Photoactivity Performance Research Articles

Synthesis, characterization and environmental application of silica grafted photoactive substances isolated from urban biowaste

Hybrid biowaste-derived silica-based systems show good photoactive performance, stability and reusability in the degradation of water pollutants.

Tungsten Oxide Coated Copper Oxide Nanowire Arrays for Enhanced Activity and Durability with Photo-Electro-Chemical Water Splitting

: Recently, we showed that the deposition of crystalline WO3 layers on copper oxide nanowire arrays to result in a five-fold improvement of photocurrent density (~ 1.7 mA/cm2 at 0 V 10 vs. RHE) over titania coated copper oxide NW arrays. Most importantly, the deposition of WO3 or CuWO4 reduced CuO phase impurity to Cu2O. Improvement in the phase purity of nanowire arrays, led to the enhancement in photoactivity.1 The origin of degradation in photoelectrochemical performance with time is currently being investigated. Introduction : Photoelectrochemical (PEC) water splitting using sunlight is an attractive approach for producing clean hydrogen towards reducing carbon dioxide emissions and meeting the growing global energy demand. The biggest obstacle with PEC water splitting is the unavailability of a suitable semiconductor with proper band gap and band edge energetics and durability. Cu2O is a promising semiconducting material for photo-electro-chemical water splitting due to its band gap (Eg = 2.0 eV) and its band edge 25 positions straddling the water oxidation and reduction reactions. Major drawbacks with Cu2O include its poor stability in aqueous solutions, short carrier diffusion lengths (20-100 nm) and high absorption depth (10 microns). A semiconductor with a 2 eV band gap is expected to yield a photocurrent density of about 15 mA/cm2. The photoactivity performance of Cu2O has been a major concern, in addition to its rapid dissolution in aqueous solution.One-dimensional nanostructures such as nanowires have the potential to produce high photoactivity due to fast charge transport properties and reduced length scales for minority carrier diffusion expected in single crystalline nanowires. In our previous work, we developed a rapid two-step process for the scalable synthesis of copper oxide nanowire arrays on copper foils.2 This involves a wet chemical oxidation of copper foils followed by oxidation in atmospheric microwave plasma. The nanowire arrays were subsequently coated with titania using atomic layer deposition to improve the aqueous stability. The photocurrents obtained with this method are much lower (<0.4 mA/cm2) but better than the data obtained with polycrystalline thin films to that date. The presence of a mixed phase of Cu2O and CuO has been identified as a possible cause for the low photoactivity. CuO has an indirect band gap of 1.4 eV leading to inefficient photon absorption and also cannot generate sufficient energy required to split water. Hence, phase purity of Cu2O is an important factor that needs to be addressed to improve the photoactivity of the Cu2O NW arrays made through atmospheric plasma oxidation. In this contribution, we investigated the use of n-type crystalline WO3 conformal layers on Cu2O nanowire arrays using hot wire chemical vapor deposition and studied their impact on photoactivity and durability. Experimental: Copper oxide nanowire arrays were synthesized by wet chemical oxidation followed by short exposure to atmospheric plasma. Copper foils were immersed in an aqueous solution of sodium hydroxide and ammonium persulfate. The copper hydroxide nanowires produced were converted into copper oxide nanowires by means of exposure to an atmospheric plasma discharge for 2 minutes. The details of the process are described elsewhere. The deposition of WO3 or CuWO4 was performed in a HWCVD reactor using oxygen flow over hot tungsten and copper filaments used as the sources for tungsten oxide and copper oxide vapors. Results and Discussion: The copper oxide nanowires are coated with titania layer using atomic layer deposition and tungsten oxide layers and copper tungstate layers using hot-wire chemical vapor deposition. The PEC characterization studies show 3-4 times enhancement in photoactivity for copper tungstate coated copper oxide nanowire arrays compared to that titania coated copper oxide nanowire arrays. See Figure 1. Acknowledgements: Financial support from US DOE (DE-FG02-07ER46375) is acknowledged.

