A new application of high-efficient silver salts-based photocatalyst under natural indoor weak light for wastewater cleaning

Regulating band structure, charge transfer and separation, oxygen adsorption and activation by surface ion modification

As a most promising environmental technology, the substantial enhancement of photocatalytic efficiency is still a big challenge for practical applications. In this work, the surface of Bi2O2CO3 (BOC) nanotubes are modified by Cl and I. The as-obtained samples at different hydrothermal temperatures (T) are designated as T-X-BOC (X = Cl, I). X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) prove that Cl and I merely chemically adsorb on the BOC surface, rather than dope into the crystal lattice. The surface modification of Cl and I slightly increases light absorption range, while significantly promotes the photoelectron migration from bulk to the surface that greatly enhances the carrier separation efficiency. Density functional theory (DFT) calculations further prove that surface Cl and I have adjusted band structure and surface charge distribution. Besides, the surface Cl and I favor the O2 adsorption and trap the surface photoelectrons, thus promoting the formation of •O2-; while the surface Cl and I impede the surface adsorption of H2O, thus refraining the generation of •OH. In the degradation of rhodamine B (RhB), holes and •O2- radicals play the crucial role. Under ultraviolet light irradiation (λ < 420 nm) for 45 min, the RhB degradation ratios over 150-Cl-BOC (94%) and 150-I-BOC (85%) are 4.2 and 3.7 times higher than that of original BOC (18%), respectively. This work demonstrates that the simple surface halogenation modification greatly improves the photocatalytic activity.

Oxygen-deficient AgIO3 for efficiently photodegrading organic contaminants under natural sunlight

Defect engineering is regarded as an effective strategy to boost the photo-activity of photocatalysts for organic contaminants removal. In this work, abundant surface oxygen vacancies (Ov) are created on AgIO3 microsheets (AgIO3-OV) by a facile and controllable hydrogen chemical reduction approach. The introduction of surface Ov on AgIO3 broadens the photo-absorption region from ultraviolet to visible light, accelerates the photoinduced charges separation and migration, and also activates the formation of superoxide radicals (•O2-). The AgIO3-OV possesses an outstanding degradation rate constant of 0.035 min-1, for photocatalytic degrading methyl orange (MO) under illumination of natural sunlight with a light intensity is 50 mW/cm2, which is 7 and 3.5 times that of the pristine AgIO3 and C-AgIO3 (AgIO3 is calcined in air without generating Ov). In addition, the AgIO3-OV also exhibit considerable photoactivity for degrading other diverse organic contaminants, including azo dye (rhodamine B (RhB)), antibiotics (sulflsoxazole (SOX), norfloxacin (NOR), chlortetracycline hydrochloride (CTC), tetracycline hydrochloride (TC) and ofloxacin (OFX)), and even the mixture of organic contaminants (MO-RhB and CTC-OFX). After natural sunlight illumination for 50 min, 41.4% of total organic carbon (TOC) for MO-RhB mixed solution can be decreased over AgIO3-OV. In a broad range of solution pH from 3 to 11 or diverse water bodies of MO solution, AgIO3-OV exhibits attractive activity for decomposing MO. The MO photo-degradation process and mechanism over AgIO3-OV under natural sunlight irradiation has been systemically investigated and proposed. The toxicities of MO and its degradation intermediates over AgIO3-OV are compared using Toxicity Estimation Software (T.E.S.T.). Moreover, the non-toxicity of both AgIO3-OV catalyst and treated antibiotic solution (CTC-OFX mixture) are confirmed by E. coli DH5a cultivation test, supporting the feasibility of AgIO3-OV catalyst to treat organic contaminants in real water under natural sunlight illumination.

