- Email: [email protected]
Fluorescence resonance energy transfer and surface Plasmon resonance induced enhanced photoluminescence and photoconductivity property of Au–TiO2 metal–semiconductor nanocomposite
Optical Materials 40 (2015) 97–101
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fluorescence resonance energy transfer and surface Plasmon resonance induced enhanced photoluminescence and photoconductivity property of Au–TiO2 metal–semiconductor nanocomposite S. Majumder a,b,1, S.K. Jana b,⇑,1, K. Bagani b, B. Satpati b, S. Kumar a, S. Banerjee b a b
Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, India Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Saltlake, Kolkata 700064, India
a r t i c l e
i n f o
Article history: Received 23 August 2014 Received in revised form 23 November 2014 Accepted 1 December 2014 Available online 18 December 2014 Keywords: Nanocomposites Luminescence Photoconductivity Au TiO2 Sol–gel preparation
a b s t r a c t In this manuscript, we have successfully synthesized Au–TiO2 nanocomposite by sol–gel process. Photoluminescence (PL) and photoconductivity (PC) study have been performed on both bare TiO2 and Au–TiO2 composite samples. We observe enhancement of photoluminescence emission in Au–TiO2 nanocomposite compared to bare TiO2. The enhanced photoluminescence is ascribed owing to combined influence of fluorescence resonance energy transfer and surface Plasmon resonance effect due to presence of Au nanoparticles in nanocomposite sample. Also this nanocomposite exhibits enhanced UV photocurrent because of fluorescence resonance energy transfer by metallic Au attached with TiO2. Interfacial charge transfer can also be realized by the electrochemical impedance spectroscopy results. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal and metal oxide based nanocomposite have been enormously studied due to their excellent photocatalytic, electrical, optical, magnetic and mechanical properties [1]. These types of nanocomposites are widely used in gas sensor [2], catalyst [3–4] and bio-sensing application [5]. The optical property and photoconductivity study of metal–semiconductor nanocomposites are interesting because of their use in future nanoelectronic devices [1]. The effect of metal doping in TiO2 has been discussed in some earlier publications [5,6]. Presence of metal in the TiO2 matrix facilitates electron transfer from photoexcited semiconductor materials to the surroundings and decreases electrons–holes recombination rate which supports the enhanced photoelectrochemical and photocatalytic property of metal semiconductor nanocomposite system [7,8]. Photocatalytic effect of noble metal nanoparticles supported oxide semiconductors have been reported previously [9,10]. Enhanced UV photocatalytic performance can be attributed due to reduced electron–hole recombination in the semiconductor attached with noble metal nanoparticles [11]. However, the drastic PC change is obtained because of band gap
modulation of the system and electron distribution between metal and semiconductor [7,12]. In this manuscript, we have extensively studied both PL and PC property on Au–TiO2 nanocomposite system. We observe the enhancement of PL emission and PC in this composite system compared to pristine TiO2. Improved photocurrent is observed in Au–TiO2 under UV illumination while fluorescence resonance energy transfer (FRET) modifies the surface states in composite system due to Au loading which enhances the PL signal of the same sample. FRET induced band edge emission in Ag nanoparticle sensitized CdSe–ZnS/ZnO heterostructure and suppression of defect emission compared to bare ZnO NRs was analyzed in earlier report [13]. A possible energy transfer mechanism to explain the improved PL properties of ZnO NRs is proposed based upon the combined effects of FRET and surface Plasmon resonance (SPR). Here, we have shown the Au nanoparticle induced enhanced defect related emission of TiO2. FRET mechanism is demonstrated also by Electrochemical Impedance Spectroscopy (EIS) measurement. 2. Experimental 2.1. Chemical used
⇑ Corresponding author. 1
E-mail address: [email protected] (S.K. Jana). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.optmat.2014.12.001 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Chloroauric acid (HAuCl4), Trisodium citrate dehydrate (C6H5Na3O72H2O), Mercaptoundecanoic acid (MPA), NH4OH
98
S. Majumder et al. / Optical Materials 40 (2015) 97–101
