Using Silver Nanoclusters as a New Tool in Nanotechnology

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Using Silver Nanoclusters as a New Tool in Nanotechnology: Synthesis and Photocorrosion of Different Shapes of Gold Nanoparticles Á ngel M. Pérez-Mariño, M. Carmen Blanco, David Buceta, and M. Arturo López-Quintela* NANOMAG Lab. Technol. Research Institute, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain

J. Chem. Educ. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/06/19. For personal use only.

S Supporting Information *

ABSTRACT: Until now, the concept of metal (0) atomic quantum clusters or nanoclusters (NCs) and their increasing role in nanotechnology due to their novel and exceptional properties in important industrial fields (such as catalysis) has not been included at the education level. Here, syntheses of both metal nanoparticles (NPs) and metal nanoclusters (with sizes below ≈1−2 nm) are used for understanding the large differences between both types of nanomaterials. In this experiment we highlight the catalytic and photocatalytic properties of silver NCs, as well as the plasmon band, as the main optical difference between metal NPs and NCs. In the first step of the experiment, the synthesis of different sizes and shapes of AuNPs is carried out and changes in their plasmon band are discussed. In a second step, students conduct the anisotropic growth of AuNPs, catalyzed by silver NCs (Agn, where n is the number of atoms forming the cluster), which do not display a plasmon band. Finally, it is shown that Agn NCs can induce AuNP photocorrosion, which can be avoided by introducing molecules with more negative redox potential (called hole scavengers). Redox properties of Agn NCs are used in this laboratory experiment to discuss with the students several important physicochemical issues, such as absolute and hydrogen redox potential scales, electronic and optical properties of nanomaterials, semiconductor band gap, photogenerated electron−hole pairs (excitons), catalysis and photocatalysis, the role of hole scavengers, or spontaneous process and free energy. This very new area at the bottom of nanotechnology is ideal to make chemistry relevant and engaging for students. It allows them to learn fundamentals by using chemistry that is at the frontier of research, and they are able to do this in an accessible way. KEYWORDS: Catalysis, Materials Science, Metals, Nanotechnology, Oxidation/Reduction, Photochemistry, Physical Chemistry, Semiconductors, Synthesis, Upper-Division Undergraduate

■

INTRODUCTION One of the desired objectives of NP synthesis is to attain good size and shape control. It has recently been found that silver nanoclusters (Ag NCs), also called atomic quantum clusters, induce anisotropic nanoparticle growth.1 A certain number of experiments for the undergraduate level on gold and silver NP synthesis can be found in this Journal, most of them including nice demonstrations of how UV−vis spectra change as a function of NP size.2,3 However, to the best of our knowledge, only two reports on nonspherical NP syntheses have been published for didactic purposes,4,5 both regarding silver NPs, based on reduction of Ag+ by sodium borohydride in the presence of H2O2 as etching agent to tune their shape, adding bromide for size control and citrate for pH control. We report here a simple experiment in which Ag clusters are formed and act as anisotropic growth catalysts in the synthesis of different shapes of AuNPs in water. Changes in UV−vis extinction spectra as a function of both size and shape are discussed. The seed mediated method is employed here for AuNP synthesis © XXXX American Chemical Society and Division of Chemical Education, Inc.

and has also been recently used, in a different way, for educational purposes.6 AuNP solutions of spheres (NS), rods (NR), or star-shaped nanoparticles (NP) are synthesized first. Then, anisotropic NPs (NR and NP) are used to analyze silver cluster photocatalytic properties when irradiated with light of a suitable wavelength. Gold nanoparticles in the presence of Ag NCs are oxidized under UV light. This last part of the experiment leads students through an interesting discussion on reactions taking place, based on redox potential scale. The experiment can be implemented in chemistry and engineering curricula as most general chemistry courses introduce redox chemistry through discussions on spontaneous electron flow in galvanic cells. On this base, the experiment has a double education purpose: it helps students to better understand redox property fundaReceived: July 18, 2018 Revised: December 29, 2018

A

DOI: 10.1021/acs.jchemed.8b00573 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

mentals and promotes work in nanotechnology as an important area for new potential applications in the subject of materials science. To the best of our knowledge, this is the first time that the idea of atomic quantum cluster NCs (see below) has been implemented in a laboratory experiment at an educational level for undergraduate students.

