Non-stoichiometric Composition Research Articles

(Invited) How Does Intercalation Ion Species Determine the Electrochemical Properties of Cathode Materials for Rechargeable Batteries?

Alkali-ion intercalation compounds are widely used as cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. For many years, researchers have developed layered oxide cathode materials for Na- and K-ion batteries given that the layered oxides show high reversible capacity and high working voltage in Li-ion technology. However, our works have demonstrated that the layered oxide compounds may not be good cathode candidates for Na and K systems. There are two main issues:[1-4] (i) Na and K transition metal oxide compounds form non-stoichiometric composition (x<1.0 in A x MO2, A= Na and K). It leads practical difficulty of realizing Na- and K-ion batteries because all the Na and K ions should come from the cathode in rocking-chair batteries. (ii) As the insertion ion size increases, the voltage curve becomes much sloped. The sloped voltage curves result in low specific capacity and average voltage. Both the problems are attributable to much stronger Na+-Na+ and K+-K+ interaction than Li+-Li+ in the layered oxide structure. In this respect, polyanion compounds that have 3 dimensional arrangements of Na and K ions, resulting in longer Na+-Na+ and K+-K+ distance, will be better candidates. We proposed KVPO4F[5] as an example of polyanion compounds and it has stoichiometric composition and provides high average voltage of ~4.3 V (vs. K/K+) with a reversible capacity of ~105 mAh/g.We further investigated the effect of intercalation ion species in K x VPO4F (x~0).[6] Our work demonstrates the voltage for Na intercalation is even higher than that for Li insertion unlike the common belief that Li insertion voltage is always higher than Na insertion. The lower Li intercalation voltage is likely attributed to unstable Li site in large cavity, making less stable discharged product upon Li insertion vs. Na insertion. In addition, we found that Li intercalation is more sluggish than Na and K intercalation in K x VPO4F (x~0). Since Li ion is too small compared to cavity in the cathode, Li ions are undercoordinated in the transition state, in which a Li ion is coordinated by two anions only. Therefore, Li ion migration barrier is much higher than Na and K migration. This finding teaches us that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species.

Effect of Stoichiometric Composition on the Functional Characteristics of Thin-Film Surface Structures on Gyroscope Components

The results of research into the effect of stoichiometry on the properties of titanium nitride thin-film coatings on the components of floated and electrostatic gyroscopes are presented. The article also considers the results of research into the mechanical and optical characteristics of titanium nitride thin-film structures: microhardness, resistance to wear and friction, and image contrast, which is determined by the reflection coefficients of the titanium nitride base surface and the raster pattern generated by the method of local laser oxidation. The future and technological methods of generation of functional surface structures of niobium carbide and niobium nitride in the process of development of a rotor of a cryogenic gyroscope are discussed. It is shown that the formation of a coating of required composition requires a thermodynamic assessment of possible interactions during which structural-phase modification of the material is carried out, determined by the complex of possible topochemical reactions leading to the formation of compounds, including the ones with nonstoichiometric composition.

Effect of Noble Metal Addition on the Disorder Dynamics of Ni3Al by Means of Monte Carlo Simulation

In this work, the effect of the addition of noble metals on the order–order disorder process of the L12 structure corresponding to the intermetallic Ni3Al is analyzed. Stoichiometric, nonstoichiometric, and quasi-binary compositions doped with noble metals such as Ag, Au, Pd, and Pt (1 at%) were analyzed. It was observed that depending on the composition, there is a modification in the activation energies calculated from the two time constants that characterize the disorder process. The statistic of atomic jumps was typified based on the configuration of the window to be crossed and, with this, it was identified that the origin of the negative activation energy of the long disorder process is due to an increase in the corresponding energy of the AlAl-Ni jump through unnatural windows.

L-histidine controls the hydroxyapatite mineralization with plate-like morphology: Effect of concentration and media

