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Direct photocatalytic reduction of toxic formaldehyde (HCHO) in value-added chemicals and fuels is promising because that not only abates the environmental pollution, but also solves the energy shortage. Herein, self-supported MoO 2 and MoO 3 nanoparticles growing on Mo meshes were comparatively applied to the photocatalytic conversion of HCHO. Under UV–visble lights, MoO 2 reduces HCHO in methanol (CH 3 OH) while MoO 3 oxidizes HCHO in carbon oxide and water. Their contrary photocatalytic capacities were revealed. Compared with MoO 3 , the lower work function of MoO 2 enables an electron-rich interface, realizing a complete reduction of 30 ppm HCHO to CH 3 OH in 30 min. Theoretical calculations clarify that a large number of delocalized electrons on MoO 2 attracts HCHO molecule and activates its C O bond, facilitating subsequent hydrogenation and reduction of HCHO to CH 3 OH. As for MoO 3 , the wider bandgap and higher potential of valence band govern the photocatalytic oxidation of HCHO. [Display omitted] • Photocatalytic reduction of HCHO using MoO 2 was conducted for the first time. • Transforming HCHO in CH 3 OH is a pollution-free strategy. • Under irradiation, MoO 2 reduces HCHO while MoO 3 oxidizes HCHO. • Low work function of MoO 2 enables the electron-rich interface for HCHO. • Wide bandgap and high potential valence band make MoO 3 oxidative. [ABSTRACT FROM AUTHOR]

Photocatalytic reduction of UVI to UIV can help remove U from the environment and thus reduce the harmful impacts of radiation emitted by uranium isotopes. Herein, we first synthesized Bi 4 Ti 3 O 12 (B1) particles, then B1 was crosslinked with 6-chloro-1,3,5-triazine-diamine (DCT) to afford B2. Finally, B3 was formed using B2 and 4-formylbenzaldehyde (BA-CHO) to investigate the utility of the D-π-A array structure for photocatalytic UVI removal from rare earth tailings wastewater. B1 lacked adsorption sites and displayed a wide band gap. The grafted triazine moiety in B2 introduced active sites and narrowed the band gap. Notably, B3, a Bi 4 Ti 3 O 12 (donor)-triazine unit (π-electron bridge)-aldehyde benzene (acceptor) molecule, effectively formed the D-π-A array structure, which formed multiple polarization fields and further narrowed the band gap. Therefore, UVI was more likely to capture electrons at the adsorption site of B3 and be reduced to UIV due to energy level matching effects. UVI removal capacity of B3 under simulated sunlight was 684.9 mg g−1, 2.5 times greater than B1 and 1.8 times greater than B2. B3 was still active after multiple reaction cycles, and UVI removal from tailings wastewater reached 90.8%. Overall, B3 provides an alternative design scheme for enhancing photocatalytic performance. [Display omitted] Photocatalytic UVI reduction is one of the most promising methods to reduce radioactive uranium contamination in tailings wastewater. Herein, a Bi 4 Ti 3 O 12 (donor)-triazine (π-electron bridge)-aldehyde benzene (acceptor) crosslinked molecule, named B3, was formed D-π-A array structure for significantly improving photocatalytic UVI removal performance from tailings wastewater. The improved performance of B3 is due to the increased adsorption sites on the triazine and the increased electron push-pull effect of aldehyde group benzene as a weak acceptor for extended electron delocalization, resulting in the formation of multiple polarization fields. The modulation of D-π-A array structure provides an alternative means to improve photocatalytic performance. • Bi 4 Ti 3 O 12 -triazine unit-aldehyde benzene molecule formed D-π-A array structure. • The D-π-A array structure can form multiple polarization fields. • The D-π-A array structure can regulate band gap width and charge transfer. • The modulation of D-π-A structure can improve photocatalytic performance. [ABSTRACT FROM AUTHOR]

Porous metal-organic frameworks (MOFs) have been arousing a great interest in exploring the application of MOFs as photocatalyst in environment remediation. In this work, two different MOFs, Ti-benzenedicarboxylate (MIL-125(Ti)) and amino-functionalized Ti-benzenedicarboxylate (NH 2 -MIL-125(Ti)) were successfully synthesized via a facile solvothermal method. The MIL-125(Ti) and NH 2 -MIL-125(Ti) were well characterized by XRD, SEM, XPS, N 2 adsorption–desorption measurements, thermogravimetric analysis and UV–vis diffuse reflectance spectra (DRS). It is revealed that the NH 2 -MIL-125(Ti) has well crystalline lattice, large surface area and mesoporous structure, chemical and thermal stability, and enhanced visible-light absorption up to 520 nm, which was associated with the chromophore (amino group) in the organic linker. Compared with MIL-125(Ti), NH 2 -MIL-125(Ti) exhibited more efficient photocatalytic activity for Cr(VI) reduction from aqueous solution under visible-light irradiation. The addition of hole scavenger, the hole scavenger concentration and the pH value of the reaction solution played important roles in the photo-catalytic reduction of Cr(VI). The presence of Ti 3+ –Ti 4+ intervalence electron transfer was the main reason for photo-excited electrons transportation from titanium-oxo clusters to Cr(VI), facilitating the Cr(VI) reduction under the acid condition. It was demonstrated that amino-functionalized Ti(IV)-based MOFs could be promising visible-light photocatalysts for the treatment of Cr(VI)-contained wastewater. [ABSTRACT FROM AUTHOR]

