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Title: Synthesis, characterization and optical properties of ligand-protected indium nanoparticles This work reports the synthesis of a series of indium nanoparticles by controlling the mode of reduction and the temperature. The optical absorption of these particles show a red shift of the absorption maxima in the UV region with decrease in particles size and thus can provide the candidate materials for UV plasmonics. See Sukhendu Mandal et al., Phys. Chem. Chem. Phys., 2015, 17, 7109.

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Synthesis, characterization and optical properties of ligand-protected indium nanoparticles† Anu George,a Harish K. Choudhary,a Biswarup Satpatib and Sukhendu Mandal*a

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Unlike silver and gold, indium has material properties that enable strong resonances extended up to the Received 9th December 2014, Accepted 17th January 2015 DOI: 10.1039/c4cp05743k

ultraviolet. This extended response, combined with low cost, and ease of synthesis process, makes indium a highly promising material for applications. In this work, we have synthesized ligand-protected indium nanoparticles by a metal reduction method. Powder X-ray diffraction and EDX analyses are consistent with the presence of metallic indium in the nanoparticles. Ligand binding was proven by IR spectroscopy and TGA experiments. TEM analyses reveal that the particle size ranges from 6.3 to 4.8 nm. Optical

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measurements show that the absorption maximum is red shifted as the particle size decreases.

Introduction There is a great demand to understand and control the properties of particles with dimensions of a few nanometers. These nanoparticles with a few nanometer dimensions (sometimes called ‘clusters’) exhibit properties that depend strongly on the size of the nanoparticles, which allow one to tune the desired properties for the novel metals.1–3 The optical properties of large (420 nm) metal nanoparticles are characterized by the plasmon excitation, which is a collective oscillation of the conduction electrons. However, the size-dependence of the optical properties, which is important for the aforementioned applications, is poorly understood in the small particle range, due to the complex physical and chemical effects.4–15 The optical properties of smaller nanoparticles are strongly dependent on the size of the metal nanoparticles. This is especially true for particles smaller than 10 nm due to the quantum size effects.16–19 Even though, noble metal nanoparticles that exhibit strong SPRs have been the focus of much work, the literature on indium is surprisingly limited.20 Unfortunately, Au and Ag do not support good SPRs in the UV,21 but indium exhibits the opposite trend and thus can provide candidate materials for UV plasmonics.22 The accessibility of the electromagnetic spectrum in the UV region may be of interest for some applications. For example, organic and biomolecules can have strong absorptions in the UV, and for chemical sensing purposes it seems worthwhile pursuing SP-enhanced spectroscopies, such a

Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala-695016, India. E-mail: [email protected] b Surface Physics & Material Science Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata-700064, India † Electronic supplementary information (ESI) available: Table for absorption maximum in different solvents, EDAX mapping for different size of nanoparticles, TGA plot, HRTEM, absorption spectra for oleic acid, absorption plot in different time and solvents. See DOI: 10.1039/c4cp05743k

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as surface enhanced Raman spectroscopy (SERS), in the UV region.23 Indium, being a group III element, is well known for its applications in the fields of catalysis,24 electronics25 etc. Indium materials have shown to be superconducting,26 active for surface plasmon27 and useful components in low melting solders,28 where bulk indium has a melting point of 156 1C. Surprisingly, the surface plasmon resonance of indium nanoparticles with size variation have not been explored as widely in applications like catalysis,24 and or for their superconducting26 and others properties. Herein, we report a one-pot room temperature synthesis of indium nanoparticles stabilized by oleic acid with size distribution in the range of 4.8–6.3 nm by varying temperature and the mode of addition of the reducing agent, but keeping the ratios of the metal precursor, ligand and reducing agent constant. All the particles were characterized by powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We measured the optical absorption of these particles in chloroform solvent and the results show a red shift of the absorption maxima as the particles size decreases. We also measured the optical absorption of these particles in different solvents. It was observed that the position of the surface plasmon band was greatly influenced by the refractive index of the solvent. The plasmon band is red shifted as the refractive index increases.

Experimental Materials Anhydrous indium(III) chloride (InCl3, Z99.999%), oleic acid (Z99%), sodium borohydride (NaBH4, 99%) and anhydrous methanol (99.8%) were purchased from Sigma-Aldrich. All the chemicals were used as received.

