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gold nanoparticle incorporated silica thin films
Thin Solid Films 525 (2012) 172–176
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Photochemical oxygen reduction by zinc phthalocyanine and silver/gold nanoparticle incorporated silica thin films Manas Pal, Vellaichamy Ganesan ⁎, Uday Pratap Azad Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, UP, India
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 18 October 2012 Accepted 23 October 2012 Available online 27 October 2012 Keywords: Photocatalysis Zinc phthalocyanine Silver nanoparticles Gold nanoparticles Oxygen reduction Mesoporous silica
a b s t r a c t Silver or gold nanoparticles are synthesized using a borohydride reduction method and are anchored simultaneously into/onto the mercaptopropyl functionalized silica. Later, zinc phthalocyanine is adsorbed onto the above materials. Thin films of these materials are prepared by coating an aqueous colloidal suspension of the respective material onto glass plates. Visible light irradiation of these films in oxygen saturated, stirred aqueous solutions effectively reduces oxygen to hydrogen peroxide. The photocatalytic reduction of oxygen is explained on the basis of the semiconducting properties of the silica films. The back electron transfer reaction is largely prevented by means of a sacrificial electron donor, triethanolamine. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electron transfer reactions are very important in the photoinduced chemistry of molecules [1] and this is the preliminary/ fundamental process of solar energy conversion in natural and artificial photosynthetic procedures. Light-induced electron transfer reactions have merited increasing attention among researchers [2–4] because of several useful photocatalytic reactions. Artificial photosynthesis is the replication of the function of a green plant in producing high-energy chemicals from sunlight [5]. In fact, photoinduced electron transfer reactions at heterogeneous surfaces have been realized in an artificially constructed membrane system [5,6]. Enhancing/improving the photo-efficiency of a photocatalyst is a major challenge in the field of photochemical processes taking place in heterogeneous systems. H2O2 is a clean and mild oxidant due to the formation of H2O as a byproduct [7]. Therefore, researchers in the field of photocatalysis have been extensively focusing their study on photocatalytic formation of H2O2 by reducing oxygen [7–9], which requires the development of efficient visible-light photocatalysts driven by solar energy. Concurrently, it would be advantageous for these photocatalysts to fulfill the requirements of the future environmental and energy technologies. Photochemically, oxygen can be reduced to hydrogen peroxide using metal phthalocyanines (MPcs) [10], which can also act as
⁎ Corresponding author. Tel.: +91 542 2307321; fax: +91 542 2368127. E-mail addresses: [email protected], [email protected] (V. Ganesan). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.049
photosensitizers [11] for photoredox reactions both in homogeneous and heterogeneous media [5,6,10]. Some other approaches are the utilization of microheterogeneous reaction media (i.e. solid–solution interfaces) such as vesicles, micelles, monolayers and polyelectrolytes [1,12–14], which include heterogeneous catalyst systems and chemically modified electrodes. Likewise, noble metal nanoparticles (NPs) having plasmon resonance exhibit strong UV–vis absorption by means of the collective oscillations of surface electrons. These metal NPs have unique physiochemical thermodynamic properties and also have unique optical [15], electronic [16] and catalytic [17] (including photocatalytic [18]) properties which are distinctly different from their bulk counterparts and thereby lead to several applications in heterogeneous catalysts [17,19,20] and molecular sensors [21]. Several studies on metal NPs show that they represent plasmonic photocatalysts [22], and can facilitate electron transfer events by acting as a tiny electron conduction center [23]. While fundamentally promising, the development of future applications for metal NPs will depend on the adherence of the nano-scaled materials on suitable substrates (which are in two- or three-dimensional arrangements) in order to take advantage of their fascinating properties under different conditions. Hence, our purpose has been focused on increasing the photocatalytic efficiency of NPs using templated silica [24,25]. In this work, catalytic materials (zinc phthalocyanine (ZnPc) and metal NPs) have been immobilized within a mesoporous silica network, which is beneficial due to its easy separation from the reaction medium and also due to the small amount of catalyst material needed for the reaction. The utility of immobilized catalysts is drastically evident since they are not leached from the network and are quite active and stable.