SiO2/carbon nitride composite materials: The role of surfaces for enhanced photocatalysis

The effect of SiO2 nanoparticles on carbon nitride (C3N4) photoactivity performance is described. The composite SiO2–C3N4 materials exhibit a higher activity in the photo degradation of RhB dye. A detailed analysis of the chemical and optical properties of the composite C3N4 materials shows that the photo activity increases with higher SiO2 concentration. We found out that the presence of SiO2 nanoparticles strongly affects the fluorescence intensity of the matrix and life time by the creation of new energy states for charge transfer within the C3N4. Furthermore, the use of SiO2 in the synthesis of C3N4 leads to new morphology with higher surface area which results in another, secondary improvement of C3N4 photoactivity. The effect of different surfaces within C3N4 on its chemical and electronic properties is discussed and a tentative mechanism is proposed. The utilization of SiO2 nanoparticles improves both photophysical and chemical properties of C3N4 and opens new possibilities for further enhancement of C3N4 catalytic properties by the formation of composites with many other materials.

Microwave Synthesis of Zinc Oxide/Reduced Graphene Oxide Hybrid for Adsorption-Photocatalysis Application

This work reports on synthesis of zinc oxide/reduced graphene oxide (ZnO/rGO) nanocomposites in the presence of diethylenetriamine (DETA) via a facile microwave method. The X-ray diffraction (XRD) patterns of the nanocomposites correspond to the ZnO hexagonal phase wurtzite structure. The high-resolution transmission electron microscopy (HRTEM) images revealed that the ZnO nanorods, with an average length : diameter ratio of 10, were successfully deposited on the rGO sheets. Under the irradiation of sunlight, the nanocomposites showed enhanced adsorption-photocatalysis by more than twofold and photocurrent response by sixfold compared to the ZnO. The excellent photoactivity performance of the nanocomposites is contributed by smaller ZnO nanorod and the presence of rGO that acts as a photosensitizer by transferring electrons to the conduction band of ZnO within the nanocomposite during sunlight illumination.

Tailoring the current density to enhance photocatalytic activity of CuO/HY for decolorization of malachite green

CuO supported on HY zeolite (CuO/HY) catalyst was prepared via a simple electrolysis method under different levels of current density, and its properties and photoactive performance were investigated. The physicochemical properties of the catalyst were examined using X-ray Diffraction (XRD), surface area analysis, Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV–Vis DRS), Fourier Transform Infrared (FTIR) spectroscopy, and pyridine adsorption FTIR. The analyses indicated that CuO/HY catalyst contains both CuO nanoparticles and incorporation of copper into the HY framework. In parallel with the formation of Cu0 in the electrolysis system, the Cu2+ ions also underwent isomorphous substitution subsequent to dealumination to form a Si–Cu–O bond, as confirmed by the FTIR result. The formation of the Si–Cu–O bond was found to decrease at elevated current density and this lowered the photocatalytic decolorization of malachite green (MG) by CuO/HY. The CuO/HY prepared at 10mAcm−2 was the optimum catalyst and produced complete and 50% decolorization of 10 and 15mgL−1 of MG, respectively. The results indicating the decrease in chemical oxygen demand (COD) and total organic carbon (TOC) demonstrated the degradability of MG molecules. The CuO/HY is also stable and showed no leaching effect even after six reaction cycles, with just small decreases in the decolorization percentage (<89%). The catalyst has the potential to be applied in textile wastewater treatment which is always in low concentration level. It is also believed that this study will be useful for synthesis of other catalysts that necessitate a degree of isomorphous substitution of metal ions in a zeolite network.

