Dual Functional CuO1âx Clusters for Enhanced Photocatalytic Activity

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Dual functional CuO1-x clusters for enhanced photocatalytic activity and stability of Pt cocatalyst in overall water-splitting reaction Yuan Lin, Yunpeng Liu, Yuhang Li, Yonghai Cao, Jiangnan Huang, HongJuan Wang, Hao Yu, Hong Liang, and Feng Peng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04889 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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ACS Sustainable Chemistry & Engineering

Dual functional CuO1-x clusters for enhanced photocatalytic activity and stability of Pt co-catalyst in overall water-splitting reaction Yuan Lina, Yunpeng Liua, Yuhang Lia, Yonghai Caoa, Jiangnan Huanga, Hongjuan Wanga, Hao Yua, Hong Liangb, Feng Pengb,*

a

School of Chemistry and Chemical Engineering, South China University of Technology, Wushan Road 381#, Guangzhou, 510640, China. b

School of Chemistry and Chemical Engineering, Guangzhou University, 230 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou, 510006, China

* Corresponding author, Email: [email protected] (F. Peng)

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract：Photocatalytic overall water splitting to produce hydrogen is one of the most promising means to utilize solar energy and water. Pt has been widely used as the optimal co-catalyst in photocatalytic water splitting. However, it still has an obvious drawback that H2 evolution rate shows an obvious decrease for overall water splitting after a certain reaction time due to the back reaction between H2 and O2 to produce water. To date, the mechanism of the back reaction under solar light irradiation is still unclear, and the method of preventing the back reaction has been rarely researched in recent years. Herein, a hybrid Pt/CuO1-x/TiO2 catalyst was prepared via an in-situ photodeposition method. Compared to Pt/TiO2, the as-prepared Pt/CuO1-x/TiO2 showed enhanced photocatalytic activity and stability in water-splitting reaction. Characterization results revealed that CuO1-x clusters are highly dispersed on the exposed TiO2 surface, and Pt particles are mainly deposited upon the CuO1-x clusters. The electrochemical hydrogen evolution reaction (HER) tests and transient photocurrent responses manifested the fast and efficient transfer of e- from CuO1-x to Pt. It was found that O2- plays an important role for the reverse reaction of photocatalytic water splitting. The CuO1-x clusters in Pt/CuO1-x/TiO2 catalyst restrain the back reaction of water-splitting due to the decreased formation of O2- on the surface of TiO2 (like Ying character in Taoism). In addition, the fast transfer of e- from TiO2 to CuO1-x and Pt particles accordingly improves the photocatalytic activity (like Yang character in Taoism). These two characters of CuO1-x synergistically improve the photocatalytic activity and stability during overall water-splitting process. This study reveals the mechanism on the back reaction of water splitting over Pt/TiO2 under solar light irradiation; and provides an innovative and practical way to efficiently enhance the photocatalytic activity and prevent the back reaction of water-splitting. Keywords: photocatalysis; overall water splitting; interfacial charge transfer; co-catalyst; hydrogen evolution 2


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INTRODUCTION As an energy carrier with high energy density and environmental protection, hydrogen has attracted more and more attention from researchers around the world due to the depletion of fossil fuels and the destruction of environment1-3. Photocatalytic water-splitting is a promising avenue to produce hydrogen because of the abundant resources of water and solar energy in the earth. Metal oxide semiconductors especially TiO2 as photocatalyst has attracted most attentions of researchers thanks to its high activity and stability under ultraviolet irradiation4-6. However, the efficiency of photocatalytic water-splitting reaction is still very limited under solar-light illumination due to the high band gap of TiO2 and the recombination of inspired electron-hole pairs. To address this issue, noble metals such as Pt, Ag or Au are solely or collectively used as cocatalysts for the H2 evolution at present technical stage7-14. Although Pt as co-catalyst shows lots of advantages, it still has an obvious drawback that it also can catalyze H2 and O2 recombination to water12-14. However, the mechanism of the back reaction under solar light irradiation is still unclear, and the method of preventing the back reaction has been rarely researched in recent years. Considering solubility of O2 in water is much more than that of H2, the recombination reaction of initial formed hydrogen by dissolved oxygen will seriously restrain the H2 evolution net rate during photocatalytic watersplitting15, 16. Due to the high energy consumption and the pollution of chemical regent, the traditional loading methods of Pt (such as impregnation method and wet-chemical method) have gradually been abandoned17-19. In this study, a series of Pt/TiO2 catalysts have been prepared by an in-situ 3



photodeposition method. Compared to the methods mentioned above, preparing Pt/TiO2 photocatalyst via photodeposition method can overcome the shortcomings of these methods mentioned above20-22. And it offers a simple and easy way to gain Pt/TiO2 with highly dispersed Pt nanoparticles and closed contact between Pt particles and TiO2. The optimal Pt/TiO2 catalyst for photocatalytic H2 revolution reaction in glycerol aqueous solution is 2Pt/TiO2 (2 wt.% Pt loading), which shows a considerable hydrogen production rate of 3181 μmol·h-1·g-1 under solar-light irradiation. While, the optimal Pt/TiO2 catalyst for photocatalytic overall water-splitting is 1Pt/TiO2 (1 wt.% Pt loading), which harvests 99.7 μmol·h-1·g-1 of H2 and 48.8 μmol·h-1·g-1 of O2 under solarlight illumination. However, the obvious decrease in photocatalytic overall water-splitting activity (based on H2 evolution rate) over Pt/TiO2 catalysts was observed under solar light irradiation. The deactivated mechanism of Pt/TiO2 in the photocatalytic water-splitting process was discussed detailedly, and it was proven that O2- plays an important role in this process. In our previous work, we found that CuO1-x can efficiently enhance the separation of photoinduced e--h+ pairs23. To improve the efficiency of Pt for H2 production, metal Pt is further loaded on CuO1-x/TiO2. The asprepared Pt/CuO1-x/TiO2 produces H2 of 220 μmol·h-1·g-1 in photocatalytic water-splitting reaction in pure water, which yields solar-to-hydrogen energy conversion efficiency (STH) of 0.25 % under solar light irradiation, and its photocatalytic activity is efficiently enhanced compared with that of 1Pt/TiO2. Meanwhile, the existence of CuO1-x mitigates the decrease of H2 evolution rate in photocatalytic water-splitting process under solar irradiation. The results demonstrate that CuO1-x plays a dual role in photocatalytic water-splitting process under solar irradiation. One is to improve 4


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the efficiency of Pt co-catalyst for overall water-splitting reaction and the other is to prevent the back reaction of photocatalytic overall water-splitting reaction. In this work, the electrocatalytic hydrogen evolution reaction (HER) was operated to further understand the function of loading Pt and the effect of CuO1-x on Pt/CuO1-x/TiO2 catalyst. The method used in this work provides an original and applicable way to efficiently improve the photocatalytic water-splitting activity of noble-metal modified TiO2 catalyst and to abate the back reaction on the catalyst.

