Cu nanostructures of various shapes and sizes as superior catalysts for nitro-aromatic reduction and co-catalyst for Cu/TiO2 photocatalysis
Graphical abstract
The lengthy Cu nanowires (CuNW) and CuNW/TiO2 exhibit superior catalytic effect and co-catalytic efficiency for m-chloronitrobenzene reduction and photooxidation of acetic acid as compared to nanorods (NR) and spherical (NS) shaped Cu nanoparticles, respectively.
Introduction
Gold, silver and copper nanostructures (NST) have received considerable attention due to their improved photo-physical [1], [2], opto-electronic [3], [4], [5], [6], [7], [8], [9], catalytic [10], [11], [12], [13], [14] and co-catalytic properties [15], [16], [17] in many instances. Amongst coinage metal nanoparticles (NPs), shape and size dependent catalytic activity of Au and AgNST [10], [11], [12], [13], [14] has been widely studied as compared to the relatively oxidative nature of Cu nanostructures which limits their usability for stable catalytic activity. There were perspective reports on the preparation and optical properties of CuNPs [18], [19], [20], [21], [22], [23], [24], yet a comparative account of their catalytic activity as a function of geometric morphology is rarely investigated. Anisotropic NPs (rods and wires) relative to the isotropic particles (spheres) brings break in the symmetry as well as alterations in the confinement of electrons in all the three dimensions, and hence would expect to show modified catalytic activity. The exposure of higher surface active atoms, edges, corners, per particle surface area, and reaction active sites in a lengthy rod or wire like NST than spherical particles largely affects the efficiency of chemical reactions. It was reported that for the detection of trace biomolecules [25], [26], anisotropic NPs with novel properties were found to show improved sensitivity than spherical particles. The comparative size effect of Cu nanospheres for the conversion of iodobenzene to biphenyl has been reported [27] to be the highest yield with 5 and 8 nm CuNPs relative to 66 nm particles. The reduction of aromatic nitro-compounds to their corresponding amino derivatives with high yield was achieved by Cu nanospheres having greater BET surface area than nanorods [28]. CuNPs were used as catalysts for many homogeneous and heterogeneous hydroxylation of phenol by hydrogen peroxide to dihydroxybenzenes [29], oxidation of alcohol was carried out by utilizing CuO and Cu catalysts immobilized on polymer resins [30], [31].
These CuNPs were also employed as co-catalysts for improving the photocatalytic efficiency of TiO2 where Cu/TiO2 interface [16], [17], [32], [33], [34], [35], [36] exhibits a shift in the Fermi level. Various CuNST in touch with the TiO2 might lead to Fermi levels equilibration to a dissimilar extent as a function of interfacial contact that depends on the shape and size of NPs. Subramaniam et al. [17] observed size-dependent Fermi level shift of the TiO2–Au composite, 20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles. Kaur et al. [16], [38] observed the size and shape dependent co-catalytic activity of Au and AgNPs for enhancement of TiO2 photoactivity. Copper (Cu2O, Cu2+, CuO, etc.) based TiO2 heterojunctions [32], [33], [34], [35], [36], [37], [38], [39], [40] are proven to be the efficient photocatalysts for H2 production, water splitting and many photooxidation reactions. Most of these hetero-systems (Cu/TiO2) utilize spherical shaped co-catalysts only, though modification in the morphology of CuNPs co-catalyst is of great interest to pace the co-catalytic activity and has not been reported so far.
While particle shape is an important parameter, many other factors such as chemical/physical environment, adsorption behavior, surface charge and electronic movement within the nanostructures also plays an important role in determining NPs reactivity. Hence, this article demonstrates the relative optical, electrokinetic and surface morphology of CuNST having different sizes and shapes for evaluating the catalysis and co-catalysis effects for some model reaction. The three dissimilar sized Cu nanospheres (3–20 nm), nanorods and nanowires were utilized as catalysts for the reduction of nitrobenzene, m-chloronitrobenzene and m-nitrotoluene and as co-catalysts (CuNPs/TiO2) for acetic acid photooxidation. The influence due to presence of electron donating (CH3) and withdrawing (Cl) group on the product selectivity and yield during nitroaromatics reduction are also investigated here. Scheme 1 outlines the difference in the rate of catalytic reduction ability by various CuNST. The anisotropic lengthy Cu nanowires (CuNW) and Cu nanorods (CuNR) possessing higher surface exposed atoms, per particle surface area and better delocalization of plasmon electrons led to improved catalysis and co-catalysis properties relative to Cu nanospheres (CuNS) of different sizes.
Section snippets
Materials
Poly(N-vinylpyrrolidone) (PVP, MW = 40,000), copper sulfate, copper(II)chloride, copper nitrate, lauric acid, aluminum chloride, ethylene glycol (EG), ethylenediamine (EDA), sodium hydroxide, acetic acid, sodium borohydride, ascorbic acid, hydrazine, nitrobenzene, m-chloronitrobenzene, m-nitrotoluene, aniline, m-chloroaniline and m-toludine was purchased from Loba chemie, India. P25-TiO2 photocatalyst (size ≈ 30–50 nm, 70% anatase and 30% rutile) is provided as a gift sample from Degussa Company,
Optical properties of Cu nanostructures
The surface plasmon resonance band (SPR) for CuNS has been displayed in Fig. 1a. The light brown colored aqueous suspension of CuNS-1 particles exhibit SPR at 590 nm which corresponds to their spherical shape as reported elsewhere [28]. CuNS-2 and CuNS-3 particles with dark reddish brown appearance showed their SPR band at 597 and 645 nm, respectively. A red shift in the SPR band for CuNS-2 and CuNS-3 particles relative to CuNS-1 represents an increase in their particle size [24]. The two SPR
Conclusion
This research demonstrated how the shape and size of Cu nanoparticles has a significant effect on catalysis and co-catalysis activity. It has been demonstrated here that the anisotropy in the particle shape shows dominance over the isotropic particles and hence nanowires and nanorods are found to be more active than conventional isotropic particles (nanospheres). Lengthy Cu nanostructures due to much exposed surface active atoms results in effective yield in a short span of time relative to
Acknowledgments
R. Kaur gratefully thanks to the Department of Science and Technology, Government of India, for financial support under Women Young Scientist Fellowship Scheme (No. SR/WOS-A/CS-61/2012). Authors would also like to thanks Dr. B. K. Chudasama and Ms. Chandani (School of Physics and Material Science, Thapar University) for Zeta potential and conductance analysis. We would also like to thank to Dr. Satnam Singh (School of Chemistry and Biochemistry, Thapar University) for BET surface area analysis
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