Wan In Lee

Size-dependent light-scattering effects of nanoporous TiO2 spheres in dye-sensitized solar cells

Submicron-sized monodispersed TiO2 spheres (SPs) with high porosity were synthesized by a controlled hydrolysis of titanium tetraisopropoxide (TTIP) and subsequent hydrothermal treatment at 230 °C. By adjusting the ratio of TTIP to water (the r-factor) in the hydrolysis reaction, the diameters of SPs were selectively controlled to 260, 350, 450, 560, 800, and 980 nm. The prepared SPs in the pure anatase phase were highly porous structures with crystallite sizes of ∼15 nm and surface areas of 101–121 m2g−1. The synthesized nanoporous SPs in different sizes were then applied as the light-scattering layer (LSL) of dye-sensitized solar cells (DSCs) for efficient utilization of solar spectrum, and the size-dependent light-scattering effects of those SPs were systematically investigated. The 450 nm sized SP (SP450) provided the highest light-scattering efficiency among those in the 260–800 nm range. Relatively higher efficiency is caused by the characteristic light-scattering effect based on its unique diameter and also by the photonic reflection effect originating from its size-uniformity and long-range ordering. As a result the photovoltaic conversion efficiency (η) of DSC was improved from 6.92 to 9.04% with introducing the nanoporous SP450 as LSL.

Design of visible-light photocatalysts by coupling of narrow bandgap semiconductors and TiO2: effect of their relative energy band positions on the photocatalytic efficiency

According to relative energy band positions between TiO2 and visible-light-absorbing semiconductors, three different types of heterojunction were designed, and their visible-light photocatalytic efficiencies were analyzed. In Type-A heterojunction, the conduction band (CB) level of sensitizer is positioned at a more negative side than that of TiO2, whereas in Type-B system its valence band (VB) level is more positive than that of TiO2 and in Type-C system the sensitizer energy level is located between the CB and VB of TiO2. In evolving CO2 from gaseous 2-propanol (IP) under visible-light irradiation, the Type-B systems such as FeTiO3/TiO2, Ag3PO4/TiO2, W18O49/TiO2, and Sb-doped SnO2 (ATO)/TiO2 demonstrated noticeably higher photocatalytic efficiency than the Type-A such as CdS/TiO2 and CdSe/TiO2, while the Type-C such as NiTiO3/TiO2, CoTiO3/TiO2, and Fe2O3/TiO2 did not show any appreciable improvement. Remarkably high visible-light photocatalytic activity of Type-B heterojunction structures could be explained by inter-semiconductor hole-transfer mechanism between the VB of the sensitizer and that of TiO2. The evidence for the hole-transport between sensitizer and TiO2 was also obtained by monitoring the hole-scavenging reactions with iodide (I−) and 1,4-terephthalic acid (TA).

Porous silicon as an agent for cancer thermotherapy based on near-infrared light irradiation†

In recent years, new thermotherapies based on nanoshells and more recently single-wall carbon nanotubes (SWCNTs) in combination with near-infrared (NIR) light irradiation have received significant attention as efficient techniques to destroy cancer cells selectively. Very recently we have reported that porous silicon (PSi) can also be utilized as a therapeutic agent that generates heat sufficient to kill cancer cells without toxicity upon exposure to NIR light. In this paper, we report the heat generation abilities of a PSi suspension, a PSi/NaCl suspension and a PSi/phosphate-buffered saline (PBS) suspension during continuous irradiation with NIR light and the in vitrocell test results obtained by using thermotherapy based on PSi and NIR light irradiation. The PSi/NaCl suspension showed heat generation ability superior to those of the PSi suspension and PSi/PBS suspension. The temperature of the PSi/NaCl suspension was elevated to 55 and 76 °C after 3 and 20 min NIR irradiation at 300 mW cm−2, respectively, while that of the control was elevated to 31 and 39 °C after 3 and 20 min, respectively. In vitrocell test results suggest that thermotherapy based on PSi in combination with NIR light irradiation is an efficient technique to destroy cancer cells selectively without damaging the surrounding healthy cells and that heterochromatic NIR radiation can also be utilized successfully for this purpose.

