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Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2–ZnO under visible light
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Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light Meng-Yu Xie a, Kang-Yang Su a, Xin-Yuan Peng a, Ren-Jang Wu a,∗, Murthy Chavali b, Wei-Chen Chang c a
Department of Applied Chemistry, Providence University, Shalu, Taichung 43301, Taiwan, ROC CoExAMMPC and Division of Chemistry, Department of Sciences and Humanities, Vignan’s Foundation for Science, Technology and Research University (VFSTR) University; Vignan’s University, Vadlamudi, Guntur 522 213, Andhra Pradesh, India c Institute of Nuclear Energy Research, Atomic Energy Council, 1000 Wenhua Rd., Chiaan Village, Lungtan, Taoyuan 325, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 4 June 2016 Revised 8 October 2016 Accepted 20 October 2016 Available online xxx Keywords: Photocatalysts Pt/TiO2 –ZnO Hydrogen production
a b s t r a c t Pt/TiO2 –ZnO photocatalytic materials were prepared for the generation of hydrogen gas from water splitting. The photocatalysts were characterized by X-ray diffraction (XRD), UV-visible spectra (UV–Vis), scanning electron microscope (SEM) and transmission electron microscope (TEM). The absorption wavelength of TiO2 was shifted to longer wavelength on Pt/TiO2 –ZnO. A 400 W mercury arc lamp with a cutoff filter which filter out all the wavelengths under 400 nm as the visible light source. It reached up to the optimum value of 203 μmolh−1 g−1 on TiO2 –ZnO (Ti/Zn = 10). A catalyst of 0.5 wt% Pt/TiO2 –ZnO (Ti/Zn = 10) was used and a best hydrogen production rate through water splitting was reaching to 2150 μmolh−1 g−1 . The 0.1 wt% Pt/TiO2 –ZnO showed a satisfying long-term stability, and 88% and 77% of initial hydrogen productivity still remained after 7 days and 14 days, respectively. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The main renewable energy sources are solar energy, wind, water, and biomass. The biomass is the other energy source below the petroleum, the gas, the coal. It provides about 11% of the basic energy demand and 88% of applying for a global renewable energy source. The hydrogen energy source of the biomass, has a variety of recycling and clean, is thought one kind of perfect and have the potential of energy source [1]. There are many ways to produce hydrogen, such as solar energy pyrogenic decomposition [2], photo-electrochemistry method [3], photocatalytic method, [4] and solar energy combination with electrolytic process [5,6]. In 1972, a phenomenon for hydrogen production from photodecomposition of water on TiO2 electrode was discovered [3]. It was found that when ultraviolet light irradiating on TiO2 , the electron is excited from the valence band to conduction band, then the exciting electron form anode of TiO2 moves to the cathode of Platinum, let the water resolve and hydrogen produce. This display that the possibility to translate from solar energy to chemical energy. Now, the decomposition water to produce hydrogen gas turns into an important technology in the world [6].
∗
Corresponding author. Fax: +886426327554. E-mail addresses: [email protected], [email protected] (R.-J. Wu).
For the past few years, the photocatalysis [7–10] activity has been studied with some semiconducting metal oxides like TiO2 [7], ZnO [8], WO3 [9], SnO2 [10], and Ta2 O5 [11]. They become commonly studied and hot photocatalysis material. Photocatalysis takes wavelength energy of photo in special wavelength shining. The electron is stirred up from valence band to conductance band; the excitation electron and electron hole resolve the water to hydrogen [7]. Some scholars focused the visible light as the light source to the water splitting reaction for the sake of there are near 50% visible light from the sun. TiO2 nanotubes possess many benefits such as providing unidirectional electric channel, large specific surface area, high yield of energy conversion and single-step synthesis for preparation [12–14]. To improve the hydrogen yields, metal oxides composites or low cost metals addition are attempted to dope on TiO2 nanotubes [15–26]. It obtained a very high promotion on hydrogen evolution of water splitting reaction [12–27]. The photocatalyst of SrTiO3 possessed photocatalytic activity for water splitting under visible light thereby produced the average rate of H2 evolution of 122.6 μmol h−1 [28]. A water splitting performance based on H2 generation photocatalyst of TiO2 /C3 N4 , and obtained a hydrogen production as 50.2 μmoh−1 [29]. The mesoporous Pt/TiO2 photocatalyst prepared by the single step sol-gel method showed higher photocatalytic activity for H2 evolution of 0.6 wt% Pt loading content and got a hydrogen production to 1400 μmoh−1 g−1 [30]. The visible light photocatalytic activity of water splitting
http://dx.doi.org/10.1016/j.jtice.2016.10.034 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 1. Equipment setup for the photocatalytic H2 evolution by water splitting reaction.
reaction towards hydrogen production was promoted by Pt/N-TiO2 , and the hydrogen yield is obtained to 772 μmoh−1 g−1 [31]. The TiO2 –ZnO mixed oxides photocatalysts were prepared by the solgel method and used for the H2 production from water splitting, and the hydrogen yield is obtained to 1300 μmoh−1 [32]. In this study, the addition of ZnO to TiO2 by a sol–gel method then uses impregnation method to add Pt on to form Pt/TiO2 – ZnO. No researcher has used the catalyst of Pt/TiO2 –ZnO. A 0.5 wt% Pt/TiO2 –ZnO (Ti/Zn = 10) was used and a best hydrogen production rate through water splitting was reaching to 2150 μmolh−1 g−1 .
all samples was examined in 250 mL methanol aqueous solution (10% methanol by volume) containing 0.5 g photocatalyst under magnetic stirring at 25 °C. Methanol was used in the solution as a sacrificial reagent which scavenges photo-generated holes. A 400 W mercury lamp with UV cut filter (>400 nm) was used as the light source [33]. Before irradiation, the system was thoroughly degassed in order to remove air. The gaseous products were analysed by an on-line gas chromatography (Gas Chromatography Personal GC 10 0 0, China) equipped with a packed column (MS-5A, 3 m in length) and a TCD detector, using Ar as the carrier gas [33]. In addition, each experiment was carried out at least three times.
