A facile synthesis methodology for preparation of Ag–Ni-reduced graphene oxide: a magnetically separable versatile nanocatalyst for multiple organic reactions and density functional study of its electronic structures

Here, we report a simple ‘in situ’ co-precipitation reduction synthesis method for the preparation of nanocatalysts composed of Ag, Ni nanoparticles, and reduced graphene oxide (RGO). First-principles calculations based on Density Functional Theory (DFT) were performed to obtain the electronic structures and properties of Ag–Ni-graphene superlattice and to understand the interfacial interactions which exist at the interface between Ag, Ni, and graphene. The catalytic performance of the synthesized catalysts (Ag_xNi_(1−x))_yRGO_(100−y) were evaluated for four reactions (i) reduction of 4-nitrophenol (4-NP) in the presence of excess NaBH₄ in aqueous medium, (ii) A3 coupling reaction for the synthesis of propargylamines, (iii) epoxidation of styrene, and (iv) ‘Click reaction’ for the synthesis of 1,2,3-triazole derivatives. For all of these reactions the catalyst composed of Ag, Ni, and RGO, exhibited significantly higher catalytic activity than that of pure Ag, Ni, and RGO. Moreover, an easy magnetic recovery of this catalyst from the reaction mixture after completion of the catalytic reactions and the good reusability of the recovered catalyst is also reported here. To the best of our knowledge, this is the first time the demonstration of the versatile catalytic activity of (Ag_xNi_(1−x))_yRGO_(100−y) towards multiple reactions, and the DFT study of its electronic structure have been reported.

Here, we report a simple ‘in situ’ co-precipitation reduction synthesis method for the preparation of nanocatalysts composed of Ag, Ni nanoparticles, and reduced graphene oxide (RGO).     

Solvothermal synthesis of octahedral and magnetic CoFe2O4–reduced graphene oxide hybrids and their photo-Fenton-like behavior under visible-light irradiation

We report a facile solvothermal synthesis of novel octahedral CoFe₂O₄–reduced graphene oxide (RGO) hybrid and pure CoFe₂O₄ that were used as heterogeneous photo-Fenton catalysts for the degradation of organic dyes in water. We investigated the structures, morphologies and catalytic activity of both the CoFe₂O₄ nanoparticles and CoFe₂O₄–RGO hybrids. The morphology of CoFe₂O₄ nanoparticles displays size-dependent shapes changing from granular (or sheet) to octahedral shapes with the introduction of RGO. Compared with bare CoFe₂O₄, the octahedral CoFe₂O₄–RGO hybrids serve as novel bifunctional materials displaying higher saturation magnetization values and excellent heterogeneous activation of H₂O₂ at nearly neutral pH. The high saturation magnetization (41.98 emu g⁻¹) of CoFe₂O₄–RGO hybrids aids their separation from the reaction mixture. In addition, the remarkable enhancement in the photo-Fenton activity of the CoFe₂O₄–RGO hybrids under visible light irradiation was attributed to the graphene/CoFe₂O₄ heterojunction, which aided the separation of excited electrons and holes. Furthermore, the CoFe₂O₄–RGO hybrids exhibited better removal efficiency for cationic methylene blue (MB) dye than for anionic methyl orange (MO) dye. Meanwhile, the CoFe₂O₄–RGO hybrids displayed acceptable photocatalytic stability, and we proposed an activation mechanism of H₂O₂ by the octahedral CoFe₂O₄–RGO hybrids.

Schematic illustration of an active heterogeneous photo-Fenton mechanism based on CoFe₂O₄–RGO hybrids.     

α-Fe2O3 hollow meso–microspheres grown on graphene sheets function as a promising counter electrode in dye-sensitized solar cells

Although nanoparticles, nanorods, and nanosheets of α-Fe₂O₃ on graphene sheets have been synthesized, it remains a challenge to grow 3D α-Fe₂O₃ nanomaterials with more sophisticated compositions and structures on the graphene sheets. Herein, we demonstrate a facile solvothermal route under controlled conditions to successfully fabricate 3D α-Fe₂O₃ hollow meso–microspheres on the graphene sheets (α-Fe₂O₃/RGO HMM). Attributed to the combination of the catalytic features of α-Fe₂O₃ hollow meso–microspheres and the high conductivity of graphene, α-Fe₂O₃/RGO HMM exhibited promising electrocatalytic performance as a counter electrode in dye-sensitized solar cells (DSSCs). The DSSCs fabricated with α-Fe₂O₃ HMM displayed high power conversion efficiency of 7.28%, which is comparable with that of Pt (7.71%).

