Recent progress in ZnCr and NiCr layered double hydroxides and based photocatalysts for water treatment and clean energy production

In pursuit of advancing photocatalysts for superior performance in water treatment and clean energy generation, researchers are increasingly focusing on layered double hydroxides (LDHs) which have garnered significant attention due to their customizable properties, morphologies, distinctive 2D layered structure and flexible options for modifying anions and cations. No review has previously delved specifically into ZnCr and NiCr LDH-based photocatalysts and therefore, this review highlights the recent surge in ZnCr and NiCr-based LDHs as potential photocatalysts for their applications in water purification and renewable energy generation. The structural and fundamental characteristics of layered double hydroxides and especially ZnCr-LDHs and NiCr-LDHs are outlined. Further, the various synthesis techniques for the preparation of ZnCr-LDHs, NiCr-LDHs and their composite and heterostructure materials have been briefly discussed. The applicability of ZnCr-LDH and NiCr-LDH based photocatalysts in tackling significant issues in water treatment and sustainable energy generation is the main emphasis of this review. It focuses on photocatalytic degradation of organic pollutants in wastewater, elucidating the principles and advancements for enhancing the efficiency of these materials. It also explores their role in H2 production through water splitting, conversion of CO2 into valuable fuels and NH3 synthesis from N2, shedding light on their potential for clean energy solutions. The insights presented herein offer valuable guidance for researchers working towards sustainable solutions for environmental remediation and renewable energy generation.

Enhanced visible light photocatalytic hydrogen production over poly(dibenzothiophene-S,S-dioxide)-based heterostructures decorated by Earth-abundant layered double hydroxides

Layered double hydroxides (LDHs) have emerged as one of the promising catalyst substitutes to noble metals in photocatalytic water splitting due to their unique optoelectronic properties. Herein, a series of novel PSO@NiFeLDH composites have been designed and synthesized to investigate photocatalytic performance. Various physicochemical techniques characterized their structural, nanomorphological, and optical properties. These results demonstrated the existence of NiFeLDH particles on the surface of PSO and the strong interaction between NiFeLDH and PSO. The photocatalytic performance was much increased in the case of PSO@NiFeLDH as compared to that of Pt-modified PSO because of the synergistic effect between PSO and NiFeLDH. Remarkably, PSO@NiFeLDH-15 exhibits the highest photocatalytic activity with a rate of 52.8 mmol h^-1 g^-1 at an optimal content without a Pt cocatalyst under visible light irradiation.

Recent Breakthrough in Layered Double Hydroxides and Their Applications in Petroleum, Green Energy, and Environmental Remediation

The fast development of the world civilization is continuously based on huge energy consumption. The extra-consumption of fossil fuel (petroleum, coal, and gas) in past decades has caused several political and environmental crises. Accordingly, the world, and especially the scientific community, should discover alternative energy sources to safe-guard our future from severe climate changes. Hydrogen is the ideal energy carrier, where nanomaterials, like layered double hydroxides (LDHs), play a great role in hydrogen production from clean/renewable sources. Here, we review the applications of LDHs in petroleum for the first time, as well as the recent breakthrough in the synthesis of 1D-LDHs and their applications in water splitting to H2. By 1D-LDHs, it is possible to overcome the drawbacks of commercial TiO2, such as its wide bandgap energy (3.2 eV) and working only in the UV-region. Now, we can use TiO2-modified structures for infrared (IR)-induced water splitting to hydrogen. Extending the performance of TiO2 into the IR-region, which includes 53% of sunlight by 1D-LDHs, guarantees high hydrogen evolution rates during the day and night and in cloudy conditions. This is a breakthrough for global hydrogen production and environmental remediation.

2D-2D lattice engineering route for intimately coupled nanohybrids of layered double hydroxide and potassium hexaniobate: Chemiresistive SO2 sensor

