Photodegradation of CuBi2 O4 Films Evidenced by Fast Formation of Metallic Bi using Operando Surface-sensitive X-ray Scattering

CuBi2 O4 has recently emerged as a promising photocathode for photo-electrochemical (PEC) water splitting. However, its fast degradation under operation currently poses a limit to its application. Here, we report a novel method to study operando the semiconductor-electrolyte interface during PEC operation by surface-sensitive high-energy X-ray scattering. We find that a fast decrease in the generated photocurrents correlates directly with the formation of a metallic Bi phase. We further show that the slower formation of metallic Cu, as well as the dissolution of the electrode in contact with the electrolyte, further affect the CuBi2 O4 activity and morphology. Our study provides a comprehensive picture of the degradation mechanisms affecting CuBi2 O4 electrodes under operation and poses the methodological basis to investigate the photocorrosion processes affecting a wide range of PEC materials.

Enhancement of CdS resistance to photocorrosion and photocatalytic removal of uranyl by complexation with N-deficient g-C3N4under aerobic conditions

The effect of oxygen on the reduction of uranyl and photocorrosion of CdS remains a pressing issue when CdS is used as a photocatalyst for the removal of uranyl in uranium-containing wastewater. In this study, composites (CdS/PCN) were prepared by designing N-deficient g-C₃N₄ composite with CdS for efficient photocatalytic reduction of uranyl under aerobic condition. Meanwhile, a series of characterizations of the CdS/PCN composites were carried out by XRD, FT-IR, XPS, EDS and UV-vis. Surprisingly, the CdS/PCN not only showed very high photocatalytic reduction activity for uranyl under aerobic condition, but also the photocorrosion of CdS by oxygen and h⁺ was inhibited. With a starting uranium (VI) concentration of 20 ppm, the uranium (VI) removal efficiency could reach 97.33% (dark: 30 min, light: 10 min). Interestingly, the removal efficiency was better in air condition than in pure nitrogen or 30% oxygen atmosphere, i.e. a proper amount of oxygen has accelerated the reduction reaction, while excess oxygen weakened the reduction. Finally, a new mechanism of reduction of uranyl by CdS/PCN photocatalyst was given under aerobic condit ions. This work presents a novel strategy for reduction of U(VI) by photocatalysis and the inhibition of photocorrosion of photocatalysts under aerobic conditions.

Evaluating high temperature photoelectrocatalysis of TiO2 model photoanode

Sunlight concentration has been demonstrated as one promising strategy for practically photoelectrochemical (PEC) water splitting with exceeding 10% solar-to-hydrogen efficiency. However, the operating temperature of PEC devices, including the electrolyte and photoelectrodes, can be elevated to 65 ℃ naturally due to the concentrated sunlight and the thermal effect of near-infrared light. In this work, high temperature photoelectrocatalysis is evaluated using titanium dioxide (TiO₂) photoanode as a model system, which is believed to be one of the most stable semiconductors. During the studied temperature range of 25-65 ℃, a linear increment of photocurrent density with a positive coefficient of 5.02 μA cm^-2 K^-1 can be observed. The onset potential for water electrolysis shows a significant negative shift by 200 mV. An amorphous titanium hydroxide layer and a number of oxygen vacancies generate on the surface of TiO₂ nanorods, promoting the water oxidation kinetics. During long-term stability testing, the NaOH electrolyte degradation and TiO₂ photocorrosion at high temperatures could cause the decaying photocurrent. This work evaluates the high temperature photoelectrocatalysis of TiO₂ photoanode and reveals the mechanism of temperature effects on TiO₂ model photoanode.

Highly efficient photodegradation of magnetic GO-Fe3O4@SiO2@CdS for phenanthrene and pyrene: Mechanism insight and application assessment

A novel magnetic core-shell Fe₃O₄@SiO₂@CdS embedded graphene oxide (GO) composite was prepared for the visible-light-driven photodegradation of high ring number polycyclic aromatic hydrocarbons (PAHs). The potential application of GO-Fe₃O₄@SiO₂@CdS was evaluated through the photodegradation of phenanthrene and pyrene in deionized water, tap water, and lake water, respectively. It was found that GO-Fe₃O₄@SiO₂@CdS could remove 86.4 % of phenanthrene and 93.4 % of pyrene, suggesting its potential for the degradation of high-ring number PAHs. The density functional theory (DFT) calculations demonstrate that pyrene has more active sites attacked by free radicals. The photoelectrochemical measurement and quenching experiments indicate that GO can transfer photoelectrons efficiently, resulting in the crucial radicals (O₂^-, OH and ¹O₂). More importantly, the photocatalytic activity kept almost constant during five cycles, confirming the significant anti-photocorrosion of GO-Fe₃O₄@SiO₂@CdS. This work provides some new insights into the removal of PAHs with high-ring numbers in the natural water environment.

