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Nanostructured TiO2 — morphological and structural changes
Materials Letters 64 (2010) 140–143
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Nanostructured TiO2 — morphological and structural changes F. Armani a, M. Gougis a, S.A. Impey b, A.C. James a, K. Lawson b, L. Lihrmann b, M. Stock a, S. Dunn a,⁎ a b
Nanotechnology Centre, Building 30, Cranfield University, MK43 0AL, UK Surface Science and Engineering, Building 61, Cranfield University, MK43 0AL, UK
a r t i c l e
i n f o
Article history: Received 23 July 2009 Accepted 8 October 2009 Available online 17 October 2009 Keywords: TiO2 Oxidation Surface analysis
a b s t r a c t Chemical modification of titanium to form TiO2 is being extensively pursued. We compare the influence of acid pickling and post processing on the crystallinity and morphology of TiO2 grown by H2O2 oxidation. Subsequent oxidation using two concentrations of H2O2 (15% and 30% by mass) indicates a variation in the fine surface morphology, apparent at low magnifications. No discernable difference in the quality of TiO2 that was produced when evaluated by SEM, Raman or XRD (glancing angle and θ:2θ) could be detected between the samples oxidised in different H2O2 concentrations. The photoactivity of TiO2 produced from ultra thin Ti films was confirmed by the photocatalytic reduction of Ag+ cations to Ag0. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Early reports of the photoactive behaviour of TiO2 [1] sparked an interest in applications and materials science of wide band gap semi-conductors. Materials investigated range from complex oxides PbZrxTi1 −x O3 [2], BaTiO3 [3] and simple binary systems [4] such as TiO2 [5]. TiO2 is interesting due its range of applications; ability to split water into H2 and O2 [6] and use in dye sensitised solar cell (DSSC) or Graetzel cells [7] being very topical. Recently there has been a focus on methods to produce highly nanostructured TiO2 from Ti substrates. The textured surface is suitable to many applications for which TiO2 is used. It can positively influence kinetics of surface reactions, through increased contact area, and reduce diffusion lengths of photoexcited carriers in optoelectronic devices such as the DSSC. Anatase is a metastable low temperature form of TiO2 and generally accepted as the most suitable for photochemical devices [8,9]. When processing TiO2 precursors there is a limit to the upper processing temperature. This limit is process dependent e.g. direct oxidation or sol–gel, and varies from 500 to 1000 °C [10,11]. Low temperature processing is considered valuable, as it affords low energy operations, and an ability to use a wider variety of substrates. Especial importance is the ability to use heat treated polyimide or polyesters in DSSC. One promising technique is using H2O2 to oxidise the surface [12–14]. The substrate is bathed in H2O2 and etching– oxidation–redeposition occurs producing nanotextured surfaces composed mainly of amorphous TiO2. Addition of counter cations to the H2O2 modifies the amount of amorphous material [14] and phase of TiO2 produced. To fully crystallise the sample a post oxidation step is required using a simple thermal anneal. A second more interesting alternative provides a low temperature route, analogous to hydro⁎ Corresponding author. E-mail address: s.c.dunn@cranfield.ac.uk (S. Dunn). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.022
thermal techniques, involves ageing in a water bath [15]. Here we show the evolution of TiO2 from the chemical oxidation of Ti using H2O2. We have investigated the influence of pre-oxidation processing, the concentration of H2O2 on the final phase after no treatment, thermal annealing or ageing in water. We show that it is possible to produce photochemicaly active TiO2 from ca. 200 nm thick sputtered films of Ti. 2. Experimental methods Ti was degreased by ultrasonicating under acetone, then isopropyl alcohol for 5 min. Samples were pickled in HF(1%)/H2SO4(25%) at room temperature for 50 or 120 s and washed with de-ionised water. Compositional analysis was performed using EDX (Oxford Instruments INCA) on an SEM (Philips XL 30 S FEG SEM). EDX was calibrated to give qualitative and quantitative analysis. SEM analysis was performed using the same machine. The sample was placed into 50 mL of either 15 or 30% H2O2(aq) at 90 °C for 24 h for oxidation then rinsed with de-ionised water and left to dry. Raman spectroscopy was performed on a Renishaw system 1000 Raman microspectrometer. XRD analysis was completed on a Siemens D5005 using Cu Kα radiation. Ageing was performed in a water bath at 90 °C for 24 h. Annealing was performed on a hotplate at 300 °C for 1 h. Thin film Ti was sputtered from a 99.99% Ti target using a Nordico sputtering system onto a glass substrate. Silver nitrate (99.99%) for was purchased from Sigma-Aldrich. Fresh 0.01 M solutions of silver nitrate were made when required. A Honle Fe doped Mg lamp was used to irradiate the sample. 3. Results and discussion The impact of cleaning and acid pickling in HF/H2SO4 solution on the Ti was investigated using EDX, as shown in Table 1. The as-
