Siyuan Yang a , Hongjuan Wang a , Hao Yu a , Shengsen Zhang b, Yueping Fang b , Shanqing Zhang c , Feng Peng a,* a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, 510640, China b College of Materials and Energy, South China Agricultural University, Guangzhou, 510642, China c Centre for Clean Environment and Energy, Griffifith School of Environment, Gold Coast Campus, Griffifith University, QLD 4222, Australia

Abstract

The hierarchical Ag NPs with a bimodal size distribution were auto-decorated on the surface of rice-like N-TiO2 (named h-Ag/N-TiO2) by semiconductor-metal double excitation process under solar light irradiation. The photoinduced growth process of self-assembled h-Ag/N-TiO2 and its photocatalytic mechanism of photo-splitting ethanol into hydrogen have been discussed in detail. The synthesized h-Ag/N-TiO2 as a plasmonic photocatalyst for photo-splitting ethanol exhibited a higher hydrogen production rate (4.7 mmol h
1 g 1) than Ag NPs universally loaded N-TiO2 (3.4 mmol h
1 g 1 ) and pure N-TiO2 (2.1 mmol h 1 g 1 ). This work presents a reversed photo-induced synthesis strategy to design a stable Ag-TiO2 plasmonic photocatalyst with high photocatalytic effificiency under solar light. A better understanding of this solar light induced self-assembled process for Ag-TiO2 is crucial in developing metal-semiconductor system for solar energy materials. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

As a recent energetic photocatalytic technology, the plasmonic photocatalysis has sparked a surge of research on introducing plasmonic metal (such as Au, Ag and Cu) into a photochemical reaction [1]. Since the localized surface plasmon resonance (LSPR) of these metal nanoparticles can be excited within the visible and near infrared light, the metalsemiconductor plasmonic photocatalysts usually have an enhanced visible light adsorption range and a notable improvement on the photocatalytic property [2e4]. Thanks to the Schottky junction resulting from the noble metal and semiconductor, an internal electric fifield in the metal/semiconductor interface was formed [5,6]. The Schottky barrier makes it possible that the charge can transfer between the metal and semiconductor [7]. Moreover, the internal electric fifield enables the electrons and holes to move in different directions once they are created in the semiconductor or on the plasmonic metal [8,9]. The Schottky barrier and the internal electric fifield as distinct features of plasmonic photocatalyst make the photocatalysis more effective. Therefore, combining with other benefificial effects, such as LSPR sensitization effect and localized heating effect etc., plasmonic photocatalyst typically exhibits a signifificantly improved photocatalytic ef- fificiency compared with the unmodifified semiconductor catalysts [10e14].

Due to the intriguing plasmonic properties in the visibleto-near-infrared regions, Au and Ag have received extensive attention as the most popular metals for plasmonic photocatalysis [15]. Compared with Au nanocrystals, Ag exhibits much stronger electric fifield enhancement and better performance in enhancing Raman scattering [16]. Besides, as a relatively inexpensive noble metal, Ag has more practical application prospect. Recently, Ag-TiO2 plasmonic photocatalyst has been widely investigated [17e24], because TiO2 has been regarded as one of the most promising semiconductors in the fifield of solar energy conversion.

For Ag-TiO2 plasmonic photocatalyst, there are three typical excitation states according to the irradiation light. Type I is semiconductor-excitation state. In this case, the electrons in the valance band (VB) of TiO2 are excited by UVlight into the conductive band (CB) and then transfer to the noble metal particles through the Schottky junction. The plasmonic metal here acts as an electron acceptor and reactive sites to enhance the photocatalytic effificiency of the semiconductor [25e30]. Type II is metal-excitation state. It is well known that the LSPR of noble metal usually happens in the visible-light region, while TiO2 can't be excited by visiblelight. In this case, the noble metal acts as light-harvesting antennae and the TiO2 acts as an electron fifilter instead of as a participant in the light conversion process [18,31e33]. Type III is semiconductor-metal double excitation state. In this case, TiO2 is excited by the ultraviolet light (l < 400 nm) and the metal is excited by the longer wavelength light (l > 400 nm), simultaneously. It is worth noting that this type is the ultimate goal of researchers due to the effective utilization of solar energy. However, the study on the photocatalytic mechanism of this type is still rare due to the complicated electron excitation and transport process. Therefore, it is technically interesting to study double excitation process and design an efficient synthetic strategy for developing a stable Ag-TiO2 plasmonic photocatalyst with a high effificiency under ultravioletevisible light.

