Atomistic understanding of enhanced selectivity in photocatalytic oxidation of benzyl alcohol to benzaldehyde using graphitic carbon nitride loaded with single copper atoms.

The loading of graphitic carbon nitride (gCN) with transition metals has received significant attention for efficient light-driven catalysis. However, the contribution of the loaded metals to enhanced performance remains unclear. In this study, Cu is loaded onto gCN to understand how photocatalytic activity is regulated by the loaded metals. Loading gCN with 3 wt% of Cu increases the electron population by 8.1 and 4.6 times under UV (λ < 370 nm) and visible light (390 < λ < 740 nm), respectively. This sample shows nearly 100% selectivity for oxidizing benzyl alcohol to benzaldehyde and a high yield-to-power ratio, reaching 0.35 mmol g-1 h-1 W-1. The loaded Cu species exist as single atoms with a +1-oxidation state. Each Cu+ cation is coordinated to two (at 3 wt% Cu) or four (at 6 wt% Cu) N atoms within the cavity of the gCN framework. Doubling the Cu loading results in a smaller electron population and coordinatively more saturated Cu+ cations, making it catalytically less reactive. Ab initio molecular dynamics simulations show that Cu+ cations produce filled mid-gap states above the valence band, which function as hole traps and hence oxidation centers. The Cu+ cation and the neighboring N atoms are electron-depletion and electron-accumulation sites due to Cu → N electron transfer, making it highly reactive for oxidative transformations via the hole transfer pathway. The role of Cu as a hole-transfer site updates the received understanding that surface-loaded Cu serves as an electron-accumulation site. A strong correlation is observed between the electron population at steady-state and the product yield, indicating that it could serve as a promising performance indicator for the design of future photocatalysts.

Reduced TiO2with prolonged electron lifetime for improving photocatalytic water reduction activity.

The reduction of anatase TiO2with NaBH4under argon atmosphere at a high temperature resulted in a longer electron lifetime and a larger electron population. The reduced gray anatase sample with disorder layer showed a higher evolution rate of H2(130.2µmol h-1g-1) compared to pristine TiO2(24.1µmol h-1g-1) in the presence of Pt co-catalyst in an aqueous glucose solution under exposure to ultraviolet light (λ⩽ 400 nm). Ti3+and oxygen vacancy defects were proposed to exist in the reduced TiO2. A continuum tail forms above the valence band edge top as a result of these two defects, which contribute to the lattice disorder. This is presumably also the case with the conduction band, which has a continuum tail composed of mid-gap states as a result of the defects. The Ti3+and oxygen vacancy defects operate as shallow traps for photoexcited electrons, thereby preventing recombination. Since the defects are primarily located at the surface, i.e. in the disorder layer, the photoexcited electrons in shallow traps hence become readily available for the reduction of H3O+into H2. The prolonged electron lifetime increases the photoexcited electron population in the reduced TiO2, resulting in enhanced water reduction activity.

Efficient electron extraction by CoS2loaded onto anatase TiO2for improved photocatalytic hydrogen evolution.

Titanium dioxide (TiO2) as a benchmark photocatalyst has been attracting attention due to its photocatalytic activity combined with photochemical stability. In particular, TiO2with anatase polymorph holds promise for driving reduction reactions, such as proton reduction to evolve H2via photocatalysis. In this study, anatase TiO2is loaded with CoS2through the hydrothermal route to form a CoS2@TiO2photocatalyst system. X-ray absorption near edge structure confirms the +2-oxidation state of the Co cation, while extended x-ray absorption fine structure shows that each Co2+cation is primarily coordinated to six S-anions forming a CoS2-like species. A small fraction of the Co2+species is also coordinated to O2-anions forming CoxOyspecies and substitutionally resides at the Ti4+-sites. Further investigations with steady-state IR absorption induced by UV-light and time-resolved microwave conductivity suggest an efficient electron transfer from the conduction band of TiO2to the surface-loaded CoS2which acts as a metallic material with no bandgap. The CoS2shallowly traps electrons at the host surface and facilitates proton reduction. An appreciably enhanced H2evolution rate (8 times) is recognised upon the CoS2loading. The CoS2is here proposed to function as a proton reduction cocatalyst, which can potentially be an alternative to noble metals.

Mechanistic understanding of the increased photoactivity of TiO2 nanosheets upon tantalum doping.

Anatase TiO2 is doped with Ta cations through a hydrothermal route. Based on X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectroscopy, the Ta dopants exist in the 5+-oxidation state. The oxidation state is insensitive to the Ta loading amount. Extended X-ray absorption fine structure spectroscopy confirms that the local structure around Ta cations is not identical between the Ta-doped samples. The Ta-O distance monotonically increases with the Ta loading amount due to a gradually expanding lattice. The Ta-doped samples show higher activity than pristine TiO2 for photomineralizing recalcitrant organics. The enhanced photocatalytic activity is proposed to be due to an enhanced population of photoexcited electrons, as probed using light-induced IR absorption spectroscopy, and an extended electron lifetime, as probed using time-resolved microwave conductivity, which are associated with the formation of Ti3+ defect states acting as shallow electron traps. The maximum photocatalytic activity is observed for TiO2 doped with 2 mol% of Ta, which shows enhancement of mineralization efficiency (about 3 times) and enhancement of electron population (up to 20 times), as compared to those of pristine TiO2. The fundamental question of why a proper metal doping into TiO2 increases photocatalytic activity is discussed in this study.

