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Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis Manoj B. Gawande,*,† Anandarup Goswami,†,‡,§ François-Xavier Felpin,∥ Tewodros Asefa,‡,§ Xiaoxi Huang,‡ Rafael Silva,⊥ Xiaoxin Zou,# Radek Zboril,*,† and Rajender S. Varma*,† †
Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University, Šlechtitelů 11, 783 71 Olomouc, Czech Republic ‡ Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States § Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, United States ∥ UFR Sciences et Techniques, UMR CNRS 6230, Chimie et Interdisciplinarité: Synthèse, Analyse, Modélisation (CEISAM), Université de Nantes, 2 Rue de la Houssinière, BP 92208, Nantes 44322 Cedex 3, France ⊥ Department of Chemistry, Maringá State University, Avenida Colombo 5790, CEP 87020-900 Maringá, Paraná, Brazil # State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China ABSTRACT: The applications of copper (Cu) and Cu-based nanoparticles, which are based on the earth-abundant and inexpensive copper metal, have generated a great deal of interest in recent years, especially in the field of catalysis. The possible modification of the chemical and physical properties of these nanoparticles using different synthetic strategies and conditions and/or via postsynthetic chemical treatments has been largely responsible for the rapid growth of interest in these nanomaterials and their applications in catalysis. In addition, the design and development of novel support and/or multimetallic systems (e.g., alloys, etc.) has also made significant contributions to the field. In this comprehensive review, we report different synthetic approaches to Cu and Cu-based nanoparticles (metallic copper, copper oxides, and hybrid copper nanostructures) and copper nanoparticles immobilized into or supported on various support materials (SiO2, magnetic support materials, etc.), along with their applications in catalysis. The synthesis part discusses numerous preparative protocols for Cu and Cu-based nanoparticles, whereas the application sections describe their utility as catalysts, including electrocatalysis, photocatalysis, and gas-phase catalysis. We believe this critical appraisal will provide necessary background information to further advance the applications of Cu-based nanostructured materials in catalysis.
CONTENTS 1. Introduction 2. Scope of This Review 3. Synthesis and Characterization 3.1. Cu and Copper Oxide (CuOx) NPs 3.2. Supported Cu-Based NPs 3.2.1. Carbon-Supported Cu-Based NPs 3.2.2. Metal Oxide-Supported Cu NPs 3.2.3. Polymer-Supported Cu NPs 3.2.4. Silica- or Silicon-Supported Cu NPs 3.2.5. Zeolite-Supported Cu NPs 3.3. Other Cu-Based NPs 3.4. Growth Mechanisms of Cu NPs 3.4.1. Growth of Cu NPs 3.4.2. Growth of CuOx NPs 3.4.3. Growth of Other Cu-Based NPs 3.5. Characterization
© 2016 American Chemical Society
4. Catalytic Applications of Cu NPs in Organic Transformations 4.1. Azide−Alkyne Cycloaddition via Click Chemistry 4.2. Coupling Reactions 4.3. Reduction Reactions 4.4. Oxidation Reactions 4.5. “One-Pot” Multicomponent Reactions 4.6. Miscellaneous Reactions 5. Photocatalysis 5.1. Cu and Copper(II) Oxide/Hydroxide NPs as Cocatalysts for the Hydrogen Evolution Reaction (HER) 5.2. Copper(I) Oxide NPs for HER
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Chemical Reviews 5.3. Cu and CuOx NPs as Cocatalysts for the CO2 Reduction Reaction 5.4. CuOx-Based NPs for Self-Cleaning 5.5. Cu NPs as Plasmonic Photocatalysts for Organic Transformations 6. Electrocatalysis 6.1. Electrocatalysis of Fuel-Cell-Related Reactions 6.2. Reduction of CO2 to Liquid Fuels 6.3. Water Splitting 6.4. Cu MOFs (Metal−Organic Frameworks) as Electrocatalysts 7. Gas-Phase Catalysis 7.1. NOx Reduction 7.2. CO Oxidation 7.3. Water−Gas Shift Reaction 8. Conclusions and Future Scope Author Information Corresponding Authors Notes Biographies Acknowledgments Abbreviations References
Review
