Tungsten oxide (WO
3
) is a promising photocatalytic material, but it has some limitations on its optoelectronic properties. Compared with binary materials, ternary compounds provide a much greater ...variety of compositions and hence properties, which can be tuned to suit particular applications. In this work, the effect of introducing a second metal cation into tungsten oxide is studied by Density Functional Theory (DFT) calculations. The compounds investigated include AWO
4
tungstates (A = Sn, Fe), M
2
WO
6
tungstates (M = Bi, Sb), tungstite (WO
3
·H
2
O) and hydrotungstite (WO
3
·2H
2
O). The tungstates studied are found to have either a small band gap (SnWO
4
, FeWO
4
, WO
3
·H
2
O and WO
3
·2H
2
O), and thus potentially improved visible-light activity compared with WO
3
, or a more negative conduction band edge than WO
3
(Bi
2
WO
6
, Sb
2
WO
6
), which means they may be able to achieve overall water splitting, in contrast to WO
3
. The band gap narrowing and the band edge changes are attributed to the introduction of new electronic states due to the second metal cation, as well as structural changes, particularly a larger spacing between layers of WO
6
octahedra. All the materials studied have a relative high static dielectric constant (
r
> 10), allowing for exciton dissociation, and a small enough electron effective mass (
m
e
* < 0.5
m
0
) along at least one direction for carrier diffusion. The performance of all the compounds is likely to be limited by poor hole mobility, except for Sb
2
WO
6
and the hydrated compounds which also have a relatively small hole effective mass (
m
h
* < 0.5
m
0
). Through this comparative study, the key trends in properties as a function of composition for a family of complex materials have been identified, allowing appropriate compositions to be selected and tuned for specific applications.
The electronic properties of ternary tungstates can be tuned for photocatalytic water splitting applications by appropriate choice of composition.
Yttria-stabilized tetragonal zirconia polycrystal has been used as a dental biomaterial for several decades because the fracture toughness and bend strength are increased by a stress-induced ...transformation-toughening mechanism. However, its esthetics are compromised by its poor translucency and grayish-white appearance.
The purpose of the present systematic review was to assess information on the mechanical, chemical, and optical requirements of monolithic zirconia dental restorations.
The following databases (2010 to 2015) were electronically searched: ProQuest, EMBASE, SciFinder, MRS Online Proceedings Library, Medline, Compendex, and Journal of the American Ceramic Society. The search was limited to English-language publications, in vitro studies, experimental reports, and modeling studies.
The data from 57 studies were considered in order to review the intrinsic and extrinsic characteristics of zirconia and their effects on the optical properties.
The materials and microstructural issues relevant to the esthetics and long-term stability of zirconia have been considered in terms of monolithic restorations, while there also are restorations specifically for esthetic applications. Although zirconia-toughened lithium silicate offers the best esthetic outcomes, transformation-toughened zirconia offers the best mechanical properties and long-term stability; cubic stabilized zirconia offers a potential compromise. The properties of these materials can be altered to some extent through the appropriate application of intrinsic (such as, annealing) and extrinsic (such as, shade-matching) parameters.
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There is considerable interest in the pH-dependent, switchable, biocatalytic properties of cerium oxide (CeO2) nanoparticles in biomedicine, where these materials exhibit beneficial antioxidant ...activity against reactive oxygen species (ROS) at a basic physiological pH but cytotoxic prooxidant activity in an acidic cancer cell pH microenvironment. While the general characteristics of the role of oxygen vacancies are known, the mechanism of their action at the atomic scale under different pH conditions has yet to be elucidated. The present work applies density functional theory (DFT) calculations to interpret, at the atomic scale, the pH-induced behavior of the stable {111} surface of CeO2 containing oxygen vacancies. Analysis of the surface-adsorbed media species reveals the critical role of pH on the interaction between ROS (•O2 – and H2O2) and the defective CeO2 {111} surface. Under basic conditions, the superoxide dismutase (SOD) and catalase (CAT) biomimetic reactions can be performed cyclically, scavenging and decomposing ROS to harmless products, making CeO2 an excellent antioxidant. However, under acidic conditions, the CAT biomimetic reaction is hindered owing to the limited reversibility of Ce3+ ↔ Ce4+ and formation ↔ annihilation of oxygen vacancies. A Fenton biomimetic reaction (H2O2 + Ce3+ → Ce4+ + OH– + •OH) is predicted to occur simultaneously with the SOD and CAT biomimetic reactions, resulting in the formation of hydroxyl radicals, making CeO2 a cytotoxic prooxidant.
