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Enhanced adsorption and visible-light-induced photocatalytic activity of hydroxyapatite modified Ag–TiO2 powders
Applied Surface Science 256 (2010) 6390–6394
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced adsorption and visible-light-induced photocatalytic activity of hydroxyapatite modified Ag–TiO2 powders Y. Liu a , C.Y. Liu a , J.H. Wei a,b,∗ , R. Xiong a , C.X. Pan a , J. Shi a a b
School of Physics Science and Technology, Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China Key Laboratory of Low Dimensional Materials & Application Technology, Xiangtan University, Ministry of Education, Hunan 411105, PR China
a r t i c l e
i n f o
Article history: Received 5 March 2010 Received in revised form 7 April 2010 Accepted 7 April 2010 Available online 14 April 2010 Keywords: Hydroxyapatite modified Ag–TiO2 catalyst Photocatalytic activity Acetone Visible light
a b s t r a c t In order to get a kind of materials with enhanced adsorption and photocatalytic performance, hydroxyapatite modified Ag–TiO2 powders (Ag–TiO2 –HAP) were prepared by a facile wet chemical strategy. The powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis absorption spectroscopy, N2 adsorption–desorption measurement (BET), photoluminescence spectroscopy (PL), etc. The photocatalytic activities were evaluated by photocatalytic oxidation decomposition of acetone in air under visible-light illumination. The results showed that the coupled system indicated a highest photocatalytic activity and photochemical stability under visible-light irradiation than all the other catalysts. The intensively improved visible-light-induced photocatalytic activity of the Ag–TiO2 –HAP hybrids could be attributed to its strong absorption in the visible-light region, low recombination rate of the electron–hole pair and large BET specific surface area. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The application of photocatalysis as a remedy to the environmental problems has been increased tremendously in recent years. As one of the most popular photocatalysts, nanosized TiO2 has long been investigated for photocatalytic degradation of organic pollutants, photocatalytic dissociation of water, and solar energy conversion due to its biological and chemical inertness, strong oxidizing power, nontoxicity and long-term stability against photo and chemical corrosion [1–3], etc. However, the low quantum yields and the lack of visible-light utilization hinder its practical application. To solve these problems, much effort has been made to enhance the photocatalytic efficiency and visible-light utilization of TiO2 by impurity doping, sensitization, and metallization [4–10], etc. Among these techniques, the doping with noble metal has been proved as an effective method to improve the photocatalytic efficiency of TiO2 -based photo catalyst [11–13]. Some investigations indicated that slightly depositing noble metals (e.g. Ag or Au) on TiO2 surface can capture the photo-induced electrons or holes, eliminate the recombination of electron–hole pairs, and also extend the light response of TiO2 to the visible-light region.
∗ Corresponding author at: School of Physics Science and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education , Wuhan University, Wuhan 430072, China. Tel.: +86 27 68754613; fax: +86 27 68752569. E-mail address: [email protected] (J.H. Wei). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.022
In addition to the low quantum yields and the lack of visiblelight utilization, the low adsorption capacity of TiO2 is another barrier to hinder its practical application. For example, due to the lack of action to attract substances, TiO2 can only decompose substances that happened to be come into contact with it, and this decomposing action fails to work when there is no impinging light. In contrast, hydroxyapatite (Ca10 (PO4 )6 (OH)2 , HAP) is known to have excellent adsorption capacity as well as biocompatibility [14,15]. However, since HAP is capable of only adsorption rather than decomposition, saturation will be reached over time, and the adsorption action weakens as well. Recently, Gholami et al. [16–19] found that silver–hydroxyapatite–titania hybrid can destruct bacteria under visible light and in the dark. However, to the best of our knowledge, little work has been carried out on the degradation of air pollutant under visible light by such hybrid until now. The volatile air pollutants such as acetone, formaldehyde and other have been the subject of numerous complaints regarding health disorders, such as leukemia, nausea, headache and fatigue. In order to improve indoor air quality, these volatile organic compounds must be eliminated. In the study, in order to get a kind of materials with enhanced adsorption and photocatalytic performance, hydroxyapatite modified Ag–TiO2 powders (Ag–TiO2 –HAP) were prepared by a facile wet chemical strategy. The photocatalytic activities were evaluated by photocatalytic oxidation decomposition of gaseous acetone under visible-light illumination. The resultant Ag–TiO2 –HAP photocatalyst showed interesting photocatalytic activity, suggesting that the Ag–TiO2 –HAP photocatalyst system could be a potential environmental catalyst system.
