Bringing Catalysis with Gold Nanoparticles in ... - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

Bringing Catalysis with Gold Nanoparticles in Green Solvents to Graduate Level Students Vikram Singh Raghuwanshi,*,†,‡ Robert Wendt,†,§ Maeve O’Neill,∥ Miguel Ochmann,⊥ Tirtha Som,§ Robert Fenger,# Marie Mohrmann,† Armin Hoell,§ and Klaus Rademann*,† †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia § Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany ∥ Trinity College Dublin, College Green, Dublin 2, Ireland ⊥ Institute of Nanostructure and Solid State Physics, University of Hamburg, Luruper 49, 22761 Hamburg, Germany # Daimler AG, Bela-Barenyi-Strasse, D-71059 Sindelfingen, Germany ‡

S Supporting Information *

ABSTRACT: We demonstrate here a novel laboratory experiment for the synthesis of gold nanoparticles (AuNPs) by using a low energy gold-sputtering method together with a modern, green, and biofriendly deep eutectic solvent (DES). The strategy is straightforward, economical, ecofriendly, rapid, and clean. It yields uniform AuNPs of 5 nm in diameter with high reproducibility. Moreover, catalytic applications of AuNPs in DES are shown by studying the conversion of p-nitrophenol to p-aminophenol. Systematic in situ UV−vis spectra are recorded during the catalytic reaction. While the experimental procedures are simple, our laboratory experiments can be applied in a variety of settings depending upon how detailed the instructor requires the depth of the analyses. The laboratory experiments described herein were successfully conducted and evaluated by students with a bachelor degree. KEYWORDS: High School/Introductory Chemistry, Graduate Education/Research, Inorganic Chemistry, Catalysis, Green Chemistry, Nanotechnology, Spectroscopy, General Public, Hands-On Learning/Manipulatives

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are biodegradable and have favorable properties such as high polarity, low vapor pressure, and nontoxicity and show high ionic conductivity.14 Therefore, DES can be considered as one of the novel green solvent matrices for the synthesis of NPs.15,16 The size of the NPs can be characterized by the SAXS method.15−19 Synthesis of AuNPs can be obtained by multiple methodologies. Sputtering is one of the solvent free, ecofriendly, and fast methods to prepare NPs on low vapor pressure liquids. In this method, a metal target is bombarded with low energy argon ions, which eject atoms from the target. The ejected atoms fall directly onto the surface of liquid DESs and form NPs. In this article, we report a simple laboratory experiment, which was performed in a graduate practical course in Physical Chemistry, for 20 graduate level students to synthesize AuNPs in DES (choline chloride−urea mixtures) and apply them for catalytic applications. In situ UV−vis spectra are recorded to observe the rate of the catalytic process. The complete cycle of

etallic nanoparticles (NPs) have attracted great attention in scientific research and in various industrial or even technological applications.1,2 Due to their small size range from 1 to 100 nm, large surface to volume ratios are observed. These NPs exhibit new exciting properties and are considered as promising materials for catalytic applications. Bimetallic NPs, which consist of two different metal elements, together with monometallic systems, are becoming very interesting candidates for fundamental studies of catalytic applications.2−6 Clearly, gold nanoparticles (AuNPs) used as model catalysts have been one of the most attractive subjects of recent research activities.7−9 Haruta et al. reported that AuNPs are highly active catalysts for CO oxidation processes.7,8 Usually, gold is considered to be a precious and expensive metal. However, on the nanoscale, gold is highly reactive, catalytically active, and inexpensive. Therefore, gold can be introduced to master students, and chemistry teachers are encouraged to use AuNPs for classroom demonstrations. Syntheses of NPs via ecofriendly methods in nontoxic and biodegradable green solvents are required to evoke the environmental awareness to students.10−12 Deep eutectic solvents (DESs) are formed by simply mixing quaternary ammonium salts and hydrogen bonded donors.13 Some DESs © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: May 25, 2016 Revised: February 15, 2017

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DOI: 10.1021/acs.jchemed.6b00388 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

all experiments including evaluation processes can be conducted easily in a short time of 3 days while the single synthesis step of AuNPs and the catalytic experiment can be performed in about 30 min.

