The effects of nickel tungstate nanoparticles (NiWO4 NPs) on freshwater microalga Raphidocelis subcapitata (Chlorophyceae)

Among the vast array of functional nanoparticles (NPs) under development, nickel tungstate (NiWO4) has gained prominence due to its potential applications as a catalyst, sensor, and in the development of supercapacitors. Consequently, new studies on the environmental impact of this material must be conducted to establish a regulatory framework for its management. This work aims to assess the effects of NiWO4 (NPs) on multiple endpoints (e.g., growth, photosynthetic activity, and morphological and biochemical levels) of the freshwater microalga Raphidocelis subcapitata (Chlorophyceae). Quantification data revealed that the fraction of dissolved Ni and free Ni2+ increased proportionally with NiWO4 NP concentrations, although these levels remained relatively low. Biological results indicated that NiWO4 NPs did not inhibit the growth of algal cells, except at 7.9 mg L-1, resulting in a 9% decrease. Morphological changes were observed in cell size and complexity, accompanied by physiological alterations, such as a reduction in chlorophyll a fluorescence (FL3-H) and signs of impaired photosynthetic activity, indicated by the effective quantum yield, quenchings, and chlorophyll a (Chl a) content. Furthermore, the rapid light curves showed that the NPs in high concentrations affected microalga ability to tolerate high light intensities, as corroborated by the significant decrease in the relative electron transport rate (rETRmax) and saturation irradiance (Ek). Based on the present study results, we emphasize the importance of applying integrative approaches in ecotoxicological studies, since each endpoint evaluated showed different sensitivity.

Band-Gap Energy and Electronic d-d Transitions of NiWO4 Studied under High-Pressure Conditions

We report an extensive study of the optical and structural properties of NiWO4 combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO4 showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d8 (t2g6eg2) configuration of Ni2+. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity that decreases as pressure increases.

Tuning the Surface Electronic Structure of Amorphous NiWO4 by Doping Fe as an Electrocatalyst for OER

Water electrolysis is considered as one of the alternative potential approaches for producing renewable energy. Due to the sluggish kinetic nature of oxygen evolution reaction (OER), it encounters a significant overpotential to achieve water electrolysis. Hence, the advancement of cost-effective transition metal-based catalysts toward water splitting has gained global attention in recent years. In this work, the doping of Fe over amorphous NiWO₄ increased the OER activity effectively and achieved stable oxygen evolution in the alkaline medium, which show better electrocatalytic activity as compared to crystalline tungstate. As NiWO₄ has poor activity toward OER in the alkaline medium, the doping of Fe³⁺ will tune the electronic structure of Ni in NiWO₄ and boost the OER activity. The as-synthesized Fe-doped amorphous NiWO₄ exhibits a low overpotential of 230 mV to achieve a current density of 10 mA cm^-2 and a lower Tafel slope value of 48 mV dec^-1 toward OER in 1.0 M KOH solution. The catalyst also exhibits long-term static stability of 30 h during chronoamperometric study. The doping of Fe improves the electronic conductivity of Ni-3d states in NiWO₄ which play a dominant role for better catalytic activity via synergistic interaction between Fe and active Ni sites. In future, these results offer an alternative route for precious metal-free catalysts in alkaline medium and can be explicitly used in various tungstate-based materials to increase the synergism between the doped atom and metal ions in tungstate-based materials for further improvement in the electrocatalytic performance.

Design and Development of Novel Composites Containing Nickel Ferrites Supported on Activated Carbon Derived from Agricultural Wastes and Its Application in Water Remediation

Recently, efficient decontamination of water and wastewater have attracted global attention due to the deficiency in the world's water sources. Herein, activated carbon (AC) derived from willow catkins (WCs) was successfully synthesized using chemical modification techniques and then loaded with different weight percentages of nickel ferrite nanocomposites (10, 25, 45, and 65 wt.%) via a one-step hydrothermal method. The morphology, chemical structure, and surface composition of the nickel ferrite supported on AC (NFAC) were analyzed by XRD, TEM, SEM, EDX, and FTIR spectroscopy. Textural properties (surface area) of the nanocomposites (NC) were investigated by using Brunauer-Emmett-Teller (BET) analysis. The prepared nanocomposites were tested on different dyes to form a system for water remediation and make this photocatalyst convenient to recycle. The photodegradation of rhodamine B dye was investigated by adjusting a variety of factors such as the amount of nickel in nanocomposites, the weight of photocatalyst, reaction time, and photocatalyst reusability. The 45NFAC photocatalyst exhibits excellent degradation efficiency toward rhodamine B dye, reaching 99.7% in 90 min under a simulated source of sunlight. To summarize, NFAC nanocomposites are potential photocatalysts for water environmental remediation because they are effective, reliable, and reusable.