Construction of BiOIO3/AgIO3 Z-Scheme Photocatalysts for the Efficient Removal of Persistent Organic Pollutants under Natural Sunlight Illumination

The efficient removal of persistent organic pollutants (POPs) in natural waters is vital for human survival and sustainable development. Photocatalytic degradation is a feasible and cost-effective strategy to completely disintegrate POPs at room temperature. Herein, we develop a series of direct Z-scheme BiOIO₃/AgIO₃ hybrid photocatalysts via a facile deposition-precipitation method. Under natural sunlight irradiation, the light intensity of which is ∼40 mW/cm², a considerable rate constant of 0.185 min^-1 for photodecomposing 40 mg/L MO is obtained over 0.5 g/L Bi@Ag-5 composite photocatalyst powder, about 92.5 and 5.3 times higher than those of pristine AgIO₃ and BiOIO₃. The photoactivity of Bi@Ag-5 for photodecomposing MO under natural sunlight illumination surpasses most of the reported photocatalysts under Xe lamp illumination. After natural sunlight irradiation for 20 min, 95% of MO, 82% of phenol, 78% of 2,4-DCP, 54% of ofloxacin, and 88% of tetracycline hydrochloride can be photodecomposed over Bi@Ag-5. Relative to the commercial photocatalyst TiO₂ (P25), Bi@Ag-5 exhibits greatly higher photoactivity for the treatment of MO-phenol-tetracycline hydrochloride mixture pollutants in the scale-up experiment of 500 mL of solution, decreasing COD, TOC, and chromaticity value by 52, 19, and 76%, respectively, after natural sunlight irradiation for 40 min. The photodegradation process and mechanism of MO have been systematically investigated and proposed. This work provides an archetype for designing efficient photocatalysts to remove POPs.

Novel Ag3PO4/boron-carbon-nitrogen photocatalyst for highly efficient degradation of organic pollutants under visible-light irradiation

Ag₃PO₄ is an indirect bandgap semiconductor with excellent photocatalytic activity. However, it has not been widely used so far for the treatment of polluted wastewaters. This scarce use in wastewater treatment can be mainly attributed to its large crystallite size, which would be due to rapid agglomeration during the synthesis process, as well as to the photo-corrosion problem affecting this material. Hence, it would be crucial to develop a photocatalytic system involving Ag₃PO₄ nanoparticles with enhanced properties, such as higher specific surface area and excellent photocatalytic stability. To meet this demand, a novel Ag₃PO₄/boron carbon nitrogen (Ag₃PO₄/BCN) composite photocatalyst was successfully prepared in the present study via electrostatically driven self-assembly and ion exchange processes. After characterization and assessment, it was shown that the as-prepared Ag₃PO₄/BCN nanocomposite photocatalyst not only contains smaller Ag₃PO₄ nanoparticles, but also exhibits an enhanced visible-light photocatalytic activity for Rhodamine B (RhB) Methyl Orange (MO) and Tetracycline (TC) and improved stability, without decrease after 5 cycles, compared with pure Ag₃PO₄ nanoparticles. Positive synergy between Ag₃PO₄ nanoparticles and BCN nanosheets, including the increase in the number of active adsorption sites, and the restriction of the formation of Ag due to the recombination of photogenerated electron-hole pairs in Ag₃PO₄ nanoparticles, are mainly responsible for the enhanced properties of the prepared catalyst. This study shows that Ag₃PO₄/BCN composite photocatalyst would be promising for wastewater treatment, which would be of clearly environmental and public health relevance.

Porous hollow Ag/Ag2S/Ag3PO4 nanocomposites as highly efficient heterojunction photocatalysts for the removal of antibiotics under simulated sunlight irradiation

Antibiotic pollutants are a serious and growing threat to human health and the environment that efficient measures must be taken to eliminate them. Here, we report the facile fabrication of porous hollow Ag/Ag₂S/Ag₃PO₄ heterostrucutres for efficient photocatalytic degradation of tetracycline under simulated sunlight irradiation. The morphology manipulation and hetero-nanocomposites construction through a coprecipitation-refluxing approach were applied to enhance the photocatalytic performance of the Ag/Ag₂S/Ag₃PO₄ products. The photodegradation outcomes indicated that the heterojunction Ag/Ag₂S/Ag₃PO₄ photocatalyst with a suitable band gap energy of 2.17 eV, has better degradation performance (∼95%) than individual Ag₂S and Ag₃PO₄ structures after 120 min of simulated sunlight irradiation, even after five recycles. The good photocatalytic activity of Ag/Ag₂S/Ag₃PO₄ nanocomposites could be mainly attributed to the unique hierarchical architectures, promoted visible-light harvesting, reduced a recombination and boosted separation of electron-hole pairs originated from the as-formed heterojunctions. Moreover, we proposed a photocatalytic degradation mechanism based on the radical scavenging results, which disclosed that the ^•O₂^- and ^•OH species perform essential tasks for the photodegradation of antibiotics by Ag/Ag₂S/Ag₃PO₄ nanocomposites.

