Revisiting the mechanisms of nitrite ions and ammonia removal from aqueous solutions: photolysis versus photocatalysis.

Nitrite ions and ammonia are widespread forms of inorganic water pollutants. Nevertheless, the mechanisms of their photolytic and photocatalytic reactions under UV-A irradiation are still fully undisclosed, particularly, at different pH values under aerobic and inert atmospheres. Herein, we have studied the photolytic decomposition of nitrite ions under different conditions using 365 nm UV-A LED as a light source instead of mercury lamps that emit photons in the UV-B region and generate a lot of heat. The results indicated that the rate of nitrite disproportionation in the dark at pH ≤ 3.0 is remarkably high relative to the rate of the photolytic decomposition. At pH ˃ 3, the photolytic reaction is negligible and nitrite ions showed considerable stability. In contrast, the photocatalytic oxidation of nitrite ions over TiO2 photocatalysts, namely, TiO2P25, TiO2UV100, and TiO2 anatase/brookite mixture proceeds at pH ˃ 3.0. TiO2 P25 exhibited the highest photocatalytic activity at pH 5. Interestingly, the photolytic simultaneous removal of nitrite ions and ammonia was possible at pH 9.0 in the absence of oxygen (Ar atmosphere). A 42.69 ± 0.66%, 27.75 ± 1.7%, and 32.74 ± 0.59% of nitrogen calculated based on nitrite, ammonia, and both of them, respectively, can be removed after 6 h of UV-A irradiation. The selectivity of N2 evolution was 77.6%. The nitrogen removal rate was significantly reduced in the presence of TiO2 photocatalyst evincing that TiO2 photocatalysis is applicable for nitrite ions oxidation, whereas the photolytic process is better suited for the simultaneous removal of nitrite ions and ammonia.

Semi-automated EPR system for direct monitoring the photocatalytic activity of TiO2 suspension using TEMPOL model compound.

The photocatalytic activity of TiO2 nanoparticles in aqueous solutions is commonly evaluated by monitoring the rate of methylene blue bleaching and phenols degradation, but both substrates suffer from many drawbacks, e.g., the high capacity of dark adsorption, self-degradation, and photosensitization. Besides, filtration is always required to separate the particulate photocatalyst before the analysis. Herein, we investigated the potential use of electron paramagnetic resonance (EPR) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) to directly monitor the photocatalytic activity of TiO2 suspensions without the need for filtration. The results showed that TEMPOL aqueous solution is in the dark and under UV-A illumination, does not absorb UV-A and visible light, and has negligible dark adsorption. The influence of TEMPOL concentration, light intensity, and TiO2 loading on the photocatalytic deactivation rate has been investigated. The mechanisms of TEMPOL deactivation in the presence and absence of oxygen as well as in the presence of methanol •OH radicals' scavenger have been discussed. The photocatalytic deactivation products have been analyzed using EPR, 1H-NMR, and 13C-NMR spectroscopies. It is found that the deactivation of TEMPOL is initiated by •OH radicals and α-H abstraction from the 4-piperidine position followed by the formation of TEMPONE (4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl) and 4-oxo-2,2,6,6-tetramethylpiperidine). In the presence of methanol, the formed α-hydroxyl radicals (•CH2OH) attack the nitroxide side of TEMPOL and produce 4-hydroxy-tetramethylpiperidine. Same activity trends have been observed for the photocatalytic methanol oxidation and TEMPOL deactivation over different types of TiO2 photocatalysts evincing that the proposed method has a potential for direct monitoring of the activities of photocatalyst suspensions.

Structure-Activity Relationships of Hierarchical Three-Dimensional Electrodes with Photosystem II for Semiartificial Photosynthesis.

Semiartificial photosynthesis integrates photosynthetic enzymes with artificial electronics, which is an emerging approach to reroute the natural photoelectrogenetic pathways for sustainable fuel and chemical synthesis. However, the reduced catalytic activity of enzymes in bioelectrodes limits the overall performance and further applications in fuel production. Here, we show new insights into factors that affect the photoelectrogenesis in a model system consisting of photosystem II and three-dimensional indium tin oxide and graphene electrodes. Confocal fluorescence microscopy and in situ surface-sensitive infrared spectroscopy are employed to probe the enzyme distribution and penetration within electrode scaffolds of different structures, which is further correlated with protein film-photoelectrochemistry to establish relationships between the electrode architecture and enzyme activity. We find that the hierarchical structure of electrodes mainly influences the protein loading but not the enzyme activity. Photoactivity is more limited by light intensity and electronic communication at the biointerface. This study provides guidelines for maximizing the performance of semiartificial photosynthesis and also presents a set of methodologies to probe the photoactive biofilms in three-dimensional electrodes.

