Volume 16 Number 29 7 August 2014 Pages 14973–15718
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Physical Chemistry Chemical Physics www.rsc.org/pccp
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PAPER Stephen B. Cronin et al. Plasmon-enhanced photocatalytic water purification
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Plasmon-enhanced photocatalytic water purification Leyre Gomez,ab Victor Sebastian,ac Manuel Arruebo,ac Jesus Santamariaac and Stephen B. Cronin*bde Increasing water demand and water scarcity around the world requires the development of robust and efficient methods for water purification in the coming decades. Here, we report a photocatalytic water purification method using visible light (532 nm) utilizing 5 nm gold nanoparticles and their enhancement when attached on the surface of silica nanospheres as an inactive support to prevent nanoparticle
Received 16th January 2014, Accepted 3rd February 2014
coalescence or sintering. This is a non-toxic, low-cost, and easy photocatalytic process which provides
DOI: 10.1039/c4cp00229f
trichloroethylene is selected as an example of a real water pollutant. When irradiated at their plasmon
high decomposition rates. Decomposition of the methyl orange dye is tested as a reaction model and resonant frequency, the gold nanoparticles generate hydroxyl radicals that degradate organic pollutants
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into non-toxic molecules representing a basic mechanism of photocatalytic water purification.
1. Introduction Water purification is an important research field due to the high demand of water for human consumption around the world. Water contamination arises from multiple sources such as pesticides, BTEX (benzene, toluene, ethylbenzene, and xylenes) petroleum derivatives, organic solvents such as trichloroethylene, pharmaceutical drugs, etc. Furthermore, the water scarcity foreseen for the coming decades makes necessary the development of more effective, and robust methods to decontaminate water.1 Many investigations address water purification by physical processes, for instance adsorption2 and filtration (including ultrafiltration and biological filtration processes).3 Chemical or biochemical processes are also widely employed, with precipitation and oxidation being widely used. Nanomaterials of different kinds offer high potential for water purification4 including physical (adsorption with nanostructured materials) and chemical (nanocatalysts and redox active nanoparticles) routes. The decomposition of organic compounds can be driven by the energy of photons in photocatalytic processes that convert light into chemical energy. Among other materials, this property a
Department of Chemical Engineering, Aragon Institute of Nanoscience (INA), University of Zaragoza, Campus Rı´o Ebro, 50018 Zaragoza, Spain b Department of Electrical Engineering, University of Southern California, Los Angeles, CA, 90089, USA c CIBER de Bioingenierı´a, Biomateriales y Nanomedicina (CIBER-BBN), Campus Rı´o Ebro, 50018 Zaragoza, Spain d Department of Chemistry, University of Southern California, Los Angeles, CA, 90089, USA. E-mail: [email protected] e Department of Physics, University of Southern California, Los Angeles, CA, 90089, USA
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can be found in Surface Plasmon Resonant (SPR) nanoparticles. Plasmons are the collective oscillation of free electrons in a conducting material. When the frequency of the incident light matches the resonant oscillation frequency, the nanoparticles absorb the light efficiently.5 The frequency of this resonance can be tuned by varying the nanoparticle size, shape, and material.6,7 For example, it is possible to shift the plasmon resonance of gold8 over the visible and infrared wavelength ranges. Gold nanoparticles involved in photocatalytic processes need to fulfill at least two requirements: (1) should have an adequate SPR and (2) small size. Although gold is supposed to be an inert material, it has been reported that Au nanoparticles with sizes equal to or less than 5 nm are catalytically active for several chemical reactions.9 Photocatalytic decomposition of organic compounds is the common model reaction used in the literature to discuss plasmonenhanced photocatalysis.10 Typically dye molecules (e.g. methyl orange) are used, since their decomposition rates can be easily observed by optical means. The enhanced photodegradation of organic compounds after addition of Au nanoparticles to several semiconductor photocatalysts under both UV and visible light has been reported on supports such as TiO2,11–13 ZrO2,14 Fe2O3,14 Cu2O.15 Translating photocatalytic decomposition of pollutants into real life applications for water decontamination requires the use of non-toxic, affordable and reproducibly produced materials. Au nanoparticles supported on silica nanospheres fulfill these requirements. They can be synthesized with a high degree of homogeneity in a continuous flow reactor16 and they have already been successfully applied in laser-driven liquid phase reactions for the production of 4-benzoylmorpholine.17 Here, we report the photocatalytic activity of small gold
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nanoparticles and its enhancement when attached to nonporous SiO2 nanospheres under green laser illumination (532 nm) in order to decompose methyl orange (MO) and TCE (trichloroethylene) as models of organic compounds representative of water pollutants.