Hydroxyapatite (HA) is the main inorganic component of bone and dentin, and their non-stoichiometric compositions and plate-shaped morphology is responsible for their bioactivity and osteoconductive nature. Collagenous (CPs) and non-collagenous proteins (NCPs) facilitate mineralization and regulate structural properties of HA through their side-chains. The bioactivity of synthetic HA does not usually match with the HA found in bone and, therefore, there is a need to understand the role of biomolecules in bone mineralization in order to develop non-stoichiometric plate-shaped HA for bone grafts. Role of several amino acids has been investigated but the role of L-his has been rarely investigated under physiological conditions even though it is a part of HA inhibitor proteins, like albumin, amelogenin, and histidine-rich proteins. In this study, L-his and L-glu were used to modify the structural properties of HA in different experimental conditions and buffer systems (tris and hepes). The results showed that L-his was able to regulate the plate-shaped morphology of HA in every experimental condition, unlike the L-glu, where the crystal morphology was regulated by experimental conditions. Both amino acids behaved differently in DI water, tris, and hepes buffer, and the media used influenced the precipitation time and structural properties of HA. Hepes and tris buffers also influenced the HA precipitation process. Overall, the studies revealed that L-his may be used as an effective regulator of plate-shaped morphology of HA, instead of large NCPs/proteins, for designing biomaterials for bone regeneration applications and the choice of buffer system is important in designing and evaluating the systems for mineralization. In cell culture studies, mouse osteoblast precursor cells (MC3T3-E1) showed highest proliferation on the bone-like plate-shaped HA, among all the HA samples investigated.

Electronic structure and sodium-ion diffusion in glaserite-type A3−хNa1+х(MoO4)2 (A = Cs, K) studied with first-principles calculations

Here we present an ab initio insight into the electronic structure and sodium diffusion in A3−хNa1+х(MoO4)2 with trigonal (A = Cs) and monoclinic (A = K) glaserite-type structures. Our DFT calculations predict the stability of К3−xNa1+x(МoO4)2 to high sodium content and a significantly smaller homogeneity region for Cs3−xNa1+x(МoO4)2 in accordance with experimental findings. These molybdates are wide-gap insulators with a band gap of 4.2 eV, which is very weakly dependent on the sodium content. The calculated electric field gradients at the Na and A sites were related to the anisotropy of electronic charge distribution and used to predict the quadrupole frequencies of the 133Cs, 39K and 23Na NMR lines in Cs3−хNa1+х(MoO4)2 and K3−хNa1+х(MoO4)2. We examined the possible diffusion paths of Na+ ions and showed that the direct Na−Na migration is unlikely due to a large energy barrier in both molybdates. Instead, the Na+ ions can migrate through the A positions with much lower barriers in A3−xNa1+x(МoO4)2. Therefore, designing stable non-stoichiometric compositions may be a way to reach good sodium-ion diffusion in double molybdates with the glaserite-type structure.

Improving the soft X-ray reflectivity of Cr/Ti multilayers by co-deposition of B4C.

The `water window', covering 2.4-4.4 nm, is an important wavelength range particularly essential to biology research. Cr/Ti multilayers are one of the promising reflecting elements in this region because the near-normal-incidence reflectivity is theoretically as high as 64% at 2.73 nm. However, due to multilayer imperfections, the reported reflectivity is lower than 3% for near-normal incidence. Here, B and C were intentionally incorporated into ultra-thin Cr/Ti soft X-ray multilayers by co-deposition of B4C at the interfaces. The effect on the multilayer structure and composition has been investigated using X-ray reflectometry, X-ray photoelectron spectroscopy, and cross-section electron microscopy. It is shown that B and C are mainly bonded to Ti sites, forming a nonstoichiometric TiBxCy composition, which hinders the interface diffusion, supresses the crystallization of the Cr/Ti multilayer and dramatically improves the interface quality of Cr/TiBxCy multilayers. As a result, the near-normal-incidence reflectivity of soft X-rays increases from 4.48% to 15.75% at a wavelength of 2.73 nm.

Effect of quenching rate on crystalline and impedance properties of NiO nanoparticles

NiO nanopowder is produced through plasma-chemical synthesis in a low-pressure arc reactor in an atmosphere of 80% Ar + 20% O 2 at a pressure of 40 (sample 1) and 180 Pa (sample 2). The crystalline, morphological, and impedance properties are studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and impedance spectroscopy. It is shown that an increase in pressure leads to the synthesis of nanomaterials with fundamentally different crystalline and electrical properties. Sample 1 shows a non-stoichiometric composition (Ni 42 O 58 ), a face-centered cubic lattice, and a size of coherent scattering regions (CSR) of 16 nm. The impedance studies shows that the frequency dependences are well described by the Debye relaxation model, at low frequencies the real part of the dielectric constant is equal to 35, and thermal studies show a negative temperature coefficient of resistance (NTCR). Sample 2 has an amorphous structure, almost stoichiometric composition (Ni 48 O 52 ). At low frequencies, the real and imaginary parts of the dielectric constant reaches 105. The anomaly is explained by the formation of a double electric layer near the electrodes due to proton-ion conductivity.