Nanocomposite of Cu–TiO2–SiO2 with High Photoactive Performance for Degradation of Rhodamine B Dye in Aqueous Wastewater

The nanocomposite of Cu-TiO2-SiO2 photocatalyst have been prepared by a sol-gel method, which is used for the degradation of Rhodamine B (RB) as a probe that is notorious organic compound present in dyes wastewater. Morphological and structural characteristics of the Cu-TiO2-SiO2 nanocomposite were studied with low temperature N2 adsorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis diffuse reflectance spectroscopy (DRS). The Fourier transformed infrared spectroscopy (FT-IR) analysis shows the enhanced chemical bonding of O-Ti and O-Ti-O after the composition of Cu and SiO2 species into TiO2. It is found that the Cu-TiO2-SiO2 nanocomposite exhibits much higher photocatalytic activity under both UV light and visible light irradiation as compared with that over commercial titania (Degussa P25) toward the dyes wastewater containing RB. The photodegradation rate of RB (5 mg/L) can reach above 95.0% under sunlight after 3 h. The addition of SiO2 not only inhibites the crystal growth and anatase-to-rutile transformation of TiO2 nanocatalyst, but also enhances the adsorption of organic compounds. Cu-doping extends the light response to the visible region. Synergetic effects between Cu-SiO2 and TiO2 have been investigated, which provides a good way and material in the degradation field of dyes wastewater.

Evaluation of microwave plasma oxidation treatments for the fabrication of photoactive un-doped and carbon-doped TiO2 coatings

The photoactivity of both un-doped and carbon-doped titanium dioxide (TiO2) coatings has been widely reported. In this paper, the use of a microwave plasma as a novel oxidation treatment for the fabrication of these coatings is evaluated. The photoactivity performance of the microwave plasma-formed coatings is benchmarked against those fabricated through air furnace oxidation as well as those deposited using reactive magnetron sputtering. The un-doped and carbon-doped TiO2 coatings were prepared respectively by microwave plasma-oxidizing titanium metal sheets and sputter deposited titanium carbide thin films. The resulting oxides were characterized using XPS, XRD, FEG-SEM, and optical profilometry. The oxide layer thicknesses achieved over the 15 to 45minute oxidation times were in the range of 0.15 to 3.44μm. These coatings were considerably thicker than those obtained by air furnace oxidation. The microwave plasma-formed oxides also exhibited significantly higher surface roughness values compared with the magnetron-sputtered coatings. The photoactivity performance of both un-doped and carbon-doped coatings was assessed using photocurrent density measurements. Comparing the un-doped TiO2 coatings, it was observed that those obtained using the microwave plasma oxidation route yielded photocurrent density measurements that were 4.3 times higher than the TiO2 coatings of the same thickness that were deposited by sputtering. The microwave plasma-oxidized titanium carbide coatings did not perform as well as the un-doped TiO2 probably due to the presence of un-oxidized carbide in the coatings, which reduced their photoactivity.

Characterization and Photocatalytic Activity of N, Cd-Codoped TiO2

N, Cd-codoped TiO2 has been synthesized by sol-gel method coupling with thermolysis. The products were characterized by XRD, SEM, UV-visible DRS, XPS and BET analysis, respectively. The photocatalytic activities of samples were evaluated on the degradation of methyl orange (MO). It was showed that particle sizes of anatase photacatalysts were about 10-15 nm. The maximum specific surface area of the products was 102.7 m2⋅g-1. N atoms were incorporated into the TiO2 crystal lattice, while Cd atoms existed on the crystal surface. The optical absorption edge of N, Cd-codoped TiO2 was 435 nm, which illustrated that the product represented photoactivity in the visible region. It indicated a strong N-Cd synergistic interaction appeared to play a decisive role in driving the excellent photoactivity performance of N,Cd-codoped TiO2.

