Nanostructured Tungsten Trioxide Thin Films Synthesized for Photoelectrocatalytic Water Oxidation: A review

Abstract

The recent developments of nanostructured WO3 thin films synthesized through the electrochemical route of electrochemical anodization and cathodic electrodeposition for the application in photoelectrochemical (PEC) water splitting are reviewed. The key fundamental reaction mechanisms of electrochemical anodization and cathodic electrodeposition methods for synthesizing nanostructured WO3 thin films are explained. In addition, the effects of metal oxide precursors, electrode substrates, applied potentials and current densities, and annealing temperatures on size, composition, and thickness of the electrochemically synthesized nanostructured WO3 thin films are elucidated in detail. Finally, a summary is given for the general evaluation practices used to calculate the energy conversion efficiency of nanostructured WO3 thin films and a recommendation is provided to standardize the presentation of research results in the field to allow for easy comparison of reported PEC efficiencies in the near future.

Capacitance Enhancement in a Semiconductor Nanostructure‐Based Supercapacitor by Solar Light and a Self‐Powered Supercapacitor–Photodetector System

“…Moreover, as a semiconductor with a narrow bandgap of 2.6-2.8 eV, WO 3 can absorb visible light to generate electron-hole pairs. [28,56] Furthermore, h-WO 3 has plenty of large hexagonal tunnels to accommodate small cations coming from the electrolyte, which can neutralize and trap photoexcited electrons. [5,49,54,55] …”

Multihierarchical electrodes based on titanate nanotubes and zinc oxide nanorods for photoelectrochemical water splitting

“…Photoelectrocatalytic water splitting using sunlight is an interesting approach for producing H 2 and O 2 ; however, the chosen photocatalyst must fulfill certain requirements, such as optimal band gap levels (aligned with the energy required to perform the reactions of H 2 /H 2 O and O 2 /H 2 O), high stability and low recombination rates 2 . Since TiO 2 photoelectrodes were first used for the water splitting reaction 3 , many other catalysts based on metal oxides have been investigated, especially those involving nanomaterials [4][5][6][7] . Among them, we highlight the titanate nanotubes (TiNTs -see Table 1), which exhibit an open and mesoporous morphology, high surface area, wide band gap energy and good electrical conductivity 8 .…”

On the electrochemical synthesis and charge storage properties of WO3/polyaniline hybrid nanostructures

“…To obtain porous structures (instead of a compact metal oxide layer), it is necessary to carry out the anodization in a solution of a complexing agent (in our case F − ). Although there is wealth of examples in the literature for this process [4,22,31], its reproducibility falls below that of TiO 2 ; therefore, precise control and monitoring of the synthesis parameters are indeed important. Figure 1a shows current density vs. time traces recorded during anodization of different W samples at a constant voltage of 60 V. At the first stage, which is magnified in the inset, the voltage was gradually increased to 60 V, with a rate of 500 mV s −1 .…”

“…As a relatively narrow bandgap semiconductor (E g =2.6 eV), it is a promising candidate for visible light-harvesting photoanodes [3,4]; furthermore, it possesses electrochromic properties which makes it an ideal component of smart windows and displays [5,6]. However, unlike other transition metal oxides (e.g., RuO 2 ), the pseudocapacitive nature of this oxide remains as yet incompletely exploited.…”

“…Anodization is a well-known method for preparing transition metal oxides [4,17] with effective control over the morphology of the produced nanostructures (pore size [18], layer thickness [19][20][21], and structural features [22,23]). Furthermore, it circumvents the use of binding agents, as the nanostructured metal oxide is directly grown from the respective metal foil.…”