Although nanoparticles, nanorods, and nanosheets of α-Fe₂O₃ on graphene sheets have been synthesized, it remains a challenge to grow 3D α-Fe₂O₃ nanomaterials with more sophisticated compositions and structures on the graphene sheets.     

Enhanced electron transfer mediated detection of hydrogen peroxide using a silver nanoparticle–reduced graphene oxide–polyaniline fabricated electrochemical sensor

The current study aims at the development of an electrochemical sensor based on a silver nanoparticle–reduced graphene oxide–polyaniline (AgNPs–rGO–PANI) nanocomposite for the sensitive and selective detection of hydrogen peroxide (H₂O₂). The nanocomposite was fabricated by simple in situ synthesis of PANI at the surface of rGO sheet which was followed by stirring with AEC biosynthesized AgNPs to form a nanocomposite. The AgNPs, GO, rGO, PANI, rGO–PANI, and AgNPs–rGO–PANI nanocomposite and their interaction were studied by UV-vis, FTIR, XRD, SEM, EDX and XPS analysis. AgNPs–rGO–PANI nanocomposite was loaded (0.5 mg cm⁻²) on a glassy carbon electrode (GCE) where the active surface area was maintained at 0.2 cm² for investigation of the electrochemical properties. It was found that AgNPs–rGO–PANI–GCE had high sensitivity towards the reduction of H₂O₂ than AgNPs–rGO which occurred at −0.4 V vs. SCE due to the presence of PANI (AgNPs have direct electronic interaction with N atom of the PANI backbone) which enhanced the rate of transfer of electron during the electrochemical reduction of H₂O₂. The calibration plots of H₂O₂ electrochemical detection was established in the range of 0.01 μM to 1000 μM (R² = 0.99) with a detection limit of 50 nM, the response time of about 5 s at a signal-to-noise ratio (S/N = 3). The sensitivity was calculated as 14.7 μA mM⁻¹ cm⁻² which indicated a significant potential as a non-enzymatic H₂O₂ sensor.

The current study aims at the development of an electrochemical sensor based on a silver nanoparticle–reduced graphene oxide–polyaniline (AgNPs–rGO–PANI) nanocomposite for the sensitive and selective detection of hydrogen peroxide (H₂O₂).     

Synthesis of Cu2O–CuFe2O4 microparticles from Fenton sludge and its application in the Fenton process: the key role of Cu2O in the catalytic degradation of phenol

This paper presents the key role of Cu₂O in Fenton catalysis using Cu₂O–CuFe₂O₄ magnetic microparticles, which were prepared using Fenton sludge as an iron source. The catalytic activity of the as-prepared Cu₂O–CuFe₂O₄ and CuFe₂O₄ microparticles was evaluated in a heterogeneous Fenton system for the degradation of recalcitrant phenol. The Cu₂O–CuFe₂O₄ microparticles demonstrated relatively superior catalytic performance as compared to CuFe₂O₄ microparticles when used as a Fenton catalyst. The relatively higher catalytic activity of Cu₂O–CuFe₂O₄ for phenol degradation during the Fenton process could be attributed to the availability of both monovalent [Cu(i)] and divalent [Cu(ii)] as well as Fe(ii)/Fe(iii) redox pairs, which could react quickly with H₂O₂ to generate hydroxyl radicals (HO˙). An electron bridge was formed between Cu(i) and Fe(iii), which accelerates the formation of Fe(ii) species in order to boost the reaction rate. Highly reactive and excessively available Cu(i) species for as prepared Cu₂O–CuFe₂O₄ microparticles could be considered to be rather crucial for the generation of highly reactive HO˙ radical species. In addition, the as-prepared Cu₂O–CuFe₂O₄ magnetic microparticles exhibited sound stability and reusability.