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2. Experimental 2.1. Preparation of photocatalysts Ti(OC4 H9 )4 was chosen as a precursor of Ti, which is less reactive than titanium chloride and titanium isopropoxide. Firstly, 3 ml diethanolamide was dissolved in 10 ml ethanol, afterwards, Ti(OC4 H9 )4 was added drop wise into the above solution under vigorous stirring, then, definite amount of AgNO3 dissolved by ethanol was added into the mixture. The molar ratio of AgNO3 to Ti(OC4 H9 )4 was controlled to be 5%. During the synthesis, the mixtures were stirred at 400–500 rmp for 2 h at room temperature. Whereafter, 0.1 g of hydroxyapatite was dispersed in 5 ml n-hexane and was added into the solution. The mixed solution was continuously stirred until the gel was formed. Finally, the gel was dried at 110 ◦ C for half-hour, heat treated at 400 ◦ C for 4 h, and then ground to obtain the Ag/TiO2 /HAP nanoparticles. For comparison, undoped TiO2 was prepared with the same method by the absence of AgNO3 . P25 powder (composing of anatase and rutile, is widely studied and well known to have good photocatalytic activity) was obtained from the Degussa Company. 2.2. Characterization The crystalline phases of the catalysts were evaluated by X-ray diffraction (XRD D8-Advance Cu K␣, = 1.54184 nm) with a scanning range of 20–80◦ . The surface compositions of the catalysts were evaluated by X-ray photoelectron spectra (XPS, ESCA-3400). The Brunauer–Emmett–Teller (BET) surface area (SBET ) of the powders was analyzed by nitrogen adsorption in an ASAP2020 surface area and porosity analyzer (Micromeritics). All the samples were degassed at 180 ◦ C prior to nitrogen adsorption measurements. UV–visible diffused reflectance spectra of samples were obtained for the dry-pressed disk samples using a UV–visible spectrophotometer (UV–vis, VARIAN Cary-5000), BaSO4 was used as a reflectance standard in a UV–visible diffuse reflectance experiment. 2.3. Analysis of hydroxyl radicals (• OH) The analysis of • OH radical’s formation on the sample surface was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule, which readily reacted with • OH radicals to produce highly fluorescent product, The method relies on the PL signal at 425 nm of 2-hydroxyterephthalic acid, the PL intensity of 2-hydroxyterephtalic acid is proportional to the amount of • OH produced on the surface of TiO2 [20,21]. Experimental procedures were as follows: at ambient temperature, 0.1 g of powder sample was dispersed in a 20-ml of the 5 × 10−4 M terephthalic acid aqueous solution with a concentration of 2 × 10−3 M NaOH in a dish with a diameter of about 9.0 cm. A 125-W high-pressure Hg lamp (10 cm above the dishes) was used as a light source. The integrated visible-light intensity measured with a visible-light radiometer (Model: FZ-A, made in Photoelectric Instrument Factory of Beijing Normal University) was 2.9 ± 0.1 mW/cm2 , the wavelength range was 400–1000 nm. PL spectra of the generated 2-hydroxyterephthalic acid were measured on a Hitachi F-4600 fluorescence spectrophotometer. After irradiation every 10 min, the reaction solution was filtrated to measure the increase in the PL intensity at 425 nm. 2.4. Adsorption/photocatalytic activity measurements Photocatalytic activity was evaluated by the removal of acetone in air with initial equilibrium concentration (2500 ppm) in an 8-l reactor at room temperature (20 ◦ C). The weight of photocatalysts used for each experiment was kept at 0.5 g. A 125-W
Fig. 1. XRD patterns of (a) HAP, (b) TiO2 , (c) Ag–TiO2 and (d) Ag–TiO2 –HAP (A, anatase; R, rutile; H, hydroxyapatite).