2. Experimental 2.3. Photo-electrochemical measurement 2.1. Preparation of photocatalysts The photocatalysts of TiO2 (ST21) and ZnO were purchased by commercial company, TiO2 –ZnO and Pt/TiO2 –ZnO were prepared by followings in our lab. 0.34 g Zinc sulphate (ZnSO4 ), 18 ml D.I. water and 40 ml methanol (CH3 OH) were mixed to form a solution, then added appropriate amounts of aqua ammonia (NH4 OH) to adjust the pH value to 9. Then 10 ml tetra butyl ortho titanate Ti(C4 H9 O)4 was added, and stirred for three hours become white colloid, and next heat with 70 °C stirring until it becomes powder. Take this powder into bake oven for two hours, then grind sample ball for eight hours. Put the sample in the oven and calcined at 500 °C for 2 h, and it will get the sample of TiO2 –ZnO (Ti/Zn = 10). Various ratios of Ti/Zn photocatalyst blended material can be obtained as following the same process. By taking 1 g TiO2 –ZnO, 30 ml methanol and appropriate amounts of chloroplatinic acid (H2 PtCl6 ·6H2 O) into the aqueous solution, stir it continuously for three hours, heat with 70 °C and stir continue until it becomes powder, put in bake oven for two hours, grind sample ball for eight hours. Next to put it in 300 °C calcination for two hours, the various platinum weight ratio percentage (wt%) of Pt/TiO2 –ZnO photocatalytic samples were obtained.
For photocurrent measurements, the electrodes were immersed in a solution of 1 M NaOH. The working electrode was scanned from −0.1 to 0.7 V (versus Ag/AgCl electrode) at a rate of 1 mV/s. The TiO2 , TiO2 –ZnO and 0.5% Pt/TiO2 –ZnO samples coated on working electrodes were illuminated with a 400 W mercury lamp with UV cut filter (>400 nm), and it was used as the light source. 2.4. Characterised by XRD, UV–Vis and TEM The photocatalytic samples were analysed by powder X-ray diffraction spectra using an XRD-60 0 0 Shimadzu X-RAY Diffrac˚ at 35 kV and tometer, with Cu Kα radiation (Cu Kα 1, 1.54060 A) 35 mA between 10º and 80º (2θ ) with 2º / min. Spectroscopy Ultraviolet (UV)-Vis absorption spectra were obtained using a UV–Vis spectrometer (UV-2550, Shimadzu, Japan). The surface morphology of all samples were characterized by field emission scanning electron microscopy (JEOL JEM-2100F). The elemental composition was determined by energy dispersive X-ray spectroscopy (EDX) and elemental mapping. Transmission electron microscopy (TEM) images were taken using JEOL JEM-2100F. All the characterization samples of TiO2 –ZnO is fixed at Ti/Zn = 10.
2.2. Water splitting system
3. Results and discussion
It showed a schematic diagram of the experimental arrangement used for the water splitting reaction in the production of hydrogen in Fig. 1 [31,33]. Photocatalytic reactions were conducted in a Pyrex glass reactor that contained with a cooling water system. Photocatalytic activity measurement hydrogen production of
3.1. Characterization of the catalysts using XRD, TEM and UV–Vis The XRD patterns of TiO2 , TiO2 –ZnO, 3.0 wt% Pt/TiO2 –ZnO and ZnO are shown in Fig. 2(a), (b), (c) and (d), respectively. Diffraction peaks that are attributable to anatase TiO2 are clearly observed in
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 2. XRD spectra of (a) TiO2 , (b) TiO2 –ZnO, (c) 3.0 wt% Pt/TiO2 ––ZnO, (d) ZnO.