Effect of MgO/SiO2 ratio and Al2O3 content on crystallization behavior and properties of cordierite-based glass–ceramics

Cordierite-based glass-ceramics with non-stoichiometric compositions were prepared by powder sintering method. The effects of MgO/SiO2 ratio and Al2O3 content on the crystallization behavior, microstructure and thermal expansion coefficient of the glass-ceramics were studied. Results indicated that the decreasing Al2O3 content showed a greater effect on decreasing the melting temperature and liquid viscosity than the increasing MgO/SiO2. DTA results showed that, at low temperatures, the MgO/SiO2 ratio dominated crystallization of cordierite due to the lack of liquid. At a high sintering temperature of 1340 °C, the Al2O3 content dominated the crystal growth of cordierite due to its great effect on decreasing the liquid viscosity. The chemical composition close to the stoichiometric cordierite and the absence of quartz were two essential factors for obtaining low thermal expansion coefficients. The decreasing MgO/SiO2 ratio and Al2O3 content caused phase separation with an interconnected structure, which followed the crystallization of cordierite in the Al2O3-rich phase.

New synthesis route to kesterite Cu2ZnSnS4 semiconductor nanocrystalline powders utilizing copper alloys and a high energy ball milling-assisted process

Presented is a new mechanochemically assisted synthesis route to kesterite Cu2ZnSnS4 semiconductor for perspective photovoltaic applications. Initially, the combined copper Cu, zinc Zn, and tin Sn powders in atomic ratio 2:1:1 were subjected to high energy ball milling in a planetary ball mill under the highest reported to date rotation speeds of 900 and 1000 rpm to yield a definite nanopowder mixture of the copper intermetallic phases, i.e., γ Cu5Zn8-type and η’ Cu6Sn5-type of non-stoichiometric compositions. Upon addition of a suitable amount of sulfur S, the reactive milling was continued affording a pure sphalerite-type cubic phase tentatively called a pre-kesterite. As expected from our earlier research, this by-product did not exhibit any 65Cu and 119Sn MAS NMR spectra or clearly-defined absorption in the UV–vis range. The disordered and very much random Cu-Zn-Sn metal site occupation of all metal layers in the structure is proposed to account for the observed properties of pre-kesterite. The subsequent annealing under argon at temperatures around 500−550 °C resulted in the cubic phase conversion to the tetragonal phase of kesterite, prevailingly, by metal center reconfiguration into distinct Cu-Sn and Cu-Zn layers accompanied by crystal growth/particle recrystallization. The micro-Raman spectra were consistent with similar bonding and lattice phonon characteristics in both related materials. The XPS data confirmed the same oxidation states and comparable energy level characteristics of the constituent elements. The annealed nanocrystalline powders showed the anticipated 65Cu and 119Sn MAS NMR features, while supporting remnant paramagnetism, and UV–vis semiconducting characteristics of the target kesterite.

Effects of sintering temperature on ac conductivity, permittivity, and permeability of manganese doped cobalt ferrite nanoparticles

This paper reports on the effects of magnetic and electric properties of a non-stoichiometric composition of cobalt ferrite nanoparticles doped with manganese content at the weight percent 0.25. Its toroid and disc-shaped samples were sintered at two different temperatures 750 °C and 1050 °C. The studied results show the dependence of the permittivity and permeability on the microstructural features, particularly crystallite/grain size that change with the sintering temperature. The anomalous behavior of ac conductivity is marked outside a hump region with a decreased sintering temperature. A rightward shift of the characteristic frequency in the absorption of electric modulus is observed with the increased sintering temperature. Similarly, a rightward shift of the resonance frequency in the spectra of the imaginary part of the magnetic modulus is marked with the increased sintering temperature. The higher magnitude of the imaginary part of the electric modulus at the characteristic frequency is expected to dissipate as eddy current loss. Similarly, the higher magnitude of the imaginary part of the magnetic modulus at the resonance frequency is assumed to dissipate as a residual loss. Both of them are selectable by the sintering temperature. This behavior may make this material suitable to be used in thermoelectric and thermomagnetic sensors/switches.

