Sonochemical Assisted Solvothermal Synthesis of Gallium Oxynitride Nanosheets and their Solar-Driven Photoelectrochemical Water-Splitting Applications

Template-assisted growth of Ga-based nanoparticle clusters on Si: effect of post-annealing process on the Ga ion beam exposed 2D arrays fabricated by focused ion beam nanolithography

We demonstrate template-assisted growth of gallium-based nanoparticle clusters on silicon substrate using a focused ion beam (FIB) nanolithography technique. The nanolithography counterpart of the technique steers a focussed 30 kV accelerated gallium ion beam on the surface of Si to create template patterns of two-dimensional dot arrays. Growth of the nanoparticles is governed by two vital steps namely implantation of gallium into the substrate via gallium beam exposure and formation of the stable nanoparticles on the surface of the substrate by subsequent annealing at elevated temperature in ammonia atmosphere. The growth primarily depends on the dose of implanted gallium which is in the order of 107atoms per spot and it is also critically influenced by the temperature and duration of the post-annealing treatment. By controlling the growth parameters, it is possible to obtain one particle per spot and particle densities as high as 109particles per square centimetre could be achieved in this case. The demonstrated growth process, utilizing the advantages of FIB nanolithography, is categorized under the guided organization approach as it combines both the classical top-down and bottom-up approaches. Patterned growth of the particles could be utilized as templates or nucleation sites for the growth of an organized array of nanostructures or quantum dot structures.

Surface Bioengineering of Mo2Ga2C MAX Phase to Develop Blended Loose Nanofiltration Membranes for Textile Wastewater Treatment

The potential of blended loose nanofiltration membranes (LNMs) to fractionate dyes and inorganic salts in textile wastewater has become a focus of attention in recent years. In this research work, we fabricated LNMs based on polysulfone (PSf) membranes blended with l-histidine amino acid-functionalized Mo2Ga2C MAX phase (His-MAX). Scanning electron microscopy (SEM), atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), contact angle, ζ-potential, porosity, and pore size analyses were employed to characterize the LNMs. Blending 0.75 wt % of His-MAX additive with the PSf tailored the LNM's features by making it more water-friendly, increasing its porosity, enlarging its pores, and making its surface smoother. The pure water flux of 127.6 L/m2 h was achieved by LNM containing 0.75 wt % His-MAX, which was 2.5 times greater than the bare one. The mentioned LNM displayed a flux recovery ratio (FRR) of 68.27 and 98.57, 98.31, and 99.7% rejections for Direct red 23, Acid brown 75, and Reactive blue 21 solutions (100 mg/L), respectively. The 0.75 wt % His-MAX LNM could reject 99.1% of dye and 11.5% of salt while maintaining an FRR of 91.19% after four cycles of filtering a binary mixture solution containing Reactive blue 21 and Na2SO4. These findings highlight the potential of the fabricated LNM for desalinating dye solutions.

Oxynitrides enabled photoelectrochemical water splitting with over 3,000 hrs stable operation in practical two-electrode configuration

Solar photoelectrochemical reactions have been considered one of the most promising paths for sustainable energy production. To date, however, there has been no demonstration of semiconductor photoelectrodes with long-term stable operation in a two-electrode configuration, which is required for any practical application. Herein, we demonstrate the stable operation of a photocathode comprising Si and GaN, the two most produced semiconductors in the world, for 3,000 hrs without any performance degradation in two-electrode configurations. Measurements in both three- and two-electrode configurations suggest that surfaces of the GaN nanowires on Si photocathode transform in situ into Ga-O-N that drastically enhances hydrogen evolution and remains stable for 3,000 hrs. First principles calculations further revealed that the in-situ Ga-O-N species exhibit atomic-scale surface metallization. This study overcomes the conventional dilemma between efficiency and stability imposed by extrinsic cocatalysts, offering a path for practical application of photoelectrochemical devices and systems for clean energy.