Mechanistic insights into the adsorption of methylene blue by particulate durian peel waste in water.

This study aims to investigate the adsorption of methylene blue (MB) over particulate durian peel waste, which is chemically activated with hydrogen peroxide. The equilibrium data are well described by the Freundlich isotherm model, which indicates that the MB adsorption takes place predominantly on multilayers and heterogeneous surfaces of the biosorbent. The Freundlich adsorption constants, KF and n, are 11.06 L/g and 2.94, respectively. Thermodynamic data suggest that the MB adsorption occurs spontaneously and endothermically. The enthalpy and entropy for the MB adsorption are obtained as 10.26 kJ/mol and 0.058 kJ/mol K, respectively, in the temperature range of 303-323 K. Based on the stepwise desorption method, the adsorption of MB is dominated by physical interactions, particularly hydrogen bonding.

The role of the shell in core-shell-structured La-doped NaTaO3 photocatalysts.

NaTaO3, a semiconductor with a perovskite structure, has long been known as a highly active photocatalyst for overall water splitting when appropriately doped with La cations. A profound understanding of the surface feature and why and how it may control the water splitting activity is critical because redox reactions take place at the surface. One surface feature characteristic of La-doped NaTaO3 is a La-rich layer (shell) capping La-poor bulk (core). In this study, we investigate the role of the shell in core-shell-structured La-doped NaTaO3 through systematic chemical etching with an aqueous HF solution. We find that the La-rich shell plays a role in electron-hole recombination, electron mobility and water splitting activity. The shallow electron traps populating the La-rich shell trap the photoexcited electrons, decreasing their mobility. The shallowly trapped electrons remain reactive and are readily available on the surface to be extracted by the cocatalysts for the reduction reaction evolving H2. The presently employed chemical etching method also confirms the presence of a La concentration gradient in the core that regulates the steady-state electron population and water splitting activity. Here, we successfully reveal the nanoarchitecture-photoactivity relationship of core-shell-structured La-doped NaTaO3 that thereby allows tuning of the surface features and spatial distribution of dopants to increase the concentration of photoexcited electrons and therefore the water splitting activity. By recognizing the key factors that control the photocatalytic properties of a highly active catalyst, we can then devise proper strategies to design new photocatalyst materials with breakthrough performances.

ZnO nanoparticles for photodegradation of humic acid in water.

Humic acid (HA) is the most important precursor of toxic disinfection byproducts upon chlorination. Removing HA from water body is therefore critical in drinking water acquisition. In this research, ZnO nanoparticles are employed for photocatalysis under UV light at neutral pH to remove HA from a water environment. Almost 100% degradation of HA was achieved using 0.3 g/L of ZnO in 180 min with UV-A and UV-C light. Under identical experimental conditions, total organic carbon (TOC) removals reach 67% and 21% with UV-A and UV-C light, respectively. A higher degree of mineralization of HA is achieved with UV-A light although the degradation of HA is slightly better with UV-C light. This indicates that ZnO/UV-A has relatively low selectivity to degrade different compounds, including various intermediates from HA degradation. The use of UV-A light is therefore recommended for ZnO as it possesses higher mineralization ability. Negligible TOC is observed on the ZnO surface after photocatalytic reactions. In contrast, the adsorption of HA in dark conditions reaches 42% in 180 min. This strongly indicates that the adsorption of HA plays an important role in the photocatalytic degradation of HA, but it is not the main process for HA removal.

Dopant site in indium-doped SrTiO3 photocatalysts.

Strontium titanate, SrTiO3, with the perovskite ABO3 structure is known as one of the most efficient photocatalyst materials for the overall water splitting reaction. Doping with appropriate metal cations at the A site or at the B site substantially increases the quantum yield to split water into H2 and O2. The site occupied by the guest dopant in the SrTiO3 host thus plays a key role in dictating the water splitting activity. However, little is known about the detailed structure of the dopant site in the host lattice. In this study, the local structure of In3+ cations, which were shown to improve the water splitting activity of SrTiO3, is investigated with X-ray absorption fine structure spectroscopy and density functional theory (DFT) calculations. The In3+ cations exclusively substitute for Ti4+ cations at the B site to form InO6 octahedra. Further optical experiments using UV-Vis diffuse reflectance spectroscopy and DFT calculations of the density of states indicate that the substitution of In3+ for Ti4+ does not alter the band structure and bandgap energy (remaining at 3.2 eV). The mechanism underlying the increased water splitting activity is discussed in relation to occupation of the B site by In3+ cations.

The atomic-scale structure of LaCrO3-NaTaO3 solid solution photocatalysts with enhanced electron population.