(28%), Titanium isopropoxide (TTIP), Triethanolamine (TEOA) were purchased from Merck India Pvt. Ltd. Technical grade ethanol and milli-pore water (18.2 MX) were used during the preparation of the samples. All the reagents were used as received. 2.2. Detail preparation method The following section describes step by step synthesis processes of Au–TiO2 nanocomposite [14]. 2.2.1. Synthesis of Au nanoparticle 1 mM HAuCl4 (25 ml) solution was used as stock solution for synthesis of Au NP. At first, the stock solution was heated under constant stirring condition. When the temperature of the solution was reached to 80 °C, then 2.4 ml (1 wt%) trisodium citrate solution was added under continuous stirring. Within few minutes, we observed the color change of the solution from yellow to grey and ultimately red wine colored solution visualized. This red wine solution confirms gold (Au) NP solution. 2.2.2. Synthesis of MPA coated Au nanoparticle We have prepared 0.5 mM mercraptopropanoic acid solution (MPA) and dissolved in 2.5% aqueous ammonia solution. The mixture was added to the Au NP solution and incubated for 2 h. Then the solution was centrifuged further at 3800 rpm. The resultant MPA coated Au NPs were collected. Subsequently, 20 ml of water was added to the resultant MPA coated Au NPs solution and re-dispersed again in 20 ll of 28 wt% ammonia solution. This was used as further stock solution of Au sol to prepare Au–TiO2 nanocomposite. 2.2.3. Preparation of Ti4+ stock solution A stock solution of Ti4+ was prepared by mixing titanium isopropoxide (TTIP) with triethanolamine (TEOA) under Ar atmosphere at a molar ratio of 2:1 (TEOA/TTIP) to form a stable complex of Ti4+–TEOA. This complex agent was used to control the hydrolysis reaction velocity of TTIP [13]. A stable Ti4+ compound was prepared at room temperature by adding the TEOA. Double distilled water was then added to make 0.50 M Ti4+ solutions. The stock solution was filtered for further use. 2.2.4. Preparation of Au–TiO2 nanocomposite The concentrations of Ti4+ in the stock solution were adjusted to 0.01–0.3 mM. Then, 4.95 ml of the MPA coated Au colloid was added to 150 ml of the Ti4+ stock solution. The mixture was then refluxed for 90 min at 200 °C. During this process, TiO2 was deposited onto the surface of Au nanoparticle. The resulting nanocomposite was washed and dried at 65 °C under vacuum. 3. Results and discussions Room temperature XRD (Bruker D8 Advanced Diffractometer) pattern shown in Fig. 1a was obtained to identify the structural information and crystal phase Au–TiO2 nanocomposite system. All the diffraction peaks are indexed as a mixture of Au and TiO2. The diffraction peaks at 2h = 38.40°, 44.47°, 64.76°, 77.77° corresponds to (1 1 1), (2 0 0), (2 2 0), (3 1 1) planes of Au and the diffraction peaks at 2h = 39.46°, 46.00°, 67.24° are matched with (0 4 4), (2 0 0), (2 2 0) planes of anatase TiO2 respectively. Fig. 1b and c show the SEM (FEI Quanta 200) and TEM (FEI, Tecnai F30) images of Au–TiO2 nanocomposite structure respectively. In the SEM image, it is observed that Au NPs are almost homogeneously distributed, although there is a little signature of agglomerations of Au nanoparticles. From the TEM image we see that metallic gold nanoparticles are embedded in TiO2 matrix. Inset image of Fig. 1c
shows the HRTEM image of metallic gold nanostructures attached with TiO2. Fig. 2d shows the high angle annular dark-field scanning/transmission electron microscopy (STEM–HAADF) images of Au–TiO2 nanocomposite. Selected area EDX analysis of Au–TiO2 nanocomposite is shown in the inset of Fig. 1d which confirms elemental composition of the system. UV–Vis spectra recorded by JASCO V-630 spectrophotometer of bare TiO2 and Au–TiO2 nanocomposites are shown in Fig. 2a. The UV–Vis spectra of TiO2 shows an intense peak at around 300 nm, which is the excitonic peak of TiO2 itself. The spectrum of Au–TiO2 nanocomposite shows two peaks at around 300 nm due to TiO2 and another peak at around 500 nm due to surface Plasmon peak of Au metal NPs respectively [7]. Room temperature PL measurements (shown in Fig. 2b) of both the samples have been carried out in JASCO FP-6700 spectrophotometer at an excitation wavelength of 310 nm. In Fig. 2(b), PL spectra of TiO2 shows six distinct peaks within the wavelength range 350–500 nm where a sharp peak at around 356–398 nm is due to band edge emission of TiO2 semiconductor [15]. Moreover, a blue–green emission band, which consists of few peaks at 450, 468, 482 and 495 nm in the PL spectrum, is observed. The origin of blue–green emission band is because of transition of electrons from shallow donor level of the oxygen vacancies to the valence band [16]. PL emission spectra of Au–TiO2 with peak intensity around 450 nm compared to bare TiO2 is observed in the same wavelength region. Enhanced PL emission in