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2.1
Materials
(3-Aminopropyl)triethoxysilane (Z98%), ammonium hydroxide solution (30%), gold(III) chloride hydrate (Z99.999%), tetraethyl orthosilicate (498%), tetrakis(hydroxymethyl)phosphonium chloride solution (80%), and trichloroethylene (Z99.5%) were purchased from Sigma Aldrich. Methyl orange was purchased from Fluka. 2.2
Catalyst synthesis
Silica nanoparticles with gold seeds on their surface are an intermediate stage in the synthesis of silica–gold nanoshells first reported by Halas and co-workers.18 The first step is the production ¨ber process.19 of silica nanoparticles following the well-known Sto Ammonium hydroxide solution and tetraethyl orthosilicate (TEOS) were mixed in ethanol as a solvent for 1 h. The surface of the resulting SiO2 nanoparticles was functionalized with 3-aminopropyltriethoxy-silane (APTES) while stirring for 2 h at room temperature plus one additional hour at 65 1C. The synthesis of gold seeds was carried out separately following the procedure of Duff et al.20 Tetrakis(hydroxymethyl)phosphonium chloride (THPC) was used to reduced a chloroauric solution. The pH of the gold solution was adjusted around 2.5 by adding HCl. Silica functionalized nanoparticles were shaken in contact with an excess of gold seeds and the mixture was allowed to settle for 2 hours at room temperature in the dark. Unbounded seeds were removed by several centrifugation washing steps. 2.3
Catalyst characterization
The shape and size distribution of gold seeds and gold-seeded silica nanoparticles were studied by transmission electron microscopy (TEM) using a FEI Tecnai T20 operating at 200 kV. To evaluate the mean diameter of gold seeds, more than 200 nanoparticles were measured. Absorbance spectra of gold seeds and silica gold-seeded nanoparticles were recorded on a PerkinElmer Lambda 950 UV/Vis/NIR spectrometer with an integrating sphere detector, which allows data collection from highly scattered samples. The mass composition of SiO2–Au seeded nanoparticles was determined by Atomic Absorption Spectroscopy, resulting in a 21 wt% of gold. 2.4
Decomposition experiments
The photocatalytic activity of gold seeds and silica–gold seeded nanoparticles was tested using an aqueous solution, 20 mg L 1, of methyl orange (MO). Bare silica nanoparticles were also tested. All experiments were carried out with the same concentration of nanoparticles 0.5 g L 1. 1 mL samples were irradiated in a plastic cuvette for 1 h at different wavelengths. We used a red
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laser at 633 nm (3 mW), a 532 nm green laser at different intensities (0.2 W, 1 W and 2 W), and an UV mercury lamp with two bandpass filters at 365 nm and 254 nm (0.02 W). The reference experiment was to illuminate the MO solution without catalyst under those conditions. The MO absorbance decay at 463 nm was monitored by UV/Vis/NIR spectroscopy measured using an integrating sphere detector. In order to avoid absorbance interferences between the plasmonic catalyst and MO, the baseline for absorbance measurements was a catalyst aqueous solution of 0.5 g L 1. To test the decomposition model with a second pollutant, we selected trichloroethylene (TCE). The concentration of nanoparticles was 0.5 g L 1, and that of TCE was 1 mM in water. The 1 mL sample was exposed for 1 h under visible light (532 nm laser) at intensities of 1 W and 2 W. Decomposition of TCE under green light exposure was also tested just with silica nanoparticles and no catalyst. The TCE decomposition rate was monitored using an Agilent gas chromatograph (GC) equipped with an HP-PLOT/U capillary and a flame ionization detector (FID).