Conversion of amorphous TiO2 coatings into their crystalline form using a novel microwave plasma treatment

Crystalline titanium dioxide (TiO2) coatings have been widely used in photo-electrochemical solar cell applications. In this study, TiO2 and carbon-doped TiO2 coatings were deposited onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron sputtering system. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO2. This study investigates the use of a microwave plasma heat treatment as a means of achieving this crystalline conversion. The treatment involved placing the sputtered coatings into a 2.45GHz microwave-induced nitrogen plasma where they were heated to approximately 550°C. It was observed that for treatment times as short as 1min, the 0.25-μm thick coatings were converted into the anatase crystalline phase of TiO2. The coatings were further transformed into the rutile crystalline phase after treatments at higher temperatures. The doping of TiO2 with carbon was found to result in a reduction in this phase transformation temperature, with higher level of doping (up to 5.8% in this study) leading to lower anatase-to-rutile transition temperature. The photoactivity performance of both doped and un-doped coatings heat-treated using both furnace and microwave plasma was compared. The carbon-doped TiO2 exhibited a 29% increase in photocurrent density compared to that observed for the un-doped coating. Comparing carbon-doped coatings heat-treated using the furnace and microwave plasma, it was observed that the latter yielded a 19% increase in photocurrent density. This enhanced performance may be correlated to the differences in the coatings' surface morphology and band gap energy, both of which influence the coatings' photoabsorption efficiency.

N- and/or W-(co)doped TiO2-anatase catalysts: Effect of the calcination treatment on photoactivity

A series of nanosized W,N-codoped and single-doped N- and W-anatase-TiO2 catalysts have been prepared by a microemulsion method and calcined at different temperatures. The activity in the sunlight selective photo-oxidation of toluene and styrene has been correlated with structural, electronic, and surface examinations of the catalysts done with the help of XRD-Rietveld, N2 physisorption, X-ray photoelectron, infrared, electron paramagnetic resonance (EPR) and UV–vis spectroscopies. Irrespective of the reaction, W,N-codoped nanocatalysts showed an enhanced photoactivity with respect to bare anatase and single-doped N-TiO2 and W-TiO2 materials. A strong W–N synergistic interaction appears to play a decisive role in driving the excellent photoactivity performance of W,N-codoped materials by affecting (i) electronic properties, particularly maximizing the anatase band gap decrease and enhancing the subsequent visible light photon absorption, and (ii) surface properties, in turn related to the formation of OH radicals upon light excitation. The maximum photoactivity is reached by calcination at 450°C and is concomitantly observed with a near complete selectivity to partial oxidation products. Higher calcination temperatures yielded solids with significantly inferior photocatalytic performance. The properties of the W–N interaction are discussed as a function of the calcination temperature.

TiO2-based building materials: Above and beyond traditional applications

In the 1910s, TiO2 began to be used in building materials as pigments and opacifier due to its excellent optical property. Since the photocatalytic property of TiO2 was observed in 1972, its application field was expanded to air cleaning and sterilization. Thereafter, people added TiO2 into building materials to develop novel and facile building materials. These materials were widely used for air cleaning, sterilization, self-cleaning, anti-fogging, decoration, and building cooling. The combination of building and other functions can serve simultaneously. Although TiO2-based building materials have bright prospects, some aspects such as improving the stability and enhancing photoactive performance of the materials are of importance for future research.

Carbon coating of anatase-type TiO2 and photoactivity

Carbon coating of photocatalytic anatase-type TiO2 powders was successfully carried out by heating a powder mixture of poly(vinyl alcohol) and TiO2 under N2 gas flow at temperatures above 700 °C. All particles of TiO2 were covered by a carbon layer and looked black. For methylene blue (MB) in aqueous solution, carbon-coated anatase powders thus prepared showed marked adsorption and decomposition under UV irradiation and were confirmed to keep almost the same photoactivity as that without any carbon coating. Carbon coating of anatase-type TiO2 was found to give different merits for photoactive performance; suppression of phase transformation from anatase to rutile at high temperatures, quick adsorption of MB into the carbon layer, decomposition of adsorbed MB by TiO2 and prevention of the interaction of TiO2 with binder resin. Repeated cycles of adsorption and decomposition of methylene blue in aqueous solution were experimentally confirmed.