Manash J. Baruah‡
a,
Eramoni Saikia‡b,
Nand Kishor Gourc,
N. Priyanshu Singhb,
Bitupon Borthakurb,
Uttam Mohand,
Arup Jyoti Dase,
Rahul Kempraif,
Bikash K. Sarmahg,
Rupjyoti Duttahi,
Young-Bin Parkj,
Biraj Das*b and
Mukesh Sharma*f
aDepartment of Chemistry, D. C. B. Girls College, Jorhat, Assam, India 785001. E-mail: manashjbom@gmail.com
bDepartment of Chemistry, D. D. R. College, Chabua, Dibrugarh, Assam, India 786184. E-mail: eramonisaikia@gmail.com; birajdaschm@gmail.com; npriyanshusingh27@gmail.com; bituponborthakur@gmail.com
cDepartment of Chemical Sciences, Tezpur University, Napaam, Tezpur, Assam, India 784028. E-mail: nkgour1@tezu.ernet.in
dDepartment of Chemistry, D. H. S. K. College, Dibrugarh, Assam, India 786001. E-mail: uttamohan@rediffmail.com
eDepartment of Chemistry, Indian Institute of Technology, Kanpur, India 208016. E-mail: arupjyoti1994@gmail.com; mcotton233@gmail.com
fDepartment of Chemistry, Suren Das College, Hajo, Kamrup, Assam, India 781102. E-mail: rahulkemprai827@gmail.com
gDepartment of Chemistry, Sonari College, Sonari, Charaideo, Assam, India 785690. E-mail: bikashsarmah93@gmail.com
hCSIR-North East Institute of Science and Technology, Jorhat, Assam, India 785006. E-mail: rjydutta@gmail.com
iAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad, India 201002
jDepartment of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea. E-mail: ypark@unist.ac.kr
First published on 25th November 2024
Herein we report the first successful synthesis of ethanol-assisted in situ generated reduced graphene oxide as a support for CuO/NiO nanoparticles. Through the strategic incorporation of Cu and Ni precursors into ethanol, followed by thermal treatment, we achieved the fabrication of reduced graphene oxide-supported CuO/NiO nanoparticles. The material underwent thorough characterization using FT-IR, XRD, TEM, XPS, Raman, and UV-DRS analysis. This method promises a breakthrough approach unveiling an unparalleled potential leading to paradigm shifts in graphene oxide synthesis. A theoretical study has also been performed in support of GO formation from ethanol. The synthesized CuO/NiO nanoparticles over reduced graphene oxide were found to be effective for the reduction of 4-nitrophenol within 5 min.
Several methods have been developed for the preparation of graphene oxide (GO), and reduced graphene oxide (rGO) each offering distinct advantages regarding scalability, reproducibility, and control over material properties.11–16 Prominent methods include the Hummers' method, and the Staudenmaier method.11,12 A comparative table showcasing different approach for GO synthesis is provided in Table S1.† However, the Hummers and Staudenmaier methods, despite their widespread adoption, come with drawbacks such as safety concerns due to the use of strong acids, potential environmental risks, and the necessity for careful handling of hazardous chemicals.11,12 These challenges underscore the need for alternative approaches each with its own set of advantages and limitations.
Ethanol acts as both a solvent and a template, stabilizing the dispersion of GO and the metal precursors. Additionally, it serves as a mild reducing agent, facilitating the reduction of GO to rGO, enhancing the composite's electrical conductivity, and improving the interaction between the NPs and the graphene matrix. Several previous studies also support the role of ethanol as reducing agent. Huba et al. reported the role of ethanol as reducing agent for Co and Ni NPs.17 Recently, Zhao et al. also reported the role of ethanol as reducing agent for cobalt-based lithium-ion battery cathodes.18 Ethanol as solvent is generally considered safer and environmentally friendly than many traditional solvents. Furthermore, since the synthesis process was conducted in the solid state, as a result it enhances the sustainability of the synthesis process and minimizes environmental impact.
Researchers are constantly seeking innovative techniques to enhance the quality, scalability, and functionalization of GO and rGO for diverse applications.19 Recent advancements in metal oxide-based GO and rGO synthesis encompass various methods such as green synthesis, microwave-assisted synthesis, hydrothermal and solvothermal methods, and bottom-up approaches.20–23 However, as of now, there is no documented instance of ethanol serving as a template for the design of rGO to support CuO/NiO NPs. Given the significance of rGO synthesis, we present the pioneering method for ethanol-assisted rGO synthesis as a support for CuO/NiO NPs, CuO/NiO/rGO.