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Synthetic procedure The synthesis of In nanoparticles was carried out under dynamic nitrogen purging. All the purification and analytical steps were performed under air. The one-pot synthesis of In nanoparticles was carried out by dissolving 0.04 M anhydrous InCl3 in 30 mL methanol in a dried 3-necked round-bottom flask. 0.2 M oleic acid was added as the protecting ligand, followed by stirring for 2 h by purging nitrogen gas at room temperature. 0.4 M aqueous NaBH4 was rapidly added to the solution under vigorous stirring. The color of the solution changed to dark grey, indicating the reduction of In(III) to In(0). The reaction was exothermic in nature. The reaction was allowed to proceed for B4 h under constant stirring. The product was centrifuged and washed repeatedly with MeOH and dried to obtain the nanoparticle as a grey precipitate. The size-controlled synthesis was driven by changing the temperature and the mode of addition of NaBH4, rather than by changing the concentration of the ligand or the reducing agent. The fast addition of NaBH4 at room temperature gives the smallest particle size (4.8  0.07 nm) and the fast addition of NaBH4 at 120 1C gives the largest particle size (6.3  0.06 nm). A change in the mode of addition (i.e., a slow addition of NaBH4) at 120 1C and at room temperature gives intermediatesized particles (5  0.04 nm). The product was dispersed in chloroform solvent for further analysis. Instrumentation The Powder X-ray diffraction data were collected on a Bruker D8-ADVANCE diffractometer equipped with CuKa radiation. The optical absorption spectra of all the samples were collected on a Shimadzu UV/Vis/NIR spectrophotometer. For these measurements, indium nanoparticles were dispersed in chloroform and taken in a 3 mL quartz cuvette. For comparison, the recorded spectra were normalized on the intensity of oleic acid absorption at 240 nm. Since the lowest wavelength of measurement of chloroform is 240 nm, the absorption spectra was recorded from 240 nm to 450 nm. The optical behavior of the nanoparticles in hexane and dichloromethane were studied using 40 mL of the nanoparticle dispersion in chloroform in 2 mL of the appropriate solvent. For the Transmission Electron Microscopy (TEM) measurements, we used a FEI, Tecnai G2 F30, S-Twin microscope operating at 300 kV. The samples for TEM analysis were prepared by dropping the In nanoparticles dispersed in chloroform solution on the surface of a carbon-coated copper grid. The Fourier transform infrared spectrum was recorded from KBr pellets in the range of 4000–400 cm 1 on a SHIMADZU FT-IR spectrometer. Thermogravimetry analysis was carried out using a SDT Q600 V20.9 Build 20. The experiments were conducted on nanoparticles in the heating range of 25–700 1C, at a heating rate of 15 1C per minute.

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the nanoparticle is uniformly deposited on the glass slide. The PXRD pattern of three different sized nanoparticles all show the characteristic Bragg peaks of elemental indium (with a tetragonal crystal lattice with sp. Gr. I/4mmm), as shown in Fig. 1. To study the spatial distribution of elemental indium in the nanoparticles, elemental mapping was carried out using an energy-dispersive analysis of X-rays (EDAX). Fig. S1–S3 (ESI†) show the EDAX spectrum collected from the oleic acid-capped nanoparticles. We measured the FT-IR spectra for pure oleic acid (a) and for the In nanoparticles (b–d), as shown in Fig. 2. The carboxylic stretching mode of carbonyl in the free oleic acid was observed at 1714 cm 1, while the stretching of carbonyl in the adsorbed oleic acid was observed at 1566 cm 1. Compared with the peak (1714 cm 1) of the carbonyl in free oleic acid, the adsorption of oleic acid on indium causes a significant red shift of the stretching of the carbonyl. It is reasonable to hypothesize that the adsorption weakens the stretching of the carbonyl groups. Simultaneously, a peak with low intensity at 1714 cm 1 is also observed in the IR of the In nanoparticles, indicating that some oleic acid molecules are physically blended in the adsorbed layers, besides the densely adsorbed oleic acid molecules.32 A thermal analysis of In nanoparticles was performed to understand the effect of temperature on the stability of the particles, as well as the weight loss due to the presence of organic ligands around the particles. The TGA profile of In nanoparticles revealed that the weight loss mainly takes place at 325 1C, which indicates the loss of bonded organic ligands. There is a total weight loss of about 70% (Fig. S4, ESI†), which could be due to the loss of oleic acid and solvent molecules. Fig. 3 shows the transmission electron microscopy images of the In nanoparticles of various sizes from B4.8 nm to B6.3 nm. The particles seem to be nearly spherical in shape. The size distribution in these nanoparticles were obtained from histograms plotted using the data from the TEM images.

