Visible Light Sensitive Photocatalyst Research Articles

Preparation of Hydroxynaphthalene-Modified TiO2 via Formation of Surface Complexes and their Applications in the Photocatalytic Reduction of Nitrobenzene under Visible-Light Irradiation

Visible light-sensitive photocatalysts were designed by the surface modification of TiO(2) using hydroxynaphthalenes, i.e., mono- and dihydroxy derivatives of naphthalene, via formation of surface complexes. The differences in the coordination ability on TiO(2) were confirmed depending on the numbers and substitution patterns of hydroxy groups. The relatively intense visible light absorption was observed after modification of TiO(2) surface by colorless 2,3-dihydroxynaphthalene (2,3-DN) having two neighboring hydroxy groups. Platinized TiO(2) (Pt-TiO(2)) modified with suitable amount of 2,3-DN showed higher photocatalytic performances and good reusability in the reduction of nitrobenzene to aminobenzene with relatively high selectivity under irradiation of visible light (λ > 420 nm).

Preparation and characterization of visible light sensitive nano titanium dioxide photocatalyst

Dye sensitizers loaded on TiO2 decrease the electron excitation energy, thereby improving the photocatalytic performance by causing an increase in sensitivity under visible light irradiation. Three dye sensitizer precursors, Mordant Orange 1, N3 (red dye) and Cobalt (II) Phthalocyanine Tetrasulfonate (CoPcTs), were utilized to load the photocatalyst. The rate of the electron trapping process on platinum is clearly compatible with the migration rate of boundary electrons. Consequently, the migration of boundary electrons from the conduction band towards electron acceptors is increased by loading platinum onto the titanium dioxide.In this research, TiO2 was synthesized from a titanium isopropoxide precursor using the sol–gel method, and the product was compared with commercial TiO2-P25 (consisting of 80% anatase and 20% rutile phases). Evaluation of the effect of calcination temperature on the formation of the nanosize photocatalyst and the phase formed was performed through the XRD technique. The specific surface area was determined by BET measurement. Ultimately, the absorption of all samples and their efficiencies were compared using the Diffuse Reflectance Spectroscopy (DRS) technique under visible light irradiation.

Effect of α-Fe2O3 addition on the morphological, optical and decolorization properties of ZnO nanostructures

Visible light sensitive photocatalysts of Fe2O3/ZnO nanocomposites were prepared by a simple solid-state reaction method, using zinc acetate, α-Fe2O3 and sodium hydroxide at room temperature. The products were characterized by scanning electron microscopy, powder X-ray diffraction, N2 adsorption–desorption measurement, UV–vis absorption, and photoluminescence spectroscopy and used for photodecolorization of Congo red. The characterization results showed that the morphology, crystallite size, BET surface area and optical absorption of the samples varied significantly with the Fe3+ to Zn2+ ratios. The nanocomposites show two absorption edges at ultraviolet and visible region. The optical band gap values of these nanocomposites were calculated to be about 3.98–3.81eV and 2.88–2.98eV, which show a red shift from that of pure ZnO. These red shifts are related to the formation of Fe s-levels below the conductive band edge of ZnO and effectively extend the absorption edge into the visible region. The growth mechanisms of the samples are proposed. These nanocomposites showed high decolorization ability in visible light with wavelength up to about 400nm. Among the samples, Fe2O3/ZnO nanoflower (molar ratio of Fe3+ to Zn2+ is 1:100) exhibited higher decolorization efficiency than the other nanocomposites. It could be considered as a promising photocatalyst for dyes treatment.

Synthesis and photocatalytic activity of mesoporous cerium doped TiO 2 as visible light sensitive photocatalyst

Cerium doped titania materials were synthesized varying the cerium concentration from 0 to 10 wt%. Materials are characterised by XRD, TEM, XPS and N 2 adsorption desorption method. Surface area and visible light absorption substantially increases and crystallite size decreases with the increasing cerium content. Cerium doping stabilizes the anatase phase and surface area even at 600 °C calcination. Photocatalytic activity towards methylene blue decomposition and selenium (IV) reduction is found to increase with the cerium content up to 5 wt% and then decreases. Materials calcined at 600 °C shows better activity than that calcined at 400 °C, even though surface area decreases. Anatase crystallinity mostly decides the photocatalytic activity rather than only surface area. It can be concluded that the optimum visible light absorption and oxygen vacancy with 5% cerium doping enhances the photocatalytic activity. In addition photocatalytic performance is found to depend on the presence of Ce 4+/Ce 3+ rather than only visible light absorption.