Cu is a 3d transition metal and has some interesting physical and chemical properties.32 Cu-based materials can promote and undergo a variety of reactions due to Cu’s wide range of accessible oxidation states (Cu0, CuI, CuII, and CuIII), which enable reactivity via both one- and two-electron pathways. Because of their unique characteristics and properties, Cu-based nanocatalysts have found many applications in nanotechnology, including catalytic organic transformations, electrocatalysis, and photocatalysis.33−46 The main challenge in the development of catalytic NPs is to prepare nanomaterials that are highly active, selective, stable, robust, and inexpensive.47,48 One economical way of creating advanced Cu-based nanomaterials for catalysis is to anchor Cu NPs (e.g., Cu, CuO, or Cu2O) on supports such as iron oxides, SiO2, carbon-based materials, or polymers. Additionally, Cu’s high boiling point makes it compatible with high-temperature and -pressure chemical reactions, including continuous flow reactions, microwave-assisted reactions, vaporphase reactions, and various organic transformations.49−53 Such unique properties, conducive for the development of reactive and selective catalytic systems, have made Cu and its alloys among the most valuable metals in the past, and will ensure that they remain important in the future.
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2. SCOPE OF THIS REVIEW Despite sporadic attempts, there is no systematic and comprehensive overview covering the established catalytic uses of Cu-based NPs.33 This review discusses the uses of Cu and Cu-based NPs as catalysts for diverse reactions, including reduction, oxidation, A3 coupling, electrocatalysis, photocatalysis, and gas-phase reactions (Figure 1). The synthetic
1. INTRODUCTION The catalytic activity of nanoparticles (NPs) represents a rich resource for chemical processes, employed both in industry and in academia.1,2 NPs have applications in diverse fields, including energy conversion and storage, chemical manufacturing, biological applications, and environmental technology.3,4 The great interest in catalysis using nanomaterials has prompted the synthesis and investigation of diverse highly functionalized NPs, including graphene-based catalysts, nanocarbon catalysts, core/ shell nanocatalysts, magnetite-supported nanocatalysts, integrated nanocatalysts, and also various metal nanostructures.5−16 The study of these systems has been facilitated by rapid developments in synthetic methodology that have enabled the preparation of NPs with tunable compositions, shapes, sizes, and structures, either on their own or supported on other materials.17−24 Nanomaterials prepared from earth-abundant and inexpensive metals have attracted considerable attention because of their potential as viable alternatives to the rare and expensive noble-metal catalysts used in many conventional commercial chemical processes.25 These metal NPs often exhibit activity different from that of the corresponding bulk materials because of their different sizes and shapes, which give rise to distinctive quantum properties. In this context, Cu NPs are particularly attractive because of copper’s high natural abundance and low cost and the practical and straightforward multiple ways of preparing Cu-based nanomaterials.26−31 Despite the strong background on the applications of bulk Cu in various fields (e.g., optics, electronics, etc.), the use of Cu NPs is restricted by Cu’s inherent instability under atmospheric conditions, which makes it prone to oxidation. Many efforts to develop the methods and supporting materials that increase the stability of Cu NPs by altering their sensitivity to oxygen, water, and other chemical entities has encouraged the exploration of alternative Cu-based NPs with more complex structures, such as core/shell Cu NPs or systems based on copper oxides.
Figure 1. Applications of Cu-based nanocatalysts.
strategies and relevant examples of Cu and Cu-based NPs are discussed with brief descriptions of relevant characterization techniques such as X-ray diffraction (XRD), scanning electronic microscopy (SEM), high-resolution transmission electronic microscopy (HR-TEM), UV−vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic absorption spectroscopy (AAS). We believe that a detailed overview of the established procedures for preparing Cu NPs and their applications would be helpful to a broad community of scientists working in the fields of nanotechnology, chemical engineering, organic chemistry, inorganic chemistry, and materials chemistry. In addition, most of the chemistries detailed in this review, namely, reactions in benign media, such as water, high turnover numbers (TON) and turnover 3723
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frequencies (TOF), and magnetic recyclability will address current important developments in the field.