the samples of Fig. 2(a), (b) and (c), which are in good accordance with those in the standard JCPSD cards No.21-1272. In addition, the pattern for the all samples exhibit diffraction peaks appeared at 2θ = 25.28º, 37.80º, 48.05º, 53.89º, 62.69º and 75.03º corresponding to (101), (0 04), (20 0), (105), (204) and (215) planes can be seen. It should be noted that the deposited Pt do not affect the crystal structure of anatase TiO2 and one distinct additional diffraction peak of metallic Pt is found in Fig. 2(c). Fig. 2(c) revealed the peak at 39.76º attributed to Pt (111) plane are apparently observed in the XRD patterns. Fig. 2(d) presented the ZnO sample exhibit diffraction peaks appeared at 2θ = 31.77º, 34.42º, 36.25º and 47.54º corresponding to (100), (002), (101), and (102) planes can be seen. In order to examination the morphology of TiO2 –ZnO and 0.5 wt% Pt/TiO2 –ZnO, SEM images and EDX spectra were completed, which are showed in Fig. 3(a), (b) and (c) and Fig. 4(a), (a) and (c). It can be found that TiO2 –ZnO shows non uniform-shape of Fig. 3(a) in micrometer scale. From Fig. 3(b) and (c), it can be observed that the Ti, O, Zn and C were observed in the EDX spectrum and elemental mapping of the TiO2 –ZnO. Fig. 4(a) revealed some sphere-like structure on the surface of 0.5 wt% Pt/TiO2 –ZnO. In Fig. 4(b) and (c), it can be observed that the Ti, O, Zn, Pt and C were observed in the EDX spectrum and elemental mapping of the 0.5 wt% Pt/TiO2 –ZnO. The TEM images of TiO2 , ZnO and 0.5 wt% Pt/TiO2 –ZnO are shown in Fig. 5(a), (b) and (c), respectively. Fig. 5(a) showed a set of regular lattice fringes of 0.36 nm is consistent with the (101) crystal plane of the TiO2 . A lattice fringe of 0.26 nm is consistent with the (002) crystal plane of the ZnO in Fig. 5(b). It can be identified that there are TiO2 (101) d = 0.36 crystal form and Pt (111) d = 0.22 crystal form, but there is no ZnO crystal form in Fig. 5(c). The possible microstructure of TiO2 –ZnO blended material is TiO2 dispersed on ZnO surface, so it can’t form crystal structure by XRD (Fig. 2(b) and (c)) and TEM (Fig. 5(c)) [32]. In order to research the optical absorption properties of the photocatalysts, the UV-visible absorption spectra of TiO2 , ZnO, TiO2 –ZnO and 0.5 wt% Pt/TiO2 –ZnO were investigated in Fig. 6(a), (b), (c) and (d). Fig. 6(a), (b) and (c) present the absorption spectra of ZnO, TiO2 and TiO2 –ZnO, respectively. It is seen that the ZnO and TiO2 –ZnO possess longer wavelength absorption than TiO2 in Fig. 6(a), (b) and (c). In Fig. 6(d), 0.5 wt% Pt/TiO2 –ZnO shows a strongest photoabsorption in the visible light region (wavelength >430 nm). In addition, the band gap was calculated from the UVvisible absorption spectra (Table 1). Table 1 presents the band gap
Fig. 3. (a) SEM images of TiO2 –ZnO, (b) EDX layered images of TiO2 –ZnO, (c) elemental EDX mapping of TiO2 –ZnO.
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 4. (a) SEM images of 0.5% Pt/TiO2 –ZnO (b) EDS layered images of 0.5% Pt/TiO2 – ZnO (c) elemental EDX mapping of 0.5% Pt/TiO2 –ZnO.
Table 1 Band gap of various photocatalysts. Catalysts
Band gap (eV)
TiO2 ZnO TiO2 –ZnO (Ti/Zn = 10) Pt/TiO2 –ZnO (Ti/Zn = 10)
3.23 3.02 3.18 3.08
Fig. 5. TEM images of (a) TiO2 , (b) ZnO, (c) 0.5%Pt/TiO2 –ZnO (Ti/Zn = 10).
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 6. Reflective spectra of UV visible (a) ZnO (b) TiO2 (c) TiO2 –ZnO (d) 0.5 wt% Pt/TiO2 –ZnO.
Table 2 Hydrogen gas yield of various Ti/Zn ratios of TiO2 –ZnO photocatalysts. Catalyst
Hydrogen yield (μmolh−1 g−1 )
TiO2 ZnO
68 3.0 83 107 144 171 203 152
TiO2 –ZnO
Ti/Zn = 2 Ti/Zn = 4 Ti/Zn = 6 Ti/Zn = 8 Ti/Zn = 10 Ti/Zn = 12
Table 3 Hydrogen yield of various Pt loadings on TiO2 –ZnO (Ti / Zn = 10). Catalyst
Hydrogen yield (μmolh− 1 g− 1 )
TiO2 –ZnO 0.01 wt% Pt/TiO2 –ZnO 0.05 wt% Pt/TiO2 –ZnO 0.1 wt% Pt/TiO2 –ZnO 0.5 wt% Pt/TiO2 –ZnO 1.0 wt% Pt/TiO2 –ZnO
203 916 1313 1789 2150 820
values of commercial TiO2 , ZnO, TiO2 –ZnO and 0.5 wt% Pt/TiO2 – ZnO are 3.23 eV, 3.02 eV, 3.18 eV and 3.08 eV, respectively. 3.2. Yields of hydrogen generation Table 2 reveals the hydrogen yields of TiO2 , ZnO and various ratios of TiO2 –ZnO samples from water splitting. It can be found that the hydrogen production rates of TiO2 and ZnO are 68 μmolh−1 g−1 and 3.0 μmolh−1 g−1 , respectively. The hydrogen yields of various ratios from Ti/Zn = 2 to 12 of TiO2 –ZnO are higher than TiO2 and ZnO. It reached up to the optimum value of 203 μmolh−1 g−1 while the Ti/Zn = 10. All the TiO2 –ZnO (Ti/Zn = 10) samples with various Pt loading from 0.01 wt% to 1.0 wt% exhibited photocatalytic activity for hydrogen production from water splitting in Table 3 and Fig. 7. The optimum Pt loading is 0.5 wt%, and it can achieve the maximum hydrogen yield to 2150 μmolh−1 g−1 . From Fig. 7, it presented that the hydrogen production rate of 0.5 wt% Pt/TiO2 – ZnO reached up to a stable yield 2150 μmolh−1 g−1 after 420 min under visible light irradiation. Meanwhile, the hydrogen production rate of 0.01 wt% Pt/TiO2 –ZnO, 0.05 wt% Pt/TiO2 –ZnO and 0.1 wt% Pt/TiO2 –ZnO reached up to 916 μmolh−1 g−1 , 1313 μmolh−1 g−1
Fig. 7. Hydrogen yield versus reaction time of various Pt loadings on TiO2 –ZnO (Ti / Zn = 10): (a) 0.01 wt% Pt/TiO2 –ZnO, (b) 0.05 wt% Pt/TiO2 –ZnO, (c) 0.1 wt% Pt/TiO2 – ZnO, (d) 0.5 wt% Pt/TiO2 –ZnO, (e) 1.0 wt% Pt/TiO2 –ZnO.