3. Results and discussion Fig. 1a and b show TEM images of the silica nanospheres B100 nm in diameter with gold seeds on their surface, while bare gold seeds are shown in Fig. 1c. The spherically integrated UV-Vis absorption spectra of both SiO2–Au seeded nanoparticles and bare Au seeds show a SPR absorption band at around 520 nm (Fig. 1d) which means that they can successfully absorb the 532 nm green laser.21 The size distribution of the gold seeds is depicted in Fig. 1e, which shows a mean diameter of 5 1.6 nm. According to Mie theory,22 the contribution of scattering is greater with increasing diameters. Nanoparticles smaller than 10 nm exhibit a SPR band with a broad envelope and it could disappear for particles with a size of 2 nm or less. The shoulder below 500 nm can be attributed to interband transitions.23 Fig. 2a and b show the catalytic activity of SiO2–Au seeded nanoparticles and bare Au seeds under illumination of a 532 nm green laser, respectively. As could be expected, increasing the laser intensity leads to a higher MO decomposition rate. As the MO molecule decomposes, the absorbance peak at 463 nm decreases,11,24 while another peak at 340 nm appears due to new N-nitrophenyl derivatives.25 The absorbance of the MO solution, and hence its concentration, drops by (a) 15%, 42%, 61% and (b) 1.4%, 28% and 29% for the two samples and different light intensities, respectively. Clearly, SiO2–Au seeded nanoparticles have a higher catalytic activity than bare Au seeds with the same amount of catalyst. Although the photocatalytic decomposition does occur in the gold seeds due to their size and enhanced plasmon resonance,9 gold supported on silica nanoparticles shows improved catalytic effects even with a lower amount of gold seeds. Support materials can be active (e.g. TiO2, Fe2O3) or inactive (e.g. SiO2, Al2O3), but both can influence the reactivity.26 In particular, the high dispersion of the metal in an inactive support and the presence of low-coordinated gold surface sites, which are essential for reactivity, increase the catalytic activity of the gold.27,28
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Fig. 1 TEM images of (a) SiO2 nanoparticles B100 nm with gold seeds uniformly attached on their surface, (b) magnified TEM image of SiO2–Au nanoparticles and (c) gold seeds. (d) Spherically integrated visible absorption spectra of the nanoparticles and seeds shown in (a and b) and (c). (e) Size distribution of the gold seeds, showing an average diameter of 5 1.6 nm.
Fig. 2 UV-Vis spherically integrated spectra of methyl orange aqueous solution with (a) SiO2–Au seeded nanoparticles and (b) Au seeds, before (black) and after 1 h of 532 nm laser exposure at different light intensities.
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To verify the catalytic activity of the plasmonic gold, we carried out several blank experiments, as depicted in Fig. 3. A methyl orange aqueous solution without catalyst (Fig. 3a) and MO aqueous solution with bare silica nanoparticles (Fig. 3b) were tested for 1 h using different light wavelengths under an incident intensity of 2 W for the 532 nm green laser. The MO peak absorbance at 463 nm in both experiments remained stable after laser exposure. Fig. 4 shows the stability of the MO aqueous solution with SiO2–Au seeded nanoparticles after irradiating the sample with wavelengths far from the gold nanoparticle SPR absorption band. Again, a good stability of the MO spectra is seen, with negligible decomposition after 1 h. A barely discernible decrease of absorbance can only be ascertained for the experiment using the 633 nm laser. Fig. 5 shows the time-dependence of the MO decomposition with the SiO2–Au seeded catalyst under visible light (532 nm) at different intensities ((a) 0.2 W, (b) 1 W, and (c) 2 W). The sample was irradiated for 1 h at the desired intensity, then the absorbance spectrum was recorded, and the same sample was further exposed for an additional 1 hour period. This experimental procedure was carried out to complete 3 h of irradiation. These results show that the catalyst is active for several cycles. In addition, Fig. 6 shows that the catalyst is stable at the laser power and exposure time tested in this manuscript. At a fixed intensity, the MO photodegradation follows a first order reaction kinetics and the kinetic rate constant is enhanced as the irradiation power increases. The rate constants k calculated
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Fig. 5 Methyl orange aqueous solution decomposition with SiO2–Au seeded nanoparticles under 532 nm laser exposure at (a) 0.2 W, (b) 1 W and (c) 2 W. (d) Color of methyl orange solutions after 3 h laser exposure. The 3 h experiment was carried out with the same sample for each intensity test.
Fig. 3 UV-Vis spectra of (a) methyl orange aqueous solution and (b) methyl orange aqueous solution with SiO2 nanoparticles before (black) and after 1 h illumination at different light wavelengths.
Fig. 4 UV-Vis spectra of methyl orange aqueous solution with SiO2–Au seeded nanoparticles before (black) and after 1 h light irradiation with different wavelength off-resonance of the catalyst SPR absorption peak.
for green laser irradiation at 0.2 W, 1 W and 2 W are 0.001 min 1, 0.0028 min 1 and 0.0163 min 1, respectively. Moreover, as described previously, the higher the 532 nm laser intensity, the higher the decomposition rate. Fig. 5d shows the color change after 3 h of visible light exposure at various light intensities. Nearly complete MO decomposition can be achieved with 2 W green laser exposure after 3 h. The MO characteristic peak at 463 nm disappears, and the intensity of the 340 nm peak increases due to the residual broken-down molecules. In addition, the orange color of MO disappears leaving a more reddish dispersion attributed to the catalyst.
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Fig. 6 TEM images of the SiO2–Au seeded catalyst (a) and their stable morphology after 532 nm laser irradiation for 3 h at (b) 0.2 W, (c) 1 W and (d) 2 W.
