Gallium-based nascent electrode materials towards promising supercapacitor applications: a review.

To meet the energy requirement of the modern era, supercapacitors are promising candidates for energy storage devices, which possess the potential to compete with the future battery technology. To accomplish this pivotal task, it is vital to choose electrode materials that have high power and energy density as well as superb electrochemical stability. For the past few years, the use of gallium-based materials for energy storage applications has attracted attention because of their excellent activity towards electrochemical energy storage applications despite the single oxidation state (i.e., +3 which is redox inactive and does not contribute towards pseudo capacitance). Recently, research on gallium-based materials has started and will be continued further owing to the fact that gallium-based materials possess numerous excellent properties such as fast charge and discharge rate, high power density, long cycle life, stability over a wide range of temperatures, excellent electron velocity, superior chemical and physical stabilities and high voltage application capability, which make them a potential class of electrode materials for supercapacitors. The enhancement in the electrochemical performance upon the introduction of gallium into the system can make it a futuristic candidate for electrochemical energy storage devices. Herein, we systematically outline the synthesis and characterization of gallium-based materials and their composites as explored by esteemed researchers focusing only on their supercapacitive performance via electrochemical techniques. For a better understanding, the underlying charge storage mechanism and identified characteristics are presented to give a crystal-clear idea about the field. In addition, the key challenges and impending perspectives of gallium-based electrodes for supercapacitor applications are debated.

Defect Control via Compositional Engineering of Zn-Cu-In-S Alloyed QDs for Photocatalytic H2O2 Generation and Micropollutant Degradation: Affecting Parameters, Kinetics, and Insightful Mechanism.

Photocatalytic H2O2 production and recalcitrant pollutant degradation are regarded as promising clean technology toward achieving sustainable solar-to-chemical energy conversion. Herein, nonstoichiometric Zn-Cu-In-S (ZCIS) quaternary alloyed quantum dots (QDs) are rationally fabricated via a reflux method toward H2O2 generation and ciprofloxacin degradation under visible light irradiation. The optimum catalyst (ZCIS-2) exhibits a notable H2O2 production of 1685.2 µmole h-1 g-1 (solar-to-chemical conversion efficiency (SCC), 0.19%), which is 5.3 times higher than that of CuInS2 (CIS), and a ciprofloxacin (CIP) degradation efficiency of 96% in 2 h. The observed improvement in activity corresponds to optimized exciton separation/transfer, broad photon absorption, tunable band alignment, and effective adsorption/activation. In addition, oxygen reduction goes through both direct two-electron single-step reduction and single-electron two-step superoxide radical pathways, whereas CIP degradation proceeds via direct •O2- and indirect •OH radical pathways, as confirmed by scavenger experiments. An appropriate amount of defects improves the adsorption/activation of O2 toward H2O2 and active oxygen species generation that facilitates CIP degradation. The effect of operational parameters, such as pH, surrounding environment, presence of ions, sacrificial agent, etc., on both H2O2 formation and CIP removal is vividly studied. Hence, the current study will provide an in-depth insight into O2 photoreduction and micropollutant removal, which encourages further advancement of potent alloyed quantum dot-oriented photocatalytic systems.

Photocatalytic and photo-electrochemical ammonia synthesis over dimensional oriented cobalt titanate/nitrogen-doped reduced graphene oxide junction interface catalyst.

Nitrogen reduction to ammonia is vital for chemical industries and renewable clean energy. Denying the harsh reaction conditions adopted in the Haber-Bosch process and stimulation research for ammonia production through sustainable technologies is a smart approach. Hitherto, photocatalyst acquiring the potential to attain high nitrogen reduction reaction (NRR) efficiency is a challenging task. Here, this study demonstrated cobalt titanate (CoTiO3) rods (p-type) straddled with two-dimensional (2D) sheets of nitrogen-doped reduced graphene oxide (N-rGO, n-type) via, reflux method; realizing the advantages of dissimilar dimensionalities and strong interfacial junction coupling for efficient NRR under visible light irradiation. The successful interface junction establishment between CoTiO3 and N-rGO has been witnessed from Raman, x-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis. Moreover, a well-defined type-II band structure is capable to curl the charge anti-recombination process; reflected in upgraded photo-catalytic/electrocatalytic upshots. The CoTiO3 modified with an optimized concentration of N-rGO exhibits high stability with an improved photocatalytic (1722.22 µmolL-1h-1) and photo-electrocatalytic (16.8 µg cm-1h-1) nitrogen reduction to ammonia production; multiple times higher than counterparts. This improved photo-activity of CoTiO3/N-rGO junction hybrid stems from the built-in electric field existing across the dissimilar junction interface, triggering charge transfer channels for reduction reaction in mild reaction conditions. The result of these materials might strategies the way for future development of new functionalities bearing highly active catalyst materials for sustainable energy-related conversion.