The higher catalytic activity of Cu₂O–CuFe₂O₄ could be attributed to the availability of both Cu(i) and Cu(ii) as well as Fe(ii)/Fe(iii).     

A magnetically separable plate-like cadmium titanate–copper ferrite nanocomposite with enhanced visible-light photocatalytic degradation performance for organic contaminants

A novel magnetic cadmium titanate–copper ferrite (CdTiO₃/CuFe₂O₄) nanocomposite, in which spherical CuFe₂O₄ nanoparticles were loaded onto the surface of CdTiO₃ nanoplates, was successfully synthesized via a sol–gel hydrothermal route at 180 °C. The structure, morphology, magnetic and optical properties of the as-prepared nanocomposite were respectively characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area analysis, UV-visible diffuse reflectance spectroscopy (DRS), vibrating sample magnetometry (VSM) and photoluminescence (PL) spectroscopy. The photocatalytic activity of this novel CdTiO₃-based magnetic nanocomposite was investigated for the degradation of organic dye pollutants such as methylene blue (MB), rhodamine B (RhB), and methyl orange (MO) in the presence of H₂O₂ under visible light irradiation. The results showed that the photocatalyst completely degraded three dyes within 90–100 min. Compared with pure CdTiO₃ and CuFe₂O₄, the heterogeneous CdTiO₃/CuFe₂O₄ nanocomposite exhibited significantly enhanced photocatalytic efficiency. On the basis of the results of the OH trapping and photoluminescence (PL) experiments, the enhanced photocatalytic performance was mainly ascribed to the efficient separation of photo-induced electron–hole pairs and the formation of highly active hydroxyl radicals (OH) species in the CdTiO₃/CuFe₂O₄ photocatalytic oxidation system. The PL measurements of the CdTiO₃/CuFe₂O₄ nanocomposite also indicated an enhanced separation of photo-induced electron–hole pairs. Moreover, the nanocomposite could be easily separated and recycled from contaminant solution using a magnet without a decrease in their photocatalytic activity due to their good magnetic separation performance and excellent chemical stability. Based on these findings, CdTiO₃/CuFe₂O₄ nanocomposite could be a promising visible-light-driven magnetic photocatalyst for converting solar energy to chemical energy for environmental remediation.

A novel magnetic cadmium titanate–copper ferrite (CdTiO₃/CuFe₂O₄) nanocomposite, in which spherical CuFe₂O₄ nanoparticles were loaded onto the surface of CdTiO₃ nanoplates, was successfully synthesized via a sol–gel hydrothermal route at 180 °C.     

Green syntheses of silver nanoparticle decorated reduced graphene oxide using l-methionine as a reducing and stabilizing agent for enhanced catalytic hydrogenation of 4-nitrophenol and antibacterial activity

Herein, we have reported a facile and green synthesis approach of Ag NP decorated reduced graphene oxide (RGO) through an in situ self-assembly method in the presence of l-methionine (l-Met) as reducing and stabilizing agent. The electronic properties, crystal structure, and morphology of the as-synthesized RGO–Ag nanocomposite were investigated by UV-Visible (UV-Vis) spectroscopy, Fourier transform-infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques. UV-Vis and FTIR show the effective reduction of GO and the formation of Ag NPs using l-Met. FESEM, TEM, and XRD analysis show the successful impregnation of Ag NPs into RGO with a 23 nm average crystallite size. The RGO–Ag nanocomposite with NaBH₄ shows a fast-catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AMP). The enhanced catalytic activity of RGO–Ag nanocomposites can be attributed to the synergistic effect of improved adsorption capacity and the absence of agglomeration of Ag nanoparticles. Moreover, RGO–Ag showed strong antibacterial activity against B. subtilis and E. coli.

Herein, we have reported a facile and green synthesis approach of Ag NP decorated reduced graphene oxide (RGO) through an in situ self-assembly method in the presence of l-methionine (l-Met) as reducing and stabilizing agent.     

