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Synthesis and room temperature photoluminescence of AgI nanoparticles embedded in silica sol–gel coating
Solid State Ionics 175 (2004) 651 – 654 www.elsevier.com/locate/ssi
Synthesis and room temperature photoluminescence of AgI nanoparticles embedded in silica sol–gel coating Haiping He, Yuxia Wang*, Hongwei Chen Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Received in revised form 20 July 2004; accepted 20 July 2004
Abstract AgI nanoparticles embedded in sol–gel silica coating were synthesized by a soaking–heating method. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), optical reflection, and photoluminescence (PL) measurements were used to characterize the coating. The size of the AgI particles was about 10–40 nm. A red shift of ~10 nm on the absorption edge as well as the formation of a band tail was observed. The PL spectrum of the coating exhibits main emissions at 333, 370, 435, 470, and 510 nm. The former two corresponding to nonbridging oxygen hole centers (NBOHC) and EV centers in silica, respectively. The emission at 435 nm is ascribed to radiative excitonic recombination of AgI, while the 470-nm one is associated with a transition between the tail of the conduction band (CB) and the valence band (VB). The emission at 510 nm may arise from free Ag+ ions or defects. D 2004 Elsevier B.V. All rights reserved. PACS: 78.55-m; 81.20-Fw Keywords: AgI nanoparticles; Sol–gel; Photoluminescence
1. Introduction Recently, there has been an increasing interest in silver iodide (AgI) due to its superionic properties and its use as sensitive detector in the visible range. In the past decade, substantial progress has been made in nanotechnology. Several approaches were developed to synthesize AgI nanoparticles [1–3] which show properties different from normal AgI. It was reported [4] that AgI clusters may find applications as photocatalysts for solar energy conversion and as a medium for optical information or image storage. The optical properties of semiconductor nanocrystals are of much current interest, as they provide information about the evolution of physical characteristics from atom to bulk and about the three-dimensional confinement of carriers and excitons [5]. Photoluminescence (PL) is a useful tool for monitoring electronic changes at the nanoparticle surface.
* Corresponding author. Tel.: +86 551 3601834; fax: +86 551 3631760. E-mail address: [email protected] (Y. Wang). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.068
However, data about optical properties of both bulk and microstructured AgI are rather poor. Sol–gel-derived porous silica has been extensively used as matrix for synthesizing various nanoparticles. Although the AgI–SiO2 composite was intensively investigated for its enhanced ionic conductivity [6], AgI nanoparticles embedded in silica as well as their optical properties were seldom reported. In the present work, we report synthesis and room temperature PL property of AgI nanoparticles embedded in silica sol–gel coating.
2. Experiment Tetraethoxysilane (TEOS), ethanol (EtOH), and 0.1 M HNO3 solution were mixed slowly in a 1:3:0.004 mol ratio and the mixture was stirred for 30 min at 25 8C. After 1 day, the clear sol was spin-coated on a Si (111) substrate at 2000 rpm followed by 24-h exposure in air and a 2-h heat treatment at 400 8C at a 1 8C/min ramp. The sample was then soaked in a 0.2 M AgNO3 solution for 2 days, washed
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H. He et al. / Solid State Ionics 175 (2004) 651–654
Fig. 1. XRD pattern of the AgI–silica coating.
extensively with deionized water. Subsequently, it was soaked in 0.2 M KI solution for 2 days and was also washed with deionized water. The sample was finally dried and heated at 200 8C for 2 h. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), reflection, PL, and PL excitation (PLE) measurements were used to characterize the composite coating.
3. Results and discussion Fig. 1 illustrates the XRD pattern of the coating. The broad diffraction band around 238 is assigned to amorphous silica. Four peaks associated with crystalline h- and g-AgI were observed at positions of 22.38, 23.98, 39.28, and 46.38, indicating the formation of the AgI–silica composite. It is well known that h- and g-AgI coexist at room temperature. Fig. 2 is a SEM image of the AgI–silica coating. The surface of the coating is very smooth. The bright protrusions
Fig. 2. SEM micrograph of the AgI nanoparticles embedded in silica sol– gel coating.