■

BRIEF INTRODUCTION TO METAL NANOPARTICLES AND METAL CLUSTERS When the size of a piece of metal is reduced to the nanoscale, the resulting nanoparticles (NPs) have some physical properties that differ from those of the bulk. This is the case for optical properties. It is well-known that the extinction spectrum of a given metal nanoparticle displays a characteristic UV−vis band, called localized surface plasmon resonance (LSPR). This band is due to a coherent and collective oscillation of the electrons on the nanoparticle surface when interacting with light of a wavelength much bigger than the particle size.7 The shape and position of LSPR depends on several factors: the material, the particle size and shape, and the particle surrounding. When the size is further reduced to only a few metal atoms (typically less than 50−100), optical and electronic properties change dramatically, given the fact that quantum effects govern the material properties in this size range. These small particles, called nanoclusters (NCs), no longer present an LSPR, since all conducting electrons are now quantized, losing all metallic properties. This means that in this size range the material possesses semiconductor electronic properties because electrons, that have discrete energy levels, display an increasing forbidden energy band as the particle size decreases.8 Such a forbidden energy band is called the band gap (see Figure 1). Therefore, metal properties (such as electrical conductivity) not only depend on the type of atoms forming the particle but also on the number of atoms. This is a new and important idea, to be implemented in a material science course, that has not been introduced/tested before at the education level. In this experiment, AuNP photocorrosion, i.e., AuNP oxidation upon irradiation with light, occurs in the presence of small silver clusters (Ag3 NCs) because the latter are a semiconductor material. As any conventional semiconductor,9 NCs exhibit luminescent properties: when such clusters are illuminated with light of higher energy than their band gap, the photon promotes an electron from the conduction band CB (HOMO) to the valence band VB (LUMO) and the resulting exciton (electron−hole pair)10 can be deactivated in different ways: by re-emission of a photon of less energy (luminescence), by chemical reaction of photoelectrons (photoreduction), or by chemical reaction of photoholes (photo-oxidation). Many properties of NCs have been studied over the past few years.11−16 Among them, this laboratory exercise is focused on two important applications: anisotropic NP growth catalysis and NP photo-oxidation catalysis.

Figure 1. Electron energy levels in metal nanoparticles (NP) and nanoclusters (NCs) are compared. The continuous bar means an electron populated level. It is shown that metal NPs have the same electronic structure than the bulk metal with EF being the Fermi level. However, NCs display a forbidden energy band (black double arrow) called “band gap”. The nature of the metal or semiconductor behavior depends on the particle size: below 0.5 nm a metal behaves like a semiconductor with band gap energy Eg.

irradiation and quartz cuvettes are used in the last part of the experiment to induce AuNP photo-oxidation catalyzed by Ag clusters. Stock solutions HAuCl4 1 mM, AgNO3 4 mM, and CTAB (hexadeciltrimethylammonium bromide) 0.2 mM are prepared in advance by senior students. Ascorbic acid 0.0788 M and NaBH4 0.1 M are prepared by students in the lab. Deionized water is used in the full experimental procedure which is detailed in the Supporting Information (SI). The experiment has been carried out, for the past 3 years, with 60 students, in groups of three or four students per group, each sharing one spectrophotometer and one thermostatic bath. The total time needed at the laboratory is 10 h, distributed in 3 days as follows: 4, 4, and 2 h per day. The detailed procedure is included in the Supporting Information, although a general view of lab activities follows. • Lab activities for day 1 (4 h): (a) initial test, (b) spherical AuNP (called NS1), and (b) bigger spherical AuNP (called NS2) synthesis. Characterization of final samples, reactants, and the intermediate steps is carried out by UV−vis spectroscopy. • Lab activities for day 2 (4 h): (a) gold nanorod (NR) and/or star-shaped gold nanoparticle (NP) synthesis and characterization, (b) setup for UV irradiation. NS, NR, and NP plus another ethanol diluted NR sample are left overnight to be irradiated (8 h). • Lab activities for day 3 (2 h): UV−vis spectra measurement of irradiated samples and discussion of the observed photocatalytic processes. Final test.