EXPERIMENTAL Material preparation Pt was supported on TiO2 by photo-deposition reaction according to our previous report23. Typically, TiO2 (Degussa P25) of 200 mg was dispersed into ethyl alcohol of 200 mL, and then a certain quantity of 0.01 M H2PtCl6 aqueous solution was injected into the mixture under stirring. The resultant solution was vacuumed into below -9×104 Pa, and then illuminated by a Xenon lamp (0.3 KW) for 1 h. After filtration, the obtained precipitates were washed with deionized H2O and ethyl alcohol for five times. Finally, the samples were dried in vacuum at 75oC for 12 h to gain Pt/TiO2. The calculated mass contents of Pt to (Pt+TiO2) were 0.25, 0.5, 1, 2, 4 wt.%, and these samples were denoted as 0.25Pt/TiO2, 0.5Pt/TiO2, 1Pt/TiO2, 2Pt/TiO2 and 4Pt/TiO2 respectively. Similarly, Cu2(OAc)4, HAuCl4 or AgNO3 aqueous solutions were used instead of H2PtCl6 under the same conditions, and the nominal mass fraction of Cu (Au or Ag) was 1 wt.%. The resulting samples were CuO1-x/TiO2, Au/TiO2, and Ag/TiO2, respectively. The photo-deposition method was also used to prepare Pt/CuO1-x/TiO2 samples. A certain 5



amount of H2Pt2Cl6 aqueous solution (0.01 M) was injected into the mixture of 0.2 g CuO1-x/TiO2 and absolute ethanol under stirring. The calculated mass content of Pt to (CuO1-x/Pt+TiO2) were 1 wt.%, and the mixtures were vacuumed and illuminated by a Xenon lamp for 1 h. The mixtures were filtrated and washed using deionized water and ethanol. Finally, the prepared catalysts were dried in vacuum at 75oC for 12 h. Material characterizations XRD (X-ray powder diffraction) was used to detect the crystal structures using a D8 Advance diffractometer. TEM (transmission electron microscopy) images and element mapping of materials were taken by a JEM-2100F microscopy with X-Flash 5030T EDX detector. XPS (X-ray photoelectron spectrum) was gained to measure surface elemental state on Axis Ultra DLD spectroscope at 15 kV using Al Kα as X-ray source. AAS (atomic absorption spectrum) was adopted to test the actual contents of metal in the as-prepared materials by a Hitachi Z-2000 spectrometer using flame furnace. DRS (UV-vis diffuse reflectance spectrum) was obtained to analyze optical property of materials on a Hitachi U3010 equipment with an attached integrated sphere. PL (Photoluminescence) spectra were tested on an F-7000 spectrophotometer. The sweep speed of 1200 nm·min-1 and the excitation wavelength of 280 nm were used. Both excitation and emission slit widths were 5.0 nm, and the voltage of photomultiplier tubes was 700 V. ESR (electron spin resonance) measurement was accomplished using a Bruker EMX-10/12 spectrometer with a 9.866 GHz magnetic field. Photocatalytic hydrogen evolution and overall water-splitting reaction 6


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The photocatalytic activities were tested in a photocatalytic performance evaluation system (CEL-SPH2N, China). The prepared catalyst of 20 mg was added in water and glycerol (95:5 vol. ratio) mixtures of 200 mL or deionized water under ultrasonic stirring. Photocatalytic reaction was executed under vacuum condition of below -9×104 Pa with a simulate solar illumination (300 W Xenon light with AM1.5 filter) or UV light (UV reflector and AM1.5 filter) illumination. The produced H2 and O2 were on-line analyzed by gas chromatograph. In photocatalytic process, the superoxide ion (O2-) produced was tested by ESR in the dark (25oC) or visible light illumination (λ > 420 nm). In addition, nitroblue tetrazolium (NBT) of 1 mM was also used as O2- sacrificial agent. The chemical reaction of NBT with superoxide ion leads to the formation of purple formazan24, 25 (see Fig. S1). Since the formazan is insoluble in water, the formazan obtained during reaction was precipitated on the surface of TiO2 and made TiO2 purple. The superoxide ion generated in the photocatalytic reaction was quantitated by analyzing the absorbance intensity of NBT in the solution with a spectrophotometer. During test, the TiO2 powder was separated by centrifugation. Electrochemical and photoelectrochemical measurements Electrochemical and photoelectrochemical tests were carried out with an electrochemical analyzer (CHI 760) at room temperature of 25oC in a typical three-electrode cell. The as-prepared catalysts coating glassy carbon electrode (diameter of 5 mm and catalyst loading of 152.8 mg·cm2

), a KCl saturated Ag/AgCl electrode and a Pt wire electrode were used as working electrode,

reference electrode and counter electrode, respectively. In this work, the potentials were converted 7



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into RHE (reversible hydrogen electrode) using the below formula: E(RHE) = E(Ag/AgCl) + 0.059pH + 0.197

(1)

Where pH = 13.6 in 1 M KOH solution without special instruction. During tests, the nitrogen flow of 50 mL·min-1 was kept. LSV (line scan voltammetry) curves were obtained at the sweep rate of 5 mV·s-1. CV (cyclic voltammetry) curves were obtained at different scan rates from -0.877 to 0.977 V vs. Ag/AgCl. The electrochemical double-layer capacitances (Cdl) is used to represent electrochemical surface area (ECSA). Mott−Schottky plot was measured at 1 kHz of frequency and 10 mV·s-1 of scan rate in the same electrolyte and electrochemical equipment under dark environment. Photoelectrochemical tests were carried out in three-electrode system using a 300W Xenon lamp (CEL-HXUV300) with AM 1.5G filter as illumination source. The counter and reference electrodes were respectively Pt sheet and Ag/AgCl, and 1 M KOH was used as the electrolyte. For the preparation of working electrode, the as-synthesized catalyst of 10 mg was first added into ethyl alcohol (0.5 mL) by ultrasonic stirring to form a homogeneous ink. The catalyst ink (20 μL) was dropped on the pretreated conductive glass substrate (FTO, 20  30  2 mm3), and then dried at 60°C for 3 h. The transient photocurrent response was tested at 0.23 V (vs Ag/AgCl) under on-off cycling of light illumination. Energy conversion efficiency of solar to hydrogen (STH) The overall water-splitting reaction was executed with a Xenon lamp through AM1.5 filter as solar light. The STH is calculated by the following equation: 8