3. SYNTHESIS AND CHARACTERIZATION To simplify our overview of the synthesis of Cu and Cu-based NPs, we have grouped them into three different categories: (i) Cu and copper oxide (CuOx) NPs, (ii) supported Cu-based NPs, and (iii) other Cu-based NPs (vide infra). The discussion of materials in the first category covers the synthesis of Cu metal and or copper oxide NPs. This section emphasizes the synthetic protocols which are prevalent in the literature. In the second category, the preparations of Cu and Cu-based NPs on supported systems are emphasized, focusing more on the diversity and the versatility of support materials. Despite some recent efforts,33 and the considerable recent progress in the development of supported catalysts, the literature currently lacks such a systematic review of this field. The final part of this section focuses on the synthesis of other Cu-containing NPs, including metal alloys and multimetallic systems. The main objective of this section is to review the procedures that have been used to synthesize Cu and Cu-based NPs with special emphasis on recent examples. Three tables are presented, listing the reported synthetic strategies for preparing materials in each of the three categories. In the “product description” columns in all three tables, Cu-based species (Cu-oxides and others) are specifically described, whereas the entries without any such mentions represent that the corresponding synthetic procedures lead to the formation of Cu NPs.
Figure 2. Major synthetic approaches for the preparation of Cu-based nanomaterials (MW = microwave, CVD = chemical vapor deposition, ALD = atomic layer deposition, IL = ionic liquid, and DBD = dielectric battery discharge).
chemical treatment, electrochemical synthesis, photochemical techniques, sonochemical methods, and thermal treatment. The section labeled “others” includes relatively sophisticated procedures that have been reported recently and are receiving increasing attention. Among these, the most popular procedure is based on “chemical treatment”, and some of the more modern methods could be regarded as an extension of this protocol providing advantages in terms of shape and/or size selectivity. The following section briefly discusses some of the important examples for the synthesis of Cu NPs (where, in most cases, external reducing agents have been used) and copper oxide NPs. Depending on the materials, reaction environments, and synthetic methods involved, this section can be broadly divided into several subcategories: wet chemical, reverse micelle, MWassisted, and biosynthesis and IL-assisted methods. The results of a thorough survey of the literature in this area are summarized in Table 1. The “wet chemical” technique is a long-established approach for the preparation of metallic Cu NPs mostly involving reducing agents that provide electrons for the reduction of Cu salts (such as CuSO4, copper(II) acetylacetonate,58 CuCl2,59 or Cu(NO3)2). Reducing agents used for this purpose often include sodium borohydride,59 hydrazine,60−62 1,2-hexadecanediol,58 glucose,63 ascorbic acid,64−66 CO,90 or more recently introduced borane compounds.48 Various capping agents have also been used to stabilize the ensuing Cu NPs and to control the particle growth. An alternative approach is the so-called “reverse micelle method”, which involves the formation of an oil-in-water (O/ W)-type microemulsion by adding a surfactant to a vessel containing otherwise-immiscible polar and nonpolar solvents.171 Figure 3 shows the typical structure of a reverse micelle, featuring hydrocarbon tails that project into the oil phase and a sphere of polar head groups that encapsulate a small quantity of the aqueous phase in an inner core. Such reverse micelles can serve as uniformly sized nanoreactors for NP production that enable precise control over the shape and size of the ensuing nanomaterials.171 By tuning the reaction conditions to produce micelles of different sizes, one can adjust the size of the resulting NPs.91−96 For instance, following the methods initially developed by Chen and co-workers,172 Rivas and co-workers94 synthesized metallic Cu nanoclusters using a microemulsion system prepared by mixing the surfactant SDS (sodium lauryl sulfate or sodium dodecyl sulfate) with the cosurfactant isopentanol, cyclohexane as the oil phase, and an aqueous solution of CuSO4 as the copper source. After removal of oxygen from the system by purging with nitrogen, a dilute solution of aqueous sodium borohydride was added to allow
3.1. Cu and Copper Oxide (CuOx) NPs