Pyrazinamide photodegradation on NiWO4-palygorskite nanocomposites under polychromatic irradiation

In this work, antibiotic pyrazinamide (PZA) photodegradation on palygorskite (Pal), NiWO₄ crystals, and NiWO₄-Pal (2, 6, and 10%) nanocomposites was evaluated under polychromatic irradiation. In the characterization of the samples, XRD patterns displayed good crystallinity for NiWO₄ crystals and nanocomposites. In addition, the diffractograms were used in the Rietveld refinement for phase indexing, revealing a wolframite-type monoclinic structure with the space group P2/c. The active vibrational modes related to the characteristic groups of the samples were identified using Raman and FTIR spectroscopy. Photoluminescence (PL) spectra revealed that NiWO₄ and NiWO₄-Pal (2%) nanocomposite have the highest electron-hole pair recombination rate, and the contribution of the green component in the NiWO₄-Pal (2%) nanocomposite indicates a greater contribution of deep energy levels to the PL profile. DRS in the UV-visible region indicated that NiWO₄ crystals have indirect band-gap energy (E_gap) 2.64 eV; NiWO₄-Pal (2, 6, and 10%) nanocomposites have 2.62, 2.58, and 2.59 eV, respectively; and Pal has 2.83 eV. The catalytic tests showed that the NiWO₄-Pal (2%) nanocomposite samples, under polychromatic radiation, exhibit greater efficiency in photodegradation at 110 min, with yield of 98.5%. The ROS tests indicated that the studied reactive species play a similar role in PZA photodegradation.

Synthesis, Characterization and Enhanced Visible Light Photocatalytic Performance of ZnWO4-NPs@rGO Nanocomposites

ZnWO4 nanoparticles on reduced graphene oxide (ZnWO4-NPs@rGO) nanocomposites were synthesized using the hydrothermal method. Structural, morphological, optical, and photocatalytic studies of the ZnWO4-NPs@rGO nanocomposites were successfully investigated. Photo-catalytic performances of the ZnWO4-NPs@rGO nanocomposites were examined for the degradation of hazardous methylene blue dye (HMBD) in a neutral medium. ZnWO4-NPs@rGO nanocomposites show superior photo-catalytic performances over pure ZnWO4 nanoparticles. ZnWO4-NPs@rGO nanocomposites degrade ~98% dye while pure ZnWO4 nanoparticles degrade ~53% dye in 120 min. The prepared nanocomposites also show excellent recycled photo-catalytic efficiencies over multiple cycles.

Effect of the Anionic Counterpart: Molybdate vs. Tungstate in Energy Storage for Pseudo-Capacitor Applications

Nickel-based bimetallic oxides (BMOs) have shown significant potential in battery-type electrodes for pseudo-capacitors given their ability to facilitate redox reactions. In this work, two bimetallic oxides, NiMoO4 and NiWO4, were synthesized using a wet chemical route. The structure and electrochemical properties of the pseudo-capacitor cathode materials were characterized. NiMoO4 showed superior charge storage performance in comparison to NiWO4, exhibiting a discharge capacitance of 124 and 77 F.g-1, respectively. NiMoO4, moreover, demonstrates better capacity retention after 1000 cycles with 87.14% compared to 82.22% for NiWO4. The lower electrochemical performance of the latter was identified to result from the redox behavior during cycling. NiWO4 reacts in the alkaline solution and forms a passivation layer composed of WO3 on the electrode, while in contrast, the redox behavior of NiMoO4 is fully reversible.

A novel p-n Mn0.2Cd0.8S/NiWO4 heterojunction for highly efficient photocatalytic H2 production

Constructing a p-n heterojunction has been regarded as an effective way to restrain charge recombination and boost photocatalytic H2 production activity. Herein, a novel Mn0.2Cd0.8S/NiWO4 composite was fabricated by a hydrothermal process and which exhibited enhanced H2 production activity and excellent photostability. Particularly, the composite with 30 wt% of NiWO4 achieved the optimal H2 production rate of 17.76 mmol g-1 h-1, which was 2.9 times higher than that of Mn0.2Cd0.8S. The increased H2 production property was mainly due to the p-n heterojunction between Mn0.2Cd0.8S and NiWO4, which provided an efficient path for charge transfer and inhibited the photocorrosion of Mn0.2Cd0.8S. This work can offer technical support for the design and development of p-n heterojunctions that can be applied for photocatalytic H2 production.