and 1789 μmolh−1 g−1 , respectively after 300 min. The reason is that the photocatalytic reaction reached equilibrium. On another way to understand the Pt promotion effect on TiO2 –ZnO to water splitting reaction. It can be achieved by photocurrent measurements in Fig. 8, the TiO2 , TiO2 –ZnO and 0.5% Pt/TiO2 –ZnO samples coated on working electrodes were illuminated with a 400 W mercury lamp with UV cut filter (>400 nm), and it was used as the light source. Fig. 8(a), (b) and (c) presented the 0.5% Pt/TiO2 – ZnO electrode can generate most high photochemical current than others. In Table 3, the addition of Pt can increase the hydrogen yield about four to ten times with TiO2 –ZnO. These results prove that Pt can promote the separation efficiency of electron–hole pairs on the Pt/TiO2 –ZnO surface, which plays a role of electron transfer intermediate. Meanwhile, Pt can effectively prevent the direct recombination of electrons and holes, which retards the electron–hole recombination rate in water splitting reaction. These two effects may combine and result in a further increment in photocatalytic activity of Pt/TiO2 –ZnO. Every seven days take once determine, finding that the experiment material has good reproducibility in Fig. 9. The 0.1 wt% Pt/TiO2 –ZnO showed a satisfying long-term stability, and 88% and 77% of initial hydrogen productivity still remained after 7 and 14 days, respectively, as illustrated in Fig. 9. Therefore, the Pt/TiO2 – ZnO was high activity, efficiency and long-term stability.
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 8. Current-potential curves with chopped light measured in 1 M NaOH with a scan rate of 1 mV s−1 for various samples of (a) TiO2 –ZnO, (b) 0.5% Pt/TiO2 –ZnO, (c) TiO2 .
Fig. 9. Re-use of photocatalysts of 0.1 wt% Pt/TiO2 –ZnO.
3.3. Probable photocatalytic mechanism A photocatalytic mechanism of splitting water reaction was proposed by following Eqs. (1)–(5) [30,32,33]. Fig. 10(a) revealed that when Pt/TiO2 –ZnO is irradiated, an electron may be excited from the valence band to the conduction band on the TiO2 –ZnO, as shown in Eq. (1). Simultaneously, it is very likely that some excited electrons in conduction band can come back to valence band to recombine with the holes on TiO2 –ZnO according to the Eq. (2). Pt nanoparticles were placed on the surface can act as electron traps and inhibit the hole-electron recombination by accumulating the electrons [31–33]. Then, the holes react with H2 O molecules to form the products of H+ and • OH, as designated in Eq. (3). Eventually, the H+ and • OH are reduced and oxidised by the photogenerated electrons and holes to produce H2 and O2 , respectively, as shown in Eqs. (4) and (5). Eqs. (1), (3), (4) and (5) are summarised as Eq. (6). Additionally, in a water-splitting reaction Pt is loaded on the surface as a dispersion of nanoparticles to produce active sites and accelerate the reduce reaction of H+ to generate hydrogen on Pt in Fig. 10(a) and (b) [30,33]. Furthermore, the probable active centre
of the photocatalytic water splitting reaction based on Pt/TiO2 –ZnO is the Pt deposited on TiO2 structure, and ZnO are surrounded by the structure as shown in Fig. 10(b).
TiO2 –ZnO → TiO2 –ZnO (e− + h+ )
(1)
TiO2 –ZnO (e− + h+ ) → TiO2 –ZnO
(2)
h+ + H2 O → H+ + • OH
(3)
2e− + 2H+ H2
(4)
•
(5)
OH + • OH → H2 O + 1/2O2
H2 O → H2 + 1/2O2
(6)
4. Conclusion The photocatalysis of Pt/TiO2 –ZnO for water-splitting which has the benefits of preparing easily, environmentally friendly and good re-utilization. The optimal experimental conditions, the titanium
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 10. (a) Energy diagram of hydrogen production of water splitting on Pt/TiO2 – ZnO, (b) Possible simple nanostructure of Pt/TiO2 –ZnO.