Results and discussion The oleic acid-capped nanoparticle in chloroform solution was drop-casted on a glass slide for the X-ray diffraction (XRD) analysis. Chloroform is evaporated in room temperature and

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Fig. 1 Powder X-ray diffraction pattern of oleic acid-capped In nanoparticles (a)–(c), and the theoretically calculated PXRD pattern of elemental In.

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Fig. 2 FT-IR spectra of (a) pure oleic acid and the In NPs synthesized in the sizes: (b) 4.8  0.07 nm, (c) 6.3  0.06 nm, (d) 5  0.04 nm.

Fig. 3 TEM images of the In NPs of various diameters. (a) 4.8  0.07 nm, (b) 5  0.04 nm, and (c) 6.3  0.06 nm. Inset: size distribution of the nanoparticles.

The average diameters were calculated for the In nanoparticles synthesized by the fast addition of NaBH4 at room temperature (4.8  0.07 nm), by the slow addition of NaBH4 at 120 1C (5  0.04 nm), and by the fast addition of NaBH4 at 120 1C (6.3  0.06 nm). The high resolution TEM image of an indium nanoparticle (4.8  0.07 nm) showing d spacing (d = 0.27 nm) matches well with the 011 and 101 planes of the indium metal, as shown in Fig. S5 (ESI†). Due to their identical surface coatings, these In nanoparticles represent an ideal class of materials for studying the

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Fig. 4 Size-dependent absorption spectra of different In nanoparticles in CHCl3 solvent.

size dependence plasmon resonance. In Fig. 4, we plot the absorption spectra of the In nanoparticles with different sizes in chloroform solvent. For comparison, the recorded spectra were normalized on the intensity of oleic acid absorption at 240 nm (Fig. S6, ESI†). Since the lowest wavelength of measurement of chloroform is 240 nm, the absorption spectra were recorded from 240 nm to 450 nm. From the figure, we see that the absorption peak red shifts from 262 nm to 275 nm as the sizes are reduced from 6.3 nm to 4.8 nm. As the particle size decreases, the absorption peak heights are seen to decrease. It is worth noting that the dependence of the absorption peak position on the particle size is quite interesting and may be due to a quantum effect. Thus, the red shift with decreasing particle size could be either due to oxide coating on the particles or a quantum size effect and/or chemical interaction with the ligand shell.29–32 It was noted that surface oxidation leads to a red shift of the plasmon resonance.33 However, our experimental results exclude the existence of oxide layers on the surfaces of the In nanoparticles. First, the chemicals are transferred to a reaction flask in an inert N2-filled glove box and the reactions were carried out under inert N2 purging. Second, the absorption spectra of the In nanoparticles were measured right after synthesis, as well as after the NPs have been stored for seven months under ambient environment, which did not show any variation in terms of both peak position or intensity, thereby indicating the stability of the synthesized In nanoparticles (Fig. S9, ESI†). This type of red shift is similar to that observed from small-ligand-stabilized Ag and Al nanoparticles.29,31 There are few experiments on Ag and Al nanoparticles pointing out a red shift of the surface plasmon band as the particles size decreases in smaller particle size ranges. In the case of Ag nanoparticles, Sun and coworkers synthesized colloidal-ligandstabilized Ag nanoparticles with uniform morphology and a narrow size distribution in the range of 2–20 nm.29 They showed that the SPR band position of the nanoparticles exhibits an exceptional size dependence: as the size decreases from 20 nm, it blue-shifts, but reverses over near 12 nm and then strongly red-shifts. A model based on the multilayer Mie theory agrees well with the observations, indicating that the lower