Robust direct Z-scheme exciton transfer dynamics by architecting 3D BiOI MF-supported non-stoichiometric Cu0.75In0.25S NC nanocomposite for co-catalyst-free photocatalytic hydrogen evolution.

Designing promising photocatalytic systems with wide photon absorption and better exciton separation ability is a cutting-edge technology for enhanced solar-light-driven hydrogen production. In this context, non-stoichiometric Cu0.75In0.25S nanocrystals (CIS NCs) coupled with three-dimensional (3D) BiOI micro-flowers (BOI MFs) were synthesized through an ultra-sonication strategy forming a CIS-BOI heterojunction, which was well supported by XRD, photocurrent, XPS and Mott-Schottky analyses. Further, the co-catalyst-free CIS-BOI binary hybrid shows improved hydrogen evolution, i.e., 588.72 µmol h-1, which is 3.2 times greater than the pristine CIS NC (183.97 µmol h-1). Additionally, the binary composite confers an apparent conversion efficiency (ACE) of 9.44% (8.90 × 1016 number of H2 molecule per sec), which is extensively attributed to the robust charge carrier separation and transfer efficiency via the direct Z-scheme mechanism (proved through superoxide and H2 evolution activity). Moreover, the broad photon absorption range and productive exciton separation over the CIS-BOI composite are substantially justified by UV-Vis DRS, PL, EIS and photocurrent measurements.

Efficient Photon Conversion via Double Charge Dynamics CeO2-BiFeO3 p-n Heterojunction Photocatalyst Promising toward N2 Fixation and Phenol-Cr(VI) Detoxification.

For better exciton separation and high catalytic activity, the most trailblazing stratagem is to construct defect engineered low-dimensional p-n heterojunction framed photocatalytic systems. In this context, we have developed a rod-sheet (1D-2D) p-n heterojunction of MCeO2-BiFeO3 by a simple hydrothermal method and scrutinized its photocatalytic performance toward N2 fixation and phenol/Cr(VI) detoxification. The intimate contact between MCeO2 and BiFeO3 in the junction material is well established via X-ray diffraction (XRD), UV-vis diffuse reflectance spectrosopy (DRS), transmission electron microscopy (TEM), and photoelectrochemical studies. Further, scanning electron microscopy (SEM) and TEM pictures clearly support the decoration of MCeO2 nanorods over BiFeO3 sheets and also depict the junction boundary. Additionally, photoluminescence (PL), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and Raman measurements give solid evidence toward the presence of an oxygen vacancy. Moreover, the Mott-Schottky result indicates a feasible band edge potential favoring the p-n heterojunction with a built-in electric field between BiFeO3 and MCeO2 favoring a double charge dynamic. The MCeO2-BFO p-n junction displays a notable catalytic activity, i.e., 98.2% Cr(VI) reduction and 85% phenol photo-oxidation, and produces 117.77 µmol h-1 g-1 of ammonia under light irradiation. Electrochemical analysis suggests a four-electron/five proton-coupled N2 photoreduction pathway. The designed oxygen vacancy oriented p-n heterojunction suffering double charge migration shows significant catalytic performance due to effective electron-hole separation as justified via PL, electrochemical impedance spectra (EIS), and Bode phase analysis.

Bimetallic co-effect of Au-Pd alloyed nanoparticles on mesoporous silica modified g-C3N4 for single and simultaneous photocatalytic oxidation of phenol and reduction of hexavalent chromium.

