Highly Ordered TiO₂Branched Nanorod Array Composites For Photoelectrochemical（PEC） Water Splitting

Photoelectrochemical (PEC) water splitting is one of the “holy grail”technologies for clean and renewable hydrogen production. Titanium dioxide (TiO2)has been extensively investigated as a photoanode for PEC water splitting because ofits favorable band-edge position, strong optical absorption, excellent chemicalstability, photocorrosion resistance, and low cost. However, TiO2-based PECs has twomajor problems:(i) no light absporption in the visible region due to its large band-gap(e.g.,~3.2eV for anatase), and ii) fast recombination of excitons due to the shortdiffusion paths of charge carriers.Compared to TiO2films comprised of nanoparticles, highly ordered TiO2nanorodarrays (TiO2NRs) offer direct electrical pathways for photogenerated electrons,leading to high electron transport rate, which in turn suppress the recombination ofelectron-hole pairs. Additionally, single crystalline branched TiO2nanorod arrays(TiO2BNRs) can result in increased surface area and enhanced electron conductivityfor improved photocurrent harvest efficiencies. Here, the modification of electronicand structural properties of TiO2with other small band-gap semiconductors or withnoble metals (e.g., Au NPs) were used to increase the absorption of visible light,which could further enhance the efficiency of solar energy conversion.This paper describes the synthesis of CdS quantum dots sensitized TiO2BNRs(CdS/TiO2BNRs) by a sequential chemical bath deposition (S-CBD) method. TheCdS/TiO2BNRs were used as photoanode in photoelectrochemical water splitting. Asthe amount of CdS increased by adding the S-CBD sensitized cycles to nine, the sizeof the CdS QDs was about5-10nm. The maximum photocurrent of CdS/TiO2BNRsis3.98mA/cm2at a potential of0V versus Ag/AgCl (the standard deviation is±0.1mA/cm2) under the AM1.5G simulated sunlight illumination, which is about fourtimes larger than the pristine TiO2BNRs. The photocurrent of CdS/TiO2BNRs issignificantly higher compared to CdS QDs sensitized on TiO2NRs as reportedpreviously. Furthermore, CdS/TiO2B-NRs exhibit a largest incident photon to currentconversion efficiency (IPCE) of55.3%over the UV region. Under visible region,CdS/TiO2B-NRs shows IPCEs of17.9%,12.3%, and2.3%at450nm,475nm, and500nm, respectively. These results clearly confirm that CdS sensitization on TiO2B-NRs improves visible light absorption, and electrons generated in CdS can betransferred to TiO2, which greatly increases the IPCE in the visible region. The plasmonic Au/TiO2BNRs were synthetized by a photoreduction method, andthe particle size of Au is6.5±0.5nm. Such Au/TiO2BNR composites exhibit highphotocatalytic activity in photoelectrochemical (PEC) water splitting. As Au particlesattached on TiO2branched nanostructure, the Au/TiO2BNRs shows strong absorptionof visible light at534nm, which was caused by the surface plasmon resonance ofsmall Au nanoparticles. Also the UV light absorption was strengthened due to theband-gap transition of TiO2semiconductor. The novel Au/TiO2BNRs showsenhanced activity with a photocurrent of0.125mA/cm2under visible light (≥420nm)and2.32mA/cm2under AM1.5G illumination (100mW/cm2), which is significantlyhigher and comparable to the highest value ever reported. The superior PECperformance is mainly due to the plasmonic effect of Au nanoparticles, whichenhances the visible light absorption, together with the large surface area, efficientcharge separation and high carrier mobility of the TiO2BNRs.