Fig. 3. XPS core level spectrum of Ag3d5/2. The band was fitted by two Gaussian peak (dotted line) at 368.0 and 369.7 eV.
with sizes ranging in about 10–40 nm on the surface are AgI particles. Fig. 3 shows the XPS Ag3d5/2 core-level spectrum of the AgI–silica coating. The band can be decomposed in two components by a Gaussian fit: a main peak at 369.7 eV and a weak shoulder at 368.0 eV. The former is ascribed to Ag in AgI; however, its binding energy is about 1.6 eV higher than the reported value of isolated AgI [7]. We suggest that the large chemical shift may be induced by the interaction between silica network and AgI to form aZSi Od d d d d d Ag I structure, by which the charge distribution around Ag atoms is pulled by the oxygen atom due to its larger electronegativity. This effect reduces covalence and increases ionicity of Ag I bond, thus reducing the screening effect and resulting in an increase of the binding energy. The weak peak at 368.0 eV is most likely corresponding to the inner part of the AgI particles not affected by the oxygen from the silica matrix. In Fig. 4, reflection spectra for virgin AgI (curve a) and AgI–silica coating (curve b) are illustrated. For virgin AgI,
Fig. 4. Reflection spectra for (a) virgin AgI and (b) AgI–silica coating.
H. He et al. / Solid State Ionics 175 (2004) 651–654
Fig. 5. PL spectra of (a) virgin silica and (b) AgI–silica coating excited at 230 nm. A 290-nm cutoff filter was used. Inset shows the PLE spectrum monitored at 510 nm.
the absorption edge at ~430 nm (2.88 eV) is sharp and the reflectance is large. While for AgI–silica, the absorption edge undergoes a red shift of ~10 nm, as well as formation of a band tail on the long-wavelength side. We suggest that these phenomena may result from AgI–silica interaction and/or the surface and defect states in AgI nanoparticles. The interaction between AgI and silica can reduce the covalence of the Ag–I bond and alter the electronic structure of AgI. Moreover, the relatively large surface-to-volume ratio of AgI nanoparticles may create unsaturated dangling bonds [8] and substantial reconstructions in the atomic positions [5], invariably leading to energy levels within the energetically forbidden gap. These energy levels can overlap with the conduction band (CB) or valence band (VB) of AgI and form band tail states. Room temperature PL spectra of virgin silica and AgI– silica coatings are plotted in Fig. 5. When excited with 230 nm radiation, virgin silica exhibits an intense PL band at 337 nm (3.68 eV) which has been ascribed to nonbridging oxygen hole centers (NBOHC) in silica [9]. For AgI–silica coating, two additional bands around 370 and 500 nm emerge. The band at 370 nm is ascribed to EV centers (ZSiz) in silica [10]. Three bands can be discriminated at 435, 470, and 510 nm, respectively, on the basis of the band around 500 nm. The band at 435 nm is ascribed to the radiative excitonic recombination [11] of AgI, with a little red shift from the excitonic absorption at 425 nm (for bulk), consistent with results in Fig. 4. As mentioned earlier, energy levels can be introduced into the band gap of AgI; accordingly, we suggest the transition from these levels to AgI VB give rise to the emission at 470 nm. Kumar and Sunandana [12] reported five prominent peaks at about 440, 450, 470, 485, and 495 nm in the PL spectrum of vaporquenched AgI films. Chen et al. [13] observed a broad PL emission at 510 nm with a shoulder at 474 nm from AgI nanoclusters in zeolites and they ascribed the 474 nm emission to AgI. These results support our assignment.
653
The inset in Fig. 5 illustrates the PLE spectrum for a AgI–silica coating monitored at 510 nm. On the basis of an absorption-edge-like profile, two shoulders at 229 and 255 nm, respectively, are observed. The former agrees quite well with the value of 228.6 nm which is given for the parityforbidden transition 4d10 (1S0)Y4d95s1 (3D1) in free Ag+ ion [14]. In silver-doped silica, Borsella et al. [15] observed a broad PL band peaked at 480 nm (excitation wavelength 266 nm) and assumed it originated from the 3D manifold excited state to the ground state of free Ag+ ions. In the present system, excessive Ag+ ions can be expected due to the large size of I ions, which makes it more difficult for them to diffuse into the silica network and combine with Ag+ ions. Accordingly, we suggest the emission at 510 nm may correlate with Ag+ ions. However, we note it may also result from a transition involving defect-related states within AgI band gap because the energy of this emission is only ~0.1 eV lower than that of 470 nm. More investigations should be conducted to further understand the actual mechanism.
