Enhanced Photoelectrochemical Properties of Cu2O- loaded Short TiO2 Nanotube Array Electrode Prepared by Sonoelectrochemical Deposition

Yanbiao Liu, Haibin Zhou, Jinhua Li, Hongchong Chen, Di Li, Baoxue Zhou* and Weimin Cai Copper and titanium remain relatively plentiful in earth crust. Therefore, using them in solar energy conversion technologies are of significant interest. In this work, cuprous oxide (Cu2O)-modified short TiO2 nanotube array electrode was prepared based on the following two design ideas: first, the short titania nanotubes obtained from sonoelectrochemical anodization possess excellent charge separation and transportation properties as well as desirable mechanical stability; second, the sonoelectrochemical deposition technique favours the improvement in the combination between Cu2O and TiO2 nanotubes, and favours the dispersion of Cu2O particles. UV-Vis absorption and photoelectronchemical measurements proved that the Cu2O coating extended the visible spectrum absorption and the solar spectrum-induced photocurrent response. Under AM1.5 irradiation, the photocurrent density of the composite electrode (i.e. sonoelectrochemical deposition for 5 min) was more than 4.75 times as high as the pure nanotube electrode. Comparing the photoactivity of the Cu2O/TiO2 electrode obtained using sonoelectrochemical deposition with others that synthesized using plain electrochemical deposition, the photocurrent density of the former electrode was ~2.2 times higher than that of the latter when biased at 1.0 V (vs. Ag/AgCl). The reproducible photocurrent response under intermittent illumination demonstrated the excellent stability of the composite electrode. Such kind of composite electrode material will have many potential applications in solar cell and other fields.

From the view point of TiO2, the highly ordered TiO2 nanotube array (TNA) obtained from the anodization of titanium in HF or [F -]-based electrolyte can reduce the scattering of free electrons and enhance electron mobility [15], which offers the potential for improved electron transport leading to higher photocatalytic efficiency. Therefore, the coupled Cu2O/TNA photoanode might facilitate kinetic separation of photogenerated charges and decrease the recombination rate within the electrode materials [16,17]. However, the structural parameters (i.e. tube length and tube diameter) of the TNA film directly influence the transport resistance of photogenerated electrons and the recombination rate among photogenerated charges as well as the practical engineering application of electrode materials. Literatures [18,19] have also demonstrated that the increase in length of nanotubes may not contribute positively to the photoelectrochemical performance of electrode materials. Recently, a short TNA (referred as STNA) film electrode prepared via sonoelectrochemical anodization route (anodization under irradiation of ultrasonic wave) was reported by our group [20]. Compared with the long nanotubes synthesized by conventional magnetic agitation technique [15,21], the STNA electrode shows excellent charge separation and transfer properties and desirable mechanical stability.
As for Cu2O, the combination of Cu2O with nanotube layer and the morphological structure of Cu2O were significantly affected by the Cu2O preparation pathways. To date, Cu2O can be fabricated by many different techniques including thermal, anodic and chemical oxidation as well as reactive sputtering [22].
Among these different synthesis techniques, the electrochemical deposition technique offers the simplest and most controllable way of synthesizing Cu2O/TNA electrodes [23]. However, obtaining a composite electrode with better quality, evenly distributed Cu2O particles and a strong combination of Cu2O and TiO2 nanotubes may be more useful if ultrasonic waves can be integrated into the electrochemical process, since the sonochemical process can help increase the mass transfer throughout the reaction system and accelerate the diffusion of [Cu + ] ions onto the nanotubes.
Based on the above design ideas and our previous work [19][20][21][24][25][26], an efficient photoelectrochemical photoanode (Cu2O-decorated STNA) was prepared via sonoelectrochemical anodization and sonoelectrochemical deposition (SED, electrochemical deposition under ultrasonic wave irradiation) methods in this work. To our best knowledge, no study has been reported to date regarding the application of SED technique into fabrication of Cu2O/TiO2 composite electrode. In our research, the detailed synthesis process, characterization, and photo-electrochemical property testing for this composite catalyst were also discussed.

Materials
Titanium sheets (0.25 mm thick, 99.9% purity) were sup- gent Company without further treatment prior to use. All solutions were prepared using high-purity deionized (DI) water.

Preparation of STNA
The detailed methodology of the preparation of short, robust and highly-ordered titania nanotube array have been published in our previous work (see Fig. S1 in Supplementary Information) [20]. The anodized samples were then rinsed with DI water and dried in air. Subsequently, the as-prepared STNA samples were crystallized by annealing in air atmosphere for 3 h at 450℃ with heating and cooling rate of 1℃/min.

Apparatus and methods
The photoelectrochemical experiments were performed in a rectangular shaped quartz reactor (20×40×50 mm) using a threeelectrode system with a platinum foil counter electrode, a saturated Ag/AgCl reference electrode and a TiO2 work electrode.
The supply bias and work current were controlled using a CHI     Figure   4(b) shows the Cu 2p core level XPS scans, at higher resolution over smaller energy windows. As can be seen, the Cu 2p core level XPS spectrum has two sharp peaks at 932.3 eV (Cu 2p3/2) and 952.2 eV (Cu 2p1/2), which is in good agreement with the reported values for Cu2O [14,27].  The stability of the composite electrode was examined by potentiostatic (current vs. time, I-t) measurements. Figure 7 shows the I-t curves obtained from the Cu2O/STNA electrode at two different bias potentials under visible-light illumination.

Results and discussion
Upon switching off the light, the photocurrent density goes down to zero; whereas the photocurrent increases rapidly to the original value when under illumination again. This indicates that the current observed for this system is mainly due to the photoreactivity of the catalyst and the charge transfer within the composite electrode is very fast. Moreover, composite electrode exhibits excellent reproducibility of the I-t curves.

Conclusions
In