Perception of surface plasmonic resonance in heterogeneous photocatalysis not only has impact on basic science of sustainable energy development, but also generates green technologies for wastewater treatment, selective oxidation and reduction reactions. In the present study Au/Pd bimetallic alloyed nanoparticles were effectively decorated on mesoporous silica modified g-C3N4 (graphitic carbon nitride) nanosheets by a simple one-pot calcinations strategy. The formation of Au/Pd alloyed nanoparticles has been supported by XRD, UV-vis DRS, TEM and XPS studies. The photocatalytic performance of the photocatalysts were investigated by performing tandem reaction for simultaneous oxidation of phenol and reduction of Cr (VI). The photocatalytic performances were found to be significant for either single phenol species or single Cr (VI), but quite appreciable photocatalytic performance was observed for a solution containing Cr (VI)-phenol mixture. The synergetic effect of Au/Pd alloyed nanoparticle and enhanced photocurrent (1.4 mA/cm2) generated by the nano-composite further supports the activity. The results of tandem reaction not only reveals the feasibility of carrying out degradation of two important pollutants simultaneously from waste water, but also gives us an enlightenment to efficiently degrade mixture of pollutants without using any additional chemical as trapping agent in the photocatalytic process.

Enhanced photocatalytic activities of polypyrrole sensitized zinc ferrite/graphitic carbon nitride n-n heterojunction towards ciprofloxacin degradation, hydrogen evolution and antibacterial studies.

Fusion of heterogeneous photocatalysts with conducting polymers has paid a rising stratagem in the field of photocatalysis owing to its biocompatibility and environment friendliness. In this work a series of polypyrrole (PPY) sensitized zinc ferrite/graphitic carbon nitride (ZFCN) n-n heterojunction (ZFCN@10PPY, ZFCN@20PPY, and ZFCN@30PPY) nanocomposite were fabricated by in-situ polymerization method. Due to low band gap of polypyrrole, it behaves as a photo-sensitizer, supplies surplus numbers of electrons to ZnFe2O4/g-C3N4 n-n heterojunction and improves the photocatalytic performance. The fabricated ZFCN@20PPY exhibits highest photocatalytic activity in comparison to others nanocomposites. The superior photocatalytic performance of ZFCN@20PPY was ascribed to the tunable band structure, synergistic effect of broad absorption upto NIR region, delayed electron-hole recombination and efficient charge transfer across the junction interface which has been well confirmed from UV-Vis DRS, PL and EIS measurement. Further the photocatalytic activity of ZFCN@20PPY was supported by both n-type and p-type photocurrent density i.e. 2.4 and 3.9 mA/cm2 respectively. ZFCN@20PPY shows good photocatalytic performance towards ciprofloxacin degradation (92%) and generation of hydrogen energy (567 µmol). Along with pollutant degradation and energy production ZFCN@20PPY also shows its potential towards antibacterial activities against human pathogenic bacteria like Escherichia coli. These newly designed polymer sensitized n-n heterojunction may offer a promising strategy for maximum light absorption and be authoritative in meeting the environmental claims in the future.

An energy band compactable B-rGO/PbTiO3 p-n junction: a highly dynamic and durable photocatalyst for enhanced photocatalytic H2 evolution.

Reduced graphene oxide (rGO) intentionally doped with boron atoms is a promising tactic to extract bandgap energy and p-type semiconducting behavior from graphene-based materials. Moreover, the integration of p-type boron-doped rGO with an n-type material through a heterojunction interface exhibits complementary properties to restrict the fast recombination of charge carriers and enhance the photoreaction towards energy applications. Herein, we have prepared boron-doped rGO/PbTiO3 p-n heterojunctions via a hydrothermal method. The successful formation of an excellent p-n heterojunction was demonstrated by TEM, XPS and Raman analysis. The constructed boron-doped rGO/PbTiO3 p-n heterojunctions exhibit dramatic increases in photoelectrochemical and photocatalytic performance due to the presence of a space charge region at the interface of the two materials. The scenario also revealed the double-edge sword functions of B-rGO: the material itself (i) acts as a visible light active photocatalyst with a band gap energy of 2.7 eV and (ii) participates in enhanced charge transfer via the band edge alignment between B-rGO and PbTiO3, as elucidated from photoluminescence and electrochemical impedance studies. Furthermore, the optimal 2B-rGO/PT p-n heterojunction possesses outstanding repeatability and exhibited the highest rate of hydrogen evolution, i.e. 293.79 µmol h-1 under visible light irradiation. The results for these materials may provide advanced insight into the design of next-generation high-efficiency black graphene-based heterojunctions.