Optimization and characterization of magnetite–reduced graphene oxide nanocomposites for demulsification of crude oil in water emulsion

Oily wastewater from the oil and gas industry negatively affects the environment. Oily wastewater typically exists in the form of an oil-in-water emulsion. Conventional methods to treat oily wastewater have low separation efficiency and long separation time and use large equipment. Therefore, a simple but effective method must be developed to separate oil-in-water emulsions with high separation efficiency and short separation times. Magnetite–reduced graphene oxide (M–RGO) nanocomposites were used as a demulsifier in this work. Magnetite nanoparticles (Fe₃O₄) were coated on reduced graphene oxide (rGO) nanosheets via an in situ chemical synthesis method. The synthesized M–RGO nanocomposites are environmentally friendly and can be recovered after demulsification by an external magnetic field. M–RGO characterization was performed using X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning microscopy, Raman spectroscopy, and vibrating sample magnetometry. Demulsification performance was evaluated in terms of M–RGO dosage, effects of pH, and brine concentration. The demulsification capability of M–RGO was determined based on the residual oil content of the emulsion, which was measured with a UV-vis spectrometer. The response surface method was used to determine the optimum conditions of the input variables. The optimum demulsification efficiency achieved at pH 4 and M–RGO dosage of 29 g L⁻¹ was approximately 96%. This finding demonstrates that M–RGO nanocomposites are potential magnetic demulsifiers for oily wastewater that contains oil-in-water emulsions. Also, the recyclability of this nanocomposite has been tested and the results shown that it is a good recyclable demulsifier.

Magnetite reduced graphene oxide were synthesized for separation of crude oil in water emulsion.     

Sulfate radical-induced transformation of trimethoprim with CuFe2O4/MWCNTs as a heterogeneous catalyst of peroxymonosulfate: mechanisms and reaction pathways

Trimethoprim (TMP), a typical antibiotic pharmaceutical, has received extensive attention due to its potential biotoxicity. In this study, CuFe₂O₄, which was used to decorate MWCNTs via a sol–gel combustion synthesis method, was introduced to generate powerful radicals from peroxymonosulfate (PMS) for TMP degradation in an aqueous solution. The results showed that almost 90% of TMP was degraded within 24 min with the addition of 0.6 mM PMS and 0.2 g L⁻¹ CuFe₂O₄/MWCNTs. The degradation rate was enhanced with the increase in initial PMS doses, catalyst loading and pH. A fairly low leaching of Cu and Fe was observed during the reaction, indicating the high potential recyclability and stability of CuFe₂O₄/MWCNTs. Electron paramagnetic resonance analysis confirmed that the CuFe₂O₄/MWCNT-PMS system had the capacity to generate ·OH and SO₄˙⁻, whereas quenching experiments further confirmed that the catalytic reaction was dominated by SO₄˙⁻. A total of 11 intermediate products of TMP was detected via mass spectrometry, and different transformation pathways were further proposed. Overall, this study showed a systematic evaluation regarding the degradation process of TMP by the CuFe₂O₄/MWCNT-PMS system.

The degradation of trimethoprim (TMP) in heterogeneously activated peroxymonosulfate (PMS) oxidation processes using CuFe₂O₄/MWCNTs as the catalyst.     

A facile heterogeneous system for persulfate activation by CuFe2O4 under LED light irradiation

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated. The effect of vital experimental factors, including initial solution pH, CuFe₂O₄ dosage, PS concentration, co-existing anion and initial RB concentration on the removal of RB was systematically studied. The removal of RB was in accordance with the pseudo first-order reaction kinetics. Over 96% of 20 mg L⁻¹ RB was removed in 60 min using 0.5 g L⁻¹ CuFe₂O₄ catalyst and 0.2 mM PS at neutral pH. In addition, free radical quenching experiments and electron spin resonance (EPR) experiments were performed, which demonstrated the dominant role of sulfate radical, photogenerated holes and superoxide radical in the CuFe₂O₄/PS/LED system. The morphology and physicochemical properties of the catalyst were characterized by XRD, SEM-EDS, TEM, N₂ adsorption–desorption isotherm, UV-vis DRS, and XPS measurements. Moreover, 18.23% and 38.79% total organic carbon (TOC) removal efficiency was reached in 30 min and 60 min, respectively. The catalyst revealed good performance during the reusability experiments with limited iron and copper leaching. Eventually, the major intermediates in the reaction were detected by GC/MS, and the possible photocatalytic pathway for the degradation of RB in the CuFe₂O₄/PS/LED system was proposed. The results suggest that the CuFe₂O₄/PS/LED system has good application for further wastewater treatment.