■

EXPERIMENTAL SETUP, REACTANTS, AND PROCEDURE A UV−vis spectrophotometer (Thermo Evolution 300 UV−vis spectrophotometer), provided with transparent plastic cuvettes, is used for NP characterization. AuNPs are prepared inside a thermostatic bath at 28 °C. A UV lamp (254 nm wavelength UVP Pen-Ray model 11SC-1) for sample B




Figure 2. UV−vis spectra showing LSPR bands for different shapes of Au nanoparticles. The maximum in spherical AuNPs shifts from 534 to 542 nm as the size increases from 22 nm (NS1, black line) to 50 nm (NS2, red line). Spectrum of the nanorods (NR, green line) shows two maxima. Star-shaped nanoparticles (NP, blue line) show three maxima.

■

In the first part of the experiment, spherical AuNPs are prepared by the seed mediated method6 (day 1). Then, using exactly the same procedure and concentrations previously used, a small amount of AgNO3 is added to the reactant solution, which produces the Ag NCs responsible for the changes in shape of the resulting AuNPs (day 2). Changes in spectra are discussed as a function of size and shape. The last part of the experiment is the UV light irradiation of the AuNP samples (NS, NR, and NP) overnight, and the discussion (day 3) of the changes in the UV−vis spectra (also noticeable to the naked eye). The main feature observed is the big decrease and, eventually, total disappearance of the LSPR bands in the anisotropic AuNPs (rods and starlike nanoparticles) due to the presence of Ag NCs. They suffer from photocorrosion when irradiated with UV light (i.e., gold is oxidized and dissolved) as a consequence of the semiconductor properties of Ag NCs located on the anisotropic NP surface. At the same time, inhibition of this process can be observed by addition of ethanol, as hole scavenger, to the samples before irradiation. Discussions of reactions taking place, guided by the instructor, are based on the potential redox scale and are of capital importance in this part.

HAZARDS Gloves, safety glasses, and lab coats are required. Skin contact with AgNO3 and HAuCl4 causes stains and may result in burns. These two reagents and sodium borohydride may also stain or decolorize clothing. The recipient containing the sodium borohydride 0.1 M solution should be left open in order to avoid overpressure due to hydrogen release. Aqua regia is a highly corrosive chemical that should only be handled by instructors in a fume hood. Exposure to UV light is dangerous for the eyes and skin and must be avoided. Experiments using this light must be carried out inside a recipient opaque to this wavelength range.

■

RESULTS AND DISCUSSION

Synthesis of Gold Nanoparticles Changing Size and Shape

The seed mediated method used in the AuNP synthesis implies the controlled growth of separately prepared gold seeds. These seeds are small unstable particles of around 2 nm, which will be added to a growth solution and act as nucleation points. This method has two steps: (a) synthesis of seeds and (b) increase of the seeds’ size when they are added to a growth solution containing more gold ions and reducing agent. The C




Figure 3. Representation of uncatalyzed or site-specific catalyzed growth of seeds into different shapes of gold nanoparticles by NCs as a function of R = [NCs]/[seeds]. On the left, yellow cubes represent gold seeds and three gray dots depict Ag3 nanoclusters adsorbed on them. On the right, the final nanoparticle shape is displayed for each value of R. Absence of nanoclusters during the synthesis results in the isotropic growth of the seed into spherical nanoparticles (NS), while the presence of one or more Ag3 per seed catalyzes the growth in one or more directions, producing cylindrical (NR) or star-shaped nanoparticles (NP).

the anisotropic growth. They cause the different nanoparticle shapes (nanorods, NR, and star-shaped nanoparticles, NP, in this case) depending on the R = [NC]/[seeds] ratio used. As can be seen in Figure 3, when R = 1, one-site catalytic preferential growth of the seed’s crystal planes where NCs are preferentially adsorbed is observed, with the corresponding formation of nanorods. When R is increased, clusters are adsorbed at different crystal planes of the seeds, so that the growth simultaneously occurs at different planes, giving rise to different shapes. Changes in the UV−vis spectra, due to changes in nanoparticle shape, can be easily identified by students as being promoted by the silver added. Isotropic NPs have only one dimension for surface plasmon resonance, and therefore, only one band is observed in the spectrum. However, NR and NP display several bands as corresponding to different geometrical parameters for each specific shape.7 In Figure 2, the NR spectrum displays two bands: one longitudinal LSPR at 820 nm and one transverse LSPR at 540 nm. The NP spectrum, with 3 LSPR bands, can also be observed in the same figure. See TEM micrographs of all AuNPs in Supporting Information. At this stage, students are provided with enough information to understand LSPR dependence on size and shape. Therefore, two facts can be learned at this point: (a) LSPR position depends on nanoparticle size and shape, and (b) Ag+ ions added to the growth solution induce AuNP anisotropic growth. However, Ag NC formation is not evident for students until

