- Email: [email protected]
Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 91 (2007) 54–61 www.elsevier.com/locate/solmat
Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film Ringo C.W. Lama, Michael K.H. Leunga,, Dennis Y.C. Leunga, Lilian L.P. Vrijmoedb, W.C. Yamc, S.P. Ngc a
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, China c Department of Microbiology, The University of Hong Kong, Pokfulam Road, Hong Kong, China
b
Received 11 January 2006; received in revised form 30 June 2006; accepted 7 July 2006 Available online 14 September 2006
Abstract Sol–gel nano titanium dioxide (TiO2) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO2 lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO2 thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO2 thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO2 photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO2coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates. r 2006 Elsevier B.V. All rights reserved. Keywords: Photocatalytic; Titanium dioxide; Chromium ion implantation; Visible light
1. Introduction The photocatalytic oxidation effect of TiO2 irradiated by artificial ultraviolet (UV) light on degradation of gaseous volatile organic compounds (VOCs) was demonstrated in previous studies [1–5]. Ching et al. [6] further showed the effectiveness of using solar UV to activate sol–gel TiO2 thin film to decompose gaseous formaldehyde. The solar photocatalysis can be enhanced by modifying the TiO2 Corresponding author. Tel.: +852 2859 2628; fax: +852 2858 5415.
E-mail address: [email protected] (M.K.H. Leung). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.07.004
photocatalyst to extend the activating spectrum from UV to visible light. Based on the studies reported in the literature, metal ion implantation [7], ion doping [8–11], and dye sensitization [12–15] are effective TiO2 modification methods. Among the above-mentioned methods, TiO2 modified by metal ion implantation showed the highest photocatalytic oxidation effect under visible light irradiation. When TiO2 is bombarded with transitional metal ions accelerated in a high-voltage field, the high-energy ions are injected into the TiO2 lattice. This process modifies the TiO2 electronic structure and extends its photoresponse to the visible range
ARTICLE IN PRESS R.C.W. Lam et al. / Solar Energy Materials & Solar Cells 91 (2007) 54–61
up to 700 nm. In this research, sol–gel TiO2 thin films were modified by Cr ion implantation and the effects on visiblelight photocatalytic degradation of formaldehyde vapor were evaluated experimentally. 2. Fabrication of Cr/TiO2 photoreactors The basic component of the photoreactors tested in this study was a thin borosilicate glass plate coated on top with a sol–gel TiO2 thin film with subsequent modification by Cr ion implantation. A single coated plate or multiple parallel plates were placed inside a rectangular channel with a borosilicate glass window. Therefore, visible light from an external source can transmit into the Cr/TiO2 thin films. The detailed preparation procedures are described in the following subsections. 2.1. Sol–gel TiO2 thin film coating Sol–gel TiO2 thin-films were prepared according to the procedures described in the previous study [6]. First, 15-ml titanium (IV) isopropoxide and 1-ml nitric acid were added to 150-ml deionized water. Then, the solution was refluxed in a stirring flask at 80 1C for 3 days to produce a sol–gel solution. A borosilicate glass plate (24 mm 24 mm; 0.1 mm thick) used as a substrate was cleaned with ethanol and deionized water. The sol–gel solution was then evenly applied onto one side of a glass plate. After it was air dried at room temperature for 24 h, the glass plate was calcinated to 400 1C with a heating rate of 3 1C min1. The calcination was maintained at 400 1C for 2 h. The weight gain due to the sol–gel thin film was 0.6 mg, equivalent to 0.3% of the weight of the glass substrate. The plate was stored under the room condition for 1 day in order to ensure that the adsorption of moisture by the thin film reached the equilibrium state before conducting any further modification of the photocatalyst. 2.2. Cr ion implantation The glass plate coated with sol–gel TiO2 thin film was placed inside an ion implanter. A vacuum pump was used to achieve a pressure below 105 Torr in order to avoid any reactions between the Cr ions and air. The Cr ions were accelerated in an electric field of 80 keV and injected into the TiO2 lattice. The ion implantation process was carried out with an electrical current of 0.4 mA. The processing time was regulated to control the amount of Cr ion loading on a sample. 2.3. Photoreactors Two different photoreactor configurations were built for experimental parametric tests. First, a single-plate photoreactor was built by placing one Cr/TiO2-coated glass plate inside a rectangular channel. The internal dimensions of the channel were 100 mm in length, 24 mm in width and