A Combination of Two Visible-Light Responsive Photocatalysts for Achieving the Z-Scheme in the Solid State

The light reaction in natural photosynthesis is generally recognized as one of the most efficient mechanisms for converting solar energy into other energy sources. We report herein on a novel strategy for generating H(2) fuel via an artificial Z-scheme mechanism by mimicking the natural photosynthesis that occurs in green plants. Designing a desirable photocatalyst by mimicking the Z-scheme mechanism leads to a conduction band that is sufficiently high to reduce protons, thus decreasing the probability of charge recombination. We combined two visible light sensitive photocatalysts, CdS and carbon-doped TiO(2), with different band structures. The used of this combination, that is, CdS/Au/TiO(1.96)C(0.04), resulted in the successful transfer of photogenerated electrons to a higher energy level in the form of the letter 'Z'. The system produced about a 4 times higher amount of H(2) under irradiation by visible light than CdS/Au/TiO(2). The findings reported herein describe an innovative route to harvesting energy by mimicking natural photosynthesis, and is independent of fossil fuels.

A new family of BiO(ClxBr1−x) visible light sensitive photocatalysts

A new family of BiO(ClxBr1 − x) photocatalysts have been synthesized by a hydrothermal method. The photocatalytic activity of the new material was measured on the degradation of Rhodamine B (RhB) and Acetophenone (AP) and photooxidation of potassium iodide in water under UV–vis and visible light (λ ≥ 420 nm) irradiations. In comparison to Degussa P25, the new photoactive material with x = 0.5 demonstrated 3 times higher rate in removing aqueous RhB under visible light irradiation. A morphology, chemical composition and crystalline structure of BiO(ClxBr1 − x) were characterized using scanning electron microscopy and powder X-ray diffraction. The values of their unit cell parameters depend on Cl/Br ratio and they change according to the Vegard's law.

Development of visible light sensitive titania photocatalysts by combined nitrogen and silver doping

In this study we present the effects of non-metal (nitrogen) and metal/non-metal (silver/nitrogen) dopants on titanium dioxide (TiO 2) in synthesising visible-light reactive photocatalysts. Nanopowders of TiO 2-N and TiO 2-N–Ag were synthesised using a simple procedure at room temperature. For nitrogen doping, a dispersion of Degussa P-25 was treated with ammonium hydroxide. The obtained modified catalyst was further treated with silver nitrate powder to facilitate silver-nitrogen co-doping. The produced catalysts were characterised using X-ray diffraction, X-ray photoelectron spectroscopy, and specific surface area measurements. Scanning electron microscopy/energy dispersive X-ray and transmission electron microscopy were adapted to detect changes in the morphology and in the chemical composition of synthesised catalysts. The results show that both the morphology and appearance of catalysts were modified to yield nanopowders of yellowish color and relatively high specific surface area. Methylene blue (MB) dye was used as a model aquatic contaminant in 23 mg/L concentration to study the performance of these novel photocatalysts in an aerobic mixed batch reactor system under white light irradiation. Both nitrogen and nitrogen-silver co-doping lead to visible light sensitivity and the new catalysts showed remarkable activities in the decolorisation of MB.

Influence of Indium Doping on the Activity of Gallium Oxynitride for Water Splitting under Visible Light Irradiation

Visible-light-active indium-doped gallium oxynitrides with a wurtzite-like structure were synthesized from nitridation of In(OH)3-containing Ga(OH)3 under NH3 flow at 625 °C and used for photocatalytic water splitting. This synthesis method yielded a homogeneous In distribution in gallium oxynitride solid solutions for Ga replacement levels of up to 1%. An appropriate amount of In substitution for Ga, approximately 0.5%, significantly enhanced the activity of gallium oxynitride in the visible-light-induced evolutions of H2 and O2 gases from methanol and AgNO3 solutions, respectively. X-ray photoelectron spectroscopic analysis indicated that In doping increased the dispersion of hybridized orbitals in the valence band of gallium oxynitride. This can enhance charge mobility in gallium oxynitride and thus photocatalytic activity. A higher degree of In doping resulted in nucleation of InN-like oxynitride on the gallium oxynitride surface and degraded the photocatalytic activity. This study demonstrates that band structure engineering of gallium oxynitride powders with In doping is a facile procedure for obtaining visible-light sensitive photocatalysts with high activities.

