Woo-Jin An

Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals

A facile development of highly efficient Pt-TiO(2) nanostructured films via versatile gas-phase deposition methods is described. The films have a unique one-dimensional (1D) structure of TiO(2) single crystals coated with ultrafine Pt nanoparticles (NPs, 0.5-2 nm) and exhibit extremely high CO(2) photoreduction efficiency with selective formation of methane (the maximum CH(4) yield of 1361 μmol/g-cat/h). The fast electron-transfer rate in TiO(2) single crystals and the efficient electron-hole separation by the Pt NPs were the main reasons attributable for the enhancement, where the size of the Pt NPs and the unique 1D structure of TiO(2) single crystals played an important role.

Enhanced water photolysis with Pt metal nanoparticles on single crystal TiO2 surfaces.

Two novel deposition methods were used to synthesize Pt-TiO(2) composite photoelectrodes: a tilt-target room temperature sputtering method and aerosol-chemical vapor deposition (ACVD). Pt nanoparticles (NPs) were sequentially deposited by the tilt-target room temperature sputtering method onto the as-synthesized nanostructured columnar TiO(2) films by ACVD. By varying the sputtering time of Pt deposition, the size of deposited Pt NPs on the TiO(2) film could be precisely controlled. The as-synthesized composite photoelectrodes with different sizes of Pt NPs were characterized by various methods, such as SEM, EDS, TEM, XRD, and UV-vis. The photocurrent measurements revealed that the modification of the TiO(2) surface with Pt NPs improved the photoelectrochemical properties of electrodes. Performance of the Pt-TiO(2) composite photoelectrodes with sparsely deposited 1.15 nm Pt NPs was compared to the pristine TiO(2) photoelectrode with higher saturated photocurrents (7.92 mA/cm(2) to 9.49 mA/cm(2)), enhanced photoconversion efficiency (16.2% to 21.2%), and increased fill factor (0.66 to 0.70). For larger size Pt NPs of 3.45 nm, the composite photoelectrode produced a lower photocurrent and reduced conversion efficiency compared to the pristine TiO(2) electrode. However, the surface modification by Pt NPs helped the composite electrode maintain higher fill factor values.

Thermal conduction effects impacting morphology during synthesis of columnar nanostructured TiO2 thin films

The aerosol chemical vapor deposition (ACVD) process allows for the synthesis of nanostructured films with well tuned morphologies that can be controlled based on the desired functionality and application. A robust understanding of the process parameters that result in desired features of the film is elucidated. One dimensional TiO2 nanostructured columns that have superior properties for solar energy harvesting and conversion applications were deposited on tin doped indium oxide (ITO) substrates. The sintering of the deposited particles was a key factor in the growth of the 1D structure with desired crystal planes. By ensuring that the sintering rate is faster than the arrival rate of deposited particles; a 1D columnar structure could be obtained. The sintering rate was controlled by the temperature and depositing particle size. As the columns grew in length, the increased thermal conduction resistance resulted in a drop in temperature and subsequently a slowing of the sintering process in upper regions of the film. This led to growth of branched structures rather than continued growth in a preferred direction. The growth of the branched structure could be overcome by enhancing the sintering rate by increasing the substrate temperature or reducing the depositing particle size (by lowering the feed rate of the precursor). The phenomenon was also confirmed by using different deposition substrates, such as FTO and glass. Dye sensitized solar cell performance efficiencies with different column lengths of 2 and 7 µm were determined to be 1.8 and 2.7% respectively.

Single-crystal semiconductor thin films for biohybrid photovoltaic devices

Although solar energy is freely available, the utility cost for electricity generated from photovoltaic (PV) modules is higher than that from coal or natural gas. Existing silicon (Si)-based PV devices are efficient, but the high manufacturing costs eventually contribute to the high utility cost. Energy generation from artificial photosynthesis, which borrows partial steps from natural photosynthesis, is a promising strategy for future energy supply.1 Photocatalytic metal oxides, such as titanium dioxide (TiO2/ thin films, are used for artificial photosynthesis to harvest solar energy. However, for the wide-scale use of metal oxide solar cells, it is essential to have low production costs and high efficiency. Thin-film morphology is a very important determinant of solar energy conversion efficiency. A 1D structure is particularly advantageous in photoelectrochemical applications. Several methods have been introduced to synthesize 1D metal oxide electrodes,2 but most of them are multistep processes that are hard to scale up. Our lab recently developed a novel thinfilm coating system called aerosol chemical vapor deposition (ACVD)3 that is simple and operates at atmospheric pressure. Conventional chemical vapor deposition takes place at low pressure, and particle formation is a nuisance for homogeneous and uniform deposition. In contrast, ACVD involves intentionally forming particles and depositing them on a substrate to form thin films with high surface area. ACVD involves feeding the precursor, for example, titanium tetra isopropoxide (TTIP) for TiO2 films, into a heated reactor (see Figure 1). The precursor decomposes at high temperatures into monomers that collide to form particles. The concentration gradient results in particles diffusing toward Figure 1. Aerosol chemical vapor deposition (ACVD) (left) and particle deposition in ACVD (right). N2: Nitrogen gas. ITO: Tin-doped indium oxide.