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Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light Meng-Yu Xie a, Kang-Yang Su a, Xin-Yuan Peng a, Ren-Jang Wu a,∗, Murthy Chavali b, Wei-Chen Chang c a
Department of Applied Chemistry, Providence University, Shalu, Taichung 43301, Taiwan, ROC CoExAMMPC and Division of Chemistry, Department of Sciences and Humanities, Vignan’s Foundation for Science, Technology and Research University (VFSTR) University; Vignan’s University, Vadlamudi, Guntur 522 213, Andhra Pradesh, India c Institute of Nuclear Energy Research, Atomic Energy Council, 1000 Wenhua Rd., Chiaan Village, Lungtan, Taoyuan 325, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 4 June 2016 Revised 8 October 2016 Accepted 20 October 2016 Available online xxx Keywords: Photocatalysts Pt/TiO2 –ZnO Hydrogen production
a b s t r a c t Pt/TiO2 –ZnO photocatalytic materials were prepared for the generation of hydrogen gas from water splitting. The photocatalysts were characterized by X-ray diffraction (XRD), UV-visible spectra (UV–Vis), scanning electron microscope (SEM) and transmission electron microscope (TEM). The absorption wavelength of TiO2 was shifted to longer wavelength on Pt/TiO2 –ZnO. A 400 W mercury arc lamp with a cutoff filter which filter out all the wavelengths under 400 nm as the visible light source. It reached up to the optimum value of 203 μmolh−1 g−1 on TiO2 –ZnO (Ti/Zn = 10). A catalyst of 0.5 wt% Pt/TiO2 –ZnO (Ti/Zn = 10) was used and a best hydrogen production rate through water splitting was reaching to 2150 μmolh−1 g−1 . The 0.1 wt% Pt/TiO2 –ZnO showed a satisfying long-term stability, and 88% and 77% of initial hydrogen productivity still remained after 7 days and 14 days, respectively. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The main renewable energy sources are solar energy, wind, water, and biomass. The biomass is the other energy source below the petroleum, the gas, the coal. It provides about 11% of the basic energy demand and 88% of applying for a global renewable energy source. The hydrogen energy source of the biomass, has a variety of recycling and clean, is thought one kind of perfect and have the potential of energy source [1]. There are many ways to produce hydrogen, such as solar energy pyrogenic decomposition [2], photo-electrochemistry method [3], photocatalytic method, [4] and solar energy combination with electrolytic process [5,6]. In 1972, a phenomenon for hydrogen production from photodecomposition of water on TiO2 electrode was discovered [3]. It was found that when ultraviolet light irradiating on TiO2 , the electron is excited from the valence band to conduction band, then the exciting electron form anode of TiO2 moves to the cathode of Platinum, let the water resolve and hydrogen produce. This display that the possibility to translate from solar energy to chemical energy. Now, the decomposition water to produce hydrogen gas turns into an important technology in the world [6].
∗
Corresponding author. Fax: +886426327554. E-mail addresses: [email protected], [email protected] (R.-J. Wu).
For the past few years, the photocatalysis [7–10] activity has been studied with some semiconducting metal oxides like TiO2 [7], ZnO [8], WO3 [9], SnO2 [10], and Ta2 O5 [11]. They become commonly studied and hot photocatalysis material. Photocatalysis takes wavelength energy of photo in special wavelength shining. The electron is stirred up from valence band to conductance band; the excitation electron and electron hole resolve the water to hydrogen [7]. Some scholars focused the visible light as the light source to the water splitting reaction for the sake of there are near 50% visible light from the sun. TiO2 nanotubes possess many benefits such as providing unidirectional electric channel, large specific surface area, high yield of energy conversion and single-step synthesis for preparation [12–14]. To improve the hydrogen yields, metal oxides composites or low cost metals addition are attempted to dope on TiO2 nanotubes [15–26]. It obtained a very high promotion on hydrogen evolution of water splitting reaction [12–27]. The photocatalyst of SrTiO3 possessed photocatalytic activity for water splitting under visible light thereby produced the average rate of H2 evolution of 122.6 μmol h−1 [28]. A water splitting performance based on H2 generation photocatalyst of TiO2 /C3 N4 , and obtained a hydrogen production as 50.2 μmoh−1 [29]. The mesoporous Pt/TiO2 photocatalyst prepared by the single step sol-gel method showed higher photocatalytic activity for H2 evolution of 0.6 wt% Pt loading content and got a hydrogen production to 1400 μmoh−1 g−1 [30]. The visible light photocatalytic activity of water splitting
http://dx.doi.org/10.1016/j.jtice.2016.10.034 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 1. Equipment setup for the photocatalytic H2 evolution by water splitting reaction.
reaction towards hydrogen production was promoted by Pt/N-TiO2 , and the hydrogen yield is obtained to 772 μmoh−1 g−1 [31]. The TiO2 –ZnO mixed oxides photocatalysts were prepared by the solgel method and used for the H2 production from water splitting, and the hydrogen yield is obtained to 1300 μmoh−1 [32]. In this study, the addition of ZnO to TiO2 by a sol–gel method then uses impregnation method to add Pt on to form Pt/TiO2 – ZnO. No researcher has used the catalyst of Pt/TiO2 –ZnO. A 0.5 wt% Pt/TiO2 –ZnO (Ti/Zn = 10) was used and a best hydrogen production rate through water splitting was reaching to 2150 μmolh−1 g−1 .
all samples was examined in 250 mL methanol aqueous solution (10% methanol by volume) containing 0.5 g photocatalyst under magnetic stirring at 25 °C. Methanol was used in the solution as a sacrificial reagent which scavenges photo-generated holes. A 400 W mercury lamp with UV cut filter (>400 nm) was used as the light source [33]. Before irradiation, the system was thoroughly degassed in order to remove air. The gaseous products were analysed by an on-line gas chromatography (Gas Chromatography Personal GC 10 0 0, China) equipped with a packed column (MS-5A, 3 m in length) and a TCD detector, using Ar as the carrier gas [33]. In addition, each experiment was carried out at least three times.