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•Cu oxidation state was tailored by heat treatment in a reducing (H2/Ar) atmosphere.•Initially present CuO was reduced to Cu2O by photoexcited electrons.•H2-treated metallic Cu was ...retained over the entire photocatalytic reaction.•Both Cu2O and metallic Cu on TiO2 are able to photocatalytically generate hydrogen, albeit by distinctly different mechanisms.
In-depth characteristic studies with H2 activity and theoretical calculations were used to reveal the copper oxidation states most effective for photocatalytic hydrogen production when loaded on TiO2. When the copper was originally present as CuO, photogenerated electrons initially reduced the Cu2+ to Cu+ in preference to proton reduction. The resulting Cu2O then behaved as a secondary photocatalyst on the TiO2 surface acting to improve the hydrogen production rate (1.4 times greater than neat TiO2). When the copper was originally present as Cu0, an improved hydrogen generation rate was also evident (2.4 times greater than Cu2O/TiO2) and the metallic state was retained over the course of the reaction. In this case, the Cu0 deposits function as a co-catalyst for proton reduction. The findings reconcile past disagreements associated with this system, demonstrating both Cu+ (following reduction from Cu2+) and Cu0 are able to photocatalytically generate hydrogen, albeit by distinctly different mechanisms.
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In this study, scalable, flame spray synthesis is utilized to develop defective ZnO nanomaterials for the concurrent generation of H2 and CO during electrochemical CO2 reduction reactions (CO2RR). ...The designed ZnO achieves an H2/CO ratio of ≈1 with a large current density (j) of 40 mA cm−2 during long‐term continuous reaction at a cell voltage of 2.6 V. Through in situ atomic pair distribution function analysis, the remarkable stability of these ZnO structures is explored, addressing the knowledge gap in understanding the dynamics of oxide catalysts during CO2RR. Through optimization of synthesis conditions, ZnO facets are modulated which are shown to affect reaction selectivity, in agreement with theoretical calculations. These findings and insights on synthetic manipulation of active sites in defective metal‐oxides can be used as guidelines to develop active catalysts for syngas production for renewable power‐to‐X to generate a range of fuels and chemicals.
Defective ZnO is reported to generate syngas with a H2/CO ratio of ≈1 and a current density of 40 mA cm−2 during continuous CO2 reduction reactions (CO2RR) at a cell voltage of 2.6 V. Through in situ atomic pair distribution function analysis, the remarkable stability of these ZnO structures, is explored, addressing the knowledge gap in understanding the dynamics of oxide catalysts during CO2RR.
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•Developed a printed, low-cost, flexible thick film Ag/AgCl reference electrode.•Integrated KCl electrolyte and PDMS junction improves performance.•Comparable potential stability to commercial ...electrodes in various environments.•Minimal potential drift across the substantial working lifetime.