M. Pal et al. / Thin Solid Films 525 (2012) 172–176
2. Experimental details 2.1. Physicochemical characterization techniques Fourier transform infrared (FT-IR) spectra were recorded using a PerkinElmer FT-IR Spectrometer (Spectrum Two) or a Varian 3100 FT-IR (Excalibur series, USA) spectrometer. UV–vis absorption spectrophotometers (UV 1700 Pharma Spec, Shimadzu for diffuse reflectance measurements and 2802 PC UNICO, USA for absorbance measurements), X-ray powder diffractometer (XRD) with transmission geometry and Cu-Kα radiation source (ID 3000 SEIFERT, Germany) and transmission electron microscope (TEM, TECNAI 20G 2 FEI microscope, operating at 200 kV) were used for the respective characterization of the materials. A home-made photolysis set-up with a 500 W tungsten lamp and focusing lenses was used to irradiate the thin silica films. X-ray photoelectron spectra (XPS) were collected from a VG Microtech Multilab ESCA 3000 spectrometer with a non-monochromatized Al Kα X-ray (hν = 1486.6 eV). Binding energy was calibrated with the Au 4f7/2 core level at 83.9 eV [26]. To remove the atmospheric surface contamination (water and CO2) on the catalyst pellet surface, the pellet was scraped freshly just before the experiment. 2.2. Chemicals and reagents Mercaptopropyltriethoxysilane, tetraethoxysilane, zinc phthalocyanine (ZnPc) and hydrogen tetrachloroaurate (III) solution (HAuCl4) from Aldrich, cetyltrimethylammonium bromide (CTAB) from Himedia and silver nitrate from Merck were used in this study. The remaining reagents (analytical grade) were purchased from Sd fine chemicals, India. HAuCl4 and AgNO3 were used as metal NP precursors and NaBH4 was used as the reducing agent [24]. Our previously reported synthetic approach [24] was implicated to prepare the silica materials. Therefore the mercaptopropyl functionalized silica (MPS), Ag or Au nanoparticles (Ag or Au NPs respectively) anchored MPS (represented as Ag-MPS or Au-MPS respectively), ZnPc adsorbed Ag-MPS or Au-MPS (represented as Ag-MPS-ZnPc or Au-MPS-ZnPc respectively) materials were available from our previous study [24]. 2.3. Preparation of thin films Thin films of the above materials were prepared on clean glass plates of 1.0 cm 2 area. 1.0% colloidal solution of the respective materials containing 0.01% of poly(vinyl alcohol) was used for the film preparations. 100 μL of a colloidal solution of the respective material was dropped on a glass plate, which was then allowed to dry in air in the absence of light. Since the films were all prepared similarly, the amount of material present on the glass plate is expected to be the same for all cases. However, small changes in the film thickness are expected since it depends also on the density of the respective materials. After the preparation of the films, they were used within 24 h for photochemical studies. 2.4. Photocatalysis experiments Photocatalytic oxygen reduction was performed using thin films of MPS, Ag-MPS, Ag-MPS-ZnPc, Au-MPS, Au-MPS-ZnPc and MPS-ZnPc materials at different light irradiation times (20, 40 and 60 min). The film was dipped into a photolysis cell containing 15 mL of oxygen saturated solution along with 0.1 M triethanolamine (TEA) and 0.1 M HClO4. Then, the film was illuminated using a tungsten lamp with two consecutive convex lenses. Inside the cell compartment, the oxygen reduced product (hydrogen peroxide) was determined by employing a typical spectrophotometric method [27,28]. Each time, 1.5 mL of the sample solution was taken out (from the cell) in a 5 mL standard measuring flask, to which 1.5 mL
173
of 0.2 M sulfuric acid was added followed by the addition of 5% potassium iodide (2 mL). As the absorption maximum of liberated iodine is 352 nm, the absorbance value was monitored at this wavelength. The corresponding molar extinction coefficient was calculated as 21,102 L mol−1 cm−1 from a known concentration of an H2O2 sample. The reference cell was filled with solution prepared in an identical way as that of the sample solution, but in the absence of the silica films. Pure grade nitrogen and oxygen gases were used for deaeration and O2 saturation. Continuous O2 purging was sustained in the experimental solution throughout the experiment. The films were stable for weeks and can be reused. 3. Results and discussion 3.1. Characterization of photocatalysts Characterization of the prepared silica materials was elaborately discussed in our previous report [24]. Structures of the silica materials, anchoring of metal NPs onto/into the silica materials and adsorption of ZnPc were analyzed by several techniques including XPS and electrochemistry [24,25,29]. No peaks were observed in the diffuse reflectance spectra in the wavelength region of 300 to 900 nm for mercaptopropyl functionalized silica (MPS). However, Ag and Au NPs (in Ag-MPS and Au-MPS) showed two distinct surface plasmon bands at 465 nm and 550 nm, respectively, indicating the incorporation of the respective metal NPs. These two peaks were not distinctly visible in Ag-MPS-ZnPc and Au-MPS-ZnPc materials due to the ZnPc molecules, which completely cover the metal NPs, thereby blocking the corresponding absorbance. Also, the aggregation of the metal NPs prevents the observation of discrete peaks, due to the merging of the NP surface plasmon band with the ZnPc absorption band [24]. MPS-ZnPc, Ag-MPS-ZnPc and Au-MPS-ZnPc materials showed characteristic absorption bands for ZnPc, which is comparable with the reported ZnPc films [30] and in solution [24]. The TEM study revealed spherical structure (of diameter 250 to 600 nm) of the silica microspheres, and uniform distribution of Ag and Au NPs in Ag-MPS, AgMPS-ZnPc, Au-MPS and Au-MPS-ZnPc materials [24]. It should also be noted that the size of Ag and Au NPs is decreased to some extent in the presence of ZnPc [24]. The size of the Ag and Au NPs present on the Ag-MPS, Au-MPS, Ag-MPS-ZnPc and Au-MPS-ZnPc materials is measured from the TEM images [24] and the range of their sizes is given in Table 1. Ag and Au NP containing materials showed XRD reflection lines corresponding to a cubic structure of Ag and Au NPs [24]. Fig. 1 reveals the FT-IR spectra of the samples in the range of 500 to 4000 cm−1. A broad band in the region of 3400 to 3750 cm−1 (and a hump merged with the broad peak) represents the O\H stretching vibration modes of free and hydrogen bonded (`SiOH···OSi`, at around 3678 cm−1) terminal silanol groups (`SiO\H) of the mesoporous materials [31]. However, the O\H stretching vibration of trace amounts of adsorbed water also contributes in the same region. The corresponding bending vibration mode of H\O\H (adsorbed water) is signified by a band at 1633 cm−1. The intensive characteristic bands located at 1087, 797 cm−1 and 464 cm−1 (Fig. 1) can be attributed to the asymmetric stretching, symmetric stretching and bending vibrations, respectively, of `Si\O bonds of the silica framework [32]. Table 1 Size of the metal NPs formed on the different materials (measured from TEM images). Material
Size of the metal NPs (radius in nm)
Ag-MPS Ag-MPS-ZnPc Au-MPS Au-MPS-ZnPc
2 to 10 ≤3 3 to 15 2 to 4
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M. Pal et al. / Thin Solid Films 525 (2012) 172–176
Fig. 2. Core level XPS spectra of the Au-MPS-ZnPc material. Fig. 1. FT-IR spectra of (a) MPS (b) Ag-MPS (c) Au-MPS (d) MPS-ZnPc (e) Ag-MPS-ZnPc and (f) Au-MPS-ZnPc. Magnified peak position for the \SH moiety is shown in the inset.