Different nanoparticles (e.g. Au, Ag, Pt) and supports (e.g. TiO2, ZnO) have been tested for the decomposition of azo-dyes. A common conclusion of those experiments is the first order MO decomposition reaction while the rate constant k changes as a function of parameters such as the gold amount deposited onto the support,29 the metal nanoparticles used,30 or how the hybrid nanostructure is synthesized.31 The efficiency of SiO2– Au seeded nanoparticles under 2 W green light reported in this manuscript is similar to the highest results achieved with Au nanoparticles in the previous studies. According to these promising results observed in the MO decomposition system, we tested SiO2–Au photocatalytic nanoparticles on a second, harder to degrade pollutant, the organic
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light with the plasmon resonance of the catalyst. For a fixed amount of contaminant and a determined amount of catalyst, the decomposition rate increased with light intensity. In addition, the gold-supported silica nanoparticle photocatalysts can be used in several reaction cycles. According to the results obtained with MO and TCE as model compounds, we can conclude that the decomposition of organic compounds by plasmonically-driven photocatalysis could be used with other water pollutants.
Acknowledgements This work has been supported by the Spanish Ministry of Science and Innovation (MICINN) with the abroad research stay of pre-doctoral students grant EEBB-I-13-06033.
References
Fig. 7 Gas chromatograph of TCE after 1 h exposure under 532 nm laser with (a) SiO2 nanoparticles and no catalyst and (b) SiO2–Au nanoparticles as the catalyst.
solvent TCE. The decomposition results follow similar trends to those already described for MO. In the control experiments (Fig. 7a), TCE with bare SiO2 nanoparticles and TCE solution without nanoparticles showed no decomposition under 532 nm laser exposure. As seen in Fig. 7b, 1 mM TCE solution under 1 W 532 nm laser illumination experiences 34% decomposition while the rate increased up to 50% when 2 W were applied. As reported in the literature, highly reactive species such as hydroxyl radicals (OH ) are mainly responsible for the decomposition of organic compounds in aqueous phase photocatalytic processes. These radicals are generated by superoxide radicals (HO2 ), which react with H2O. The photodegradation begins by the photogenerated electrons, which reduce absorbed oxygen to form superoxide radicals,32 which carry out the photodecomposition of the organic compounds into inorganic final products.33,34
4. Conclusions We demonstrate the photocatalytic activity of 5 nm gold nanospheres under visible light irradiation and their enhanced performance when deposited on silica nanoparticles as an inactive support using the decomposition of the methyl orange dye as a decomposition model reaction. The photocatalytic activity of SiO2–Au nanoparticles was also verified with trichloroethylene. As the experimental data show, in order to achieve a high catalytic activity, it is necessary to match the wavelength of the incident
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1 M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marinas and A. M. Mayes, Nature, 2008, 452, 301–310. 2 A. Dabrowski, Adv. Colloid Interface Sci., 2001, 93, 135–224. 3 T. Heberer, D. Feldmann, K. Reddersen, H.-J. Altmann and T. Zimmermann, Acta Hydrochim. Hydrobiol., 2002, 30, 24–33. 4 N. Savage and M. S. Diallo, J. Nanopart. Res., 2005, 7, 331–342. 5 H. Chen, T. Ming, L. Zhao, F. Wang, L.-D. Sun, J. Wang and C.-H. Yan, Nano Today, 2010, 5, 494–505. 6 T. Atay, J.-H. Song and A. V. Nurmikko, Nano Lett., 2004, 4, 1627–1631. 7 K. Kolwas, A. Derkachova and M. Shopa, J. Quant. Spectrosc. Radiat. Transfer, 2009, 110, 1490–1501. 8 S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 4212–4217. 9 B. Hvolbaek, T. V. W. Jannssens, B. S. Clausen, H. Falsig, C. H. Christensen and J. K. Norskov, Nano Today, 2007, 2, 14–18. 10 W. Hou and S. B. Cronin, Adv. Funct. Mater., 2012, 23, 1612–1619. 11 W. Hou, Z. Liu, P. Pavaskar, W. H. Hung and S. B. Cronin, J. Catal., 2011, 277, 149–153. 12 M. Alvaro, B. Cojocaru, A. A. Ismail, N. Petrea, B. Ferrer, F. A. Harraz, V. I. Parvulescu and H. Garcia, Appl. Catal., B, 2010, 99, 191–197. 13 J. Yu, G. Dai and B. Huang, J. Phys. Chem. C, 2009, 113, 16394–16401. 14 X. Chen, H.-Y. Zhu, J.-C. Zhao, Z.-F. Zheng and X.-P. Gao, Angew. Chem., Int. Ed., 2008, 47, 5353–5356. 15 M. A. Mahmoud, W. Qian and M. A. El-Sayed, Nano Lett., 2011, 11, 3285–3289. 16 L. Gomez, M. Arruebo, V. Sebastian, L. Gutierrez and J. Santamaria, J. Mater. Chem., 2012, 22, 21420–21425. 17 A. Pineda, L. Gomez, A. M. Balu, V. Sebastian, M. Ojeda, M. Arruebo, A. A. R. Reyes, J. Santamaria and R. Luque, Green Chem., 2013, 15, 2043–2049.