2D-2D lattice engineering route is used to synthesize intimately coupled nanohybrids of layered double hydroxide (LDH) and potassium hexaniobate. The 2D-2D lattice engineering route is based on the electrostatically derived self-assembly of delaminated zinc-chromium-layered double hydroxide (ZC-LDH) nanosheets and potassium hexaniobate (HNb) nanosheets (ZCNb nanohybrids). The 2D-2D lattice-engineered ZCNb nanohybrids display expanded surface area, mesoporous anchored nanosheets network morphology, and intimate coupling between nanosheets. The 2D-2D lattice engineered ZCNb nanohybrids are used for the low temperature operated gas sensor. The ZCNb nanohybrids display outstanding selectivity for the SO₂, with the high response of 61.5% compared to pristine ZC-LDH (28.08%) and potassium niobate (8%) at 150 °C. Moreover, ZCNb sensors demonstrate superior response and recovery periods of 6 and 167 s at 150 °C, respectively. This result underscores the exceptional functionality of the ZCNb nanohybrids as efficient SO₂ sensors. Moreover, these findings vividly demonstrate that the 2D-2D lattice-engineered ZCNb nanohybrids are quite effective not only in improving the gas sensor activity but also in developing of new type of intimately coupled mesoporous LDH-metal-oxide based hybrid materials.

Effect of cobalt doping on photocatalytic water splitting activity of NiTi-layered double hydroxide

Cobalt-doping a NiTi layered double hydroxide affords CoNiTi-LDH with smaller plate sizes and a higher degree of order, which allows the band gap to shrink from 2.7 eV to 2.4 eV. CoNiTi-LDH is active for both water oxidation and reduction.

[Ru(NꓥNꓥN)2-Ce]-based Framework for Photocatalytic Sulfide Oxidation and Hydrogen Production

Using photosensitized Ru(NꓥNꓥN)2-metalloligand, a series of Ce-frameworks were synthesized. The incorporation of visible-light-responsive Ru(NꓥNꓥN)2-unit endows the Ce-frameworks with photocatalytic activities in both sulfide oxidation and hydrogen production. The doping of...

Zinc-chromium layered double hydroxides anchored on carbon nanotube and biochar for ultrasound-assisted photocatalysis of rifampicin

In this study, ZnCr layered double hydroxide (LDH), ZnCr LDH/carbon nanotube (CNT), and ZnCr LDH/Biochar (BC) were synthesized and characterized by various analyses. The successful synthesis and the great crystallinity of the samples were consented by XRD analysis. SEM and TEM were applied to study the morphology of the synthesized samples. The simultaneous presence of C, Zn, and Cr elements was well confirmed by EDX and dot mapping analyses demonstrating the successful preparation of nanocomposites. According to the BET analysis, ZnCr LDH nanocomposites with BC and CNT had more specific surface area compared to ZnCr LDH alone. The catalytic performances of the samples were determined for the degradation of rifampicin (RF). The degradation efficiency of the sonophotocatalytic process in the presence of 0.6 g L^-1 of ZnCr LDH/BC toward 15 mg L^-1 of RF under 150 W ultrasound and visible light irradiation was found to be about 100% within 40 min. The influence of the reactive species on the sonophotocatalytic process was assessed via the addition of different scavengers (para-benzoquinone (p-BQ), formic acid (FA), isopropyl alcohol (IPA)), and enhancers (hydrogen peroxide and potassium persulfate). The GC-MS analysis was carried out and eleven by-products during the RF decomposition were detected.

Graphene-based ZnCr layered double hydroxide nanocomposites as bactericidal agents with high sonophotocatalytic performances for degradation of rifampicin

Herein, ZnCr layered double hydroxide (ZnCr LDH), and its nanocomposites with GO and rGO were synthesized using the co-precipitation method. The samples were characterized using XRD, FT-IR, SEM, TEM, BET, and XPS techniques. The sonophotocatalytic activity of the ZnCr LDH, ZnCr LDH/GO, and ZnCr LDH/rGO was investigated via the degradation of rifampicin (RIF) in the ultrasonic bath under visible light irradiation. The synergy index of more than 1 determined for ZnCr LDH/rGO indicated the positive interaction of sonocatalysis and photocatalysis resulted by hybridizing the LDH nanosheets with rGO. The maximum sonophotocatalytic degradation efficiency of 87.3% was achieved in the presence of ZnCr LDH/rGO nanocomposite with the concentration of 1.5 g L^-1 for degradation of RIF with an initial concentration of 15 mg L^-1 within 60 min sonication under visible light irradiation. The addition of different scavengers indicated that hydroxyl radicals, superoxide anion radicals, and the generated holes played a dominant role in the degradation of the pollutant molecules. A possible degradation mechanism was suggested based on the intermediates. The antibacterial tests confirmed the higher antibacterial activity of ZnCr LDH/GO compared with ZnCr LDH and ZnCr LDH/rGO against Gram-positive Staphylococcus aureus.