Influence of A-site non-stoichiometry on electromechanical properties of Sr(Hf0.5Zr0.5)O3-modified Bi0.5(Na0.8K0.2)0.5TiO3 piezoelectric ceramics

Effect of Na non-stoichiometry in Sr(Hf0.5Zr0.5)O3-modified Bi0.5(Na(0.8+x)K0.2)0.5TiO3 ceramics was examined in the range of − 0.04 ≤ x ≤ 0.04. The effect of Na non-stoichiometry on microstructure, ferroelectric, piezoelectric, dielectric properties and high-temperature impedance was explored. Compared to the stoichiometric compositions, Na excess and deficiency exhibit the same effect as observed for the acceptor and donor doping, respectively. All non-stoichiometric and stoichiometric compositions exhibit a pure ABO3 perovskite phase with pseudocubic symmetry. The deficiency of Na in ceramics decreases the grain size and drives the system towards ergodic relaxor as evident by pinched ferroelectric loop and the negligible negative strain along with an increase in maximum strain (Sm). The corresponding normalized strain (Sm/Emax.) increased from 275 to 800 pm/V when Na content decreased from x = + 0.04 to − 0.04. In contrast, excess Na exhibits the converse effect and drives the system towards ferroelectricity. Composition with deficient Na also exhibits lower dielectric loss and increase in resistivity by an order of magnitude compared to excess Na as observed from the high-temperature dielectric and impedance measurements. The results of this study demonstrate that Na non-stoichiometry significantly affects the electric properties of the studied system and thus can be effectively modulated to achieve improved electromechanical properties for materials having actuator-based applications.

Structural and Energy Characteristics of NiAl Alloys with Deviations from Stoichiometric Composition. Part 2

The analysis of the atomic and phase structure confirmed the difference between the structural-phase states at the heating and cooling stages. Based on the analysis of the influence of the deviation of the atomic composition from the stoichiometric condition of the intermetallic compound during cooling, it is established that the deviation is a significant factor in the area of low-stability pre-transitional structural-phase states before the transformation. The behavior of the long-range parameter temperature dependence curves during the cooling of alloys of non-stoichiometric compositions differs significantly from the behavior of the corresponding alloy curve of a stoichiometric composition alloy. It is shown that in the case of cooling of alloys of non-stoichiometric compositions, a significant supercooling is required to establish a long-range order, and the appearance of ordered phases occurs at significantly lower temperatures. It is found that the temperature dependence curve of the long-range parameter of the Ni45Al55 alloy is significantly lower than the corresponding curve of the Ni55Al45 alloy. This indicates different mechanisms for establishing the long-range order of alloys with non-stoichiometric compositions. It is noted that the deviation of the system composition from the stoichiometric one causes a significant reduction of ordered and disordered regions. This article is a continuation of the work published earlier in the journal Izvestiya AltGU. 2020. №1 (111).

Platinum Group Element Enrichment of Natural Quenched Sulfide Solid Solutions, the Norilsk 1 Deposit, Russia

Abstract The deepest terminations of the Mount Rudnaya subvertical massive sulfide offshoots of the Norilsk 1 orebody are composed of exceptionally fine grained sulfides that are believed to be natural quenched sulfide solid solutions. Copper-rich intermediate solid solution (ISS) and Fe-rich monosulfide solid solution (MSS) form an equigranular and lamellar matrix hosting MSS- and ISS-dominant globules. The nonstoichiometric chemical compositions of the solid solutions plot within their high-temperature fields known from experiments. MSS contains 19 to 35 wt % Ni, 0.09 to 0.45 wt % Co, and up to 0.6 wt % Cu and is heterogeneously enriched in Rh (up to 32 ppm), Ir (up to 0.6 ppm), Pt (up to 65 ppm), and Pd (up to 168 ppm). ISS occurs as the lamellar intergrowths of the chalcopyrite (Ccpss) and cubanite (Cubss) solid solutions, which bear up to 4.74 wt % Ni and 0.2 wt % Co and are heterogeneously enriched in Zn, Ag, and In. The assemblage of platinum group minerals (PGMs) is hosted mostly in the ISS and is dominated by Pt-Fe alloys and minerals of the rustenburgite-atokite series, like the set of PGMs at the Norilsk 1 deposit. Similar Pt-Pd-Sn compounds in the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) spectra of profiles through MSS and ISS are interpreted to be trapped microinclusions. The pentlandite contains up to 0.13 wt % Pt, up to 4.62 wt % Pd, &amp;lt;0.53 wt % Co, and &amp;lt;0.4 wt % Cu according to electron microprobe analysis. LA-ICP-MS data and mapping show that Pd content in the pentlandite increases toward contacts with ISS and decreases toward contacts with MSS, supporting a reaction origin of pentlandite. The wide variations of the concentrations of major and trace elements in the solid solutions, as well as the coexistence of Pd-poor (a few ppm Pd) and Pd-rich (over 4.62 wt % Pd) pentlandite within a single sample, seem to characterize the different generations of the MSS to MSS-ISS globules, antecrysts, and phenocrysts with the distinct histories of enrichment due to exchange with fractionated Cu-platinum group element-rich residue. The directional distribution of Pd of high-temperature primary magmatic origin is preserved due to rapid quenching of the sulfides from ~650°C.

