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TiO2-based photocatalysis has been considered to be one of the most promising avenues for environmental remediation including water purification. However, several technical issues such as the limited surface area of bulk TiO2, the large band gap of TiO2, and rapid charge recombination still limit the practical application of TiO2 photocatalysts. Therefore, here we focus on two structural design strategies: (i) monolithic three-dimensional (3D) nanostructuring, and (ii) heterostructuring with graphitic carbon nanomaterials. A monolithic 3D nanostructure enables maximal surface area in a given volume and efficient reuse of the photocatalyst without recollection. Heterostructuring with carbon nanomaterials helps achieve maximal utilization of the solar spectrum and charge separation and provides efficient TiO2 photocatalysts. In this review, recent progress on TiO2 photocatalysts toward the abovementioned strategies will be summarized. Further discussion and direction will provide insights into the rational design of highly efficient TiO2 photocatalysts, and help develop advanced photocatalyst models.

Considering the environmental issues, it is essential to develop highly efficient and recyclable photocatalysts in purification systems. Conventional TiO2 nanoparticles have strong intrinsic oxidizing power and high surface area, but are difficult to collect after use and rarely absorb visible light, resulting in low photocatalytic efficiency under sunlight. Here we develop a new type of highly efficient and recyclable photocatalyst made of a three-dimensional (3D) nanostructured N-doped TiO2 monolith with enhanced visible light absorption. To prepare the sample, an ultrathin TiN layer (∼10 nm) was conformally coated using atomic layer deposition (ALD) on 3D nanostructured TiO2. Subsequent thermal annealing at low temperature (550 °C) converted TiN to anatase phase N-doped TiO2. The resulting 3D N-doped TiO2 showed ∼33% enhanced photocatalytic performance compared to pure 3D TiO2 of equivalent thickness under sunlight due to the reduced bandgap, from 3.2 eV to 2.75 eV through N-doping. The 3D N-doped TiO2 monolith could be easily collected and reused at least 5 times without any degradation in photocatalytic performance.

ZnO is a potential thermoelectric material because of its non-toxicity, high thermal stability, and relatively high Seebeck coefficient (S) of metal oxides. However, the extremely low figure of merit (zT), which comes from a high thermal conductivity (κ) over 40 W m-1 K-1, limits the thermoelectric application of ZnO. In particular, below 500 K, ZnO exhibits a nearly negligible zT (

TiO2-based photocatalysis has been considered to be one of the most promising avenues for environmental remediation including water purification. However, several technical issues such as the limited surface area of bulk TiO2, the large band gap of TiO2, and rapid charge recombination still limit the practical application of TiO2 photocatalysts. Therefore, here we focus on two structural design strategies: (i) monolithic three-dimensional (3D) nanostructuring, and (ii) heterostructuring with graphitic carbon nanomaterials. A monolithic 3D nanostructure enables maximal surface area in a given volume and efficient reuse of the photocatalyst without recollection. Heterostructuring with carbon nanomaterials helps achieve maximal utilization of the solar spectrum and charge separation and provides efficient TiO2 photocatalysts. In this review, recent progress on TiO2 photocatalysts toward the abovementioned strategies will be summarized. Further discussion and direction will provide insights into the rational design of highly efficient TiO2 photocatalysts, and help develop advanced photocatalyst models.

Titania has attracted considerable interest for use in water purification applications due to its excellent photocatalytic activity. To further improve the efficiency of photocatalysis, numerous nanostructures (i.e. nanoparticles, nanotubes, and nanowires) have been proposed to increase the surface area of titania. Despite the high photocatalytic performance of the nanostructured titania, subsequent difficulties encountered in recollection and reuse of titania inhibit the practical application for water purification systems. Here we successfully fabricate monolithic, three dimensional (3D) nanoshell titania with high uniformity over large areas (~1 × 1 inch(2)) through proximity field nanopatterning (PnP) and low-temperature atomic layer deposition (ALD) techniques. The higher surface area of 3D nanoshell titania increases the photocatalytic performance more than three-fold relative to that of a thin film of equivalent sample size. Also, the monolithic form of titania enables it to be reused without any degradation of photocatalytic activity. The newly developed nanomaterials in this study can serve as an efficient and reusable photocatalyst for water purification systems.