Enhanced photocatalytic performance of heterogeneous hydrotalcite by spontaneously polarized ferroelectric

Two-dimensional photocatalytic materials have attracted great attention due to their large specific surface area and abundant active sites. Suppressing the recombination of photo-excited carriers is an effective approach to improve the performances of photocatalytic materials. Herein, we introduced ferroelectric PbTiO₃ into the two-dimensional layered double hydroxides (LDHs) to improve the carrier separation efficiency and photocatalytic performances. A built-in electric field was generated in the polarized PbTiO₃, resulting in the improvement of the carrier separation efficiency and the promotion of the lifetime of photo-excited carriers in the LDHs-PbTiO₃ composites. As a result, the LDHs-PbTiO₃ composites showed the decent photocatalytic performances towards water splitting under visible light irradiation. The oxygen production rate of the proposed LDHs-PbTiO₃ composites was almost twice than that of pristine LDHs. These results have addressed the significance of photo-excited carriers in photocatalytic materials. This approach could undoubtedly provide the valuable information in design and construction of high efficiency photocatalysts.

Recovering heavy metals from electroplating wastewater and their conversion into Zn2Cr-layered double hydroxide (LDH) for pyrophosphate removal from industrial wastewater

This work incorporated technological values into Zn₂Cr-layered double hydroxide (LDH), synthesized from unused resources, for removal of pyrophosphate (PP) in electroplating wastewater. To adopt a resource recovery for the remediation of the aquatic environment, the Zn₂Cr-LDH was fabricated by co-precipitation from concentrated metals of plating waste that remained as industrial by-products from metal finishing processes. To examine its applicability for water treatment, batch experiments were conducted at optimum M²⁺/M³⁺, pH, reaction time, and temperature. To understand the adsorption mechanisms of the PP by the adsorbent, the Zn₂Cr-LDH was characterized using Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses before and after adsorption treatment. An almost complete PP removal was attained by the Zn₂Cr-LDH at optimized conditions: 50 mg/L of PP, 1 g/L of adsorbent, pH 6, and 6 h of reaction. Ion exchange controlled the PP removal by the adsorbent at acidic conditions. The PP removal well fitted a pseudo-second-order kinetics and/or the Langmuir isotherm model with 79 mg/g of PP adsorption capacity. The spent Zn₂Cr-LDH was regenerated with NaOH with 86% of efficiency for the first cycle. The treated effluents could comply with the discharge limit of <1 mg/L. Overall, the use of the Zn₂Cr-LDH as a low-cost adsorbent for wastewater treatment has contributed to national policy that promotes a zero-waste approach for a circular economy (CE) through a resource recovery paradigm.

Reusability and stability of a novel ternary (Co-Cd-Fe)-LDH/PbI2 photoelectrocatalytst for solar hydrogen production

The design of highly active and cost-effective photoelectrocatalysts for effective hydrogen generation becomes a mandatory issue due to the demands on sustainable solar fuels. Herein a novel ternary Co-Cd-Fe LDH/PbI2 nanocomposite (T-LDH/PbI2NC) was fabricated by combining strategies of doping and in-situ loading of ternary Co-Cd-Fe LDH. The morphological, structural, and optical properties of PbI2, T-LDH, and T-LDH/PbI2 NC were studied by different techniques. LDH narrows the bandgap of the nanocomposite to 2.53 eV which prolongs the lifetime of the photo-induced electrons. Subsequently, the use of T-LDH/PbI2 NC improves the photoelectrocatalytic (PEC) H2 production rate. T-LDH/PbI2 NC shows a catalytic H2 production rate of 107.53 mmol h-1 cm-2 with IPCE% of 83.8% for 307 nm and 67.3% for 508 nm. The ABPE% reaches its supreme of 4.24% for - 0.58 V and 5.41% for - 0.97 V, these values are the highest values yet for LDH-based photocatalysts. The influences of the operating temperature and monochromatic illumination on the PEC performance were studied. Also, the electrochemical surface area, thermodynamic parameters, and Tafe slopes are calculated to label the hydrogen evolution mechanism. Moreover, the stability and reusability of the T-LDH/PbI2 NC photoelectrode were investigated. This work not only illustrated a simplistic and accessible way to produce a new category of highly efficient photocatalysts compared to the previously reported LDH-based PEC catalysts but also demonstrates a new point of view for improving PEC performance towards industrial water splitting under sunlight irradiation.