Construction of M-BiVO4/T-BiVO4 isotype heterojunction for enhanced photocatalytic degradation of Norfloxacine and Oxygen evolution reaction.

To conquer the issues of poor compatibility, confined intimate contact and limited improvement of charge anti-recombination process of a traditional heterojunction formed by interfacing of two different semiconductors, a simplistic strategy has been espoused for the fabrication of isotype heterojunction flanked with two dissimilar crystal phases of a single semiconducting material. Herein, we account the fabrication of an in-situ formed M-BiVO4/T-BiVO4 (MT-BiVO4) isotype heterojunction by a simple co-precipitation method followed by altering the calcinations temperatures. The physico-chemical properties of the fabricated MT-BiVO4 isotype hetrojunctions were analyzed by using X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-Visible Diffuse reflectance spectroscopy (UV-Vis DRS) techniques. The FESEM image of MT-BiVO4 photodeposited by Au and MnOx particles was provided strong evidence for the spatial separation of photogenerated charge carriers between M and T phase of BiVO4 in an isotype heterojunction. The interfacing of T-BiVO4 with M-BiVO4 in an isotype heterojunction affords the well-built close contact between them was confirmed by the High resolution transmission electron microscopy (HRTEM) analysis. The photocatalytic reactions of all the prepared MT-BiVO4 isotype heterojunctions were examined by monitoring the degradation of Norfloxacine and oxygen evolution reaction under visible light irradiation. The optimized 65% MT-BiVO4 isotype heterojunction discloses higher photocatalytic activity around 91% of Norfloxacine degradation in 150 min and 808 µmol of O2 evolution in 2 h under visible light irradiation. On the other hand, the photoelectrochemical measurements reveals that the optimized 65% MT-BiVO4 isotype heterojunction exhibits superior photocurrent i.e. 584 µA/cm2 which is approximately 5.1 and 25.3 times higher than the neat T-BiVO4 and M-BiVO4, and these results are well consistent with the photocatalytic activities. The higher PEC and photocatalytic activities are due to the well-built close contact, superior compatibility and matching band structure between T-BiVO4 and M-BiVO4, which provides strapping driving force for the efficient enhancement of charge separation process. The Electrochemical impedance spectroscopy (EIS), photoluminescence (PL), photoelectrochemical (PEC) and bode analysis confirms the effectual diminish of charge recombination process in MT-BiVO4 isotype heterojunction relative to the neat materials. The chronoamperometric analysis authenticated that the isotype heterojunctions are more stable than the neat materials.

Deciphering Z-scheme Charge Transfer Dynamics in Heterostructure NiFe-LDH/N-rGO/g-C3N4 Nanocomposite for Photocatalytic Pollutant Removal and Water Splitting Reactions.

A series of heterostructure NiFe LDH/N-rGO/g-C3N4 nanocomposite were fabricated by combining calcinations-electrostatic self-assembly and hydrothermal steps. In this method, negatively charged N-rGO was electrostaticaly bonded to the self-assembled interface of n-n type g-C3N4/NiFe LDH hybrid. XRD and AFM results revealed successful formation of heterostructure nanocomposite due to the coupling effect of exfoliated NiFe LDH nanosheets with N-rGO and g-C3N4. Among the as synthesized heterostructure, CNNG3LDH performed superior photocatalytic activities towards 95 and 72% mineralization of RhB and phenol. Furthermore, CNNG3LDH could achieve the highest photocatalytic H2 evolution rate of 2508 µmolg-12h-1 and O2 evolution rate of 1280 µmolg-12h-1 under visible light irradiation. The CNNG3LDH possess lowest PL intensity, reduced arc of the Nyquist plot (43.8 Ώ) and highest photocurrent density (-0.97 mA cm-2) which revealed effective charge separation for superior photocatalytic activities. TRPL spectral results reveal the synergistic effect of layered component in CNNG3LDH for achievable higher life time of excitons of ~16.52 ns. In addition, N-rGO mediator based Z-scheme charge transfer mechanisms in CNNG3LDH were verified by the ESR and TA-PL studies. Enriched oxygen vacancy type defects in NiFe LDH and N-rGO mediated Z-scheme charge transfer mechanistic path strongly manifest the superior photocatalytic activities of the heterostructure materials.