metal precursor in any case is HAuCl4 which will either be reduced with a strong reducing agent (sodium borohydride) to produce the small seeds or be reduced with a soft reducing agent (ascorbic acid) on the surface of the previously formed seeds. This methodology is particularly interesting for education purposes as it gives an intuitive understanding of the “bottomup” approach in nanoparticle synthesis. As nucleation and growth here are physically separated steps, students can easily control the synthesis and prepare gold nanoparticles of different sizes and shapes. To illustrate this, spherical NP synthesis (NS1) is completed following the procedure mentioned above (see Experimental Details in Supporting Information), and then the method is slightly modified to produce bigger spherical nanoparticles (NS2). Nanoparticles formed in NS1 are added to the growth solution instead of seeds to further increase its size. Changes in LSPR position as a function of AuNP size (and therefore as a function of the “available length” for collective electron oscillation) are discussed. For spherical nanoparticles only one band is observed (Figure 2). The dependence of its maximum with particle size is exemplified with NS1 (maximum at 534 nm, 22 nm in size) and NS2 (maximum at 542 nm, 50 nm in size). Anisotropic AuNPs are grown from the same seeds used before for NS1 synthesis but now with addition of silver nitrate to the same growth solution as used before. Silver nitrate is reduced by ascorbic acid producing Ag3 NCs,15 which catalyze D




photocatalytic experiments are observed in the last part of the experiment.

The photocorrosion of the gold nanoparticles can be partially avoided by the addition of a hole scavenger, which will be oxidized instead of gold since its reduction potential is lower (see Figure S5 in Supporting Information). Although ethanol is used in this laboratory practice, some pollutants, dust, and many organic molecules (e.g., dyes) can act as hole scavengers and be degraded.16 In Figure 5B, partial inhibition of NR photocorrosion in the presence of ethanol can be observed in comparison with the as-prepared particles, not containing the hole scavenger (Figure 5A). In this last part of the experiment, students learn the photocorrosion effect. On this basis, other different experiments can be designed for learning how self-cleaning surfaces work or even how pollutants present in water can be degraded.

■

GOLD NANOPARTICLE PHOTOCORROSION Photocorrosion of AuNPs can be described as their oxidation by action of light, in the presence of a semiconductor, Ag3 NCs in this case. The poor stability of the anisotropic AuNPs against UV light is due to the photocorrosion carried out by the in situ generated Ag3, as has been identified before.1,15 Photons of enough energy to overcome the band gap of the NCs are absorbed by these semiconductors to form an exciton. The photogenerated holes are rapidly quenched by an electron coming from the AuNP surface, releasing gold ions to the solution, while the photogenerated electrons reduce the oxygen present in solution (see Figure 4 and Photocorrosion Experiment in Supporting Information for a more detailed discussion).

■

OBJECTIVES AND STUDENT OUTCOMES This laboratory experiment was part of the Material Science I course in the fourth year of the chemistry degree. The experiment is performed in the last part of the course, when students have already been lectured on mechanical, chemical, electronic, thermal, magnetic, and optical properties of the materials they are going to obtain in the laboratory practice. Nanomaterial properties are addressed in each area, with an insight into quantum confinement of quantum dots and NCs. The aim of this laboratory experiment is to provide the students with the opportunity to prepare, observe, and discuss the properties for nanomaterials prepared by themselves and strengthen the related key concepts: • electronic and optical properties of nanomaterials • absolute and hydrogen redox potentials • semiconductor band gap dependence with particle size (quantum confinement) • photocatalysis with semiconductors To evaluate the pedagogical effectiveness of this work, almost 200 students were subjected to the same brief quiz for the last 3 years both before and after carrying out the laboratory experiment. See Supporting Information for a representative subset of test questions. The percentage of right answers per question is presented on Table 1. As a general conclusion, we found a systematic increase in the right answers after students had been in the laboratory. The increase in percentage of students that answered correctly depends on the specific question, increasing on average about 28.5%.

Figure 4. AuNP photocorrosion under UV light in the presence of Ag3 NCs. Blue and yellow arrows are indicating electron shifts. Light is absorbed by the Ag3 NC (three gray circles connected) producing a photogenerated and positively charged hole in the HOMO and a negatively charged electron in the LUMO (both orbitals are depicted by gray rectangles). The hole produces the oxidation of the nanoparticle, which releases gold ions into the solution and decreases the length of the rod. The photogenerated electron reduces O2 to H2O.