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electron conductivity in the outermost atomic layer, due to chemical interactions, is the cause of the red shift. Recently, Jensen and coworkers showed that the absorption maximum exhibits a red shift when the Al nanoparticles (ligand-stabilized) sizes decrease from 4 to 1.5 nm.31 They used two-layer Mie theory and predicted that the electron conductivity in the Al core is reduced due to a combination of quantum size effects and chemical interaction with the ligand shell, resulting in the observed red shift with decreasing size. In the present case, we also observed the same kind of phenomena as with Al nanoparticles, where the absorption maximum red shifts with decreasing size. We assume that the behaviour of Al and In would be the same due to their similar type of electronic configuration. So this red shifts with decreasing particle size could be due to the combined effects of quantum size effects and chemical interaction with the ligand shell. To gain better insights into the dependence of the surface plasmon band position of In nanoparticles, we measured the optical absorption in different solvents (chloroform, dichloromethane and hexane) (Fig. 4 and Fig. S7 and S8, ESI†). The lmax of the surface plasmon resonance of In nanoparticles in these solvents gradually shifts toward the red with the increase in the solvent refractive index (Table S1, ESI†). The linear variation of the maxima of surface plasmon resonance with the refractive index of the solvent can be treated within the framework of the Drude model. The effect of the surrounding solvent on the plasmon resonance wavelength of spherical nanoparticles is easily understood using the plasmon resonance conditions: l2 = lp2 (ep + 2em), where lp = (2pc)2/op2, (op = bulk plasmon frequency) is the bulk metal plasmon wavelength, ep is the high frequency dielectric constant due to the inter band and core transitions, and em (= n2, n = refractive index of the surrounding medium) is the optical dielectric function of the medium.34,35 We plotted the l2 vs. 2em in Fig. 5, which shows that the surrounding medium dielectric function varies linearly with l2. The linear variation of lmax of the surface plasmon resonance with the medium dielectric function is indicative of the fact that the solvent refractive index influences the surface plasmon maximum according to the Drude model.

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Therefore, it can be inferred that although the surfactant molecules surrounding the nanoparticle act as a barrier preventing solvent penetration to the surface, the electromagnetic fields of indium remain extended to sense the refractive index changes occurring at the bulk interface.

Conclusions A series of oleic acid-capped indium nanoparticles were synthesized using a wet chemical method. The presence of elemental In in the nanoparticle was confirmed from the powder X-ray diffraction pattern and the EDAX mapping. The interaction of In with the ligand was understood from the Fourier transform infrared spectroscopy and from thermogravimetric analysis. The thermogravimetric curve helps to understand the thermal stability of the nanoparticle. The morphology and the size distribution of the nanoparticles were analysed using the transmission electron microscopy image. The optical absorption of these particles show that the absorption maxima red shifts as the particle size decreases. This unusual optical behaviour could be due to a combined effect of a quantum effect and chemical interaction with the ligand shell. We measured the absorption spectra at different solvents and the results showed the linear behavior of the absorption maxima with the solvent refractive index according to the Drude model. Presently, we are working on the synthesis of a series of In nanoparticles with the same ligand but narrow size distribution, which can further underscore the molecular mechanism of plasmonic behaviour.

Funding sources Science and Engineering Research Board (SERB), Govt. of India, through a grant SB/S1/IC-14/2013.

Acknowledgements AG, HC and SM acknowledge financial support from Science Engineering and Research Board through a grant SB/S1/IC-14/2013 and Prof. V. Ramakrishnan for encouragement. AG acknowledges UGC for fellowship.

Notes and references

Fig. 5 Plot of the square of the absorption maxima as a function of twice the medium dielectric function (where, em = n2) of different particle sizes in different solvents [chloroform (n = 1.445), dichloromethane (n = 1.424) and hexane (n = 1.375)].

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1 P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and R. D. Kornberg, Science, 2007, 318, 430–433. 2 M. W. Heaven, A. Dass, P. S. White, K. M. Holt and R. W. Murray, J. Am. Chem. Soc., 2008, 130, 3754–3755. ¨kkinen and 3 J. Akola, M. Walter, R. L. Whetten, H. Ha ¨nbeck, J. Am. Chem. Soc., 2008, 130, 3756–3757. H. Gro 4 P. Nagpal, N. C. Lindquist, S. H. Oh and D. J. Norris, Science, 2009, 325, 594–597. 5 M. J. Banholzer, J. E. Millstone, L. Qin and C. A. Mirkin, Chem. Soc. Rev., 2008, 37, 885–897. 6 S. Lal, S. Link and N. J. Halas, Nat. Photonics, 2007, 1, 641–648.