55
10 mm in height. The upper wall was made of borosilicate glass to allow visible light transmission from an external source. Additionally, a parallel-plate photoreactor was built with three Cr/TiO2-coated glass plates to increase the apparent photoactivated surface area. The internal height of the rectangular channel was 30 mm and the separation between two adjacent plates was 10 mm. The other dimensions and the window on top were same as the single-plate photoreactor. 3. Characterization of TiO2 and Cr/TiO2 thin films A series of characterization tests were conducted to analyze the chemical contents and crystal structures of the TiO2 and Cr/TiO2 photocatalysts fabricated for the experimental tests in this study. The characteristics found are presented below. 3.1. X-ray diffraction (XRD) A Bruker D8 Advance diffractometer with Cu Ka was used to obtain the XRD pattern of a sol–gel TiO2 thin film. A scan rate of 0.051 s1 was used. The XRD pattern shown in Fig. 1 was compared with the 21–1272 anatase American Society for Testing and Materials (ASTM) card and the 21–1276 rutile ASTM card. Only the anatase phase was found without the presence of the rutile phase. 3.2. Differential scanning calorimetry (DSC) and thermal gravity (TG) analysis The DSC and TG analysis were preformed using a NETZSCH instrument. The TiO2 photocatalyst test sample was prepared by heating the sol–gel solution at 75 1C for 24 h to drive out the liquid content. A 10 mg sample was tested with a heating rate of 10 1C min1 in flowing air. As shown by the TG curve in Fig. 2, two rates of weight loss were observed. From room temperature to 380 1C, the weight loss was due to desorption of physically adsorbed water. From 380 to 970 1C, the lower rate of weight loss was due to the removal of the residual organics and chemisorbed water. The DSC curve in Fig. 2 showed two endothermic peaks at 120 and 240 1C corresponding to the boiling point of nitric acid and that of titanium (IV) isopropoxide, respectively. The absence of exothermic peak implied that there was no TiO2 crystallization, normally found between 400 and 500 1C for crystallization of amorphous hydrated TiO2 to the anatase phase [16]. The results indicated that the anatase phase was formed during the heating process at low temperature under 75 1C. 3.3. UV–Vis spectrophotometry A Perkin Elmer Lambda 900 UV–Vis–NIR spectrophotometer was used to measure the UV–Vis absorbance
ARTICLE IN PRESS R.C.W. Lam et al. / Solar Energy Materials & Solar Cells 91 (2007) 54–61
56
Intensity (a.u.)
A : Anatase phase
A
20
30
A
40
A
50
60
70
2θ (°)
Fig. 1. XRD patterns of sol–gel TiO2 calcinated at 400 1C. 100
4 Exo 3 TG
DSC
90
2
1
DSC (mW/mg)
TG (%)
95
0 85
120°C -1 240°C
80 0
200
400 600 Temperature (°C)
800
-2 1000
Fig. 2. DSC–TG analysis of sol–gel TiO2 photocatalyst.
of the TiO2 and Cr/TiO2 thin films. As shown in Fig. 3, the sol–gel TiO2 thin film without Cr ion implantation could mostly absorb UVA radiation. As Cr ions were implanted into TiO2, the absorption band was extended to the visible range. The visible light absorbance of the Cr/TiO2 thin film increased with the Cr ion loading. It was observed that, in comparison with other UV–Vis absorbance measurements, the UVA absorbance of the Cr/ TiO2 thin film sample (c) with Cr ion loading of 2 1016 ions cm2 was abnormally low but the visible light absorbance was in a proper magnitude with respect to its Cr ion loading. This incident occurred possibly because the thin film (c) had a relatively small thickness so it absorbed less UVA; however, as the same ion acceleration energy (80 keV) was used in all the Cr ion implantation processes, the Cr ions were implanted into the TiO2 thin film at the
same depth that was independent of the TiO2 thin film thickness. Thus, the measured visible light absorbance properly increased with the Cr ion loading. 3.4. Scanning-electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis The surface topography of the TiO2 and Cr/TiO2 thin films were measured by a LEO1530 SEM equipped with an Oxford Instruments EDX system. As shown in Fig. 4a, the crystallinity of the sol–gel TiO2 thin film was observed and the diameter of the nano-particles was about 8 nm. After modification by Cr ion implantation, interstitial dislocation loops of about 1 mm diameter were produced as shown in Fig. 4b. The EDX system was used to measure the chemical composition of the Cr/TiO2 thin films in terms of atomic
ARTICLE IN PRESS R.C.W. Lam et al. / Solar Energy Materials & Solar Cells 91 (2007) 54–61
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70 Ion acceleration energy: 80 keV Cr ion loadings (ions×cm-2):
60
(a) 0, (b) 1×1016, (c) 2×1016 and (d) 8×1016
Absorbance (%)
50
(d)
40
(c)
30
(b) 20
(a)
10 0 300
400
500 Wavelength (nm)
600
700
Fig. 3. UV–Vis spectra of TiO2 and Cr/TiO2 thin films.
Fig. 4. SEM images: (a) sol–gel TiO2 thin film and (b) Cr/TiO2 thin film (Cr ion loading ¼ 8 1016 ions cm2).
