Photo-deposition of ZnO/Co3O4 core-shell nanorods with p-n junction for efficient oxygen evolution reaction

Straightforward electrochemical synthesis of a Co3O4 nanopetal/ZnO nanoplate p-n junction for photoelectrochemical water splitting

Hydrogen production through photoelectrochemical (PEC) reactions is an innovative and promising approach to producing clean energy. The PEC working electrode of a Co3O4/ZnO-based p-n heterojunction was prepared by a straightforward electrochemical deposition with different deposition times onto an FTO (Fluorine-doped Tin Oxide) glass substrate. The successful synthesis of the materials was confirmed through analysis using XRD, FTIR, SEM-EDX, DRS, and PL techniques. Mott-Schottky plots and some characterization studies also checked the determination of the formation of the p-n junction. Co3O4/ZnO/FTO with a Co3O4 deposition time of 2 minutes exhibited the lowest onset potential of 0.82 V and the lowest overpotential of 470 mV at a current density of 10 mA cm -2. Furthermore, the photo-conversion efficiency of the Co3O4/ZnO/FTO sample showed 1.4 times higher current density than the ZnO/FTO sample. A mechanism is also proposed to enhance the Co3O4/ZnO/FTO electrode photo-electrocatalytic activity involved in the water-splitting reaction. The Co3O4/ZnO/FTO electrode shows significant potential as a promising PEC electrode to produce hydrogen.

Construction of a ZnO Heterogeneous Structure Using Co3O4 as a Co-Catalyst to Enhance Photoelectrochemical Performance

Recently, heterostructured photocatalysts have gained significant attention in the field of photocatalysis due to their superior properties compared to single photocatalysts. One of the key advantages of heterostructured photocatalysts is their ability to enhance charge separation and broaden the absorption spectrum, thereby improving photocatalytic efficiency. Zinc oxide is a widely used n-type semiconductor with a proper photoelectrochemical activity. In this study, zinc oxide nanorod arrays were synthesized, and then the surfaces of ZnO nanorods were modified with the p-type semiconductor Co3O4 to create a p-n junction heterostructure. A significant increase in the photocurrent for the ZnO/Co3O4 composite, of 4.3 times, was found compared to pure ZnO. The dependence of the photocurrent on the morphology of the ZnO/Co3O4 composite allows for optimization of the morphology of the ZnO nanorod array to achieve improved photoelectrochemical performance. The results showed that the ZnO/Co3O4 heterostructure exhibited a photocurrent density of 3.46 mA/cm2, while bare ZnO demonstrated a photocurrent density of 0.8 mA/cm2 at 1.23 V. The results of this study provide a better understanding of the mechanism of charge separation and transfer in the heterostructural ZnO/Co3O4 photocatalytic system. Furthermore, the results will be useful for the design and optimization of photocatalytic systems for water splitting and other applications.

A highly sensitive TiO2-based molecularly imprinted photoelectrochemical sensor with regulation of imprinted sites by Photo-deposition

Molecularly imprinted photoelectrochemical sensors (MIPES) have gained significant attention in the detection field due to their high selectivity and accuracy. However, their sensitivity still needs improvement. Here we developed a TiO2-based MIPES (TiO2 NRs/NiOOH/rMIP) to detect ciprofloxacin (CIP). We identified the photoactive sites of TiO2 by NiOOH photo-deposition and anchored the imprinted sites on the photoactive sites by complexation between CIP and NiOOH. By regulating the imprinted sites, the photocurrent difference before and after the addition of CIP increases and the detection sensitivity of CIP is improved. Moreover, a PN heterojunction is formed between TiO2 and NiOOH, which enables rapid transfer of photoexcited holes and electrons to different semiconductors under the built-in electric field. This leads to improved photoactivity of TiO2 and further increases the sensitivity of MIPES. Compared with sensors prepared by the traditional electro-polymerization CIP and Molecularly imprinted polymers (TiO2 NRs/NiOOH/eMIP), TiO2 NRs/NiOOH/rMIP as constructed in this work displays higher sensitivity, wider linear detection range, and lower limit of detection (LOD). Additionally, TiO2 NRs/NiOOH/rMIP shows good selectivity, stability, and recovery rate, and has a promising application prospect in the actual detection of antibiotics.

