Fabrication and photoelectrochemical properties of porous ZnWO4 film

Network Structure and Luminescent Properties of ZnO-B2O3-Bi2O3-WO3:Eu3+ Glasses

In this study, we investigated the influence of Bi2O3 and WO3 on both structure and optical properties of 50ZnO:(49 - x)B2O3:1Bi2O3:xWO3; x = 1, 5, 10 glasses doped with 0.5 mol% Eu2O3. IR spectroscopy revealed the presence of trigonal BØ3 units connecting superstructural groups, [BØ2O]- metaborate groups, tetrahedral BØ4- units in superstructural groupings (Ø = bridging oxygen atom), borate triangles with nonbridging oxygen atoms, [WO4]2- tetrahedral, and octahedral WO6 species. Neutron diffraction experimental data were simulated by reverse Monte Carlo modeling. The atomic distances and coordination numbers were established, confirming the short-range order found by IR spectra. The synthesized glasses were characterized by red emission at 612 nm. All findings suggest that Eu3+ doped zinc borate glasses containing both WO3 and Bi2O3 have the potential to serve as a substitute for red phosphor with high color purity.

The stability and electronic and photocatalytic properties of the ZnWO4 (010) surface determined from first-principles and thermodynamic calculations

We present the results of the generalized-gradient approximation of Perdew, Burke and Ernzerhof (GGA-PBE) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional calculations of the atomic and the electronic structures of ZnWO₄ (010) surfaces. The total energies obtained from these calculations are used to analyze the thermodynamic stability of the surfaces. The surface phase diagrams are constructed by surface Gibbs free energies obtained as a function of temperature and oxygen partial pressure. Our results suggested that the stable area of the surface terminations of ZnWO₄ (010) has little correlation with the functional selected. The stability phase diagram shows that O-Zn, DL-W, and DL-Zn terminations of ZnWO₄ (010) can be stabilized under certain thermodynamic equilibrium conditions. Based on the HSE06 hybrid functional, we calculate the electronic structures for three possible stability surface terminations. It is found that there is a fat band of the surface states in DL-W termination, which shows a delocalized feature. This fat band acts as an electron transition bridge between the valence band (VB) and conduction band (CB). It contributes to visible-light absorption by two-step optical transition with the first transition from the VB to the fat band and the second from the fat band to the CB. Significantly, the conduction band minimum (CBM) band edge position of DL-W termination is favourable for H₂ evolution as the CBM edge is located above the water reduction level (H⁺/H₂). Simultaneously, DL-W termination's valence band maximum (VBM) potential shows a strong potential for O₂ generation from water oxidation because of the higher VBM edge with respect to the water oxidation level (H₂O/O₂). These results may help explore ZnWO₄ (010) surfaces' intrinsic properties, providing a helpful strategy for experimental studies of ZnWO₄-based photocatalysts in the future.

Reactive Dual Magnetron Sputtering: A Fast Method for Preparing Stoichiometric Microcrystalline ZnWO4 Thin Films

Thin films of ZnWO4, a promising photocatalytic and scintillator material, were deposited for the first time using a reactive dual magnetron sputtering procedure. A ZnO target was operated using an RF signal, and a W target was operated using a DC signal. The power on the ZnO target was changed so that it would match the sputtering rate of the W target operated at 25 W. The effects of the process parameters were characterized using optical spectroscopy, X-ray diffraction, and scanning electron microscopy, including energy dispersive X-ray spectroscopy as well as X-ray photoelectron spectroscopy. It was found that stoichiometric microcrystalline ZnWO4 thin films could be obtained, by operating the ZnO target during the sputtering procedure at a power of 55 W and by post-annealing the resulting thin films for at least 10 h at 600 °C. As FTO coated glass substrates were used, annealing led as well to the incorporation of Na, resulting in n+ doped ZnWO4 thin films.

Modulating formation rates of active species population by optimizing electron transport channels for boosting the photocatalytic activity of a Bi2S3/BiO1−xCl heterojunction

Bi₂S₃/BiO_1−xCl heterojunction with OVs can regulate PS to produce ASP through optimizing ETCs and modulating ASP formation rates by cooperation of OVs with IEF to hinder the invalid consumption of ASP and significantly enhance the catalytic efficiency.