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27 I. N. Remediakis, N. Lopez and J. K. Nørskov, Angew. Chem., Int. Ed., 2005, 44, 1824–1826. 28 N. Weiher, E. Bus, L. Delannoy, C. Louis, D. E. Ramaker, J. T. Miller and J. A. van Bokhoven, J. Catal., 2006, 240, 100–107. 29 I. M. Arabatzis, T. Stergiopoulos, D. Andreeva, S. Kitova, S. G. Neophytides and P. Falaras, J. Catal., 2003, 220, 127–135. 30 N. Gupta, H. P. Singh and R. K. Sharma, J. Mol. Catal. A: Chem., 2011, 335, 248–252. 31 F. Xiao, F. Wang, X. Fu and Y. Zheng, J. Mater. Chem., 2012, 22, 2868–2877. 32 A. Primo, A. Corma and H. Garcia, Phys. Chem. Chem. Phys., 2011, 13, 886–910. 33 C. Yogi, K. Kojima, N. Wada, H. Tokumoto, T. Takai, T. Mizoguchi and H. Tamiaki, Thin Solid Films, 2008, 516, 5881–5884. 34 E. Kowalska, O. O. P. Mahaney, R. Abe and B. Ohtani, Phys. Chem. Chem. Phys., 2010, 12, 2344–2355.
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Physical Chemistry Chemical Physics www.rsc.org/pccp
ISSN 1463-9076
PAPER Stephen B. Cronin et al. Plasmon-enhanced photocatalytic water purification
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Published on 10 February 2014. Downloaded by Tufts University on 27/10/2014 17:35:20.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 15111
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Plasmon-enhanced photocatalytic water purification Leyre Gomez,ab Victor Sebastian,ac Manuel Arruebo,ac Jesus Santamariaac and Stephen B. Cronin*bde Increasing water demand and water scarcity around the world requires the development of robust and efficient methods for water purification in the coming decades. Here, we report a photocatalytic water purification method using visible light (532 nm) utilizing 5 nm gold nanoparticles and their enhancement when attached on the surface of silica nanospheres as an inactive support to prevent nanoparticle
Received 16th January 2014, Accepted 3rd February 2014
coalescence or sintering. This is a non-toxic, low-cost, and easy photocatalytic process which provides
DOI: 10.1039/c4cp00229f
trichloroethylene is selected as an example of a real water pollutant. When irradiated at their plasmon
high decomposition rates. Decomposition of the methyl orange dye is tested as a reaction model and resonant frequency, the gold nanoparticles generate hydroxyl radicals that degradate organic pollutants
www.rsc.org/pccp
into non-toxic molecules representing a basic mechanism of photocatalytic water purification.
1. Introduction Water purification is an important research field due to the high demand of water for human consumption around the world. Water contamination arises from multiple sources such as pesticides, BTEX (benzene, toluene, ethylbenzene, and xylenes) petroleum derivatives, organic solvents such as trichloroethylene, pharmaceutical drugs, etc. Furthermore, the water scarcity foreseen for the coming decades makes necessary the development of more effective, and robust methods to decontaminate water.1 Many investigations address water purification by physical processes, for instance adsorption2 and filtration (including ultrafiltration and biological filtration processes).3 Chemical or biochemical processes are also widely employed, with precipitation and oxidation being widely used. Nanomaterials of different kinds offer high potential for water purification4 including physical (adsorption with nanostructured materials) and chemical (nanocatalysts and redox active nanoparticles) routes. The decomposition of organic compounds can be driven by the energy of photons in photocatalytic processes that convert light into chemical energy. Among other materials, this property a
Department of Chemical Engineering, Aragon Institute of Nanoscience (INA), University of Zaragoza, Campus Rı´o Ebro, 50018 Zaragoza, Spain b Department of Electrical Engineering, University of Southern California, Los Angeles, CA, 90089, USA c CIBER de Bioingenierı´a, Biomateriales y Nanomedicina (CIBER-BBN), Campus Rı´o Ebro, 50018 Zaragoza, Spain d Department of Chemistry, University of Southern California, Los Angeles, CA, 90089, USA. E-mail: [email protected] e Department of Physics, University of Southern California, Los Angeles, CA, 90089, USA
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can be found in Surface Plasmon Resonant (SPR) nanoparticles. Plasmons are the collective oscillation of free electrons in a conducting material. When the frequency of the incident light matches the resonant oscillation frequency, the nanoparticles absorb the light efficiently.5 The frequency of this resonance can be tuned by varying the nanoparticle size, shape, and material.6,7 For example, it is possible to shift the plasmon resonance of gold8 over the visible and infrared wavelength ranges. Gold nanoparticles involved in photocatalytic processes need to fulfill at least two requirements: (1) should have an adequate SPR and (2) small size. Although gold is supposed to be an inert material, it has been reported that Au nanoparticles with sizes equal to or less than 5 nm are catalytically active for several chemical reactions.9 