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EXPERIMENTS

AuNPs in DES

In a first experiment, students will prepare pure DES samples typically less than 100 gby mixing choline chloride (ChCl, ≥ 98%) and urea (ACS reagent, 99.0−100%) in a molar ratio of 1(ChCl):2(urea). After mixing, the DES is heated to 80 °C while being stirred until a homogeneous colorless liquid is formed as shown in Figure 1. The final mixture is degassed and Figure 2. Conversion of p-nitrophenol (yellow color) solution to paminophenol (colorless) in the presence of NaBH4 and by using DESAuNPs (red color, as inset transmission electron microscopy image) as catalyst.

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HAZARDS Urea and choline chloride are mildly hazardous on ingestion and irritants on skin and eye contact and on inhalation. Sodium borohydride (corrosive) and p-nitrophenol are hazardous on ingestion, with skin or eye contact, and on inhalation. Inflammation, redness, and itching are the major characteristics on contact with eyes and skin. Moreover, inhalation causes coughing, sneezing, and unconsciousness on overexposure (see Supporting Information). All students must read the material safety data sheet (MSDS) prior to the experiment and use proper personal protective equipment (PPE) such as laboratory coats, suitable gloves, safety glasses, and closed shoes during experiments. Moreover, direct contact with any chemicals should be avoided.

Figure 1. Preparation of pure DES sample by mixing choline chloride and urea in a molar ratio of 1:2 and heating at 80 °C.

dried under vacuum. In a second experiment, the sputter deposition technique was used to synthesize AuNPs in DES. Sputtering was carried out using a sputter coater fitted with a gold target, and argon was used as an inert carrier gas. During sputtering the chamber pressure was between ambient and 0.001 mbar. Different samples have been prepared by selecting gold-sputtering times between 60 and 150 s. Immediately after having completed the sputtering, the samples have to be stirred for 60 s at room temperature. Further details of the complete experiments are provided in the Supporting Information.

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RESULTS AND DISCUSSION AuNPs used as catalysts have been one of the most attractive subjects of recent research activities.9 Figure 3a shows a pure DES sample and the gold-sputtered DES sample with sputtering time of 60 s. The pure DES sample is clearly transparent, and the pink color of the gold-sputtered DES sample reveals the formation of AuNPs. UV−vis investigations

UV−vis Experiment

UV−vis measurements were performed to obtain information on the growth of AuNPs in DES samples with respect to the gold-sputtering times. Spectra were recorded in standard poly(methyl methacrylate) (PMMA) or polystyrene (PS) cuvettes with an optical path length of d = 10 mm. The spectra were recorded between 300 and 800 nm at room temperature (Supporting Information). Catalysis of p-Nitrophenol

The catalytic activity of AuNPs in DES is evaluated by the reduction of p-nitrophenol to p-aminophenol by using sodium borohydryide (NaBH4). In a typical reaction, 2 mL of 0.1 mM distilled water solution of p-nitrophenol was mixed with the 1 mL of 10 mM of NaBH4 freshly prepared ice cold distilled water solution.19 Addition of NaBH4 in p-nitrophenol solution alters the color of the mixture from light yellow to dark yellow. Moreover, addition of DES-AuNPs triggers the catalytic reaction which can be observed by a gradual change in color of the solution from yellow to colorless as shown in Figure 2.