A Surface Conformal Laser-Assisted Alloying Reaction for 3D-Printable Solid/Liquid Biphasic Conductors

Recently, electronics research has made major advances toward a new platform technology facilitating form factor-free devices. 3D printing techniques have attracted significant attention in the context of fabricating arbitrarily shaped circuits. Herein, a 3D-printable metallic ink comprising multidimensional eutectic gallium indium (EGaIn)/Ag hierarchical particles is proposed to fabricate arbitrarily designable solid/liquid biphasic conductors that can be inherently self-healed/chip bonded and do not suffer from liquid flood out due to their liquid and solid nature, respectively. The EGaIn/Ag hierarchical particles are designed to have plasmonic optical absorption at the visible green-red wavelength regime, which is elucidated by an optical simulation study, and also enable the direct transfer of thermal energy, generated in the vicinity of the Ag nanoparticles, to the surface of the EGaIn particles. The 3D surface conformal green laser irradiation process activates the evolution of the biphasic conductive layer from the as-printed insulating particulate one. The chemical/physical evolution is elucidated along with a photothermal simulation study for clarifying the suppression of undesirable side reactions. It is demonstrated that the biphasic conductors formed by successive 3D printing and the surface conformal green laser irradiation process exhibit electrical properties that have thus far been unexplored in solid metallic conductors.

Advanced Photoelectrochemical Hydrogen Generation by CdO-g-C3N4 in Aqueous Medium under Visible Light

Herein, hydrothermal fabrication of CdO-g-C3N4 photocatalyst for a substantially better photocatalytic recital in water splitting is presented. The XRD analysis confirms the cubic phase of CdO-g-C3N4, whereas FTIR and UV-VIS studies revealed the presence of respective groups and a median band gap energy (2.55 eV) of the photocatalyst, respectively, which further enhanced its photo-electrochemical (PEC) properties. The SEM displays the oblong structures of g-C3N4 sheets and nano rod-like morphology of CdO and CdO-g-C3N4, respectively. The HR-TEM exhibits morphology & orientation of the grains and substantiates the polycrystal-line nature of CdO-g-C3N4 nanocomposite. The photocatalytic water-splitting concert is evaluated by PEC experiments under 1 SUN visible light irradiation. Linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) comprehend the CdO-g-C3N4 as a hydrogen evolution photocatalyst. A photocurrent density beyond ≥5 mA/cm2 is recorded from CdO-g-C3N4, which is 5-6 folds greater than pure CdO and g-C3N4. The efficient separation and transfer of charges allocated to CdO-g-C3N4 and fabricating heterojunctions between g-C3N4 and CdO suppresses the unfavorable electron-hole pairs recombination process. Thus, it recesses charge transfer resistance, augmenting enhanced photocatalytic performance under 1 SUN irradiation.

Tailoring g-C3N4 with Lanthanum and Cobalt Oxides for Enhanced Photoelectrochemical and Photocatalytic Activity

Herein, the synthesis, characterization, and photoelectrochemical and photocatalytic characteristics of hydrothermally prepared La2O3–g-C3N4, CoO–g-C3N4, and La2O3–CoO–g-C3N4 are discussed. The XRD analysis and crystalline phases unveiled the impregnation of La2O3 and CoO into g-C3N4. The microscopic analysis supports the formation of g-C3N4 nanoflakes and La2O3 and CoO nanoparticles embedded homogeneously in the La2O3–CoO–g-C3N4 nanocomposite, whereas the EDX comprehended their respective elemental composition and ratios. A bandgap energy of 2.38 eV for La2O3–CoO–g-C3N4 was calculated using the Tauc plot method, complementing high visible-light activity. The solar-driven water-splitting reaction exhibited significant photocurrent efficiency (~3.75 mA/cm2), augmenting the hydrogen generation by La2O3–CoO–g-C3N4 compared to that by pure g-C3N4, La2O3–g-C3N4, and CoO–g-C3N4 in 0.5 M Na2SO4 electrolyte. The synergistic effect of La2O3 and CoO impregnation with g-C3N4 led to effective division of the photogenerated charge transporters, enhancing the photocatalytic hydrogen generation by the photocatalysts. Furthermore, photocatalytic pollutant removal, namely greater than 90% decomposition of methylene blue (MB) from water, was investigated with a pseudo-first-order reaction kinetics under 1 sun visible-light irradiation. Thus, La2O3–CoO–g-C3N4 nanocomposite was found to be a prospective material for harnessing solar energy.

High shear in situ exfoliation of 2D gallium oxide sheets from centrifugally derived thin films of liquid gallium

A diversity of two-dimensional nanomaterials has recently emerged with recent attention turning to the post-transition metal elements, in particular material derived from liquid metals and eutectic melts below 330 °C where processing is more flexible and in the temperature regime suitable for industry. This has been explored for liquid gallium using an angled vortex fluidic device (VFD) to fabricate ultrathin gallium oxide (Ga2O3) sheets under continuous flow conditions. We have established the nanosheets to form highly insulating material and have electrocatalytic activity for hydrogen evolution, with a Tafel slope of 39 mV dec-1 revealing promoting effects of the surface oxidation (passivation layer).