The conversion of hexavalent chromium (Cr(VI)), a highly poisonous heavy metal found in natural environment, to less poisonous trivalent chromium (Cr(III)) has attracted a lot of interest. However, little interest has been paid to the development of metal-free catalysts. Here, we demonstrate for the first time a molecular engineering strategy to synthesize a range of donor-acceptor conjugated polymer photocatalysts, which can significantly increase the reduction efficiency of Cr(VI) by a factor of 5.2, corresponding to a significant change in the reduction reaction rate constant (from 0.0337 to 0.1740 min−1). In addition, the apparent quantum efficiency (AQE) of Cr(VI) removal was obtained, and the optimized photocatalyst (Py-SO 1) could achieve the highest apparent quantum efficiency at wavelength of 420 nm in those samples. Despite the narrow light absorption of Py-SO 1 polymer, its excellent exciton separation efficiency and efficient electron output enabled it to achieve excellent performance in photoreduction of Cr(VI), surpassing that of the reported metal-free photocatalysts. The results show that the present work provides a new perspective for designing suitable environmental remediation catalysts based on molecular engineering strategies. [Display omitted] • Various donor-acceptor conjugated polymers synthesized by molecular engineering. • Metal-free polymers exhibit excellent activity on the photocatalytic reduction of Cr(VI). • Exciton separation efficiency is more influential than light absorption in photocatalysis. • The reduction efficiency of Cr(VI) can be significantly increased to be 5.2 times. [ABSTRACT FROM AUTHOR]

Abstract: Hexavalent chromium is of particular environmental concern due to its toxicity and mobility and is challenging to remove from industrial wastewater. It is a strong oxidizing agent that is carcinogenic and mutagenic and diffuses quickly through soil and aquatic environments. It does not form insoluble compounds in aqueous solutions, so separation by precipitation is not feasible. While Cr(VI) oxyanions are very mobile and toxic in the environment, Cr(III) cations are not. Like many metal cations, Cr(III) forms insoluble precipitates. Thus, reducing Cr(VI) to Cr(III) simplifies its removal from effluent and also reduces its toxicity and mobility. In this review, we describe the environmental implications of Cr(VI) presence in aqueous solutions, the chemical species that could be present and then we describe the technologies available to efficiently reduce hexavalent chromium. [Copyright &y& Elsevier]

Abstract: The photocatalytic HCrO4 − reduction was investigated in air equilibrated solution using the spinel CuFe2O4 nanoparticles as sensitizers. The oxide is p-type semi conductor, prepared from nitrates decomposition. The catalytic performance increases with decreasing pH and the concomitant oxidation of salicylic acid contributes significantly to the photoactivity through the charges separation of electron/hole pairs (C7H6O3 +6 O2 +4h+ +3 H2O→7 CO2 +4 H3O+). Evidence has been given to show the advantages of the hetero-system CuFe2O4/CdS in the chromate reduction. CuFe2O4 acts as electrons pump and the electron transfer to chromate is mediated via CdS hexagonal variety (greenockite). A reduction of 60% occurs and the process is well described by a pseudo first order kinetic with a half life of ∼2.8h and a quantum yield of ∼0.12% for an initial HCrO4 − concentration of 3×10−4 M. An improvement up to 72% is obtained when the reaction occurs in a stirred reactor and no cadmium was detected after 6h illumination. The results indicate a competitive effect with the water reduction. The hydrogen evolutions are found to be 0.236 and 0.960cm3 mn−1 g−1 in presence and in absence of HCrO4 −, respectively. [ABSTRACT FROM AUTHOR]

Abstract: In this study, immobilized TiO2 electrode is applied to reduce toxic Cr(VI) to non-toxic Cr(III) in aqueous solution under UV irradiation. To overcome the limitation of powder TiO2, a novel technique of immobilization based on anodization was applied and investigated under various experimental conditions. The anodization was performed at 20V–5°C for 45min with 0.5% hydrofluoric acid, and then the anodized samples were annealed under oxygen stream in the range 450–850°C. Based on the results of the experiments, the photocatalytic Cr(VI) reduction was favorable in acidic conditions, with ∼98% of the Cr(VI) being reduced within 2h at pH 3. Among the samples tested under same anodizing condition, the nanotubular TiO2 annealed at 450 and 550°C showed highest reduction efficiencies of Cr(VI). In addition, the surface characterizations (zeta potential, XRD, and SEM) of these samples proved that the Cr(VI) reduction efficiency was higher under acidic conditions and at a lower annealing temperature. From this research, it was concluded that the anodized TiO2 has the potential to be a useful technology for environmental remediation as well as photocatalytic hydrogen production from water. [Copyright &y& Elsevier]