Au–TiO2 nanocomposite can be illustrated by FRET mechanism [13] shown schematically in Fig. 3. Au nanoparticles attached with TiO2 quench the band edge emission of TiO2 as shown in Fig. 2b. In bare TiO2, some non-radiative transition may possible caused by dangling bonds and surface states as shown in Fig. 3. Defects induced blue–green emission of TiO2 attribute to the non-radiative energy transfer from TiO2 to Au NPs. This FRET could cause surface Plasmon resonance (SPR) of the Au nanoparticles, as the SPR occurs around 500 nm which is observed in Fig. 2a. The possible energy transfer mechanism involved with FRET process is described sequentially as follows. (i) Electron transfer from V.B to C.B in TiO2 under UV excitation (310 nm). (ii) Electron transfer from C.B of TiO2 to Au Nanoparticles through non radiative process. (iii) Radiative emission from donor states to V.B of TiO2 (400–500 nm) originates the SPR (as the absorption band of Au lies between 420 and 530 nm). (iv) At SPR all electrons (those are involved with non-radiative transition and surface charge of Au Nanoparticles at SPR) relax to the V.B of TiO2 through radiative emission. Near field enhancement of the Au nanoparticles can affect the photon flux to the TiO2. In addition, defect related emission by TiO2 induced surface Plasmon resonance of Au NPs which increases the radiative recombination rates. Synergistic effect of both FRET and SPR mechanism is the reason for the enhancement of defect related emission of Au–TiO2 compared to bare TiO2. Thus energy transfer occurs between Au and TiO2 and electron accumulation takes place in Au. This electron accumulation due to charge transfer within Au–TiO2 reduces trap level and majority of electrons relax radiatively to the valance band. Enhanced PL emission is not only due to reducing trap level but also because of coupling effect between local surface plasmons (SPs) and excitons [17]. This is the reason why we observed the enhancement of emission peaks of Au–TiO2 nanocomposite compared to bare TiO2. The photoconductivity response under steady state illumination of both the samples is shown in Fig. 4a and b for bare TiO2 and Au–TiO2 respectively. These graphs show the different dark value of the two samples which were kept in the dark condition until the current reached at equilibrium. It is clear from both figures, initially dark current starts decreasing slowly until it achieves the stable value (marked by upward arrow in both Fig. 4a and b respectively). This may be attributed to adsorption of oxygen
99
S. Majumder et al. / Optical Materials 40 (2015) 97–101
Fig. 1. (a) XRD pattern, (b) SEM image, (c) TEM and HRTEM image (inset), and (d) STEM-HAADF image of Au–TiO2 nanocomposite and inset figure shows the EDX analysis of the nanocomposite.
Fig. 2. (a) UV–Vis and (b) photoluminescence study of both bare and Au–TiO2 nanocomposite respectively.
molecule as well as due to presence of defects [18]. In the absence of UV light, oxygen is absorbed by taking a free electron (e) from the surface of both bare TiO2 and Au–TiO2 particles to form chemisorbed surface state and develop a depletion layer near the surface of both samples with low conductivity. We have described these in the following reaction.
O2 þ e ! O2
ð1Þ
Thus the surface of both particles is almost depleted with charge carriers leads to a high resistance in the dark state [19,20]. Upon UV illumination, the photogenerated holes are produced and release the captured species (O 2 ion) by leaving behind
þ
an electron, i.e. O 2 þ h ! O2 ðgÞ. The adsorbed oxygen molecules are released in air, which lowers the barrier height of electron in TiO2. This mechanism was proposed by Muraoka et al. [21]. Once all photoinduced holes react with O 2 , photo current have a tendency to stabilize for both TiO2 and Au–TiO2 cases. As soon as UV is OFF (indicated by downward arrow) oxygen gets absorbed and starts to develop a depletion layer at the surface of material, so current tends to decrease as shown in both Fig. 4. Also it is notified that the ratio photocurrent to dark current (photosensitivity 5) of Au–TiO2 is higher compared to bare TiO2 (photosensitivity 2.7). Under UV illumination, the electron transfer between Au and TiO2 continues until two systems attain equilibrium position
100
S. Majumder et al. / Optical Materials 40 (2015) 97–101
Fig. 3. Enhanced photoluminescence emission through FRET mechanism.
Fig. 5. EIS spectra of both bare TiO2 and Au–TiO2 composite under UV illumination and the Randle circuit shown in the inset of this graph.
Fig. 4. Photoconductivity response curve of (a) bare TiO2 and (b) Au–TiO2 nanocomposite respectively.