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percentage. The presence of Cr was found. The quantitative results are summarized in Table 1. 3.5. Visible-light-assisted photocatalysis The visible-light-assisted photocatalytic effect of Cr/ TiO2 thin films was assessed by decolorization test of Acid Blue 80 dye solution. At the beginning of the test, two Cr/ TiO2-coated glass plates with Cr loading of 1 1016 ions cm2 were placed in a 90 mm petri dish and 10 ml of Acid Blue 80 dye solution (10 ppm and pH 5.78) was then poured into the petri dish. The setup was kept in the dark for 30 min to allow the adsorption of dye to reach a state of equilibrium. Then, a 250-W metal halide lamp (Philips) placed above the petri dish was turned on to produce visible light irradiance of 1.2 mW cm2. Samples of the dye solution were taken at designated time intervals and analyzed by a calibrated UV–Vis spectrophotometer (Spectronic Instrument, Spectronic Genesys 2). The decolorization test results in terms of percentage reduction of dye concentration are plotted in Fig. 5. The results clearly showed that when Cr/TiO2 was irradiated by visible light, apparent photocatalytic decolorization effect was obtained.
effect on gaseous formaldehyde. The single-pass reduction in formaldehyde concentration was measured as the contaminated air was drawn through an irradiated photoreactor. The experimental details are presented below.
4.1. Test apparatus The single-pass circulation test equipment was set up as illustrated in Fig. 6. The photoreactor, U-tube, and cubic chamber (200 mm each side) were sealed to form a closed system. A 250-W metal halide lamp was used as an external source of visible light. A UV filter was placed on the window of the photoreactor in order to block any unwanted UV light emitted by the lamp. The air circulation was regulated by a variable-speed fan. Two sampling ports were available at the inlet and outlet of the photoreactor for simultaneous collection of air samples.
4. Experimental setup and procedures The performance of a Cr/TiO2 photoreactor was experimentally tested for its photocatalytic degradation Table 1 EDX analysis of Cr/TiO2 thin films Sample (c)
Sample (d)
1 1016
2 1016
8 1016
32.89 65.77 1.34
32.54 65.09 2.37
29.58 59.17 11.25
Fig. 6. Experimental setup for testing parallel-plate Cr/TiO2 photoreactor.
Photocatalytic degradation of Acid Blue 80 Visible light irradiance = 1.2 mW cm-2
15 Decolorization effect (%)
Cr ion loading (ions cm2) Atomic percentage Ti (%) O (%) Cr (%)
Sample (b)
Visible light only
10
Visible light + TiO2 Visible light + Cr/TiO2 5
0 0
50
100 150 Irradiation time (min.)
200
250
Fig. 5. Decolorization of Acid Blue 80 dye solution by Cr/TiO2 under visible-light irradiation.
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4.2. Test procedures The single-plate and parallel-plate photoreactors were tested separately for various Cr ion loading and visible light irradiance. At the beginning of each test, the Cr/TiO2 reactor was covered by a piece of opaque fabric to avoid exposure to the indoor lighting. Then, the vapor in a bottle of 37% MERCK formaldehyde solution was charged to the chamber until the formaldehyde concentration reached about 400 ppm. The variable-speed fan was regulated to drive the air circulation through the reactor at a rate of 3 104 m3 s1. The air movement promoted a fully mixed condition inside the test chamber, as well as the U-tube. Meanwhile, the Cr/TiO2 thin films were allowed to adsorb and desorb formaldehyde to reach the state of equilibrium. Air samples were collected and analyzed to ensure that the system stabilized. The time taken for a steady condition was about 2 h. The procedures of sampling analysis using a formaldehyde meter were reported in the previous studies [6,17]. The photocatalytic degradation of gaseous formaldehyde was started by exposing the Cr/TiO2 photoreactor under the 250-W metal halide lamp. In each set of measurements, 1-ml air samples were collected by separate injection needles at the inlet and outlet of the Cr/TiO2 photoreactor simultaneously. For testing each combination of material and operational parameters, four sets of air sampling measurements were taken to obtain the maximum, minimum, and mean values of the photocatalytic degradation effect. 5. Results and discussion 5.1. Single-plate photoreactor The photocatalytic degradation effect of a single-plate photoreactor on gaseous formaldehyde is presented in
59
Fig. 7. The single-pass photocatalytic degradation effect (Z) referring to the percentage reduction of formaldehyde is calculated by Z¼
Ci Co 100%, Ci
(1)
where Ci and Co are the formaldehyde concentrations at the inlet and outlet of the photoreactor, respectively. For the sol–gel TiO2 thin film without the presence of Cr ion, the visible light irradiance (I) of 1.2 mW cm2 could not activate the photocatalytic reaction. After Cr ion implantation, the Cr/TiO2 thin film became sensitive to the visible light. The photocatalytic degradation of formaldehyde (Z) increased with increasing loading of Cr ion until Z reached 4.2% at a loading of 2 1016 ions cm2. Any excessive Cr ion loading would not enhance Z. This phenomenon indicated that sufficient Cr ion was already implanted to absorb all the visible light useful for the photocatalytic effect. At this point, the overall degradation rate was limited by the adsorption of formaldehyde into the Cr/TiO2 photocatalyst. 5.2. Parallel-plate photoreactor A design of stacking multiple single-plate photoreactors to form a parallel-plate photoreactor, as illustrated in the experimental setup in Fig. 6, can possibly increase the photocatalytic degradation effect. It is because the apparent light activated area can be increased if sufficient light can irradiate and transmit through multiple parallelcoated plates. Moreover, the residence time will increase, as the cross-sectional area of the photoreactor is larger while the volumetric airflow rate remains unchanged. In this investigation, the performance of parallel-plate photoreactors was studied experimentally. Each photoreactor tested consisted of three identical parallel Cr/ TiO2-coated plates with the same Cr ion loading. The
Photocatalytic degradation of gaseous formaldehyde by single-plate photoreactor
6
Degradation, η (%)
Visible light irradiance : 1.2 mW cm-2
4
Max Mean
2
Experiment
Min
0 0
2
4
6
8
10
Cr ion loading (x 1016 ions cm-2)
Fig. 7. Visible-light-assisted photocatalytic degradation of formaldehyde by single-plate photoreactor.