Over the years, many methods have been also developed to synthesize Ag-TiO2 nanostructures, such as the wettingthermal decomposition, photo-reduction deposition, hydrothermal synthesis and chemical reduction deposition [26,34,35]. In these methods, the final Ag NPs all come from Agþ ion of the reactant (such as AgNO3). By carefully controlling and adjusting the synthesis process, uniform Ag NPs can be loaded onto the TiO2 (u-Ag/TiO2). However, Tada et al. have recently demonstrated that TiO2-Au with a bimodal size distribution of hierarchical Au nanoparticles exhibited a high activity and stability [36]. The different Fermi energy between the bimodal Au NPs benefited the hot electrons to transport from small Au nanoparticles to large ones through the CB of TiO2, resulting in an increased photo-induced electron effi- ciency and more operable reaction sites than unimodal TiO2- Au. Coincidentally, Gao et al. also reported that a different size distribution of Ag NPs nanoparticles was more favorable for enlarging the light adsorption range and improving the charge separation [37]. The fabrication, composition structure and morphology, irradiation light type, and the electron transfer direction of Ag and TiO2 composited catalysts were summarized in Table 1.

To this end, we have designed a reversed photo-induced synthesis strategy to prepare hierarchical Ag NPs by semiconductor-metal double excitation process under solar light irradiation. Moreover, in view of the fact that nitrogen doped TiO2 (N-TiO2) has better visible light response property [38], a stable plasmonic photocatalyst of hierarchical Ag NPs auto-decorated on N-TiO2 (h-Ag/N-TiO2) has been successfully prepared for the first time. The growth mechanism of hierarchical Ag NPs by photoinduced synthesis has been revealed. The as-prepared h-Ag/N-TiO2 as an ethanol-splitting photocatalyst exhibits a much higher H2 production rate than uniform Ag NPs loaded N-TiO2 (u-Ag/N-TiO2) and pure N-TiO2. The electron transfer route of double excitation process and the solar-light induced ethanol-splitting mechanism have also been discussed in detail.

Experimental section

Photoinduced synthesis of hierarchical Ag NPs auto-decorated on N-TiO2 (h-Ag/N-TiO2

Firstly, Ag NPs (150 mL of the above synthesized Ag NPs solution) and N-TiO2 (50 mg) were mixed in ethanol solution (150 mL). Then, the mixed solution was ultrasonicated for 10 min. Finally, the mixture was irradiated with a solar light (300 W, Xenon lamp, CEL-HXF 300E) for 3 h. The autodecorated h-Ag/N-TiO2 plasmonic photocatalyst was prepared. The above operations were executed in a sealed Pyrex glass reactor.

Synthesis of uniform Ag NPs loaded on N-TiO2 (u-Ag/N-TiO2) N-TiO2 loaded Ag NPs with uniform size was synthesized by a photo-deposition method. 50 mL of AgNO3 water solution (0.2 M) were dispersed in ethanol (150 mL) containing 50 mg of N-TiO2. The solution was de-aerated by a flflow of nitrogen gas for 30 min and then exposed to UVevis light for 3e5 min with continuous stirring. The as-synthesized powder was washed with ethanol for several times.

Photocatalytic test

Photocatalytic splitting ethanol into H2 was carried out in a CEL-SPH2N hydrogen evolution system (Beijing China Education Au-light Co., Ltd) equipped with a 300 W Xe lamp (CEL-HXF 300E) simulating solar light. In a typical reaction, 20 mg of the as-prepared samples was dispersed in a Pyrex glass reactor containing 150 mL of ethanol solution (100%). After the mixed solution with ultrasonic treatment for 30 min, the system was sealed and vacuumed to keep the pressure at  0.09 MPa. A circular cooling water system was turned on and the reactor was irradiated with Xe lamp (300 W) under magnetic stirring. The gases evolved were analyzed on line with a gas chromatograph (GC 9790 T-2, TCD, with N2 as carrier gas) every 30 min of illumination.