Abstract: A series of Si-doped TiO2 (Si/TiO2) photocatalysts supported on woven glass fabric were prepared by hydrothermal method for photocatalytic reduction of NO with NH3. The photocatalytic activity tests were carried out in a continuous Pyrex reactor with the flow rate of 2000mL/min under UV irradiation (luminous flux: 1.1×104 lm, irradiated catalyst area: 160cm2). The photocatalysts were characterized by X-ray diffraction (XRD), BET, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectrophotometer, transmission electron microscopy (TEM), photoluminescence (PL) and temperature-programmed desorption (TPD). The experiment results showed that NO conversion on Si/TiO2 at 323K could exceed 60%, which was about 50% higher than that on Degussa P25 and pure TiO2. With the doping of Si, photocatalysts with smaller crystal size, larger surface area and larger pore volume were obtained. It was also found that Ti–O–Si bands were formed on the surface of Si/TiO2 and that the surface hydroxyl concentration was greatly increased. As a result, total acidity and NH3 chemisorption amount were enhanced for Si/TiO2 leading to its photocatalytic activity improvement. [Copyright &y& Elsevier]

The metal ions M2+ (Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+) are potentially toxic. Their electro deposition has been carried out in aqueous air-equilibrated CuCrO2 suspension upon visible illumination. The delafossite CuCrO2 is p-type semiconductor characterized by a low band gap (1.28eV) and a long-term chemical stability. The corrosion rate is found to be 10−2 μmolm−2 month−1 in aqua regia. The oxide has been elaborated through nitrate route where the specific surface area is increased via the surface/bulk ratio. A correlation exists between the dark M2+ adsorption, the redox potential of M2+/0 couple and the conduction band of CuCrO2 positioned at −1.06 VSCE. Ag+ cannot be photoreduced because of its positive potential located far above the valence band. By contrast, Zn2+ is efficiently deposited due to the large driving force at the interface. The improved photoactivity of copper with a deposition percentage (90%) is attributed to the strong dark adsorption onto the surface catalyst. The results indicate a competitive effect with the water reduction; it has been observed that the M2+ deposition goes parallel with the hydrogen evolution. Such behavior is attributed to the low H2 over voltage when ultra fine aggregate of M islands are photodeposited onto CuCrO2 substrate. [Copyright &y& Elsevier]

• Ag-TiO 2 nanotube array thin films were prepared via magnetron sputtering and anodization treatment. • Ag additions resulted in the preferred orientation of crystals in the TiO 2 nanotube arrays. • Ag species enhanced the visible-light absorption of the thin film photocatalysts. • The TNA-3Ag exhibits superior photocatalytic performance in treating wastewater containing heavy metal. Ag-TiO 2 nanotube array films with the preferential orientation of crystals were fabricated on ITO glass by magnetron sputtering and anodization. Comprehensive characterization was performed to ascertain the composition and microstructure characteristics of thin films. The photocatalytic activities were evaluated through the reduction of hexavalent chromium (Cr 2 O 7 2− (Cr (VI)) as a model compound under visible light irradiation. XRD and XPS studies reveal the development of preferred orientation along [001] in anatase TiO 2 nanotubes by adjusting the Ag content during magnetron sputtering. Such unusual behavior is attributed to the minimization of anatase (001) surface energy assisted by Ag. The Ag-TiO 2 nanotube arrays having preferred crystal orientation exhibit superior separation/transfer of photo-induced charges. Furthermore, the Ag-TiO 2 nanotube arrays show improved absorption of visible light due to the SPR effect induced by Ag and the formation of heterojunction between the TNAs and Ag 2 O. TNA-3Ag exhibits the highest photocatalytic activities by removing 99.1 % Cr (VI) in 90 min under visible light illumination. [ABSTRACT FROM AUTHOR]