Procedures for synthesizing Cu-based NPs generally rely on the same techniques that have been used to prepare other metal NPs. Most methods are either “bottom-up” (in which atomiclevel precursors are used to synthesize nanosized materials) or “top-down” (in which a bulk solid is broken down into progressively smaller components). While both approaches have their advantages and disadvantages, the bottom-up approach has become more popular because it offers greater scope for controlling the shape and size of the resulting NPs.54−57 In addition, the susceptibility of Cu NPs to undergo oxidation allows the formation of more stable copper oxide NPs in many cases. The difficulty in classifying the synthetic techniques for the formation of such nanomaterials mainly stems from the similarity of the associated chemical processes. The synthesis of Cu and copper oxide NPs essentially centers around mainly four chemical reaction types, namely, (1) reduction, (2) hydrolysis, (3) condensation, and (4) oxidation. Depending on the choice of final materials, either one or a combination of aforementioned chemistries can be applied. The synthesis of Cu NPs often entails the reduction of Cu(I) or Cu(II) sources. The synthesis of copper oxide NPs, on the other hand, basically requires hydrolysis of the precursors followed by a dehydration process leading to the final materials. Additionally, an oxidation process (sometimes unavoidable for Cu-based NPs) can be deployed for the preparation of Cubased NPs with higher oxidation numbers from their respective precursors of lower oxidation states. In synthetic processes, the techniques that are applied provide a suitable environment and energy to facilitate the process of choice while additional constraints are imposed to modulate the stability, properties, and morphology of the final NPs. On the basis of the literature precedence, the methods for synthesizing Cu-based nanomaterials can be classified into five major categories (Figure 2): 3724
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Table 1. Reported Methods for the Synthesis of Cu and Copper Oxide NPs
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the formation of colloidal Cu clusters, and finally colloidal Cu NPs. An expeditious newer approach called microwave (MW)assisted synthesis, which uses microwaves as an alternate energy source for the reaction, has been utilized for the synthesis of
Cu-based NPs. In this process, the reaction mixture is irradiated with microwaves whose electromagnetic energy gets converted into heat, accelerating the synthesis of Cu-based NPs. This process offers several advantages compared to traditional heating methods such as noncontact heating, quick energy 3729
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Figure 4. Ionic-liquid-induced dissociation of Cu microparticles into nanoparticles. Reprinted from ref 118. Copyright 2009 American Chemical Society.
Utrasonic frequencies between 20 kHz and 15 MHz have also been used to synthesize various nanostructures. Sonication promotes chemical reactions via acoustic cavitation, in which minute bubbles are formed and then implode in a way that generates very high localized temperatures and pressures. Sonochemical techniques make it possible to prepare nanomaterials under ambient conditions without the need for high temperatures, high pressures, or extended reaction times.125,178,179 Sabbaghzadeh and co-workers reported a sonication-based protocol for preparing monodisperse metallic Cu NPs of different sizes using ethylene glycol as a capping agent and hydrazine monohydrate as the reducing agent; sonication made it possible to avoid the formation of polydispersed Cu materials.123 A similar protocol has been used to prepare amorphous Cu NPs, nanocrystalline Cu2O embedded in a polyaniline matrix,124 and NPs based on CuO,125 Cu2O,125 and copper hydride.180 Photonic energy, especially in the form of lasers, can offer an advantage for the synthesis of high-purity metal NPs with specific shapes and chemical compositions.129 This predominantly “top-down” synthetic approach was used by Shimotsuma et al. to synthesize Cu nanomaterials from Cu flakes. Subsequently, nanowires and nanospheres of 50 nm were prepared from the initial flakes via ultrafast pulse laser irradiation by taking advantage of the energy generated from the interference between the laser light field and the surface plasmon polariton waves.130 In another study, Mahdi and coworkers synthesized Cu NPs in virgin coconut oil using a laser ablation technique;131 the setup of their apparatus is shown in Figure 5. Laser ablation has also been recently used to make similar Cu NPs in various solvents, including water,132,133 acetone,132,133,181 and ethanol.132 Electrochemical synthesis has attracted considerable attention in nanomaterial fabrication mainly due to its low cost, lowtemperature operation, high product purity, simplicity, and environmental friendliness.182−184 In electrochemical syntheses of Cu-based nanomaterials, a steady current flow is applied through an electrolytic cell containing an aqueous solution of a Cu salt such as CuSO4. This causes electrons to be transferred from the anode to the cathode, where the Cu ions are reduced to Cu atoms that subsequently agglomerate to form Cu NPs.96 In another case, electrochemical deposition was used to synthesize finer Cu NPs than were obtainable by chemical reduction, with control over the products’ size.97 In addition, it is also possible to obtain Cu NPs with specific morphologies by performing electrochemical synthesis in the presence of templates. For example, electrochemical deposition was used to prepare Cu nanowires by confining a CuSO4 solution within the nanochannels of porous anodic alumina templates.136 Yang et al. have prepared Cu nanorods (with diameters of around 30
Figure 3. A typical reverse micelle.