Figure 3. (a) UV−vis cuvettes with pure DES (left) and gold nanoparticles in DES with sputtering time of 60 s. (b) UV−vis spectra for DES sample with gold-sputtering times of 60 and 150 s. B




⎛A ⎞ ln⎜ t ⎟ = −kappt ⎝ A0 ⎠

were performed to further confirm the formation of AuNPs in DES. Figure 3b shows the UV−vis spectra for DES samples with gold-sputtering time of 60 and 150 s, respectively. UV−vis spectra show the surface plasmon resonance (SPR) peak. It is a resultant peak from resonance between incident photon frequency and the collective excitation of conduction electrons at about 524 nm, which reveals the formation of AuNPs. Previously reported small-angle X-ray scattering (SAXS) investigations on the same samples with different goldsputtering times show the formation of spherically shaped particles with a mean diameter of 5 nm.15,16 SAXS is a powerful characterization method to investigate structural parameters of the particles as prepared in different matrices.17,18 It is observed that the average diameter of the AuNPs in DES is constant and independent of the sputtering time. With prolonged sputtering times only the number density of AuNPs increases linearly. Figure 4 shows the UV−vis spectra recorded at different reaction times during the reduction of p-nitrophenol by AuNPs

In this equation, At is the concentration at time t and A0 is the concentration at t = 0. These results help to determine the kinetics of the catalysis reaction in relation to the theoretical kinetic modeling.3,19 The kinetics of the catalytic activity can be modeled by assuming a Langmuir−Hinshelwood (LH) mechanism.3,20,21 Detailed description on the LH mechanism is provided in the Supporting Information. This can be easily contrasted with the Eley−Rideal (ER) mechanism. The detailed relation between these two models and the laboratory experiments can be seen as follows: In the LH mechanism two molecules are adsorbed simultaneously at neighboring sites and the adsorbed molecules undergo simultaneously the reaction (Figure 1 of the Supporting Information). However, in the ER mechanism, only one of the molecules adsorbs on the gold surface and the other molecule reacts directly from the solution. Both mechanisms give completely different reaction rate constants. We can easily distinguish between these two models by comparing the apparent rate constant of the experiments with the theoretically expected data. Our results are interpreted in terms of the LH model.3 This LH interpreation3 is well recognized in the catalysis community working with AuNPs. In a broader perspective, AuNPs are very promising materials for many direct applications in the modern fields of biodiagnostics, catalysis, cancer therapy, sensors, plasmonics, and even energy conversion processes.22−25

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AIM AND ASSESSMENT OF THE LEARNING PROGRESS The very first aim of the proposed laboratory experiment is to make students familiar with the principles of green chemistry and enhance their awareness for the importance of sustainability and responsibility for environmental science. The novel laboratory experiment provides an opportunity to the students to develop their skills and understanding to conduct environmentally friendly research. The students comprehend the fundamentals and applications of nanotechnology and catalysis. These experiments allow the students to think about the main purpose of their experiment. Sequences of tasks need to be performed in order to complete investigations and collect, analyze, and process data. Finally, the students summarize and communicate their findings and discussions in the form of an elaborated laboratory report with the aim being to publish their findings. As detailed in the Supporting Information, a questionnaire serves the purpose of monitoring the quantitative and qualitative progress of the learning process of students at different levels. Figure 5 shows the progress of a group of 20 master level students on answering the questionnaire based on the 12 principles of green chemistry. It is found that in the beginning of the 3 day course (round 1) almost all master students fail to answer the questionnaire. It cannot be tolerated in modern chemistry education that only 19 correct answers (out of 240) are given by heart. Typical overall success rates are below 10%. This rate changes dramatically during the second and third days with a success rate of 117 correct answers (round 2) and 178 correct answers (round 3), respectively. The progress of learning green chemistry aspects, and hence the pedagogical value of our laboratory experiment, becomes

Figure 4. In situ UV−vis spectra recorded after different reaction times during the reduction of p-nitrophenol (black dashed spectra at 310 nm). The fast decrease at 400 nm is used to determine the reaction kinetics data of p-nitrophenolate conversion.