Enhanced photocatalytic performance of a Ti-based metal-organic framework for hydrogen production: Hybridization with ZnCr-LDH nanosheets

Novel hybrid composites of NH₂-MIL-125(Ti) and ZnCr-layered double hydroxide nanosheets (ZnCr-LDH NSs) are developed for use as visible-light-active photocatalysts for hydrogen production based on water photolysis. The hybrid composites are obtained by growing NH₂-MIL-125(Ti) in the presence of exfoliated ZnCr-LDH NSs using a solvothermal reaction. Hybridization of NH₂-MIL-125(Ti) with exfoliated ZnCr-LDH NSs leads to significant effects on the morphology and optical properties of NH₂-MIL-125(Ti). To find the optimum photocatalytic activity for hydrogen production by the hybrid composite photocatalysts, the content of ZnCr-LDH in this work is controlled. Compared to that of pristine NH₂-MIL-125(Ti) and ZnCr-LDH, the hybrid composites exhibit an improved photocatalytic activity for hydrogen production under visible-light irradiation. In addition, the hybrid composite photocatalyst shows excellent photo-chemical stability. The improved photocatalytic activity is believed to benefit from the synergy of strong electronic coupling between NH₂-MIL-125(Ti) and ZnCr-LDH NSs, expanded light absorption and band alignment to enhance the lifetime of photo-induced electrons and holes.

Fabrication of a Co(OH)2/ZnCr LDH "p-n" Heterojunction Photocatalyst with Enhanced Separation of Charge Carriers for Efficient Visible-Light-Driven H2 and O2 Evolution

Photocatalytic generation of H₂ and O₂ by water splitting remains a great challenge for clean and sustainable energy. Taking into the consideration promising heterojunction photocatalysts, analogous energy issues have been mitigated to a meaningful extent. Herein, we have architectured a highly efficient bifunctional heterojunction material, i.e., p-type Co(OH)₂ platelets with an n-type ZnCr layered double hydroxide (LDH) by an ultrasonication method. Primarily, the Mott-Schottky measurements confirmed the n- and p-type semiconductive properties of LDH and CH material, respectively, with the construction of a p-n heterojunction. The high resolution transmission electron microscopy results suggest that surface modification of ZnCr LDH by Co(OH)₂ hexagonal platelets could form a fabulous p-n interfacial region that significantly decreases the energy barrier for O₂ and H₂ production by effectively separating and transporting photoinduced charge carriers, leading to enhanced photoreactivity. A deep investigation into the mechanism shows that a 30 wt % Co(OH)₂-modified ZnCr LDH sample liberates maximum H₂ and O₂ production in 2 h, i.e., 1115 and 560 μmol, with apparent conversion efficiencies of H₂ and O₂ evolution of 13.12% and 6.25%, respectively. Remarkable photocatalytic activity with energetic charge pair transfer capability was illustrated by electrochemical impedance spectroscopy, linear sweep voltammetry, and photoluminescence spectra. The present study clearly suggests that low-cost Co(OH)₂ platelets are the most crucial semiconductors to provide a new p-n heterojunction photocatalyst for photocatalytic H₂ and O₂ production on the platform of ZnCr LDH.

A review on fabricating heterostructures from layered double hydroxides for enhanced photocatalytic activities

LDH is a controllable 2D material for fabricating heterostructures with another semiconductor.

Porous Layered Double Hydroxides Synthesized using Oxygen Generated by Decomposition of Hydrogen Peroxide

Porous magnesium-aluminium layered double hydroxides (LDH) were prepared through intercalation and decomposition of hydrogen peroxide (H₂O₂). This process generates oxygen gas nano-bubbles that pierce holes in the layered structure of the material by local pressure build-up. The decomposition of the peroxide can be triggered by microwave radiation or chemically by reaction with iodide (I⁻) ions. The carbonate LDH version [Mg_0.80Al_0.20(OH)₂](CO₃)_0.1∙mH₂O was synthesized by microwave-assisted urea coprecipitation and further modified by iodide or H₂O₂ intercalation. High resolution Scanning Electron Microscopy (HR-SEM) and Brunauer-Emmet-Teller (BET) analysis were used to assess the morphology and surface area of the new porous materials. The presence of H₂O₂ in the interlayer region and later decomposition triggered by microwave radiation generated more pores on the surface of the LDH platelets, increasing their specific surface area from initially 9 m²/g to a maximum of 67 m²/g. X-Ray Diffraction showed that the formation of the pores did not affect the remaining crystal structure, allowing possible further functionalization of the material.