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experimental results plotted in Fig. 8 showed how the Cr ion loading affected the overall photocatalytic degradation effect. Among the different Cr ion loadings tested with a visible light irradiance (I) of 1.2 mW cm2, a loading of 1 1016 ions cm2 yielded the highest Z equal to 13%. Increasing the Cr ion loading above 1 1016 ions cm2 caused a reduction in Z. The phenomenon could be explained that the poor light transmission of the Cr/TiO2 thin films at high Cr ion loading reduced the irradiation of the lower Cr/TiO2-coated glass plates. Although the Cr/TiO2 thin film on the top plate would absorb more light, its photocatalytic degradation effect had already reached the maximum as presented by the singleplate test results shown in Fig. 7. Therefore, the overall Z decreased with increasing Cr ion loading above 1 1016 ions cm2.
More tests were conducted to obtain the effect of irradiance. The parallel-plate photoreactor exhibiting the best performance at I equal to 1.2 mW cm2 (three Cr/TiO2 glass plates with Cr loading of 1 1016 cm2) was tested at higher irradiance up to 3 mW cm2. The experimental results shown in Fig. 9 indicated that as I increased, Z increased at a decreasing rate. The average value of measured Z was 18% when I was 3 mW cm2. The trend of Z versus I could be explained by the effects of electron–hole formation rate and light-saturated photocatalyst [6,18,19]. As the visible-light irradiance onto a parallel-plate photoreactor increased from zero, the abovementioned effects would initially become significant on the uppermost Cr/TiO2 thin film. As I continued to increase, the lower thin films would become light saturated one by one. When all the parallel thin films became
15 Photocatalytic degradation of gaseous formaldehyde by parallel-plate photoreactor
Degradation, η (%)
Visible light irradiance = 1.2 mW cm-2 10
5
Max Experiment
Mean Min 0 0
2
4
6
8
10
Cr ion loading (x 1016 ions cm-2)
Fig. 8. Visible-light-assisted photocatalytic degradation of formaldehyde by parallel-plate photoreactor.
25 Photocatalytic degradation of gaseous formaldehyde by parallel plate photoreactor Cr loading = 1 × 1016 ions cm-2
Degradation, η (%)
20
15
10 Max Mean 5
Experiment
Min
0 0
1
2
3
4
Visible light irradiance, I (mW cm-2) Fig. 9. Visible-light-assisted photocatalytic degradation effect of parallel-plate photoreactor versus visible light irradiance.
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light-saturated, the photoreactor would reach its peak performance to yield the maximum photocatalytic degradation effect.
6. Conclusions Sol–gel TiO2 thin film was successfully modified by Cr ion implantation to achieve visible-light photocatalysis. The experimental results revealed the effectiveness of the visible-light-assisted Cr/TiO2 photoreactors in degradation of gaseous formaldehyde. It implied that the photoreactors could be effectively applied to achieve solar photocatalysis. The study also demonstrated the effects of Cr ion loading and visible-light irradiance on photocatalytic degradation of formaldehyde vapor. For future design optimization of parallel-plate solar photoreactor, the number of parallel Cr/TiO2-coated glass plates and the amount of Cr ion loading should be properly controlled to maximize the efficiency of the use of the solar irradiation.
Acknowledgement The study was fully supported by a grant from the Innovation and Technology Fund of the Hong Kong SAR Government (ITS/076/03).