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated.     

A ZrO2-RGO composite as a support enhanced the performance of a Cu-based catalyst in dehydrogenation of diethanolamine

The sintering resistance of supported Cu nanoparticle (NP) catalysts is crucial to their practical application in the dehydrogenation of diethanolamine (DEA). In this paper, co-precipitation, hydrothermal synthesis, and sol–gel condensation are used to form a new support material through chemical bonding between graphene oxide and ZrO₂. The composite carriers prepared by the three methods are mixed with copper nitrate and ground using a ball mill. A series of Cu/ZrO₂-reduced graphene oxide (RGO) composites were prepared by calcination under nitrogen at 450 °C for 3 h and hydrogen reduction at 250 °C for 4 h. The conversion of DEA to iminodiacetic acid (IDA) reached 96% with the Cu/ZrO₂-RGO catalyst prepared by hydrothermal synthesis. The conversion rate of DEA is more than 80% following the reuse of the CZG-2 catalyst for twelve cycles. The various physicochemical characterization techniques show that the Cu/ZrO₂-RGO layered and wrinkled nanostructures can improve catalytic stability and suppress the sintering of the supported Cu NPs during the catalytic dehydrogenation of diethanolamine. A synergistic effect between the RGO and the Cu nanoparticles is observed. The Cu nanoparticles with RGO have a better dispersibility, and a new nano-environment is created, which is the key to improving the efficiency of diethanolamine dehydrogenation. These new Cu/ZrO₂-RGO catalysts show increased durability compared to commercially produced Cu/ZrO₂ catalysts and show promise for practical applications involving diethanolamine dehydrogenation.

A Cu/ZrO₂-RGO catalyst prepared by hydrothermal synthesis of a ZrO₂-RGO carrier has highly dispersed Cu nanoparticles and resistance to sintering.     

Correlating the effect of preparation methods on the structural and magnetic properties,and reducibility of CuFe2O4 catalysts

CuFe₂O₄ spinel oxide has attracted research interest because of its versatile practical applications, especially for catalysis. In this study, nanometre-sized CuFe₂O₄ particles were prepared by three different methods, including nanospace confinement in SBA-15, hard template removal, and sol–gel combustion. The relationship between structure, size, magnetic behaviour, and reducibility of the catalysts was further investigated by various advanced techniques. Samples prepared by impregnation and hard template removal show high surface area and small crystallite size with superparamagnetic behaviour. In contrast, the sol–gel sample exhibits ferromagnetic properties with a large crystallite size and low surface area. Although all samples present a tetragonal crystal structure, the distributions of Fe and Cu cations in tetrahedral and octahedral sites in the spinel structure are different. The reducibility results demonstrate that the supported CuFe₂O₄/SBA-15 shows the lowest reduction profile. These results could suggest that the synthesis method strongly affects the crystal properties and cation distribution in the spinel structure, microstructure, surface area and reducibility, which are among the most relevant physicochemical properties for the catalytic activity.

The preparation method plays an important role in the structural properties and catalytic performance of CuFe₂O₄ catalysts.     

12-Molybdophosphoric acid anchored on aminopropylsilanized magnetic graphene oxide nanosheets (Fe3O4/GrOSi(CH2)3–NH2/H3PMo12O40): a novel magnetically recoverable solid catalyst for H2O2-mediated oxidation of benzylic alcohols under solvent-free conditions