A small red shift of the symmetric stretching vibration (from 797 cm−1 to 779 cm−1) is observed for MPS-ZnPc, Ag-MPS-ZnPc, and Au-MPSZnPc materials, which could be due to the weak interaction of the silica framework with immobilized ZnPc. The bands present at 2970 and 2929 cm−1 are assigned to the stretching vibrations caused by \CH2\ groups, indicating the presence of a mercaptopropyl moiety after the co-condensation procedure (MPS), after anchoring metal NPs (Ag-MPS and Au-MPS), and also after the adsorption of ZnPc (Ag-MPS-ZnPc and Au-MPS-ZnPc). The presence of ZnPc is clearly noticeable in the MPS-ZnPc, Ag-MPS-ZnPc and Au-MPS-ZnPc materials due to the presence of characteristic FT-IR bands of ZnPc at 886, 1061, 1285 cm−1 and 1408–1486 cm−1 [33]. Two strong bands at 728 and 777 cm−1 may be ascribed as the C\H vibrations and another band at 1455 cm−1 is due to the \CN\C_C\ moiety of ZnPc [33]. A weak vibration band at 2562 cm−1 associated with the \SH stretching vibration [34] is observed in the MPS materials (shown in the inset of Fig. 1). This band is less pronounced due to low amounts of \SH groups in the MPS material and due to the fact that they protrude into the channels of the mesoporous silica [24]. This specific band (\SH stretching) disappears in Ag-MPS, Au-MPS, Ag-MPS-ZnPc, Au-MPS-ZnPc materials, which could be due to chemisorption of metal NPs with \SH groups [34]. Detailed core level XPS of Ag-MPS, Au-MPS and Ag-MPS-ZnPc materials were monitored and already discussed in our previous article [25,29]. In this study, we focus on the XPS of the Au-MPS-ZnPc material. No impurities in the silica materials are revealed from XPS. Distinctive peaks for the S2p, C1s, Au4f, N1s and Zn2p are found in the respective materials. Fig. 2 shows the XPS spectra of the Au-MPS-ZnPc material, where all the components can be seen clearly. Data are analyzed by single/multiple Gaussian fit (Fig. 3). All the silica materials show peaks for unbound alkanethiol (\SH) with little amounts of dialkyl disulfide peaks (\S\S\) for oxidized species, where both are typically of S2p3/2 contribution. The \SH peak position in Au-MPS-ZnPc is slightly shifted down field (compared to MPS and Au-MPS) by the insertion of ZnPc, indicating that the chemical environment around the S atom has been changed considerably by the introduction of ZnPc. The C1s XP spectrum in Au-MPS-ZnPc shows four-peak curve fitting, responsible for benzene carbons (C\C), pyrrole carbons (C\N), propyl carbons (C\C) and a shake-up transition associated with the photoionization of the benzene carbons (data not shown), which is similar to the Ag-MPS-ZnPc materials [29]. Chemisorption/interaction of the metal NPs with \SH groups is recognized by a slight alteration of the peak positions. As expected for all ZnPc containing material, the N1s peak contribution is present which corresponds to the N chelated with zinc in
ZnPc. A sharp doublet peak with a peak-to-peak distance of about 3.68 eV for the Au 4f level [35] has been observed in Au-MPS and Au-MPS-ZnPc (Fig. 3). It shows two binding energies at 84.6 and 83.9 eV, which signifies the 4f7/2 level, whereas 88.3 and 87.6 eV indicate the 4f5/2 level for Au-MPS and Au-MPS-ZnPc, respectively [35]. 3.2. Photocatalytic reduction of oxygen The dry thin silica films are dipped into an acidic solution containing TEA. The films are irradiated and the products are analyzed using UV– vis absorption spectroscopy for the formation of H2O2. The yield of H2O2 formed (in μM) at different light irradiation times using different materials is determined (Fig. 4). The MPS material by itself shows no photocatalytic reduction of oxygen. The highest amounts of H2O2 are produced at the Au-MPS-ZnPc material followed by the Ag-MPS-ZnPc material. Ag-MPS and Au-MPS materials produce much less H2O2 when compared to the Ag-MPS-ZnPc and Au-MPS-ZnPc materials. The MPS-ZnPc material itself produces similar amounts of H2O2 to the Ag-MPS material. It was reported that ZnPc ion-exchanged/adsorbed on Nafion® membranes photocatalytically produces H2O2 from oxygen [10]. Similarly in this study, we believe that the photocatalytic reduction of oxygen is due to ZnPc. Throughout the photocatalytic process, ZnPc absorbs light (photon) according to its band gap photoexcitation energy and an electron is promoted from the valence band to the conduction band to produce electron (e−) hole (h+) pairs. These photogenerated electrons inside the silica film are then localized on the surface, poised
Fig. 3. Core level XPS spectra of Au 4f in the range of 82–91 eV for Au-MPS and Au-MPS-ZnPc materials together with the two-component fit.