BiVO4 Photoanode with NiV2O6 Back Contact Interfacial Layer for Improved Hole-Diffusion Length and Photoelectrochemical Water Oxidation Activity.

The short hole diffusion length (HDL) and high interfacial recombination are among the main drawbacks of semiconductor-based solar energy systems. Surface passivation and introducing an interfacial layer are recognized for enhancing HDL and charge carrier separation. Herein, we introduced a facile recipe to design a pinholes-free BiVO4 photoanode with a NiV2O6 back contact interfacial (BCI) layer, marking a significant advancement in the HDL and photoelectrochemical activity. The fabricated BiVO4 photoanode with NiV2O6 BCI layer exhibits a 2-fold increase in the HDL compared to pristine BiVO4. Despite this improvement, we found that the front surface recombination still hinders the water oxidation process, as revealed by photoelectrochemical (PEC) studies employing Na2SO3 electron donors and by intensity-modulated photocurrent spectroscopy measurements. To address this limitation, the surface of the NiV2O6/BiVO4 photoanode was passivated with a cobalt phosphate electrocatalyst, resulting in a dramatic enhancement in the PEC performance. The optimized photoanode achieved a stable photocurrent density of 4.8 mA cm-2 at 1.23 VRHE, which is 12-fold higher than that of the pristine BiVO4 photoanode. Density Functional Theory (DFT) simulations revealed an abrupt electrostatic potential transition at the NiV2O6/BiVO4 interface with BiVO4 being more negative than NiV2O6. A strong built-in electric field is thus generated at the interface and drifts photogenerated electrons toward the NiV2O6 BCI layer and photogenerated holes toward the BiVO4 top layer. As a result, the back-surface recombination is minimized, and ultimately, the HDL is extended in agreement with the experimental findings.

Brookite versus anatase TiO2 photocatalysts: phase transformations and photocatalytic activities.

Titanium dioxide nanoparticles consisting of pure anatase, anatase-rich, brookite-rich, and pure brookite modifications were synthesized and characterized by X-ray diffraction, field emission-scanning electron microscopy and nitrogen adsorption. The phase transformations among the three modifications of TiO2 (anatase, brookite, and rutile) and the photocatalytic activities of these nanoparticles were investigated by heat treatment over the temperature range from 400 to 800 °C and by the photooxidation of methanol, respectively. Direct transformation of anatase and brookite to rutile was observed, while in the case of the anatase-brookite mixture, anatase transforms firstly to brookite and then to rutile. The photocatalytic activity measurements indicate that brookite nanoparticles exhibit higher photocatalytic activities than anatase, and a comparable activity to that of the anatase-rich nanoparticles. The phase transformations and photocatalytic results are discussed regarding their dependence on crystallite size, surface area, and phase composition.

Mesoporous TiO2 nanostructures: a route to minimize Pt loading on titania photocatalysts for hydrogen production.

Mesostructured TiO(2) nanocrystals have been prepared using Pluronic F127 as the structure-directing agent. Platinum nanoparticles at different contents (0.1-1.0 wt%) have been photochemically deposited onto the mesoporous TiO(2). TEM investigation of 0.2 wt% Pt/TiO(2) calcined at 450 °C reveals that the TiO(2) particles are quite uniform in size and shape with the particle sizes of TiO(2) and Pt being 10 and 3 nm, respectively. The photocatalytic activities of the Pt loaded TiO(2) have been assessed and compared with those of nonporous commercial Pt/TiO(2)-P25 by determining the rates and the photonic efficiencies of molecular hydrogen production from aqueous methanol solutions. The results show that the amount of hydrogen evolved on Pt/TiO(2)-450 at low Pt loading (0.2 wt%) is three times higher than that evolved on Pt/TiO(2)-P25 and twelve times higher than that evolved on Pt/TiO(2)-350. Despite the BET surface area of the TiO(2)-450 photocatalyst being 3.5 times higher than that of TiO(2)-P25, a 60% smaller amount of the Pt co-catalyst is required to obtain the optimum photocatalytic hydrogen production activity. The reduced Pt loading on the mesoporous TiO(2) will be important both from a commercial and an ecological point of view.

New application for TiO2 P25 photocatalyst: A case study of photoelectrochemical sensing of nitrite ions.