4. Conclusion In summary, AgI nanoparticles with sizes ranging in 10– 40 nm were synthesized in a silica matrix by a spin-coating and soaking method. Reflection spectra reveal a red shift of the absorption edge and the formation of a band tail. Room temperature PL bands at 435, 470, and 510 nm are observed from AgI nanoparticles. The 435-nm emission is attributed to excitonic recombination in AgI nanoparticles. The 470nm one is associated with a transition between tail of conduction band and valence band, while the emission at 510 nm may arise from free Ag+ ions or defects.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China under Grant No. 59972034.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
S. Chen, T. Ida, K. Kimura, J. Phys. Chem., B 102 (1998) 6169. Y. Wang, J. Mo, W. Cai, L. Yao, J. Mater. Res. 16 (2001) 990. Y. Wang, L. Huang, H. He, M. Li, Physica, B 325 (2003) 357. W. Chen, Z. Wang, Z. Lin, L. Lin, K. Fang, Y. Xu, M. Su, J. Lin, J. Appl. Phys. 83 (1998) 3811. A.P. Alivisatos, Science 271 (1996) 933. K. Tadanaga, K. Imai, M. Tatsumisago, T. Minami, J. Electrochem. Soc. 149 (2002) A773. NIST X-ray Photoelectron Spectroscopy Database, 1997 Gaitherburg, MD. J. Nayak, S.N. Sahu, Appl. Surf. Sci. 182 (2001) 407. B.D. Yao, H.Z. Shi, X.Y. Zhang, L.D. Zhang, Appl. Phys. Lett. 78 (2001) 174.
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H. He et al. / Solid State Ionics 175 (2004) 651–654
[10] K. Kim, M.S. Suh, T.S. Kim, C.J. Youn, E.K. Suh, Y.J. Suh, Y.J. Shin, K.B. Lee, H.J. Lee, M.H. An, H.J. Lee, H. Ryu, Appl. Phys. Lett. 69 (1996) 3908. [11] S. Mochizuki, Y. Ohta, J. Lumin. 87–89 (2000) 299. [12] P.S. Kumar, C.S. Sunandana, Nano Lett. 2 (2002) 975.
[13] W. Chen, A.G. Joly, J. Roark, Phys. Rev., B 65 (2002) 245404. [14] E. Paje, J. Llopis, M.A. Villegas, J.M. Ferna´ndez Navarro, Appl. Phys., A 63 (1996) 431. [15] E. Borsella, G. Battaglin, M.A. Garcı`a, F. Gonella, R. Polloni, A. Quaranta, Appl. Phys., A 71 (2000) 125.
Synthesis and room temperature photoluminescence of AgI nanoparticles embedded in silica sol–gel coating Haiping He, Yuxia Wang*, Hongwei Chen Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Received in revised form 20 July 2004; accepted 20 July 2004
Abstract AgI nanoparticles embedded in sol–gel silica coating were synthesized by a soaking–heating method. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), optical reflection, and photoluminescence (PL) measurements were used to characterize the coating. The size of the AgI particles was about 10–40 nm. A red shift of ~10 nm on the absorption edge as well as the formation of a band tail was observed. The PL spectrum of the coating exhibits main emissions at 333, 370, 435, 470, and 510 nm. The former two corresponding to nonbridging oxygen hole centers (NBOHC) and EV centers in silica, respectively. The emission at 435 nm is ascribed to radiative excitonic recombination of AgI, while the 470-nm one is associated with a transition between the tail of the conduction band (CB) and the valence band (VB). The emission at 510 nm may arise from free Ag+ ions or defects. D 2004 Elsevier B.V. All rights reserved. PACS: 78.55-m; 81.20-Fw Keywords: AgI nanoparticles; Sol–gel; Photoluminescence
1. Introduction Recently, there has been an increasing interest in silver iodide (AgI) due to its superionic properties and its use as sensitive detector in the visible range. In the past decade, substantial progress has been made in nanotechnology. Several approaches were developed to synthesize AgI nanoparticles [1–3] which show properties different from normal AgI. It was reported [4] that AgI clusters may find applications as photocatalysts for solar energy conversion and as a medium for optical information or image storage. The optical properties of semiconductor nanocrystals are of much current interest, as they provide information about the evolution of physical characteristics from atom to bulk and about the three-dimensional confinement of carriers and excitons [5]. Photoluminescence (PL) is a useful tool for monitoring electronic changes at the nanoparticle surface.
* Corresponding author. Tel.: +86 551 3601834; fax: +86 551 3631760. E-mail address: [email protected] (Y. Wang). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.068
However, data about optical properties of both bulk and microstructured AgI are rather poor. Sol–gel-derived porous silica has been extensively used as matrix for synthesizing various nanoparticles. Although the AgI–SiO2 composite was intensively investigated for its enhanced ionic conductivity [6], AgI nanoparticles embedded in silica as well as their optical properties were seldom reported. In the present work, we report synthesis and room temperature PL property of AgI nanoparticles embedded in silica sol–gel coating.