ZnO/Cu2O/g-C3N4 heterojunctions with enhanced photocatalytic activity for removal of hazardous antibiotics

In view of the environmental pollution caused by antibiotics, the creation of an efficient photocatalytic material is an effectual way to carry out water remediation. Herein, we developed a smart strategy to synthesize ZnO/Cu₂O/g-C₃N₄ heterojunction photocatalysts for the photodegradation of hazardous antibiotics by one-pot synthesis method. In this system, the Cu₂O nanoparticles with electrons reducing capacity were coupled with g-C₃N₄ composites. The photocarriers were generated from the electric field of type Ⅰ heterojunction between ZnO and g-C₃N₄ and type Ⅱ heterojunction between Cu₂O and g-C₃N₄. ZnO as a co-catalyst was doped to Cu₂O/g-C₃N₄ catalyst system for removal of broad-spectrum antibiotics with the condition of visible light to protect Cu₂O from photocorrosion, which thereby accelerated photocatalytic reactivity. Benefiting by new p-n-n heterojunction, the resulting ZnO/Cu₂O/g-C₃N₄ composites had an excellent degradation performance of broad-spectrum antibiotics such as tetracycline (TC), chlortetracycline (CTC), oxytetracycline (OTC) and ciprofloxacin (CIP), the degradation of which were 98.79%, 99.5%, 95.35% and 73.53%. In particular, ZnO/Cu₂O/g-C₃N₄ photocatalysts showed a very high degradation rate of 98.79% for TC in first 30 min under visible light, which was 1.35 and 10.62 times higher than that of Cu₂O/g-C₃N₄ and g-C₃N₄, respectively. This work gives a fresh visual aspect for simultaneously solving the instability deficiencies of traditional photocatalysts and improving photocatalytic performance.

Construction of bismuth-based porous carbon models by 3D printing technology for light-enhanced removal of chloride ions in wastewater

The bismuth oxide (Bi₂O₃) based chloride (Cl^-) removal method is one of the chemical precipitation methods possessing good selectivity and high removal efficiency of Cl^- ions, but Bi₂O₃ often appears in the powder form, which is difficult to be recovered for regeneration. In this work, the combination of 3D printing technology and the Bi₂O₃ method was explored to construct the resin model including the Bi-precursors. In the optimum carbonization process at 400 °C for 30 min, the Bi³⁺ ions of the Bi-precursor were reduced into the metallic Bi⁰ nanoparticles, whose surfaces were covered by the thin Bi₂O₃ layers to form the heterostructured Bi⁰/Bi₂O₃ core/shell nanoparticles with an average size of 43 nm. These Bi⁰/Bi₂O₃ nanoparticles were tightly adhered to the internal and external surfaces of the hierarchical porous carbon model (Bi-PCM), which greatly facilitated their regeneration and ensured the stable Cl^- removal performance. After five cycles of Cl^- removal, the chloride removal efficiency over the multiple Bi-PCMs in the dark and pH 1 conditions maintained at about 26%, which then largely increased to 63.6% with UV light irradiation. The light-enhanced mechanism was related to the improved release rate of Bi³⁺ ions caused by photocorrosion and the Cl^• radicals produced from the holes and the ^•OH and O₂^•- radicals, which quickly reacted with Bi₂O₃ to form BiOCl. The construction of Bi-PCMs by using 3D printing technology provides a very promising strategy for the removal of Cl^- ions from wastewater.

Research progress in metal sulfides for photocatalysis: From activity to stability

Metal sulfides are a type of reduction semiconductor photocatalysts with narrow bandgap and negative conduction band potential, which make them have unique photocatalytic performance in solar-to-fuel conversion and environmental purification. However, metal sulfides also suffer from low quantum efficiency and photocorrosion. In this review, the strategies to improve the photocatalytic activity of metal sulfide photocatalysts by stimulating the charge separation and improving light-harvesting ability are introduced, including morphology control, semiconductor coupling and surface modification. In addition, the recent research progress aiming at improving their photostability is also illustrated, such as, construction of hole transfer heterojunctions and deposition of hole transfer cocatalysts. Based on the electronic band structures, the applications of metal sulfides in photocatalysis, namely, hydrogen production, degradation of organic pollutants and reduction of CO₂, are summarized. Finally, the perspectives of the promising future of metal-sulfide based photocatalysts and the challenges remaining to overcome are also presented.

