles were deposited on HSTiO₂ by liquid reduction method. Au-HSTiO₂ was characterized by XRD, UV-vis, SEM, TEM, XPS and photocurrent intensity. The experimental results show that after 150 minutes radiation, Au-HSTiO₂ has a photocatalytic hydrogen production with 5.129 mL H₂.

  2. EXPERIMENTAL

  2.1 Materials

  Methanol (99.9%), ethanol (99.9%), titanium dioxide (> 99.5%), sodium borohydride (99%), chloroauric acid (99%), tetrabutyl titanium (> 98%), hydrochloric acid (> 95%), acetic acid (> 95%) and polyethylene glycol(propanediol)-polyethylene glycol(propanediol)-polyethylene glycol(ethylene glycol)(F127) are supported by Shanghai Chemical Co., Ltd.. The dissolved water came from the analytical laboratory. All the above reagents are analytical levels.

  2.2 Synthesis of samples

  2.2.1   Synthesis of high specific surface area TiO₂ (HSTiO₂)

  The experimental method for obtaining HSTiO₂ is as follows. First, 1.6 g of F127 was added to 30 mL ethanol, to which 3.5 mL of tetrabutyl titanium, 2.3 mL of acetic acid and 0.7 mL of hydrochloric acid were then further added. Second, the mixture was stirred at 60℃until completely dried. The beaker containing the mixed solution was placed in in an electric blast drying oven for 10 h and the sample was burned to 450 ℃ for 4 h at a heating rate of 5 ℃/min.

  2.2.2   Preparation of Au-HSTiO₂

  In an ice water bath, 0.4 g HSTiO₂ was dropped into a pale yellow HAuCl₄ solution with a concentration of 3 mmol/L. Then 1 mL NaBH₄ (0.1 mmol/L) was added to the above resulting solution followed by continuous stirring at a speed of 800 rpm. It can be observed that the solution gradually turns wine red under continuous stirring for 15 minutes, obtaining Au-HSTiO₂ which was respectively washed with water and ethanol for 3 times each to remove ions. The Au-HSTiO₂ was stirred at 60 ℃ until being completely dried.

  2.3 Characterization

  The pattern of the sample was obtained by X-ray diffraction pattern (XRD) through a Shimadzu XRD-6000 diffraction system having a high intensity CuKa radiation (40 kV, 200 mA) of 20~70 and a scanning step of 10 °/min. The surface morphology was discussed by scanning electron microscopy (SEM) (JSM-6510) at an accelerating voltage of 15 KV. The transmission electron microscopy (TEM) image was acquired on JEM-2100 transmission electron microscopy at an accelerating voltage of 200 kV. UV-vis diffuse reflectance spectra (DRS) were obtained on a Shimadzu UV-3600 spectrometer by using BaSO₄ as a reference. X-ray photoelectron spectroscopy (XPS) was operated on PHI-5300 ESCA. Brunauer-Emmett-Teller (BET) surface area of the samples was determined from a N₂ adsorption-desorption isotherm study at liquid nitrogen temperature (77 K) using Micromeritics TRiStarII3020.

  2.4 Photocatalytic experiments

  The H₂ evolution experiments were carried out in the two-neck double walled borosilicate round-bottomed flask (100 mL) at room temperature. In order to prevent gas leakage from the reaction, the neck of the reactor was sealed with a rubber septum which is coated with vacuum silicone grease. A 300W Xe lamp was used as the artificial light source. 50 mg of photocatalyst was dispersed in aqueous methanol solution (CH₃OH: H₂O 1:3 by volume) by continuous stirring. Then He was bubbled through the reaction mixture for 30 minutes to fill the reactor with an inert gas. H₂ evolution was sampled every 30 minutes for 150 minutes. The evolved hydrogen was detected by an on-line gas chromatograph (TECHCOMP GC7900) equipped with a thermal conductivity detector (TCD).