Directed electron regulation promoted sandwich-like CoO@FeBTC/NF with p-n heterojunctions by gel electrodeposition for oxygen evolution reaction

Metal organic framework (MOF) is currently-one of the key catalysts for oxygen evolution reaction (OER), but its catalytic performance is severely limited by electronic configuration. In this study, cobalt oxide (CoO) on nickel foam (NF) was first prepared, which then wrapped it with FeBTC synthesized by ligating isophthalic acid (BTC) with iron ions by electrodeposition to obtain CoO@FeBTC/NF p-n heterojunction structure. The catalyst requires only 255 mV overpotential to reach a current density of 100 mA cm^-2, and can maintain 100 h long time stability at 500 mA cm^-2 high current density. The catalytic properties are mainly related to the strong induced modulation of electrons in FeBTC by holes in the p-type CoO, which results in stronger bonding and faster electron transfer between FeBTC and hydroxide. At the same time, the uncoordinated BTC at the solid-liquid interface ionizes acidic radicals which form hydrogen bonds with the hydroxyl radicals in solution, capturing them onto the catalyst surface for the catalytic reaction. In addition, CoO@FeBTC/NF also has strong application prospects in alkaline electrolyzers, which only needs 1.78 V to reach a current density of 1 A cm^-2, and it can maintain long-term stability for 12 h at this current. This study provides a new convenient and efficient approach for the control design of the electronic structure of MOF, leading to a more efficient electrocatalytic process.

Multifunctionality exploration of NiCo2O4-rGO nanocomposites: photochemical water oxidation, methanol electro-oxidation and asymmetric supercapacitor applications

Different weight percentages of NiCo₂O₄-rGO nanocomposites were prepared via a facile hydrothermal method. The prepared nanocomposites were structurally and morphologically characterized by X-ray diffraction, Raman spectroscopy, and electron microscopy. The structural studies show the formation of rGO-NiCo₂O₄ nanocomposites by embedment of porous NiCo₂O₄ rods on rGO sheets. The effect of the NiCo₂O₄ content on photochemical water oxidation was investigated. It revealed that the catalysts NiCo₂O₄-rGO with 1 : 26 ratio (NCO26) and 1 : 13 ratio (NCO13) are efficient in generating oxygen under light illumination. It proves that NCO26 works far more effectively as a photocatalyst compared to NCO13. Methanol electro-oxidation of the NCO26 nanocomposite shows a current density of 24 mA cm^-2 at a potential of 0.45 V in cyclic voltammetry and maintains the current for 3600 s at 0.45 V in chronoamperometry. An onset potential of 0.344 V was observed for 0.5 M methanol oxidation. The specific capacitance values were found to be 354.75 F g^-1 and 375.32 F g^-1 at 1 mV s^-1 and 1 A g^-1, respectively, for NCO26 in supercapacitor studies. The charge stored via capacitive and diffusion-controlled processes was determined using Power's law and Trasatti plot. An asymmetric supercapacitor device shows a specific capacitance of 122.2 F g^-1 at a current density of 1 A g^-1 and exhibits a retention of 74.3% after 5000 cycles. An energy density of 67.89 W h kg^-1 and a power density of 1 kW kg^-1 at a current density of 1 A g^-1 are observed.

Efficient Recovery of Lithium Cobaltate from Spent Lithium-Ion Batteries for Oxygen Evolution Reaction

Owing to technological advancements and the ever-increasing population, the search for renewable energy resources has increased. One such attempt at finding effective renewable energy is recycling of lithium-ion batteries and using the recycled material as an electrocatalyst for the oxygen evolution reaction (OER) step in water splitting reactions. In electrocatalysis, the OER plays a crucial role and several electrocatalysts have been investigated to improve the efficiency of O₂ gas evolution. Present research involves the use of citric acid coupled with lemon peel extracts for efficient recovery of lithium cobaltate from waste lithium-ion batteries and subsequent use of the recovered cathode material for OER in water splitting. Optimum recovery was achieved at 90 °C within 3 h of treatment with 1.5 M citric acid and 1.5% extract volume. The consequent electrode materials were calcined at 600, 700 and 800 °C and compared to the untreated waste material calcined at 600 °C for OER activity. The treated material recovered and calcined at 600 °C was the best among all of the samples for OER activity. Its average particle size was estimated to be within the 20–100 nm range and required a low overpotential of 0.55 V vs. RHE for the current density to reach 10 mA/cm² with a Tafel value of 128 mV/dec.

Electrodeposited Co-W-P ternary catalyst for hydrogen evolution reaction

In order to reduce the overpotential of hydrogen evolution reaction (HER), the ternary coating Co-W-P was deposited on the surface of the nickel foam by electrochemical deposition to obtain a highly active electrode. Based on the measured double layer capacitance (C_dl) and HER activity, there is volcanic behavior between the intrinsic activity of Co-W-P and the Co:W ratio in the electrolyte. W and P play different roles in the formation of nanoparticles, and work together to achieve the large electrochemical surface area and excellent activity. When applied to the modification of other catalysts (Ni-P and Fe-P), the higher intrinsic activity was obtained after the introduction of W.