Photocatalytic decomposition of organic compounds is the common model reaction used in the literature to discuss plasmonenhanced photocatalysis.10 Typically dye molecules (e.g. methyl orange) are used, since their decomposition rates can be easily observed by optical means. The enhanced photodegradation of organic compounds after addition of Au nanoparticles to several semiconductor photocatalysts under both UV and visible light has been reported on supports such as TiO2,11–13 ZrO2,14 Fe2O3,14 Cu2O.15 Translating photocatalytic decomposition of pollutants into real life applications for water decontamination requires the use of non-toxic, affordable and reproducibly produced materials. Au nanoparticles supported on silica nanospheres fulfill these requirements. They can be synthesized with a high degree of homogeneity in a continuous flow reactor16 and they have already been successfully applied in laser-driven liquid phase reactions for the production of 4-benzoylmorpholine.17 Here, we report the photocatalytic activity of small gold
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Paper
nanoparticles and its enhancement when attached to nonporous SiO2 nanospheres under green laser illumination (532 nm) in order to decompose methyl orange (MO) and TCE (trichloroethylene) as models of organic compounds representative of water pollutants.
2. Materials and methods Published on 10 February 2014. Downloaded by Tufts University on 27/10/2014 17:35:20.
2.1
Materials
(3-Aminopropyl)triethoxysilane (Z98%), ammonium hydroxide solution (30%), gold(III) chloride hydrate (Z99.999%), tetraethyl orthosilicate (498%), tetrakis(hydroxymethyl)phosphonium chloride solution (80%), and trichloroethylene (Z99.5%) were purchased from Sigma Aldrich. Methyl orange was purchased from Fluka. 2.2
Catalyst synthesis
Silica nanoparticles with gold seeds on their surface are an intermediate stage in the synthesis of silica–gold nanoshells first reported by Halas and co-workers.18 The first step is the production ¨ber process.19 of silica nanoparticles following the well-known Sto Ammonium hydroxide solution and tetraethyl orthosilicate (TEOS) were mixed in ethanol as a solvent for 1 h. The surface of the resulting SiO2 nanoparticles was functionalized with 3-aminopropyltriethoxy-silane (APTES) while stirring for 2 h at room temperature plus one additional hour at 65 1C. The synthesis of gold seeds was carried out separately following the procedure of Duff et al.20 Tetrakis(hydroxymethyl)phosphonium chloride (THPC) was used to reduced a chloroauric solution. The pH of the gold solution was adjusted around 2.5 by adding HCl. Silica functionalized nanoparticles were shaken in contact with an excess of gold seeds and the mixture was allowed to settle for 2 hours at room temperature in the dark. Unbounded seeds were removed by several centrifugation washing steps. 2.3
Catalyst characterization
The shape and size distribution of gold seeds and gold-seeded silica nanoparticles were studied by transmission electron microscopy (TEM) using a FEI Tecnai T20 operating at 200 kV. To evaluate the mean diameter of gold seeds, more than 200 nanoparticles were measured. Absorbance spectra of gold seeds and silica gold-seeded nanoparticles were recorded on a PerkinElmer Lambda 950 UV/Vis/NIR spectrometer with an integrating sphere detector, which allows data collection from highly scattered samples. The mass composition of SiO2–Au seeded nanoparticles was determined by Atomic Absorption Spectroscopy, resulting in a 21 wt% of gold. 2.4
Decomposition experiments
The photocatalytic activity of gold seeds and silica–gold seeded nanoparticles was tested using an aqueous solution, 20 mg L 1, of methyl orange (MO). Bare silica nanoparticles were also tested. All experiments were carried out with the same concentration of nanoparticles 0.5 g L 1. 1 mL samples were irradiated in a plastic cuvette for 1 h at different wavelengths. We used a red
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laser at 633 nm (3 mW), a 532 nm green laser at different intensities (0.2 W, 1 W and 2 W), and an UV mercury lamp with two bandpass filters at 365 nm and 254 nm (0.02 W). The reference experiment was to illuminate the MO solution without catalyst under those conditions. The MO absorbance decay at 463 nm was monitored by UV/Vis/NIR spectroscopy measured using an integrating sphere detector. In order to avoid absorbance interferences between the plasmonic catalyst and MO, the baseline for absorbance measurements was a catalyst aqueous solution of 0.5 g L 1. To test the decomposition model with a second pollutant, we selected trichloroethylene (TCE). The concentration of nanoparticles was 0.5 g L 1, and that of TCE was 1 mM in water. The 1 mL sample was exposed for 1 h under visible light (532 nm laser) at intensities of 1 W and 2 W. Decomposition of TCE under green light exposure was also tested just with silica nanoparticles and no catalyst. The TCE decomposition rate was monitored using an Agilent gas chromatograph (GC) equipped with an HP-PLOT/U capillary and a flame ionization detector (FID).
