Nanosheets of In2S3/S-C3N4-Dots for Solar Water-Splitting in Saline Water.

Hydrogen generation from splitting of water under the photoelectrochemical (PEC) pathway is considered as the most promising strategy for covering the upcoming fuel crisis by taking care of all environmental issues. In this context, In2S3 can be explored as it is a visible light-active semiconductor with an appropriate band alignment with the water redox potential. Herein, In2S3 nanosheets are developed by the chemical method. The nanosheets of In2S3 absorb high visible light due to the manifold inside scattering and reflection. The PEC activity of In2S3 is enhanced because of the increase in the light absorbance of the materials. In the present work, at 1.18 V versus RHE in 3.5 wt % NaCl, a maximum 2.07 mA/cm2 photocurrent density can be achieved by In2S3 nanosheets. However, In2S3 suffers strongly due to photo-corrosion. To improve the efficacy of the In2S3 nanosheets in saline water, the charge-carrier transportation ability of In2S3 is aimed to increase by decorating S-C3N4-dots on In2S3. The heterostructure of type-II is developed by sensitization of S-C3N4-dots on In2S3. It increases both the transportation of charge carriers as well as separation. In the heterostructure, the transient decay time (τ) increases, which indicates a decrease in photogenerated charge-carrier recombination. S-C3N4-dots also act as an optical antenna and increase the range of visible light absorbance of In2S3. The heterostructure can generate ∼2.38-fold higher photocurrent density of 1.18 V versus RHE in 3.5 wt % NaCl. The photoconversion efficiency of the heterostructure is 0.88% at 0.95 V versus RHE. The nanosheets of In2S3 and In2S3/S-C3N4-dots are stable, and photocurrent density is measured up to 2700 s under continuous back-illumination conditions.

Doping of MoS2 by "Cu" and "V": An Efficient Strategy for the Enhancement of Hydrogen Evolution Activity.

To replace Pt-based compounds in the electrocatalytic hydrogen evolution reaction (HER), MoS2 has already been established as an efficient catalyst. The electrocatalytic activity of MoS2 is further improved by tuning the morphology and the electronic structure through doping, which helps the band energy position to be modified. Presently, thin sheets of MoS2 (MoS2-TSs) are synthesized via a microwave technique. Thin sheets of MoS2 can outperform nanosheets of MoS2 in the HER. Further, the efficiency of the thin sheets is improved by doping with different metals like Cu, V, Zn, Mn, Fe, Sn, etc. "Cu"- and "V"-doped MoS2-TSs are highly efficient for the HER. At a fixed potential of -0.588 V vs RHE, Cu-doped MoS2 (Cu-MoS2-TS), V-doped MoS2 (V-MoS2-TS), and MoS2-TS can generate current densities of 327.46, 308.45, and 127.82 mA/cm2, respectively. The electrochemically active surface area increases nearly 7.7-fold and 2.5-fold for Cu-MoS2-TS and V-MoS2-TS than for MoS2-TS, respectively. Cu-MoS2-TS shows exceptionally high electrocatalytic stability up to 140 h in an acidic medium (0.5 M H2SO4). First-principles calculations using density functional theory (DFT) are performed, which are well matched with the experimental observations. DFT calculations dictate that after doping with "V" and "Cu" both valance band maxima and conduction band minima are uplifted, which indicates the higher hydrogen-ion-reducing ability of M-MoS2-TS (M = Cu, V) compared to bare MoS2-TS.

Aggregates of Ni/Ni(OH)2/NiOOH Nanoworms on Carbon Cloth for Electrocatalytic Hydrogen Evolution.

The development of an efficient electrocatalyst for hydrogen evolution reaction (HER) is essential to facilitate the practical application of water splitting. Here, we aim to develop an electrocatalyst, Ni/Ni(OH)2/NiOOH, via electrodeposition technique on carbon cloth, which shows efficient activity and durability for HER in an alkaline medium. Phase purity and morphology of the electrodeposited catalyst are determined using powder X-ray diffraction and electron microscopic techniques. The compositional and thermal stability of the catalyst is checked using X-ray photoelectron spectroscopy and thermogravimetry analysis. Electrodeposited Ni/Ni(OH)2/NiOOH material is an efficient, stable, and low-cost electrocatalyst for hydrogen evolution reaction in a 1.0 M KOH medium. The catalyst exhibits remarkable performance, achieving a current density of 10 mA/cm2 at a potential of -0.045 V vs reversible hydrogen electrode (RHE), and the Tafel slope value is 99.6 mV/dec. The overall electrocatalytic water splitting mechanism using Ni/Ni(OH)2/NiOOH catalyst is well explained, where formation and desorption of OH- ion on the catalyst surface are significant at alkaline pH. The developed electrocatalyst shows significant durability up to 200 h in a negative potential window in a highly corrosive alkaline environment along with efficient activity. The electrocatalyst can generate 165.6 µmol of H2 in ∼145 min of reaction time with 81.5% faradic efficiency.