Formation of Ag+–N–TiO2 nanochains and their HPA-composites as highly visible light-sensitive photocatalysts toward two-color solar cells

Ag+-coupled N-doped TiO2 (Ag+–N–TiO2) nanochains were newly synthesized by combination of sol–gel and hydrothermal methods, and their structures were characterized by TEM, SEM, and XPS. The UV-visible diffuse reflectance absorption spectrum of Ag+–N–TiO2 nanochains was observed to absorb visible light in the broad range of 400–800 nm as compared to neat N–TiO2, exhibiting efficient visible light-photocatalytic activity. Their visible light absorption band became much broader and stronger upon formation of composites with heteropoly phosphotungstic acid (HPA), and their photocatalytic efficiency was observed to be further enhanced. The Ag+–N–TiO2 nanochains also exhibited improved photovoltaic performance with significantly high photoenergy conversion efficiency (PCE) (5.6%) under AM 1.5 (100 mW cm−2) as compared to N–TiO2 nanochains. The PCE was observed to be further enhanced to 7.6% by formation of HPA-composites. These results suggest that Ag and HPA play important roles in enhancement of PCE through the cooperative Z-scheme process of the electron transport triggered by two-color visible lights as in the plant photosynthetic system. The HPA-composites were also found to be useful for the dye-sensitized solar cells (DSSCs) as proved by synergistic enhancement of PCE to 10.2% as they are additionally incorporated with Ru-dye.

Fabrication and characterization of multi-metal co-doped titania films for a water-splitting reaction

Anchored visible-light-absorbing TiO(2) films have been synthesized by the layer-by-layer method on a quartz slide substrate as a new class of visible light-sensitive photocatalyst. UV-vis, XRD and XPS spectra show that W and Mn enter the TiO(2) lattices and partially substitute for Ti, and that W appears to have a solubility limit into the anatase structure. The Mn and W dopants cause new electronic states above the valence-band edge of pure TiO(2), and the new electronic states may be directly related to the visible-light absorption of doped TiO(2) films. A constant H(2) generation rate is obtained for long periods of time for all the investigated TiO(2) films, and the H(2) production rates for titania films doped with 0.74 at% W (relative to Ti) are 4.1 and 3.3 times higher than that of non-doped TiO(2) under UV and visible light, respectively, as the dopant atoms not only restrict the band gap to the visible region, but also facilitate the detrapping of charge carriers to the surface of the catalyst.

Facile preparation of high performance visible light sensitive photo-catalysts

Since the conversion of solar energy to electric or chemical forms is essential for achieving a sustainable society, the development of novel photo-catalytic materials is a subject of considerable interest. Here we report on the facile synthesis of a visible light sensitive photo-catalyst by doping a large amount of carbon into TiO2 nanoparticles. The findings show that carbon is clearly incorporated into the TiO2 framework, as evidenced by TPO, XANES, and EXAFS. The photo-catalytic activity of the carbon-doped TiO2 is increased by more than 4 times, compared to commercial TiO2 P25 for the degradation of phenol under irradiation by white light. Additionally, we observed for the first time that the carbon-doped TiO2 shows catalytic activity under irradiation by visible light at wavelengths longer than 600nm. The synthetic method developed in this study is very simple, and doping level of carbon can be easily controlled by changing the synthetic conditions used.

Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates

Anchored visible light-absorbing metal oxide nanoclusters, consisting of trivalent cerium ions and Keggin-type polyoxotungstates (PW12O403− and CuIIPW11O397−), have been synthesized on the pore surface of mesoporous silica (MCM-41) as a new class of visible light sensitive photocatalysts. The diffuse reflectance UV−vis, FT-Raman, and Ce LIII-edge X-ray absorption measurements showed that grafting of trivalent cerium ions to the preimmobilized PW12O403− cluster on the silica surface provided a broad absorption tail extending from the UV region to 530 nm assigned to a metal-to-metal charge transfer (MMCT) transition of an oxo-bridged binuclear WVI−O−CeIII unit. Photoexcitation of WVI−O−CeIII → WV−O−CeIV MMCT in the anchored clusters initiated the photooxidation reactions of gaseous isopropanol to CO2, whereby WV and CeIV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW12O403− cluster, it was also confirmed that the present methods can be extended to metal-substituted polyoxotungstates. Grafting of trivalent cerium ions to the copper-monosubstituted cluster (CuIIPW11O397−) produced an oxo-bridged trinuclear CuII/WVI/CeIII unit and exhibited photoactivity 4 times higher than that for the binuclear WVI/CeIII cluster. CuII-substituted polyoxotungstates are known to initiate a multielectron dioxygen reduction reaction and have a unique redox property in that the two-electron reduction of CuII to Cu0 occurs in a concerted manner. Therefore, the enhanced photoactivity for the trinuclear unit demonstrates that the activation of a single Cu center from CuII to Cu0 is accomplished by sequential photoinduced MMCT through the confined WVI−O−CeIII linkages, which results in acceleration of the photocatalytic two-electron O2 reduction process.

Substitutional nitrogen-doped tin oxide single crystalline submicrorod arrays: Vertical growth, band gap tuning and visible light-driven photocatalysis

High-density substitutional N-doped SnO 2 submicrorod arrays were grown on Si and quartz substrates by catalysts-free reactive sputtering. Scanning electron microscope and high-resolution transmission electron microscopy results show that the submicrorods are vertically aligned single crystal with quasi-tetrahedral pyramid shape nanotip at the top end. The density and the shape of the submicrorods can be modulated by the nitrogen partial pressure. Ellipsometry and optical absorption characterization show that after substitutional N-doping, the band gap of N-doped SnO 2 submicrorod shifts toward visible light region (up to 624 nm), and the visible light absorption are significantly enhanced due to the band gap narrowing. The photodegradation of methylene blue by N-doped SnO 2 submicrorod under visible light illumination is demonstrated, and it was found that the surface-to-volume ratio plays an important role in achieving high photocatalytic reactivity. The SnO 2:N submicrorod arrays with visible light band gap may have potential applications in solar cells electrode and visible light sensitive photocatalyst.

Visible-light Sensitive Photocatalysts Triggered by Interfacial Charge Transfer, Grafting Metal Ions onto Titanium Dioxide

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Effects of molybdenum substitution on the photocatalytic behavior of BiVO4

Transition metal doping has always suffered from the disadvantage of reduction of photocatalytic activity because the dopant ions can form a discrete level in the forbidden band of the photocatalyst resulting in low mobility of electrons and holes in the dopant level and thus lowered activity. However, in this study the photocatalytic activity of an efficient visible-light sensitive photocatalyst, BiVO4, for water oxidation or organic compound degradation was found to be remarkably enhanced by molybdenum doping. The role of molybdenum doping in enhancing the photocatalytic activity of BiVO4 was investigated and discussed based on the changes of the surface acidity of the photocatalyst.

Photocatalytic activities of AgSbO 3 under visible light irradiation

A novel visible light sensitive photocatalyst, AgSbO 3 was prepared by a conventional solid-state reaction method. This oxide belonging to a cubic-pyrochlore structure can absorb visible light with wavelength up to about 480 nm. From the band structure calculation, we found that the top of the valence band consists of the hybridized Ag 4d and O 2p orbitals and the bottom of the conduction band mainly consists of the Ag 5s and the Sb 5s orbitals. Photocatalytic activities were evaluated using O 2 evolution from an aqueous silver nitrate solution and decomposition of gaseous 2-propanol under visible light irradiation. We found that AgSbO 3 shows a higher O 2 evolution activity than WO 3 and 2-propanol can be mineralized by the AgSbO 3 photocatalysis under visible light irradiation.