  2.5 Photoelectrochemical (PEC) measurement

  Photoelectric electrochemical measurements were performed on the chi-760e electrochemical instrument of Shanghai Chenhua Instrument Co., Ltd.. Before the test was performed. The following steps were performed to prepare the electrode required for the test. 0.5 g sample was dispersed in 3 mL ethanol and evenly distributed on 1cm × 1cm fluorinated tin glass oxide electrode. After drying with a hairdryer, the resulting photocatalyst film was used as a working electrode, platinum as the counter electrode, and silver/silver chloride as the reference electrode. The 500W shaft lamp serves as a light source for lighting.

  3. RESULTS AND DISCUSSION

  3.1 Characterization of Ag-HSTiO₂ composite photocatalyst

  Fig. 1 shows the XRD patterns to facilitate phase structures of prepared samples. Fig. 1a, 1b and 1c are diffraction patterns of TiO₂, HSTiO₂ and Au-HSTiO₂, respectively. All the diffraction peaks of HSTiO₂ (Fig. 1b) are consistent with TiO₂ (Fig. 1a). All the diffraction peaks of Au-HSTiO₂ (Fig. 1c) are well consistent with TiO₂ and Au (JCPDS: 04-0784). It is fully proved that the Au-HSTiO₂ sample is the composite structures of Au and TiO₂ without other impurities.

  Figure 1

  

  Figure 1.  XRD patterns of (a) TiO₂, (b) HSTiO₂ and (c) Au-HSTiO₂

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  In order to study the morphology of Au-HSTiO₂ composite photocatalyst, we presented its SEM and TEM images. As shown in Fig. 2a, Au nanoparticles adhere to the surface of flaky TiO₂. We give TEM images (Fig. 2b) as further evidence. Au particles attach to TiO₂ and bind closely to TiO₂. It is well known that photocatalytic activity of photocatalysts is related to the lifetime of photo-producing electrons and holes. The structure of Au particles combined with TiO₂ is conducive to extend the lifetime of photo-producing electrons, thereby increasing photocatalytic activity. The following work will confirm this inference.

  Figure 2

  

  Figure 2.  (a) SEM and (b) TEM images of Au-HSTiO₂

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  In order to explore the light absorption ability of different samples, ultraviolet absorption spectrum is given. As shown in Fig. 3, TiO₂ and HSTiO₂ samples show an intrinsic absorption peak in the UV region. The absorption peak of Au-HSTiO₂ in the ultraviolet region is the same as that of TiO₂. However, the Au-HSTiO₂ sample exhibits absorption peaks from the ultraviolet region to the visible range. It can be concluded that Au-HSTiO₂ has a wider light absorption area than TiO₂ and can fully absorb visible light, resulting in higher photocatalytic activity.

  Figure 3

  

  Figure 3.  UV-visible absorption spectra of (a) TiO₂, (b) HSTiO₂ and (c) Au-HSTiO₂

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  The results of BET specific surface area data for TiO₂, HSTiO₂ and Au-HSTiO₂ are shown in Fig. 4. The specific surface area values of TiO₂, HSTiO₂ and Au-HSTiO₂ are 2.255, 121.122 and 88.124 m²·g⁻¹, respectively. The results indicate HSTiO₂ and Au-HSTiO₂ prepared in our work possess high specific surface area.

  Figure 4

  

  Figure 4.  BET specific surface area data for TiO₂, HSTiO₂ and Au-HSTiO₂

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  To study the chemical composition of Au-HSTiO₂, we used XPS to test the sample. The full spectrum of Au-HSTiO₂ confirmed that the sample consisted of Au, O and Ti, and the XPS and XRD test results confirmed each other. The formation of Au-HSTiO₂ heterostructure is demonstrated. In addition, the C element is derived from chamber contamination in the XPS device.