transfer, or material-selective heating, etc.,173,174 and often allows reactions to proceed at much higher rates than can be achieved by conventional heating. Yin and co-workers developed a rapid microwave-based method for the preparation of metallic Cu NPs by reduction of CuSO4·5H2O with NaH2PO2·H2O in ethylene glycol.98 This procedure offers high reaction rates and yields a product with a narrow particle size distribution. Kawasaki et al. reported the preparation of fluorescent 2 nm Cu nanocrystals via MW-assisted polyol synthesis, which was performed in a nonaqueous solvent to avoid oxidation of the Cu surface.99 A similar report revealed that MW irradiation of a solution containing copper acetylacetonate in benzyl alcohol produced crystalline Cu nanospheres that were 150 nm in size and stable in air for a few months.100 Similarly, Cu2O105 and CuO103 NPs have also been prepared utilizing MW energy. Another rapidly emerging method that requires special mention utilizes the plant-derived extracts that serve both as reducing and/or capping agents. For instance, an extract from the plant Terminalia arjuna was used to reduce Cu(NO3)2 under the MW irradiation conditions,101 and an extract from the leaves of henna (Lawsonia inermis) could reduce Cu(II) salt (CuSO4·5H2O).108 Under these conditions, the biomolecules in the extract served as capping agents, resulting in the formation of stable Cu NPs. These methods have also been successfully applied to synthesize copper oxide-based nanomaterials,114,116 in which such bioextracts acted mostly as capping agents/stabilizers. Among other methods, ionic-liquid (IL)-assisted synthesis of Cu-based nanoparticles has gained attention because of the unique solvating and other physicochemical properties of ILs.175−177 A relatively straightforward “top-down” approach to Cu NP synthesis was reported by Kim et al., who prepared nanoparticles with diameters of 20−200 nm by simply stirring Cu microparticles (1−5 μm in diameter) in the IL 1-butyl-3methylimidazolium tetrafluoroborate (BMIM+BF4−).118 This caused the Cu microparticles to dissociate into NPs because of the strong interaction between the Cu surface and the IL’s anionic components, as shown in Figure 4. A similar method was employed by Han et al. to prepare Cu NPs using other ILs such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6−) and 1-ethyl-3-methylimidazolim tetrafluoroborate (EMIM+BF4−).119 A more recent report demonstrated that the reaction times required for such syntheses could be reduced by using the IL BMIM+NO3−,120 whose anions are mostly in the free ion state, facilitating the efficient dissociation of Cu flakes. 3730
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nanoparticles. The vast availability of reducing and capping agents, different synthetic environments, and simpler laboratory techniques make this category relatively economical and userfriendly. In this class, the reaction parameters play important roles in determining the shape, size, and composition of the final nanomaterials. In addition to the reagents, by providing appropriate energy input (through thermal or MW-assisted methods) and modifying the reaction environments (with reverse micelles or ionic liquids) for the reactions, the process can be made more selective for specific products. Moreover, greener synthetic approaches using naturally abundant plant extracts as reducing and/or as capping agents (biosynthesis) have been utilized. Decomposition of Cu precursors at a relatively high temperature (calcination or pyrolysis) has also been fruitful for the generation of such species, especially copper oxide species (thermal treatment). The use of alternate energy sources offers the synthesis of nanoparticles with uniform, small sizes (e.g., sonochemical, photochemical, electrochemical, etc.) from their respective precursors. For example, in the case of electrochemical synthesis, the rate of reduction of Cu species can be controlled by varying the voltage used, which in turn can lead to a variety of Cu and CuOx nanoparticles. Barring the above-mentioned methods, the relatively sophisticated deposition and sputtering techniques provide more size-selective synthesis of Cu and Cu-based nanoparticles. In this respect, the experimental conditions as well as the type of copper precursors play a controlling role in the final outcome. The rational choice of the synthetic technique is becoming increasingly important to design such nanoparticles with the desired shape, size, and morphology for various applications and to study their structure−activity relationships.