2. Experimental 2.3. Photo-electrochemical measurement 2.1. Preparation of photocatalysts The photocatalysts of TiO2 (ST21) and ZnO were purchased by commercial company, TiO2 –ZnO and Pt/TiO2 –ZnO were prepared by followings in our lab. 0.34 g Zinc sulphate (ZnSO4 ), 18 ml D.I. water and 40 ml methanol (CH3 OH) were mixed to form a solution, then added appropriate amounts of aqua ammonia (NH4 OH) to adjust the pH value to 9. Then 10 ml tetra butyl ortho titanate Ti(C4 H9 O)4 was added, and stirred for three hours become white colloid, and next heat with 70 °C stirring until it becomes powder. Take this powder into bake oven for two hours, then grind sample ball for eight hours. Put the sample in the oven and calcined at 500 °C for 2 h, and it will get the sample of TiO2 –ZnO (Ti/Zn = 10). Various ratios of Ti/Zn photocatalyst blended material can be obtained as following the same process. By taking 1 g TiO2 –ZnO, 30 ml methanol and appropriate amounts of chloroplatinic acid (H2 PtCl6 ·6H2 O) into the aqueous solution, stir it continuously for three hours, heat with 70 °C and stir continue until it becomes powder, put in bake oven for two hours, grind sample ball for eight hours. Next to put it in 300 °C calcination for two hours, the various platinum weight ratio percentage (wt%) of Pt/TiO2 –ZnO photocatalytic samples were obtained.
For photocurrent measurements, the electrodes were immersed in a solution of 1 M NaOH. The working electrode was scanned from −0.1 to 0.7 V (versus Ag/AgCl electrode) at a rate of 1 mV/s. The TiO2 , TiO2 –ZnO and 0.5% Pt/TiO2 –ZnO samples coated on working electrodes were illuminated with a 400 W mercury lamp with UV cut filter (>400 nm), and it was used as the light source. 2.4. Characterised by XRD, UV–Vis and TEM The photocatalytic samples were analysed by powder X-ray diffraction spectra using an XRD-60 0 0 Shimadzu X-RAY Diffrac˚ at 35 kV and tometer, with Cu Kα radiation (Cu Kα 1, 1.54060 A) 35 mA between 10º and 80º (2θ ) with 2º / min. Spectroscopy Ultraviolet (UV)-Vis absorption spectra were obtained using a UV–Vis spectrometer (UV-2550, Shimadzu, Japan). The surface morphology of all samples were characterized by field emission scanning electron microscopy (JEOL JEM-2100F). The elemental composition was determined by energy dispersive X-ray spectroscopy (EDX) and elemental mapping. Transmission electron microscopy (TEM) images were taken using JEOL JEM-2100F. All the characterization samples of TiO2 –ZnO is fixed at Ti/Zn = 10.
2.2. Water splitting system
3. Results and discussion
It showed a schematic diagram of the experimental arrangement used for the water splitting reaction in the production of hydrogen in Fig. 1 [31,33]. Photocatalytic reactions were conducted in a Pyrex glass reactor that contained with a cooling water system. Photocatalytic activity measurement hydrogen production of
3.1. Characterization of the catalysts using XRD, TEM and UV–Vis The XRD patterns of TiO2 , TiO2 –ZnO, 3.0 wt% Pt/TiO2 –ZnO and ZnO are shown in Fig. 2(a), (b), (c) and (d), respectively. Diffraction peaks that are attributable to anatase TiO2 are clearly observed in
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 2. XRD spectra of (a) TiO2 , (b) TiO2 –ZnO, (c) 3.0 wt% Pt/TiO2 ––ZnO, (d) ZnO.