■

NCs present in these solutions are located on the tips of the NPs, where they catalyzed the anisotropic growth during the synthesis.1 Therefore, photocorrosion also starts on the tips, as can be observed for gold nanorods (Figure 5A) where the longitudinal plasmon band is the first one to disappear. Similar behavior is observed for NP, since these samples also contain Ag3 NCs. Nevertheless, the gold NS solution does not contain Ag NCs and shows only a negligible decrease in the LSPR band within 8 h of irradiation in this experiment (see Figure S8). In the case irradiation is continued (at least 3 days) for the samples containing Ag NCs (NR an NP); the UV−vis spectrum shows the absorption features of the Au3+−CTAB complex indicating complete oxidation of gold NPs (see Figures S6 and S7). It is interesting to mention that oxidation of the anisotropic nanoparticles is a reversible process. The addition of more reducing agent and more gold seeds is enough to again achieve anisotropic growth, given the fact that Ag3 NCs are still present in the solution.1

CONCLUSION The experiment has already been implemented in a material science practical course for chemistry undergraduate level students. We found students take good advantage of this practical insight related to many fundamental physical chemistry concepts that are involved in the experiment, increasing scores on the subject in theoretical course evaluation. In summary, UV−vis spectroscopy is used for characterization of size and shape of gold nanoparticles (AuNP), which are synthesized in the lab. Students are aware of the presence of Ag NCs at the tips of the Au nanorods because of their photocatalytic behavior: Upon illumination of the as-prepared nanoparticle solutions overnight, strong photocorrosion of the nanorods and star-shaped nanoparticles and their corresponding changes in the UV−vis spectra are observed. On the contrary, samples containing ethanol are more stable, since this alcohol acts as a hole scavenger. This E




Figure 5. Time evolution for UV−vis spectra of anisotropic gold nanoparticles under irradiation: (A) NR as-prepared, (B) NR in the presence of ethanol as hole scavenger, and (C) NP. Each irradiation time in parts A, B, and C is plotted as a different color line as specified in part B.

David Buceta: 0000-0002-3297-6695 M. Arturo López-Quintela: 0000-0002-4842-8028

Table 1. Comparative Student Results for Answering Each Lab Quiz Question Correctly

Notes

Correct Answers, % Question

Before Lab

After Lab

Increase, % (N = 183)

1 2 3 4 5 6

11% 50% 47% 48% 58% 59%

37% 79% 75% 81% 81% 91%

26% 29% 28% 33% 23% 32%

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This paper is dedicated to the memory of the Physical Chemistry Professor Julio Casado Linarejos. This work has received funding from Conselleriá de Cultura, Educación e Ordenación Universitaria, Xunta de Galicia, Spain (Grupos ref. Comp. ED431C 2017/22); “la Caixa” Foundation (ref: LCF/ PR/PR12/11070003); Ministry of Economy and Competitiveness, MINECO, Spain (MAT2015-67458-P, Co-financed with FEDER Funds, and CTQ2013-44762-R); “Ramon Aceres” Foundation (Project CIVP18A3940); and the Ministry of Education, Culture and Sports, MECD, Spain (FPU grant 13/ 0062).

fact is discussed by students to prove both the presence of silver clusters and their photocatalytic properties. We think the role of clusters in anisotropic NP growth and the photocorrosion phenomenon are new tools in nanotechnology experiments for educational purposes. This is because the experiment is suitable for wide discussion involving physicochemical concepts other than localized surface plasmon resonance in metal NPs. In fact, the experiment has been tested as a useful tool for clarifying often unclear concepts, e.g., standard and absolute electron potential scale; band gap and other electronic and optical properties of semiconductors in opposition to metals; Fermi level; differences between bulk metal, NPs, and NCs; as well as conditions for the spontaneity of redox and photocatalytic processes.