high-pressure Hg lamp with a cutoff filter ( ≥ 400 nm) as a visiblelight source were passed into the reactor for 1 h. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was conducted on line with a Photoacoustic IR Multigas Monitor (INNOVA Air Tech Instruments Model 1312). The acetone vapor was allowed to reach adsorption equilibrium with catalysts in the reactor prior to visible-light irradiation. The photocatalytic activity of the samples was quantitatively evaluated according to the equation: ln(C/C0 ) = −kt [21]. Here C0 and C represent the initial equilibrium concentration and reaction concentration of acetone, respectively, k represents the apparent reaction rate constant, and t represents reactive time. The photocatalytic oxidation of the acetone is based on the following reaction: CH3 COCH3 + 4O2 → 3CO2 + 3H2 O
(1)
The adsorption capacity of the catalysts was measured in the similar way to that of photocatalytic activity measurements. The only difference is that the adsorption process was carried out without light irradiation. 3. Results and discussion 3.1. XRD and XPS analysis XRD is used to investigate the phase structure and the phase composition of materials. Fig. 1 shows the XRD patterns of HAP, TiO2 , Ag–TiO2 , and Ag–TiO2 –HAP respectively. The crystallinity of the HAP (Fig. 1a) is confirmed by the reflections observed at 2 values of 25.8◦ , 31.7◦ , 32.17◦ , 33.0◦ , 39.8◦ , 46.6◦ and 49.5◦ (JCPDS cards No. 74-565). Both the anatase (at 2 values of 25.3◦ , 48.0◦ and 62.5◦ , JCPDS cards No. 21-1272) and rutile phases (at 2 values of 27.4◦ , 36.0◦ , 41.2◦ and 56.5◦ , JCPDS cards No. 21-1276) could be observed in pure TiO2 and its corresponding hybrids (Fig. 1b–d). Based on the equation given by Spurr and Myers [22], the relative contents of anatase phase were 65.77%, 72.23% and 73.43% for TiO2 , Ag–TiO2 , Ag–TiO2 –HAP, respectively. As a result, it is reasonable to suggest that a small amount of rutile phase was transformed into anatase TiO2 with the addition of additives. From Fig. 1c and d, it is also found the metallic Ag phase (at 2 values of 38.12◦ , JCPDS cards No. 89-3722) in the Ag-doped catalysts, while the peaks belongs to Ag are too less and weak to describe the nature of Ag in the catalysts. To further investigate the nature of the Ag and HAP species introduced into the hybrids, we performed X-ray photoelectron spectroscopy (XPS) measurements. The XPS full spectrum of
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Fig. 3. UV–vis adsorption spectra of different samples.
the isotherms of type IV [26], with a hysteresis loop at relative pressure (P/P0 ) between 0.4 and 1.0, indicating the presence of mesopores. The shape of the hysteresis loop for Ag–TiO2 –HAP is a type H3, associated with plate-like particles giving rise to narrow slit-shaped pores. The shape of the hysteresis loop for the other catalysts are of type H2, associated with the pores with narrow necks and wider bodies (ink-bottle pores) [27], these mesopores are usually from the agglomeration of primary crystallite. As shown in Fig. 4, the order of BET specific surface areas is as follows: Ag–TiO2 –HAP > Ag–TiO2 > P25 > TiO2 . It is widely accepted that photocatalysts with high specific surface areas and porous structures are beneficial to the enhancement of photocatalytic performance, as a large amount of adsorbed organic molecules promotes the reaction rate [28]. So, Ag–TiO2 –HAP hybrids are expected to exhibit improved photocatalytic performance. Fig. 2. XPS spectra of (a) different samples and (b) Ag peak.
Ag–TiO2 –HAP hybrids confirms the existence of Ti, O, Ag, Ca, and P elements on the surface of the sample, mirroring the successful introduction of Ag and HAP into Ag–TiO2 –HAP hybrids (Fig. 2a). The high resolution XPS spectrum of the Ag 3d region (Fig. 2b) shows that the presence of silver in the form of Ag 0 in Ag–TiO2 and Ag–TiO2 –HAP samples, for Ag 3d5/2 peak appears at a binding energy of 368.2 eV, and the splitting of the 3d doublet is 6 eV [23,24], these results agree well with those obtained by XRD analysis.