Type-II Heterostructure of ZnO and Carbon Dots Demonstrates Enhanced Photoanodic Performance in Photoelectrochemical Water Splitting.

Hydrogen evolution through ecofriendly photoelectrochemical (PEC) water splitting is considered to be one of the most cost-effective and desirable methods for meeting ever-growing energy demands. However, the low photoconversion efficiency limits the practical applicability of PEC water splitting. To develop an efficient photoelectrode, here the morphology of ZnO is tuned from 0D to 3D. It is observed that vertically grown 2D nanosheets outperform other morphologies in PEC water splitting by generating nearly 0.414 mA cm-2 at 0 V vs Ag/AgCl. Furthermore, these perpendicularly developed 2D nanosheets of ZnO are sensitized by metal-free carbon (C) dots to improve the photoconversion efficiency of ZnO. The prepared ZnO/C dots work as an effective photoanode, which can produce a 0.831 mA cm-2 photocurrent density upon application of 0 V vs Ag/AgCl under constant illumination, which is 2 times higher than that of bare ZnO. The enhanced PEC performance of ZnO/C dots is confirmed by the photoconversion efficiency (η). The ZnO/C dots exhibit a 2-fold-higher photoconversion efficiency (η) compared to that of ZnO. Additionally, the enhancement in PEC activity of ZnO/C dots is attributed to the higher carrier concentrations in the heterostructure. Bare ZnO has a 1.77 × 1020 cm-3 carrier density, which becomes 3.70 × 1020 cm-3 after sensitization with C dots. Enhanced carrier density successively leads to higher PEC water splitting efficiency. Band alignments of ZnO and C dots indicate the creation of the type-II heterostructure, which facilitates successful charge transportation among C dots and ZnO, producing a charge-carrier separation. Two-dimensional sheets of ZnO and ZnO/C dots exhibit appreciable stability under continuous illumination for 1 and 2 h, respectively.

2D Thin Sheet Heterostructures of MoS2 on MoSe2 as Efficient Electrocatalyst for Hydrogen Evolution Reaction in Wide pH Range.

Two-dimensional layered transition metal dichalcogenides, MoSe2 and MoS2, have drawn potential attention in the field of water splitting. Coupling of MoS2 and MoSe2 provides a sustainable route to improve the electrocatalytic activity for the hydrogen evolution reaction (HER). Here, the heterostructures of thin sheets (ts) of MoSe2 and MoS2 are combined to develop the MoSe2-ts@MoS2-ts heterostructure via multiple-step methodology. First, thin sheets of MoSe2 are synthesized following the stepwise hydrothermal method. After the successful synthesis of MoSe2-ts, MoS2-ts is synthesized on it to develop the heterostructure: MoSe2-ts@MoS2-ts. By tuning the amount of MoS2-ts and MoSe2-ts in the heterostructure separately, the optimum condition is obtained for HER. The unique heterostructure is efficient for HER under wide pH conditions like 1 M KOH, pH-7 phosphate buffer, 3.5% saline water, and finally 0.5 M H2SO4. MoSe2-ts@MoS2-ts can generate 10 mA/cm2 current density under the application of -0.186 V vs RHE with a low Tafel value of 71 mV/decade. The formation of the heterojunction plays an essential role in facilitating charge transportation. Furthermore, the heterostructure provides the more active sites for the adsorption of hydrogen to generate H2. An excess amount of any of the bare counter parts in the heterostructure leads to a decrease in electrocatalytic efficiency because of the lowered heterojuction formation. MoSe2-ts@MoS2-ts has very high stability during the electrocatalytic reaction, which is determined from 1000 consecutive cycles and a 24 h prolonged scan. MoSe2-ts@MoS2-ts can generate 147 µmol of H2 in ∼50 min of reaction time with 100% Faradaic efficiency.