Cr(VI)-imprinted polymer wrapped on urchin-like Bi2S3 for reduced photocorrosion and improved photoreduction of aqueous Cr(VI)

Just like other metal sulfides, the misfortune of photocorrosion and undesired photogenerated electron-hole recombination for Bi₂S₃ was inevitable. In this work, a viable route to reduce photocorrosion of Bi₂S₃ and improve photoreduction of aqueous Cr(VI) was developed via "dressed" a Cr(VI) imprinting polymer (Cr(VI)-IP) on urchin-like Bi₂S₃ (U-Bi₂S₃). Cr(VI)-IP wrapped on the three dimensional U-Bi₂S₃ was implemented by a bulk polymerization. The wrapped Cr(VI)-IP enabled to fast enrich and adsorb Cr(VI) on U-Bi₂S₃ leading to improve the photoreduced efficiency of photogenerated carriers and restrain the photogenerated electron-hole recombination. What's more, Cr(VI)-IP wrapped on U-Bi₂S₃ was just like an "armor" which could support the three dimensional construction of U-Bi₂S₃ from the structural collapse of photocorrosion and retard the direct contact of oxygen and H₂O from the surrounding media. As expected, the obtained U-Bi₂S₃@Cr(VI)-IP exhibited higher photostability, adsorption, photoreduction capacities towards the target Cr(VI) than the bare U-Bi₂S₃. The photocatalytic kinetic constant of U-Bi₂S₃@Cr(VI)-IP was 6 times higher than U-Bi₂S₃. After 3 times recycling uses, the morphology, crystal structure and chemical constitution of U-Bi₂S₃@Cr(VI)-IP were maintained. In addition, the removal efficiency of Cr(VI) by U-Bi₂S₃@Cr(VI)-IP was kept at 58% whereas U-Bi₂S₃ was almost lost to zero.

Novel magnetically separable tetrahedral Ag3PO4/NrGO/CuFe2O4 photocatalyst for efficient detoxification of 2,4-dichlorophenol

An effective as well as eco-friendly photodegradation methods by atoxic and easily reusable photocatalysts are essential for wastewater treatment. Silver phosphate (Ag₃PO₄) specifically in tetrahedral shape is one of the superior catalysts under visible light but its photocorrosion, poor electron transfer ability and low surface adsorption properties limits its applications. Combination of Ag₃PO₄ and nitrogen doped reduced graphene oxide (NrGO) having higher in surface area, ample functional groups and hetero atom doping is expected to get over the problem. Further addition of a spinel ferrite (CuFe₂O₄) could enhance the visible light response activity and helps in easy separation of catalyst for reuse. Given the merits of Ag₃PO₄, NrGO and CuFe₂O₄ we rationally integrated a novel magnetically separable stable Ag₃PO₄/NrGO/CuFe₂O₄ photocatalyst for efficient detoxification of 2,4-dichlorophenol (2,4-DCP). About 95.3% degradation efficiency was achieved by Ag₃PO₄/NrGO/CuFe₂O₄ (k = 0.01978 min^-1) which was ~2.6 times higher than pure Ag₃PO₄ (k = 0.00747 min^-1) in 60 min of visible light irradiation. The Ag₃PO₄/NrGO/CuFe₂O₄ heterojunction was able to separate and recycle easily using an external magnetic field due to its strong magnetism, and after 5 recycles it showed 88.6% of degradation efficiency revealed its higher stability and recyclability. The photocatalytic mechanism of Ag₃PO₄/NrGO/CuFe₂O₄ was explained by heterojunction energy-band theory. In addition, the plausible intermediate products of 2,4-dichlorophenol were analyzed by ESI/LC-MS and proposed the pathway. Moreover, the phytotoxicity was also studied on V. radiata in which GI (germination index) was found to be 11.97% before degradation, while 80.31% of GI was observed in 60 min of degradation which revealed that more significant reduction in toxicity was attained on this photodegradation.