Strong Photo-Oxidative Capability of ZnWO4 Nanoplates with Highly Exposed {0 1 1} Facets

ZnWO4 nanoplates with highly exposed {0 1 ¯ 1} facets were synthesized via a hydrothermal technique. The phase, morphology, and optical characteristics of ZnWO4 nanoplates were characterized with scanning electron microscopy, transmission electron microscopy, X–ray diffraction, diffuse ultraviolet–visible light (UV–Vis) reflectance spectroscopy, photoluminescence (PL) spectrophotometry, and PL lifetime spectroscopy. Optical characterizations, along with the density functional calculations, confirm that the strong blue PL band of ZnWO4 nanoplates originates from the intrinsic defects in ZnWO4 nanoplates. Furthermore, photocatalytic tests show that ZnWO4 nanoplates exhibit strong photo-oxidative capability of complete mineralization of the organic pollutant (methyl orange) in water, whereas ZnWO4 nanoparticles can only cleave the organic molecules into fragments. The superior photo-oxidative capability of ZnWO4 nanoplates can be attributed to the specific chemical bonding and stereochemistry on the exposed facets. This work demonstrates that crystal facet engineering is an efficient strategy to endow ZnWO4 with strong photo-oxidative capability.

Construction of noble-metal-free TiO2 nanobelt/ZnIn2S4 nanosheet heterojunction nanocomposite for highly efficient photocatalytic hydrogen evolution

A binary nanocomposite composed of two-dimensional (2D) ultrathin ZnIn₂S₄ nanosheets and one-dimension (1D) TiO₂ nanobelts was prepared and applied as a noble-metal-free photocatalyst for hydrogen evolution under solar-light irradiation. The TiO₂ nanobelt/ZnIn₂S₄ nanosheet heterojunction nanocomposites show higher light absorption capacity, larger surface area and higher separation of charge carriers in comparison to pristine TiO₂ and ZnIn₂S₄. As a result, the hydrogen production over the TiO₂/ZnIn₂S₄ nanocomposite with 15 wt% TiO₂ can reach up to 348.21 μmol · g^-1 · h^-1, even without noble metals, which is about 26 and 2.3 times higher than the pristine TiO₂ and ZnIn₂S₄, respectively. Meanwhile, a possible photocatalytic mechanism of TiO₂/ZnIn₂S₄ heterojunction nanocomposites was proposed and corroborated by photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) results. This work paves a way for developing low-cost and high-efficiency noble-metal-free photocatalytic systems for solar-to-hydrogen evolution.

Intrinsic Defect Engineering in Eu3+ Doped ZnWO₄ for Annealing Temperature Tunable Photoluminescence

Eu³⁺ doped ZnWO₄ phosphors were synthesized via the co-precipitation technique followed by subsequent thermal annealing in the range of 400–1000 °C. The phase, morphology, elemental composition, chemical states, optical absorption, and photoluminescence (PL) of the phosphors were characterized by X-ray diffraction, scanning electron microscopy, dispersive X-ray spectroscopy, X-ray photoelectron spectrometry, diffuse UV–vis reflectance spectroscopy, PL spectrophotometry, and PL lifetime spectroscopy, respectively. It is found that the PL from Eu³⁺ doped ZnWO₄ is tunable through the control of the annealing temperature. Density functional calculations and optical absorption confirm that thermal annealing created intrinsic defects in ZnWO₄ lattices play a pivotal role in the color tunable emissions of the Eu³⁺ doped ZnWO₄ phosphors. These data have demonstrated that intrinsic defect engineering in ZnWO₄ lattice is an alternative and effective strategy for tuning the emission color of Eu³⁺ doped ZnWO₄. This work shows how to harness the intrinsic defects in ZnWO₄ for the preparation of color tunable light-emitting phosphors.

Eu2+ and Eu3+ Doubly Doped ZnWO₄ Nanoplates with Superior Photocatalytic Performance for Dye Degradation

Eu²⁺ and Eu³⁺ doubly doped ZnWO₄ nanoplates with highly exposed {100} facets were synthesized via a facile hydrothermal route in the presence of surfactant cetyltrimethyl ammonium bromide. These ZnWO₄ nanoplates were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectrometry, diffuse UV-vis reflectance spectroscopy, photoluminescence spectrophotometry, and photoluminescence lifetime spectroscopy to determine their morphological, structural, chemical, and optical characteristics. It is found that Eu-doped ZnWO₄ nanoplates exhibit superior photo-oxidative capability to completely mineralize the methyl orange into CO₂ and H₂O, whereas undoped ZnWO₄ nanoparticles can only cleave the organic molecules into fragments. The superior photocatalytic performance of Eu-doped ZnWO₄ nanoplates can be attributed to the cooperative effects of crystal facet engineering and defect engineering. This is a valuable report on crystal facet engineering in combination with defect engineering for the development of highly efficient photocatalysts.