Ag2S/Ag Heterostructure: A Promising Electrocatalyst for the Hydrogen Evolution Reaction.

Different metal chalcogenides, being a potential candidate for hydrogen evolution catalysts, have attracted enormous attention in the field of water splitting. In the present study, Ag2S/Ag is revealed as an efficient catalyst for hydrogen evolution. When a sacrificial template of the CuS nanostructure is used, Ag2S/Ag heterostructures are synthesized following a simple wet-chemical technique. Two different routes, wet chemical and hydrothermal, are followed to modulate the morphology of the CuS templates from flower ball to wirelike structures, which subsequently results in the formation of Ag2S nanostructure. Finally, the Ag layer is deposited on Ag2S with the help of a photoreduction technique. The unique heterostructure of Ag2S/Ag shows efficient catalytic activity in the H2 evolution reaction. A Ag2S/Ag wire can successfully generate a 10 mA/cm2 current density at a -0.199 V potential. Ag2S/Ag contains the micronanostructure where nanoplates of Ag2S/Ag assemble to give rise to microstructures such as flower balls and wire.

Synthesis of Monometallic (Au and Pd) and Bimetallic (AuPd) Nanoparticles Using Carbon Nitride (C3N4) Quantum Dots via the Photochemical Route for Nitrophenol Reduction.

In this study, we report the synthesis of monometallic (Au and Pd) and bimetallic (AuPd) nanoparticles (NPs) using graphitic carbon nitride (g-C3N4) quantum dots (QDs) and photochemical routes. Eliminating the necessity of any extra stabilizer or reducing agent, the photochemical reactions have been carried out using a UV light source of 365 nm where C3N4 QD itself functions as a suitable stabilizer as well as a reducing agent. The g-C3N4 QDs are excited upon irradiation with UV light and produce photogenerated electrons, which further facilitate the reduction of metal ions. The successful formation of Au, Pd, and AuPd alloy nanoparticles is evidenced by UV-vis, powder X-ray diffraction, X-ray photon spectroscopy, and energy-dispersive spectroscopy techniques. The morphology and distribution of metal nanoparticles over the C3N4 QD surface has been systematically investigated by high-resolution transmission electron microscopy (HRTEM) and SAED analysis. To explore the catalytic activity of the as-prepared samples, the reduction reaction of 4-nitrophenol with excellent performance is also investigated. It is noteworthy that the synthesis of both monometallic and bimetallic NPs can be accomplished by using a very small amount of g-C3N4, which can be used as a promising photoreducing material as well as a stabilizer for the synthesis of various metal nanoparticles.

Heterostructure of Si and CoSe2: A Promising Photocathode Based on a Non-noble Metal Catalyst for Photoelectrochemical Hydrogen Evolution.

Development of a solar water splitting device requires design of a low-cost, efficient, and non-noble metal compound as alternative to noble metals. For the first time, we showed that CoSe2 can function as co-catalyst in phototoelectrochemical hydrogen production. We designed a heterostructure of p-Si and marcasite-type CoSe2 for solar-driven hydrogen production. CoSe2 successively coupled with p-Si can act as a superior photocathode in the solar-driven water splitting reaction. Photocurrents up to 9 mA cm(-2) were achieved at 0 V vs. reversible hydrogen electrode. Electrochemical impedance spectroscopy showed that the high photocurrents can be attributed to low charge transfer resistance between the Si and CoSe2 interfaces and that between the CoSe2 and electrolyte interfaces. Our results suggest that this CoSe2 is a promising alternative co-catalyst for hydrogen evolution.

Selective and sensitive recognition of Cu2+ in an aqueous medium: a surface-enhanced Raman scattering (SERS)-based analysis with a low-cost Raman reporter.

In the present study, surface-enhanced Raman spectra of a bifunctional Raman reporter, 2-mercaptobenzimidazole, has been found to be responsive exclusively towards Cu(2+) ions while the reporter remains anchored on the Au nanoparticle surface. Thus a specific Cu(2+)-ion-detection protocol emerges. The simplicity, sensitivity, and reproducibility of the method allow routine and quantitative detection of Cu(2+) ions. An interference study involving a wide number of other metal ions shows the procedure to be uniquely selective and analytically rigorous. A theoretical study was carried out to corroborate the experimental results. Finally, the method is promising for real-time assessment of Cu(2+) ions in aqueous samples and also has the ability to discriminate Cu(I) and Cu(II) ions in solution.