• Bi 2 S 3 thin film synthesized via PS-PED as a simple, one-step and repeatable approach for the first time. • Pumice and fractal structures of Bi 2 S 3 films obtained on the substrate. • Stainless steel mesh used as a suitable substrate with proper characters. • The synthesized films applied for photocatalytic reduction of Cr (VI). • Response surface methodology used for optimization of film procedure with high reduction efficiency. In this work, for the first time, bismuth sulfide (Bi 2 S 3) film formed on the stainless steel mesh surface as a suitable substrate. Different films were synthesized by various combinations of the two methods (sonochemistry and electrochemistry) in continuous and pulse modes. The Bi 2 S 3 films characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Atomic Force Microscope (AFM). To reach the best film for photocatalytic reduction of Cr (VI), different films were deposited on the substrate by changing three independent variables including pulse times (t on , t off) and sonication amplitude. Response Surface Methodology (RSM) applied for optimization of independent variables by using Central Composite Design (CCD). Here, the films prepared by sono-electrodeposition in pulse modes led to high photocatalytic efficiency in comparison with other films. The results confirmed that ultrasound affected the morphology of film due to the production of cavitation, micro jets and acoustic streaming. On the other hand, ultrasound decreased double layer thickness and dissolved diffusion problems. Sono-electrodeposition in pulse modes produced films with pumice and fractal like structures. The high photocatalytic activity attributed to special morphologies that have key roles in separation of hole/electron pairs and light multi-scattering. [ABSTRACT FROM AUTHOR]

The band structure of BiOCl 0.3 Br 0.7 solid solution, BiOCl 0.7 I 0.3 solid solution, and BiOBr 0.7 I 0.3 solid solution, and the synergy of adsorption and photocatalytic reduction for toxicity elimination of Cr(VI). • BiOCl x Br 1-x , BiOCl x I 1-x and BiOBr x I 1-x were prepared by facile solvothermal method. • BiOCl x Br 1-x , BiOCl x I 1-x and BiOBr x I 1-x possesses high internal hole-scavenging capacity. • BiOCl x Br 1-x , BiOCl x I 1-x and BiOBr x I 1-x showed efficient toxicity elimination of Cr (VI). • Toxicity elimination of Cr (VI) are due to the synergy of adsorption and photocatalysis. • BiOCl x Br 1-x , BiOCl x I 1-x and BiOBr x I 1-x are promising candidates for Cr (VI) remediation. The ecological toxicity of photocatalysts and toxicity reduction of Cr(VI) have attracted much attention. The development of environmentally-friendly photocatalysts with adsorption and internal hole-scavenging capacity for toxicity reduction of aqueous Cr(VI) via facile preparation method is desirable. In this study, visible-light-active BiOCl x Br 1-x , BiOCl x I 1-x and BiOBr x I 1-x solid solutions were prepared by simple solvothermal strategy. The BiOCl 0.3 Br 0.7 , BiOCl 0.7 I 0.3 and BiOBr 0.7 I 0.3 solid solutions exhibited excellent adsorption capacity and photoreduction ability. The removal efficiency of Cr(VI) by BiOCl 0.3 Br 0.7 was 97.7% within 60 min. BiOCl 0.7 I 0.3 and BiOBr 0.7 I 0.3 can remove Cr(VI) almost completely within less than 30 min, which were much higher than those by BiOCl (81.6%) and BiOBr (67.4%) due to joint effect of adsorption and photoreduction. More importantly, the toxicity evaluation confirmed nontoxicity of BiOCl x Br 1-x , BiOCl x I 1-x and BiOBr x I 1-x , and rapid toxicity elimination process of Cr(VI). [ABSTRACT FROM AUTHOR]

• Synthetization of new Bi 2 S 3 thin film via simple and repeatable method of SWV. • Using stainless steel mesh as substrate bearing low cost, high chemical stability, and available for scale up applications. • Stainless steel mesh shows capability in light harvesting and multi-scattering. • Surface voids of prepared thin film with nanometers to micrometers diameters. • Surface voids as harvesting and multi‒scattering light sites, play heterojunction about hole-electron separation. In this work a new method of square‒wave voltammetry was performed for electrodepositing of Bi 2 S 3 film on the stainless steel mesh surface as low-cost and effective substrate in visible light harvesting. First, the obtained porous film was characterized by X‒ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and Raman analysis. In following, photocatalytic activity of film for reduction of hexavalent Cr was evaluated under sunlight irradiation. Results suggested that the presence of voids having several nanometers to 2 μm diameters on the film surface plays key role in photocatalytic processes. Indeed, surface voids as trapping cites and producing routs leads to multiple scattering of visible light. It can be noted that morphology of thin film, substrate structure, and diameters of voids are the main parameters to achieving the high reduction efficiency. Cr(VI) was completely reduced within 100 min under sunlight irradiation. Photoreduction mechanistic studies induced by Bi 2 S 3 film suggested that active oxygen species such as HOO• and O 2 have significant role in photocatalytic reaction. Finally, the evaluation of simultaneous photocatalytic process of binary mixture (Cr(VI) and Rhodamin B) was shown synergistic improvement of Cr(VI) and Rhodamin B degradation due to rapid surface reactions. [ABSTRACT FROM AUTHOR]