M. Pal et al. / Thin Solid Films 525 (2012) 172–176
175
Fig. 4. Yields of H2O2 (μM) in an oxygen saturated solution containing 0.1 M HClO4 and 0.1 M TEA at thin films of MPS; Ag-MPS; Au-MPS; Ag-MPS-ZnPc and Au-MPS-ZnPc materials with different irradiation times (20, 40 and 60 min).
to interact with molecular oxygen and reduce it to O2•− as well as some other species such as HO2⋅ , HO2−, and H2O2 [36,37]. The photogenerated holes can be scavenged by the use of a hole scavenger or sacrificial electron donor like TEA, which prevents electron/hole recombination and hence the efficiency of the energy conversion system is found to be satisfactory. In fact, in the aerated media, this molecular semiconductor readily adsorbs molecular oxygen on its surface [38], and accepts photogenerated conduction band electrons, giving rise to hydrogen peroxide, while the sacrificial electron donor (TEA) is oxidized by photogenerated holes [39,40]. The yield of H2O2 reached a maximum at longer irradiation time (above 60 min). The photocatalysis of Ag-MPS and Au-MPS could be due to the semiconducting nature of the metal NPs. The presence of metal NPs enhances the oxygen reduction efficiency of ZnPc by acting as a small electron conducting center. In addition, the photocatalytic reduction of oxygen by Ag or Au NPs present in the Ag-MPS-ZnPc and Au-MPS-ZnPc materials conveniently synergizes with the reduction of oxygen by ZnPc. The introduction of ZnPc decreases the metal NP size (Table 1) and the observed decrease in the size of the Ag and Au NPs, present on the Ag-MPS-ZnPc and Au-MPS-ZnPc materials (in comparison with Ag-MPS and Au-MPS materials) could also be a reason for the increased efficiency of the Ag-MPS-ZnPc and Au-MPS-ZnPc materials. A schematic representation of this photocatalytic oxygen reduction is given in Scheme 1. The photocatalytic reactions occurring at the ZnPc adsorbed silica films can be given as in Eqs. (1)–(3) hv
−
ZnPc → e
cb
cb þ
TEA þ h
þ vb
2Hþ
−
O2 þ 2e
þh
→ H2 O2
vb →ðTEAÞox
ð1Þ ð2Þ ð3Þ
where cb and vb represent the conduction band and valance band of the ZnPc film, respectively, and (TEA)ox represents the oxidized products of the sacrificial electron donor TEA. In general, films of MPcs have been described by several investigators [10,41] as p-type semiconductors with a high density of intermediate energy levels, but the origins of such behavior have not been fully explored. The active photoconversion junction is formed on the interface between the molecular semiconductor (ZnPc) and the cell solution. According to the literature [10], the turnover number for the production of H2O2 was higher for ZnPc compared to the other MPcs, which was ascribed to a better interaction of ZnPc with oxygen than what is seen in other MPcs. Under the conditions employed, the concentration of TEA is sufficiently large and scavenges
Scheme 1. Schematic illustration of photocatalytic reduction of oxygen at metal NPs and ZnPc adsorbed silica thin films. TEA = triethanolamine, cb = conduction band, vb = valance band and Ef = Fermi level.
the h+ of ZnPc efficiently. The films are stable during the photolysis, and the detachment of complete films or particles from the film surface is rarely observed. 4. Summary Ordered mesoporous organosilica spheres with Ag or Au NPs and ZnPc were prepared and characterized by IR. Metal NPs and ZnPc photocatalyze the reduction of oxygen to H2O2 efficiently. No photocatalysis was observed for the MPS material alone which contains neither metal NPs nor ZnPc. An increased yield of H2O2 was observed in the presence of metal NPs as well as ZnPc. The presence of metal NPs and the synergic effect of ZnPc with metal NPs are believed to be the cause for the enhanced photocatalytic oxygen reduction. Among the materials studied, Au-MPS-ZnPc shows high photocatalytic activity. Acknowledgments The Council of Scientific and Industrial Research (CSIR, 01(2098)/07/ EMR-II), New Delhi, India is gratefully acknowledged for funding. We also acknowledge Prof. B. Viswanathan, Department of Chemistry, Indian Institute of Technology, Chennai, for XPS studies and Dr. Matthew T. Meredith for fruitful suggestions. MP acknowledges CSIR for SRF. References [1] In: M. Graetzel (Ed.), Energy Resources Through Photochemistry and Catalysis, Academic Press, London, 1983. [2] R. Ramaraj, M. Kaneko, Adv. Polym. Sci. 123 (1995) 215. [3] D. Schlettwein, M. Kaneko, A. Yamada, D. Wohrle, N.I. Jaeger, J. Phys. Chem. 95 (1991) 1748. [4] N. Sutin, J. Photochem. 10 (1979) 19. [5] K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, London, 1987. [6] R. Ramaraj, Res. Chem. Intermed. 27 (2001) 407. [7] F. Cavani, J.H. Teles, ChemSusChem 2 (2009) 508. [8] A. Das, V. Joshi, D. Kotkar, V.S. Pathak, V. Swayambunathan, P.V. Kamat, P.K. Ghosh, J. Phys. Chem. A 105 (2001) 6945. [9] W. Song, J. Ma, C. Chen, J. Zhao, J. Photochem. Photobiol., A 183 (2006) 31. [10] J. Premkumar, R. Ramaraj, J. Mol. Catal. A: Chem. 142 (1999) 153. [11] E.A. Lukyanets, J. Porphyrins Phthalocyanines 3 (1999) 424.