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STH (%) = (r(H2)  Gr)/(Psolar  A)  100%

(2)

Where r(H2), Gr, Psolar and A represent for the hydrogen evolution rate, the Gibbs energy from H2O(l) to H2(g) and O2(g), the power intensity of solar light, and the effective irradiated sample area (5.4  103 cm2), respectively. The used light source is very close to the standard AM1.5G as shown in Fig. S2. The power densities of the used solar light and AM1.5G were respectively 84.3 and 79.1 mW·cm-2 from 350 to 1100 nm. Computational method The adsorption energy of O2 on Pt (200) and anatase TiO2 (101) were calculated using Materials Visualizer (Materials Studio, V3.1, Accelrys, Inc.). In this work, Perdew-Burke-Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA)26 coupled with the precise numerical basis set DNP (double numerical plus polarization)27 was used. In our calculations, fine quality mesh size was employed, and three-dimensional periodic boundary conditions were applied to the unit cell. The lattice constants of anatase TiO2 were optimized to be a = 10.886 Å and c = 18.656 Å. We used them to build a TiO2 (101) slab and a vacuum of 15 Å. Atoms in the last layer at the bottom were fixed to their bulk positions, while the rest were allowed to fully relax. A 3 × 3 × 1 kpoint mesh was used. As for Pt, the lattice constants were optimized to be a = 8.324 Å and c = 20.886 Å. Then we built a Pt (200) model and a vacuum of 15 Å. Also, atoms in the last layer at the bottom were fixed to their bulk positions, and a 3 × 3 × 1 k-point mesh was used.

RESULTS AND DISCUSSION Structure and morphology characterizations 9



The XRD patterns of the Pt/TiO2 samples prepared via a photodeposition method are shown in Fig.1A. The main characteristic peaks at 2θ = 25.5º (101), 48º (200) indicate the existence of plenty of anatase and the weak characteristic peak at 2θ =27.4º (110) indicates the existence of a small amount of rutile TiO228-30. For all Pt/TiO2 photocatalysts with different Pt contents, the characteristic peaks of Pt were undetected due to its low loading content and small particle size. For CuO1-x/TiO2, the characteristic peaks of Cu, CuO or Cu2O were also undetected. In our previous work, we found that the loaded CuO1-x mainly existed in an amorphous form. After loading Pt, Pt/CuO1-x/TiO2 shows no characteristic diffraction peaks of Pt, demonstrating that the dispersity of Pt barely changes after inducing CuO1-x. In addition, the width and intensity of characteristic peaks of TiO2 have no obvious change, which means that the phase structure and crystalline size of TiO2 are well maintained during the photodeposition process. Furthermore, for the as-prepared Pt/TiO2 catalysts, the diffraction peak positions have no obvious shift, indicating that the supported Pt is not doped into the lattice of TiO2. A

Intensity / a.u.

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A: anatase

R g f e d c b a

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A

A

R R

20

30

40

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50

2 theta /

A

60

R: rutile

A

70

A

A

80

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o

Fig. 1. (A) XRD patterns of (a) TiO2 (P25), (b) 0.5Pt/TiO2, (c) 1Pt/TiO2, (d) 2Pt/TiO2, (e) 4Pt/TiO2, 10


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(f) CuO1-x/TiO2 and (g) Pt/CuO1-x/TiO2. In our previous work, the deposited CuO1-x on the surface of TiO2 mainly exists as amorphous clusters. In the absent of CuO1-x, PtCl62- tends to capture the inspired electron from TiO2 and is reduced on the surface of TiO2 to form uniform Pt particles. The Pt nanoparticles are highly dispersed on the surface of TiO2, and the average particle size of Pt on the as-prepared Pt/TiO2 is 1.75-2.22 nm (see Fig. S3), meanwhile the particle size increases gradually as the raise of Pt loading content. After loading CuO1-x, the inspired electrons from TiO2 are mostly collected by CuO1-x due to IFCT effect (interfacial charge transfer) between TiO2 and CuO1-x. With the adding of H2PtCl6 aqueous solution, PtCl62- tends to capture e- collected in CuO1-x, and then is in-situ reduced and deposited on CuO1-x clusters (as shown in Fig. 2).

Fig. 2. Schematic of materials preparation process and the corresponding TEM images of 1Pt/TiO2 and Pt/CuO1-x/TiO2

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Fig. 3. (A, D) SEM images, (B, E) corresponding line-scanning EDS spectra and (C, F) HRTEM images of 1Pt/TiO2 (A-C) and Pt/CuO1-x/TiO2 (D-F). Fig. 3 shows HRTEM, STEM images and the corresponding line-scanning EDS spectra of 1Pt/ TiO2 and Pt/CuO1-x/TiO2 samples. The results illustrate that the highly dispersed Pt particles mainly expose (220) and (200) planes of lattice31, 32 and TiO2 is in the form of anatase. Fig. 3E further proves that Pt nanoparticles in Pt/CuO1-x/TiO2 are mainly deposited on the CuO1-x. Photocatalytic water-splitting performances Firstly, the solar-driven photocatalytic activities of Pt/TiO2 nanocomposites with different Pt contents are evaluated for hydrogen evolution in 5% (v/v) glycerol aqueous solution and deionized water respectively. Without Pt loading, the pure TiO2 shows extremely low H2 evolution rate (5.9 μmol·h-1·g-1) even in glycerol aqueous solution, indicating that pure TiO2 is relatively inert for 12


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photocatalytic H2 evolution reactions process, possibly because of the fast recombination of photogenerated charge carriers and high energy barrier for H2 evolution reaction on the surface of TiO2. When Pt nanoparticles were integrated onto TiO2, based on the UV-Vis DRS (Fig. S4A), Pt/TiO2 samples display a higher absorbance of light in the visible range (λ > 400 nm) than that of pure TiO2, attributed to the light scattering, intraband and interband transitions of Pt particles. The absorbance of the catalysts obviously increases in the visible region and slightly decreases in UV region with the Pt loadings increasing because of the increase in the number of Pt particles. The optimal Pt/TiO2 catalyst with 2 wt.% Pt content exhibits the highest H2 evolution rate of 3181 μmol·h-1·g-1 in glycerol aqueous solution under solar light illumination (Fig. 4A), which reaches 539 times that of pure TiO2. While the optimal Pt content is 1 wt.% in photocatalytic overall watersplitting reaction, and this optimal catalyst produces 99.7 μmol·h-1·g-1 of H2 and 48.8 μmol·h-1·g-1 of O2 under UV (350-400 nm) light irradiation (Fig. 4B). The molar ratio of H2 to O2 is close to 2, indicating a reliable overall water-splitting reaction. However, the photocatalytic activity decreases as the further increase of Pt content obviously, which is likely ascribed to the decrease of lightharvesting efficiency resulted from the overmuch deposition of Pt nanoparticles. In addition, the obvious difference of the hydrogen generation rates between these two reaction systems is caused by the different rates of oxidation reaction on the surface of TiO2, since the oxidation of glycerol is thermodynamically easier than that of H2O.