ZnWO4 nanocrystals: synthesis, morphology, photoluminescence and photocatalytic properties

The best photocatalytic properties for monoclinic ZnWO₄ nanocrystals are related to the surface energy and the types of clusters formed on their surface.

Impacts of ionic liquid capping on the morphology and photocatalytic performance of SbPO4 crystals

A novel hydrothermal route to prepare SbPO₄ with [BMIM][PO₄] or Na₂HPO₄·12H₂O as the phosphorus source has been developed. The morphology transition of SbPO₄ photocatalysts from microparticles to microspheres was shown by changing the reaction temperature.

Enhanced photoelectrocatalytic degradation of ammonia by in situ photoelectrogenerated active chlorine on TiO2 nanotube electrodes

TiO₂ nanotube (TiNT) electrodes anodized in fluorinated organic solutions were successfully prepared on Ti sheets. Field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) were performed to characterize the TiNT electrodes. The linear voltammetry results under irradiation showed that the TiNT electrode annealed at 450°C presented the highest photoelectrochemical activity. By combining photocatalytic with electrochemical process, a significantly synergetic effect on ammonia degradation was observed with Na₂SO₄ as supporting electrolyte at pH10.7. Furthermore, the photoelectrocatalytic efficiency on the ammonia degradation was greatly enhanced in presence of chloride ions without the limitation of pH. The degradation rate was improved by 14.8 times reaching 4.98×10^-2min^-1 at pH10.7 and a faster degradation rate of 6.34×10^-2min^-1 was obtained at pH3.01. The in situ photoelectrocatalytic generated active chlorine was proposed to be responsible for the improved efficiency. On the other hand, an enhanced degradation of ammonia using TiNT electrode fabricated in fluorinated organic solution was also confirmed compared to TiNT electrode anodized in fluorinated water solution and TiO₂ film electrode fabricated by sol-gel method. Finally, the effect of chloride concentration was also discussed.

In situ synthesis and photocatalytic performance of WO3/ZnWO4 composite powders

The WO₃/ZnWO₄ composite powders were synthesized through an in situ reaction process with tunnel structure K₁₀W₁₂O₄₁·11H₂O filiform crystallites used as a precursor.

Rationally designed n-n heterojunction with highly efficient solar hydrogen evolution

In most of the reported n-n heterojunction photocatalysts, both the conduction and valence bands of one semiconductor are more negative than those of the other semiconductor. In this work, we designed and synthesized a novel n-n heterojunction photocatalyst, namely CdS-ZnWO4 heterojunctions, in which ZnWO4 has more negative conduction band and more positive valence band than those of CdS. The hydrogen evolution rate of CdS-30 mol %-ZnWO4 reaches 31.46 mmol h(-1)  g(-1) under visible light, which is approximately 8 and 755 times higher than that of pure CdS and ZnWO4 under similar conditions, respectively. The location of the surface active sites is researched and a plausible mechanism of performance enhancement by the tuning of the structure is proposed based on the photoelectrochemical characterization. The results illustrate that this kind of nonconventional n-n heterojunctions is also suitable and highly efficient for solar hydrogen evolution.

Novel In2S3/ZnWO4 heterojunction photocatalysts: facile synthesis and high-efficiency visible-light-driven photocatalytic activity

Novel In₂S₃/ZnWO₄ heterojunction photocatalysts with high-efficiency visible-light-driven photocatalytic activities were synthesized by a hydrothermal and surface-functionalized method.

Yolk–shell ZnWO4 microspheres: one-pot synthesis, characterization and photocatalytic properties

Novel yolk–shell ZnWO₄ microspheres have been synthesized via a mild and one-pot hydrothermal route by using l-aspartic acid as the chelating agent and shape modifier.

Synthesis and photocatalytic properties of ZnWO4 nanocrystals via a fast microwave-assisted method

High crystallinity of ZnWO4 nanoparticles has been successfully synthesized via a highly effective and environmentally friendly microwave route by controlling the reaction time and temperature. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Fourier infrared spectrum (FT-IR). The crystallinity was enhanced with the increase of the reaction temperature and time. The photocatalytic activities of ZnWO4 nanocrystals were evaluated by testing the photodegradation of rhodamine B (RhB) dye under ultraviolet (UV) light irradiation. The results indicated that as-prepared ZnWO4 was highly effective for the degradation of RhB. The degradation rate of RhB reached 98.01% after 6 h of UV illumination.