in DES sample with a sputtering time of 150 s. In the UV−vis spectra, the absorption at about 300 nm corresponds to the reactant p-nitrophenol. Addition of NaBH4 shifts this peak to 400 nm which corresponds to the p-nitrophenolate. No changes in the peak position and intensity are observed without catalyst. After addition of 30 μL of AuNPs-DES solution, the catalytic process begins as can be observed by diminishing of the p-nitrophenolate peak intensity with respect to the reaction time. The catalytic reaction was followed by recording in situ UV− vis spectra of extinction of the peak at 400 nm at short intervals of 2 s until the peak diminishes completely. It took about 90 s for the conversion reaction of p-nitrophenol to p-aminophenol by using DES-AuNPs. The kinetics of the catalytic reaction can be estimated by plotting the logarithm of the peak intensity value at 400 nm A400(t) divided by the initial intensity at the beginning of the reaction A400(0) versus reaction time as given in the Figure 9 of the Supporting Information. Kinetic apparent rate (kapp) constants depend on the slope of the data points which can be obtained by a linear fit. The equation for determining the kapp is given as C




prepare AuNPs in DES. Furthermore, we have demonstrated the catalytic activity of AuNPs for the chemical reduction of pnitrophenol to p-aminophenol. These experiments are simple, ecofriendly, and economical and can be performed in a laboratory practical session. From our experience with many chemistry students we can assess that the learning process of students can be reinforced by integrating state of the art research based on most recent publications. Moreover, these experiments can help students to understand the basic growth and formation mechanisms of nanoparticles. Students can work with nontoxic, ionic matrices and can study fundamental catalytic mechanisms as part of their education in the area of chemical reaction kinetics. Catalysis is still the most important concept in academic research and real world chemistry. Here, the young master level students get a chance to become familiar with these concepts. Such laboratory experiments lead graduate level students to gain further research experience and positively engender their appreciation toward chemistry for the 21st century.

Figure 5. Plot of evaluation of number of students giving correct answers on the questionnaire based on 12 principles of green chemistry. The maximum number of correct answers in total could be 240.

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obvious. It is not easy to design such a laboratory experiment from scratch, but on a long-term basis it will be well worth the effort. The results of rounds 1, 2, and 3 are plotted in Figure 5. The mean value of correct answers is about 5.85 (round 2) and increases to 8.9 (round 3). It is remarkable that all students responded with surprise and curiosity when preparing their own deep eutectic solvent. The students also agreed that DES is a perfect example for introducing green chemistry due to the fact that it is a chicken food additive. It is a system for enabling progress during learning and conducting practices. The second aim of the completed laboratory experiment is the strengthening of the specialized multidisciplinary chemistry knowledge, combining the field of chemistry with energy and environmental studies. The knowledge gained by students after a successful completion of this experiment is as follows: • the students become familiar with the element gold and its uniqueness in green chemistry • they gain an increased knowledge of metal nanoparticles’ synthesis and its preparation and characterization • they enjoy a general laboratory experience in the synthesis of near-monodisperse colloidal nanoparticles • they develop an understanding of mechanisms of nanoparticle nucleation and growth • they reinforce their understanding of theoretical principles of particle sizing • they receive a fundamental and practical understanding of catalytic testing, evaluating rate constants and comparing them with respect to different mechanisms, LH and ER • they get insights into green solvents, deep eutectic solvents (DES) in particular • they improve their skills in collecting, processing, and analyzing data followed by writing a comprehensive laboratory report which reinforces lecture material

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00388. Objective, Instructor’s notes, experimental procedure, and hazards and safety instructions (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Authors

*(V.S.R.) E-mail: [email protected]. *(K.R.) E-mail: [email protected]. ORCID

Vikram Singh Raghuwanshi: 0000-0001-9524-1314 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All of us acknowledge the master and bachelor students who took part in the laboratory practical sessions as well as Antonia Klaas from the Robert Blum Gymnasium in Berlin.

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REFERENCES

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CONCLUSION In summary, we have shown an uncomplicated laboratory experiment to synthesize gold nanoparticles in a deep eutectic solvent, which is based on choline chloride and urea. This solvent can be considered as truly green. A low energy sputtering method which is simple and effective was applied to D




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Bringing Catalysis with Gold Nanoparticles in ... - ACS Publications

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