Redox-switchable superhydrophobic silver composite.

Unique packaging of Ag(2)O on the surface of polycrystalline AgCl allows fabrication of a new useful, superhydrophobic composite material. This pure inorganic material with surface porosity of submicrometer aperture size fabricates air pockets, which make the composite material superhydrophobic. The new material behaves like lotus leaves, butterfly wings, or water strider's leg in relation to superhydrophobicity. Visible light induces photoreduction of solid Ag(2)O surface layer and generates Ag(0), making the composite surface superhydrophilic. Reoxidation of Ag(0) on the composite surface gives back the hydrophobicity that represents the redox-switchable wetting property of the material.

Fabrication of large-scale hierarchical ZnO hollow spheroids for hydrophobicity and photocatalysis.

We report here the preparation of a crystalline, pure hexagonal phase of ZnO as hollow 500-800 nm spheroids in the presence of organic bases, such as pyridine, using zinc acetate as the precursor salt. The spheroids exhibit unique 3D hierarchical architectures, like cocoons, and demonstrate improved superhydrophobic (water contact angle, 150 degrees) character due to the inherited air-trapped capillarity within the cocoon structure. The simple synthetic strategy used in this process is modified hydrothermolysis (MHT), which represents a general approach and may contribute to the formation mechanism of the hollow nanostructures with highly improved porosity. Depending on the concentration of the precursor salt, it has been possible to cover glass plates or the inner wall of a reaction vessel with ZnO nanocrystals. A low salt concentration (<0.01 M) allows the easy preparation of a superhydrophobic glass surface, whereas a high salt concentration (>0.01 M) results in the precipitation of cocoons at the bottom of the reaction vessel as a solid mass together with a deposited thin film of ZnO nanocrystals covering the inner wall of the glass vessel. The thickness of the film successively grows through repetitive hydrothermolysis processes for which a low salt concentration (<0.01 M) was employed. Because of the hollow cocoon-like morphology, the surface area of the film is greatly increased, which makes it accessible for functionalization by incoming substrates from both sides (internally and externally) and helps to drive a competent photocatalytic dye degradation pathway. The heterocyclic base pyridine exclusively develops cocoons. Thus, the mechanism of self-aggregation of ZnO nanocrystals under MHT reaction conditions has been studied and the characterization of the compounds has been supported with physical measurements.

Electrostatic field force directed gold nanowires from anion exchange resin.

We have developed a polarization-induced growth process to synthesize gram quantity of gold nanowire (Au NW) on the outer surface of an anion exchange resin matrix. This new, simple, modified hydrothermolysis (MHT) procedure involving resin-bound HAuCl(4) produced micrometer long Au nanowire on resin surface. The charged resin matrix responsibly imposes electrostatic field effect (EFF) for 1D growth of Au NWs in the presence of different amines or derivatives of amines. The Au nanowire is separated from resin by sonication. Again, the synthesis of MnO(2) nanowire with resin support through similar MHT strengthens the 1D growth proposition, that is, EFF-induced polarization effect.

Hierarchical superparamagnetic magnetite nanowafers from a resin-bound [Fe(bpy)3]2+ matrix.

The brilliant red [Fe(bpy)(3)](2+) complex upon immobilization on a strongly acidic cation exchanger or in situ formation of the same cationic complex onto a resin matrix and subsequent modified hydrothermolysis (MHT) at approximately 110 degrees C produces unusually stable hierarchical magnetite (Fe(3)O(4)) nanowafers. The slow hydrothermolysis, oxidation, and subsequent dehydration of the complex on the solid-liquid interface produce stable hierarchical nanostructures. The isolation of neat Fe(3)O(4) (uncapped) particles from the resin matrix as hierarchical nanowafers was achieved by magnetically stirring a CH(3)CN suspension of nanocomposites. The solid resin support not only aids nanowafer formation on its surface but also provides unique stability to the magnetite particles, where nanowafer oxidation is largely retarded. The utility of the as-prepared porous nanocomposite and characterization of the nanoparticles are promising for nanotechnological and soft ferromagnetic applications.