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4.0

B Evolution rate / mmolh-1g-1

H2 evolution rate / mmolh-1g-1

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0

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4

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Irradiation time / h

Irradiation time / h

Fig. 4. Photocatalytic H2 and O2 evolution rate of Pt/TiO2 catalysts with different Pt content in (A) 5% glycerol solution under solar light illumination and (B) deionized water under UV light for 5 h; (C) photocatalytic water splitting performances of (a) 1Pt/TiO2, (b) Pt/CuO1-x/TiO2 under solar light illumination and (c) 1Pt/TiO2 under UV light illumination; (D) cycled photocatalytic water splitting reaction with 1Pt/TiO2 in deionized water under solar light illumination. It is noteworthy that the H2 evolution rate on 1Pt/TiO2 decreases gradually during the photocatalytic overall water-splitting reaction. And the total amount of hydrogen is no longer increasing after 5 hours under solar light irradiation, indicating that the photocatalytic overall 14


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water-splitting reaction is in equilibrium (Fig. 4C-a). While the H2 evolution rate on 1Pt/TiO2 hardly decreased in the glycerol-water system (Fig. S4B) or under UV light irradiation (Fig. 4C-c) for 7 hours. In addition, we designed a cycle experiment to study the reason of H2 evolution rate decreasing under solar light irradiation for overall water-splitting system. We extracted the generated gas after every 2-hour’ reaction for 4 cycles, meaning that the catalyst has continuously reacted for 8 h. As exhibited in Fig. 4D, the H2 and O2 evolution rates are still maintained 95% of initial activity in the 4th cycle, which demonstrates that the decrease of reaction rate for overall water splitting after 4 h is not caused by the inactivation of photocatalyst. It was hypothesized that the deactivation is due to the back reaction between evolved H2 and O212-14. Most of the researches deem that the main reason is the co-adsorption and recombination of H2 and O2 on the surface of noble metals such as Pt and Au12, 15. Hence, we carried out an experiment for the combination reaction of stoichiometry H2 and O2 (H2/O2 = 2/1) in dark. Refer to the water splitting reaction on 1Pt/TiO2 under solar light irradiation, 13.4 mol of H2 and 6.7 mol of O2 were injected into the reacted system with the existence of 1Pt/TiO2, and then reacted for 5 hours in dark. As showed in Fig. S5, the remained amount of H2 and O2 after 5 hours’ reaction are 12.2 and 4.5 mol. The decrease rates are 12 mol·h-1·g-1 for H2 and 22 mol·h-1·g-1 for O2, respectively. It exhibits that the combination rate of H2 and O2 in dark is much slower than the evolution rate of H2 and O2 during the photocatalytic water splitting reaction on 1Pt/TiO2 catalyst. Besides, the solubility of H2 and O2 in water is quite different. About 9.0 mg of O2 can be dissolved in 1 L water, and it is about 5.5 times to H2 at the same conditions15. It means the produced H2 is hard to be dissolved into water 15



and adsorbed by Pt particles, especially at vacuum condition. But, O2 is more soluble in the same condition and can be more easily adsorbed on the photocatalysts.

Fig. 5. (A) Computation model of O2 adsorption on Pt (200) and anatase TiO2 (101), (B) schematic of the reduction of O2 by e- inspired from Pt nanoparticles. According to the results of simulated calculation33, 34, the adsorbed energy of O2 on the surface of TiO2 is about 6 times to that on the surface of Pt particle (Fig. 5A), which means O2 is more likely to be adsorbed on the surface of TiO2 rather than the surface of Pt particles. Besides, the amount of H2 evolved from the photocatalytic water-splitting system is a little over twice the amount of O2, indicating the larger solubility of O2 than H2. Some researchers reported that Pt nanoparticles also harvest visible light, which is assigned to the intraband transition of electrons from the sp band to the sp-conduction band (SPR absorption) and the interband transition of d band e- to the sp-conduction band35-37 (Fig. 5B), and Pt nanoparticles loaded on anatase TiO2 is able to efficiently promote aerobic oxidation reaction under visible light irradiation38-40, which is attributed to the transfer of e- from photoactivated Pt particles to anatase TiO2 and the reduction of adsorbed O2 on the surface of TiO2 to produce active oxygen (O2-)38-40.

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Unlike H2 evolution reaction, the oxidation of H2O to produce O2 is mainly happened on the surface of TiO2. Thus, due to the strong adsorption of TiO2 to O241-43, the generated O2 molecules are in-situ adsorbed on the surface of TiO2 and reduced into O2- by e- transferred from the visiblelight activated Pt particles. The produced O2- continues to consume H+ and H2 to form H2O2, which is instable and whereafter decomposed into O2 and H2O43-44. In other words, O2- plays an important role for the reverse reaction of water splitting. Hence, during the overall water-splitting process, the main reason for the decrease of photocatalytic activity of Pt/TiO2 is originated from the accumulation of absorbed O2 and the formation of O2- on the surface of TiO2 under sunlight illumination. The hypothetical processes of H2-O2 recombination reaction are as follows: 2H2O → 2H+ + 2OH-

(3)

2H+ + 2e- → H2

(4)

2OH- + 4h+ → O2 + 2H+

(5)

O2a + O2 + 2e- b → 2O2-

(6)

2O2- + 2H+ → 2HO2•

(7)

2HO2• + H2 → 2H2O2

(8)

2H2O2 → 2H2O + O2a

(9)

Where O2a represents for the O2 accumulated on the surface of TiO2 and 2e- b is electrons from the interband transition of Pt. As the photocatalytic water splitting reaction begins, the produced H2 and O2 start to accumulate on the surface of Pt particles and TiO2 respectively. The adsorbed H2 can easily break into two H atoms on the surface of Pt and then continue to react with O2. The directed combination of H2 and O2 on the surface of Pt particles might causes the decrease of the