the samples of Fig. 2(a), (b) and (c), which are in good accordance with those in the standard JCPSD cards No.21-1272. In addition, the pattern for the all samples exhibit diffraction peaks appeared at 2θ = 25.28º, 37.80º, 48.05º, 53.89º, 62.69º and 75.03º corresponding to (101), (0 04), (20 0), (105), (204) and (215) planes can be seen. It should be noted that the deposited Pt do not affect the crystal structure of anatase TiO2 and one distinct additional diffraction peak of metallic Pt is found in Fig. 2(c). Fig. 2(c) revealed the peak at 39.76º attributed to Pt (111) plane are apparently observed in the XRD patterns. Fig. 2(d) presented the ZnO sample exhibit diffraction peaks appeared at 2θ = 31.77º, 34.42º, 36.25º and 47.54º corresponding to (100), (002), (101), and (102) planes can be seen. In order to examination the morphology of TiO2 –ZnO and 0.5 wt% Pt/TiO2 –ZnO, SEM images and EDX spectra were completed, which are showed in Fig. 3(a), (b) and (c) and Fig. 4(a), (a) and (c). It can be found that TiO2 –ZnO shows non uniform-shape of Fig. 3(a) in micrometer scale. From Fig. 3(b) and (c), it can be observed that the Ti, O, Zn and C were observed in the EDX spectrum and elemental mapping of the TiO2 –ZnO. Fig. 4(a) revealed some sphere-like structure on the surface of 0.5 wt% Pt/TiO2 –ZnO. In Fig. 4(b) and (c), it can be observed that the Ti, O, Zn, Pt and C were observed in the EDX spectrum and elemental mapping of the 0.5 wt% Pt/TiO2 –ZnO. The TEM images of TiO2 , ZnO and 0.5 wt% Pt/TiO2 –ZnO are shown in Fig. 5(a), (b) and (c), respectively. Fig. 5(a) showed a set of regular lattice fringes of 0.36 nm is consistent with the (101) crystal plane of the TiO2 . A lattice fringe of 0.26 nm is consistent with the (002) crystal plane of the ZnO in Fig. 5(b). It can be identified that there are TiO2 (101) d = 0.36 crystal form and Pt (111) d = 0.22 crystal form, but there is no ZnO crystal form in Fig. 5(c). The possible microstructure of TiO2 –ZnO blended material is TiO2 dispersed on ZnO surface, so it can’t form crystal structure by XRD (Fig. 2(b) and (c)) and TEM (Fig. 5(c)) [32]. In order to research the optical absorption properties of the photocatalysts, the UV-visible absorption spectra of TiO2 , ZnO, TiO2 –ZnO and 0.5 wt% Pt/TiO2 –ZnO were investigated in Fig. 6(a), (b), (c) and (d). Fig. 6(a), (b) and (c) present the absorption spectra of ZnO, TiO2 and TiO2 –ZnO, respectively. It is seen that the ZnO and TiO2 –ZnO possess longer wavelength absorption than TiO2 in Fig. 6(a), (b) and (c). In Fig. 6(d), 0.5 wt% Pt/TiO2 –ZnO shows a strongest photoabsorption in the visible light region (wavelength >430 nm). In addition, the band gap was calculated from the UVvisible absorption spectra (Table 1). Table 1 presents the band gap
Fig. 3. (a) SEM images of TiO2 –ZnO, (b) EDX layered images of TiO2 –ZnO, (c) elemental EDX mapping of TiO2 –ZnO.
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Fig. 4. (a) SEM images of 0.5% Pt/TiO2 –ZnO (b) EDS layered images of 0.5% Pt/TiO2 – ZnO (c) elemental EDX mapping of 0.5% Pt/TiO2 –ZnO.
Table 1 Band gap of various photocatalysts. Catalysts
Band gap (eV)
TiO2 ZnO TiO2 –ZnO (Ti/Zn = 10) Pt/TiO2 –ZnO (Ti/Zn = 10)
3.23 3.02 3.18 3.08
Fig. 5. TEM images of (a) TiO2 , (b) ZnO, (c) 0.5%Pt/TiO2 –ZnO (Ti/Zn = 10).
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Fig. 6. Reflective spectra of UV visible (a) ZnO (b) TiO2 (c) TiO2 –ZnO (d) 0.5 wt% Pt/TiO2 –ZnO.
Table 2 Hydrogen gas yield of various Ti/Zn ratios of TiO2 –ZnO photocatalysts. Catalyst
Hydrogen yield (μmolh−1 g−1 )
TiO2 ZnO
68 3.0 83 107 144 171 203 152
TiO2 –ZnO
Ti/Zn = 2 Ti/Zn = 4 Ti/Zn = 6 Ti/Zn = 8 Ti/Zn = 10 Ti/Zn = 12
Table 3 Hydrogen yield of various Pt loadings on TiO2 –ZnO (Ti / Zn = 10). Catalyst
Hydrogen yield (μmolh− 1 g− 1 )
TiO2 –ZnO 0.01 wt% Pt/TiO2 –ZnO 0.05 wt% Pt/TiO2 –ZnO 0.1 wt% Pt/TiO2 –ZnO 0.5 wt% Pt/TiO2 –ZnO 1.0 wt% Pt/TiO2 –ZnO
203 916 1313 1789 2150 820
values of commercial TiO2 , ZnO, TiO2 –ZnO and 0.5 wt% Pt/TiO2 – ZnO are 3.23 eV, 3.02 eV, 3.18 eV and 3.08 eV, respectively. 3.2. Yields of hydrogen generation Table 2 reveals the hydrogen yields of TiO2 , ZnO and various ratios of TiO2 –ZnO samples from water splitting. It can be found that the hydrogen production rates of TiO2 and ZnO are 68 μmolh−1 g−1 and 3.0 μmolh−1 g−1 , respectively. The hydrogen yields of various ratios from Ti/Zn = 2 to 12 of TiO2 –ZnO are higher than TiO2 and ZnO. It reached up to the optimum value of 203 μmolh−1 g−1 while the Ti/Zn = 10. All the TiO2 –ZnO (Ti/Zn = 10) samples with various Pt loading from 0.01 wt% to 1.0 wt% exhibited photocatalytic activity for hydrogen production from water splitting in Table 3 and Fig. 7. The optimum Pt loading is 0.5 wt%, and it can achieve the maximum hydrogen yield to 2150 μmolh−1 g−1 . From Fig. 7, it presented that the hydrogen production rate of 0.5 wt% Pt/TiO2 – ZnO reached up to a stable yield 2150 μmolh−1 g−1 after 420 min under visible light irradiation. Meanwhile, the hydrogen production rate of 0.01 wt% Pt/TiO2 –ZnO, 0.05 wt% Pt/TiO2 –ZnO and 0.1 wt% Pt/TiO2 –ZnO reached up to 916 μmolh−1 g−1 , 1313 μmolh−1 g−1
Fig. 7. Hydrogen yield versus reaction time of various Pt loadings on TiO2 –ZnO (Ti / Zn = 10): (a) 0.01 wt% Pt/TiO2 –ZnO, (b) 0.05 wt% Pt/TiO2 –ZnO, (c) 0.1 wt% Pt/TiO2 – ZnO, (d) 0.5 wt% Pt/TiO2 –ZnO, (e) 1.0 wt% Pt/TiO2 –ZnO.