and the Fermi level (EF) of the total system lies on a single line. The transfer of electron from TiO2 semiconductor to Au under UV illumination results the accumulation of electron in Au–TiO2 nanocomposite. The accumulation of electron shifts the Fermi level of Au metal and reduces the barrier height, and thereby delays of
the recombination of electron hole pair which is responsible for enhancement of the photosensitivity of Au–TiO2 nanocomposite [7]. Electron accumulation and interfacial charge transfer can also be verified electrochemical impedance spectroscopy measurement. EIS spectra taken at frequency range between 100 Hz and 100KHz was recorded by CH Instruments 660 in three electrode configuration adapted with ITO coated sample (both bare and Au–TiO2 nanocomposite) as working electrode, Pt wire as counter electrode and Ag/AgCl as reference electrode. The electrochemical measurement was done in 0.1M Na2SO4 solution. EIS spectrum of both bare and Au nanoparticle attached TiO2 under UV illumination is depicted in Fig. 5. EIS spectra can be divided into two regions: a semicircle formed in the higher frequency region and the straight line formed in the lower frequency region. An equivalent circuit model (shown in the inset of Fig. 5) is proposed to
S. Majumder et al. / Optical Materials 40 (2015) 97–101
simplify the electrochemical process at electrode–electrolyte interface, where Rs is the solution resistance indicated by the first intercept of the semicircle to the real impedance axis, the second intercept corresponds to the charge transfer resistance (Rct), straight line signifies the Warburg impedance, Cdl is the Helmholtz double layer capacitance formed at the working electrode and electrolyte interface. The smaller Rct value of Au–TiO2 than that of bare TiO2 reveals lower charge transfer resistance at the electrode/ electrolyte interface, which is attributed to the improved interfacial charge transfer between the electroactive TiO2 and conductive Au nanoparticle attached with TiO2. Under UV illumination electrons and holes pairs (EHPs) are generated and some fractions of EHP can recombine at the surface of the bare TiO2 while in composite sample these photogenerated EHPs transferred from TiO2 surface to Au nanoparticles. Thus accumulation of charge in the composite system through interfacial charge transfer can be realized.
4. Conclusions In summary, we have successfully synthesized Au–TiO2 nanocomposite by sol–gel process. The enhancement of PL emission and UV photosensitivity study of this nanostructure have been investigated. Significant enhanced PL emission of Au–TiO2 nanocomposite spectra is obtained because of increasing radiative recombination rate and coupling effect of SPR and FRET mechanism. Under UV illumination, this nanocomposite exhibits also improved photosensitivity due to electron accumulation by the interfacial charge transfer in the composite system. So Au–TiO2 nanocomposite can be used for UV photo detection, optical switches and solar cells applications.
101
Acknowledgement Authors would like to acknowledge to Dr. S. K. Mishra, currently post-doctoral fellow of Lucknow University, India, for PC measurements of the samples. References [1] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Am. Chem. Soc. 126 (2004) 4943– 4950. [2] W. Jia, L. Su, Y. Ding, A. Schempf, Y. Wang, Y. Lei, J. Phys. Chem. C 113 (2009) 16402–16407. [3] D.I. Enache, J.K. Edwards, P. Landon, B.S. Espriu, A.F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362–365. [4] L. Su, W. Jia, A. Schempf, Y. Lei, Electrochem. Commun. 11 (2009) 2199–2202. [5] S.F. Chen, J.P. Li, K. Qian, W.P. Xu, Y. Lu, W.X. Huang, S.H. Yu, Nano Res. 3 (2010) 244–255. [6] G. Zhao, H. Kozuka, T. Yoko, Thin Solid Films 277 (1996) 147–154. [7] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Phys. Chem. B 105 (2001) 11439– 11446. [8] S.K. Jana, T. Majumder, S. Banerjee, J. Electroanal. Chem. 727 (2014) 99–103. [9] B. Kraeutler, A.J. Bard, J. Am. Chem. Soc. 100 (1978) 4317–4318. [10] Y. Inel, D. Ertek, Faraday Trans. 89 (1993) 129–133. [11] W. Zhou, H. Fu, Chem. Cat. Chem. 5 (2013) 885–894. [12] A. Bumajdad, M. Madkour, Phys. Chem. Chem. Phys. 16 (2014) 7146–7158. [13] J.Y. Chang, T.G. Kim, Y.M. Sung, Nanotechnology 22 (2011) 425708–425714. [14] H.W. Kwon, Y.M. Lim, S.K. Tripathy, B.G. Kim, Jpn. J. Appl. Phys. 46 (2007) 2567–2570. [15] K.M. Rahulan, S. Ganesan, P. Aruna, Adv. Nat. Sci. Nanosci. Nanotechnol. 2 (2011) 025012–025016. [16] B. Choudhury, M. Dey, A. Choudhury, Appl. Nanosci. 4 (2014) 499–506. [17] A. Neogi, C.W. Lee, H.O. Everitt, T. Kuroda, A. Tackeuchi, E. Yablonovitch, Phys. Rev. B: Condens. Matter Mater. Phys. 66 (2002) 153305–153308. [18] S. K Mishra, R.K. Srivastava, S.G. Prakash, R.S. Yadav, A.C. Pandey, Electron. Mater. Lett. 7 (2011) 31–38. [19] Q.H. Li, T. Gao, T.H. Wang, Appl. Phys. Lett. 86 (2005) 193109–193111. [20] T. Gao, Q.H. Li, T.H. Wang, Appl. Phys. Lett. 86 (2005) 173105–173107. [21] Y. Muraoka, N. Takubo, Z. Hiroi, J. Appl. Phys. 105 (2009) 103702–103708.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fluorescence resonance energy transfer and surface Plasmon resonance induced enhanced photoluminescence and photoconductivity property of Au–TiO2 metal–semiconductor nanocomposite S. Majumder a,b,1, S.K. Jana b,⇑,1, K. Bagani b, B. Satpati b, S. Kumar a, S. Banerjee b a b
Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, India Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Saltlake, Kolkata 700064, India
a r t i c l e
i n f o
Article history: Received 23 August 2014 Received in revised form 23 November 2014 Accepted 1 December 2014 Available online 18 December 2014 Keywords: Nanocomposites Luminescence Photoconductivity Au TiO2 Sol–gel preparation
a b s t r a c t In this manuscript, we have successfully synthesized Au–TiO2 nanocomposite by sol–gel process. Photoluminescence (PL) and photoconductivity (PC) study have been performed on both bare TiO2 and Au–TiO2 composite samples. We observe enhancement of photoluminescence emission in Au–TiO2 nanocomposite compared to bare TiO2. The enhanced photoluminescence is ascribed owing to combined influence of fluorescence resonance energy transfer and surface Plasmon resonance effect due to presence of Au nanoparticles in nanocomposite sample. Also this nanocomposite exhibits enhanced UV photocurrent because of fluorescence resonance energy transfer by metallic Au attached with TiO2. Interfacial charge transfer can also be realized by the electrochemical impedance spectroscopy results. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal and metal oxide based nanocomposite have been enormously studied due to their excellent photocatalytic, electrical, optical, magnetic and mechanical properties [1]. These types of nanocomposites are widely used in gas sensor [2], catalyst [3–4] and bio-sensing application [5]. The optical property and photoconductivity study of metal–semiconductor nanocomposites are interesting because of their use in future nanoelectronic devices [1]. The effect of metal doping in TiO2 has been discussed in some earlier publications [5,6]. Presence of metal in the TiO2 matrix facilitates electron transfer from photoexcited semiconductor materials to the surroundings and decreases electrons–holes recombination rate which supports the enhanced photoelectrochemical and photocatalytic property of metal semiconductor nanocomposite system [7,8]. Photocatalytic effect of noble metal nanoparticles supported oxide semiconductors have been reported previously [9,10]. Enhanced UV photocatalytic performance can be attributed due to reduced electron–hole recombination in the semiconductor attached with noble metal nanoparticles [11]. However, the drastic PC change is obtained because of band gap
modulation of the system and electron distribution between metal and semiconductor [7,12]. In this manuscript, we have extensively studied both PL and PC property on Au–TiO2 nanocomposite system. We observe the enhancement of PL emission and PC in this composite system compared to pristine TiO2. Improved photocurrent is observed in Au–TiO2 under UV illumination while fluorescence resonance energy transfer (FRET) modifies the surface states in composite system due to Au loading which enhances the PL signal of the same sample. FRET induced band edge emission in Ag nanoparticle sensitized CdSe–ZnS/ZnO heterostructure and suppression of defect emission compared to bare ZnO NRs was analyzed in earlier report [13]. A possible energy transfer mechanism to explain the improved PL properties of ZnO NRs is proposed based upon the combined effects of FRET and surface Plasmon resonance (SPR). Here, we have shown the Au nanoparticle induced enhanced defect related emission of TiO2. FRET mechanism is demonstrated also by Electrochemical Impedance Spectroscopy (EIS) measurement. 2. Experimental 2.1. Chemical used
⇑ Corresponding author. 1
E-mail address: [email protected] (S.K. Jana). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.optmat.2014.12.001 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Chloroauric acid (HAuCl4), Trisodium citrate dehydrate (C6H5Na3O72H2O), Mercaptoundecanoic acid (MPA), NH4OH
98
S. Majumder et al. / Optical Materials 40 (2015) 97–101