Figure 5. Laser ablation system for preparing Cu NPs, comprising a laser, a lens, a cell, a stirrer, a Cu target, and a N2 tank. The fluid cell is double-shelled and connected to a supply of N2 gas, which passes through virgin coconut oil and escapes through the outlet valve. Reprinted with permission from ref 131. Copyright 2013 Materials Research Society.
nm and lengths of 400−1000 nm) via an electrochemical method in the presence of surfactants, which serve as both templates and stabilizers.137 It is relatively difficult to put “thermal treatment” as a separate procedure because other methods often involve heating (in one form or another) during the syntheses of Cubased nanomaterials (especially copper oxides). However, in view of the substantial reports of such thermal treatment, which not only entails thermal degradation but also high-temperature heat treatment of different Cu precursors, this type of synthesis deserves a separate discussion. Thermal decomposition is among the most widely used methods in this category. For instance, the thermal decomposition of bis(2-hydroxy-1naphthaldehydato)copper(II) in the presence of oleylamine as a capping agent was used to prepare metallic copper nanocrystals.144 The same group also reported the preparation of 40 nm Cu NPs via the thermal decomposition of copper oxalate145 as well as the thermal synthesis of Cu and Cu2O NPs from bis(salicylidiminato)copper(II).146 Similarly, a copper oleate complex prepared by the reaction of CuCl2 and an aqueous solution of sodium oleate was used as a precursor in the thermal synthesis of well-dispersed Cu NPs.148,149 GarciaCerda reported the synthesis of Cu NPs by thermal decomposition of copper oleate using phenyl ether,150 whereas the thermal decomposition of copper(II) acetylacetonate reportedly afforded Cu and copper oxide NPs.151,152 Apart from previously described methods, with the advancement of sophisticated instrumentation, the field keeps witnessing different techniques embodying various deposition techniques (atomic layer deposition (ALD),163 etc.), chemical dealloying,168 dielectric battery discharge (DBD),169 and the recrystallization-induced self-assembly (RISA)170 process, among others. The aforementioned sections and Table 1 provide an extensive review of the different synthetic methods available for making the Cu and Cu-based nanoparticles. “Chemical treatment” pertains to methods in which Cu precursors are treated with external reagents that lead to Cu and Cu-based
3.2. Supported Cu-Based NPs
As mentioned in the Introduction, special attention should be given to the materials used to support Cu and Cu-based NPs because the interactions between the NPs and support can have profound effects on the resulting material’s physical and chemical properties. Consequently, by choosing an appropriate support, it is possible to tailor the properties of the NPs to suit specific applications. In this section, supporting materials for Cu NPs are categorized on the basis of their fundamental chemical compositions, and subcategorized on the basis of their prevalence and utility. Representative examples are given for each category, and a comprehensive list of supporting materials described in the literature to date is provided in Table 2. 3.2.1. Carbon-Supported Cu-Based NPs. Recent advances in the use of metal-embedded carbon networks, especially in electrocatalysis, have prompted the exploration of several carbon-based materials as supports for Cu NPs. One such supporting material is activated carbon, which has been used in a number of applications. For example, Alonso et al. prepared Cu NPs on activated carbon187,248−250 by stirring activated carbon with a freshly prepared Cu NP solution. The NPs were prepared by reducing anhydrous CuCl2 with Li metal in THF containing a small amount of 4,4′-di-tert-butylbiphenyl (DTBB; 10 mol %) for a few minutes at room temperature. The resulting supported Cu NPs efficiently catalyzed a range of reactions without needing any pretreatment. Eskandari and coworkers described the preparation of Cu NPs on activated carbon via a two-step procedure;191,251 activated carbon was initially refluxed in nitric acid solution for a few hours, and the ensuing oxidized carbon material was than refluxed in an 3731