Molybdenum impregnated g-C3N4 nanotubes as potentially active photocatalyst for renewable energy applications

Molybdenum (Mo) impregnated g-C3N4 (Mo-CN) nanotubes are fabricated via a thermal/hydrothermal process to augment photoelectrochemical properties during solar-driven water-splitting (SDWS) reactions. Graphitic-C3N4 is an attractive material for photocatalysis because of its suitable band energy, high thermal and chemical stability. The FE-SEM and HR-TEM comprehend the nanotube-like morphology of Mo-CN. The spectroscopic characterization revealed bandgap energy of 2.63 eV with high visible-light activity. The x-ray diffraction of pristine g-C3N4 and Mo-CN nanotubes discloses the formation of triazine-based nanocrystalline g-C3N4, which remains stable during hydrothermal impregnation of Mo. Furthermore, Mo-CN nanotubes possess high sp2-hybridized nitrogen content, and metallic/oxidized Mo nanoparticles (in a ratio of 1:2) are impregnated into g-C3N4. The XPS analysis confirms C, N, and Mo for known atomic and oxidation states in Mo-CN. Furthermore, high photocurrent efficiency (~ 5.5 mA/cm2) is observed from 5%-Mo-CN nanotubes. That displays efficient SDWS by 5%-Mo-CN nanotubes than other counterparts. Impedance spectroscopy illustrated the lowest charge transfer resistance (Rct) of 5%-Mo-CN nanotubes, which further confirms the fast electron transfer kinetics and efficient charge separation resulting in high photocurrent generation. Hence, 5%Mo-CN composite nanotubes can serve as a potential photocatalytic material for viable solar-driven water splitting.

Metal-organic framework (MOF) composite materials for photocatalytic CO2 reduction under visible light

The tridentate ligand 2,4,6-tris(2-(pyridin-4-yl)vinyl)-1,3,5-triazine (TPVT) was designed and synthesized. We prepared metal-organic framework (TPVT-MOFs) crystals containing TPVT, 1,3,5-benzenetricarboxylic acid and cobalt by solvothermal reaction. Then, a series of composite materials with different contents of TPVT-MOFs were obtained by combining TPVT-MOFs with g-C3N4. Due to the interaction between TPVT-MOFs and g-C3N4, the composite materials have a more favorable valence band (VB) and conduction band (CB) for photocatalytic reduction of CO2 and oxidation of H2O. Under the conditions of visible light and a gas-solid system without a co-catalyst, a photosensitizer and a sacrificial agent, the yield of CO2 reduction by TPVT-MOFs@g-C3N4-10 can reach 56.4 μmol·g-1·h-1, which is 3.2 times that of pure g-C3N4 (17.5 μmol·g-1·h-1). The results of DFT calculations showed that the adsorption of H2O on the TPVT-MOFs@g-C3N4 composite material was more preferential, which promoted the adsorption and reduction of CO2.

Experimental Investigation on the Sputtering Process for Tantalum Oxynitride Thin Films

Metal oxynitrides are compounds between nitrides and oxides with a certain level of photocatalytic functions. The purpose of this study is to investigate an appropriate range of oxygen flow rate during sputtering for depositing tantalum oxynitride films. The sputtering process was carried out under fixed nitrogen but variable oxygen flow rates. Post rapid thermal annealing was conducted at 800 °C for 5 min to transform the as-deposited amorphous films into crystalline phases. The material characterizations of annealed films include X-ray diffraction and Raman spectroscopy for identifying crystal structures; scanning electron microscope for examining surface morphology; energy-dispersive X-ray spectroscopy to determine surface elemental compositions; four-point probe and Hall effect analysis to evaluate electrical resistivity; UV-visible-NIR spectroscopy for quantifying optical properties and optical bandgaps. To assess the photocatalytic function of oxynitride films, the degradation of methyl orange in de-ionized water was examined under continuous irradiation by a simulated solar light source for six hours. Results indicate that crystalline tantalum oxynitride films can be obtained if the O2 flow rate is chosen to be 0.25–1.5 sccm along with 10 sccm of N2 and 20 sccm of Ar. In particular, films deposited between 0.25 and 1.5 sccm O2 flow have higher efficiency in photodegradation on methyl orange due to a more comprehensive formation of oxynitrides.