Novel hollow Pt-ZnO nanocomposite microspheres with hierarchical structure and enhanced photocatalytic activity and stability

Noble metal/semiconductor nanocomposites play an important role in high efficient photocatalysis. Herein, we demonstrate a facile strategy for fabrication of hollow Pt-ZnO nanocomposite microspheres with hierarchical structure under mild solvothermal conditions using Zn (CH(3)COO)(2)·2H(2)O and HPtCl(4) as the precursors, and polyethylene glycol-6000 (PEG-6000) and ethylene glycol as the reducing agent and solvent, respectively. The as-synthesized ZnO and Pt-ZnO composite nanocrystals were well characterized by powder X-ray diffraction (XRD), nitrogen-physical adsorption, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectra (DRS), and photoluminescence (PL) emission spectroscopy. It was found that Pt content greatly influences the morphology of Pt-ZnO composite nanocrystals. Suitable concentration of HPtCl(4) in the reaction solution system can produce well hierarchically hollow Pt-ZnO nanocomposite microspheres, which are composed of an assembly of fine Pt-ZnO nanocrystals. Photocatalytic tests of the Pt-ZnO microspheres for the degradation of the dye acid orange II revealed extremely high photocatalytic activity and stability compared with those of pure ZnO and corresponding Pt deposited ZnO. The remarkable photocatalytic performance of hollow Pt-ZnO microspheres mainly originated from their unique nanostructures and the low recombination rate of the e(-)/h(+) pairs by the platinum nanoparticles embedded in ZnO nanocrystals.

A survey of photocatalytic materials for environmental remediation

Heterogeneous photocatalysis is an advanced oxidation process which has been the subject of a huge amount of studies related to air cleaning and water purification. All these processes have been carried out mainly by using TiO(2)-based materials as the photocatalysts and ca. 75% of the articles published in the last 3 years is related to them. This review illustrates the efforts in the search of alternative photocatalysts that are not based on TiO(2), with some exceptions concerning particularly innovative modifications as nanoassembled TiO(2) or TiO(2) composites with active carbon, graphite and fullerene. Papers reporting preparation, characterization and testing of binary, ternary and quaternary compounds, have been reviewed. Despite many of these photocatalysts being effective for the photodecomposition of many pollutants, most of them do not allow a complete mineralization of the starting compounds, differently from TiO(2).

Investigation of photocatalytic activities over Bi₂WO₆/ZnWO₄ composite under UV light and its photoinduced charge transfer properties

Bi(2)WO(6)/ZnWO(4) composite photocatalysts have been successfully synthesized by a facile hydrothermal process. The catalysts were characterized by powder X-ray diffraction (XRD), transmission electron microcopy (TEM), and UV-vis diffuse reflectance spectrum (DRS). The results show that Bi(2)WO(6) nanoparticles grow on the primary ZnWO(4) nanorods. The Bi(2)WO(6)/ZnWO(4) composites have better UV light photocatalytic activities compared to single ZnWO(4) nanorods. Furthermore, the photoinduced charge transfer properties of Bi(2)WO(6)/ZnWO(4) composites were investigated by means of transient photovoltage (TPV) technique in detail. The interconnected interface of Bi(2)WO(6)/ZnWO(4) composites led to the low recombination ratios of photoinduced electron-hole pairs and enhanced photocatalytic activities.

Fluorination of ZnWO4 photocatalyst and influence on the degradation mechanism for 4-chlorophenol

The fluorine doped ZnWO4 photocatalyst was synthesized by hydrothermal synthesis and annealing treatment. The existing states of fluorine in the crystal were elucidated, and the effects of fluorine on the crystal structure, photocatalytic activity, and degradative intermediates were investigated. The doping concentration of fluorine in the interstitial lattice of ZnWO4 crystal can be controlled bythe annealing conditions. The photocatalytic activity can be enhanced about 50% after the doped ZnWO4 was annealed at 450 degrees C for 1 h due to perfect crystal structure. The enhancement of photocatalytic activity after fluorine doping could be attributed to the higher separation efficiency of electron-hole pairs, which results in a large number of holes participated in the photocatalytic process. The fluorine doping does not change the degradation pathway of 4-chlorophenol (4-CP) in our system. 4-CP was mainly transformed into hydroxylated aromatic intermediates such as benzoquinone (BQ), hydroquinone (HQ), and 4-chlorocatechol (4-CC). The photodegradation of 4-CP in powdered F-doped ZnWO4 system proceeded via direct holes oxidation reactions.