Visible light sensitization of sodium tantalate (NaTaO3), a highly UV-active material, is critical for realizing its practical application in photocatalytic water splitting under solar light. Double doping of a half-filled transition metal together with another metal for cationic charge balance is a promising way of sensitizing NaTaO3 to visible light. One fundamental issue is that the atomic-scale structure of such doubly doped NaTaO3 is not yet fully understood. In this study, we doubly doped NaTaO3 with La3+ and Cr3+ through a solid-state route. The occupation preference of La3+ in a doubly doped system was particularly studied by the extended X-ray absorption fine structure technique. We revealed the substitution of La3+ for Na+, and Cr3+ for Ta5+, forming a LaCrO3-NaTaO3 solid solution. We then showed that doping NaTaO3 with La3+ and Cr3+ appreciably increased the population of electrons photoexcited by either visible light or UV light. Photoactivation of the doubly doped NaTaO3 with visible light produced a population of electrons comparable to that under UV light. The charge compensation scheme of double doping with La3+ and Cr3+ is shown here to be a good option for the sensitization of NaTaO3 to visible light.

Structural properties and catalytic activity of a novel ternary CuO/gC3N4/Bi2O3 photocatalyst.

In the present study, CuO/gC3N4/Bi2O3 composite is constructed as a ternary visible light active photocatalyst. Since CuO plays a critical role in enhancing the photocatalytic activity of the formed composite, its structural properties are particularly studied using synchrotron X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES confirms the presence of Cu species with +2 oxidation state in the form of CuO. EXAFS furthermore confirms that each Cu cation is coordinated to four O anions in an approximately square planar configuration. The length of the Cu-O coordination is estimated to be 1.92 Å, slightly shorter than that of bulk CuO (1.95 Å). The CuO/gC3N4/Bi2O3 composite exhibits highly enhanced photocatalytic activity in the 2,4-dichlorophenol decomposition under visible light. The enhanced photocatalytic activity is due to the increased population of electrons and the successful consumption of the photoproduced electrons by the dissolved oxygen through the one-electron transfer reaction.

Comparison and mechanism of photocatalytic activities of N-ZnO and N-ZrO2 for the degradation of rhodamine 6G.

N-doped ZnO (N-ZnO) and N-doped ZrO2 (N-ZrO2) are synthesized by novel, simple thermal decomposition methods. The catalysts are evaluated for the degradation of rhodamine 6G (R6G) under visible and UV light. N-ZnO exhibits higher dye degradation under both visible and UV light compared to N-ZrO2 due to possessing higher specific surface area, lower crystalline size, and lower band gap. However, it is less reusable than N-ZrO2 and its photocatalytic activity is also deteriorated at low pH. At the same intensity of 3.5 W/m(2), UVC light is shown to be a better UV source for N-ZnO, while UVA light is more suitable for N-ZrO2. At pH 7 with initial dye concentration of 10 mg/L, catalyst concentration of 1 g/L, and UVC light, 94.3 % of R6G is degraded by N-ZnO within 2 h. Using UVA light under identical experimental conditions, 93.5 % degradation of R6G is obtained by N-ZrO2. Moreover, the type of light source is found to determine the reactive species produced in the R6G degradation by N-ZnO and N-ZrO2. Less oxidative reactive species such as superoxide radical and singlet oxygen play a major role in the degradation of R6G under visible light. On the contrary, highly oxidative hydroxyl radicals are predominant under UVC light. Based on the kinetic study, the adsorption of R6G on the catalyst surface is found to be the controlling step.

Rapid enhanced photocatalytic degradation of dyes using novel N-doped ZrO2.

A novel N-doped ZrO2 (N-ZrO2) photocatalyst is synthesized through thermal decomposition of zirconium hydroxide-urea complex and is characterized using various techniques, including XRD, FTIR, TGA, SEM, TEM, UV-DRS, XPS, XANES, and BET. The N-ZrO2 possesses pure monoclinic structure with high crystallinity. By using the proposed facile route of synthesis, both interstitial and substitutional N doping with high dopant stability can be realized. The optical properties of the catalyst are significantly altered after N doping, giving an optical response in the visible and near infrared regions and an additional strong absorption peak in the UVA region. The N-ZrO2 showed a higher photocatalytic activity than pristine ZrO2 for the degradation of amaranth (AM) and methylene blue (MB) under visible or UV light irradiation, which could be attributed to the band gap narrowing, higher specific area, smaller crystalline size, and higher availability of surface hydroxyl groups. Due to its molecular structure and light absorption characteristics, MB is easier to degrade than AM. Overall removal efficiencies, including adsorption and photolysis, for AM and MB by N-ZrO2 at pH 7 with initial dye concentration of 10 mg/L, catalyst concentration of 1 g/L, and visible light irradiation of 144.7 W/m(2) are 67.2 and 96%, respectively. Using UVA light of only 3.5 W/m(2) under identical experimental conditions, complete removal of MB and AM is obtained. The photocatalytically treated solution of either AM or MB is nontoxic against Bacillus cereus, an agriculturally important soil microorganism.