Au nanoparticles decorated β-Bi2O3 as highly-sensitive SERS substrate for detection of methylene blue and methyl orange

In this work, Au/Bi2O3 was synthesized by loading Au nanoparticles (NPs) onto β-Bi2O3 by a simple solution reduction method. β-Bi2O3 was synthesized by a precipitation-thermal decomposition procedure, which results in significantly improved SERS detection limits down to 10-9 M for methylene blue (MB) and 10-7 M for methyl orange (MO) as probe molecules, comparable to those reported for the best semiconductor SERS substrates. In particular, further deposition of Au NPs (5.20% wt%) onto β-Bi2O3 results in a two-order-of-magnitude enhancement in detection sensitivity, achieving a detection limit of 10-11 M for MB and 10-9 M for MO. Under ultraviolet/visible irradiation, the Au/Bi2O3 hybrids substrate exhibits superior self-cleaning ability due to its photocatalytic degradation ability which can be applied repeatedly to the detection of pollutants. The advanced composite substrate simultaneously achieved ultra-low mass loading of Au NPs, outstanding detection performance, good reproducibility, high stability and self-cleaning ability. The development strategy of low load noble metal coupled high performance semiconductor β-Bi2O3 to obtain nano-hybrid materials provides a method to balance SERS sensitivity, cost effectiveness and operational stability, and can be synthesized in large quantities, which is a key step towards commercialization and has good reliability prospects.

Green synthesis of reduced graphene oxide/chitosan/gold nanoparticles composites and their catalytic activity for reduction of 4-nitrophenol

Gold nanoparticles (AuNPs) have attracted extensive attention in the past few years due to their unique properties and great potential application in catalysis. However, the application of AuNPs remains a significant challenge due to the lack of high efficiency and stability caused by aggregation. Immobilization of AuNPs on appropriate support shows promising results in avoiding aggregation and improving catalytic activity. In this work, reduced graphene oxide/chitosan/gold nanoparticles (rGO/CHS/AuNPs) composites were prepared using chitosan with different molecular weights (MW) as a reducing agent and stabilizer, and characterized by FT-IR, XRD, XPS, SEM, FESEM, EDS, TEM, HRTEM, and TGA. The preparation conditions of rGO/CHS/AuNPs composites, including chitosan MW, CHS/GO mass ratio, reaction temperature and time, and HAuCl₄ concentration were investigated in detail. The results indicated that reduction activity of chitosan for GO increased with the decrease of chitosan MW. The C/O ratio of rGO reduced by low molecular weight chitosan (LMWC) with viscosity-average molecular weight (Mv) of 21 kDa was 6.34. Small spherical AuNPs were uniformly immobilized on the rGO surface. The particle size of AuNPs increased from 9.29 to 13.03 nm as chitosan MW decreased from 465 to 21 kDa. The rGO/CHS/AuNPs showed good catalytic activity for the reduction of 4-NP in the presence of NaBH₄. The catalytic activity of rGO/CHS/AuNPs was closely related to chitosan MW. rGO/CHS/AuNPs synthesized by LMWC with Mv of 21 kDa showed the highest kinetic rate constant of 0.2067 min^-1. The results of this experimental study could be useful in the development of effective catalysts for the reduction of aromatic nitro compounds.

Facile synthesis of green and efficient magnetic nanocomposites of carrageenan/copper for the reduction of nitrophenol derivatives

A green and facile method for preparation of Kappa-Carrageenan or Iota-Carrageenan grafted N,N'-methylenebisacrylamide/Fe₃O₄/Cu nanoparticles (κC-g-MBA/MNPs/Cu and ιC-g-MBA/MNPs/Cu) catalysts was developed to place copper on a magnetic carrageenan surface. The structure and morphology of the prepared catalysts were identified using FT-IR, XRD, BET, VSM, TGA, EDX, mapping, FE-SEM, TEM, and ICP-OES analyses. The catalytic activity of the catalysts was investigated to reduce 4-nitrophenol, 2-nitrophenol, 3-nitroaniline, and 4-nitroaniline compounds using the UV-Vis spectrum. To reduce 4-nitrophenol using κC-g-MBA/MNPs/Cu and ιC-g-MBA/MNPs/Cu, the rate constants (K_app) obtained were 0.37 and 0.25 min^-1, and the activity factors (k') were 134 and 193 s^-1 g^-1, respectively. The catalysts had a good performance in reducing the nitrophenol compounds and due to the magnetic properties of the catalysts, they could easily be separated and used multiple times.

Microwave-assisted continuous flow phytosynthesis of silver nanoparticle/reduced graphene oxide composites and related visible light catalytic performance

The creation of an environmentally friendly synthesis method for silver nanomaterials (AgNPs) is an urgent concern for sustainable nanotechnology development. In the present study, a novel straightforward and green method for the preparation of silver nanoparticle/reduced graphene oxide (AgNP/rGO) composites was successfully developed through the combination of phytosynthesis, continuous flow synthesis and microwave-assistance. Oriental persimmon (Diospyros kaki Thunb.) extracts were used as both plant reducing and capping agents for fast online synthesis of AgNP/rGO composites. The experimental parameters were optimized and the morphologies of the prepared materials were investigated. The characterization results reveal that spherical AgNPs were quickly synthesized and uniformly dispersed on rGO sheets using the proposed online system. Fourier transform infrared spectroscopy analysis confirmed that phenols, flavonoids, and other substances in the plant extracts played a decisive role in the synthesis of AgNP/rGO composites. Using sodium borohydride (NaBH₄) degradation of p-nitrophenol (4-NP) as a model, the catalytic activity of the prepared AgNP/rGO materials was evaluated. The complete degradation of 4-NP was achieved within 12 min through the use of AgNP/rGO materials, and the composite had a much better catalytic activity than the bare AgNPs and rGO had. Compared with the conventional chemical method, our online method is facile, fast, cost-efficient, and environmentally friendly.