  Figure 5

  

  Figure 5.  XPS spectra of Au-HSTiO₂

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  3.2 Photocatalytic activity

  Fig. 6 shows the yield of hydrogen produced by photocatalytic decomposition of water using TiO₂, HSTiO₂ and Au-HSTiO₂, respectively with methanol as a hole scavenger. It can be concluded from the observation that after using a Xe lamp for 150 minutes, the amount of hydrogen generated by using Au-HSTiO₂ as a photocatalyst to decompose water is much larger than that of TiO₂ and HSTiO₂. Compared with TiO₂, the yield of hydrogen produced by Au-HSTiO₂ as a photocatalyst decomposed water is increased by 24 times to 5.129 mL. The heterojunction structure of Au and HSTiO₂ significantly increases the yield of hydrogen. In order to explore the cyclic stability of Au-HSTiO₂, we conducted four cycles of experiments. After four cycles of test, the hydrogen generation capacity of the sample decreased slightly, which may be due to the shedding of gold nanoparticles. But overall, the sample has good cycle stability.

  Figure 6

  

  Figure 6.  (a) Time profile of photocatalytic H₂ production through aqueous solution of methanol using Au-HSTiO₂, (b) Cycle curve of the photocatalytic H₂ production by Au-HSTiO₂

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  3.3 Photocatalytic mechanism

  Fig. 7 shows photocurrent tests of TiO₂, HSTiO₂ and Au-HSTiO₂. By comparing the photocurrent time curves of different samples in dark and visible light under the switching cycle mode, it can be concluded that Au-HSTiO₂ has the highest photocurrent strength compared to TiO₂ and HSTiO₂. The results demonstrate that Au-HSTiO₂ has higher electron hole separation efficiency, which leads to higher catalytic activity of Au-HSTiO₂.

  Figure 7

  

  Figure 7.  Photocurrent response curves of (a) TiO₂, (b) HSTiO₂ and (c) Au-HSTiO₂

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  Fig. 8 shows the mechanism of Au-HSTiO₂ composite photocatalytic hydrogen production. Under light conditions, the valence band (VB) electrons (e^-) of titanium dioxide are excited to the conduction band (CB) and a hole (h⁺) is generated on the valence band. Because the gold has good electrical conductivity, the electrons on the titanium dioxide conduction band quickly transfer to Au, which has a good electron storage energy and effectively extends the lifetime of light producing electrons. The methanol group as a victim combines to the holes in the valence band to produce oxidation of methanol, which prevents the recombine of hole and electrons. The electrons generated by light react with the H⁺ adsorbed on the surface of the Au to form H₂.

  Figure 8

  

  Figure 8.  Photocatalytic mechanism of the Au-HSTiO₂ under visible light irradiation

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  4. CONCLUSION

  The Au-HSTiO₂ composite photocatalyst was prepared successfully by liquid reduction method, and the photocatalytic properties of the material were good. The photocatalytic activity of Au-HSTiO₂ was found to be the highest among composite materials. In short, Au-HSTiO₂ is an excellent photocatalytic hydrogen production. This study also laid the foundation for further exploration of photocatalysts in the future.

  
  
• 1. [1]

     Kruczek, G.; Przybyła, G.; Ziółkowski, Ł.; Adamczyk, W. P. Comparative assessment of the application of methane and biogas in energy production: an experimental and numerical investigation. Renew. Energy 2019, 143, 1519–1530. doi: 10.1016/j.renene.2019.05.087

  2. [2]

     Dostanić, J.; Lončarević, D.; Pavlović, V. B.; Papan, J.; Nedeljković, J. M. Efficient photocatalytic hydrogen production over titanate/titania nanostructures modified with nickel. Ceram. Int. 2019, 45, 19447–19455. doi: 10.1016/j.ceramint.2019.06.200

  3. [3]

     Chen, K.; Zhang, S.; Peng, W.; Qian, X.; Huang, J. Modification of g-C₃N₄ quantum dots by Ni–Ni₃C@C nanoparticles for hydrogen production. J. Phys. Chem. Solids 2019, 133, 100–107. doi: 10.1016/j.jpcs.2019.05.009

  4. [4]

     Tang, G.; Li, H.; Cheng, C. Au nanoparticles embedded in BiVO₄ films photoanode with enhanced photoelectrochemical performance. Nanotechnology 2019, 30, 445402–11. doi: 10.1088/1361-6528/ab37ee