Enhancing photocatalytic degradation of methyl orange by crystallinity transformation of titanium dioxide: A kinetic study

This work aimed to enhance the photocatalytic degradation of methyl orange (MO) by crystallinity transformation of titanium dioxide (TiO₂ ). In addition, the kinetic degradation of MO was determined. To transform its crystallinity, TiO₂ was synthesized using a sol-gel method and calcined at between 200°C to 600°C. Calcination at a temperature of 250°C resulted in TiO₂ that showed the best performance, corresponding to MO removal of 87% ± 7%. MO removal by TiO₂ calcined between 250°C to 400°C was higher than for commercial TiO₂ powder (Sigma-aldrich) (62% ± 4%). TiO₂ with a small crystallite size and high anatase fraction enhanced the photocatalytic degradation of MO, while the specific surface area and surface roughness seemed to play a minor role. The photocatalytic degradation of MO was NaCl-independent, while the photocatalytic activity increased with decreased pH. Reused TiO₂ showed similar photocatalytic degradation of MO compared with pristine TiO₂ , at 84 ± 2%. The oxidation kinetics of TiO₂  calcined at 250°C were fitted to the Langmuir-Hinshelwood model (R²  = 0.9134). The k_r and K_s values were 0.027 mg L^-1  min^-1  and 0.621 L/mg, respectively. Crystallinity transformation was a major factor in the enhancement of photocatalytic degradation of MO. PRACTITIONER POINTS: Photocatalytic activity of TiO₂ depends on calcination temperature, pH, and a number of UVC lamps. TiO₂ with a small crystallite size and high anatase fraction enhanced the photocatalytic degradation of MO.

Developing boron nitride-pyromellitic dianhydride composite for removal of aromatic pollutants from wastewater via adsorption and photodegradation

A series of boron nitride-pyromellitic dianhydride composites have been successfully synthesized by calcinating the mixtures of boron nitride (BN) and pyromellitic dianhydride (PA) at 350 °C, in which the composite (BNPA2) has the largest adsorption quantity (65.1 mg/g) for rhodamine B (RhB) and the best photo-removal efficiency for RhB under visible light irradiation. ¹H NMR characterizations for BN, PA and BNPA2 suggest that this composite is formed via the reaction between the OH groups in BN and PA. BNPA2 can also adsorb neutral red (NR), methyl orange (MO), tetracycline (TC) and atrazine (AT). NR and MO can be photo-removed by BNPA2 under visible light irradiation. Colorless TC and AT can also be degraded by BNPA2 under visible light irradiation, suggesting that BNPA2 is visible light responsible photocatalyst. BNPA2 has the highest photo-removal efficiency for the cationic RhB and NR, followed by the anionic MO. This is from that BNPA2 has negative surface. When anionic MO mixes with cationic RhB (or NR) together, BNPA2 prefers to remove cationic RhB (or NR) from the mixture solution under visible light irradiation and the removal efficiency of anionic MO by BNPA2 is also increased. Thus, electrostatic interactions between dyes and BNPA2 as well as between dyes play significant role in the removal process. •O^2- makes a main contribution for this photo-removal of these aromatic pollutants (dyes, TC and AT) by BNPA2 under visible light irradiation. Furthermore, the removal performance of BNPA2 for RhB, TC and AT can be effectively regenerated by visible light irradiation.

Antibiotic removal from water: A highly efficient silver phosphate-based Z-scheme photocatalytic system under natural solar light

Photocatalytic degradation is an alternative method to remove pharmaceutical compounds from water, however it is hard to achieve efficient rate because of the low efficiency of photocatalysts. In this study, an efficient Z-Scheme photocatalyst was constructed by integrating graphitic carbon nitride (CN) and reduced graphene oxide (rGO) with AP via a simple facile precipitation method. Excitedly, ternary AP/rGO/CN composite showed superior photocatalytic and anti-photocorrosion performances under both intense sunlight and weak indoor light irradiation. NOF can be completely degraded in only 30 min and about 85% of NOF can be mineralized after 2 h irradiation under intensive sunlight irradiation. rGO could work not only as a sheltering layer to protect AP from photocorrosion but also as a mediator for Z-Scheme electron transport, which can protect AP from the photoreduction. This strategy could be a promising method to construct photocatalytic system with high efficiency for the removal of antibiotics under natural light irradiation.