Advances in Graphene/Inorganic Nanoparticle Composites for Catalytic Applications

Graphene-based nanocomposites with inorganic (metal and metal oxide) nanoparticles leads to materials with high catalytic activity for a variety of chemical transformations. Graphene and its derivatives such as graphene oxide, highly reduced graphene oxide, or nitrogen-doped graphene are excellent support materials due to their high surface area, their extended π-system, and variable functionalities for effective chemical interactions to fabricate nanocomposites. The ability to fine-tune the surface composition for desired functionalities enhances the versatility of graphene-based nanocomposites in catalysis. This review summarizes the preparation of graphene/inorganic NPs based nanocomposites and their use in catalytic applications. We discuss the large-scale synthesis of graphene-based nanomaterials. We have also highlighted the interfacial electronic communication between graphene/inorganic nanoparticles and other factors resulting in increased catalytic efficiencies.

Magnetically recyclable reduced graphene oxide nanosheets/magnetite-palladium aerogel with superior catalytic activity and reusability

A two-step method was employed to synthesize reduced graphene oxide nanosheets/magnetite-palladium (rGSs/Fe₃O₄-Pd) aerogel, with excellent catalytic activity and recyclability. Firstly, graphene oxide nanosheet (GS) hydrogels were formed by the self-assembly of GSs during the hydrothermal process. Meanwhile, hematite (α-Fe₂O₃) and Pd nanoparticles (NPs) were synthesized and anchored onto the surface of the hydrogel. During heat-treatment, GSs were reduced to rGSs, while nonmagnetic α-Fe₂O₃NPs were converted to magnetic Fe₃O₄ NPs. The as-obtained rGSs/Fe₃O₄-Pd aerogel displayed a three-dimensional interconnected hierarchical porous architecture, which was rich in mesopores and macropores. Such a structure was suitable for catalysis, since it not only improved the mass diffusion and transport, but also readily exposed the catalytic Pd NPs to the reactants. The typical reduction of 4-nitrophenol was chosen as a model reaction to evaluate the catalytic performance of the aerogel. As anticipated, both the reaction rate constant and turn over frequency of the aerogel were much higher than those of the commercial Pd/C catalyst. Moreover, due to incorporation of Fe₃O₄ NPs, the rGSs/Fe₃O₄-Pd aerogel could be magnetically separated from the reaction solution and reused, without obvious loss of catalytic activity.

Mechanistic insights into the photocatalytic properties of metal nanocluster/graphene ensembles. Examining the role of visible light in the reduction of 4-nitrophenol

Metal nanoclusters (M_NCs) based on silver and gold, abbreviated as Ag_NCs and Au_NCs, respectively, were synthesized and combined with functionalized graphene, abbreviated as f-G, forming novel M_NC/f-G ensembles. The preparation of M_NCs/f-G was achieved by employing attractive electrostatic interactions developed between negatively charged M_NCs, attributed to the presence of carboxylates due to α-lipoic acid employed as a stabilizer, and positively charged f-G, attributed to the presence of ammonium units as addends. The realization of M_NC/f-G ensembles was established via titration assays as evidenced by electronic absorption and photoluminescence spectroscopy as well as scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectroscopy analyses. Photoinduced charge-transfer phenomena were inferred within M_NCs/f-G, attributed to the suppression of M_NC photoluminescence by the presence of f-G. Next, the M_NC/f-G ensembles were successfully employed as proficient catalysts for the model reduction of 4-nitrophenol to the corresponding 4-aminophenol as proof for the photoinduced hydrogen production. Particularly, the reduction kinetics decelerated by half when bare M_NCs were employed vs. the M_NC/f-G ensembles, highlighting the beneficial role of M_NCs/f-G in catalysing the process. Furthermore, Au_NCs/f-G displayed exceptionally higher catalytic activity both in the dark and under visible light illumination conditions, which is ascribed to three synergistic mechanisms, namely, (a) hydride transfer from Au-H, (b) hydride transfer from photogenerated Au-H species, and (c) hydrogen produced by the photoreduction of water. Finally, recycling and re-employing M_NCs/f-G in successive catalytic cycles without loss of activity toward the reduction of 4-nitrophenol was achieved, thereby highlighting their wider applicability.

ZnO nanosheet-assisted immobilization of Ag nanoparticles on graphene/Ni foam for highly efficient reduction of 4-nitrophenol

ZnO nanosheet-assisted immobilization of Ag nanoparticles on graphene/Ni foam forming a novel hybrid structure for catalysis.