F. Armani et al. / Materials Letters 64 (2010) 140–143 Table 1 Compositional analysis performed by EDX for Ti samples at different stages of processing. Sample
As-received 50s pickle 120s pickle Cross section
Atomic ratios / (%) Ti
O
C
52.1 73.0 92.8 97.3
17.0 15.12 0 0
30.7 11.8 7.2 2.7
received Ti sheet sample contains significant amounts of surface oxygen originating from the passivation layer (TiO2). After 120 s (for the sample tested) of acid pickling metallic Ti was exposed. The carbon present on the surface in the as-received sample stems from the photodegradation of organic materials in contact with the surface. However, it should be noted that the core of the sample, where no contamination due to surface contact would be possible, contained almost 3% carbon. The source of this carbon is unknown and is likely to be the supplied Ti. SEM examination of the surface after reaction with the H2O2, of two concentrations, is shown in Fig. 1. A difference in the morphology of the surface associated with the concentration of the H2O2(aq) is observed. There is no such variation in morphology caused by a variation of time in the acid bath. In the case of the 15% solution the surface looks cracked, whereas for the 30% sample it looks smooth with local nano/microstructured texture. There are few large cracks in the surface. The observed changes related to the acid exposure stem from the fact that the acid bath removes the tenacious oxide layer from the surface of the Ti [16,17]. The oxide layer of the as-received Ti samples is of an unknown thickness. H2O2(aq) is capable of dissolving [13] both Ti and TiO2 to produce a highly reticulated surface. Hence the incomplete removal of the TiO2 in the acid bath has made little change to overall nanostructured morphology of the surface. In the case of the lower concentration peroxide (15%) the cracks in the micrographs are commensurate with the grain structure of the Ti substrates. The removed material is then redeposited in the nanostructure reticulated structure, as shown in the insets of Fig. 1. Fig. 1 also shows that whilst there is a difference with the 30% solution surface morphology when compared to the 15% specimen, there is only evidence of grain etching in the sample that was pickled for 50 s in acid. The lower time in the acid bath may not completely remove
141
the TiO2, see Table 1. TiO2 is less soluble in H2O2(aq) resulting in selective etching at grain boundaries [16]. As dissolution of the surface takes place at a faster rate in concentrated H2O2(aq) [17] the most notable change is the overall large scale surface morphology. This result confirms the hypothesis that there is a dynamic equilibrium [18] which involves the dissolution of surface TiOx, x can be 0 or 2, and selective oxidation of dissolved Ti cations before precipitation onto the surface of as TiO2. The TiO2 layer grows into the Ti due a combination of variations in surface free energy and the porous nature of the TiO2 matrix. Samples that were pickled in acid for 120 s and oxidised under peroxide were tested to determine the influence of post processing. The results of post processing on crystallinity, are shown in Fig. 2, and show measureable differences in the crystallinity. The TiO2 produced in 30% H2O2(aq) exhibits a more crystalline structure prior to post processing. A comparison between thermal anneal or water bath ageing shows that water bath annealing produces more crystalline material. A thermal anneal of 300 °C was selected to maintain anatase [14] material. Variation in the crystallinity of the post-processed materials stems from the additional crystallinity of the pre-processed material which acts as a template for crystal growth or gives additional counts to the XRD baseline. Glancing angle XRD, Fig. 2 inset, shows little variation in the crystallinity of the material and good agreement with the anatase (101) peak is found [19]. Glancing angle XRD shows no secondary phase present. Raman spectroscopy was used to analyse the surface of the sample, Fig. 3. Raman spectroscopy is highly surface sensitive and shows that anatase TiO2 was present after the initial oxidation process, with the presence of a secondary phase. The action of post process ageing in water reduces the full width half maximum of the peaks indicating an improvement of surface quality and removal of the secondary phase. The exact nature of the secondary phase is not clear as brookite exhibits a strong peak at 153 cm− 1 and rutile at 232 cm− 1. However, there are reports of a brookite band [20] at 247 cm− 1 which might be represented in the nanostructure as the peaks between 250 and 265 cm− 1 in the samples produced. This indicates that the surface after ageing is primarily formed of anatase TiO2 and that the secondary phase detected by glancing angle XRD is sub-surface, and composed of material that has not been fully converted to anatase during ageing. Samples of ultra thin films of Ti (200 nm thick) were sputtered onto a glass substrate. The Ti layer was oxidised using 30% H2O2(aq) before
Fig. 1. SEM micrographs of TiO2 as formed on Ti sheet substrate after 50 or 120 s pickle under 15 or 30% H2O2(aq). Inset images show local nanostructured texture.