  5. [5]

     Son, S.; Lee, J. M.; Kim, S. J.; Kim, H.; Jin, X.; Wang, K. K.; Kim, M.; Hwang, J. W.; Choi, W.; Kim, Y. R.; Kim, H.; Hwang, S. J. Understanding the relative efficacies and versatile roles of 2D conductive nanosheets in hybrid-type photocatalyst. Appl. Catal. B-Environ. 2019, 257, 117875–9. doi: 10.1016/j.apcatb.2019.117875

  6. [6]

     Huang, H.; Jin, Y.; Chai, Z.; Gu, X.; Liang, Y.; Li, Q.; Liu, H.; Jiang, H.; Xu, D. Surface charge-induced activation of Ni-loaded CdS for efficient and robust photocatalytic dehydrogenation of methanol. Appl. Catal. B-Environ. 2019, 257, 117869–8. doi: 10.1016/j.apcatb.2019.117869

  7. [7]

     Yang, Y.; Li, X.; Lu, C.; Huang, W. g-C₃N₄ Nanosheets coupled with TiO₂ nanosheets as 2D/2D heterojunction photocatalysts toward high photocatalytic activity for hydrogen production. Catal. Lett. 2019, 149, 2930–2939. doi: 10.1007/s10562-019-02805-8

  8. [8]

     Hunge, Y. M.; Yadav, A. A.; Mathe, V. L. Photocatalytic hydrogen production using TiO₂ nanogranules prepared by hydrothermal route. Catal. Lett. 2019, 731, 136582–4.

  9. [9]

     Kumar, M.; Negi, K.; Chauhan, S.; Umar, A.; Kumar, R.; Masuda, Y.; Chauhan, M. S.; Rajni. Synthesis, characterization, photocatalytic and sensing properties of Mn-doped ZnO nanoparticles. J. Nanosci. Nanotechnol. 2019, 19, 8095–8103. doi: 10.1166/jnn.2019.16758

  10. [10]

      Khan, M. A. M.; Siwach, R.; Kumar, S.; Alhazaa, A. N. Role of Fe doping in tuning photocatalytic and photoelectrochemical properties of TiO₂ for photodegradation of methylene blue. Opt. Laser Technol. 2019, 118, 170–178. doi: 10.1016/j.optlastec.2019.05.012

  11. [11]

      Fajrina, N.; Tahir, M. Engineering approach in stimulating photocatalytic H₂ production in a slurry and monolithic photoreactor systems using Ag-bridged Z-scheme pCN/TiO₂ nanocomposite. Chem. Eng. J. 2019, 374, 1076–1095. doi: 10.1016/j.cej.2019.06.011

  12. [12]

      Yao, L.; Wang, H.; Zhang, Y.; Wang, S.; Liu, X. Fabrication of N doped TiO₂/C nanocomposites with hierarchical porous structure and high photocatalytic activity. Microporous Mesoporous Mat. 2019, 288, 109604–6. doi: 10.1016/j.micromeso.2019.109604

  13. [13]

      Ida, S.; Wilson, P.; Neppolian, B.; Sathish, M.; Karthik, P.; Ravi, P. Ultrasonically aided selective stabilization of pyrrolic type nitrogen by one pot nitrogen doped and hydrothermally reduced graphene oxide/titania nanocomposite (N-TiO₂/N-RGO) for H₂ production. Ultrason. Sonochem. 2019, 57, 62–72. doi: 10.1016/j.ultsonch.2019.04.041

  14. [14]

      Bae, J. Y.; Jang, S. G. Preparation and characterization of CuO-TiO₂ composite hollow nanospheres with enhanced photocatalytic activity under visible light irradiation. J. Nanosci. Nanotechnol. 2019, 19, 6363–6368. doi: 10.1166/jnn.2019.17050

  15. [15]