An aminolytic approach toward hierarchical ß-Ni(OH)(2) nanoporous architectures: a bimodal forum for photocatalytic and surface-enhanced raman scattering activity.

A surfactantless, trouble-free, and gentle wet chemistry approach has been used to interpret the precisely controlled growth of ß-Ni(OH)(2) with the assistance of ammonia and nickel acetate from seedless mild hydrothermal conditions. A thorough investigation of the reaction kinetics and product morphology with varied concentration of NH(3) and different reaction times suggests that a putative mechanism of dissolution, recrystallization, and oriented attachment supports the intelligent self-assembly of nanobuilding blocks. Associated characterizations (FTIR, PXRD, FESEM, EDAX, HRTEM, and Raman) have identified it to be pure ß-Ni(OH)(2) without any signature of contamination. The assembled units result in porous frameworks (nanoflowers and nanocolumns) and are indeed full of communally intersecting nanopetals/nanoplates with both lengths and widths on the order of micrometer to nanometer length scale. The as-synthesized material could also be used as a precursor for nanometric black NiO under calcination. The hydroxide has been found to be a potent and environmentally benign material because it warrants its photocatalytic activity through dye mineralization. Finally, Ni(OH)(2) has been photochemically derivatized with dosages of silver nanoparticles bringing a competent composite authority Ag@Ni(OH)(2), to give a full-proof enhanced field effect of prolific SERS activity. In a nutshell, these results are encouraging and fetch new promise for the fabrication of a low-cost and high-yielding greener synthetic protocol for a functional material with promising practicability.

Evolution of hierarchical hexagonal stacked plates of CuS from liquid-liquid interface and its photocatalytic application for oxidative degradation of different dyes under indoor lighting.

Blue solution of copper(II) acetylacetonate complex, [Cu(acac)(2)] in dichloromethane (DCM) and an aqueous alkaline solution of thioacetamide (TAA) constitute a biphasic system. The system in a screw cap test tube under a modified hydrothermal (MHT) reaction condition produces a greenish black solid at the liquid-liquid interface. It has been characterized that the solid mass is an assembly of hexagonal copper sulfide (CuS) nanoplates representing a hierarchical structure. The as-synthesized CuS nanoplates are well characterized by several physical techniques. An ethanolic dispersion of CuS presents a high band gap energy (2.2 eV) which assists visible light photocatalytic mineralization of different dye molecules. Thus a cleanup measure of dye contaminated water body even under indoor light comes true.

Layer-by-layer deposition of silver/gold nanoparticles for catalytic reduction of nitroaromatics.

A general method has been fabricated to achieve normal as well as inverted core-shell architectures of silver/gold through a layer-by-layer deposition technique on a commercial anion exchange resin. Electrostatic field force of the charged resin beads supports immobilization of anionic metal precursors [MX(n)]-, in turn deposition of silver/gold nanoparticles onto the solid resin matrix and reduction of 2-nitrobenzoic acid to obtain the corresponding amines through effective catalysis. The shell thickness has been tailored made by exploiting a new method of cyclic and repetitive deposition of the desired metal precursors. Thermodynamic parameters for the reduction reaction have been presented. Kinetic study reveals a comparative account of rates between the mono- and bi-metallic nanoparticles where silver stands to be a better catalyst for the reduction of nitroaromatics.

Nanosheets of MoSe2@M (M = Pd and Rh) function as widespread pH tolerable hydrogen evolution catalyst.

In this present study we have developed method for the synthesis of MoSe2 nanosheets following a simple hydrothermal technique. Palladium (Pd) and rhodium (Rh) nanoparticles were decorated on the surface of MoSe2 following a simple wet-chemical route. Pd and Rh nanoparticles decorated MoSe2 were applied for hydrogen evolution reaction (HER) in different pH conditions like acidic (0.5 M H2SO4), neutral (pH-7 buffer) and in alkaline (1 M KOH) medium and 3.5 wt% of saline water. Pd and Rh decorated MoSe2 show efficient activity towards HER irrespective of the applied electrolyte. In 0.5 M H2SO4, MoSe2 can produce 10 mA/cm2 current density with applied potential of -0.256 V vs. RHE. Rh decorated MoSe2 shows more shift in the onset potential. Upon applied potential of -0.192 V vs. RHE, MoSe2/Rh can produce 10 mA/cm2 current density. MoSe2/Rh is electrocatalytically more active than MoSe2/Pd which is established from the calculated electrochemically active surface area (ECSA) value. Significantly lower (47 mV/decade) Tafel value is observed for MoSe2/Rh in 0.5 M H2SO4 which indicates the superior activity. MoSe2/Rh is more stable in neutral and alkaline medium compared to acidic medium and it can retain its own activity even after continuous 12 h reaction.