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7 P. K. Jain, K. S. Lee, I. H. El-Sayed and M. A. El-Sayed, J. Phys. Chem. B, 2006, 110, 7238–7248. 8 N. Fang, H. Lee, C. Sun and X. Zhang, Science, 2005, 308, 534–537. 9 H. Atwater, Sci. Am., 2007, 296, 56–63. 10 E. Ozbay, Science, 2006, 311, 189–193. 11 R. Yan, P. Pausauskie, J. Huang and P. Yang, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 21045–21050. 12 Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 2005, 127, 7632–7637. 13 S. D. Standridge, G. C. Schatz and J. T. Hupp, J. Am. Chem. Soc., 2009, 131, 8407–8409. 14 M. S. Yavuz, Y. Cheng, J. Chen, C. M. Cobley, Q. Zhang, M. Rycenga, J. Xie, C. Kim, K. H. Song, A. G. Schwartz, L. V. Wang and Y. Xia, Nat. Mater., 2009, 8, 935–939. 15 (a) S. Lal, S. E. Clare and N. J. Halas, Acc. Chem. Res., 2008, 41, 1842–1851; (b) K. Biswas and C. N. R. Rao, J. Phys. Chem. B, 2006, 110, 842–845; (c) S. Ghosh, K. Biswas and C. N. R. Rao, J. Mater. Chem., 2007, 17, 2412–2417. 16 W. P. Halperin, Rev. Mod. Phys., 1986, 58, 533–606. 17 W. A. de Heer, Rev. Mod. Phys., 1993, 65, 611–676. 18 S. M. Morton, D. W. Silverstein and L. Jensen, Chem. Rev., 2011, 111, 3962–3994. 19 (a) J. M. McMahon, G. C. Schatz and S. K. Gray, Phys. Chem. Chem. Phys., 2013, 15, 5415–5423; (b) M. W. Knight, N. S. King, L. Liu, H. O. Everitt, P. Nordlander and N. J. Halas, ACS Nano, 2013, 8, 834–840. 20 (a) N. H. Chou, X. Ke, P. Schiffer and R. E. Schaak, J. Am. Chem. Soc., 2008, 130, 8140–8141; (b) C. Kind and C. Feldmann, Chem. Mater., 2011, 23, 4982–4987; (c) Y. Zhao, Z. Zhang and H. J. A. Dang, J. Phys. Chem. B, 2003, 107, 7574–7576;

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Paper

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

(d) S. Cingarapu, Z. Yang, C. M. Sorensen and K. J. Klabunde, Inorg. Chem., 2011, 50, 5000–5005. P. B. Johson and R. W. Christy, Phys. Rev. B: Solid State, 1972, 6, 4370–4379. D. Y. Smith, in Handbook of Optical Constants of Solids, ed. E. D Palik, Academic, Orlando, FL, 1985. T. Dorfer, M. Scmitt and J. Popp, J. Raman Spectrosc., 2007, 38, 1379–1382. U. Schneider, M. Ueno and S. Kobayashi, J. Am. Chem. Soc., 2008, 130, 13824–13825. T. Strupeit, C. Klinke, A. Kornowski and H. Weller, ACS Nano, 2009, 3, 668–672. F. Y. Wu, C. C. Yang, C. M. Wu, C. W. Wang and W. H. Li, J. Appl. Phys., 2007, 101, 09G111. P. K. Khanna, K.-W. Jun, K. B. Hong, J. O. Baeg, R. C. Chikate and B. K. Das, Mater. Lett., 2005, 59, 1032–1036. M. Abtew and G. Selvaduray, Mater. Sci. Eng., R, 2000, 27, 95–141. S. Peng, J. M. McMahon, G. C. Schatz, S. K. Gray and Y. P. Sun, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 14530–14534. J. A. Scholl, A. L. Koh and J. A. Dionne, Nature, 2012, 483, 421–427. S. Mandal, J. Wang, R. E. Winans, L. Jensen and A. Sen, J. Phys. Chem. C, 2013, 117, 6741–6746. J. Lv, Y. Shen, L. Peng, X. Guo and W. Ding, Chem. Commun., 2010, 46, 5909–5911. G. H. Chan, J. Zhao, G. C. Schatz and R. P. V. Duyne, J. Phys. Chem. C, 2008, 112, 13958–13963. P. Mulvaney, Langmuir, 1996, 12, 788–800. K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz, J. Phys. Chem. B, 2003, 107, 668–677.

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Synthesis, characterization and optical properties of ligand-protected indium nanoparticles.

Unlike silver and gold, indium has material properties that enable strong resonances extended up to the ultraviolet. This extended response, combined ...
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