      Cui, L.; Song, Y. H.; Wang, F. K.; Sheng, Y.; Zou, H. F. Electrospinning synthesis of SiO₂-TiO₂ hybrid nanofibers with large surface area and excellent photocatalytic activity. Appl. Surf. Sci. 2019, 488, 284–292. doi: 10.1016/j.apsusc.2019.05.151

  16. [16]

      Piña-Díaz, A. J.; Torres-Torres, D.; Trejo-Valdez, M.; Torres-SanMiguel, C. R.; Martínez-González, C. L.; Torres-Torres, C. Decision making two-wave mixing with rotating TiO₂-supported Au-Pt nanoparticles. Opt. Laser Technol. 2019, 119, 105638–7. doi: 10.1016/j.optlastec.2019.105638

  17. [17]

      Hassanzadeh-Tabrizi, S. A.; Nguyen, C. C.; Do, T. O. Synthesis of Fe₂O₃/Pt/Au nanocomposite immobilized on g-C₃N₄ for localized plasmon photocatalytic hydrogen evolution. Appl. Surf. Sci. 2019, 489, 741–754. doi: 10.1016/j.apsusc.2019.06.010

  18. [18]

      Matsubara, K.; Inoue, M.; Hagiwara, H.; Abe, T. Photocatalytic water splitting over Pt-loaded TiO₂ (Pt/TiO₂) catalysts prepared by the polygonal barrel-sputtering method. Appl. Catal. B-Environ. 2019, 254, 7–14. doi: 10.1016/j.apcatb.2019.04.075

  19. [19]

      Pincella, F.; Isozaki, K.; Miki, K. A visible light-driven plasmonic photocatalyst. Light-Sci. Appl. 2014, 3, e133–6. doi: 10.1038/lsa.2014.14

  20. [20]

      Ma, X. C.; Dai, Y.; Yu, L.; Huang, B. B. Energy transfer in plasmonic photocatalytic composites. Light-Sci. Appl. 2016, 5, e16017–6. doi: 10.1038/lsa.2016.17

  21. [21]

      Sun, M. L.; Liu, X. L.; Zhao, G. D.; Kong, W. C.; Xuan, J. Y.; Tan, S. G.; Sun, Y. P.; Wei, S. L.; Ren, J. F.; Yin, G. C. Sn⁴⁺ doping combined with hydrogen treatment for CdS/TiO₂ photoelectrodes: an efficient strategy to improve quantum dots loading and charge transport for high photoelectrochemical performance. J. Power Sources 2019, 430, 80–89. doi: 10.1016/j.jpowsour.2019.05.019

  22. [22]

      Zhao, G. D.; Sun, M. L.; Liu, X. L.; Xuan, J. Y.; Kong, W. C.; Zhang, R. N.; Sun, Y. P.; Jia, F. C.; Yin, G. C.; Liu, B. Fabrication of CdS quantum dots sensitized ZnO nanorods/TiO₂ nanosheets hierarchical heterostructure films for enhanced photoelectrochemical performance. Electrochim. Acta 2019, 304, 334–34. doi: 10.1016/j.electacta.2019.03.022

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  Figure 1  XRD patterns of (a) TiO₂, (b) HSTiO₂ and (c) Au-HSTiO₂

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  Figure 2  (a) SEM and (b) TEM images of Au-HSTiO₂

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  Figure 3  UV-visible absorption spectra of (a) TiO₂, (b) HSTiO₂ and (c) Au-HSTiO₂

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  Figure 4  BET specific surface area data for TiO₂, HSTiO₂ and Au-HSTiO₂

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  Figure 5  XPS spectra of Au-HSTiO₂

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  Figure 6  (a) Time profile of photocatalytic H₂ production through aqueous solution of methanol using Au-HSTiO₂, (b) Cycle curve of the photocatalytic H₂ production by Au-HSTiO₂

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  Figure 7  Photocurrent response curves of (a) TiO₂, (b) HSTiO₂ and (c) Au-HSTiO₂

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  Figure 8  Photocatalytic mechanism of the Au-HSTiO₂ under visible light irradiation

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