Improving photocatalytic activity by construction of immobilized Z-scheme CdS/Au/TiO2 nanobelt photocatalyst for eliminating norfloxacin from water

To improve the photocatalytic activity of TiO₂ NBs under irradiation of solar light, an immobilized Z-scheme composite photocatalyst CdS/Au/TiO₂ NBs has been constructed. For the unique architectures, the TiO₂ NBs provide more absorption and reaction sites, the CdS nanoparticles enhance overall light harvesting, and Au acts as the electron transfer mediator, promoting the interfacial charge transfer and efficient separation of electrons and holes. The morphology, elements, crystal structure, optical and photoelectrochemical properties, and photocatalytic activity of CdS/Au/TiO₂ NBs were characterized. Results showed that CdS/Au/TiO₂ NBs possesses higher photocatalytic activity toward the degradation of antibiotic norfloxacin under irradiation of simulated sunlight, which is attributed to the synergetic interaction of increased light absorption and separation of photogenerated electrons and holes. Besides, the degradation of norfloxacin was promoted by HCO₃^-, but inhibited by NO₃^- and Cl^-. The radicals trapping experiments proved that superoxide radicals (O₂^-) was the dominating active species during the photocatalysis process. The photocatalytic degradation products of norfloxacin was analyzed, and nine intermediates were identified. Moreover, the photocatalytic degradation mechanism and photostability of CdS/Au/TiO₂ NBs were analyzed in detail. The matched energy levels and unique ternary Z-scheme design are the key for improved photocatalytic activity. The deactivation of CdS/Au/TiO₂ NBs after recycles mainly due to the release of CdS by photocorrosion and the loss of deposited Au.

Oxygen-deficient bismuth tungstate and bismuth oxide composite photoanode with improved photostability

A homogeneous layer of Bi₂O₃-Bi₁₄WO₂₄ composite (BWO/Bi₂O₃) thin film was fabricated using a combination of electrodeposition and thermal treatment. The evenly distributed Bi₁₄WO₂₄ component within the Bi₂O₃ layer was found to be important in stabilising the photoelectrochemical performances of Bi₂O₃ photoanode by promoting the photoelectron transport. The unmodified Bi₂O₃ suffered from severe photocorrosion as proven by X-ray diffraction (XRD) and inductively coupled plasma (ICP) analyses while the composite thin film was active without noticeable activity decay for at least 3 h of illumination. This strategy might be applicable to other photocatalysts with stability issues.

Hierarchical magnetic petal-like Fe3O4-ZnO@g-C3N4 for removal of sulfamethoxazole, suppression of photocorrosion, by-products identification and toxicity assessment

Herein, a petal-like photocatalyst, Fe₃O₄-ZnO@g-C₃N₄ (FZG) with different g-C₃N₄ to ZnO ratios was synthesized with hierarchical structure. The FZG1 photocatalyst, having the weight ratio of 1:1 for the initial urea and Fe₃O₄-ZnO (Fe-ZnO), presented the highest sulfamethoxazole (SMX) degradation rate of 0.0351 (min^-1), which was 2.6 times higher than that of pristine ZnO. Besides the facile separation, the performance of photocatalyst was improved due to the function of iron oxide as an electron acceptor that reduced the electron/hole recombination rate. The coating of g-C₃N₄ on the Fe-ZnO surface not only acted as a protective layer for ZnO against photocorrosion, but it also enhanced the photocatalytic activity of the catalyst for SMX degradation through the heterojunction mechanism. By using the FZG1 photocatalyst, 95% SMX removal was obtained after 90 min reaction, while 47% COD and 30% TOC removal were achieved after 60 min treatment under a low energy-consuming UV lamp (10 W). Moreover, a substantial reduction in the solution toxicity was shown after the treatment, as compared with the SMX solution before treatment. The LC-HR-MS/MS analysis results showed that the concentration of most detected by-products produced after 90 min reaction by FZG1 was considerably lower than those obtained using other synthesized photocatalysts. By performing radical scavenging experiments, OH^° radical was found to be the major reactive species. The FZG1 photocatalyst also displayed excellent reusability in five cycles and the leaching of zinc and iron ions was reduced by 54% and ∼100%, respectively, after coating Fe-ZnO with g-C₃N₄.

