rutile TiO2 photocatalyst with

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A novel highly active anatase/rutile TiO2 photocatalyst with hydrogenated heterophase interface structures for photoelectrochemical water splitting into hydrogen Jiayuan Hu, Shengsen Zhang, Yonghai Cao, HongJuan Wang, Hao Yu, and Feng Peng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02130 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Sustainable Chemistry & Engineering

A novel highly active anatase/rutile TiO2 photocatalyst with hydrogenated heterophase interface structures for photoelectrochemical water splitting into hydrogen

Jiayuan Hua, Shengsen Zhangb,*, Yonghai Caoa, Hongjuan Wanga, Hao Yua, Feng Penga, c,*

a

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

b

College of Materials and Energy, South China Agricultural University, Wushan Road 483#, Guangzhou, 510643, China

c

Guangzhou Key Laboratory for New Energy and Green Catalysis, School of Chemistry and

Chemical Engineering, Guangzhou University, Guangzhou Higher Education Mega Center, Guangzhou, 510006, China

* Corresponding author. E-mail: [email protected] (S. Zhang); [email protected] (F. Peng)

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ABSTRACT: In the past few years, anatase/rutile TiO2 heterophase junction structures with highly efficient photocatalytic performance have been explored widely, while their activities are still unsatisfactory in solar-to-hydrogen energy conversion. In this study, a novel anatase/rutile TiO2 photo-electrode with hydrogenated heterophase interface structures (A-H-RTNA) was successfully

designed

and

synthesized

for

the

first

time

via

hydrothermal

synthesis-hydrogenation-branching growth. The structure characterizations indicated that the hydrogenated interfaces between anatase-branches and rutile-TiO2-nanorod hold appropriate oxygen vacancies and Ti3+, and inferred that new energy levels of oxygen vacancy and Ti-OH lie below the band edge positions of conduction band and valance band of rutile TiO2 nanorod, respectively. The matching energy levels between anatase-branches and hydrogenated rutile-nanorod obviously reduce the recombination of the photogenerated carriers, resulting in a superior photoelectrochemical (PEC) performance. The hydrogen evolution rate on A-H-RTNA photoelectrode for PEC water splitting is 20 and 2.1 times those of unhydrogenated TiO2 nanorod arrays photoelectrode (RTNA) and surface hydrogenated anatase/rutile TiO2 photoelectrode (H-A-RTNA), respectively. This work provides a new insight into the effect of hydrogenated heterophase interface structure on PEC properties of TiO2. KEYWORDS: Nanostructured materials; Photocatalyst; Hydrogen production; Hydrogenated interface structure; Photoelectrochemical water splitting,

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INTRODUCTION Photocatalytic (PC) or photoelectrochemical (PEC) water splitting based on semiconductor, which converts the absorbed solar energy into hydrogen energy, is one of the most promising technologies in new energy fields.1 Titanium dioxide (TiO2) as an approving photocatalyst has been attracted widespread interest because of its physical and chemical stability, low cost, nontoxic and effective catalytic efficiency.2-4 Nevertheless, the defect of rapid recombination of photogenerated carriers in titanium dioxide hinders its further development.5,6 Therefore, a large quantity of approaches, including designing of TiO2 heterostructures,7,8 modification of surface morphology,9-11 synthesizing of branched structures12 and doping of noble metals and non-noble metals,13-16 have been applied to enhance the PC or PEC activity of TiO2. Plenty of studies indicated that the anatase/rutile heterophase TiO2 material, such as P25, possesses higher PC or PEC performance than that of pure anatase or rutile phase TiO2.5,8,17-20 The higher performance is ascribed to its efficient separation of the photogenerated electron-hole pairs under solar light irradiation due to the different band gaps and matched band-edge positions of rutile and anatase TiO2.5,8,17-19 For anatase/rutile TiO2 heterophase photocatalysts, there are mainly two kinds of materials, one is powder photocatalyst that is composed of anatase/rutile TiO2 composite nanoparticles,8,18,21 the other is film photocatalyst that is composed of rutile TiO2 nanorod arrays (RTNA) covered with nano-anatase TiO2.22-25 The former consists of small particles with large surface-to-volume ratios, which is appropriate for suspension solution. However, when the powder photocatalyst is used to fabricate a photoanode on the conductive FTO substrate through a typical deposition-annealing process, the

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efficiency of this film for PEC water splitting is relatively low due to huge grain boundaries that blocks the transfer and transmission of photogenerated carriers.25,26 The latter holds high PEC activity that can be attributed to an appropriate phase alignment and the direct photogenerated electron transportation pathway in the nanorods.12, 22 Many researchers have designed RTNA covered with different anatase TiO2 nanostructures, including nanoparticles,19,27 nanosheets,28,29 nanoflowers23 and nanobranches,22 and their PEC activities were significantly improved compared with pure RTNA because of their heterophase nano-structure and larger surface area. So far, their activities are still unsatisfactory in solar-to-hydrogen energy conversion. To this end, it is still of particular interest to develop highly efficient TiO2 heterophase nanostructures for PEC water splitting. In the past few years, hydrogenation treatment on photocatalyst has become an attractive method because it is a useful and simple process to increase the PC and PEC performance of photocatalysts.30-33 The increased performance is ascribed to a suitable disordered layer on the surface of photocatalysts after hydrogenation treatment. The disordered layer has high densities of oxygen vacancies (Ovac) and Ti3+, which improves its optical absorption performance, electrical conductivity, carriers transport and separation efficiency.30-34 For instance, Wang et al.35 fabricated a hydrogenated rutile TiO2 nanorod arrays (H-RTNA) as photoanode for PEC water splitting. The photo-conversion efficiency of H-RTNA was 6.8 times that of pristine RTNA. In our previous work,36 a branched hydrogenated TiO2 nanorod arrays photoelectrode (R-H-RTNA) was designed using H-RTNA as tree trunk grew unhydrogenated rutile TiO2 nano-branches. The photocurrent of R-H-RTNA was about 3.8 times that of H-RTNA, further