3. Results and discussion Fig. 1a and b show TEM images of the silica nanospheres B100 nm in diameter with gold seeds on their surface, while bare gold seeds are shown in Fig. 1c. The spherically integrated UV-Vis absorption spectra of both SiO2–Au seeded nanoparticles and bare Au seeds show a SPR absorption band at around 520 nm (Fig. 1d) which means that they can successfully absorb the 532 nm green laser.21 The size distribution of the gold seeds is depicted in Fig. 1e, which shows a mean diameter of 5 1.6 nm. According to Mie theory,22 the contribution of scattering is greater with increasing diameters. Nanoparticles smaller than 10 nm exhibit a SPR band with a broad envelope and it could disappear for particles with a size of 2 nm or less. The shoulder below 500 nm can be attributed to interband transitions.23 Fig. 2a and b show the catalytic activity of SiO2–Au seeded nanoparticles and bare Au seeds under illumination of a 532 nm green laser, respectively. As could be expected, increasing the laser intensity leads to a higher MO decomposition rate. As the MO molecule decomposes, the absorbance peak at 463 nm decreases,11,24 while another peak at 340 nm appears due to new N-nitrophenyl derivatives.25 The absorbance of the MO solution, and hence its concentration, drops by (a) 15%, 42%, 61% and (b) 1.4%, 28% and 29% for the two samples and different light intensities, respectively. Clearly, SiO2–Au seeded nanoparticles have a higher catalytic activity than bare Au seeds with the same amount of catalyst. Although the photocatalytic decomposition does occur in the gold seeds due to their size and enhanced plasmon resonance,9 gold supported on silica nanoparticles shows improved catalytic effects even with a lower amount of gold seeds. Support materials can be active (e.g. TiO2, Fe2O3) or inactive (e.g. SiO2, Al2O3), but both can influence the reactivity.26 In particular, the high dispersion of the metal in an inactive support and the presence of low-coordinated gold surface sites, which are essential for reactivity, increase the catalytic activity of the gold.27,28
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Fig. 1 TEM images of (a) SiO2 nanoparticles B100 nm with gold seeds uniformly attached on their surface, (b) magnified TEM image of SiO2–Au nanoparticles and (c) gold seeds. (d) Spherically integrated visible absorption spectra of the nanoparticles and seeds shown in (a and b) and (c). (e) Size distribution of the gold seeds, showing an average diameter of 5 1.6 nm.
Fig. 2 UV-Vis spherically integrated spectra of methyl orange aqueous solution with (a) SiO2–Au seeded nanoparticles and (b) Au seeds, before (black) and after 1 h of 532 nm laser exposure at different light intensities.
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To verify the catalytic activity of the plasmonic gold, we carried out several blank experiments, as depicted in Fig. 3. A methyl orange aqueous solution without catalyst (Fig. 3a) and MO aqueous solution with bare silica nanoparticles (Fig. 3b) were tested for 1 h using different light wavelengths under an incident intensity of 2 W for the 532 nm green laser. The MO peak absorbance at 463 nm in both experiments remained stable after laser exposure. Fig. 4 shows the stability of the MO aqueous solution with SiO2–Au seeded nanoparticles after irradiating the sample with wavelengths far from the gold nanoparticle SPR absorption band. Again, a good stability of the MO spectra is seen, with negligible decomposition after 1 h. A barely discernible decrease of absorbance can only be ascertained for the experiment using the 633 nm laser. Fig. 5 shows the time-dependence of the MO decomposition with the SiO2–Au seeded catalyst under visible light (532 nm) at different intensities ((a) 0.2 W, (b) 1 W, and (c) 2 W). The sample was irradiated for 1 h at the desired intensity, then the absorbance spectrum was recorded, and the same sample was further exposed for an additional 1 hour period. This experimental procedure was carried out to complete 3 h of irradiation. These results show that the catalyst is active for several cycles. In addition, Fig. 6 shows that the catalyst is stable at the laser power and exposure time tested in this manuscript. At a fixed intensity, the MO photodegradation follows a first order reaction kinetics and the kinetic rate constant is enhanced as the irradiation power increases. The rate constants k calculated
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Fig. 5 Methyl orange aqueous solution decomposition with SiO2–Au seeded nanoparticles under 532 nm laser exposure at (a) 0.2 W, (b) 1 W and (c) 2 W. (d) Color of methyl orange solutions after 3 h laser exposure. The 3 h experiment was carried out with the same sample for each intensity test.