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References [1] P. Pichat, J. Disdier, C. Hoang-Van, D. Mas, G. Goutailler, C. Gaysse, Catal. Today 63 (2000) 363. [2] T.N. Obee, Environ. Sci. Technol. 30 (1996) 3578. [3] M.L. Sattler, H.M. Liljestrand, J. Air Waste Manage. Assoc. 53 (2003) 3. [4] G.M. Zuo, Z.X. Cheng, M. Xu, X.Q. Qiu, J. Photochem. Photobiol. A-Chem. 161 (2003) 51. [5] S. Hager, R. Bauer, Chemosphere 38 (1999) 1549. [6] W.H. Ching, M. Leung, D.Y.C. Leung, Sol. Energy 77 (2004) 129. [7] H. Yamashita, M. Anpo, Catal. Surv. Asia 8 (2004) 35. [8] B. Wawrzyniak, A.W. Morawski, Appl. Catal. B-Environ. 62 (2005) 150. [9] K. Demeestere, J. Dewulf, T. Ohno, P.H. Salgado, H.V. Langenhove, Appl. Catal. B-Environ. 61 (2005) 140. [10] B. Kosowska, S. Mozia, A.W. Morawski, B. Grzmil, M. Janus, K. Kalucki, Sol. Energy Mater. Sol. Cells 88 (2005) 269. [11] W.Y. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669. [12] D. Chatterjee, A. Mahata, Appl. Catal. B-Environ. 33 (2001) 119. [13] H. Ding, H. Sun, Y. Shan, J. Photochem. Photobiol. A-Chem. 169 (2005) 101. [14] D. Chatterjee, S. Dasgupta, N.N. Rao, Sol. Energy Mater. Sol. Cells 90 (2006) 1013. [15] R. Espinosa, I. Zumeta, J.L. Santana, F. Martinez-Luzardo, B. Gonzalez, S. Docteur, E. Vigil, Sol. Energy Mater. Sol. Cells 85 (2005) 359. [16] C.H. Ao, S.C. Lee, J.C. Yu, J. Photochem. Photobiol. A-Chem. 156 (2003) 171. [17] M.K.H. Leung, S.M. Tang, R.C.W. Lam, D.Y.C. Leung, W.C. Yam, S.P. Ng, L.L.P. Vrijmoed, Sol. Energy 80 (2006) 949. [18] H.T. Chang, N.M. Wu, F. Zhu, Water Res. 34 (2000) 407. [19] W. Wang, Y. Ku, J. Photochem. Photobiol. A-Chem. 159 (2003) 47.
Solar Energy Materials & Solar Cells 91 (2007) 54–61 www.elsevier.com/locate/solmat
Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film Ringo C.W. Lama, Michael K.H. Leunga,, Dennis Y.C. Leunga, Lilian L.P. Vrijmoedb, W.C. Yamc, S.P. Ngc a
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, China c Department of Microbiology, The University of Hong Kong, Pokfulam Road, Hong Kong, China
b
Received 11 January 2006; received in revised form 30 June 2006; accepted 7 July 2006 Available online 14 September 2006
Abstract Sol–gel nano titanium dioxide (TiO2) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO2 lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO2 thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO2 thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO2 photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO2coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates. r 2006 Elsevier B.V. All rights reserved. Keywords: Photocatalytic; Titanium dioxide; Chromium ion implantation; Visible light
1. Introduction The photocatalytic oxidation effect of TiO2 irradiated by artificial ultraviolet (UV) light on degradation of gaseous volatile organic compounds (VOCs) was demonstrated in previous studies [1–5]. Ching et al. [6] further showed the effectiveness of using solar UV to activate sol–gel TiO2 thin film to decompose gaseous formaldehyde. The solar photocatalysis can be enhanced by modifying the TiO2 Corresponding author. Tel.: +852 2859 2628; fax: +852 2858 5415.
E-mail address: [email protected] (M.K.H. Leung). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.07.004
photocatalyst to extend the activating spectrum from UV to visible light. Based on the studies reported in the literature, metal ion implantation [7], ion doping [8–11], and dye sensitization [12–15] are effective TiO2 modification methods. Among the above-mentioned methods, TiO2 modified by metal ion implantation showed the highest photocatalytic oxidation effect under visible light irradiation. When TiO2 is bombarded with transitional metal ions accelerated in a high-voltage field, the high-energy ions are injected into the TiO2 lattice. This process modifies the TiO2 electronic structure and extends its photoresponse to the visible range
ARTICLE IN PRESS R.C.W. Lam et al. / Solar Energy Materials & Solar Cells 91 (2007) 54–61
up to 700 nm. In this research, sol–gel TiO2 thin films were modified by Cr ion implantation and the effects on visiblelight photocatalytic degradation of formaldehyde vapor were evaluated experimentally. 2. Fabrication of Cr/TiO2 photoreactors The basic component of the photoreactors tested in this study was a thin borosilicate glass plate coated on top with a sol–gel TiO2 thin film with subsequent modification by Cr ion implantation. A single coated plate or multiple parallel plates were placed inside a rectangular channel with a borosilicate glass window. Therefore, visible light from an external source can transmit into the Cr/TiO2 thin films. The detailed preparation procedures are described in the following subsections. 2.1. Sol–gel TiO2 thin film coating Sol–gel TiO2 thin-films were prepared according to the procedures described in the previous study [6]. First, 15-ml titanium (IV) isopropoxide and 1-ml nitric acid were added to 150-ml deionized water. Then, the solution was refluxed in a stirring flask at 80 1C for 3 days to produce a sol–gel solution. A borosilicate glass plate (24 mm 24 mm; 0.1 mm thick) used as a substrate was cleaned with ethanol and deionized water. The sol–gel solution was then evenly applied onto one side of a glass plate. After it was air dried at room temperature for 24 h, the glass plate was calcinated to 400 1C with a heating rate of 3 1C min1. The calcination was maintained at 400 1C for 2 h. The weight gain due to the sol–gel thin film was 0.6 mg, equivalent to 0.3% of the weight of the glass substrate. The plate was stored under the room condition for 1 day in order to ensure that the adsorption of moisture by the thin film reached the equilibrium state before conducting any further modification of the photocatalyst. 