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improved the PEC activity of H-RTNA.36 Nevertheless, because of no inherent different of bandgaps and energy level positions between the hydrogenated-rutile and rutile, the activity of hydrogenated branched RTNA photocatalyst is still not very high. Inspired by the above impressive heterophase photocatalysts and the hydrogenation effects, it is reasonably believed that hydrogenation of heterophase interfaces will be an effective strategy to develop a higher active anatase/rutile TiO2 photocatalyst. In this work, a novel anatase/rutile TiO2 photoelectrode with hydrogenated heterophase interface structures (A-H-RTNA) was successfully designed and synthesized for the first time. The roles of hydrogenated heterophase interfaces in PEC water splitting were investigated. For comparison, hydrogenated rutile TiO2 nanorod (H-RTNA) and surface hydrogenated anatase/rutile TiO2 (H-A-RTNA) photoelectrodes were fabricated at the same condition. The characterizations proved that the hydrogenated interfaces between anatase-branches and rutile TiO2 trunk hold appropriate Ovac and Ti3+. Two new energy levels of Ovac (EOV) and Ti-OH (ETi-OH) in A-H-RTNA lie below the positions of conduction band (CB) and valence band (VB) of rutile, respectively, resulting in a superior activity for PEC hydrogen evolution.

EXPERIMENTAL SECTION Preparation of Materials. The preparation process of H-A-RTNA and A-H-RTNA is shown in Scheme 1. A simple hydrothermal method was applied to synthesize the RTNA on a FTO glass slide.33 In this experimental, 20.0 mL deionized water was mixed with 20.0 mL hydrochloric acid (36.5% by weight) and stirring for 5 min, and then added 0.48 mL tetrabutyl titanate (97%

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Aldrich) under rigorous magnetic stirring for another 5 min. Two pieces of clean FTO substrates (5.0 × 2.0 cm2) were immersed in the solution, which was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 170 °C for 6 h in an oven. After cooling, the samples were washed many times with deionized water. One sample was dried at 350 °C in air for 1 h to obtain RTNA. Another sample was further hydrogenated at 350 °C in a continuous hydrogen (20.0 mL⋅min-1 ) and argon (80.0 mL⋅min-1 ) flow for 1 h to obtain H-RTNA.33 The obtained H-RTNA was further put into an aqueous solution mixed by boric acid (H3BO3) and ammonium hexafluoro-titanate ((NH4) 2TiF6) at 50 °C for 40 h23,37 to grow anatase TiO2 branches on the surface of H-RTNA. The prepared sample was named as A-H-RTNA. For comparisons, RTNA was also put into the above-mentioned aqueous solution in the same way to grow the anatase TiO2 branches, and then dried at 350 °C for 1 h in air to prepare anatase-branched RTNA (A-RTNA). The A-RTNA was hydrogenated under the same conditions stated above to obtain the surface hydrogenated A-RTNA (H-A-RTNA). Characterization Methods. The morphology was conducted by scanning electron microscope (SEM, LEO 1530VP) and transmission electron microscope (TEM, JEOL JEM-2010F, Japan). X-ray diffraction (XRD) analysis was characterized by an X-ray diffractometer (D/max-IIIA, Japan). The UV-vis absorption spectra were recorded with a UV-vis absorption spectrometer (U3010, Hitachi) and the photoluminescence (PL) spectra were obtained using an F-4500 spectrophotometer with the excitation wavelength of 260 nm (Hitachi, Japan). Raman spectroscopy was examined by a LaRAM Aramis (HORIBA Jobin Yvon S.A.S, France). X-ray photoelectron spectroscopy (XPS) was obtained on an ESCALAB250Xi Thermo scientific

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spectroscope. Electron paramagnetic resonance (EPR) spectra were characterized (E500, Bruker, Germany) at 77 K and radio frequency of 9.442 GHz. Electrochemical impedance spectroscopy (EIS, PGSTAT30 Eco Chemie B. V.) was studied under open-circuit voltage, with 1.0 M NaOH solution and frequency from 0.01 Hz to 106 Hz and amplitude of 5 mV. Mott–Schottky analysis was measured with the potential increment of 5×10−2 V, the amplitude of 5×10−3 V, and the frequency of 103 Hz. The incident photon-to-current conversion efficiency (IPCE) spectra were collected by a Keithley 2000 multimeter incorporated with a spectral product DK240 monochromator. PEC Performance Measurement. The photocurrent measurements were tested on electrochemical workstation (CHI 760) in a conventional three-electrode system reactor containing 1.0 M NaOH electrolyte solution, using a platinum foil as a counter electrode and an Ag/AgCl saturated KCl as a reference electrode. The as-prepared sample was used as working electrode and the illuminated area was 1.0 cm2. The UV-vis light (PLS-SXE 300UV, 200-1100 nm) was used as the light source without any filter and is 10 cm far away from the sample. And the light intensity was adjusted to 100 mW cm-2 (λ

rutile TiO2 photocatalyst with

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