17



H2 and O2 evolution rate as the photocatalytic reaction begins. The accumulated O2- can strongly capture and consume the produced H2 on Pt particles, which obviously accelerates the decrease of the H2 evolution rate (as shown in Fig. 6-a). When the visible region in solar light is filtered, the directed recombination of H2 and O2 on the surface of Pt particles is still retained. While the photoexcited electrons from Pt nanoparticles decrease sharply, so that few photo-excited electrons from Pt transfer to the surface of TiO2 and reduce O2. The excited electrons from TiO2 mainly transfer to the Pt particles45 and followed by reducing H+ to produce H2, so that the decrease of the H2 evolution rate is much slower under UV light illumination (Fig. 6-c). 300

H2 evolution rate / mmol·g-1·h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1h 5h 200

55.7 % 54.0 %

100

12.4 % 0

a

b

c

Fig. 6. The hydrogen evolution rates for water-splitting reaction at the 1st and 5th hour on (a) 1Pt/TiO2 and (b) Pt/CuO1-x/TiO2 under solar light illumination, (c) 1Pt/TiO2 under UV light illumination. The percentage represents the residual value of the H2 evolution rate. Recent research has reported that non-noble metal Cu has a similar electrochemical property to noble metal like Pt46, 47, so that it is also regards as a co-catalyst for photocatalytic hydrogen evolution. In our previous work23,

46

, we have discussed the role of CuO1-x co-catalyst for

18


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photocatalytic hydrogen evolution detailedly. The loaded CuO1-x efficiently facilitated the separation of photoinduced carriers and reduced the overpotential of hydrogen evolution. But, the activity of CuO1-x/TiO2 for photocatalytic water-splitting is quite weak, even lower than those of Au/TiO2 and Ag/TiO2 (Fig. S6). To reduce the usage of noble metal Pt and further enhance the photocatalytic performance of 1Pt/TiO2, the CuO1-x was introduced to promote the separation of photogenerated e--h+ pairs. As shown in Fig. 4C-b, in the first 3 hours, the average hydrogen production rate on 1Pt/TiO2 is 145 μmol·h-1·g-1 for photocatalytic overall water-splitting reaction under solar-light illumination, while it reaches to 220 μmol·h-1 ·g-1 on Pt/CuO1-x/TiO2. The asprepared Pt/CuO1-x/TiO2 shows 1.5 folds activity of 1Pt/TiO2, demonstrating that the loaded CuO1x

effectually accelerates the activity of Pt/TiO2 for photocatalytic water-splitting. The STH of

Pt/CuO1-x/TiO2 is 0.25%, which is relatively high compared with the results of reported similar photocatalytic pure water-splitting3,

48

. More importantly, the retention rate of photocatalytic

hydrogen production on Pt/CuO1-x/TiO2 is far higher than that on 1Pt/TiO2 (Fig. 6-a,b), demonstrating that CuO1-x can efficiently prevent the back reaction of water splitting on Pt/TiO2 catalyst.

19



A

1.5

B (Ah)2 / (eV)2·cm2

1.2

Absorption / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 43

0.9

Ag/TiO2 Au/TiO2 0.6

d c

Cu/TiO2

0.3

0.0 200

Pt/TiO2

b a 300

400

500

600

700

800

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

h / eV

Wavelength / nm

Fig. 7. (A) UV-vis diffuse reflection spectra and (B) (Ah)2 vs. h plots of (a) pure TiO2, (b) CuO1-x/TiO2, (c) 1Pt/TiO2 and (d) Pt/CuO1-x/TiO2 samples. To further corroborate the point of our view, we also tested the photocatalytic water-splitting performance of other noble metals (Au or Ag) as co-catalyst. Noble metal Au and Ag exist an obvious LSPR effect in visible light region, and the photo-deposited Au/TiO2 and Ag/TiO2 show obvious absorption peak at 548 nm and 463 nm respectively in the UV-Vis DRS spectra (Fig. 7A). It is reported that the combination reaction of H2 and O2 is much weaker on the surface of Au particles than on Pt particles12. As shown in Fig. S6, during the photocatalytic overall watersplitting process, the hydrogen generation rate on Au/TiO2 also exhibits obvious decrease and the evolution of hydrogen has almost stopped after 5-hour reaction. Iwase et al.12 reported that the obvious decrease of H2 evolution rate is also caused by the formation of O2-, which is from the evolved O2 being reduced by e- from LSPR inspired Au particles. The photocatalytic H2 evolution rate on Au/CuO1-x/TiO2 also decreases slowly during the photocatalytic water splitting reaction, while it is observed that the H2 amount in the reaction system is still increasing after 7-hour reaction. 20


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According to Wang et al.’ work15,

16

, it is possibly originated from the co-adsorption and

recombination of H2 and O2 on the surface of Au particles, resulting in the decrease of H2 evolution rate on Au/CuO1-x/TiO2. It further demonstrates that the existence of CuO1-x cluster can efficiently inhibit the reverse reaction of hydrogen evolution under solar light illumination. Since the surface of Au particles are both exposed in Au/TiO2 and Au/CuO1-x/TiO2, we deem that CuO1-x clusters can efficiently decrease the formation of O2- and thus inhibit the reverse reaction of the water splitting reaction. Besides, no obvious decrease of H2 evolution rate is observed on Ag/TiO2 under solar light illumination (Fig. S6), which is owing to the quite low photocatalytic activity for watersplitting on Ag/TiO2 under both solar and UV light. This may be due to the below reasons: (i) the adsorption and recombination of H2 and O2 are rarely happened on the surface of Ag particles; (ii) the photocatalytic water-splitting reaction is too slow to produce enough O2- to efficiently consume the produced H2. It also illustrates that O2- needs to reach a certain concentration to efficiently consume the produced H2, and it might stop the increase of H2 amount during photocatalytic watersplitting reaction. Electrochemical and photoelectrochemical properties Furthermore, we investigated the effect of electrochemical properties of different metals on photocatalytic H2 evolution reaction. The semiconductor photocatalyst is generally considered as a primary cell, and the loading co-catalyst is regarded as cathode. Hence, investigating the intrinsic electrochemical properties is also very important to understand the cause of differences in photocatalytic activity. Firstly, the electrochemical impedance spectrum (EIS) was adopted to 21



investigate the resistance of the as-prepared photocatalysts. As shown in Fig. S7A, the largest semicircle in the Nyquist plots indicates the poor charge mobility of pure TiO2 before loading Pt. However, the semicircle of TiO2 becomes much smaller under AM1.5 irradiation than under the dark, indicating the decrease of interface resistance caused by the accumulation of photoelectrons at the interface49. After loading Pt, the resistance decreases as increasing the loading content of Pt, which is owing to the increasing dispersion of Pt nanoparticles on the surface of TiO2. In Fig. 8A, the surface resistance of CuO1-x/TiO2 is smaller than TiO2 but larger than 1Pt/TiO2, which are ascribe to the favorable conductivity of metal Cu and the oxidation of surface Cu respectively. The surface resistance of Pt/CuO1-x/TiO2 is much smaller than 1Pt/TiO2 under dark condition, and it is attributed to the decrease of resistance between the isolate Pt nanoparticles due to the existence of dispersed CuO1-x nanoclusters. The resistance of 1Pt/TiO2 decreases obviously under AM1.5 illumination, which is owing to the decrease of surface resistance of TiO2 under illumination. Besides, the resistance of 1Pt/TiO2 becomes almost the same to Pt/CuO1-x/TiO2 under AM1.5 illumination, indicating that Pt nanoparticles possess higher charge mobility than CuO1-x clusters.