Novel Ag2ZnGeO4 photocatalyst for dye degradation under visible light irradiation

Abstract We report a multiple-metal oxide Ag2ZnGeO4 with cristobalite-related structure as a visible light-sensitive photocatalyst (Eg 2.29 eV) for dye degradation. The physical characteristics of the samples were examined by techniques such as XRD, UV–vis diffuse reflectance spectroscopy and SEM. The photocatalytic activity of Ag2ZnGeO4 was confirmed by Rhodamine B (RhB) and Orange II photodegradation in aqueous phase. After 360 min of visible light (λ > 420 nm) irradiation, the conversions of RhB and Orange II reached 100% and 69.2%, respectively. The electronic structure of Ag2ZnGeO4 was investigated by band structure calculation based on the density function theory (DFT). Results revealed that the hybridized Ag 4d10 and O 2p6 orbitals form the valence band top of Ag2ZnGeO4, which leads to a much narrowed band gap as compared with that of the Na2ZnGeO4 parent sample. We propose that the specific cristobalite structure of Ag2ZnGeO4 favors the mobility of photo-generated carriers, and that this contributes positively to the observed photocatalytic activity for dye degradation.

Design of All-Inorganic Molecular-Based Photocatalysts Sensitive to Visible Light: Ti(IV)−O−Ce(III) Bimetallic Assemblies on Mesoporous Silica

The first photocatalysis operated by the visible-light induced metal-to-metal charge transfer (MMCT) has been demonstrated for the Ti(IV)/Ce(III) bimetallic assemblies synthesized on the pore of mesoporous silica. The Ce LIII-edge XANES measurements combined with 18O-isotopic labeling as well as photoelectrochemical experiments proved the ability of Ti(IV)/Ce(III) assemblies to drive the site-specific photocatalytic reaction under visible-light irradiation. Photocatalytic experiments using the oxidative decomposition of 2-propanol showed that the quantum efficiency for Ti(IV)/Ce(III) bimetallic photocatalysts was remarkably higher than that for the one of the most active visible-light sensitive photocatalysts, nitrogen-doped TiO2. It was also found that the grafting of Ce(III) ions onto nanocrystalline TiO2 particles led to the appearance of intense MMCT band in the visible-light regions, thus verifying it as the generally applicable strategy to obtain visible-light photocatalysts. To date, the grafting o...

Visible light-assisted photocatalytic oxidation of organic pollutants using nitrogen-doped titania

An experimental research into the aqueous photocatalytic oxidation (PCO) of organic groundwater pollutants, methyl-tert-butyl ether (MTBE), tert-butyl alcohol, phenol, humic substances, 2-ethoxy ethanol and ethylene glycol was undertaken using visible light-sensitive nitrogen-doped titanium dioxide photocatalysts. Nitrogen-doped titania proved to be an effective photocatalyst for MTBE with its action comparable to and even surpassing that of Degussa P25. In contrast, with the other substances the photocatalysts showed negligible activity. This difference was explained by the poor adsorption properties of N-doped catalysts. The predominance of different PCO mechanisms dependent of the surface properties of N-TiO2 catalysts was elucidated.

Applications of Single-site Photocatalysts Implanted within the Silica Matrixes of Zeolite and Mesoporous Silica

Abstract The tetrahedrally coordinated metal oxide (titanium, vanadium, chromium, and molybdenum oxides) moieties can be implanted and isolated in the silica matrixes of microporous zeolite and mesoporous silica materials and named as “single-site photocatalysts.” Under UV-light irradiation these single-site photocatalysts form the charge-transfer excited state, i.e., the excited electron–hole pair state which localizes quite near to each other as compared to the electron and hole produced in semiconducting materials, plays a significant role in various photocatalytic reactions. Especially, the single-site titanium oxide photocatalyst demonstrates the high reactivity and selectivity under UV-light irradiation, while the single-site chromium oxide operates as a visible-light sensitive photocatalyst. These single-site photocatalysts not only can promote photocatalytic reactions but also can be utilized to synthesis of functional materials. The transparent mesoporous thin film with single-site photocatalyst generates the superhydrophilic surface. The nano-sized metal catalyst can be photodeposited on the excited single-site photocatalyst under UV-light irradiation.