Direct Z-Scheme g-C3N4/FeWO4 Nanocomposite for Enhanced and Selective Photocatalytic CO2 Reduction under Visible Light

Photocatalytic reduction of CO₂ to renewable solar fuels is considered to be a promising strategy to simultaneously solve both global warming and energy crises. However, development of a superior photocatalytic system with high product selectivity for CO₂ reduction under solar light is the prime requisite. Herein, a series of nature-inspired Z-scheme g C₃N₄/FeWO₄ composites are prepared for higher performance and selective CO₂ reduction to CO as solar fuel under solar light. The novel direct Z-scheme coupling of the visible light-active FeWO₄ nanoparticles with C₃N₄ nanosheets is seen to exhibit excellent performance for CO production with a rate of 6 μmol/g/h at an ambient temperature, almost 6 times higher compared to pristine C₃N₄ and 15 times higher than pristine FeWO₄. More importantly, selectivity for CO is 100% over other carbon products from CO₂ reduction and more than 90% over H₂ products from water splitting. Our results clearly demonstrate that the staggered band structure between FeWO₄ and C₃N₄ reflecting the nature-inspired Z-scheme system not only favors superior spatial separation of the electron-hole pair in g-C₃N₄/FeWO₄ but also shows good reusability. The present work provides unprecedented insights for constructing the direct Z-scheme by mimicking the nature for high performance and selective photocatalytic CO₂ reduction into solar fuels under solar light.

Solvothermal synthesis of NiWO4 nanostructure and its application as a cathode material for asymmetric supercapacitors

This study proposes a facile solvothermal synthesis of nickel tungstate (NiWO4) nanowires for application as a novel cathode material for supercapacitors. The structure, morphology, surface area and pore distribution were characterized and their capacitive performances were investigated. The results showed that the NiWO4 nanowires synthesized in ethylene glycol solvent could offer a high specific capacitance of 1190 F g-1 at a current density of 0.5 A g-1 and a capacitance retaining ratio of 61.5% within 0.5-10 A g-1. When used as a cathodic electrode of an asymmetric supercapacitor (ASC), the NiWO4 nanowire based device can be cycled reversibly in a high-voltage region of 0-1.7 V with a high specific capacitance of 160 F g-1 at 0.5 A g-1, which therefore contributed to an energy density of 64.2 W h kg-1 at a power density of 425 W kg-1. Moreover, 92.8% of its initial specific capacitance can be maintained after 5000 consecutive cycles (5 A g-1). These excellent capacitive properties make NiWO4 a credible electrode material for high-performance supercapacitors.

Design of ultrasensitive Ag-LaFeO3 methanol gas sensor based on quasi molecular imprinting technology

An ultrasensitive methanol gas sensing device based on the quasi-molecular imprinting technology (quasi-MIT) is studied in this work. We applied the sol-gel method (ALS denotes Ag-LaFeO₃ prepared by the sol-gel method) and combustion synthesis (ALC denotes Ag-LaFeO₃ prepared by combustion synthesis) to prepare Ag-LaFeO₃ based sensors. The morphologies and structures of the Ag-LaFeO₃ materials were examined via various detection techniques. The ALSM and ALCM sensor (ALSM and ALCM denotes the devices prepared by coating the ALS and ALC materials with methanol, respectively) fabricated using the sol-gel method and combustion synthesis combined with quasi-MIT exhibit good gas sensing properties to methanol, in contrast with the two devices (ALSW and ALCW denote the devices prepared for coating the ALS and ALC materials with water, respectively) without the use of quasi-MIT. The results show that quasi-MIT introduced the target gas in the fabrication process of the device, playing an important role in the design of the ultrasensitive methanol gas sensor. The sensing response and the optimum working temperature of ALSM and ALCM gas sensor are 52.29 and 155 °C and 34.89 and 155 °C, respectively, for 5 ppm methanol, and the highest response to other gases is 8. The ALSM and ALCM gas sensors reveal good selectivity and response for methanol.

Evaluation of photocatalytic and supercapacitor potential of nickel tungstate nanoparticles synthesized by electrochemical method

Nickel tungstate nanoparticles (NiWO₄ NPs) that were synthesized by an electrochemical method under various reaction conditions, namely, different tungstate ion concentrations, voltages, temperatures, and stirring rates, were studied here.