The FT-IR spectra further inveterate the chemical structure of CuO/NiO/rGO, identifying a strong peak at 1724 cm−1 for CO stretching, an O–H deformation at 1405 cm−1, and a peak at 1620 cm−1 for graphitic skeletal vibrations, Fig. 1b.24,29,30 The weakening of peak intensity at 1724 cm−1 indeed suggests the conversion of graphene oxide (GO) to reduced graphene oxide (rGO) during the synthesis process.31 Vibrational bands at 463 cm−1 and 588 cm−1 for Cu–O bonds and 410 cm−1 for Ni–O bonds corroborated the XRD findings of a CuO–NiO phase mixture.32,33 Raman analysis identified characteristics D, G, and 2D bands at 1354 cm−1, 1573 cm−1, 2448 cm−1, and 2578 cm−1, confirming the formation rGO during the ethanol assisted synthesis, Fig. 1c.24,34 The UV-DRS analysis exhibited a characteristic band at 258 nm for the π–π* transitions of graphene's π bonds, with additional peaks at 334 nm corresponding to NPs, further validating the composition and structure of the synthesized rGO, Fig. 1d.24,35
The TEM analysis elucidates the morphology, nanoporous architecture, and crystalline properties of highly dispersed CuO/NiO NPs and thin wrinkled rGO layers (Fig. 2a–c). The HRTEM image demonstrates stacked rGO layers with folds and wrinkles, indicating multiple layers and an interplanar spacing of approximately 0.34 nm further vindicated the formation of rGO from ethanol, Fig. 2c.36 The formation of CuO/NiO NPs was further ascertained from the obtained d-spacing value of 0.21 nm for NiO and 0.24 nm for CuO, respectively, Fig. 2c.37 Furthermore, from the particle size distribution analysis, the dimensions of the NPs were found in the range of 4–6 nm, as determined by TEM analysis (Fig. S1†). The low visibility of CuO/NiO NPs over the rGOs surface is might be due to the overlapping of the NiO surfaces completely covering the CuO particles. A similar observation was also reported by Cheng et al. during the formation of CuO–NiO.38 The selected area electron diffraction (SAED) pattern (Fig. 2d and inset depicts the crystalline plane calibration) confirms the crystalline nature of the material, showing ordered atomic arrangements.
Fig. 2 (a and b) TEM and HRTEM images showing rGO wrinkled layers, (c) interplanar distances, and (d) SAED pattern of CuO/NiO/rGO (inset depicts the crystalline plane calibration). |
XPS confirmed the presence of CuO and NiO through distinctive peaks and binding energies. The CuO was identified by the appearance of peaks at 935.1 eV and 955.1 eV for Cu 2p3/2 and Cu 2p1/2, indicating Cu2+, Fig. 3a.39 The presence of peaks at 856.8 eV and 874.3 eV for Ni 2p3/2 and Ni 2p1/2, with additional satellite peaks at 861.9 eV and 879.8 eV supporting the presence of Ni2+, Fig. 3b.40 These findings affirm the formation of CuO and NiO in the sample, providing valuable insight into its chemical composition and structure. The deconvoluted oxygen (O) spectra revealed three peaks at 530.8 eV, 531.8 eV, and 532.9 eV, indicating distinct O binding sites. The peak at 530.8 eV is associated with metal-oxide formation, the peak at 531.8 eV corresponds to oxygen vacancies within the composite, and the peak at 532.9 eV suggests adsorbed –OH groups on the surface in Fig. 3c.39 In the carbon (C) spectra, binding energy values at 284.7 eV and 287.3 eV signify the presence of sp2 carbon bonds and O–CO groups, respectively, Fig. 3d. These atomic percentages confirm a sufficient amount of oxygen content, indicating oxide formation within the graphene structures.36 The survey XPS spectra of the elements are depicted in ESI (Fig. S2†) along with the respective atomic percentages (Table S2†).
The N2 adsorption–desorption isotherm of the synthesized CuO/NiO/rGO composite, shown in Fig. S3,† exhibits a distinct hysteresis loop, characteristic of a type IV isotherm.41 The measured specific surface area of 86.6 m2 g−1 proposes a high surface area and the incorporation of rGO sheets within the CuO/NiO structure. The presence of this graphitic material appears to boost the internal pore architecture, resulting in an improved nitrogen uptake as well as increased surface area and pore volume.42 It is vital to note that a slight drop in the vertical axis reflects a reduction in nitrogen adsorption with decreasing pressure, signifying a reduced uptake of N2 by the CuO/NiO/rGO composite at lower pressures.43
The nanostructure synthesis was optimized through multiple trials under identical conditions to evaluate reproducibility. The results demonstrated consistent particle size, distribution, and composite stability across all runs. To test scalability, the synthesis process was scaled up by increasing the quantities of starting materials while maintaining the same temperature and time conditions. The larger batches yielded composites with properties comparable to those produced in smaller-scale experiments, confirming that the method is both reproducible and scalable.
Fig. 4 Optimized geometries along with bond lengths of all species at ωB97XD/6-311++G(d,p) level of theory. |
Species | Energy (in Hartree) | RE (in kcal mol−1) |
---|---|---|
CuONiO | −3599.630255 | |
C2H5OH | −155.0408365 | |
CuONiO + C2H5OH | −3754.671091 | 0.00 |
CuONiO(C2H5OH) | −3754.694888 | −14.93 |
CuONiO(H)(C2H5O) | −3754.814614 | −90.06 |
In the next step, the hydrogen atom from the OH group of ethanol shifts to a bridging oxygen atom in the CuONiO system, forming CuONiO(H)(C2H5O). In this structure, the oxygen atom of the ethoxy group (C2H5O) forms a strong interaction with the copper atom of the CuONiO matrix at a distance of 2.288 Å. The energy of CuONiO(H)(C2H5O) is even lower than that of the preceding CuONiO(C2H5OH) species, with a relative energy decrease of 90.06 kcal mol−1, confirming increased stability. On the basis of the theoretical calculations with the initial steps supporting the favorability of this reaction pathway, a proposed mechanism for the generation of rGO has been depicted in Scheme S1.† In the final step, the ethanal and alkene produced might undergo cross coupling reaction and dehydrogenation process leading to the generation of rGO, Scheme S1.†
Footnotes |
† Electronic supplementary information (ESI) available: Physical measurements of the physicochemical and spectrochemical tools, comparative table outlining different synthetic routes for rGO along with their preparation methods, BET isotherm, PSD analysis, computational details, a detailed comparison was conducted with the performance of other reported bimetallic CuO/NiO catalysts and catalytic studies of synthesized CuO/NiO/rGO nanocomposite in reduction of 4-NP. See DOI: https://doi.org/10.1039/d4ra04308a |
‡ Both the authors have equal contribution. |
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