Nanotubes of NiCo2 S4 /Co9 S8 Heterostructure: Efficient Hydrogen Evolution Catalyst in Alkaline Medium.

The most important issue in water splitting is the development of efficient, abundant, and cost-effective hydrogen and oxygen evolution catalysts. The development of an efficient electrocatalyst for the hydrogen evolution reaction (HER) under alkaline conditions is described here following a simple hydrothermal route. Here, a method for the synthesis of NiCo2 S4 /Co9 S8 , Co9 S8 , and NiCo2 S4 nanotubes has been developed. The NiCo2 S4 /Co9 S8 heterostructure has been introduced as an efficient electrocatalyst towards HER under alkaline conditions (1.0 m KOH). The vertically aligned nanotube heterostructure (NiCo2 S4 /Co9 S8 ) shows the most efficient activity as compared to bare Co9 S8 and NiCo2 S4 nanotubes. The heterostructure of NiCo2 S4 and Co9 S8 shows a significant anodic shift in the onset potential compared to the bare counterpart. NiCo2 S4 /Co9 S8 can generate a current density of 10 mA cm-2 upon application of only -0.172 V vs. RHE, whereas Co9 S8 and NiCo2 S4 require -0.293 V and -0.239 V vs. RHE, respectively. The heterostructure formation and the nanotube morphology of Co9 S8 and NiCo2 S4 facilitates a fast charge transportation which results in higher electrocatalytic activity. The hydrogen gas evolution rate of the NiCo2 S4 /Co9 S8 heterostructure was determined to be 2.29 µmol min-1 .

In-situ developed carbon spheres function as promising support for enhanced activity of cobalt oxide in oxygen evolution reaction.

Highly active, stable electrocatalyst for oxygen evolution reaction (OER) is sincerely required for the practical application of water splitting to get rid from the sluggish reaction kinetics and the stability issue. Here, Co3O4 is studied as OER catalyst and to improve the electrocatalytic activity, carbon is chosen as the conducting support. A simple and cost-effective synthetic route is developed for the synthesis of Co3O4 on carbon support following hydrothermal route using various hydrolyzing agents. The heterostructure 'Co3O4/C' perform well in OER as a non-precious metal catalyst. The best Co3O4/C electrocatalyst can generate 10 and 30 mA/cm2 current densities upon application of 1.623 V and 1.678 V vs. RHE whereas, bare Co3O4 can generate current density of 10 and 30 mA/cm2 upon application of 1.677 and 1.754 V vs. RHE. Carbon in the heterostructure helps to improve the conductivity and at the same time enhances the charge transfer ability which further leads to increase current density and stability to the catalyst. Co3O4/C can generate unaltered current density up to 1000 cycles.

Nanosheets of NiCo2O4/NiO as Efficient and Stable Electrocatalyst for Oxygen Evolution Reaction.

Development of a stable catalyst that can efficiently function for longer time for energy conversion process in water splitting is a challenging work. Here, NiCo2O4/NiO nanosheets are successfully synthesized following a simple wet-chemical route, followed by the combustion technique. Finally, the synthesized catalyst NiCo2O4/NiO can function as an efficient catalyst for oxygen evolution reaction. Nanosheets with interconnections are very useful for better electron transportation because the pores in between the sheets are useful for the diffusion of electrolyte in electrocatalysis. In oxygen evolution reaction, these sheets can generate current densities of 10 and 20 mA/cm2, respectively, upon application of 1.59 and 1.62 V potential versus reversible hydrogen electrode (RHE) under alkaline condition. In contrast, bare NiCo2O4 nanowire bundles can generate a current density of 10 mA/cm2 upon application of 1.66 V versus RHE. The presence of NiO in NiCo2O4/NiO nanosheets helps to increase the conductivity, which further increases the electrocatalytic activity of NiCo2O4/NiO nanosheets.