3.4. PL spectra PL emission spectra can be used to investigate the fate of photogenerated electron and holes in a semiconductor, since PL emission results from the recombination of free charge carriers. Fig. 5a shows the change of PL spectra with irradiation time for Ag–TiO2 –HAP samples in terephthalic acid solution. A gradual increase in PL intensity at about 425 nm is observed with increasing irradiation time, however, no PL increase is observed in the absence of visible light or Ag–TiO2 –HAP samples. This suggests that the fluores-
3.2. UV–vis diffuse reflectance spectra The light absorption characteristics of different samples were probed with UV–vis diffuse spectroscopy (Fig. 3). From the result, it can be seen that he HAP shows no obvious adsorption in the range of 250–800 nm. The undoped TiO2 and P25 show the characteristic spectra of TiO2 with its fundamental absorption sharp edge rising at 400 nm. An obvious absorption peak around 500 nm in the visible range was observed for the Ag–TiO2 nanoparticles, it is the characteristic of surface plasma absorption corresponding to Ag particles [25]. The adsorption spectrum of Ag–TiO2 –HAP are similar to that of Ag–TiO2 , which infers that both Ag–TiO2 and Ag–TiO2 –HAP can effectively extend light absorption to the range of visible light. 3.3. BET Fig. 4 shows the nitrogen adsorption–desorption isotherms and BET surface area of different catalysts. All the samples show
Fig. 4. N2 adsorption–desorption isotherms of (a) P25, (b) TiO2 , (c) Ag–TiO2 and (d) Ag–TiO2 –HAP.
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Fig. 6. Photocatalytic degradation of acetone by (a) TiO2 , (b) P25, (c) Ag–TiO2 and (d) Ag–TiO2 –HAP.
Fig. 5. Fluorescence spectral changes in 5 × 10−4 M NaOH solution of terephthalic acid for (a) Ag–TiO2 –HAP and (b) different catalysts.
cence is from the chemical reactions between terephthalic acid and ·OH formed at the Ag–TiO2 –HAP/water interface via photocatalytic reactions [29,30]. Fig. 5b shows the change of PL spectra with irradiation time for different samples. Usually, PL intensity is proportional to the amount of produced hydroxyl radicals. It can be easily seen that at a fixed time (60 min), the formation rate of OH radicals on the Ag–TiO2 nanoparticles is much faster than that of the undoped TiO2 and P25, this implies that the Ag–TiO2 nanoparticles have higher visible-light photocatalytic activity than undoped TiO2 and P25. Especially, the Ag–TiO2 –HAP sample has the strongest PL intensity, implying a highest photocatalytic activity. Then reason maybe that the Ag◦ adsorbed on the surfaces of TiO2 can attract the photogenerated electrons and then reduce the recombination rate of the photogenerated electrons and holes. Therefore, there exists large mounts of separated electrons and holes on the surface of the Ag–TiO2 and Ag–TiO2 –HAP samples, the excessive electrons and holes are accepted by surface adsorbed O2 and H2 O, and formed more •OH radicals to participate the photocatalytic reaction. What is more, the OH radicals generated on the surface of Ag–TiO2 –HAP samples more inclined to remain adsorbed on its surface for its much stronger adsorption ability than the other samples. 3.5. Adsorption and photocatalytic activity The dark adsorption study of acetone on supported catalysts is presented in Table 1. After 1 h in the dark, about 23% acetone were absorbed by HAP, 7% acetone were absorbed by Ag–TiO2 –HAP and
2% acetone were absorbed by Ag–TiO2 , but adsorption of acetone by the undoped TiO2 and P25 within the same time is approximately zero (Table 1). Another 1 h in the dark, the concentration of acetone change little, this means that adsorption equilibrium can be reached after 1 h in the dark. The kinetics of photocatalytic degradation of acetone and the apparent reaction rate constant k of different samples are shown in Fig. 6. The undoped TiO2 and P25 samples show poor photocatalytic activities in visible-light region, which can be assigned to the large band gap of the titanium dioxide (3.0 eV for rutile and 3.2 for anatase). The photocatalytic activity of Ag–TiO2 sample is much higher than that of the pure TiO2 and P25, its rate constant k reaches 0.018 min−1 . For the Ag–TiO2 –HAP composite, it shows a highest photocatalytic activity, its k value is 0.049 min−1 which is 2.72 times to that of Ag–TiO2 and 7 times to that of P25, respectively. It is well known that the mixing of an active oxidizing anatase phase with a comparatively inactive rutile phase could produce a kind of photocatalysts with unusually high activity [31,32]. According to XRD analysis, it was reasonable to conclude that a mixed effect between anatase and rutile would exist in all our prepared samples for the photocatalytic reactions. However, it should be noted that there is an obvious enhancement in the photoactivity of the TiO2 powder after modified by Ag and HAP. Therefore, it is suggested that in addition to the mixed effect of anatase and rutile phases, there must be some other factors enhancing the photocatalytic activity of the Ag–TiO2 –HAP. In a typical photocatalytic reaction, photogenerating electrons and holes are captured by O2 and H2 O absorbed by TiO2 forming superactive • OH, • O2 − and • OOH oxidants which decomposed acetone into CO2 and H2 O on the surface of TiO2 . Therefore, the effective separation of electron–hole pairs, or the enhanced photo degradation of acetone can be promoted by the increase in the concentration of surface hydroxyl groups and adsorbed molecular oxygen [33,34]. In this study, after modified by Ag and HAP, the SBET and the content of hydroxyl groups increase significantly. On the one hand, the catalysts with higher specific surface areas can provide greater reaction platform for organic pollutants decomposition. On the other hand, the increase of amount of hydroxyl not only increase the trapping sites for photogenerated holes, but also can increase the trapping sites for photogenerated electrons by adsorbing more molecule oxygen, resulting in more hydroxyl radicals to participate the photocatalytic reaction. As a result, Ag–TiO2 –HAP shows a highest photocatalytic activity than all the other catalysts.