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F. Armani et al. / Materials Letters 64 (2010) 140–143
Fig. 2. XRD patterns for Ti sheet after oxidation in H2O2(aq). Patterns have been scaled on the y axis to improve clarity. The inset shows the glancing angle XRD pattern for the sample.
thermal annealing. During the oxidation stage the sample surface transformed from reflective metal to opaque. It was not possible to generate XRD or Raman spectra from the sample. In order to determine whether a photochemicaly active surface was produced the sample was
Fig. 4. Photochemical deposition of Ag nanoparticles on ultra thin TiO2 produced using H2O2(aq) oxidation of sputtered Ti thin film and subsequent water ageing. An SEM micrograph (a) shows the morphology of the surface with regions of grown Ag (bright regions) and an EDX analysis (b) shows the presence of silver on the surface.
immersed in 0.01 M AgNO3 solution. This was irradiated with superbandgap irradiation for 30min and examined using SEM/EDX. The surface was nanotextured, Fig. 4 (a). EDX demonstrated the presence of silver on the surface, Fig. 4 (b). The sample exhibited good optical transparency at wavelengths below the band gap of 93% and a sharp absorption at 3.35 eV associated with the band gap transition. This result therefore indicates that it is possible to make highly reticulated nanostructure TiO2 surfaces on transparent substrates. 4. Conclusions Surface morphology of peroxide oxidised Ti is influenced at the large scale 5 + μm by surface preparation of the Ti substrate. Lower scale (1 μm) texture is defined by the redeposition of oxidised metal. It is possible to use the process to make ultra thin films of TiO2 that exhibit good sub band gap transparency and photocatalytic activity. Acknowledgement We would like to acknowledge Dr M Kershaw his for help with the XRD and SEM analysis. References
Fig. 3. Raman spectra for samples of TiO2 produced in peroxide solution pre or post water ageing.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Goodeve CF, Kitchener JA. Trans Faraday Soc 1938;34:902–12. Jones PM, Gallardo DE, Dunn S. Chem Mater 2008;20:5901–6. Nasby RD, Quinn RK. Mater Res Bull 1976;11:985. Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Chem Rev 1995;95(1):69–96. Hashimoto K, Irie H, Fujishima A. Jpn J Appl Phys 2005;44(12):8269–85. Fujishama A, Honda K. Nature 1972;238:37–8. O'Regan B, Grätzel M. Nature 1991;353(6346):737–40. Zhao G, Utsumi S, Kozuka H, Yoko T. J Mater Sci 1998;33:3655–9. Rogers KD, Lane DW, Painter JD, Chapman A. Thin Solid Films 2004;466:97–102. Ahn YU, Kim EJ, Kim HT, Hahn SH. Mater Lett 2003;57:4660–6. Gemelli E, Camargo NHA. Rev Mater 2007;12:525–31. Wen M, Gu J-F, Liu G, Wang Z-B, Lu J. Appl Surf Sci 2008;254:2905–10.
F. Armani et al. / Materials Letters 64 (2010) 140–143 [13] Froimovitch A, McGarry S, Smy T, Fraser J. Electrochem Solid-State Lett 2007;10: K11–2. [14] Wu J-M. J Cryst Growth 2004;269:347–55. [15] Wu J-M, Wang M, Li Y-W, Zhao F-D, Ding X-J, Osaka A. Surf Coat Technol 2006;201:755–61. [16] Weast RC. Handbook of Chemistry and Physics. 65th Edition. Boca Raton, FL.: CRC Press; 1984. p. B154–5.