and 1789 μmolh−1 g−1 , respectively after 300 min. The reason is that the photocatalytic reaction reached equilibrium. On another way to understand the Pt promotion effect on TiO2 –ZnO to water splitting reaction. It can be achieved by photocurrent measurements in Fig. 8, the TiO2 , TiO2 –ZnO and 0.5% Pt/TiO2 –ZnO samples coated on working electrodes were illuminated with a 400 W mercury lamp with UV cut filter (>400 nm), and it was used as the light source. Fig. 8(a), (b) and (c) presented the 0.5% Pt/TiO2 – ZnO electrode can generate most high photochemical current than others. In Table 3, the addition of Pt can increase the hydrogen yield about four to ten times with TiO2 –ZnO. These results prove that Pt can promote the separation efficiency of electron–hole pairs on the Pt/TiO2 –ZnO surface, which plays a role of electron transfer intermediate. Meanwhile, Pt can effectively prevent the direct recombination of electrons and holes, which retards the electron–hole recombination rate in water splitting reaction. These two effects may combine and result in a further increment in photocatalytic activity of Pt/TiO2 –ZnO. Every seven days take once determine, finding that the experiment material has good reproducibility in Fig. 9. The 0.1 wt% Pt/TiO2 –ZnO showed a satisfying long-term stability, and 88% and 77% of initial hydrogen productivity still remained after 7 and 14 days, respectively, as illustrated in Fig. 9. Therefore, the Pt/TiO2 – ZnO was high activity, efficiency and long-term stability.
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Fig. 8. Current-potential curves with chopped light measured in 1 M NaOH with a scan rate of 1 mV s−1 for various samples of (a) TiO2 –ZnO, (b) 0.5% Pt/TiO2 –ZnO, (c) TiO2 .
Fig. 9. Re-use of photocatalysts of 0.1 wt% Pt/TiO2 –ZnO.
3.3. Probable photocatalytic mechanism A photocatalytic mechanism of splitting water reaction was proposed by following Eqs. (1)–(5) [30,32,33]. Fig. 10(a) revealed that when Pt/TiO2 –ZnO is irradiated, an electron may be excited from the valence band to the conduction band on the TiO2 –ZnO, as shown in Eq. (1). Simultaneously, it is very likely that some excited electrons in conduction band can come back to valence band to recombine with the holes on TiO2 –ZnO according to the Eq. (2). Pt nanoparticles were placed on the surface can act as electron traps and inhibit the hole-electron recombination by accumulating the electrons [31–33]. Then, the holes react with H2 O molecules to form the products of H+ and • OH, as designated in Eq. (3). Eventually, the H+ and • OH are reduced and oxidised by the photogenerated electrons and holes to produce H2 and O2 , respectively, as shown in Eqs. (4) and (5). Eqs. (1), (3), (4) and (5) are summarised as Eq. (6). Additionally, in a water-splitting reaction Pt is loaded on the surface as a dispersion of nanoparticles to produce active sites and accelerate the reduce reaction of H+ to generate hydrogen on Pt in Fig. 10(a) and (b) [30,33]. Furthermore, the probable active centre
of the photocatalytic water splitting reaction based on Pt/TiO2 –ZnO is the Pt deposited on TiO2 structure, and ZnO are surrounded by the structure as shown in Fig. 10(b).
TiO2 –ZnO → TiO2 –ZnO (e− + h+ )
(1)
TiO2 –ZnO (e− + h+ ) → TiO2 –ZnO
(2)
h+ + H2 O → H+ + • OH
(3)
2e− + 2H+ H2
(4)
•
(5)
OH + • OH → H2 O + 1/2O2
H2 O → H2 + 1/2O2
(6)
4. Conclusion The photocatalysis of Pt/TiO2 –ZnO for water-splitting which has the benefits of preparing easily, environmentally friendly and good re-utilization. The optimal experimental conditions, the titanium
Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034
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Fig. 10. (a) Energy diagram of hydrogen production of water splitting on Pt/TiO2 – ZnO, (b) Possible simple nanostructure of Pt/TiO2 –ZnO.
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Please cite this article as: M.-Y. Xie et al., Hydrogen production by photocatalytic water-splitting on Pt-doped TiO2 –ZnO under visible light, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.034