In this work, 12-molybdophosphoric acid (H₃PMo₁₂O₄₀, HPMo) was chemically anchored onto the surface of aminosilanized magnetic graphene oxide (Fe₃O₄/GrOSi(CH₂)₃–NH₂) and was characterized using different physicochemical techniques, such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), BET specific surface area analysis and magnetic measurements. The results demonstrated the successful loading of HPMo (∼31.5 wt%) on the surface of magnetic aminosilanized graphene oxide. XRD patterns, N₂ adsorption–desorption isotherms and SEM images confirm the mesostructure of the sample. FT-IR and EDX spectra indicate the presence of the PMo₁₂O₄₀³⁻ polyanions in the nanocomposite. The as-prepared Fe₃O₄/GrOSi(CH₂)₃–NH₂/HPMo nanocomposite has a specific surface area of 76.36 m² g⁻¹ that is much higher than that of pure HPMo. The selective oxidation of benzyl alcohol to benzaldehyde was initially studied as a benchmark reaction to evaluate the catalytic performance of the Fe₃O₄/GrOSi(CH₂)₃–NH₂/HPMo catalyst. Then, the oxidation of a variety of substituted primary and secondary activated benzylic alcohols was evaluated with H₂O₂ under solvent-free conditions. Under the optimized conditions, all alcohols were converted into the corresponding aldehydes and ketones with very high selectivity (≥99%) in moderate to excellent yields (60–96%). The high catalytic performance of the nanocomposite was ascribed to its higher specific surface area and more efficient electron transfer, probably due to the presence of GrO nanosheets. The nanocomposite catalyst is readily recovered from the reaction mixture by a usual magnet and reused at least four times without any observable change in structure and catalytic activity.

12-Molybdophosphoric acid was anchored on magnetic aminopropylsilanized graphene oxide nanosheets and used as a magnetically recoverable catalyst for solvent-free selective oxidation of benzylic alcohols into the carbonyl compounds with H₂O₂.     

Selective synthesis of visible light active γ-bismuth molybdate nanoparticles for efficient photocatalytic degradation of methylene blue,reduction of 4-nitrophenol,and antimicrobial activity

In this study, we have reported selective synthesis of bismuth molybdate (γ-Bi₂M₂O₆) nanoparticles (NPs) under different pH conditions for photocatalytic degradation of methylene blue (MB), reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and antimicrobial activities. The synthesis of pure phase γ-Bi₂M₂O₆ at pH = 3 was confirmed by X-ray diffraction (XRD) and Raman analysis. A single hexagonal morphology was obtained at pH = 3 which shows the formation of the pure phase γ-Bi₂M₂O₆ NPs. The mixed morphologies (hexagonal and spherical) were observed at different pH values other than pH = 3. The bandgap energy of all the synthesized Bi₂M₂O₆ NPs is found in the visible region (2.48–2.59 eV). The photocatalytic activity of bismuth molybdate (BM) NPs was examined by the degradation of MB under visible light irradiation. Results show that 95.44% degradation efficiency was achieved by pure γ-Bi₂M₂O₆ NPs compared to mixed phases (γ-Bi₂M₂O₆, α-Bi₂M₂O₆ and β-Bi₂M₂O₆) synthesized at pH = 1.5 and 5. Moreover, the degradation efficiency of γ-Bi₂M₂O₆ was enhanced to 98.89% by the addition of H₂O₂. The effective catalytic activity of γ-Bi₂M₂O₆ was observed during the reduction of 4-NP to 4-AP by NaBH₄. Potential antibacterial and antifungal activity of γ-Bi₂M₂O₆ was observed, which gives a basis for further study in the development of antibiotics.

In this study, we have reported selective synthesis of γ-Bi₂M₂O₆ NPs under different pH conditions for photocatalytic degradation of methylene blue (MB), reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and antimicrobial activities.     

Fabrication of a magnetic ternary ZnFe2O4/TiO2/RGO Z-scheme system with efficient photocatalytic activity and easy recyclability

A magnetic composite based on TiO₂ nanosheets, ZnFe₂O₄ and reduced graphene oxide (RGO) was synthesized by a one-step hydrothermal synthesis method, which possessed the band structure of a Z-scheme photocatalytic system. The properties and structures of the samples were characterized by XRD, UV-Vis DRS, Raman spectroscopy, SEM, EDS, XPS and PL spectroscopy. Compared with TiO₂ nanosheets and the TiO₂/RGO composite, the obtained ternary composite with 3 wt% RGO exhibited a significant enhancement in photocatalytic activities, attributed to the efficient charge separation induced by the fabricated Z-scheme system. About 99.7% of p-nitrophenol (p-NP) degraded within 60 min under simulated solar irradiation. Trapping experiments showed that superoxide anions (˙O₂⁻) and hydroxyl radicals (˙OH) were the main active species in the p-NP photocatalytic degradation. Finally, a possible photocatalytic mechanism of Z-scheme ZnFe₂O₄/TiO₂/RGO was proposed based on the results of trapping experiments and the energy bands of TiO₂ and ZnFe₂O₄.