[17] [18] [19] [20]
143
Srivastava KG. Phys Rev 1960;199:250–5. Xie YB, Li XZ. J Appl Electromagn 2006;36:663–8. Song GB, Liang JK, Liu FS, Peng TJ, Rao GH. Thin Solid Films 2005;491:110–6. Gotic M, Ivanda M, Popovic S, Music S, Seklic A, Turkovic A, Furic K. J Raman Spectrosc 1997;28:555–8.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Nanostructured TiO2 — morphological and structural changes F. Armani a, M. Gougis a, S.A. Impey b, A.C. James a, K. Lawson b, L. Lihrmann b, M. Stock a, S. Dunn a,⁎ a b
Nanotechnology Centre, Building 30, Cranfield University, MK43 0AL, UK Surface Science and Engineering, Building 61, Cranfield University, MK43 0AL, UK
a r t i c l e
i n f o
Article history: Received 23 July 2009 Accepted 8 October 2009 Available online 17 October 2009 Keywords: TiO2 Oxidation Surface analysis
a b s t r a c t Chemical modification of titanium to form TiO2 is being extensively pursued. We compare the influence of acid pickling and post processing on the crystallinity and morphology of TiO2 grown by H2O2 oxidation. Subsequent oxidation using two concentrations of H2O2 (15% and 30% by mass) indicates a variation in the fine surface morphology, apparent at low magnifications. No discernable difference in the quality of TiO2 that was produced when evaluated by SEM, Raman or XRD (glancing angle and θ:2θ) could be detected between the samples oxidised in different H2O2 concentrations. The photoactivity of TiO2 produced from ultra thin Ti films was confirmed by the photocatalytic reduction of Ag+ cations to Ag0. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Early reports of the photoactive behaviour of TiO2 [1] sparked an interest in applications and materials science of wide band gap semi-conductors. Materials investigated range from complex oxides PbZrxTi1 −x O3 [2], BaTiO3 [3] and simple binary systems [4] such as TiO2 [5]. TiO2 is interesting due its range of applications; ability to split water into H2 and O2 [6] and use in dye sensitised solar cell (DSSC) or Graetzel cells [7] being very topical. Recently there has been a focus on methods to produce highly nanostructured TiO2 from Ti substrates. The textured surface is suitable to many applications for which TiO2 is used. It can positively influence kinetics of surface reactions, through increased contact area, and reduce diffusion lengths of photoexcited carriers in optoelectronic devices such as the DSSC. Anatase is a metastable low temperature form of TiO2 and generally accepted as the most suitable for photochemical devices [8,9]. When processing TiO2 precursors there is a limit to the upper processing temperature. This limit is process dependent e.g. direct oxidation or sol–gel, and varies from 500 to 1000 °C [10,11]. Low temperature processing is considered valuable, as it affords low energy operations, and an ability to use a wider variety of substrates. Especial importance is the ability to use heat treated polyimide or polyesters in DSSC. One promising technique is using H2O2 to oxidise the surface [12–14]. The substrate is bathed in H2O2 and etching– oxidation–redeposition occurs producing nanotextured surfaces composed mainly of amorphous TiO2. Addition of counter cations to the H2O2 modifies the amount of amorphous material [14] and phase of TiO2 produced. To fully crystallise the sample a post oxidation step is required using a simple thermal anneal. A second more interesting alternative provides a low temperature route, analogous to hydro⁎ Corresponding author. E-mail address: s.c.dunn@cranfield.ac.uk (S. Dunn). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.022
thermal techniques, involves ageing in a water bath [15]. Here we show the evolution of TiO2 from the chemical oxidation of Ti using H2O2. We have investigated the influence of pre-oxidation processing, the concentration of H2O2 on the final phase after no treatment, thermal annealing or ageing in water. We show that it is possible to produce photochemicaly active TiO2 from ca. 200 nm thick sputtered films of Ti. 2. Experimental methods Ti was degreased by ultrasonicating under acetone, then isopropyl alcohol for 5 min. Samples were pickled in HF(1%)/H2SO4(25%) at room temperature for 50 or 120 s and washed with de-ionised water. Compositional analysis was performed using EDX (Oxford Instruments INCA) on an SEM (Philips XL 30 S FEG SEM). EDX was calibrated to give qualitative and quantitative analysis. SEM analysis was performed using the same machine. The sample was placed into 50 mL of either 15 or 30% H2O2(aq) at 90 °C for 24 h for oxidation then rinsed with de-ionised water and left to dry. Raman spectroscopy was performed on a Renishaw system 1000 Raman microspectrometer. XRD analysis was completed on a Siemens D5005 using Cu Kα radiation. Ageing was performed in a water bath at 90 °C for 24 h. Annealing was performed on a hotplate at 300 °C for 1 h. Thin film Ti was sputtered from a 99.99% Ti target using a Nordico sputtering system onto a glass substrate. Silver nitrate (99.99%) for was purchased from Sigma-Aldrich. Fresh 0.01 M solutions of silver nitrate were made when required. A Honle Fe doped Mg lamp was used to irradiate the sample. 3. Results and discussion The impact of cleaning and acid pickling in HF/H2SO4 solution on the Ti was investigated using EDX, as shown in Table 1. The as-