Li2CO3-affiliative mechanism for air-accessible interface engineering of garnet electrolyte via facile liquid metal painting

Garnet based solid-state batteries have the advantages of wide electrochemical window and good chemical stability. However, at Li-garnet interface, the poor interfacial wettability due to Li₂CO₃ passivation usually causes large resistance and unstable contact. Here, a Li₂CO₃-affiliative mechanism is proposed for air-accessible interface engineering of garnet electrolyte via facile liquid metal (LM) painting. The natural LM oxide skin enables a superior wettability of LM interlayer towards ceramic electrolyte and Li anode. Therein the removal of Li₂CO₃ passivation network is not necessary, in view of its delamination and fragmentation by LM penetration. This dissipation effect allows the lithiated LM nanodomains to serve as alternative Li-ion flux carriers at Li-garnet interface. This mechanism leads to an interfacial resistance as small as 5 Ω cm² even after exposing garnet in air for several days. The ultrastable Li plating and stripping across LM painted garnet can last for 9930 h with a small overpotential.

At Li-garnet interface, the poor interfacial wettability due to Li₂CO₃ passivation causes large resistance and unstable contact. Here the authors propose a Li₂CO₃-affiliative mechanism for air-accessible interface engineering of garnet electrolyte with superior wettability via facile liquid metal painting.

Modification of 1D TiO2 nanowires with GaOxNy by atomic layer deposition for TiO2@GaOxNy core-shell nanowires with enhanced photoelectrochemical performance

As a well-known semiconductor that can catalyse the oxygen evolution reaction, TiO2 has been extensively investigated for its solar photoelectrochemical water properties. Unmodified TiO2 shows some issues, particularly with respect to its photoelectrochemical performance. In this paper, we present a strategy for the controlled deposition of controlled amounts of GaOxNy cocatalysts on TiO2 1D nanowires (TiO2@GaOxNy core-shell) using atomic layer deposition. We show that this modification significantly enhances the photoelectrochemical performance compared to pure TiO2 NW photoanodes. For our most active TiO2@GaOxNy core-shell nanowires with a GaOxNy thickness of 20 nm, a photocurrent density up to 1.10 mA cm-2 (at 1.23 V vs. RHE) under AM 1.5 G irradiation (100 mW cm-2) has been achieved, which is 14 times higher than that of unmodified TiO2 NWs. Furthermore, the band gap matching with TiO2 enhances the absorption of visible light over unmodified TiO2 and the facile oxygen vacancy formation after the deposition of GaOxNy also provides active sites for water activation. Density functional theory studies of model systems of GaOxNy-modified TiO2 confirm the band gap reduction, high reducibility and ability to activate water. The highly efficient and stable systems of TiO2@GaOxNy core-shell nanowires with ALD deposited GaOxNy demonstrate a good strategy for the fabrication of core-shell structures that enhance the photoelectrochemical performance of readily available photoanodes.

Hematite and Magnetite Nanostructures for Green and Sustainable Energy Harnessing and Environmental Pollution Control: A Review

The optoelectrical and magnetic characteristics of naturally existing iron-based nanostructures, especially hematite and magnetite nanoparticles (H-NPs and M-NPs), gained significant research interest in various applications, recently. The main purpose of this Review is to provide an overview of the utilization of H-NPs and M-NPs in various environmental remediation. Iron-based NPs are extensively explored to generate green energy from environmental friendly processes such as water splitting and CO₂ conversion to hydrogen and low molecular weight hydrocarbons, respectively. The latter part of the Review provided a critical overview to use H-NPs and M-NPs for the detection and decontamination of inorganic and organic contaminants to counter the environmental pollution and toxicity challenge, which could ensure environmental sustainability and hygiene. Some of the future perspectives are comprehensively presented in the final portion of the script, optimiztically, and it is supported by some relevant literature surveys to predict the possible routes of H-NPs and M-NPs modifications that could enable researchers to use these NPs in more advanced environmental applications. The literature collection and discussion on the critical assessment of reserving the environmental sustainability challenges provided in this Review will be useful not only for experienced researchers but also for novices in the field.