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[12] In: M. Graetzel, K. Kalyanasundaram (Eds.), Kinetics and Catalysis in Microheterogeneous System, Mercel Dekker, New York, 1991. [13] V. Ganesan, R. Ramaraj, J. Lumin. 92 (2001) 167. [14] R. Ramaraj, V.M. Kumar, C.R. Raj, V. Ganesan, J. Incl. Phenom. Macrocycl. Chem. 40 (2001) 99. [15] P. Mulvaney, Langmuir 12 (1996) 788. [16] In: H. Graber, M.H. Devoret (Eds.), Simple Charge Tunneling Coulomb Blockade Phenomena in Nanostructures, NATO ASI Series B 294, Plenum, New York, 1992. [17] L.N. Lewis, Chem. Rev. 93 (1993) 2693. [18] H. Weller, Angew. Chem. Int. Ed. 32 (1993) 41. [19] J.M. Planeix, N. Coustel, B. Coq, V. Brotons, P.S. Kumbhar, R. Dutartre, P. Geneste, P. Bernier, P.M. Ajayan, J. Am. Chem. Soc. 116 (1994) 7935. [20] V. Lordi, N. Yao, J. Wei, Chem. Mater. 13 (2001) 733. [21] J. Kong, M.G. Chapline, H. Dai, Adv. Mater. 13 (2001) 1384. [22] S. Linic, P. Christopher, D.B. Ingram, Nat. Mater. 10 (2011) 911. [23] C. Ren, Y. Song, Z. Li, G. Zhu, Anal. Bioanal. Chem. 381 (2005) 1179. [24] M. Pal, V. Ganesan, Langmuir 25 (2009) 13264. [25] M. Pal, V. Ganesan, J. Electroanal. Chem. 672 (2012) 7. [26] M. Sathish, B. Viswanathan, R.P. Viswanath, C.S. Gopinath, Chem. Mater. 17 (2005) 6349. [27] J.A. Basset, R.C. Denney, G.H. Jeffrey, J. Mendham, Vogel's Textbook of Quantitative Inorganic Analysis, Longman, London, 1978.
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Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Photochemical oxygen reduction by zinc phthalocyanine and silver/gold nanoparticle incorporated silica thin films Manas Pal, Vellaichamy Ganesan ⁎, Uday Pratap Azad Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, UP, India
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 18 October 2012 Accepted 23 October 2012 Available online 27 October 2012 Keywords: Photocatalysis Zinc phthalocyanine Silver nanoparticles Gold nanoparticles Oxygen reduction Mesoporous silica
a b s t r a c t Silver or gold nanoparticles are synthesized using a borohydride reduction method and are anchored simultaneously into/onto the mercaptopropyl functionalized silica. Later, zinc phthalocyanine is adsorbed onto the above materials. Thin films of these materials are prepared by coating an aqueous colloidal suspension of the respective material onto glass plates. Visible light irradiation of these films in oxygen saturated, stirred aqueous solutions effectively reduces oxygen to hydrogen peroxide. The photocatalytic reduction of oxygen is explained on the basis of the semiconducting properties of the silica films. The back electron transfer reaction is largely prevented by means of a sacrificial electron donor, triethanolamine. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electron transfer reactions are very important in the photoinduced chemistry of molecules [1] and this is the preliminary/ fundamental process of solar energy conversion in natural and artificial photosynthetic procedures. Light-induced electron transfer reactions have merited increasing attention among researchers [2–4] because of several useful photocatalytic reactions. Artificial photosynthesis is the replication of the function of a green plant in producing high-energy chemicals from sunlight [5]. In fact, photoinduced electron transfer reactions at heterogeneous surfaces have been realized in an artificially constructed membrane system [5,6]. Enhancing/improving the photo-efficiency of a photocatalyst is a major challenge in the field of photochemical processes taking place in heterogeneous systems. H2O2 is a clean and mild oxidant due to the formation of H2O as a byproduct [7]. Therefore, researchers in the field of photocatalysis have been extensively focusing their study on photocatalytic formation of H2O2 by reducing oxygen [7–9], which requires the development of efficient visible-light photocatalysts driven by solar energy. Concurrently, it would be advantageous for these photocatalysts to fulfill the requirements of the future environmental and energy technologies. Photochemically, oxygen can be reduced to hydrogen peroxide using metal phthalocyanines (MPcs) [10], which can also act as
⁎ Corresponding author. Tel.: +91 542 2307321; fax: +91 542 2368127. E-mail addresses: [email protected], [email protected] (V. Ganesan). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.049
photosensitizers [11] for photoredox reactions both in homogeneous and heterogeneous media [5,6,10]. Some other approaches are the utilization of microheterogeneous reaction media (i.e. solid–solution interfaces) such as vesicles, micelles, monolayers and polyelectrolytes [1,12–14], which include heterogeneous catalyst systems and chemically modified electrodes. Likewise, noble metal nanoparticles (NPs) having plasmon resonance exhibit strong UV–vis absorption by means of the collective oscillations of surface electrons. These metal NPs have unique physiochemical thermodynamic properties and also have unique optical [15], electronic [16] and catalytic [17] (including photocatalytic [18]) properties which are distinctly different from their bulk counterparts and thereby lead to several applications in heterogeneous catalysts [17,19,20] and molecular sensors [21]. Several studies on metal NPs show that they represent plasmonic photocatalysts [22], and can facilitate electron transfer events by acting as a tiny electron conduction center [23]. While fundamentally promising, the development of future applications for metal NPs will depend on the adherence of the nano-scaled materials on suitable substrates (which are in two- or three-dimensional arrangements) in order to take advantage of their fascinating properties under different conditions. Hence, our purpose has been focused on increasing the photocatalytic efficiency of NPs using templated silica [24,25]. In this work, catalytic materials (zinc phthalocyanine (ZnPc) and metal NPs) have been immobilized within a mesoporous silica network, which is beneficial due to its easy separation from the reaction medium and also due to the small amount of catalyst material needed for the reaction. The utility of immobilized catalysts is drastically evident since they are not leached from the network and are quite active and stable.