F. Armani et al. / Materials Letters 64 (2010) 140–143 Table 1 Compositional analysis performed by EDX for Ti samples at different stages of processing. Sample
As-received 50s pickle 120s pickle Cross section
Atomic ratios / (%) Ti
O
C
52.1 73.0 92.8 97.3
17.0 15.12 0 0
30.7 11.8 7.2 2.7
received Ti sheet sample contains significant amounts of surface oxygen originating from the passivation layer (TiO2). After 120 s (for the sample tested) of acid pickling metallic Ti was exposed. The carbon present on the surface in the as-received sample stems from the photodegradation of organic materials in contact with the surface. However, it should be noted that the core of the sample, where no contamination due to surface contact would be possible, contained almost 3% carbon. The source of this carbon is unknown and is likely to be the supplied Ti. SEM examination of the surface after reaction with the H2O2, of two concentrations, is shown in Fig. 1. A difference in the morphology of the surface associated with the concentration of the H2O2(aq) is observed. There is no such variation in morphology caused by a variation of time in the acid bath. In the case of the 15% solution the surface looks cracked, whereas for the 30% sample it looks smooth with local nano/microstructured texture. There are few large cracks in the surface. The observed changes related to the acid exposure stem from the fact that the acid bath removes the tenacious oxide layer from the surface of the Ti [16,17]. The oxide layer of the as-received Ti samples is of an unknown thickness. H2O2(aq) is capable of dissolving [13] both Ti and TiO2 to produce a highly reticulated surface. Hence the incomplete removal of the TiO2 in the acid bath has made little change to overall nanostructured morphology of the surface. In the case of the lower concentration peroxide (15%) the cracks in the micrographs are commensurate with the grain structure of the Ti substrates. The removed material is then redeposited in the nanostructure reticulated structure, as shown in the insets of Fig. 1. Fig. 1 also shows that whilst there is a difference with the 30% solution surface morphology when compared to the 15% specimen, there is only evidence of grain etching in the sample that was pickled for 50 s in acid. The lower time in the acid bath may not completely remove
141
the TiO2, see Table 1. TiO2 is less soluble in H2O2(aq) resulting in selective etching at grain boundaries [16]. As dissolution of the surface takes place at a faster rate in concentrated H2O2(aq) [17] the most notable change is the overall large scale surface morphology. This result confirms the hypothesis that there is a dynamic equilibrium [18] which involves the dissolution of surface TiOx, x can be 0 or 2, and selective oxidation of dissolved Ti cations before precipitation onto the surface of as TiO2. The TiO2 layer grows into the Ti due a combination of variations in surface free energy and the porous nature of the TiO2 matrix. Samples that were pickled in acid for 120 s and oxidised under peroxide were tested to determine the influence of post processing. The results of post processing on crystallinity, are shown in Fig. 2, and show measureable differences in the crystallinity. The TiO2 produced in 30% H2O2(aq) exhibits a more crystalline structure prior to post processing. A comparison between thermal anneal or water bath ageing shows that water bath annealing produces more crystalline material. A thermal anneal of 300 °C was selected to maintain anatase [14] material. Variation in the crystallinity of the post-processed materials stems from the additional crystallinity of the pre-processed material which acts as a template for crystal growth or gives additional counts to the XRD baseline. Glancing angle XRD, Fig. 2 inset, shows little variation in the crystallinity of the material and good agreement with the anatase (101) peak is found [19]. Glancing angle XRD shows no secondary phase present. Raman spectroscopy was used to analyse the surface of the sample, Fig. 3. Raman spectroscopy is highly surface sensitive and shows that anatase TiO2 was present after the initial oxidation process, with the presence of a secondary phase. The action of post process ageing in water reduces the full width half maximum of the peaks indicating an improvement of surface quality and removal of the secondary phase. The exact nature of the secondary phase is not clear as brookite exhibits a strong peak at 153 cm− 1 and rutile at 232 cm− 1. However, there are reports of a brookite band [20] at 247 cm− 1 which might be represented in the nanostructure as the peaks between 250 and 265 cm− 1 in the samples produced. This indicates that the surface after ageing is primarily formed of anatase TiO2 and that the secondary phase detected by glancing angle XRD is sub-surface, and composed of material that has not been fully converted to anatase during ageing. Samples of ultra thin films of Ti (200 nm thick) were sputtered onto a glass substrate. The Ti layer was oxidised using 30% H2O2(aq) before
Fig. 1. SEM micrographs of TiO2 as formed on Ti sheet substrate after 50 or 120 s pickle under 15 or 30% H2O2(aq). Inset images show local nanostructured texture.
