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In this work, we report on defects generation in TiO2 inverse opal (IO) nanostructures by electrochemical reduction in order to increase photocatalytic activity and improve photoelectrochemical (PEC) water splitting performance. Macroporous structures, such as inverse opals, have attracted a lot of attention for energy-related applications because of their large surface area, interconnected pores, and ability to enhance light-matter interaction. Photocurrent density of electrochemically reduced TiO2-IO increased by almost 4 times, compared to pristine TiO2-IO photoelectrodes. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses confirm the presence of oxygen vacancies in electrochemically reduced TiO2-IO photoelectrodes. Oxygen vacancies extend the absorption of TiO2 from the UV to visible region. The incident photon-to-current efficiency (IPCE) increased by almost 3 times in the absorption (UV) region of TiO2 and slightly in the visible region. Impedance studies show improved electrical conductivity, longer photogenerated electron lifetime, and a negative shift of the flatband potential, which are attributed to oxygen vacancies acting as electron donors. The Fermi level shifts to be closer to the conduction band edge of TiO2-IO.

Cadmium sulfide (CdS)is a unique semiconducting material for solar hydrogen generation applications with a tunable, narrow bandgap that straddles water redox potentials. However, its potential towards efficient solar hydrogen generation has not yet been realized due to low photon-tocurrent conversions, high charge carrier recombination and the lack of controlled preparation methods. In this work, we demonstrate a highly efficient CdS/TiO2 heterostructured photoelectrode using atomic layer deposition and solution ion transfer reactions. Enabled by the well-controlled deposition of CdS nanocrystals on TiO2 inverse opal(TiIO) nanostructures using the proposed method, a saturation photocurrent density of 9.1 mA cm−2 is realized which is the highest ever reported for CdS-based photoelectrodes. We further demonstrate that the passivation of a CdS surface with an ultrathin amorphous layer(∼1.5 nm) of TiO2 improves the charge collection efficiency at low applied potentials paving the way for unassisted solar hydrogen generation.

Photoelectrolysis of water using solar energy into storable and environment-friendly chemical fuel in the form of hydrogen provides a potential solution to address the environmental concerns and fulfill future energy requirements in a sustainable manner. Achieving efficient and spontaneous hydrogen evolution in water using solar light as the only energy input is a highly desirable but a difficult target. In this work, we report perovskite solar cell integrated CdS-based photoanode for unbiased photoelectrochemical hydrogen evolution. An integrated tandem device consisting of mesoporous CdS/TiO2 photoanode paired with a triple-cation perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) solar cell is developed via a facile fabrication route. The proposed photovoltaic integrated photoanode presents an efficient tandem configuration with high optical transparency to long-wavelength photons and strong photoelectrochemical conversions from short-wavelength photons. On the basis of this integrated tandem device, an unbiased photocurrent density of 7.8 mA/cm2 is demonstrated under AM1.5G illumination.

The research interest in photoelectrochemical (PEC) water splitting is ever growing due to its potential to contribute towards clean and portable energy. However, the lack of low energy band gap materials with high photocorrosion resistance is the primary setback inhibiting this technology from commercialisation. The ternary alloy InGaN shows promise to meet the photoelectrode material requirements due to its high chemical stability and band gap tunability. The band gap of InGaN can be modulated from the UV to IR regions by adjusting the In concentration so as to absorb the maximum portion of the solar spectrum. This paper reports on the influence of In concentration on the PEC properties of planar and nanopillar (NP) InGaN/GaN multi-quantum well (MQW) photoanodes, where NPs were fabricated using a top-down approach. Results show that changing the In concentration, while having a minor effect on the PEC performance of planar MQWs, has an enormous impact on the PEC performance of NP MQWs, with large variations in the photocurrent density observed. Planar photoanodes containing MQWs generate marginally lower photocurrents compared to photoanodes without MQWs when illuminated with sunlight. NP MQWs with 30% In generated the highest photocurrent density of 1.6 mA cm-2, 4 times greater than that of its planar counterpart and 1.8 times greater than that of the NP photoanode with no MQWs. The InGaN/GaN MQWs also slightly influenced the onset potential of both the planar and NP photoanodes. Micro-photoluminescence, diffuse reflectance spectroscopy and IPCE measurements are used to explain these results.

Nanostructured materials are crucial to the light harvesting and power conversion efficiency of solar energy conversion systems. The template-assisted fabrication method offers a versatile route to produce nanostructured materials of controlled morphology and optoelectronic properties. Toward this, self-assembled opal colloidal crystals and nanoporous anodic aluminum oxide have been used as templates to produce nanostructures of a wide range of materials with different dimensionalities and enhanced photocatalytic properties. This chapter introduces various approaches developed for producing the templates and methods of infiltrating them with the desired photoactive materials. This chapter also critically assesses the advantages and limitations of the proposed methods, and summarizes the beneficial outcomes realized using nanostructures developed through the template-assisted route for photocatalysis