Hierarchical mesoporous anatase TiO2 nanostructures with efficient photocatalytic and photovoltaic performances

Mesoporous anatase TiO2 nanostructures with diverse morphologies ranging from rod-like 1D nanostructures to diverse hedgehog-like 3D hierarchical nanostructures have been found to display highly efficient photocatalytic and photovoltaic performances. Hierarchical 3D nanostructures exhibit 5.5 times higher photocatalytic activity than rod-like 1D nanostructures, and the DSSC efficiency of hierarchical 3D nanostructures becomes as high as 5.37%, which is 50% higher than the DSSC efficiency of 1D nanostructures. The high photocatalytic performances of the hierarchical 3D anatase TiO2 nanostructures have been attributed mainly to their large specific pore volume, whereas the high photovoltaic performances of hierarchical 3D nanostructures have been attributed not only to their high specific surface area and the large specific pore volume but also to their interparticular connectivity. Comparative experimental studies have shown that the as-prepared hierarchical mesoporous anatase TiO2 nanostructures show superior photocatalytic (three times higher) and photovoltaic performances (35% higher) to commercially available Degussa P25 TiO2.

Enhancement of photovoltaic properties of CH3NH3PbBr3 heterojunction solar cells by modifying mesoporous TiO2 surfaces with carboxyl groups

In a new heterojunction solar cell employing CH3NH3PbBr3 (MAPbBr3) as the light absorber, we found that the introduction of a carboxylate monolayer on the mesoporous TiO2 surfaces significantly enhances JSC as well as VOC. In particular, the presence of a bromoacetate monolayer at the interface of TiO2/MAPbBr3 remarkably increases the photovoltaic conversion efficiency (PCE) from 2.65% to 5.57% with JSC of 5.411 mA cm−2, VOC of 1.372 V, and FF of 0.75. Time-resolved photoluminescence measurements indicate that the presence of the interfacial carboxylate groups expedites electron injection from MAPbBr3 to TiO2. Furthermore, according to pulsed light-induced transient measurements (PLITM) of photocurrent analysis, the lifetime of photoinjected electrons (τe) in the TiO2 conduction band (CB) significantly increases with the passivation of the interface, implying the suppression of charge recombination as well.

The properties of porous silicon as a therapeutic agent via the new photodynamic therapy

Photodynamic therapy (PDT) is a very useful approach for cancer treatment, but it has a few short-term and long-term side effects arising from reactive oxygen species (ROS) generation. Recently a new photodynamic therapy (PDT) based not on the ROS generation capability of photosensitizers but on the heat generation capability of carbon nanotubes (CNT) combined with a near-infrared (NIR) light irradiation technique has received significant attention. Our experimental results show that PSi can also be utilized as a therapeutic agent that generates sufficient heat to kill cancer cells without toxicity. The surface temperature of PSi increases as high and as quickly as that of CNT, but PSi was found to produce a smaller amount of ROS than CNT during NIR light irradiation. In addition, we developed a new method to effectively measure the amount of the ROS produced by nanomaterial photosensitizers including porous silicon (PSi) and CNT. The analysis results show that this method is reliable and reproducible.

Double-heterojunction structure of SbxSn1-xO2/TiO2/CdSe for efficient decomposition of gaseous 2-propanol under visible-light irradiation

Highly crystallized antimony-doped tin oxide (ATO; SbxSn1-xO2, x = 0.1) of ∼50 nm size was prepared by co-precipitation of SnCl4·5H2O and SbCl3, followed by heat-treatment at 1000 °C. The prepared ATO nanoparticles of deep blue color revealed a profound light-absorption in the visible range. ATO/TiO2 composites were prepared by covering the surface of ATO nanoparticles with TiO2 using the sol–gel method. Under visible-light irradiation (λ ≥ 420 nm), the prepared ATO/TiO2 showed a notable photocatalytic efficiency in decomposing gaseous 2-propanol (IP), which seemed to be caused by the hole-transfer mechanism between the valence bands (VB) of ATO and TiO2, since the ATOs VB level is located lower than that of TiO2. Subsequently, a double-heterojunction ATO/TiO2/CdSe structure was prepared by loading CdSe quantum dots (QDs) onto the surface of the ATO/TiO2, which dramatically enhanced the visible-light photocatalytic efficiency. In fact, the catalytic activity of ATO/TiO2/CdSe in evolving CO2 from IP, was ∼3 times that of ATO/TiO2 and twice that of typical N-doped TiO2. The unexpectedly high efficiency of ATO/TiO2/CdSe seemingly is due to the unique band matching among these semiconductors. With sensitization of ATO and CdSe, not only the holes but also the electrons are generated in the VB and CB, respectively, of TiO2 under visible-light irradiation.