Enhanced photocatalytic degradation of sulfamethoxazole by zinc oxide photocatalyst in the presence of fluoride ions: Optimization of parameters and toxicological evaluation

The presence of antibiotics in water bodies has received increasing attention since they are continuously introduced and detected in the environment and may cause unpredictable environmental hazards and risks. The photocatalytic degradation of sulfamethoxazole (SMX) by ZnO in the presence of fluoride ions (F-ZnO) was evaluated. The effects of operating parameters on the efficiency of SMX removal were investigated by using response surface methodology (RSM). Under the optimum condition, i.e. photocatalyst dosage = 1.48 g/L, pH 4.7, airflow rate = 2.5 L/min and the concentration of fluoride ions = 2.505 mM, about 97% SMX removal was achieved by F-ZnO after 30 min of reaction. The mechanism of reactions, COD removal efficiency and reaction kinetics were also investigated under optimum operating conditions. In addition, about 85% COD reduction was obtained after 90 min photocatalytic reaction. The pseudo-first-order kinetics rate constants for the photodegradation of SMX were found to be 0.099, 0.058 and 0.048 min^-1 by F-ZnO, ZnO and TiO₂ (P25), respectively. The figure-of-merit electrical energy per order (E_EO) was used for estimating the electrical energy efficiency, which was shown to be considerably lower than the energy consumption for the reported research on removal of SMX by photocatalytic degradation under UV irradiation. Toxicity assays were conducted by measuring the inhibition percentage (PI) towards E. coli bacteria strain and by agar well diffusion method. The results showed that after 30 min of reaction, the toxicity of the treated solutions by all photocatalysts fell within the non-toxic range; however, the reduction in toxicity by F-ZnO was faster than those by ZnO and P25. Despite the positive effects of surface fluorination of ZnO on the SMX and COD removal and reaction kinetics, its lower stability compared to ZnO and P25 in the repeated experiments gave rise to some doubts about its performance from a practical point of view.

Growth of Escherichia coli on the GaAs (001) surface

Detection of pathogenic bacteria and monitoring their susceptibility to antibiotics are of great importance in the fields of medicine, pharmaceutical research, as well as water and food industries. In order to develop a photonic biosensor for detection of bacteria by taking advantage of photoluminescence (PL) of GaAs-based devices, we have investigated the capture and growth of Escherichia coli K12 on bare and biofunctionalized surfaces of GaAs (001) - a material of interest for capping different semiconductor microstructures. The results were compared with the capture and growth of Escherichia coli K12 on Au surfaces that have commonly been applied for studying a variety of biological and biochemical reactions. We found that neither GaAs nor Au-coated glass wafers placed in Petri dishes inoculated with bacteria inhibited bacterial growth in nutrient agar, regardless of the wafers being bare or biofunctionalized. However, the capture and growth of bacteria on biofunctionalized surfaces of GaAs and Au wafers kept in a flow cell and exposed to different concentrations of bacteria and growth medium revealed that the initial surface coverage and the subsequent bacterial growth were dependent on the biofunctionalization architecture, with antibody-coated surfaces clearly being most efficient in capturing bacteria and offering better conditions for growth of bacteria. We have observed that, as long as the GaAs wafers were exposed to bacterial suspensions at concentrations of at least 10⁵ CFU/mL, bacteria could grow on the surface of wafers, regardless of the type of biofunctionalization architecture used to capture the bacteria. These results provide important insight towards the successful development of GaAs-based devices designed for photonic monitoring of bacterial reactions to different biochemical environments.

Microwave-assisted photocatalysis of neurotoxin compounds using metal oxides quantum dots/nanosheets composites: Photocorrosion inhibition, reusability and antibacterial activity studies

Water pollution caused by different pollutants is one of the challenging tasks for the scientific community. We have prepared and characterized a material for removal of pollutant compounds. ZnO quantum dots decorated CuO nanosheets and TiO₂ quantum dots decorated WO₃ nanosheets composites have been prepared using a hydrothermal method. The as synthesized catalysts were characterized by various techniques. The crystallite sizes of CuO NSs and WO₃ NSs were to be obtained 12.5 and 13.25nm and when dopped with ZnO and TiO₂ size reduces to 3.2 and 3.9nm, respectively. The energy band gap of the CuO NSs, WO₃ NSs, ZnO QDs/CuO NSs and TiO₂ QDs/WO₃ NSs composite are calculated to be 2.01, 2.61, 1.86 and 2.32eV, respectively. The prepared catalysts are efficiently utilized for the photocatalytic degradation of two neurotoxin compounds under UV and UV coupled with microwave irradiation. The prepared catalyst composites reveal excellent photocatalytic degradation of neurotoxin compound by degrading it up to 75% under UV and UV/microwave irradiation. The photocalysis efficiency in UV/microwave system is higher than UV system. The result shows that the ZnO QDs/CuO NSs and TiO₂ QDs/WO₃ NSs composites have excellent photocorrosion inhibition and reusability properties. Thus, prepared samples with positive surface potential upon interaction with negative surface potential of Enterococcus faecalis and Micrococcus luteus.