A magnetic separable Z-scheme composite based on ZnFe₂O₄, TiO₂ nanosheets and RGO exhibits efficient photocatalytic degradation of p-NP.     

Synchronous oxidation and sequestration for As(iii) from aqueous solution by modified CuFe2O4 coupled with peroxymonosulfate: a fast and stable heterogeneous process

Bifunctional heterogeneous catalytic processes for highly efficient removal of arsenic (As(iii)) are receiving increased attention. However, the agglomerated nature and stability of nanoparticles are major concerns. Herein, we report a new process regarding the anchoring of CuFe₂O₄ nanoparticles on a substrate material, a kind of Fe–Ni foam, to form porous CuFe₂O₄ foam (CuFe₂O₄-foam) by in situ synthesis. The prepared material was then applied to activate peroxymonosulfate (PMS) for fast and efficient removal of As(iii) from water. The results of removal experiments show that the complete removal of arsenic (<10 μg L⁻¹) from 1 mg L⁻¹ As(iii) aqueous solution can be achieved within shorter time (<10 min) using this adsorbent coupled with PMS. The maximum adsorption capability of As(iii) and As(v) on the prepared adsorbent is observed to be about 105.78 mg g⁻¹ and 120.32 mg g⁻¹, respectively. CuFe₂O₄-foam/PMS couple could work effectively in a wide pH range (3.0–9.0) and temperature range (10–60 °C), which is more beneficial to its application in actual water treatment engineering. The exhausted adsorbents can be refreshed for cyclic runs (at least 7 cycles) with insignificant capacity loss using alkaline solution as a regeneration strategy, suggesting this process has good stability. Investigation of the mechanism reveals that the route to the removal of As(iii) is synchronous oxidation and sequestration in the arsenic removal process. The large As(iii) removal capability and stability of CuFe₂O₄-foam/PMS show its potential as a promising candidate in real As(iii)-contaminated groundwater treatment.

Bifunctional heterogeneous catalytic processes for highly efficient removal of arsenic (As(iii)) are receiving increased attention.     

Three-dimensional graphene networks and RGO-based counter electrode for DSSCs

Graphene is considered to be a potential replacement for the traditional Pt counter electrode (CE) in dye-sensitized solar cells (DSSCs). Besides a high electron transport ability, a close contact between the CE and electrolyte is crucial to its outstanding catalytic activity for the I₃⁻/I redox reaction. In this study, reduced graphene oxide (RGO) and three-dimensional graphene networks (3DGNs) were used to fabricate the CE, and the graphene-based CE endowed the resulting DSSC with excellent photovoltaic performance features. The high quality and continuous structure of the 3DGNs provided a channel amenable to fast transport of electrons, while the RGO afforded a close contact at the interface between the graphene basal plane and electrolyte. The obtained energy conversion efficiency (η) was closely related to the mass fraction and reduction degree of the RGO that was used. Corresponding optimization yielded, for the DSSCs based on the 3DGN–RGO CE, a value of η as high as 9.79%, comparable to that of the device using a Pt CE and hence implying promising prospects for the as-prepared CE.

Graphene is considered to be a potential replacement for the traditional Pt counter electrode (CE) in dye-sensitized solar cells (DSSCs).     