M. Pal et al. / Thin Solid Films 525 (2012) 172–176
2. Experimental details 2.1. Physicochemical characterization techniques Fourier transform infrared (FT-IR) spectra were recorded using a PerkinElmer FT-IR Spectrometer (Spectrum Two) or a Varian 3100 FT-IR (Excalibur series, USA) spectrometer. UV–vis absorption spectrophotometers (UV 1700 Pharma Spec, Shimadzu for diffuse reflectance measurements and 2802 PC UNICO, USA for absorbance measurements), X-ray powder diffractometer (XRD) with transmission geometry and Cu-Kα radiation source (ID 3000 SEIFERT, Germany) and transmission electron microscope (TEM, TECNAI 20G 2 FEI microscope, operating at 200 kV) were used for the respective characterization of the materials. A home-made photolysis set-up with a 500 W tungsten lamp and focusing lenses was used to irradiate the thin silica films. X-ray photoelectron spectra (XPS) were collected from a VG Microtech Multilab ESCA 3000 spectrometer with a non-monochromatized Al Kα X-ray (hν = 1486.6 eV). Binding energy was calibrated with the Au 4f7/2 core level at 83.9 eV [26]. To remove the atmospheric surface contamination (water and CO2) on the catalyst pellet surface, the pellet was scraped freshly just before the experiment. 2.2. Chemicals and reagents Mercaptopropyltriethoxysilane, tetraethoxysilane, zinc phthalocyanine (ZnPc) and hydrogen tetrachloroaurate (III) solution (HAuCl4) from Aldrich, cetyltrimethylammonium bromide (CTAB) from Himedia and silver nitrate from Merck were used in this study. The remaining reagents (analytical grade) were purchased from Sd fine chemicals, India. HAuCl4 and AgNO3 were used as metal NP precursors and NaBH4 was used as the reducing agent [24]. Our previously reported synthetic approach [24] was implicated to prepare the silica materials. Therefore the mercaptopropyl functionalized silica (MPS), Ag or Au nanoparticles (Ag or Au NPs respectively) anchored MPS (represented as Ag-MPS or Au-MPS respectively), ZnPc adsorbed Ag-MPS or Au-MPS (represented as Ag-MPS-ZnPc or Au-MPS-ZnPc respectively) materials were available from our previous study [24]. 2.3. Preparation of thin films Thin films of the above materials were prepared on clean glass plates of 1.0 cm 2 area. 1.0% colloidal solution of the respective materials containing 0.01% of poly(vinyl alcohol) was used for the film preparations. 100 μL of a colloidal solution of the respective material was dropped on a glass plate, which was then allowed to dry in air in the absence of light. Since the films were all prepared similarly, the amount of material present on the glass plate is expected to be the same for all cases. However, small changes in the film thickness are expected since it depends also on the density of the respective materials. After the preparation of the films, they were used within 24 h for photochemical studies. 2.4. Photocatalysis experiments Photocatalytic oxygen reduction was performed using thin films of MPS, Ag-MPS, Ag-MPS-ZnPc, Au-MPS, Au-MPS-ZnPc and MPS-ZnPc materials at different light irradiation times (20, 40 and 60 min). The film was dipped into a photolysis cell containing 15 mL of oxygen saturated solution along with 0.1 M triethanolamine (TEA) and 0.1 M HClO4. Then, the film was illuminated using a tungsten lamp with two consecutive convex lenses. Inside the cell compartment, the oxygen reduced product (hydrogen peroxide) was determined by employing a typical spectrophotometric method [27,28]. Each time, 1.5 mL of the sample solution was taken out (from the cell) in a 5 mL standard measuring flask, to which 1.5 mL
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of 0.2 M sulfuric acid was added followed by the addition of 5% potassium iodide (2 mL). As the absorption maximum of liberated iodine is 352 nm, the absorbance value was monitored at this wavelength. The corresponding molar extinction coefficient was calculated as 21,102 L mol−1 cm−1 from a known concentration of an H2O2 sample. The reference cell was filled with solution prepared in an identical way as that of the sample solution, but in the absence of the silica films. Pure grade nitrogen and oxygen gases were used for deaeration and O2 saturation. Continuous O2 purging was sustained in the experimental solution throughout the experiment. The films were stable for weeks and can be reused. 3. Results and discussion 3.1. Characterization of photocatalysts Characterization of the prepared silica materials was elaborately discussed in our previous report [24]. Structures of the silica materials, anchoring of metal NPs onto/into the silica materials and adsorption of ZnPc were analyzed by several techniques including XPS and electrochemistry [24,25,29]. No peaks were observed in the diffuse reflectance spectra in the wavelength region of 300 to 900 nm for mercaptopropyl functionalized silica (MPS). However, Ag and Au NPs (in Ag-MPS and Au-MPS) showed two distinct surface plasmon bands at 465 nm and 550 nm, respectively, indicating the incorporation of the respective metal NPs. These two peaks were not distinctly visible in Ag-MPS-ZnPc and Au-MPS-ZnPc materials due to the ZnPc molecules, which completely cover the metal NPs, thereby blocking the corresponding absorbance. Also, the aggregation of the metal NPs prevents the observation of discrete peaks, due to the merging of the NP surface plasmon band with the ZnPc absorption band [24]. MPS-ZnPc, Ag-MPS-ZnPc and Au-MPS-ZnPc materials showed characteristic absorption bands for ZnPc, which is comparable with the reported ZnPc films [30] and in solution [24]. The TEM study revealed spherical structure (of diameter 250 to 600 nm) of the silica microspheres, and uniform distribution of Ag and Au NPs in Ag-MPS, AgMPS-ZnPc, Au-MPS and Au-MPS-ZnPc materials [24]. It should also be noted that the size of Ag and Au NPs is decreased to some extent in the presence of ZnPc [24]. The size of the Ag and Au NPs present on the Ag-MPS, Au-MPS, Ag-MPS-ZnPc and Au-MPS-ZnPc materials is measured from the TEM images [24] and the range of their sizes is given in Table 1. Ag and Au NP containing materials showed XRD reflection lines corresponding to a cubic structure of Ag and Au NPs [24]. Fig. 1 reveals the FT-IR spectra of the samples in the range of 500 to 4000 cm−1. A broad band in the region of 3400 to 3750 cm−1 (and a hump merged with the broad peak) represents the O\H stretching vibration modes of free and hydrogen bonded (`SiOH···OSi`, at around 3678 cm−1) terminal silanol groups (`SiO\H) of the mesoporous materials [31]. However, the O\H stretching vibration of trace amounts of adsorbed water also contributes in the same region. The corresponding bending vibration mode of H\O\H (adsorbed water) is signified by a band at 1633 cm−1. The intensive characteristic bands located at 1087, 797 cm−1 and 464 cm−1 (Fig. 1) can be attributed to the asymmetric stretching, symmetric stretching and bending vibrations, respectively, of `Si\O bonds of the silica framework [32]. Table 1 Size of the metal NPs formed on the different materials (measured from TEM images). Material
Size of the metal NPs (radius in nm)
Ag-MPS Ag-MPS-ZnPc Au-MPS Au-MPS-ZnPc
2 to 10 ≤3 3 to 15 2 to 4
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Fig. 2. Core level XPS spectra of the Au-MPS-ZnPc material. Fig. 1. FT-IR spectra of (a) MPS (b) Ag-MPS (c) Au-MPS (d) MPS-ZnPc (e) Ag-MPS-ZnPc and (f) Au-MPS-ZnPc. Magnified peak position for the \SH moiety is shown in the inset.