Fig. 3 UV-Vis spectra of (a) methyl orange aqueous solution and (b) methyl orange aqueous solution with SiO2 nanoparticles before (black) and after 1 h illumination at different light wavelengths.
Fig. 4 UV-Vis spectra of methyl orange aqueous solution with SiO2–Au seeded nanoparticles before (black) and after 1 h light irradiation with different wavelength off-resonance of the catalyst SPR absorption peak.
for green laser irradiation at 0.2 W, 1 W and 2 W are 0.001 min 1, 0.0028 min 1 and 0.0163 min 1, respectively. Moreover, as described previously, the higher the 532 nm laser intensity, the higher the decomposition rate. Fig. 5d shows the color change after 3 h of visible light exposure at various light intensities. Nearly complete MO decomposition can be achieved with 2 W green laser exposure after 3 h. The MO characteristic peak at 463 nm disappears, and the intensity of the 340 nm peak increases due to the residual broken-down molecules. In addition, the orange color of MO disappears leaving a more reddish dispersion attributed to the catalyst.
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Fig. 6 TEM images of the SiO2–Au seeded catalyst (a) and their stable morphology after 532 nm laser irradiation for 3 h at (b) 0.2 W, (c) 1 W and (d) 2 W.
Different nanoparticles (e.g. Au, Ag, Pt) and supports (e.g. TiO2, ZnO) have been tested for the decomposition of azo-dyes. A common conclusion of those experiments is the first order MO decomposition reaction while the rate constant k changes as a function of parameters such as the gold amount deposited onto the support,29 the metal nanoparticles used,30 or how the hybrid nanostructure is synthesized.31 The efficiency of SiO2– Au seeded nanoparticles under 2 W green light reported in this manuscript is similar to the highest results achieved with Au nanoparticles in the previous studies. According to these promising results observed in the MO decomposition system, we tested SiO2–Au photocatalytic nanoparticles on a second, harder to degrade pollutant, the organic
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light with the plasmon resonance of the catalyst. For a fixed amount of contaminant and a determined amount of catalyst, the decomposition rate increased with light intensity. In addition, the gold-supported silica nanoparticle photocatalysts can be used in several reaction cycles. According to the results obtained with MO and TCE as model compounds, we can conclude that the decomposition of organic compounds by plasmonically-driven photocatalysis could be used with other water pollutants.
Acknowledgements This work has been supported by the Spanish Ministry of Science and Innovation (MICINN) with the abroad research stay of pre-doctoral students grant EEBB-I-13-06033.
References
Fig. 7 Gas chromatograph of TCE after 1 h exposure under 532 nm laser with (a) SiO2 nanoparticles and no catalyst and (b) SiO2–Au nanoparticles as the catalyst.
solvent TCE. The decomposition results follow similar trends to those already described for MO. In the control experiments (Fig. 7a), TCE with bare SiO2 nanoparticles and TCE solution without nanoparticles showed no decomposition under 532 nm laser exposure. As seen in Fig. 7b, 1 mM TCE solution under 1 W 532 nm laser illumination experiences 34% decomposition while the rate increased up to 50% when 2 W were applied. As reported in the literature, highly reactive species such as hydroxyl radicals (OH ) are mainly responsible for the decomposition of organic compounds in aqueous phase photocatalytic processes. These radicals are generated by superoxide radicals (HO2 ), which react with H2O. The photodegradation begins by the photogenerated electrons, which reduce absorbed oxygen to form superoxide radicals,32 which carry out the photodecomposition of the organic compounds into inorganic final products.33,34
4. Conclusions We demonstrate the photocatalytic activity of 5 nm gold nanospheres under visible light irradiation and their enhanced performance when deposited on silica nanoparticles as an inactive support using the decomposition of the methyl orange dye as a decomposition model reaction. The photocatalytic activity of SiO2–Au nanoparticles was also verified with trichloroethylene. As the experimental data show, in order to achieve a high catalytic activity, it is necessary to match the wavelength of the incident
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