2.2. Cr ion implantation The glass plate coated with sol–gel TiO2 thin film was placed inside an ion implanter. A vacuum pump was used to achieve a pressure below 105 Torr in order to avoid any reactions between the Cr ions and air. The Cr ions were accelerated in an electric field of 80 keV and injected into the TiO2 lattice. The ion implantation process was carried out with an electrical current of 0.4 mA. The processing time was regulated to control the amount of Cr ion loading on a sample. 2.3. Photoreactors Two different photoreactor configurations were built for experimental parametric tests. First, a single-plate photoreactor was built by placing one Cr/TiO2-coated glass plate inside a rectangular channel. The internal dimensions of the channel were 100 mm in length, 24 mm in width and
55
10 mm in height. The upper wall was made of borosilicate glass to allow visible light transmission from an external source. Additionally, a parallel-plate photoreactor was built with three Cr/TiO2-coated glass plates to increase the apparent photoactivated surface area. The internal height of the rectangular channel was 30 mm and the separation between two adjacent plates was 10 mm. The other dimensions and the window on top were same as the single-plate photoreactor. 3. Characterization of TiO2 and Cr/TiO2 thin films A series of characterization tests were conducted to analyze the chemical contents and crystal structures of the TiO2 and Cr/TiO2 photocatalysts fabricated for the experimental tests in this study. The characteristics found are presented below. 3.1. X-ray diffraction (XRD) A Bruker D8 Advance diffractometer with Cu Ka was used to obtain the XRD pattern of a sol–gel TiO2 thin film. A scan rate of 0.051 s1 was used. The XRD pattern shown in Fig. 1 was compared with the 21–1272 anatase American Society for Testing and Materials (ASTM) card and the 21–1276 rutile ASTM card. Only the anatase phase was found without the presence of the rutile phase. 3.2. Differential scanning calorimetry (DSC) and thermal gravity (TG) analysis The DSC and TG analysis were preformed using a NETZSCH instrument. The TiO2 photocatalyst test sample was prepared by heating the sol–gel solution at 75 1C for 24 h to drive out the liquid content. A 10 mg sample was tested with a heating rate of 10 1C min1 in flowing air. As shown by the TG curve in Fig. 2, two rates of weight loss were observed. From room temperature to 380 1C, the weight loss was due to desorption of physically adsorbed water. From 380 to 970 1C, the lower rate of weight loss was due to the removal of the residual organics and chemisorbed water. The DSC curve in Fig. 2 showed two endothermic peaks at 120 and 240 1C corresponding to the boiling point of nitric acid and that of titanium (IV) isopropoxide, respectively. The absence of exothermic peak implied that there was no TiO2 crystallization, normally found between 400 and 500 1C for crystallization of amorphous hydrated TiO2 to the anatase phase [16]. The results indicated that the anatase phase was formed during the heating process at low temperature under 75 1C. 3.3. UV–Vis spectrophotometry A Perkin Elmer Lambda 900 UV–Vis–NIR spectrophotometer was used to measure the UV–Vis absorbance
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56
Intensity (a.u.)
A : Anatase phase
A
20
30
A
40
A
50
60
70
2θ (°)
Fig. 1. XRD patterns of sol–gel TiO2 calcinated at 400 1C. 100
4 Exo 3 TG
DSC
90
2
1
DSC (mW/mg)
TG (%)
95
0 85
120°C -1 240°C
80 0
200
400 600 Temperature (°C)
800
-2 1000
Fig. 2. DSC–TG analysis of sol–gel TiO2 photocatalyst.
of the TiO2 and Cr/TiO2 thin films. As shown in Fig. 3, the sol–gel TiO2 thin film without Cr ion implantation could mostly absorb UVA radiation. As Cr ions were implanted into TiO2, the absorption band was extended to the visible range. The visible light absorbance of the Cr/TiO2 thin film increased with the Cr ion loading. It was observed that, in comparison with other UV–Vis absorbance measurements, the UVA absorbance of the Cr/ TiO2 thin film sample (c) with Cr ion loading of 2 1016 ions cm2 was abnormally low but the visible light absorbance was in a proper magnitude with respect to its Cr ion loading. This incident occurred possibly because the thin film (c) had a relatively small thickness so it absorbed less UVA; however, as the same ion acceleration energy (80 keV) was used in all the Cr ion implantation processes, the Cr ions were implanted into the TiO2 thin film at the
same depth that was independent of the TiO2 thin film thickness. Thus, the measured visible light absorbance properly increased with the Cr ion loading. 3.4. Scanning-electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis The surface topography of the TiO2 and Cr/TiO2 thin films were measured by a LEO1530 SEM equipped with an Oxford Instruments EDX system. As shown in Fig. 4a, the crystallinity of the sol–gel TiO2 thin film was observed and the diameter of the nano-particles was about 8 nm. After modification by Cr ion implantation, interstitial dislocation loops of about 1 mm diameter were produced as shown in Fig. 4b. The EDX system was used to measure the chemical composition of the Cr/TiO2 thin films in terms of atomic
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70 Ion acceleration energy: 80 keV Cr ion loadings (ions×cm-2):
60
(a) 0, (b) 1×1016, (c) 2×1016 and (d) 8×1016
Absorbance (%)
50
(d)
40
(c)
30
(b) 20
(a)
10 0 300
400
500 Wavelength (nm)
600
700
Fig. 3. UV–Vis spectra of TiO2 and Cr/TiO2 thin films.