Erratic charge transfer dynamics of Au/ZnTiO3 nanocomposites under UV and visible light irradiation and their related photocatalytic activities.

The present article presents an in-depth discussion on the state-of-the-art multifarious roles of Au nanoparticles and their associated charge anti-recombination process in burgeoning photocatalysis research. Hexagonal-phase ZnTiO3 was fabricated through a sol-gel auto-combustion method by optimizing the calcination temperatures. To further improve the charge separation efficiency and visible light induced photocatalytic activity of pristine ZnTiO3, we designed a new type of Au/ZnTiO3 nanocomposite by a precipitation-deposition method. The photocatalytic activities of the Au/ZnTiO3 nanocomposites were substantiated by evaluating the rate of hydrogen evolution under both UV and visible light illumination. The photocatalytic activity of the Au/ZnTiO3 nanocomposites rises proportionally with an increase in Au content up to 1.5 wt% under UV light illumination and it produce around 285 µmol h-1 of H2 which is approximately 2.6 times higher than that produced by pristine ZnTiO3. Therefore, the Au nanoparticles present on the surface of ZnTiO3 act as electron acceptors, leading to an increase in the rate of generation and separation of charge carriers. This process helps to enhance the congregation of electrons on Au nanoparticles through the Schottky junction. The obtained results are very consistent with steady-state PL and UV light induced photocurrent measurements. Conversely, such a trend was not detected under visible light illumination. The visible light induced photocatalytic activity of Au/ZnTiO3 nanocomposites increases with a rise in Au content up to 1 wt% and thereafter decreases with further Au loading. Therefore, the initial increment in photocatalytic activity is due to the generation, separation and participation of a large number of SPR-induced charge carriers and thereafter decreases due to the recombination of SPR-generated charge carriers because of the formation of defect sites at the Au and ZnTiO3 interface. That the excess Au loading causes the recombination of SPR charge carriers was well explained by undertaking SPR-induced TRPL analysis and this result is directly followed up with the results of visible light induced photocurrent and EIS measurements. The Au/ZnTiO3 nanocomposites with optimal Au loading (1 wt%) delivered an amazingly high rate of hydrogen evolution i.e. 108 µmol h-1 with an energy conversion efficiency of 7.14%, whereas pristine ZnTiO3 shows negligible activity under visible light illumination.

Topotactic Transformation of Solvated MgCr-LDH Nanosheets to Highly Efficient Porous MgO/MgCr2O4 Nanocomposite for Photocatalytic H2 Evolution.