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Table 2. Results of a Comprehensive Literature Survey of Methods for the Synthesis of Supported Cu and Cu-Based NPs
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anhydrous ethanolic solution of CuI under a nitrogen atmosphere. Among carbon support materials, carbon nanotubes (CNTs) offer unique advantages because of their interesting physical
and chemical properties, which include high surface area, high conductivity, numerous possibilities of surface functionalization, and the inherent capability of incorporating the guest materials owing to their unique hollow geometry.252−254 Incorporating 3734
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metal NPs into carbon nanotube structures makes it possible to combine the useful properties of carbon nanonetworks and metal NPs.255−258 Wang et al. have reported the preparation of Cu NP-decorated carbon nanotubes, with the NPs either occupying the nanotubes’ inner cavities or coating their exterior surfaces.202 The hybrid nanostructures were prepared by treating carbon nanotubes with concentrated HNO3 and then mixing them with a solution of Cu(NO3)2 under sonication and magnetic stirring. After subsequent washing and drying, the intermediate products were calcined at 623 K under an argon atmosphere for 3 h to afford the final products. In this study, nanotubes with Cu NPs coating on their external surfaces were obtained by using end-closed nanotubes instead of open-ended ones. In another study, N-doped carbon nanotubes were prepared by taking carbon nanotubes that had been preoxidized with HNO3 and heating them in the presence of ammonia.203 The resulting N-doped nanotubes were then reacted with a solution of copper acetate to load them with Cu NPs,203 a process facilitated by the strong interactions between nitrogen centers and metals. Carbon nanotubes have also been used as solid supports for Cu NPs in a range of applications.200,259,260 Graphene has been recognized as an attractive catalyst support for some time, largely because of its extremely high surface area (∼2600 m2/g), high thermal/electrical conductivity, and chemical stability.188 Graphene-supported Cu NPs have been synthesized using various methods that typically involve mixing a solution of graphene oxide and a Cu salt and then reducing them together.188 For example, Guo et al. prepared metallic Cu NPs on graphene sheets by reducing Cu2O/ graphene composites in a mixture of H2 (5 vol %) and argon at 500 °C.192 Composites of reduced graphene oxide (rGO) decorated with Cu NPs have also been prepared by mixing a solution of rGO with Cu NPs and heating at 110 °C for 12 h.190 Graphene sheets adorned with Cu NPs were synthesized by electrodepositing the nanoparticles on the graphene with a modified glass carbon electrode.199 Graphite, another important carbon-based support, is a precursor for graphene where the interlayer interactions can be destroyed to offer single-layer or multilayer graphene.261 Due to their potential of having covalent and noncovalent interactions with other materials as well as relatively high natural abundance, graphitic materials have been utilized in diverse applications,262 including a benign and inexpensive support for CuO NPs. Felpin and co-workers have developed a simple procedure for the preparation of CuO NPs on graphite by stirring a solution of Cu(OAc)2 and graphite in anhydrous degassed methanol (MeOH) under an atmosphere of H2 (1 atm) for 12 h.263 Filtrations over a membrane or a glass filter provided the ready-to-use solid catalyst with no need for further treatment. This simple procedure yielded well-dispersed spherical CuO NPs with a crystallite size of