Fig. 4. SEM images: (a) sol–gel TiO2 thin film and (b) Cr/TiO2 thin film (Cr ion loading ¼ 8 1016 ions cm2).
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percentage. The presence of Cr was found. The quantitative results are summarized in Table 1. 3.5. Visible-light-assisted photocatalysis The visible-light-assisted photocatalytic effect of Cr/ TiO2 thin films was assessed by decolorization test of Acid Blue 80 dye solution. At the beginning of the test, two Cr/ TiO2-coated glass plates with Cr loading of 1 1016 ions cm2 were placed in a 90 mm petri dish and 10 ml of Acid Blue 80 dye solution (10 ppm and pH 5.78) was then poured into the petri dish. The setup was kept in the dark for 30 min to allow the adsorption of dye to reach a state of equilibrium. Then, a 250-W metal halide lamp (Philips) placed above the petri dish was turned on to produce visible light irradiance of 1.2 mW cm2. Samples of the dye solution were taken at designated time intervals and analyzed by a calibrated UV–Vis spectrophotometer (Spectronic Instrument, Spectronic Genesys 2). The decolorization test results in terms of percentage reduction of dye concentration are plotted in Fig. 5. The results clearly showed that when Cr/TiO2 was irradiated by visible light, apparent photocatalytic decolorization effect was obtained.
effect on gaseous formaldehyde. The single-pass reduction in formaldehyde concentration was measured as the contaminated air was drawn through an irradiated photoreactor. The experimental details are presented below.
4.1. Test apparatus The single-pass circulation test equipment was set up as illustrated in Fig. 6. The photoreactor, U-tube, and cubic chamber (200 mm each side) were sealed to form a closed system. A 250-W metal halide lamp was used as an external source of visible light. A UV filter was placed on the window of the photoreactor in order to block any unwanted UV light emitted by the lamp. The air circulation was regulated by a variable-speed fan. Two sampling ports were available at the inlet and outlet of the photoreactor for simultaneous collection of air samples.
4. Experimental setup and procedures The performance of a Cr/TiO2 photoreactor was experimentally tested for its photocatalytic degradation Table 1 EDX analysis of Cr/TiO2 thin films Sample (c)
Sample (d)
1 1016
2 1016
8 1016
32.89 65.77 1.34
32.54 65.09 2.37
29.58 59.17 11.25
Fig. 6. Experimental setup for testing parallel-plate Cr/TiO2 photoreactor.
Photocatalytic degradation of Acid Blue 80 Visible light irradiance = 1.2 mW cm-2
15 Decolorization effect (%)
Cr ion loading (ions cm2) Atomic percentage Ti (%) O (%) Cr (%)
Sample (b)
Visible light only
10
Visible light + TiO2 Visible light + Cr/TiO2 5
0 0
50
100 150 Irradiation time (min.)
200
250
Fig. 5. Decolorization of Acid Blue 80 dye solution by Cr/TiO2 under visible-light irradiation.
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4.2. Test procedures The single-plate and parallel-plate photoreactors were tested separately for various Cr ion loading and visible light irradiance. At the beginning of each test, the Cr/TiO2 reactor was covered by a piece of opaque fabric to avoid exposure to the indoor lighting. Then, the vapor in a bottle of 37% MERCK formaldehyde solution was charged to the chamber until the formaldehyde concentration reached about 400 ppm. The variable-speed fan was regulated to drive the air circulation through the reactor at a rate of 3 104 m3 s1. The air movement promoted a fully mixed condition inside the test chamber, as well as the U-tube. Meanwhile, the Cr/TiO2 thin films were allowed to adsorb and desorb formaldehyde to reach the state of equilibrium. Air samples were collected and analyzed to ensure that the system stabilized. The time taken for a steady condition was about 2 h. The procedures of sampling analysis using a formaldehyde meter were reported in the previous studies [6,17]. The photocatalytic degradation of gaseous formaldehyde was started by exposing the Cr/TiO2 photoreactor under the 250-W metal halide lamp. In each set of measurements, 1-ml air samples were collected by separate injection needles at the inlet and outlet of the Cr/TiO2 photoreactor simultaneously. For testing each combination of material and operational parameters, four sets of air sampling measurements were taken to obtain the maximum, minimum, and mean values of the photocatalytic degradation effect. 5. Results and discussion 5.1. Single-plate photoreactor The photocatalytic degradation effect of a single-plate photoreactor on gaseous formaldehyde is presented in
59
Fig. 7. The single-pass photocatalytic degradation effect (Z) referring to the percentage reduction of formaldehyde is calculated by Z¼
Ci Co 100%, Ci
(1)