(28%), Titanium isopropoxide (TTIP), Triethanolamine (TEOA) were purchased from Merck India Pvt. Ltd. Technical grade ethanol and milli-pore water (18.2 MX) were used during the preparation of the samples. All the reagents were used as received. 2.2. Detail preparation method The following section describes step by step synthesis processes of Au–TiO2 nanocomposite [14]. 2.2.1. Synthesis of Au nanoparticle 1 mM HAuCl4 (25 ml) solution was used as stock solution for synthesis of Au NP. At first, the stock solution was heated under constant stirring condition. When the temperature of the solution was reached to 80 °C, then 2.4 ml (1 wt%) trisodium citrate solution was added under continuous stirring. Within few minutes, we observed the color change of the solution from yellow to grey and ultimately red wine colored solution visualized. This red wine solution confirms gold (Au) NP solution. 2.2.2. Synthesis of MPA coated Au nanoparticle We have prepared 0.5 mM mercraptopropanoic acid solution (MPA) and dissolved in 2.5% aqueous ammonia solution. The mixture was added to the Au NP solution and incubated for 2 h. Then the solution was centrifuged further at 3800 rpm. The resultant MPA coated Au NPs were collected. Subsequently, 20 ml of water was added to the resultant MPA coated Au NPs solution and re-dispersed again in 20 ll of 28 wt% ammonia solution. This was used as further stock solution of Au sol to prepare Au–TiO2 nanocomposite. 2.2.3. Preparation of Ti4+ stock solution A stock solution of Ti4+ was prepared by mixing titanium isopropoxide (TTIP) with triethanolamine (TEOA) under Ar atmosphere at a molar ratio of 2:1 (TEOA/TTIP) to form a stable complex of Ti4+–TEOA. This complex agent was used to control the hydrolysis reaction velocity of TTIP [13]. A stable Ti4+ compound was prepared at room temperature by adding the TEOA. Double distilled water was then added to make 0.50 M Ti4+ solutions. The stock solution was filtered for further use. 2.2.4. Preparation of Au–TiO2 nanocomposite The concentrations of Ti4+ in the stock solution were adjusted to 0.01–0.3 mM. Then, 4.95 ml of the MPA coated Au colloid was added to 150 ml of the Ti4+ stock solution. The mixture was then refluxed for 90 min at 200 °C. During this process, TiO2 was deposited onto the surface of Au nanoparticle. The resulting nanocomposite was washed and dried at 65 °C under vacuum. 3. Results and discussions Room temperature XRD (Bruker D8 Advanced Diffractometer) pattern shown in Fig. 1a was obtained to identify the structural information and crystal phase Au–TiO2 nanocomposite system. All the diffraction peaks are indexed as a mixture of Au and TiO2. The diffraction peaks at 2h = 38.40°, 44.47°, 64.76°, 77.77° corresponds to (1 1 1), (2 0 0), (2 2 0), (3 1 1) planes of Au and the diffraction peaks at 2h = 39.46°, 46.00°, 67.24° are matched with (0 4 4), (2 0 0), (2 2 0) planes of anatase TiO2 respectively. Fig. 1b and c show the SEM (FEI Quanta 200) and TEM (FEI, Tecnai F30) images of Au–TiO2 nanocomposite structure respectively. In the SEM image, it is observed that Au NPs are almost homogeneously distributed, although there is a little signature of agglomerations of Au nanoparticles. From the TEM image we see that metallic gold nanoparticles are embedded in TiO2 matrix. Inset image of Fig. 1c
shows the HRTEM image of metallic gold nanostructures attached with TiO2. Fig. 2d shows the high angle annular dark-field scanning/transmission electron microscopy (STEM–HAADF) images of Au–TiO2 nanocomposite. Selected area EDX analysis of Au–TiO2 nanocomposite is shown in the inset of Fig. 1d which confirms elemental composition of the system. UV–Vis spectra recorded by JASCO V-630 spectrophotometer of bare TiO2 and Au–TiO2 nanocomposites are shown in Fig. 2a. The UV–Vis spectra of TiO2 shows an intense peak at around 300 nm, which is the excitonic peak of TiO2 itself. The spectrum of Au–TiO2 nanocomposite shows two peaks at around 300 nm due to TiO2 and another peak at around 500 nm due to surface Plasmon peak of Au metal NPs respectively [7]. Room temperature PL measurements (shown in Fig. 2b) of both the samples have been carried out in JASCO FP-6700 spectrophotometer at an excitation wavelength of 310 nm. In Fig. 2(b), PL spectra of TiO2 shows six distinct peaks within the wavelength range 350–500 nm where a sharp peak at around 356–398 nm is due to band edge emission of TiO2 semiconductor [15]. Moreover, a blue–green emission band, which consists of few peaks at 450, 468, 482 and 495 nm in the PL spectrum, is observed. The origin of blue–green emission band is because of transition of electrons from shallow donor level of the oxygen vacancies to the valence band [16]. PL emission spectra of Au–TiO2 with peak intensity around 450 nm compared to bare TiO2 is observed in the same wavelength