22


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Page 23 of 43

900

B

750

CuO1-x/TiO2-dark

600

1Pt/TiO2-dark

450 300

CuO1-x/TiO2-AM1.5 1Pt/CuO1-x/TiO2-dark 1Pt/CuO1-x/TiO2-AM1.5 1Pt/TiO2-AM1.5

150 0 0

150

450

300

600

750

0.0

-0.2

j / mA·cm-2

- Z'' / k

A

-0.4

CuO1-x / TiO2 1Pt/CuO1-x/TiO2 -1.0 -0.5

900

D

c

6.0

-0.2

-0.3

-0.1

0.0

3.0 2.5

a

4.5 3.0

-0.4

Overpotential / V vs RHE

9.0

Off On 7.5

1Pt / TiO2

-0.8

1/C2 / 1010F2·cm4

C

TiO2

-0.6

Z' / k

j / A·cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

1.5

2.0 1.5

1Pt/CuO1-x/TiO2 1Pt/TiO2 1Au/TiO2 1Ag/TiO2 CuO1-x/TiO2

1.0 0.5

0.0 0

60

120

180

240

0.0 0.00

300

0.05

0.10

0.15

0.20

0.25

0.30

Potential / V vs RHE

Time / s

Fig. 8. (A) Nyquist plots of electrochemical impedance under dark and AM1.5 irradiation and (B) Steady-state reductive (for HER) polarization curves and; (C) transient photocurrent responses of (a) 1Pt/TiO2, (b) CuO1-x/TiO2 and (c) Pt/CuO1-x/TiO2 samples; (D) the normalized Mott-Schottky plots of M/TiO2 (M = Pt, Au or Ag), CuO1-x/TiO2 and Pt/CuO1-x/TiO2 samples. The electrochemical hydrogen evolution reaction (HER) performance of all samples were also measured to study the active sites on the as-prepared catalysts for photocatalytic H2 evolution. The onset overpotential of catalysts are obtained from LSV curves. Compared to Pt/C, the as prepared semiconductor photocatalysts show much poorer HER activity (Fig. S7B-C), which is attributed to

23



the poor conductivity of the semiconductor substrate. Besides, the HER activity enhances obviously even after loading small amount of Pt, which is reflected in the decrease of onset overpotential. With Pt content increasing, the onset overpotential of Pt/TiO2 decrease gradually (Fig. S7C), which is ascribed to the increase of conductivity. As exhibited in Fig. 8B, the pure TiO2 shows the highest onset overpotential that demands 538 mV at 1 mA·cm-2 to overcome the thermodynamic barrier of hydrogen evolution reaction50-52. After loading CuO1-x, the onset overpotential moves positively compared with TiO2, demonstrating the slight decrease of thermodynamic barrier for H2 evolution reaction. The loading of Pt also obviously decreases the onset overpotential of CuO1-x/TiO2 for H2 evolution reaction, which illustrates that the hydrogen evolution reaction is mainly happened on the surface of Pt. The Tafel curves are plotted from LSV curves according to the equation given by η = blog(j/j0), where b is the Tafel slope and j0 is the exchange current density. Tafel slope reflects the charge transfer dynamics of the electrode reaction. And both high j0 and low b suggest an excellent performance of HER51, 52. For Pt/TiO2 catalysts, the Tafel slopes are between126 and 159 mV·dec1

(Fig. S8A), which reveals that HER on all Pt/TiO2 catalysts happens by way of Volmer-Heyrovsky

mechanism due to the rate limiting step of electrochemical H* adsorption. In alkaline media, electrochemical H2O reduction into adsorbed H* and OH- occurs on the Volmer process, which needs to break the H-O-H bond on the catalyst’ surface before adsorbing H*51, 52. Since Pt is equally effective for water dissociation as well as H* recombination step, the Tafel slope decreases gradually as the increase of Pt loading amount. 24


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The behavior of photoinduced carrier transfer in 1Pt/TiO2, CuO1-x/TiO2 and Pt/CuO1-x/TiO2 catalysts are proven by the photocurrent responses under solar light illumination and as shown in Fig. 8C. As the Pt content increasing, the photocurrent of Pt/TiO2 enhances gradually (Fig. S8B), and the optimal Pt content is 2 wt.%, which accords with the photocatalytic results in glycerol solution. The photocurrent density rapidly decreases after turning off the light, which indicates that photogenerated electron migrates to FTO electrodes to generate photocurrent under solar light irradiation53. Moreover, the photocurrent intensity of Pt/TiO2 increases gradually as time goes by under irradiation. It may be caused by the further transfer of e- from Pt particles to FTO substrate, indicating the effective separation of photoinduced electron-hole pairs by Pt particles53. The measured photocurrent of CuO1-x/TiO2 is much lower than that of 1Pt/TiO2, since the separation of photoinduced charges by Pt is more effective than by CuO1-x. However, the loading of Pt can efficiently enhance the photocurrent of CuO1-x/TiO2, and the photocurrent of Pt/CuO1-x/TiO2 is much higher than 1Pt/TiO2, which may be attributed to the fast and efficient transfer of e- from CuO1-x to Pt particles. The electrochemical catalytic HER performance of different metal-loaded TiO2 were also investigated to further understand the roles of different noble metals in H2 evolution reaction. Fig. S8C shows the LSV curves of the TiO2 catalysts with different metal co-catalysts. It demonstrates that Pt/TiO2 exhibits the best electrochemical HER activity among these TiO2 catalysts. The onset overpotential order for electrochemical H2 evolution reaction is 1Pt/TiO2 < CuO1-x/TiO2 < Au/TiO2 < Ag/TiO2. It suggests that Pt exists a much lower overpotential to HER than other metals, which 25