where Ci and Co are the formaldehyde concentrations at the inlet and outlet of the photoreactor, respectively. For the sol–gel TiO2 thin film without the presence of Cr ion, the visible light irradiance (I) of 1.2 mW cm2 could not activate the photocatalytic reaction. After Cr ion implantation, the Cr/TiO2 thin film became sensitive to the visible light. The photocatalytic degradation of formaldehyde (Z) increased with increasing loading of Cr ion until Z reached 4.2% at a loading of 2 1016 ions cm2. Any excessive Cr ion loading would not enhance Z. This phenomenon indicated that sufficient Cr ion was already implanted to absorb all the visible light useful for the photocatalytic effect. At this point, the overall degradation rate was limited by the adsorption of formaldehyde into the Cr/TiO2 photocatalyst. 5.2. Parallel-plate photoreactor A design of stacking multiple single-plate photoreactors to form a parallel-plate photoreactor, as illustrated in the experimental setup in Fig. 6, can possibly increase the photocatalytic degradation effect. It is because the apparent light activated area can be increased if sufficient light can irradiate and transmit through multiple parallelcoated plates. Moreover, the residence time will increase, as the cross-sectional area of the photoreactor is larger while the volumetric airflow rate remains unchanged. In this investigation, the performance of parallel-plate photoreactors was studied experimentally. Each photoreactor tested consisted of three identical parallel Cr/ TiO2-coated plates with the same Cr ion loading. The
Photocatalytic degradation of gaseous formaldehyde by single-plate photoreactor
6
Degradation, η (%)
Visible light irradiance : 1.2 mW cm-2
4
Max Mean
2
Experiment
Min
0 0
2
4
6
8
10
Cr ion loading (x 1016 ions cm-2)
Fig. 7. Visible-light-assisted photocatalytic degradation of formaldehyde by single-plate photoreactor.
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experimental results plotted in Fig. 8 showed how the Cr ion loading affected the overall photocatalytic degradation effect. Among the different Cr ion loadings tested with a visible light irradiance (I) of 1.2 mW cm2, a loading of 1 1016 ions cm2 yielded the highest Z equal to 13%. Increasing the Cr ion loading above 1 1016 ions cm2 caused a reduction in Z. The phenomenon could be explained that the poor light transmission of the Cr/TiO2 thin films at high Cr ion loading reduced the irradiation of the lower Cr/TiO2-coated glass plates. Although the Cr/TiO2 thin film on the top plate would absorb more light, its photocatalytic degradation effect had already reached the maximum as presented by the singleplate test results shown in Fig. 7. Therefore, the overall Z decreased with increasing Cr ion loading above 1 1016 ions cm2.
More tests were conducted to obtain the effect of irradiance. The parallel-plate photoreactor exhibiting the best performance at I equal to 1.2 mW cm2 (three Cr/TiO2 glass plates with Cr loading of 1 1016 cm2) was tested at higher irradiance up to 3 mW cm2. The experimental results shown in Fig. 9 indicated that as I increased, Z increased at a decreasing rate. The average value of measured Z was 18% when I was 3 mW cm2. The trend of Z versus I could be explained by the effects of electron–hole formation rate and light-saturated photocatalyst [6,18,19]. As the visible-light irradiance onto a parallel-plate photoreactor increased from zero, the abovementioned effects would initially become significant on the uppermost Cr/TiO2 thin film. As I continued to increase, the lower thin films would become light saturated one by one. When all the parallel thin films became
15 Photocatalytic degradation of gaseous formaldehyde by parallel-plate photoreactor
Degradation, η (%)
Visible light irradiance = 1.2 mW cm-2 10
5
Max Experiment
Mean Min 0 0
2
4
6
8
10
Cr ion loading (x 1016 ions cm-2)
Fig. 8. Visible-light-assisted photocatalytic degradation of formaldehyde by parallel-plate photoreactor.
25 Photocatalytic degradation of gaseous formaldehyde by parallel plate photoreactor Cr loading = 1 × 1016 ions cm-2
Degradation, η (%)
20
15
10 Max Mean 5
Experiment
Min
0 0
1
2
3
4
Visible light irradiance, I (mW cm-2) Fig. 9. Visible-light-assisted photocatalytic degradation effect of parallel-plate photoreactor versus visible light irradiance.
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light-saturated, the photoreactor would reach its peak performance to yield the maximum photocatalytic degradation effect.
6. Conclusions Sol–gel TiO2 thin film was successfully modified by Cr ion implantation to achieve visible-light photocatalysis. The experimental results revealed the effectiveness of the visible-light-assisted Cr/TiO2 photoreactors in degradation of gaseous formaldehyde. It implied that the photoreactors could be effectively applied to achieve solar photocatalysis. The study also demonstrated the effects of Cr ion loading and visible-light irradiance on photocatalytic degradation of formaldehyde vapor. For future design optimization of parallel-plate solar photoreactor, the number of parallel Cr/TiO2-coated glass plates and the amount of Cr ion loading should be properly controlled to maximize the efficiency of the use of the solar irradiation.
Acknowledgement The study was fully supported by a grant from the Innovation and Technology Fund of the Hong Kong SAR Government (ITS/076/03).
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