Low-cost sensor arrays are required to allow for real-time, in-situ electrochemical monitoring using Internet-of-Things (IoT) systems; however, they are currently not practical due to a lack of stable, mass-producible reference electrodes. To solve this problem, in this work we have developed a screen-printed Ag/AgCl true reference electrode with an offset salt reservoir on a flexible substrate for use in disposable, low-cost sensor arrays. A KCl-containing poly(vinyl acetate) ink was prepared as the solid-state electrolyte, and a PDMS junction membrane was deposited to suppress electrolyte leaching. The potentials of the electrodes with and without the electrolyte and junction membranes were measured versus a commercial saturated calomel reference electrode (SCE) in 0.1 M K2SO4 solution. Potential stability of −45.5 ± 3 mV vs. SCE with low drift was maintained for up to 27 days for electrodes containing both the electrolyte and PDMS layers, compared to less than 1 day without the PDMS junction. The electrodes were found to be stable in solutions at different pH and were also insensitive to most interfering ionic species, including SO42−, I−, Br−, Cl−, F−, Li+, Na+, and K+, under continuous potential measurement with an impedance of ∼ 15 kΩ at 106 Hz. The results demonstrate that the present printed reference electrodes are stable for an extended period and therefore well suited for use in electroanalytical systems for high volume IoT applications.
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Efficient hydrogen production from water by photocatalysis under sunlight requires a significant improvement in light-harvesting capability. Zinc sulfide is a promising, inexpensive hydrogen ...generation photocatalyst, but in its pure, bulk form it is only active under ultra-violet light. Here, we show clear evidence of photoelectrochemical activity of ZnS thin films under visible-light irradiation without any co-catalysts, achieved through defect engineering. Fabrication of nanostructured ZnS under controlled conditions introduces defects, and hence intermediate electronic states within the band gap, which allow significant absorption of light at energies below the band gap energy of pure, bulk ZnS. The measured band gap of the ZnS thin films is ~2.4eV, while the photocurrent density exceeds 1.5mA/cm2 under visible-light irradiation (λ≥435nm). This is the first measurement of such high photocurrents for undoped ZnS under visible light.
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•Crystal defects are introduced in ZnS using controlled film deposition conditions.•High photocurrents are measured for undoped ZnS under visible light.•Defect states allow visible-light absorption and improve charge separation.•This work shows how high photocatalytic solar-to-hydrogen efficiency could be achieved for ZnS.
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Surface coating of a protective layer can prevent the corrosion of Cu2O at electrode liquid junctions (ELJs) in photoelectrochemical water splitting. However, a facile methodology for the deposition ...of a conformal protective layer is still a challenge. Here, an ultrathin layer of amorphous ZnO is introduced on Cu2O by pulsed electrodeposition, to construct a “sandwich” structure of a composite photoelectrode of TiO2/ZnO/Cu2O on an FTO substrate. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) visualises the spatial distribution of Ti, Zn, Cu, and Sn elements of the composite. Benefiting from the homogeneous coating of a ZnO layer, visible cracks in TiO2 coating are significantly reduced, thus preventing the direct contact between the electrolyte and Cu2O. Moreover, due to the ultrathin property of the amorphous ZnO layer, the energetic electrons from the excited Cu2O can be injected via the ZnO layer into TiO2, as elucidated by time-resolved photoluminescence (TRPL) results. The resulting composite photoelectrode shows enhanced photoelectrochemical activity and stability, compared to the bare Cu2O, as well as the TiO2/Cu2O photoelectrode. This study offers a versatile and effective method for improving the stability and charge separation efficiency of Cu2O, which is useful in guiding the surface coating of other nanostructured materials for solar energy conversion.
The electronic properties of transition‐metal‐doped zinc sulfide (ZnS) have been investigated by using first‐principles calculations. Transition‐metal doping can allow electronic transitions at ...energies corresponding to visible‐light wavelengths, thus potentially resulting in increased photocatalytic efficiency under sunlight. In particular, our calculations show that transition‐metal atoms that produce little lattice strain, such as Co, Ni, Mn, and Fe, can be readily incorporated in ZnS. Due to their low formation energies and appropriate band energies, we predict that Ni‐ and Co‐doped ZnS will be promising materials for photocatalytic hydrogen production.
Mind the gap! Transition‐metal doping of ZnS is investigated by first‐principles calculations with the aim of designing a ZnS‐based photocatalyst that can absorb visible light. Formation energies and electronic properties are calculated. Cobalt‐ and nickel‐doping are found to be most promising for photocatalytic hydrogen production under visible light.
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