Some recent developments in photoelectrochemical water splitting using nanostructured TiO2: a short review

Kinetics of TiO2 photochromic response in different hole scavenging solvents

There has recently been a renewed interest in the photochromic properties and uses of TiO₂ as a potential candidate for smart windows. However, the surrounding medium of TiO₂ nanoparticles (NPs) is of equal importance, as it facilitates hole scavenging and, in turn, photochromism. Here, we investigated the impact of scavenging power on the photochromic properties of TiO₂. TiO₂ NP colloids in these solvents exhibit a photochromic response in a broad wavelength range from the visible to near-infrared region. We have shown that solvents such as ethanol have the best hole scavenging properties among the alcohols tested. The response can be further modified by the addition of a supplementary hole scavenger, such as hydroxylamine. The photochromism of TiO₂ is fully reversible and could be used for applications in smart windows.

Towards Solar Methanol: Past, Present, and Future

This work aims to provide an overview of producing value-added products affordably and sustainably from greenhouse gases (GHGs). Methanol (MeOH) is one such product, and is one of the most widely used chemicals, employed as a feedstock for ≈30% of industrial chemicals. The starting materials are analogous to those feeding natural processes: water, CO2, and light. Innovative technologies from this effort have global significance, as they allow GHG recycling, while providing society with a renewable carbon feedstock. Light, in the form of solar energy, assists the production process in some capacity. Various solar strategies of continually increasing technology readiness levels are compared to the commercial MeOH process, which uses a syngas feed derived from natural gas. These strategies include several key technologies, including solar-thermochemical, photochemical, and photovoltaic-electrochemical. Other solar-assisted technologies that are not yet commercial-ready are also discussed. The commercial-ready technologies are compared using a technoeconomic analysis, and the scalability of solar reactors is also discussed in the context of light-incorporating catalyst architectures and designs. Finally, how MeOH compares against other prospective products is briefly discussed, as well as the viability of the most promising solar MeOH strategy in an international context.

Three-Dimensional Decoupling Co-Catalyst from a Photoabsorbing Semiconductor as a New Strategy To Boost Photoelectrochemical Water Splitting

A cocatalyst is normally deposited on a photoabsorbing semiconductor (PAS) for photoelectrochemical (PEC) water splitting, but with drawbacks of limited loading, reduced light absorption, and tendency of charge recombination. To tackle these problems, a scheme of three-dimensional (3D) decoupling cocatalysts from the PAS with a pore-spanning crisscross conducting polymer host is proposed in this work. To demonstrate the concept, a facile method was developed for the in situ cogrowth of FeO _x nanoparticles and conducting polymer (CP) network in various PAS with different microstructures such as a TiO₂ nanorod array, WO₃ nanosheet array, and planar TiO₂ nanoparticle film, generating the bespoke photoanodes. The as-synthesized photoanodes exhibited a significantly enhanced PEC water splitting performance, which is clearly shown to arise from the improved light utilization, increased catalytic active sites, enhanced charge separation, and decreased electrochemical impedance of the photoelectrode. This 3D decoupling strategy is expected to open a promising direction for designing efficient PEC cells.

Self-assembled bundled TiO2 nanowire arrays encapsulated with indium tin oxide for broadband absorption in plasmonic photocatalysis

In order to enhance photocatalysis by broadening light harvesting, bundled TiO₂ nanowire bundle arrays are encapsulated with indium tin oxide (ITO) by a self-assembly technique involving anodization, electrochemical etching, and ITO deposition. The plasmonic photocatalyst, which has a multiscale structure with variable nanoscale gaps as well as microscale funnels, shows broadband localized surface plasmon resonance absorption of 84% in the wavelength range between 400 and 2500 nm. The improved photocatalytic efficiency is demonstrated by methyl orange degradation under sunlight illumination. The improvement stems from enhanced light harvesting arising from the localized surface plasmon resonance of the ITO membrane which extends the light response to the visible and NIR regions and excites hot charge carriers.