Synergetic effects of graphene–CoPc/silk fibroin three-dimensional porous composites as catalysts for acid red G degradation

The disposal of dye wastewater is one of the hotspots of scientific research. Upon combining the ability of graphene to accelerate the hydroxyl radical generation with the Fenton system, it has shown a faster degradation rate and can be recycled, showing greater degradation efficiency than the traditional dye treatment method. Herein, a catalytic system based on the regenerated silk fibroin (SF) gel integrated with cobalt tetraaminophthalocyanine (CoTAPc)-grafted-reduced graphene oxide (RGO) sheets were fabricated, and its catalytic activity was assessed via the degradation of acid red G (ARG) at varying catalyst and H₂O₂ dosages, pH values, and temperatures. The results revealed that the three-dimensional (3D) porous RGO-CoTAPc/SF gel exhibited a much stronger catalytic behavior than the other arbitrary components due to its high surface area and synergetic hydroxyl radical generation efficiency, with the dye removal ratio by RGO-CoTAPc/SF being higher in an acidic medium than in an alkaline medium. It also increases with the increase in temperature and RGO-CoTAPc/SF and H₂O₂ dosages. Further, the catalytic oxidation process of ARG was determined, and the possible degradation mechanism of ARG has been discussed. Our results suggest that the composite materials with high catalytic activity can provide a reference for future Fenton-like catalytic systems.

The disposal of dye wastewater is one of the hotspots of scientific research, and graphene–CoTAPc–SF composites exhibit more effective catalytic abilities.     

Effect of nickel ion doping in MnO2/reduced graphene oxide nanocomposites for lithium adsorption and recovery from aqueous media

Novel and effective reduced graphene oxide–nickel (Ni) doped manganese oxide (RGO/Ni-MnO₂) adsorbents were fabricated via a hydrothermal approach. The reduction of graphite to graphene oxide (GO), formation of α-MnO₂, and decoration of Ni-MnO₂ onto the surface of reduced graphene oxide (RGO) were independently carried out by a hydrothermal technique. The physical and morphological properties of the as-synthesized adsorbents were analyzed. Batch adsorption experiments were performed to identify the lithium uptake capacities of adsorbents. The optimized parameters for Li⁺ adsorption investigated were pH = 12, dose loading = 0.1 g, Li⁺ initial concentration = 50 mg L⁻¹, in 10 h at 25 °C. It is noticeable that the highest adsorption of Li⁺ at optimized parameters are in the following order: RGO/Ni3-MnO₂ (63 mg g⁻¹) > RGO/Ni2-MnO₂ (56 mg g⁻¹) > RGO/Ni1-MnO₂ (52 mg g⁻¹). A Kinetic study revealed that the experimental data were best designated pseudo-second order for each adsorbent. Li⁺ desorption experiments were performed using HCl as an extracting agent. Furthermore, all adsorbents exhibit efficient regeneration ability and to some extent satisfying selectivity for Li⁺ recovery. Briefly, it can be concluded that among the fabricated adsorbents, the RGO/Ni3-MnO₂ exhibited the greatest potential for Li⁺ uptake from aqueous solutions as compared to others.

Novel and effective reduced graphene oxide–nickel (Ni) doped manganese oxide (RGO/Ni-MnO₂) adsorbents were fabricated via a hydrothermal approach for lithium adsorption and recovery from aqueous media.     

Enhanced heterogeneous Fenton-like degradation of methylene blue by reduced CuFe2O4

To facilitate rapid dye removal in oxidation processes, copper ferrite (CuFe₂O₄) was isothermally reduced in a H₂ flow and used as a magnetically separable catalyst for activation of hydrogen peroxide (H₂O₂). The physicochemical properties of the reduced CuFe₂O₄ were characterized with several techniques, including transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and magnetometry. In the catalytic experiments, reduced CuFe₂O₄ showed superior catalytic activity compared to raw CuFe₂O₄ for the removal of methylene blue (MB) due to its relatively high surface area and loading Fe⁰/Cu⁰ bimetallic particles. A limited amount of metal ions leached from the reduced CuFe₂O₄ and these leached ions could act as homogeneous Fenton catalysts in MB degradation. The effects of experimental parameters such as pH, catalyst dosage and H₂O₂ concentration were investigated. Free radical inhibition experiments and electron spin resonance (ESR) spectroscopy revealed that the main reactive species was hydroxyl radical (˙OH). Moreover, reduced CuFe₂O₄ could be easily separated by using an external magnet after the reaction and remained good activity after being recycled five times, demonstrating its promising long-term application in the treatment of dye wastewater.

CuFe₂O₄ was reduced for activation of hydrogen peroxide and the reduced CuFe₂O₄ showed a relatively higher catalytic activity.