The hybrid structure of nanoparticles (NPs) with nanosheets has the advantage of both anisotropic properties of NPs and large specific surface areas of nanosheets, which is desirable for many technological applications. In this study, MgCr2O4 spinel NPs decorated on highly porous MgO nanosheets forming MgO/MgCr2 O4( x) nanocomposites were synthesized by a one pot coprecipitation method followed by a heat treatment process of the solvated wet gel of MgCr-LDH with polar solvent N, N-dimethylformamide (DMF) at 400 °C. This novel synthetic methodology generates materials consisting of porous metal oxides nanosheets adhered with spinel phase NPs due to the slow generation of gases such as H2O, CO2, and NH3 under moderate temperature during the heat treatment process. The synergistic effect of much wider band gap MgO nanosheets and narrow band gap MgCr2O4 NPs added increased stability due to the stronger bonding coordination of MgCr2O4 NPs with MgO nanosheets. The obtained MgO/MgCr2 O4( x) nanocomposites possess large specific surface areas, highly porous structure, and excellent interface between MgCr2O4 NPs and MgO nanosheets, which proved from N2 sorption isotherm, TEM, HR-TEM study. With metallic ratio of MgCr3:1, MgO/MgCr2O4(MgCr3:1) nanocomposites exhibit highest H2 evolution rate of 840 µmolg-12h-1, which was 2 times higher than that of pure MgCr2O4(420 µmolg-12h-1). The LSV measurement study of MgO/MgCr2O4 (MgCr3:1) nanocomposite shows an enhancement of light current density of 0.22 µA/cm2 at potential bias of -1.1 V. The Mott-Schottky analysis suggested the band edge positions of the n-type constituents and formation of n-n type heterojunctions in MgO/MgCr2O4 (MgCr3:1) nanocomposite, which facilitates the flow of charge carriers. The EIS and Bode phase plot of MgO/MgCr2O4 (MgCr3:1) nanocomposite signifies the lower interfacial charge transfer resistance and higher lifetime of electrons (2.7 ms) for enhanced H2 production. Lastly, the enhanced photocatalytic H2 production activity and long-term stability of MgO/MgCr2O4(MgCr3:1) could be attributed to maximum specific surface area, porous structure, close intimacy contact angle between two cubic phases of MgCr2O4 NPs and MgO nanosheets, abundant oxygen vacancies sites, reduced charge transfer resistance and suitable band edge potential to drive the thermodynamic energy for H2 production. This work highlighted an effective strategy for the synthesis of cost-effective 2D porous heterojunctions nanocomposite photocatalyst for promising applications in the field of clean H2 production utilizing abundant solar energy.

Pyrochlore Ce2Zr2O7 decorated over rGO: a photocatalyst that proves to be efficient towards the reduction of 4-nitrophenol and degradation of ciprofloxacin under visible light.

In the present study, a series of Ce2Zr2O7@rGO nanocomposites were synthesized using a simple solution combustion method followed by a photoreduction technique. The as-prepared samples were well characterised using various analytical techniques to determine the morphological, optical, structural, electrochemical and compositional properties. The presence of oxygen defects was observed from Raman and photoluminescence spectra. The photoreduction of GO to rGO was determined from Raman and Fourier-transform infrared (FTIR) spectroscopy results. The role of rGO proved to be quite significant for the enhanced photocatalytic activity of the nanocomposites. The synergistic communication between Ce2Zr2O7 and rGO accelerates the photoreduction of 4-nitrophenol along with the degradation of ciprofloxacin under visible light irradiation. Of the rGO nanocomposites, 3 wt% GO loaded Ce2Zr2O7 reduces 99% of 20 ppm of 4-nitrophenol to 4-aminophenol in 120 min and decomposes 10 ppm of ciprofloxacin by up to 89% in 60 min. The significant enhancement in the activity of the Ce2Zr2O7@rGO nanocomposite was ascribed to the effective charge separation of excitons through π-conjugation of graphene at the interface, which is well supported by the impedance, photoluminescence and photocatalytic results.

Highly efficient charge transfer through a double Z-scheme mechanism by a Cu-promoted MoO3/g-C3N4 hybrid nanocomposite with superior electrochemical and photocatalytic performance.

Herein, a novel Cu-MoO3/g-C3N4 hybrid nanocomposite was successfully synthesized by a two-step strategy of one-pot pyrolysis followed by the impregnation method. The structure, phase, morphology and electronic environment of MoO3, g-C3N4 and Cu in the composite were determined by various characterization methods. The oxygen vacancies of MoO3 were ascertained by UV-DRS, Raman, and XPS analysis. The formation of the heterostructure was characterised by electrochemical measurements. The photocatalytic performance of the composite was investigated by the water reduction reaction and the reduction of an important inorganic pollutant, Cr(vi). In the presence of Cu NPs, the H2 evolution of the MoO3/g-C3N4 hybrid was found to be 652 µmol h-1 with an apparent energy conversion efficiency of 13.46%, and up to 95% of Cr(vi) was reduced using citric acid as a hole scavenger. The remarkably enhanced photocatalytic performance was attributed to the combined effect of the double Z-scheme mechanism and defective MoO3. The in situ formation process of the MoO3/g-C3N4 hybrid followed a direct Z-scheme charge transfer by generating a great number of defects at the solid-solid interface, similar to that of a conductor, and offered low electrical resistance, whereas loading of Cu NPs built up an indirect Z-scheme charge transfer to establish the double Z-scheme charge transfer mechanism. This hybrid material produces a photocurrent density of 12.1 mA cm-2, in good agreement with the photocatalytic activity. This study highlights the facilitation effect of MoO3 due to oxygen vacancies and charge transfer through the double Z-scheme mechanism to open up a new window in the field of 2D nanostructured photocatalytic materials.