Page 26 of 43

means Pt needs to overcome quite lower thermodynamic barrier for a HER. The measured Cdl is used to characterize ECSA. The larger Cdl indicates more surface-active sites exposed, resulting in the higher current density51, 54. As shown in Fig. S8D, Au/TiO2, Ag/TiO2 and CuO1-x/TiO2 exhibit almost identical Cdl (120, 117 and 107 μF·cm-2, respectively), suggesting their similar surface area. 1Pt/TiO2 and Pt/CuO1-x/TiO2 exhibit much higher Cdl values (Table 1) than other catalysts, demonstrating that Pt possesses more exposed surface reactive sites in the same loading content comparing to other metals. Table 1 The electrochemical double-layer capacitance (Cdl) and calculated carrier density of different catalysts from normalized Mott-Schottky plots. Catalyst 1Pt/TiO2 1Au/TiO2 1Ag/TiO2 CuO1-x/TiO2 Pt/CuO1-x/TiO2

Cdl / nF·cm-2 178 120 117 107 207

[d(1/C2)/dV-1] a 5.53 7.07 10.54 7.37 3.37

Carrier density / cm-3 5.32  1029 4.16  1029 2.79  1029 3.98  1029 8.72  1029

a. The slopes of normalized Mott-Schottky plots. The transient photocurrent intensities of different metal-loaded TiO2 are also measured (Fig. S8B), and the results are consistent with their photocatalytic activities. To clarify the difference of charge carrier density between different metal-loaded TiO2 catalysts, Mott-Schottky test was carried out. Although these analyses are based on the planar electrode model, the calculated the carrier density is still reasonable for a relative comparison55. To compensate for surface areas’ differences, the Cdl of the samples was normalized55, and the normalized Mott–Schottky plots are exhibited in Fig. 8D. As exhibited in Table 1, the slopes of plots suggest the carrier densities in the 26


Page 27 of 43

order of Pt/CuO1-x/TiO2 > 1Pt/TiO2 > Au/TiO2 > CuO1-x/TiO2 > Ag/TiO2, which is well matched to the photocurrent intensity results discussed above, indicating that CuO1-x markedly enhances the carrier density of Pt/TiO2 catalyst, and then effectively improves its photocatalytic performance. Roles of the CuO1-x clusters in the photocatalytic water-splitting reaction Herein, we studied the role of the CuO1-x clusters on the photocatalytic reaction. As discussed in TEM characterization above (Fig. 3), the loaded Pt in Pt/CuO1-x/TiO2 is mainly located on the surface of highly dispersed CuO1-x. It suggests that CuO1-x clusters efficiently prevent the migration of photogenerated electron from Pt to TiO2 and thus prevent the reduction of the O2 adsorbed on the surface of TiO2. The photogenerated electron-hole pairs can be efficiently separated via the IFCT process56, 57. This process was expressed in the plots of (Ah)2 vs. h shown in Fig. 7B. Compared with the band gap of pure TiO2, the one of CuO1-x/TiO2 has a slight decrease, which is likely due to the electrons interfacial transfer from TiO2 to CuO1-x surface, resulting in the part of Cu(II) reduction into Cu(0)56, 57. Compared to 1Pt/TiO2, the enhanced absorbance of Pt/CuO1-x/TiO2 in 500-800 nm can be ascribed to the d-d transition of Cu(II)23. Cu 2p3/2

935.0 eV

B

932.9 eV satellite peak

934.5 eV

932.3 eV satellite peak

930

933

936

939

942

945

Binding Energy / eV

f 73.0 eV 71.0 eV

948

a

73.1 eV

70.6 eV

Intensity / a.u.

d

C

Pt 4f

c

68

70

72

b

PL intensity / a.u.

A b

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


74

76

78

80

Binding Energy / eV

c

d

300

350

400

450

500

550

Wavelength / nm

Fig. 9. The XPS spectra of Cu2p3/2 (A) and Pt4f (B) for Pt/CuO1-x/TiO2 samples; (C) PL spectra of 27



(a) pure TiO2, (b) CuO1-x/TiO2, (c) 1Pt/TiO2, (d) Pt/CuO1-x/TiO2. The XPS spectrum was used to demonstrate the electron transfer between CuO1-x and Pt particles. Fig. 9A shows that the Cu2p3/2 spectrum is matched into two peaks, which can be assigned to Cu2O (932.9 and 932.3 eV) and CuO (935.0 and 934.5 eV)23, 58, respectively. Fig. 9B shows the Pt4f XPS spectra of as-prepared 1Pt/TiO2 and Pt/CuO1-x/TiO2. The component at 70.6 and 71.0 eV is assigned to Pt (0), while the one at 73.0 and 73.1 eV corresponds to Pt (II)-O respectively59, 60. Compared to Pt/CuO1-x/TiO2, the both binding energies of Cu+ and Cu2+ in CuO1-x/TiO2 shift to higher position, while the ones of Pt in Pt/TiO2 shift to lower position. It demonstrates the strong interaction between CuO1-x cluster and deposited Pt particles. To further understand the electron transfer process, we measured the photoluminescence (PL) spectra of as-prepared 1Pt/TiO2 and Pt/CuO1-x/TiO2 (Fig. 9C). The fluorescence intensities of the two samples are obviously lower than that of TiO2. It indicates that the loading of noble metals on TiO2 efficiently reduces the recombination of photogenerated carriers under UV light illumination. Besides, the life-span order of photoinduced carriers is 1Pt/TiO2 > Au/TiO2 > Ag/TiO2 > CuO1x/TiO2

(Fig. 9C and Fig. S9), indicating the e- separating capabilities of different loading metals.