A small red shift of the symmetric stretching vibration (from 797 cm−1 to 779 cm−1) is observed for MPS-ZnPc, Ag-MPS-ZnPc, and Au-MPSZnPc materials, which could be due to the weak interaction of the silica framework with immobilized ZnPc. The bands present at 2970 and 2929 cm−1 are assigned to the stretching vibrations caused by \CH2\ groups, indicating the presence of a mercaptopropyl moiety after the co-condensation procedure (MPS), after anchoring metal NPs (Ag-MPS and Au-MPS), and also after the adsorption of ZnPc (Ag-MPS-ZnPc and Au-MPS-ZnPc). The presence of ZnPc is clearly noticeable in the MPS-ZnPc, Ag-MPS-ZnPc and Au-MPS-ZnPc materials due to the presence of characteristic FT-IR bands of ZnPc at 886, 1061, 1285 cm−1 and 1408–1486 cm−1 [33]. Two strong bands at 728 and 777 cm−1 may be ascribed as the C\H vibrations and another band at 1455 cm−1 is due to the \CN\C_C\ moiety of ZnPc [33]. A weak vibration band at 2562 cm−1 associated with the \SH stretching vibration [34] is observed in the MPS materials (shown in the inset of Fig. 1). This band is less pronounced due to low amounts of \SH groups in the MPS material and due to the fact that they protrude into the channels of the mesoporous silica [24]. This specific band (\SH stretching) disappears in Ag-MPS, Au-MPS, Ag-MPS-ZnPc, Au-MPS-ZnPc materials, which could be due to chemisorption of metal NPs with \SH groups [34]. Detailed core level XPS of Ag-MPS, Au-MPS and Ag-MPS-ZnPc materials were monitored and already discussed in our previous article [25,29]. In this study, we focus on the XPS of the Au-MPS-ZnPc material. No impurities in the silica materials are revealed from XPS. Distinctive peaks for the S2p, C1s, Au4f, N1s and Zn2p are found in the respective materials. Fig. 2 shows the XPS spectra of the Au-MPS-ZnPc material, where all the components can be seen clearly. Data are analyzed by single/multiple Gaussian fit (Fig. 3). All the silica materials show peaks for unbound alkanethiol (\SH) with little amounts of dialkyl disulfide peaks (\S\S\) for oxidized species, where both are typically of S2p3/2 contribution. The \SH peak position in Au-MPS-ZnPc is slightly shifted down field (compared to MPS and Au-MPS) by the insertion of ZnPc, indicating that the chemical environment around the S atom has been changed considerably by the introduction of ZnPc. The C1s XP spectrum in Au-MPS-ZnPc shows four-peak curve fitting, responsible for benzene carbons (C\C), pyrrole carbons (C\N), propyl carbons (C\C) and a shake-up transition associated with the photoionization of the benzene carbons (data not shown), which is similar to the Ag-MPS-ZnPc materials [29]. Chemisorption/interaction of the metal NPs with \SH groups is recognized by a slight alteration of the peak positions. As expected for all ZnPc containing material, the N1s peak contribution is present which corresponds to the N chelated with zinc in
ZnPc. A sharp doublet peak with a peak-to-peak distance of about 3.68 eV for the Au 4f level [35] has been observed in Au-MPS and Au-MPS-ZnPc (Fig. 3). It shows two binding energies at 84.6 and 83.9 eV, which signifies the 4f7/2 level, whereas 88.3 and 87.6 eV indicate the 4f5/2 level for Au-MPS and Au-MPS-ZnPc, respectively [35]. 3.2. Photocatalytic reduction of oxygen The dry thin silica films are dipped into an acidic solution containing TEA. The films are irradiated and the products are analyzed using UV– vis absorption spectroscopy for the formation of H2O2. The yield of H2O2 formed (in μM) at different light irradiation times using different materials is determined (Fig. 4). The MPS material by itself shows no photocatalytic reduction of oxygen. The highest amounts of H2O2 are produced at the Au-MPS-ZnPc material followed by the Ag-MPS-ZnPc material. Ag-MPS and Au-MPS materials produce much less H2O2 when compared to the Ag-MPS-ZnPc and Au-MPS-ZnPc materials. The MPS-ZnPc material itself produces similar amounts of H2O2 to the Ag-MPS material. It was reported that ZnPc ion-exchanged/adsorbed on Nafion® membranes photocatalytically produces H2O2 from oxygen [10]. Similarly in this study, we believe that the photocatalytic reduction of oxygen is due to ZnPc. Throughout the photocatalytic process, ZnPc absorbs light (photon) according to its band gap photoexcitation energy and an electron is promoted from the valence band to the conduction band to produce electron (e−) hole (h+) pairs. These photogenerated electrons inside the silica film are then localized on the surface, poised
Fig. 3. Core level XPS spectra of Au 4f in the range of 82–91 eV for Au-MPS and Au-MPS-ZnPc materials together with the two-component fit.