Visible-Light Photocatalytic Properties of W18O49/TiO2 and WO3/TiO2 Heterocomposites

Dark blue W18O49 nanorod (NR) with ~50xa0nm length and ~4xa0nm width was synthesized by thermally induced crystal growth process. Pale yellow WO3 NR with the same dimension was also prepared by post-annealing the as-prepared W18O49 NR at 400xa0°C in air. The W18O49/TiO2 and WO3/TiO2 nanocomposites were then formed by coupling the corresponding NRs and TiO2 by sol–gel process. Under visible-light irradiation (λxa0≥xa0422xa0nm), the W18O49/TiO2 showed a remarkably high photocatalytic activity in removing gaseous 2-propanol (IP) and in evolving CO2. Its efficiency (6.9xa0ppm CO2 in 2xa0h) was more than twice that of the WO3/TiO2, which seems to be caused by a profound visible-light absorbance of W18O49 NR.Graphical Abstract

Annealing-free preparation of anatase TiO2 nanopopcorns on Ti foil via a hydrothermal process and their photocatalytic and photovoltaic applications

Uniform and well-defined nanopopcorns of the tetragonal anatase TiO2 having an average diameter of 670 nm have been facilely grown on Ti foil via a hydrothermal method and characterized by analyzing electron microscopic images and electron diffraction patterns as well as X-ray photoelectron, photoluminescence, and Raman spectra. The morphology of TiO2 nanostructures on Ti foil has been controlled well by adjusting the volume ratio of H2O2u2006:u2006HFu2006:u2006H2O, VR(H2O2u2006:u2006HFu2006:u2006H2O). Truncated tetragonal pyramidal TiO2 nanocrystals exposing the {001} and {101} facets have grown on the surface of TiO2 nanostructures exposing the {001} facets to produce anatase TiO2 nanopopcorns. Without being treated via any annealing process, our well-defined TiO2 nanopopcorns on Ti foil have been directly employed for photocatalytic materials and dye-sensitized solar cells. Among our prepared samples, anatase TiO2 nanopopcorns grown on Ti foil at a VR(H2O2u2006:u2006HFu2006:u2006H2O) of 1u2006:u20061u2006:u20061000 have shown the most reduced oxygen vacancy luminescence, the highest photocatalytic activity for the degradation of methylene blue, and the highest photovoltaic conversion efficiency of 3.98% as the working electrode of a dye-sensitized solar cell.

Facile fabrication of anatase TiO2 nanotube arrays having high photocatalytic and photovoltaic performances by anodization of titanium in mixed viscous solvents

Highly ordered, vertically aligned, one side-opened, and regularly porous anatase TiO2 nanotube arrays (TNAs) have been facilely grown by anodizing Ti foil in mixed viscous solvents of ethylene glycol (EG) and glycerol. By changing the volume ratio of two solvents, we have controlled the structural properties of TNAs such as tube diameters, wall thicknesses, and tube lengths. Our prepared TNAs have been found to have enhanced (004) planes, which are reactive in catalysis reactions. We have demonstrated that TNAs grown in 2:1 (v/v) EG and glycerol have the lowest band-gap energy and the largest mean crystallite diameter. TNAs grown on Ti foil have been directly employed for photocatalytic materials and the working electrode of photovoltaic dye-sensitized solar cells. Among our prepared samples, TNAs grown in 2:1 (v/v) EG and glycerol have shown the best photocatalytic activity for the degradation of methylene blue and the highest photovoltaic conversion efficiency of 4.08xa0%.