We deem that the decrease of fluorescence intensity is mainly ascribed to the migration of photoexcited electrons from TiO2 to CuO1-x clusters or noble metal particles, suppressing the direct recombination of photoexcited carriers. Meanwhile, compared with CuO1-x/TiO2 and1Pt/TiO2, Pt/CuO1-x/TiO2 shows a significant decrease of fluorescence intensity around 470 nm. It is likely ascribed to the fast and efficient migration of e- from TiO2 to CuO1-x and then to Pt surface, which 28


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effectively retards the recombination of photoexcited carriers, improving the efficiency of Pt cocatalyst for overall water-splitting reaction. Besides, we also investigated the stabilities of CuO1-x clusters and Pt nanoparticles during photocatalytic water-splitting process, as shown in Fig. S10. The characterization results of XPS, TEM, and XRD indicate that the chemical state, morphology and construction of CuO1-x clusters and the particle size of Pt almost remain unchanged after longtime photocatalytic water splitting, which demonstrate that the CuO1-x clusters and Pt nanoparticles are stable during photocatalytic water-splitting process. In the Pt/TiO2 system, the photoexcited electrons from Pt particles transfer to the surface of TiO2, and then electrons reduce O2 to O2-, which is proven by ESR analysis. In Fig. S11, both bare TiO2 and 1Pt/TiO2 show no signal in the ESR spectra after treated with O2 in the dark. After irradiating ( > 420 nm) for 1 h, 1Pt/TiO2 shows obvious signals assigned to superoxide ion (gxx = 2.002, gyy = 2.009)39, 40. As the irradiation time prolonging, the intensity of the peak signals increases, indicating the accumulation of O2- on the surface of 1Pt/TiO2 after long-time irradiation. No signal is detected in bare TiO2, even it was irradiated ( > 420 nm) for 2 h. And it is because no e- is excited from TiO2 under visible light irradiation. We also designed the experiments for the semi-quantitative analysis of O2- in photocatalytic water-splitting process. When NBT is added to the reaction system, no H2 is detected in the reaction process, and the suspension shows a purple hue demonstrating the deposition of formazan on catalyst particles (Fig. S1). In the presence of NBT, the photocatalytic reaction may happen as follows25: (i) the generation of e- and h+ in photoinduced TiO2 via reaction (10); (ii) the oxidation of H2O to produce O2 via reaction (11); (iii) 29



Page 30 of 43

the reduction of O2 and H+ via reactions (12) and (13); (iv) the capture of O2- by NBT via reaction (14). Since the produced O2- is reacted with NBT to form the formazan precipitate, which is separated from the reaction system immediately, the reduction reaction of O2 is easier than the reduction of H+. The inspired e- and h+ in pure TiO2 is efficiently separated via reactions (11), (12) and (13), which lead to the purpling of pure TiO2 particles and the decreased absorbance intensity of NBT in the solution. TiO2 → h+ + e-

(10)

4h+ + 2H2O ↔ 4H+ + O2

(11)

O2 (ad.) + e- ↔ O2-

(12)

2H+ + 2e- ↔ H2

(13)

O2- + NBT → formazan↓

(14)

The changes in the absorption spectra of 1 mM NBT in deionized water during photocatalytic water-splitting reaction are showed in Fig. S12. And the production rates of O2- on pure TiO2, 1Pt/TiO2, CuO1-x/TiO2 and Pt/CuO1-x/TiO2 are shown in Table 2. No purple precipitate and decreased absorbance intensity of NBT in the solution are observed after irradiation for 1 h without photocatalyst, which suggests the stability of NBT solution under the irradiation. The production rate of O2- on 1Pt/TiO2 is about 1.7 times that on pure TiO2, which is possibly due to the reduction of O2 by the photo-excited electrons from Pt nanoparticles. Meanwhile, the production rate of O2on Pt/CuO1-x/TiO2 (2.0510-7 mol·min-1) is much slower than that on 1Pt/TiO2 (4.7510-7 mol·min1

), confirming that CuO1-x efficiently prevents the reduction of O2 by the photo-excited electrons 30


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from Pt particles. Table 2 Estimated production rates of superoxide ion using different catalysts. Catalyst /a TiO2(P25) 1Pt/TiO2 Pt/CuO1-x/TiO2

c(NBT) / mM Reacted NBT / mol Production rates of O2- / mol min-1 0.015 / / -7 0.013 3.5310 2.8210-7 0.012 5.9710-7 4.7510-7 0.014 2.5710-7 2.0510-7

a. The initial concentration of NBT aqueous solution.

Fig. 10. Schematic of (A) the inhibition of e- transfer from Pt to the surface of TiO2 by Cu2O and (B) the dual roles of CuO1-x for enhancement of photocatalytic water splitting reaction. As discussed above, the CuO1-x consists of CuO and Cu2O. The conduction band of Cu2O is more negative than that of TiO2, the Schottky barrier between Pt and Cu2O is higher than that between Pt and TiO2, as shown in Fig. 10A. Therefore, the photoexcited hot electrons in Pt is harder to overcome the Schottky barrier between Pt and Cu2O and transfer to the surface of TiO2, preventing the back reaction of Pt co-catalyst in overall water-splitting reaction. Fig. 10B shows the dual roles of CuO1-x in Pt/CuO1-x/TiO2 for photocatalytic overall water-splitting reaction. One 31



is to enhance the efficiency of co-catalyst Pt for water-splitting reaction and the other is to prevent the formation of O2- on the surface of TiO2, avoiding the decrease of photocatalytic hydrogen production rate.

CONCLUSIONS In summary, a highly dispersed Pt/TiO2 catalyst was prepared with an in-situ photodeposition method. It was found that the photocatalytic overall water-splitting activity of Pt/TiO2 sharply decreased after reacting for 4 hours under solar-light illumination, due to the formation of O2-. To improve the photocatalytic activity and stability, CuO1-x was introduced to between TiO2 and Pt. The results of electrochemical HER tests and transient photocurrent responses demonstrated a fast and efficient transfer of e- from CuO1-x clusters to Pt particles. It was proven that CuO1-x plays a dual role in photocatalytic water-splitting process under solar-light illumination. One is to enhance the efficiency of co-catalyst Pt for H2 evolution reaction and the other is to prevent the formation of O2- on TiO2 surface, avoiding the decrease of photocatalytic hydrogen production. This work reveals the mechanism on the back reaction of water splitting on Pt/TiO2 under solar-light; and provides an innovative and practical way to efficiently improve the photocatalytic hydrogen evolution of noble-metal co-catalyst and prevent the back reaction of water splitting using CuO1-x as an efficient transfer station for electrons.

SUPPORTING INFORMATION Normalization of Mott-Schottky plots, supplemental tables S1 and figures S1-S12.

32


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ACKNOWLEDGEMENTS We acknowledge the financial support from the National Natural Science Foundation of China (No. 21673080), the Provincial Science and Technology Project of Guangdong (No. 2014A030312007) and the High-Level University Construction Fund of Guangzhou University (No. 69-18ZX10015).

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to

quantitative

histochemistry.

Histochemie

1970,

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(2),

170-180,

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CuO1-x clusters can efficiently prevent the back reaction of Pt/TiO2 photocatalyst and improve the efficiency in overall water-splitting reaction.

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Dual Functional CuO1âx Clusters for Enhanced Photocatalytic Activity

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Dual Functional CuO1âx Clusters for Enhanced Photocatalytic Activity