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Fig. 4. Yields of H2O2 (μM) in an oxygen saturated solution containing 0.1 M HClO4 and 0.1 M TEA at thin films of MPS; Ag-MPS; Au-MPS; Ag-MPS-ZnPc and Au-MPS-ZnPc materials with different irradiation times (20, 40 and 60 min).
to interact with molecular oxygen and reduce it to O2•− as well as some other species such as HO2⋅ , HO2−, and H2O2 [36,37]. The photogenerated holes can be scavenged by the use of a hole scavenger or sacrificial electron donor like TEA, which prevents electron/hole recombination and hence the efficiency of the energy conversion system is found to be satisfactory. In fact, in the aerated media, this molecular semiconductor readily adsorbs molecular oxygen on its surface [38], and accepts photogenerated conduction band electrons, giving rise to hydrogen peroxide, while the sacrificial electron donor (TEA) is oxidized by photogenerated holes [39,40]. The yield of H2O2 reached a maximum at longer irradiation time (above 60 min). The photocatalysis of Ag-MPS and Au-MPS could be due to the semiconducting nature of the metal NPs. The presence of metal NPs enhances the oxygen reduction efficiency of ZnPc by acting as a small electron conducting center. In addition, the photocatalytic reduction of oxygen by Ag or Au NPs present in the Ag-MPS-ZnPc and Au-MPS-ZnPc materials conveniently synergizes with the reduction of oxygen by ZnPc. The introduction of ZnPc decreases the metal NP size (Table 1) and the observed decrease in the size of the Ag and Au NPs, present on the Ag-MPS-ZnPc and Au-MPS-ZnPc materials (in comparison with Ag-MPS and Au-MPS materials) could also be a reason for the increased efficiency of the Ag-MPS-ZnPc and Au-MPS-ZnPc materials. A schematic representation of this photocatalytic oxygen reduction is given in Scheme 1. The photocatalytic reactions occurring at the ZnPc adsorbed silica films can be given as in Eqs. (1)–(3) hv
−
ZnPc → e
cb
cb þ
TEA þ h
þ vb
2Hþ
−
O2 þ 2e
þh
→ H2 O2
vb →ðTEAÞox
ð1Þ ð2Þ ð3Þ
where cb and vb represent the conduction band and valance band of the ZnPc film, respectively, and (TEA)ox represents the oxidized products of the sacrificial electron donor TEA. In general, films of MPcs have been described by several investigators [10,41] as p-type semiconductors with a high density of intermediate energy levels, but the origins of such behavior have not been fully explored. The active photoconversion junction is formed on the interface between the molecular semiconductor (ZnPc) and the cell solution. According to the literature [10], the turnover number for the production of H2O2 was higher for ZnPc compared to the other MPcs, which was ascribed to a better interaction of ZnPc with oxygen than what is seen in other MPcs. Under the conditions employed, the concentration of TEA is sufficiently large and scavenges
Scheme 1. Schematic illustration of photocatalytic reduction of oxygen at metal NPs and ZnPc adsorbed silica thin films. TEA = triethanolamine, cb = conduction band, vb = valance band and Ef = Fermi level.
the h+ of ZnPc efficiently. The films are stable during the photolysis, and the detachment of complete films or particles from the film surface is rarely observed. 4. Summary Ordered mesoporous organosilica spheres with Ag or Au NPs and ZnPc were prepared and characterized by IR. Metal NPs and ZnPc photocatalyze the reduction of oxygen to H2O2 efficiently. No photocatalysis was observed for the MPS material alone which contains neither metal NPs nor ZnPc. An increased yield of H2O2 was observed in the presence of metal NPs as well as ZnPc. The presence of metal NPs and the synergic effect of ZnPc with metal NPs are believed to be the cause for the enhanced photocatalytic oxygen reduction. Among the materials studied, Au-MPS-ZnPc shows high photocatalytic activity. Acknowledgments The Council of Scientific and Industrial Research (CSIR, 01(2098)/07/ EMR-II), New Delhi, India is gratefully acknowledged for funding. We also acknowledge Prof. B. Viswanathan, Department of Chemistry, Indian Institute of Technology, Chennai, for XPS studies and Dr. Matthew T. Meredith for fruitful suggestions. MP acknowledges CSIR for SRF. References [1] In: M. Graetzel (Ed.), Energy Resources Through Photochemistry and Catalysis, Academic Press, London, 1983. [2] R. Ramaraj, M. Kaneko, Adv. Polym. Sci. 123 (1995) 215. [3] D. Schlettwein, M. Kaneko, A. Yamada, D. Wohrle, N.I. Jaeger, J. Phys. Chem. 95 (1991) 1748. [4] N. Sutin, J. Photochem. 10 (1979) 19. [5] K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, London, 1987. [6] R. Ramaraj, Res. Chem. Intermed. 27 (2001) 407. [7] F. Cavani, J.H. Teles, ChemSusChem 2 (2009) 508. [8] A. Das, V. Joshi, D. Kotkar, V.S. Pathak, V. Swayambunathan, P.V. Kamat, P.K. Ghosh, J. Phys. Chem. A 105 (2001) 6945. [9] W. Song, J. Ma, C. Chen, J. Zhao, J. Photochem. Photobiol., A 183 (2006) 31. [10] J. Premkumar, R. Ramaraj, J. Mol. Catal. A: Chem. 142 (1999) 153. [11] E.A. Lukyanets, J. Porphyrins Phthalocyanines 3 (1999) 424.
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