Xinrui Cao,a Guohui Tian,*ab Yajie Chen,a Juan Zhou,a Wei Zhou,a Chungui Tiana and Honggang Fu*a

In this study, hierarchical composites of reduced graphene oxide (RGO)–TiO2 nanowire arrays were prepared via an in situ controlled growth process and subsequent calcination. A more homogeneous mixture was formed and strong interaction between RGO nanosheets and anatase TiO2 nanowires existed, which help the interfacial charge transfer and separation. Electrochemical impedance spectroscopy (EIS), surface photovoltage spectroscopy (SPV) and transient photovoltage (TPV) were used to study the interfacial charge transfer process. The prepared hierarchical RGO–TiO2 nanowire array composites showed excellent H2 production activity in the absence of noble metal cocatalysts, which was much higher than that of the pure anatase TiO2 and RGO–TiO2 generated by mechanical mixing. It was mainly attributed to the synergetic effffect of the improved electron–hole pair separation rate, increased catalytic active sites and high light harvesting provided by the special hierarchical structure. The approach for the preparation of the hierarchical composite in this study may guide the way for designing new composite materials for enhanced photocatalytic and photoelechemical performance.

1. Introduction

Research progress nowadays has further shown that 1D nano structures can be assembled into nano-brous membranes to improve the photovoltage performance and self-cleaning ability.13 Especially, an array of highly ordered, vertically aligned 1D nanostructures have exhibited more excellent performance because the aligned nanostructure perpendicular to the substrate could potentially improve the charge-collection efficiency.14–17

As is well known, TiO2 is one of the most important semi conductor materials which is widely used in the  eld of photocatalysis.1,2 The photocatalytic performance of TiO2 is strongly dependent on its morphology and structure.3–5 Among the various morphologies, one dimensional (1D) TiO2 nano-materials such as nanowires and nanotubes, offering direct pathways for photogenerated electron transfer, possess advantages such as facile charge transport along the longitudinal dimensions, low electron–hole recombination rates, manifested superior photovoltaic and superhydrophilicity performance.6–9 Therefore, 1D TiO2 has attracted extensive attention as a substitute for randomly oriented titania nanoparticles.10–12 Although, among the various morphologies, 1D TiO2 showed better performance, pure 1D TiO2 shows not satisfactory photocatalytic efficiency. Coupling TiO2 with metal oxides, noble metals and other semiconductors is an effective approach to inhibit the electron–hole pair recombination and improve the photocatalytic activity.18,19

Measurement of photocatalytic hydrogen evolution

The photocatalytic H2 evolution from water was conducted using an online photocatalytic hydrogen production system (AuLight, Beijing, CEL-SPH2N). A powder sample of the catalyst (0.07 g) was suspended in a mixture of 100 mL of mixed aqueous solution containing 80 mL water and 20 mL methanol in a quartz reactor, without Pt co-catalyst. The reaction was carried out by irradiating the mixture with UV light from a 300 W Xe lamp with a 200–400 nm reaction lter which means that the wavelength of light is approximately 200–400 nm. Prior to the reaction, the mixture was stirred to remove O2 and CO2 dissolved in water. Gas evolution was observed only under photoirradiation and analyzed using an online gas chromatograph (SP7800, TCD, molecular sieve 5 ˚ A, N2 carrier, Beijing Keruida Limited).

Conclusions

In summary, hierarchical RGO–TiO2 composites were prepared via an in situ controlled growth and subsequent calcination process with a high yield. In this process, TiO2 nanowire arrays are delicately grown on 2D GO nanosheets. The morphology of TiO2 nanowires can be controlled by adjusting the reaction time, which is attributed to different nucleation processes. There exist stronger interaction and interface contact between anatase TiO2 nanowires and RGO nanosheets in the hierarchical RGO–TiO2 composites. The photocatalytic test results proved that hierarchical RGO–TiO2 composites showed much higher hydrogen evolution performance than the individual TiO2 component and RGO–TiO2 generated by mechanical mixing due to the synergistic effect of more catalytic active sites, improved charge separation and high light utilization. This work provides a facile approach to construct high photocatalytically active hierarchical structure composites using 1D and 2D nanocomponents with a high yield.