region. Enhanced PL emission in Au–TiO2 nanocomposite can be illustrated by FRET mechanism [13] shown schematically in Fig. 3. Au nanoparticles attached with TiO2 quench the band edge emission of TiO2 as shown in Fig. 2b. In bare TiO2, some non-radiative transition may possible caused by dangling bonds and surface states as shown in Fig. 3. Defects induced blue–green emission of TiO2 attribute to the non-radiative energy transfer from TiO2 to Au NPs. This FRET could cause surface Plasmon resonance (SPR) of the Au nanoparticles, as the SPR occurs around 500 nm which is observed in Fig. 2a. The possible energy transfer mechanism involved with FRET process is described sequentially as follows. (i) Electron transfer from V.B to C.B in TiO2 under UV excitation (310 nm). (ii) Electron transfer from C.B of TiO2 to Au Nanoparticles through non radiative process. (iii) Radiative emission from donor states to V.B of TiO2 (400–500 nm) originates the SPR (as the absorption band of Au lies between 420 and 530 nm). (iv) At SPR all electrons (those are involved with non-radiative transition and surface charge of Au Nanoparticles at SPR) relax to the V.B of TiO2 through radiative emission. Near field enhancement of the Au nanoparticles can affect the photon flux to the TiO2. In addition, defect related emission by TiO2 induced surface Plasmon resonance of Au NPs which increases the radiative recombination rates. Synergistic effect of both FRET and SPR mechanism is the reason for the enhancement of defect related emission of Au–TiO2 compared to bare TiO2. Thus energy transfer occurs between Au and TiO2 and electron accumulation takes place in Au. This electron accumulation due to charge transfer within Au–TiO2 reduces trap level and majority of electrons relax radiatively to the valance band. Enhanced PL emission is not only due to reducing trap level but also because of coupling effect between local surface plasmons (SPs) and excitons [17]. This is the reason why we observed the enhancement of emission peaks of Au–TiO2 nanocomposite compared to bare TiO2. The photoconductivity response under steady state illumination of both the samples is shown in Fig. 4a and b for bare TiO2 and Au–TiO2 respectively. These graphs show the different dark value of the two samples which were kept in the dark condition until the current reached at equilibrium. It is clear from both figures, initially dark current starts decreasing slowly until it achieves the stable value (marked by upward arrow in both Fig. 4a and b respectively). This may be attributed to adsorption of oxygen
99
S. Majumder et al. / Optical Materials 40 (2015) 97–101
Fig. 1. (a) XRD pattern, (b) SEM image, (c) TEM and HRTEM image (inset), and (d) STEM-HAADF image of Au–TiO2 nanocomposite and inset figure shows the EDX analysis of the nanocomposite.
Fig. 2. (a) UV–Vis and (b) photoluminescence study of both bare and Au–TiO2 nanocomposite respectively.
molecule as well as due to presence of defects [18]. In the absence of UV light, oxygen is absorbed by taking a free electron (e) from the surface of both bare TiO2 and Au–TiO2 particles to form chemisorbed surface state and develop a depletion layer near the surface of both samples with low conductivity. We have described these in the following reaction.
O2 þ e ! O2
ð1Þ
Thus the surface of both particles is almost depleted with charge carriers leads to a high resistance in the dark state [19,20]. Upon UV illumination, the photogenerated holes are produced and release the captured species (O 2 ion) by leaving behind
þ
an electron, i.e. O 2 þ h ! O2 ðgÞ. The adsorbed oxygen molecules are released in air, which lowers the barrier height of electron in TiO2. This mechanism was proposed by Muraoka et al. [21]. Once all photoinduced holes react with O 2 , photo current have a tendency to stabilize for both TiO2 and Au–TiO2 cases. As soon as UV is OFF (indicated by downward arrow) oxygen gets absorbed and starts to develop a depletion layer at the surface of material, so current tends to decrease as shown in both Fig. 4. Also it is notified that the ratio photocurrent to dark current (photosensitivity 5) of Au–TiO2 is higher compared to bare TiO2 (photosensitivity 2.7). Under UV illumination, the electron transfer between Au and TiO2 continues until two systems attain equilibrium position
100
S. Majumder et al. / Optical Materials 40 (2015) 97–101
Fig. 3. Enhanced photoluminescence emission through FRET mechanism.
Fig. 5. EIS spectra of both bare TiO2 and Au–TiO2 composite under UV illumination and the Randle circuit shown in the inset of this graph.
Fig. 4. Photoconductivity response curve of (a) bare TiO2 and (b) Au–TiO2 nanocomposite respectively.