First Principles Study on the Interaction Mechanisms of Water Molecules on TiO₂ Nanotubes

The adsorption properties of water molecules on TiO₂ nanotubes (TiO₂NT) and the interaction mechanisms between water molecules are studied by first principles calculations. The adsorption preferences of water molecules in molecular or dissociated states on clean and H-terminated TiO₂NT are evaluated. Adsorption of OH clusters on (0, 6) and (9, 0) TiO₂ nanotubes are first studied. The smallest adsorption energies are -1.163 eV and -1.383 eV, respectively, by examining five different adsorption sites on each type of tube. Eight and six adsorption sites were considered for OH adsorbtion on the H terminated (0, 6) and (9, 0) nanotubes. Water molecules are reformed with the smallest adsorption energy of -4.796 eV on the former and of -5.013 eV on the latter nanotube, respectively. For the adsorption of a single water molecule on TiO₂NT, the molecular state shows the strongest adsorption preference with an adsorption energy of -0.660 eV. The adsorption of multiple (two and three) water molecules on TiO₂NT is also studied. The calculated results show that the interactions between water molecules greatly affect their adsorption properties. Competition occurs between the molecular and dissociated states. The electronic structures are calculated to clarify the interaction mechanisms between water molecules and TiO₂NT. The bonding interactions between H from water and oxygen from TiO₂NT may be the reason for the dissociation of water on TiO₂NT.

General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting

Photoelectrochemical (PEC) water splitting is a very promising technology that converts water into clean hydrogen fuel and oxygen by using solar light. However, the characterization methods for PEC cells are diverse and a systematic introduction to characterization methods for PEC cells has rarely been attempted. Unlike most other review articles that focus mainly on the material used for the working electrodes of PEC cells, this review introduces general characterization methods for PEC cells, including their basic configurations and methods for characterizing their performance under various conditions, regardless of the materials used. Detailed experimental operation procedures with theoretical information are provided for each characterization method. The PEC research area is rapidly expanding and more researchers are beginning to devote themselves to related work. Therefore, the content of this Minireview can provide entry-level knowledge to beginners in the area of PEC, which might accelerate progress in this area.

Black titanium dioxide (TiO2) nanomaterials

Recent progress in the preparation, properties and applications of black TiO₂nanomaterials is reviewed.

Migration of Holstein Polarons in Anatase TiO2

A simple yet reliable valence bond theory was applied to ascertain the effective size and shape for the electron and hole polarons in bulk anatase TiO2 by examining the extent of polaron charge delocalization. It was found that the electron polaron is approximately 2 times as large as its hole counterpart, leading to a faster electron diffusion than hole hopping with regard to the electron-phonon coupling strength. Moreover, the oblate hole polaron exhibits a pronounced directional heterogeneity in migration, whereas the nearly spherical electron polaron tends to diffuse along all possible lattice directions. In light of the notable delocalization characteristics of both polarons, their migration should proceed in an adiabatic manner, and their rates can be calculated by the Arrhenius equation. It turns out that our calculated polaron mobilities at 300 and 1300K are both in excellent agreement with experimental values, justifying our novel approach for Holstein polaron modeling in crystalline semiconductors.

Construction of ZnO/ZnS/CdS/CuInS₂ core-shell nanowire arrays via ion exchange: p-n junction photoanode with enhanced photoelectrochemical activity under visible light

ZnO/ZnS/CdS/CuInS2 core-shell nanowire arrays with enhanced photoelectrochemical activity under visible light were successfully prepared via ion exchange and hydrothermal methods. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis absorption, X-ray photoemission spectroscopy, and photoelectrochemical response. As a p-n junction photoanode, ZnO/ZnS/CdS/CuInS2 heterostructure shows much higher visible light photoelectrocatalytic activity toward water splitting than ZnO/ZnS/CdS and ZnO/ZnS films. The ZnO/ZnS/CdS/CuInS2 film with optimal constitution exhibits the highest photocurrent of 10.5 mA/cm(2) and the highest IPCE of approximately 57.7% at 480 nm and a bias potential of 0 V versus Ag/AgCl. The critical roles of CdS and ZnS in ZnO/ZnS/CdS/CuInS2 heterostructure were investigated. ZnS, as a passivation layer, suppresses the recombination of the photogenerated charge carriers at the interface of the oxide and CuInS2. CdS enhances the absorption of visible light and forms p-n junctions with CuInS2, which promotes the transport of charge carriers and retards the recombination of electrons and holes in CuInS2 to improve the photoelectrochemical performance of ZnO/ZnS/CdS/CuInS2 heterostructure.