2. Experiment Tetraethoxysilane (TEOS), ethanol (EtOH), and 0.1 M HNO3 solution were mixed slowly in a 1:3:0.004 mol ratio and the mixture was stirred for 30 min at 25 8C. After 1 day, the clear sol was spin-coated on a Si (111) substrate at 2000 rpm followed by 24-h exposure in air and a 2-h heat treatment at 400 8C at a 1 8C/min ramp. The sample was then soaked in a 0.2 M AgNO3 solution for 2 days, washed
652
H. He et al. / Solid State Ionics 175 (2004) 651–654
Fig. 1. XRD pattern of the AgI–silica coating.
extensively with deionized water. Subsequently, it was soaked in 0.2 M KI solution for 2 days and was also washed with deionized water. The sample was finally dried and heated at 200 8C for 2 h. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), reflection, PL, and PL excitation (PLE) measurements were used to characterize the composite coating.
3. Results and discussion Fig. 1 illustrates the XRD pattern of the coating. The broad diffraction band around 238 is assigned to amorphous silica. Four peaks associated with crystalline h- and g-AgI were observed at positions of 22.38, 23.98, 39.28, and 46.38, indicating the formation of the AgI–silica composite. It is well known that h- and g-AgI coexist at room temperature. Fig. 2 is a SEM image of the AgI–silica coating. The surface of the coating is very smooth. The bright protrusions
Fig. 2. SEM micrograph of the AgI nanoparticles embedded in silica sol– gel coating.
Fig. 3. XPS core level spectrum of Ag3d5/2. The band was fitted by two Gaussian peak (dotted line) at 368.0 and 369.7 eV.
with sizes ranging in about 10–40 nm on the surface are AgI particles. Fig. 3 shows the XPS Ag3d5/2 core-level spectrum of the AgI–silica coating. The band can be decomposed in two components by a Gaussian fit: a main peak at 369.7 eV and a weak shoulder at 368.0 eV. The former is ascribed to Ag in AgI; however, its binding energy is about 1.6 eV higher than the reported value of isolated AgI [7]. We suggest that the large chemical shift may be induced by the interaction between silica network and AgI to form aZSi Od d d d d d Ag I structure, by which the charge distribution around Ag atoms is pulled by the oxygen atom due to its larger electronegativity. This effect reduces covalence and increases ionicity of Ag I bond, thus reducing the screening effect and resulting in an increase of the binding energy. The weak peak at 368.0 eV is most likely corresponding to the inner part of the AgI particles not affected by the oxygen from the silica matrix. In Fig. 4, reflection spectra for virgin AgI (curve a) and AgI–silica coating (curve b) are illustrated. For virgin AgI,
Fig. 4. Reflection spectra for (a) virgin AgI and (b) AgI–silica coating.
H. He et al. / Solid State Ionics 175 (2004) 651–654
Fig. 5. PL spectra of (a) virgin silica and (b) AgI–silica coating excited at 230 nm. A 290-nm cutoff filter was used. Inset shows the PLE spectrum monitored at 510 nm.
the absorption edge at ~430 nm (2.88 eV) is sharp and the reflectance is large. While for AgI–silica, the absorption edge undergoes a red shift of ~10 nm, as well as formation of a band tail on the long-wavelength side. We suggest that these phenomena may result from AgI–silica interaction and/or the surface and defect states in AgI nanoparticles. The interaction between AgI and silica can reduce the covalence of the Ag–I bond and alter the electronic structure of AgI. Moreover, the relatively large surface-to-volume ratio of AgI nanoparticles may create unsaturated dangling bonds [8] and substantial reconstructions in the atomic positions [5], invariably leading to energy levels within the energetically forbidden gap. These energy levels can overlap with the conduction band (CB) or valence band (VB) of AgI and form band tail states. Room temperature PL spectra of virgin silica and AgI– silica coatings are plotted in Fig. 5. When excited with 230 nm radiation, virgin silica exhibits an intense PL band at 337 nm (3.68 eV) which has been ascribed to nonbridging oxygen hole centers (NBOHC) in silica [9]. For AgI–silica coating, two additional bands around 370 and 500 nm emerge. The band at 370 nm is ascribed to EV centers (ZSiz) in silica [10]. Three bands can be discriminated at 435, 470, and 510 nm, respectively, on the basis of the band around 500 nm. The band at 435 nm is ascribed to the radiative excitonic recombination [11] of AgI, with a little red shift from the excitonic absorption at 425 nm (for bulk), consistent with results in Fig. 4. As mentioned earlier, energy levels can be introduced into the band gap of AgI; accordingly, we suggest the transition from these levels to AgI VB give rise to the emission at 470 nm. Kumar and Sunandana [12] reported five prominent peaks at about 440, 450, 470, 485, and 495 nm in the PL spectrum of vaporquenched AgI films. Chen et al. [13] observed a broad PL emission at 510 nm with a shoulder at 474 nm from AgI nanoclusters in zeolites and they ascribed the 474 nm emission to AgI. These results support our assignment.