and the Fermi level (EF) of the total system lies on a single line. The transfer of electron from TiO2 semiconductor to Au under UV illumination results the accumulation of electron in Au–TiO2 nanocomposite. The accumulation of electron shifts the Fermi level of Au metal and reduces the barrier height, and thereby delays of
the recombination of electron hole pair which is responsible for enhancement of the photosensitivity of Au–TiO2 nanocomposite [7]. Electron accumulation and interfacial charge transfer can also be verified electrochemical impedance spectroscopy measurement. EIS spectra taken at frequency range between 100 Hz and 100KHz was recorded by CH Instruments 660 in three electrode configuration adapted with ITO coated sample (both bare and Au–TiO2 nanocomposite) as working electrode, Pt wire as counter electrode and Ag/AgCl as reference electrode. The electrochemical measurement was done in 0.1M Na2SO4 solution. EIS spectrum of both bare and Au nanoparticle attached TiO2 under UV illumination is depicted in Fig. 5. EIS spectra can be divided into two regions: a semicircle formed in the higher frequency region and the straight line formed in the lower frequency region. An equivalent circuit model (shown in the inset of Fig. 5) is proposed to
S. Majumder et al. / Optical Materials 40 (2015) 97–101
simplify the electrochemical process at electrode–electrolyte interface, where Rs is the solution resistance indicated by the first intercept of the semicircle to the real impedance axis, the second intercept corresponds to the charge transfer resistance (Rct), straight line signifies the Warburg impedance, Cdl is the Helmholtz double layer capacitance formed at the working electrode and electrolyte interface. The smaller Rct value of Au–TiO2 than that of bare TiO2 reveals lower charge transfer resistance at the electrode/ electrolyte interface, which is attributed to the improved interfacial charge transfer between the electroactive TiO2 and conductive Au nanoparticle attached with TiO2. Under UV illumination electrons and holes pairs (EHPs) are generated and some fractions of EHP can recombine at the surface of the bare TiO2 while in composite sample these photogenerated EHPs transferred from TiO2 surface to Au nanoparticles. Thus accumulation of charge in the composite system through interfacial charge transfer can be realized.
4. Conclusions In summary, we have successfully synthesized Au–TiO2 nanocomposite by sol–gel process. The enhancement of PL emission and UV photosensitivity study of this nanostructure have been investigated. Significant enhanced PL emission of Au–TiO2 nanocomposite spectra is obtained because of increasing radiative recombination rate and coupling effect of SPR and FRET mechanism. Under UV illumination, this nanocomposite exhibits also improved photosensitivity due to electron accumulation by the interfacial charge transfer in the composite system. So Au–TiO2 nanocomposite can be used for UV photo detection, optical switches and solar cells applications.
101
Acknowledgement Authors would like to acknowledge to Dr. S. K. Mishra, currently post-doctoral fellow of Lucknow University, India, for PC measurements of the samples. References [1] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Am. Chem. Soc. 126 (2004) 4943– 4950. [2] W. Jia, L. Su, Y. Ding, A. Schempf, Y. Wang, Y. Lei, J. Phys. Chem. C 113 (2009) 16402–16407. [3] D.I. Enache, J.K. Edwards, P. Landon, B.S. Espriu, A.F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362–365. [4] L. Su, W. Jia, A. Schempf, Y. Lei, Electrochem. Commun. 11 (2009) 2199–2202. [5] S.F. Chen, J.P. Li, K. Qian, W.P. Xu, Y. Lu, W.X. Huang, S.H. Yu, Nano Res. 3 (2010) 244–255. [6] G. Zhao, H. Kozuka, T. Yoko, Thin Solid Films 277 (1996) 147–154. [7] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Phys. Chem. B 105 (2001) 11439– 11446. [8] S.K. Jana, T. Majumder, S. Banerjee, J. Electroanal. Chem. 727 (2014) 99–103. [9] B. Kraeutler, A.J. Bard, J. Am. Chem. Soc. 100 (1978) 4317–4318. [10] Y. Inel, D. Ertek, Faraday Trans. 89 (1993) 129–133. [11] W. Zhou, H. Fu, Chem. Cat. Chem. 5 (2013) 885–894. [12] A. Bumajdad, M. Madkour, Phys. Chem. Chem. Phys. 16 (2014) 7146–7158. [13] J.Y. Chang, T.G. Kim, Y.M. Sung, Nanotechnology 22 (2011) 425708–425714. [14] H.W. Kwon, Y.M. Lim, S.K. Tripathy, B.G. Kim, Jpn. J. Appl. Phys. 46 (2007) 2567–2570. [15] K.M. Rahulan, S. Ganesan, P. Aruna, Adv. Nat. Sci. Nanosci. Nanotechnol. 2 (2011) 025012–025016. [16] B. Choudhury, M. Dey, A. Choudhury, Appl. Nanosci. 4 (2014) 499–506. [17] A. Neogi, C.W. Lee, H.O. Everitt, T. Kuroda, A. Tackeuchi, E. Yablonovitch, Phys. Rev. B: Condens. Matter Mater. Phys. 66 (2002) 153305–153308. [18] S. K Mishra, R.K. Srivastava, S.G. Prakash, R.S. Yadav, A.C. Pandey, Electron. Mater. Lett. 7 (2011) 31–38. [19] Q.H. Li, T. Gao, T.H. Wang, Appl. Phys. Lett. 86 (2005) 193109–193111. [20] T. Gao, Q.H. Li, T.H. Wang, Appl. Phys. Lett. 86 (2005) 173105–173107. [21] Y. Muraoka, N. Takubo, Z. Hiroi, J. Appl. Phys. 105 (2009) 103702–103708.