Ag nanoparticle embedded TiO(2) composite nanorod arrays fabricated by oblique angle deposition: toward plasmonic photocatalysis

Using a unique oblique angle co-deposition technique, well-aligned arrays of Ag nanoparticle embedded TiO2 composite nanorods have been fabricated with different concentrations of Ag. The structural, optical, and photocatalytic properties of the composite nanostructures are investigated using a variety of experimental techniques and compared with those of pure TiO2 nanorods fabricated similarly. Ag nanoparticles are formed in the composite nanorods, which increase the visible light absorbance due to localized surface plasmon resonance. The Ag concentrations and the annealing conditions are found to affect the size and the density of Ag nanoparticles and their optical properties. The Ag nanoparticle embedded TiO2 nanostructures exhibit enhanced photocatalytic activity compared to pure TiO2 under visible- or UV-light illumination. Ag plays different roles in assisting the photocatalysis with different light sources. Ag can be excited and can inject electrons to TiO2, working as an electron donor under visible light. While under UV illumination, Ag acts as an electron acceptor to trap the photogenerated electrons in TiO2. Due to the opposite electron transfer direction under UV and visible light, the presence of Ag may not result in a greater enhancement in the photocatalytic performance.

Nanoscale investigation of photoinduced hydrophilicity variations in anatase and rutile nanopowders

The photoactive properties of TiO2 are employed to develop surfaces with self-cleaning capabilities. Clearly, the fine-tuning of such surfaces for different applications relies on a holistic understanding of the different aspects that induce the self-cleaning behavior. Among those, the mechanisms responsible for the photoinduced surface alteration in the TiO2 allotropes are still not completely understood. In this study, TiO2 polymorphs nanopowders are investigated by combining the high spatial resolution observables of recently developed atomic force microscopy (AFM) based force spectroscopy techniques with diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). Phase maps under irradiated and nonirradiated conditions for anatase and rutile suggest the existence of two distinct behaviors that are further discerned by energy analysis of amplitude and phase vs distance curves. Independently, surface analysis of anatase and rutile by means of DRIFTS spectroscopy reveals a readily distinguishable coexistence of dissociated water and molecular water on the two phases, confirming the stronger photoactivity of anatase. The peculiarity of the surface interaction under UV exposure is further investigated by reconstructing the force profiles between the oscillating AFM tip and the TiO2 phases with the attempt of gaining a better understanding of the mechanisms that cause the different hydrophilic properties in the TiO2 allotropes.

Controlled fabrication of Sn/TiO2 nanorods for photoelectrochemical water splitting

In this work, we investigate the controlled fabrication of Sn-doped TiO2 nanorods (Sn/TiO2 NRs) for photoelectrochemical water splitting. Sn is incorporated into the rutile TiO2 nanorods with Sn/Ti molar ratios ranging from 0% to 3% by a simple solvothermal synthesis method. The obtained Sn/TiO2 NRs are single crystalline with a rutile structure. The concentration of Sn in the final nanorods can be well controlled by adjusting the molar ratio of the precursors. Photoelectrochemical experiments are conducted to explore the photocatalytic activity of Sn/TiO2 NRs with different doping levels. Under the illumination of solar simulator with the light intensity of 100 mW/cm2, our measurements reveal that the photocurrent increases with increasing doping level and reaches the maximum value of 1.01 mA/cm2 at -0.4 V versus Ag/AgCl, which corresponds to up to about 50% enhancement compared with the pristine TiO2 NRs. The Mott-Schottky plots indicate that incorporation of Sn into TiO2 nanorod can significantly increase the charge carrier density, leading to enhanced conductivity of the nanorod. Furthermore, we demonstrate that Sn/TiO2 NRs can be a promising candidate for photoanode in photoelectrochemical water splitting because of their excellent chemical stability.

Fabrication of metal oxide nanobranches on atomic-layer-deposited TiO2 nanotube arrays and their application in energy storage

Due to the chemical stability and easy fabrication by atomic layer deposition (ALD), TiO2 nanotubes are regarded highly useful in constructing branched nanostructured electrodes for solar conversion and electrochemical energy storage devices. Here we present a facile and scalable fabrication of metal oxide nanobranches on ALD pre-formed TiO2 nanotubes. The metal oxide branches can be a wide range (nearly any) of desirable materials, including NiO and Co3O4 demonstrated herein. As an example, the TiO2/NiO nanoarray battery cathode exhibits a relatively high gravimetric capacity value of ~153 mA h g(-1) and a fairly good stability up to 12,000 cycles with a capacitance of 132 mA h g(-1) at 2 A g(-1).