Band engineering of Dirac cones in iron chalcogenides

By band engineering the iron chalcogenide Fe(Se,Te) via ab-initio calculations, we search for topological surface states and realizations of Majorana bound states. Proposed topological states are expected to occur for non-stoichiometric compositions on a surface Dirac cone where issues like disorder scattering and charge transfer between relevant electronic states have to be addressed. However, this surface Dirac cone is well above the Fermi-level. Our goal is to theoretically design a substituted crystal in which the surface Dirac cone is shifted towards the Fermi-level by modifying the bulk material without disturbing the surface. Going beyond conventional density functional theory (DFT), we apply the coherent potential approximation (BEB-CPA) in a mixed basis pseudo-potential framework to scan the substitutional phase-space of co-substitutions on the Se-sites. We have identified iodine as a promising candidate for intrinsic doping. Our specific proposal is that FeSe$_{0.325}$I$_{0.175}$Te$_{0.5}$ is a very likely candidate to exhibit a Dirac cone right at the Fermi energy without inducing strong disorder scattering.

Formation and energetics of amorphous rare earth (RE) carbonates in the RE2O3–CO2–H2O system

Amorphous materials are of interest in the genesis of crystalline solids. They are often the first to precipitate from supersaturated solutions and are in the nanometer size range, transforming into a nanocrystalline metastable phase, and then to thermodynamically stable phases. One must seek knowledge of the composition, structure, thermodynamics, kinetics, and reaction pathways that relate the amorphous and crystalline phases. The subject of this study is the amorphous phases in the system RE2O3−CO2–H2O (RE- La, Nd, Dy,Yb). They were synthesized via direct precipitation and urea hydrolysis, and characterized by powder XRD, DTA with mass spectrometric analysis of the evolved gases. Phases have the non-stoichiometric composition RE2O3·xCO2·yH2O (1 <x < 3, 3 <y <7), which is different from that of crystalline simple carbonates and hydroxycarbonates. Thus we consider them to be amorphous precursors, rather than amorphous carbonates. High temperature oxide melt solution calorimetry in molten sodium molybdate solvent was used to derive their enthalpies of formation from oxides and elements. Increase of energetic stability per mole of RE2O3 compounds occurs in the order: RE2O3 → 2RE(OH)3→ 2REO(OH) → RE2O2CO3 → Amorphous precursor → 2(REOHCO3) → RE2(CO3)3·yH2O. We calculated the enthalpies of possible transformations of amorphous precursors to crystalline phases and conclude that thermodynamics determines the dependence of the crystallizing products on the temperature and partial pressure of CO2 and H2O. Amorphous precursors are clearly intermediate in the synthesis, and ternary phases containing both H2O and CO2 compete with each other in terms of thermodynamic stability. The energy landscape obtained here will allow one to directly synthesize specific products and control their functionality.

The choice of Zn or ZnS layer in the stacked precursors for preparation of Cu2ZnSnS4 (CZTS) thin films

Cu2ZnSnS4 (CZTS) thin films are commonly used as an absorber layer in the thin film solar cell structure. In this study, CZTS thin films were produced by sulfurization of stacked precursor films that is prepared by deposition of Cu, Zn, SnS, ZnS films on glass substrate using sputtering method. The sequential sputter deposition was performed to obtain two distinct stacked precursors, Cu/SnS/Zn/Cu and Cu/SnS/ZnS/Cu respectively. Afterwards, annealing process was implemented at various reaction temperatures (500–575 °C) for 1 min utilizing rapid thermal processing (RTP). The EDX measurements revealed that all the prepared CZTS samples had Cu-poor and Zn-rich composition that are non-stoichiometric chemical composition. This non-stoichiometric composition is important for high efficient CZTS based solar cells. XRD measurements revealed that all patterns are dominated by diffraction planes of kesterite CZTS. The CuS, Cu2S and SnS2 secondary phases also were detected in the XRD pattern of some CZTS films. Raman spectroscopy measurements verified formation of kesterite CZTS phase for all films and picked out CTS phase for some samples prepared using Cu/SnS/Zn/Cu precursor films. The surface microstructure of the films that were obtained through SEM displayed polycrystalline surface structure. Room temperature PL emission spectra of the films showed broad peak at around 1.37–1.38 eV, which is near to the optical band gap of kesterite CZTS structure. Electrical characterization of the samples demonstrated that B-525 CZTS thin film has more suitable electrical resistivity and carrier concentration values for CZTS based solar cell applications.