"Dark Deposition" of Ag Nanoparticles on TiO2: Improvement of Electron Storage Capacity To Boost "Memory Catalysis" Activity

"Memory catalysis" (MC) studies have received appreciable attention recently because of the unique talent to retain the catalytic performance in the dark condition. However, the MC activity is still low owing to the relatively limited electron storage capacity of the present materials. Here, a TiO₂@Ag composite was synthesized by a "dark-deposition (DD)" method, which is based on the electron trap effect of TiO₂. Unlike traditional photodeposition (PD), an exploration of the morphology and chemical compositions of as-prepared samples shows that DD can inhibit the growth of Ag nanoparticles and the formation of Ag₂O, which greatly improve the electron storage capacity. We further demonstrated that the maximum electronic capacity was in the order of TiO₂@Ag-DD (1 μmol/mg) > TiO₂@Ag-PD (0.35 μmol/mg) > TiO₂ (0.11 μmol/mg). Moreover, the enhanced MC activity was confirmed by various degradation experiments. Especially, the use of TiO₂@Ag-DD as a round-the-clock catalyst for the degradation of multicomponent pollutants has also been achieved. This strategy opens a door for enhancing the MC activity and reveals that the coupling of photocatalysis and MC may provide a new opportunity for the continuous removal of pollutants in day and night. It also may be extended to other fields, such as energy storage and continuous disinfection.

Solvothermal synthesis of Ag hybrid BiPO4 heterostructures with enhanced photodegradation activity and stability

In this study, Ag hybrid BiPO₄ (Ag/BiPO₄) heterostructures were synthesized using a solvothermal method. The morphologies and optical properties of the Ag/BiPO₄ heterostructures were drastically different from those of BiPO₄ and were highly dependent on the AgNO₃:BiPO₄ weight percent during the synthesis. The three formulated heterostructures were evaluated for their photocatalytic degradation of methylene blue (MB) under UV light illumination; the 0.5%Ag/BiPO₄ heterostructure was observed to result in 99% degradation of MB within 60min, a remarkably higher level of photodegradation activity than the levels caused by TiO₂ and BiPO₄. Furthermore, even after use for five cycles of MB degradation, the 0.5%Ag/BiPO₄ heterostructure showed no observable loss in photodegradation activity and no change in XRD patterns, demonstrating its chemical and structural stability. According to the results of a systematic experimental investigation, the enhanced photodegradation activity of this Ag/BiPO₄ heterostructure could be ascribed to the high position of its valence band and the highly efficient separation of photogenerated electrons and holes. Moreover, hydroxyl radicals and holes were found to be the major reactive species. Successful photodegradation of standard dye solutions, including acid blue 1, methyl orange, fast green, rhodamine B, rhodamine 6G, and MB, in real water samples was demonstrated with the 0.5%Ag/BiPO₄ heterostructure, providing clear evidence of its utility for treating waste water containing organic dyes.

Efficient photocatalytic degradation of tetracycline hydrochloride by Ag3PO4 under visible-light irradiation

A facile, environmental-friendly Ag3PO4-PN photocatalyst was synthesized by a simple precipitation method at room temperature in the presence of ammonia and polyvinyl pyrrolidone (PVP). As-synthesized samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible diffuse reflectance spectroscopy (UV-vis DRS). The enhancement of photocatalytic efficiency of Ag3PO4-PN is strongly dependent on the excellent photo-absorption capacity, sharp edges and corners, and synergistic effect of PVP and NH3·H2O. The effects of catalyst dosage, TC concentration and solution pH were explored with tetracycline hydrochloride (TC) as target contamination. The mineralization was evaluated by total organic carbon (TOC) analysis and determination of the concentration of inorganic ions such as NO3 (-) and Cl(-). Radical detection experiment indicated the h(+) and ·O(2-) are major active species in the degradation of TC by Ag3PO4-PN. Moreover, photocatalyst stability and regeneration experiments exhibited the favorable stability and rejuvenation ability, suggesting a promising prospect of practical application of Ag3PO4 in the wastewater treatment.