142
F. Armani et al. / Materials Letters 64 (2010) 140–143
Fig. 2. XRD patterns for Ti sheet after oxidation in H2O2(aq). Patterns have been scaled on the y axis to improve clarity. The inset shows the glancing angle XRD pattern for the sample.
thermal annealing. During the oxidation stage the sample surface transformed from reflective metal to opaque. It was not possible to generate XRD or Raman spectra from the sample. In order to determine whether a photochemicaly active surface was produced the sample was
Fig. 4. Photochemical deposition of Ag nanoparticles on ultra thin TiO2 produced using H2O2(aq) oxidation of sputtered Ti thin film and subsequent water ageing. An SEM micrograph (a) shows the morphology of the surface with regions of grown Ag (bright regions) and an EDX analysis (b) shows the presence of silver on the surface.
immersed in 0.01 M AgNO3 solution. This was irradiated with superbandgap irradiation for 30min and examined using SEM/EDX. The surface was nanotextured, Fig. 4 (a). EDX demonstrated the presence of silver on the surface, Fig. 4 (b). The sample exhibited good optical transparency at wavelengths below the band gap of 93% and a sharp absorption at 3.35 eV associated with the band gap transition. This result therefore indicates that it is possible to make highly reticulated nanostructure TiO2 surfaces on transparent substrates. 4. Conclusions Surface morphology of peroxide oxidised Ti is influenced at the large scale 5 + μm by surface preparation of the Ti substrate. Lower scale (1 μm) texture is defined by the redeposition of oxidised metal. It is possible to use the process to make ultra thin films of TiO2 that exhibit good sub band gap transparency and photocatalytic activity. Acknowledgement We would like to acknowledge Dr M Kershaw his for help with the XRD and SEM analysis. References
Fig. 3. Raman spectra for samples of TiO2 produced in peroxide solution pre or post water ageing.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Goodeve CF, Kitchener JA. Trans Faraday Soc 1938;34:902–12. Jones PM, Gallardo DE, Dunn S. Chem Mater 2008;20:5901–6. Nasby RD, Quinn RK. Mater Res Bull 1976;11:985. Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Chem Rev 1995;95(1):69–96. Hashimoto K, Irie H, Fujishima A. Jpn J Appl Phys 2005;44(12):8269–85. Fujishama A, Honda K. Nature 1972;238:37–8. O'Regan B, Grätzel M. Nature 1991;353(6346):737–40. Zhao G, Utsumi S, Kozuka H, Yoko T. J Mater Sci 1998;33:3655–9. Rogers KD, Lane DW, Painter JD, Chapman A. Thin Solid Films 2004;466:97–102. Ahn YU, Kim EJ, Kim HT, Hahn SH. Mater Lett 2003;57:4660–6. Gemelli E, Camargo NHA. Rev Mater 2007;12:525–31. Wen M, Gu J-F, Liu G, Wang Z-B, Lu J. Appl Surf Sci 2008;254:2905–10.
F. Armani et al. / Materials Letters 64 (2010) 140–143 [13] Froimovitch A, McGarry S, Smy T, Fraser J. Electrochem Solid-State Lett 2007;10: K11–2. [14] Wu J-M. J Cryst Growth 2004;269:347–55. [15] Wu J-M, Wang M, Li Y-W, Zhao F-D, Ding X-J, Osaka A. Surf Coat Technol 2006;201:755–61. [16] Weast RC. Handbook of Chemistry and Physics. 65th Edition. Boca Raton, FL.: CRC Press; 1984. p. B154–5.
[17] [18] [19] [20]
143
Srivastava KG. Phys Rev 1960;199:250–5. Xie YB, Li XZ. J Appl Electromagn 2006;36:663–8. Song GB, Liang JK, Liu FS, Peng TJ, Rao GH. Thin Solid Films 2005;491:110–6. Gotic M, Ivanda M, Popovic S, Music S, Seklic A, Turkovic A, Furic K. J Raman Spectrosc 1997;28:555–8.