Developing photoelectrochemical (PEC) sensors based on photocatalytic materials has recently attracted great interest as an emerging technology for environmental monitoring. TiO2 P25 is a well-known highly active photocatalyst, cheap, and produced commercially on a large scale. In the current work, a practical and durable TiO2-based PEC sensor has been fabricated by immobilizing TiO2 P25 nanoparticles at disposable screen-printed carbon substrates using drop-casting method. The fabricated PEC sensor has been applied for the anodic-detection and determination of nitrite (NO2-) ions under UV(A) light (LED, 365 nm) using chronoamperometry (CA) and differential pulse voltammetry (DPV). Linear calibration curves were obtained between the photocurrent responses and the concentrations of NO2- ions in the ranges of 0.1-5.0 and 0.5-10 mg L-1 for CA and DPV, respectively. Surprisingly, the detection limits (sensitivities) of the fabricated sensor towards NO2- ions under light were enhanced by a factor of 4.75 (4.1) and 8.3 (37.4) for CA and DPV, respectively, in comparsion with those measured in the dark. It is found that the photo-excitation of TiO2 facilitates the photooxidation of NO2- ions via the photo-generated holes whereas the photogenerated electrons contribute to the enhanced photocurrent and consequently the enhanced detection limit and sensitivity. The fabricated TiO2-based PEC sensor exhibits a good stability, durability, and satisfying selectivity for NO2- ions determination. These results indicate that the TiO2-based PEC sensor fabricated by utilizing cheap and commercially available components has great potential for being transferred from lab-to-factory.

Physical Insights into Band Bending in Pristine and Co-Pi-Modified BiVO4 Photoanodes with Dramatically Enhanced Solar Water Splitting Efficiency.

Herein, a novel method is introduced to synthesize 3D hierarchically assembled BiVO4 nanosheet photoanodes. Despite the fact that the obtained photoanodes inherit the intrinsic properties of 2D and 3D structures, they generate low photocurrent under simulated solar light at 1.0 sun. Upon modification with the cobalt-phosphate (Co-Pi) cocatalyst, the photocurrent is dramatically enhanced from 0.41 to 3.32 mA cm-2 at 1.23 VRHE. Charge-transfer kinetic studies by intensity-modulated photocurrent spectroscopy indicated that the low photocurrent response is mainly due to the high density of surface states, which pin the Fermi level and suspend the band bending. The Co-Pi loading passivates these surface states, unpins the Fermi level, and thus resumes the band bending. It also greatly enhances the rate constant of charge transfer and the overall efficiency, evincing that Co-Pi exhibits a dual function (i.e., passivation and catalysis). The current results explicitly disclose the role of the Co-Pi cocatalyst in photoelectrochemical solar water splitting on BiVO4.

Visible Light-Driven Photoelectrocatalytic Water Splitting Using Z-Scheme Ag-Decorated MoS2/RGO/NiWO4 Heterostructure.

Herein, we have successfully constructed a solid-state Z-scheme photosystem with enhanced light absorption capacity by combining the optoelectrical properties of AgNPs with those of the MoS2/RGO/NiWO4 (Ag-MRGON) heterostructure. The Ag-MRGON Z-scheme system demonstrates improved photo-electrochemical (PEC) water-splitting performance in terms of applied bias photon-to-current conversion efficiency (ABPE), which is 0.52%, and 17.3- and 4.3-times better than those of pristine MoS2 and MoS2/NiWO4 photoanodes, respectively. The application of AgNPs as an optical property enhancer and RGO as an electron mediator improved the photocurrent density of Ag-MRGON to 3.5 mA/cm2 and suppressed the charge recombination to attain the photostability of ∼2 h. Moreover, the photocurrent onset potential of the Ag-MRGON heterojunction (i.e., 0.61 VRHE) is cathodically shifted compared to those of NiWO4 (0.83 VRHE), MoS2 (0.80 VRHE), and MoS2/NiWO4 heterojunction (0.73 VRHE). The improved PEC water-splitting performance in terms of ABPE, photocurrent density, and onset potential is attributed to the facilitated charge transfer through the RGO mediator, the plasmonic effect of AgNPs, and the proper energy band alignments with the thermodynamic potentials of hydrogen and oxygen evolution.

Photocatalytic hydrogen production from biomass-derived compounds: a case study of citric acid.

Highly crystalline anatase TiO2 nanoparticles with high BET surface area have been synthesized by thermal hydrolysis of titanium(IV) bis(ammoniumlactato) dihydroxide aqueous solutions. The photocatalytic H2 production from aqueous citric acid (CA) solutions over Pt-loaded TiO2 has been investigated under different experimental conditions, that is, different CA concentration, temperature, light intensity, and pH of Pt/TiO2 suspension. For comparison, the photocatalytic dehydrogenation of triethanolamine (TEA) has also been investigated. The highest H2 production rates were obtained at pH 3 and 9 for CA and TEA, respectively. This behavior is readily explained by the adsorption characteristic of the employed reagent on the surface of the charged TiO2. The effect of the photocatalyst loading and the light intensity on the H2 production rate showed the same behavior in the case of CA and TEA evincing that these parameters are catalyst dependent. The apparent activation energies have been determined to be 13.5 ± 1.8 and 14.7 ± 1.6 kJ mol(-1) for CA and TEA, respectively, indicating the existence of an activation energy barrier in a photocatalytic process which can be attributed to the desorption of adsorbed products.