Kinetics, Isotherm, and Thermodynamic Study for Ultrafast Adsorption of Azo Dye by an Efficient Sorbent: Ternary Mg/(Al + Fe) Layered Double Hydroxides.

The extremely high adsorption efficiency of malachite green (MG) was examined through a series of batch experiments by using Fe3+-doped Mg/Al layered double hydroxides (LDHs). The incorporation of iron into Mg/Al LDH with varying Al + Fe molar ratio of 4 + 1, 3 + 2, 2 + 3, and 1 + 4 increased the adsorption capacity with respect to time. The spectral analysis and N2 sorption studies showed that there was retention of surface morphology in all of the iron-modified LDH samples. The experimental evidences showed that the adsorbent Mg/(Al + Fe) with a molar ratio of 10:2 + 3 had a significant removal, i.e., 99.94% for MG with the initial concentration of 1000 mg L-1 at pH ∼ 9 and at room temperature in 5 min. With further increase in iron loading (at ratio 10:1 + 4), there was a decrease in the removal of MG due to the agglomeration of Fe2O3 on the surface. The adsorption process was best fitted to the Freundlich isotherm followed by the pseudo-second-order model. The standard thermodynamic parameters (ΔH°, ΔS°, and ΔG°) were obtained over the temperature range of 20-50 °C. It was observed that the adsorption of MG onto Mg/(Al + Fe) LDH was spontaneous, exothermic, and enthalpy driven in the physisorption mode. A worthy desorption efficiency was achieved by using ethanol and water, which was more than 90% in the three cycles. Maintaining almost the same removal efficiency of MG even after three cycles indicated Mg/(Al + Fe) LDH as a promising material for wastewater treatment. This work was anticipated to open up new possibilities in dealing with anionic dye pollutants.

Dynamics of Charge-Transfer Behavior in a Plasmon-Induced Quasi-Type-II p-n/n-n Dual Heterojunction in Ag@Ag3PO4/g-C3N4/NiFe LDH Nanocomposites for Photocatalytic Cr(VI) Reduction and Phenol Oxidation.

In this work, a series of heterostructure Ag@Ag3PO4/g-C3N4/NiFe layered double hydroxide (LDH) nanocomposites were prepared by a combination of an electrostatic self-assembly and in situ photoreduction method. In this method, positively charged p-type Ag3PO4 was electrostatically bonded to the self-assembled negatively charged surface of the n-n-type g-C3N4/NiFe (CNLDH) LDH hybrid material with partial reduction of Ag+ to metallic Ag nanoparticles (NPs) by the photogenerated electrons and available surface -OH groups of LDH under visible light irradiation. The presence of Ag3PO4 as a p-type semiconductor, the surface plasmon resonance (SPR) effect of metallic Ag NPs, and oxygen vacancies as Ov-type defects in NiFe LDH could greatly achieve the quasi-type-II p-n/n-n dual heterojunctions, which was revealed by the shifted conduction band and valence band potentials in Mott-Schottky (M-S) analysis. Among all the optimized heterostructures, CNLDHAgP4 could achieve the highest photocatalytic Cr(VI) reduction rate of 97% and phenol oxidation rate of 90% in 2 h. The heterostructure CNLDHAgP4 photocatalyst possesses a unique morphology consisting of cubic phases of both Ag NPs and Ag3PO4, which adhered to the thin and curvy layers of the CNLDH hybrid for smooth electronic and ionic charge transport. Furthermore, the intimate Schottky barriers formed at the interface of quasi-type-II p-n/n-n dual heterojunctions were verified by the photoluminescence, linear sweep voltammetry, M-S, electrochemical impedance study, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy studies. The SPR effect of Ag NPs and oxygen vacancies as Ov-type defect in NiFe LDH can effectively accelerate the threshold of charge separation and be the main reason for the enhanced activity achieved by the as-fabricated heterostructure photocatalyst.