■

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00573. Laboratory guide notes for students (PDF, DOCX) Notes for instructors (PDF, DOCX)

■

REFERENCES

(1) Attia, Y. a.; Buceta, D.; Requejo, F. G.; Giovanetti, L. J.; LópezQuintela, M. A. Photostability of gold nanoparticles with different shapes: the role of Ag clusters. Nanoscale 2015, 7 (26), 11273−11279. (2) Karunanayake, A. G.; Gunatilake, S. R.; Ameer, F. S.; Gadogbe, M.; Smith, L.; Mlsna, D.; Zhang, D. Undergraduate Laboratory Experiment Modules for Probing Gold Nanoparticle Interfacial Phenomena. J. Chem. Educ. 2015, 92 (11), 1924−1927. (3) Lee, C. F.; You, P. Y.; Lin, Y. C. Y. W.; Hsu, T. L.; Cheng, P. Y.; Wu, Y. X.; Tseng, C. S.; Chen, S. W.; Chang, H. P.; Lin, Y. C. Y. W. Exploring the Stability of Gold Nanoparticles by Experimenting with Adsorption Interactions of Nanomaterials in an Undergraduate Lab. J. Chem. Educ. 2015, 92 (6), 1066−1070. (4) Frank, A. J.; Cathcart, N.; Maly, K. E.; Kitaev, V. Synthesis of silver nanoprisms with variable size and investigation of their optical properties: A first-year undergraduate experiment exploring plasmonic nanoparticles. J. Chem. Educ. 2010, 87 (10), 1098−1101. (5) Panzarasa, G. Just What Is It That Makes Silver Nanoprisms so Different, so Appealing? J. Chem. Educ. 2015, 92 (11), 1918−1923. (6) Jenkins, J. A.; Wax, T. J.; Zhao, J. Seed-Mediated Synthesis of Gold Nanoparticles of Controlled Sizes to Demonstrate the Impact of Size on Optical Properties. J. Chem. Educ. 2017, 94 (8), 1090−1093. (7) Xia, Y.; Campbell, D. J. Plasmons: Why Should We Care? J. Chem. Educ. 2007, 84 (1), 91.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Á ngel M. Pérez-Mariño: 0000-0001-8456-3211 F




(8) Calvo Fuentes, J.; Rivas, J.; López-Quintela, M. A. Synthesis of Subnanometric Metal Nanoparticles. Encycl. Nanotechnol. 2011, 1−15. (9) Winkelmann, K.; Noviello, T.; Brooks, S. Preparation of CdS Nanoparticles by First-Year Undergraduates. J. Chem. Educ. 2007, 84 (4), 709. (10) Reid, P. J.; Fujimoto, B.; Gamelin, D. R. A simple ZnO nanocrystal synthesis illustrating three-dimensional quantum confinement. J. Chem. Educ. 2014, 91 (2), 280−282. (11) Selva, J.; Martínez, S. E.; Buceta, D.; Rodríguez-Vázquez, M. J.; Blanco, M. C.; López-Quintela, M. A.; Egea, G. Silver subnanoclusters electrocatalyze ethanol oxidation and provide protection against ethanol toxicity in cultured mammalian cells. J. Am. Chem. Soc. 2010, 132 (20), 6947−6954. (12) Corma, A.; Concepción, P.; Boronat, M.; Sabater, M. J.; Navas, J.; Yacaman, M. J.; Larios, E.; Posadas, A.; López-Quintela, M. A.; Buceta, D.; et al. Exceptional oxidation activity with size-controlled supported gold clusters of low atomicity. Nat. Chem. 2013, 5 (9), 775−781. (13) Vilar-Vidal, N.; Rivas, J.; López-Quintela, M. A. Size Dependent Catalytic Activity of Reusable Subnanometer Copper(0) Clusters. ACS Catal. 2012, 2 (8), 1693−1697. (14) Buceta, D.; Blanco, M. C.; Lopez-Quintela, M. A.; Vukmirovic, M. B. Critical Size Range of Sub-Nanometer Au Clusters for the Catalytic Activity in the Hydrogen Oxidation Reaction. J. Electrochem. Soc. 2014, 161 (7), D3113−D3115. (15) Attia, Y.; Buceta, D.; Blanco-Varela, C.; Mohamed, M. B.; Barone, G.; López-Quintela, M. A. Structure-directing and highefficiency photocatalytic hydrogen production by Ag clusters. J. Am. Chem. Soc. 2014, 136 (4), 1182−1185. (16) Vilar-Vidal, N.; Rey, J. R.; López Quintela, M. A. Green Emitter Copper Clusters as Highly Efficient and Reusable Visible Degradation Photocatalysts. Small 2014, 10 (18), 3632−3636.

G


Using Silver Nanoclusters as a New Tool in Nanotechnology

Recommend Documents