Synthesis of oxy-sulfide based nanocomposite catalyst for visible light-driven reduction of Cr(VI)

The oxy-sulfide based V₂O₅@(In,Ga)₂(O,S)₃ nanocomposite catalyst, at different weight percentages of V₂O₅, was successfully synthesized via a simplistic procedural route for the detoxification of hazardous Cr(VI). The two pure catalysts were intimately allied and used for visible light-driven reduction of hazardous Cr(VI). The nanocomposite catalysts were characterized to observe the effects of V₂O₅ on crystal phase, morphology, light absorption, catalytic activity, and electrical properties. Compared to all, 40% V₂O₅ loaded nanocomposite catalyst, designated as VOS-2, exhibited the best-reducing capability. It completely reduced toxic Cr(VI) at 2 min under visible light illumination. From the kinetics, it was found that the rate constant of the nanocomposite catalyst was improved by a factor of 3.6 compared to the host nanoflower catalyst. The plausible mechanism of charge transfer process across the interfacial region indicates the diminished recombination probability of photogenerated charge carriers. Therefore, the nanocomposite catalyst is promising for enhanced reduction of Cr(VI) in the Cr-based industrial activities, which is significantly relevant for environmental remediation.

Hydrothermal Approach to Spinel-Type 2D Metal Oxide Nanosheets

The peculiar physical and chemical properties of 2D nanostructures have aroused global research interest in developing new members, synthetic technology, and exploring their potential applications in functional nanodevices. However, it is extremely challenging to directly obtain the 2D nanosheets for these extrinsic layered structures using conventional routines. In this work, we demonstrate the facile and general synthesis of 2D spinel-type metal oxides nanosheets through a simple hydrothermal reaction. Using this method, cubic γ-Ga₂O₃, ZnGa₂O₄ and MnGa₂O₄ nanosheets with triangular/hexagonal configuration and ultrathin thickness have been synthesized, and all these nanosheets show preferential growth along (111) plane with the minimum formation energy. Microstructural and composition analyses using HRTEM, EDS, XPS, and so on reveal that the as-synthesized 2D nanosheets are well-crystallized in cubic fcc-phase and show high purity in composition, and the formation process of MGa₂O₄ nanosheets can be regarded as the competition of M²⁺ and Ga³⁺ in tetrahedral site. Spatially resolved cathodoluminescence measurement of individual 2D nanosheet shows that the γ-Ga₂O₃, ZnGa₂O₄, and MnGa₂O₄ nanosheets exhibit distinct luminescence behavior, and ZnGa₂O₄ nanosheets show the strongest emission in visible region. It is expected that the facile synthesis of spinel-type metal oxides of γ-Ga₂O₃, ZnGa₂O₄, and MnGa₂O₄ nanosheets will further promote the exploration of a variety of semiconductor nanostructures that could not be achieved using conventional technology suitable for layered structures and will also open up some opportunities for the integration of advanced functional nanodevices such as photodetectors, phosphors on the basis of them.

Sonochemical assisted hydrothermal synthesis of pseudo-flower shaped Bismuth vanadate (BiVO4) and their solar-driven water splitting application

Bismuth vanadate (BiVO₄) is a well-known photocatalyst due to its lower bandgap (E_g) and visible electromagnetic light absorption capacity. Herein, we reported the pulse ultra-sonochemical assisted hydrothermal approach to synthesize S-BiVO₄. For the comparison purpose, H-BiVO₄ is also synthesized via conventional hydrothermal approach. The surface morphology of S-BiVO₄ through scanning electron microscope (SEM) indicates condensed microarrays (MAs) having pseudo-flower shapes. The energy dispersive X-rays (EDX) spectrum also confirmed the elemental percent composition of Bi, V and O in BiVO₄. X-rays diffraction (XRD) pattern further confirmed the monoclinic scheelite phase of S-BiVO₄. Fourier transformed infrared (FTIR) spectrum showed Bi-O and Bi-V-O vibrational bands at 1382 and 1630cm^-1, respectively. The diffuse reflectance spectroscopy (DRS) indicated absorption edge at ∼515nm, corresponds to bandgap value (E_g) of 2.41eV, which is suitable range for water splitting applications. The photocurrent density from water splitting under artificial 1 SUN visible light source found at 60 and 50μA/cm² for S-BiVO₄ and H-BiVO₄, respectively. The stability test through chronoamperometry showed that S-BiVO₄ was more stable than H-BiVO₄. It can be depicted from the growth mechanism that ultrasonication played a definite role in the overall synthesis of pseudo-flower shaped S-BiVO₄ MAs.