Synthesis, Properties, and Visible Light Photocatalytic Activity of Nonstoichiometric Titanium Dioxide-Based Composites

We have synthesized oxide composites based on TiO2 modified with aliovalent cations, W6+, Co3+, Cu2+, and Fe3+, in the range 5–30 wt %. Its deviation from stoichiometry is increased by active generation of Ti3+ cations during vacuum calcination at temperatures from 500 to 800°C. The photocatalytic activity (PCA) of the composites for dye degradation reactions under illumination with visible light exceeds that of analogous samples calcined in air. The PCA of the synthesized materials is an intricate function of their composition and structure. The highest PCA, which considerably exceeds the PCA of TiO2 of similar origin and that of the commercially available Degussa P-25 TiO2, is offered by the optimal compositions of the W- and Cu-modified samples for ferroin and methylene blue degradation and the Co- and Fe-modified samples for aniline degradation.

Design of (Hf,Ta)Fe2/Fe composite with zero thermal expansion covering room temperature

MgZn2-type (Hf,Ta)Fe2 is a typical negative-thermal-expansion system, which expands during the establishment of ferromagnetic ordering in cooling. This abnormal behavior suggests that (Hf,Ta)Fe2 is a potential ingredient to produce a composite with zero thermal expansion. In this work, we find that the as-cast (Hf,Ta)Fe2 have Fe-rich grain boundaries, which disappear after annealing. It inspires us to produce a (Hf,Ta)Fe2/Fe composite by adding extra Fe in nonstoichiometric composition without annealing. After carefully tuning Hf/Ta ratio and Fe concentration, room-temperature zero-thermal-expansion is achieved in the sample with a composition of Hf0.80Ta0.20Fe2.5.

A Hydrogen-Deficient Nickel-Cobalt Double Hydroxide for Photocatalytic Overall Water Splitting.

Developing highly efficient and low-cost photocatalysts for overall water splitting has long been a pursuit for converting solar power into clean hydrogen energy. Herein, we demonstrate that a nonstoichiometric nickel-cobalt double hydroxide can achieve overall water splitting by itself upon solar light irradiation, avoiding the consumption of noble-metal co-catalysts. We employed an intensive laser to ablate a NiCo alloy target immersed in alkaline solution, and produced so-called L-NiCo nanosheets with a nonstoichiometric composition and O2- /Co3+ ions exposed on the surface. The nonstoichiometric composition broadens the band gap, while O2- and Co3+ ions boost hydrogen and oxygen evolution, respectively. As such, the photocatalyst achieves a H2 evolution rate of 1.7 μmol h-1 under AM 1.5G sunlight irradiation and an apparent quantum yield (AQE) of 1.38 % at 380 nm.

A Hydrogen‐Deficient Nickel–Cobalt Double Hydroxide for Photocatalytic Overall Water Splitting

AbstractDeveloping highly efficient and low‐cost photocatalysts for overall water splitting has long been a pursuit for converting solar power into clean hydrogen energy. Herein, we demonstrate that a nonstoichiometric nickel–cobalt double hydroxide can achieve overall water splitting by itself upon solar light irradiation, avoiding the consumption of noble‐metal co‐catalysts. We employed an intensive laser to ablate a NiCo alloy target immersed in alkaline solution, and produced so‐called L‐NiCo nanosheets with a nonstoichiometric composition and O2−/Co3+ ions exposed on the surface. The nonstoichiometric composition broadens the band gap, while O2− and Co3+ ions boost hydrogen and oxygen evolution, respectively. As such, the photocatalyst achieves a H2 evolution rate of 1.7 μmol h−1 under AM 1.5G sunlight irradiation and an apparent quantum yield (AQE) of 1.38 % at 380 nm.