A Facile Surface Passivation of Hematite Photoanodes with TiO2 Overlayers for Efficient Solar Water Splitting.

The surface modification of semiconductor photoelectrodes with passivation overlayers has recently attracted great attention as an effective strategy to improve the charge-separation and charge-transfer processes across semiconductor-liquid interfaces. It is usually carried out by employing the sophisticated atomic layer deposition technique, which relies on reactive and expensive metalorganic compounds and vacuum processing, both of which are significant obstacles toward large-scale applications. In this paper, a facile water-based solution method has been developed for the modification of nanostructured hematite photoanode with TiO2 overlayers using a water-soluble titanium complex (i.e., titanium bis(ammonium lactate) dihydroxide, TALH). The thus-fabricated nanostructured hematite photoanodes have been characterized by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Photoelectrochemical measurements indicated that a nanostructured hematite photoanodes modified with a TiO2 overlayer exhibited a photocurrent response ca. 4.5 times higher (i.e., 1.2 mA cm(-2) vs RHE) than that obtained on the bare hematite photoanode (i.e., 0.27 mA cm(-2) vs RHE) measured under standard illumination conditions. Moreover, a cathodic shift of ca. 190 mV in the water oxidation onset potential was achieved. These results are discussed and explored on the basis of steady-state polarization, transient photocurrent response, open-circuit potential, intensity-modulated photocurrent spectroscopy, and impedance spectroscopy measurements. It is concluded that the TiO2 overlayer passivates the surface states and suppresses the surface electron-hole recombination, thus increasing the generated photovoltage and the band bending. The present method for the hematite electrode modification with a TiO2 overlayer is effective and simple and might find broad applications in the development of stable and high-performance photoelectrodes.

Nano-sized quaternary CuGa2In3S8 as an efficient photocatalyst for solar hydrogen production.

The synthesis of quaternary metal sulfide (QMS) nanocrystals is challenging because of the difficulty to control their stoichiometry and phase structure. Herein, quaternary CuGa2In3S8 photocatalysts with a primary particle size of ≈4 nm are synthesized using a facile hot-injection method by fine-tuning the sulfur source injection temperature and aging time. Characterization of the samples reveals that quaternary CuGa2In3S8 nanocrystals exhibit n-type semiconductor characteristics with a transition band gap of ≈1.8 eV. Their flatband potential is located at -0.56 V versus the standard hydrogen electrode at pH 6.0 and is shifted cathodically by 0.75 V in solutions with pH values greater than 12.0. Under optimized conditions, the 1.0 wt % Ru-loaded CuGa2In3S8 photocatalyst exhibits a photocatalytic H2 evolution response up to 700 nm and an apparent quantum efficiency of (6.9±0.5) % at 560 nm. These results indicate clearly that QMS nanocrystals have great potential as nano-photocatalysts for solar H2 production.

Enhanced photocatalytic production of molecular hydrogen on TiO(2) modified with Pt-polypyrrole nanocomposites.

Titanium dioxide was modified with Pt-polypyrrole nanocomposites through the in situ simultaneous reduction of Pt(iv) and the oxidative polymerization of pyrrole monomers at ambient temperature. The modified powders were characterized using X-ray photoelectron spectroscopy (XPS), dark-field scanning transmission electron microscopy (DF-STEM), infrared spectroscopy (IR) and by the determination of the BET surface area by nitrogen adsorption. Photocatalytic hydrogen production tests were performed employing 75 ml aqueous solution containing 2250 mumol methanol as the sacrificial electron donor. The obtained results show that 0.5 and 1.0 wt% Pt and polypyrrole, respectively, are the optimum ratios for high photocatalytic H(2) production rates. The amount of H(2) evolved during 5 h of UV-vis illumination of the suspension of Pt-polypyrrole modified TiO(2) powder is three times higher than that obtained with Pt-loaded TiO(2) prepared by a photochemical deposition method. The photonic efficiencies of the H(2) production employing 75 ml aqueous solution containing 370 mmol methanol were calculated to be 10.6 +/- 0.5 and 4.5 +/- 0.2% for TiO(2) modified with Pt-polypyrrole nanocomposites and for Pt-loaded TiO(2) prepared by a photochemical deposition method, respectively. A synergistic effect between Pt nanoparticles and polypyrrole leading to a better separation of the charge carriers is proposed to explain the enhanced reactivity of the newly synthesized photocatalyst.