Controlled Synthesis of CeO2NS-Au-CdSQDs Ternary Nanoheterostructure: A Promising Visible Light Responsive Photocatalyst for H2 Evolution.

With the advancement of promising multifaceted powdered photocatalytic systems, problems related to environmental pollution and energy requirements have been addressed to a significant extent. The major reason for this great achievement lies in the combined effect of both structure modification and integration of different functional materials. Here, we report a ternary hybrid containing wide band gap CeO2 nanosheets with CdSQDs and Au nanoparticles, incorporated between this type II heterostructure through simple chemical reduction methods. Structural and morphological characterization of the fabricated samples was carried out by XRD, XPS, and TEM analysis. From a series of optical and photoelectrochemical measurements, it was found that the incorporation of Au nanoparticles into the interfaces of CeO2 and CdSQDs was the major cause of the enhancement in the photocatalytic activity. Au nanoparticles play a dual character by acting as a mediator and also inject hot electrons through LSPR (light-induced surface plasmon resonance) effects in the ternary hybrid. The photocatalytic activity of the fabricated samples was tested toward H2 evolution, where the ternary hybrid CeO2NS-Au-CdSQDs lead the activity sequence with 499 µmol/2 h followed by the binary and neat counterparts. From the Mott-Shottky and linear sweep voltammetry measurements, a heterostructure relay mechanism was predicted where electrons from CdSQDs flow to the surface of CeO2 via Au. The novelty of this work is that it provides useful information about the synergistic effect among three functional components, integrated in a nanosheet structured system, as the basic requirement for constructing good heterostructures in powdered photocatalytic systems.

CdS QDs-Decorated Self-Doped γ-Bi2MoO6: A Sustainable and Versatile Photocatalyst toward Photoreduction of Cr(VI) and Degradation of Phenol.

In this work, CdS quantum dots (QDs)-sensitized self-doped Bi2MoO6 has been synthesized using glucose as reducing agent by hydrothermal method, followed by in situ deposition of the QDs. The synthesized catalyst has been employed to reduce toxic Cr(VI) and degrade phenol from the aqueous solution. The structural, optical, and electrochemical characterizations are performed using X-ray diffraction, UV-vis diffuse reflection, photoluminescence (PL), scanning electron microscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The optical properties were precisely investigated by calculating the Urbach energy, PL, and photoluminescence excitation spectra. The orderly distribution of QDs is confirmed from the correlation between full width at half-maximum of PL spectra, Urbach energy, and TEM analysis. The versatile photocatalytic activity has been tested toward Cr(VI) reduction and degradation of phenol. 3% CdS QDs-sensitized self-doped Bi2MoO6 showed highest activity, i.e., 97 and 47.5% toward reduction of Cr(VI) and degradation of phenol under solar light. The reduction of Cr(VI) by the catalyst is supported by the kinetics and determination of the pHPZC value. In addition to this, the photostability and reusability test showed that the catalyst can be reused up to five cycles without diminishing its activity.

A facile in situ approach to fabricate N,S-TiO2/g-C3N4 nanocomposite with excellent activity for visible light induced water splitting for hydrogen evolution.

A series of novel N,S-TiO2/g-C3N4 nanocomposite (abbreviated as TuT) photocatalysts has been synthesized via a facile, cost effective, in situ thermal induced polymerization method. The as-synthesized nanocomposites were thoroughly characterized through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-vis diffuse reflectance spectroscopy (UV-Vis DRS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photo luminescence spectroscopy (PL). Using UV-Vis DRS, a gradual enhancement in visible light absorption towards the red end was observed for the xTuT photocatalyst in comparison to bare g-C3N4 (Tu). The result demonstrates that thermal reaction of a higher wt% of thiourea with respect to Ti precursor causes coupling of the N,S-TiO2 and g-C3N4 nanocomposite, however at a lower wt% only N,S-TiO2 forms. The photocatalytic activity has been evaluated through H2 evolution. The synergistic combination of small crystallite size, the crystalline anatase phase, enhanced visible light absorption ability, enhanced specific surface area and the effective charge separation properties of the 10TuT photocatalyst makes the system pivotal for photocatalytic H2 evolution under visible light irradiation.