DOI: 10.1002/asia.201300809 Synthesis of Highly Stable, Water-Dispersible Copper Nanoparticles as Catalysts for Nitrobenzene Reduction Ravneet Kaur, [a, b] Cristina Giordano, [c] Michael Gradzielski,* [b] and Surinder K. Mehta* [a] Introduction Since their inception, nanoparticles have continued to cap- ture researchers) interest owing to their wide variety of ap- plications, ranging from catalysis to antibacterial agents, to biosensors and so on. [1–6] Among all the metallic nanoparti- cles available, copper has always been one of the favorites because of its abundance and low cost compared with gold or silver, while still being relatively noble. It is particularly attractive because it only requires mild reaction conditions and is easily recyclable. Pure copper nanoparticles have al- ready been synthesized by various methods that include laser ablation, chemical reduction, thermal decomposition, and so forth. [7–9] The real challenge is to stabilize the synthe- sized copper nanoparticles because they get passivated quite easily owing to the formation of copper oxide, and in partic- ular, to have them present in active form in aqueous solu- tion, as water forms the most interesting green solvent for catalytic reactions. Recently, Miranda et al. [8] produced colloidal copper nanoparticles by using the laser-ablation method and ana- lyzed them with surface-enhanced Raman scattering. Al- though their stability had been dramatically improved using 1,10-phenanthroline (phen) and 2,2’-bipyridyl (bipy) as cap- ping agents, they still could not impair particle oxidation. Polyacrylate-assisted synthesis of stable copper nanoparti- cles has been reported by Prucek et al. [10] The nanoparticles were prepared by the addition of sodium borohydride to the aqueous mixture of copper ions and sodium polyacrylate, which resulted in an immediate color change from blue to rusty, thus indicating reduction. However, the nanoparticles (NPs) could remain stable only with the addition of sodium sulfite to the synthesized dispersion. Upon exposure to air, the nonprotected dispersion led to the formation of Cu 2 O nanocubes, that is, the CuNPs became readily oxidized. In contrast, highly stable CuNPs approximately 80 nm in diam- eter were obtained in the presence of gelatin through the re- duction of CuCl 2 by hydrazine and were stable for at least a month in an airtight container. [11] The NPs were stabilized by a rather thick (30 nm) layer of gelatin surrounding the CuNPs. Yang et al. [12] also prepared CuNPs using polyacrylic acid (PAA) with a good catalytic activity, but setting the pH of the solution to an appropriate value played an important role in pure copper nanomaterial formation. Copper nano- particles were obtained when the final pH of the solution was between 9.2 and 0.5, maintained using NaOH. Although the authors were able to prepare stable CuNPs using PAA, Abstract: We report an aqueous-phase synthetic route to copper nanoparticles (CuNPs) using a copper–surfactant complex and tests of their catalytic effi- ciency for a simple nitrophenol reduc- tion reaction under atmospheric condi- tions. Highly stable, water-dispersed CuNPs were obtained with the aid of polyacrylic acid (PAA), but not with other dispersants like surfactants or polymethacrylic acid (PMAA). The di- ameter of the CuNPs could be con- trolled in the range of approximately 30–85 nm by modifying the ratio of the metal precursor to PAA. The catalytic reduction of p-nitrophenol to p-amino- phenol takes place at the surface of CuNPs at room temperature and was accurately monitored by UV/Vis spec- troscopy. The catalytic efficiency was found to be remarkably high for these PAA-capped CuNPs, given the fact that at the same time PAA is efficiently preventing their oxidation as well. The activity was found to increase as the size of the CuNPs decreased. It can therefore be concluded that the synthe- sized CuNPs are catalytically highly ef- ficient in spite of the presence of a pro- tective PAA coating, which provides them with a long shelf life and thereby enhances the application potential of these CuNPs. Keywords: catalysis · copper · nanoparticles · reduction · surfac- tants [a] R. Kaur, Prof. S. K. Mehta Department of Chemistry and Centre for Advanced Studies in Chemistry, Panjab University Chandigarh-160014 (India) Fax: (+ 91) 172-2545074 E-mail : skmehta@pu.ac.in [b] R. Kaur, Prof. M. Gradzielski Stranski-Laboratorium für Physikalische und Theoretische Chemie Institut für Chemie, Technische Universität Berlin Strasse des 17. Juni 124, Sekr. TC7, 10623 Berlin (Germany) Fax: (+ 49) 30-314-26602 E-mail : michael.gradzielski@tu-berlin.de [c] Dr. C. Giordano Max-Planck Institute of Colloids and Interfaces Department of Colloid Chemistry Research Campus Golm, 14424 Potsdam (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300809. Chem. Asian J. 2014, 9, 189 – 198 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 189 FULL PAPER