653
The inset in Fig. 5 illustrates the PLE spectrum for a AgI–silica coating monitored at 510 nm. On the basis of an absorption-edge-like profile, two shoulders at 229 and 255 nm, respectively, are observed. The former agrees quite well with the value of 228.6 nm which is given for the parityforbidden transition 4d10 (1S0)Y4d95s1 (3D1) in free Ag+ ion [14]. In silver-doped silica, Borsella et al. [15] observed a broad PL band peaked at 480 nm (excitation wavelength 266 nm) and assumed it originated from the 3D manifold excited state to the ground state of free Ag+ ions. In the present system, excessive Ag+ ions can be expected due to the large size of I ions, which makes it more difficult for them to diffuse into the silica network and combine with Ag+ ions. Accordingly, we suggest the emission at 510 nm may correlate with Ag+ ions. However, we note it may also result from a transition involving defect-related states within AgI band gap because the energy of this emission is only ~0.1 eV lower than that of 470 nm. More investigations should be conducted to further understand the actual mechanism.
4. Conclusion In summary, AgI nanoparticles with sizes ranging in 10– 40 nm were synthesized in a silica matrix by a spin-coating and soaking method. Reflection spectra reveal a red shift of the absorption edge and the formation of a band tail. Room temperature PL bands at 435, 470, and 510 nm are observed from AgI nanoparticles. The 435-nm emission is attributed to excitonic recombination in AgI nanoparticles. The 470nm one is associated with a transition between tail of conduction band and valence band, while the emission at 510 nm may arise from free Ag+ ions or defects.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China under Grant No. 59972034.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
S. Chen, T. Ida, K. Kimura, J. Phys. Chem., B 102 (1998) 6169. Y. Wang, J. Mo, W. Cai, L. Yao, J. Mater. Res. 16 (2001) 990. Y. Wang, L. Huang, H. He, M. Li, Physica, B 325 (2003) 357. W. Chen, Z. Wang, Z. Lin, L. Lin, K. Fang, Y. Xu, M. Su, J. Lin, J. Appl. Phys. 83 (1998) 3811. A.P. Alivisatos, Science 271 (1996) 933. K. Tadanaga, K. Imai, M. Tatsumisago, T. Minami, J. Electrochem. Soc. 149 (2002) A773. NIST X-ray Photoelectron Spectroscopy Database, 1997 Gaitherburg, MD. J. Nayak, S.N. Sahu, Appl. Surf. Sci. 182 (2001) 407. B.D. Yao, H.Z. Shi, X.Y. Zhang, L.D. Zhang, Appl. Phys. Lett. 78 (2001) 174.
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H. He et al. / Solid State Ionics 175 (2004) 651–654
[10] K. Kim, M.S. Suh, T.S. Kim, C.J. Youn, E.K. Suh, Y.J. Suh, Y.J. Shin, K.B. Lee, H.J. Lee, M.H. An, H.J. Lee, H. Ryu, Appl. Phys. Lett. 69 (1996) 3908. [11] S. Mochizuki, Y. Ohta, J. Lumin. 87–89 (2000) 299. [12] P.S. Kumar, C.S. Sunandana, Nano Lett. 2 (2002) 975.
[13] W. Chen, A.G. Joly, J. Roark, Phys. Rev., B 65 (2002) 245404. [14] E. Paje, J. Llopis, M.A. Villegas, J.M. Ferna´ndez Navarro, Appl. Phys., A 63 (1996) 431. [15] E. Borsella, G. Battaglin, M.A. Garcı`a, F. Gonella, R. Polloni, A. Quaranta, Appl. Phys., A 71 (2000) 125.