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Table 1 Adsorptive ability of the catalysts for acetone. Sample
Initial concentration (%)
Concentration after 1 h in dark (%)
Concentration after 2 h in dark (%)
HAP P25 TiO2 Ag–TiO2 Ag–TiO2 –HAP
100 100 100 100 100
77 100 100 98 90
76 100 100 98 89
Ag–TiO2 and 7 times to that of P25, respectively. This research will give some new insights into the design and preparation of highly active photocatalytic nanomaterials. Acknowledgments We are grateful for the financial support from the National Program on key Basic Research Project (973 Grant No. 2009CB939700), the National Natural Science Foundation of China (No. 10974148) and the Open Project Program of Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (No. DWKF0806). References
Fig. 7. Circled photochemical experiments to observe the photostability of Ag–TiO2 and Ag–TiO2 –HAP photocatalysts.
3.6. Photochemical stability To evaluate the photochemical stability of the catalyst, the repeated experiments for the photodegradation of gaseous acetone were performed in Ag–TiO2 and Ag–TiO2 –HAP samples, and the results are shown in Fig. 7. After each photocatalytic reaction, the sample was left for reuse without further treatment. As shown in Fig. 7, 87.8% of acetone could be degraded when Ag–TiO2 sample is used for the first time. However, after six recycles, a significant decrease of photocatalytic activity for Ag–TiO2 samples is found, and only 71.7% of acetone is degraded within 60 min. In contrast, the reused Ag–TiO2 –HAP samples show less change in photocatalytic activity, 93.2% of acetone could be degraded when Ag–TiO2 –HAP sample is used for the first time and after six recycles, about 86.5% of acetone is degraded within 60 min. In other words, HAP modification cannot only improve the photocatalytic performance but also long-term stability of Ag-doped TiO2 nanocrystals. This result is significant from the viewpoint of practical application, as the enhanced photocatalytic activity and prevention of catalyst deactivation will lead to more cost-effective operation. 4. Conclusions HAP modified Ag–TiO2 powders exhibit a marked improved photocatalytic activity than that of the Ag–TiO2 , undoped TiO2 and P25 powders. After modified by HAP and Ag particles, the BET specific surface areas of the undoped TiO2 powders greatly increases and anatase content slightly increases. Meanwhile, the amount of hydroxyls on the surface of the TiO2 powders obviously increased. The Ag–TiO2 –HAP samples show a highest visible-light-induced photocatalytic activity and photochemical stability than all the other catalysts, its degradation rate constant is 2.72 times to that of
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced adsorption and visible-light-induced photocatalytic activity of hydroxyapatite modified Ag–TiO2 powders Y. Liu a , C.Y. Liu a , J.H. Wei a,b,∗ , R. Xiong a , C.X. Pan a , J. Shi a a b
School of Physics Science and Technology, Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China Key Laboratory of Low Dimensional Materials & Application Technology, Xiangtan University, Ministry of Education, Hunan 411105, PR China
a r t i c l e
i n f o
Article history: Received 5 March 2010 Received in revised form 7 April 2010 Accepted 7 April 2010 Available online 14 April 2010 Keywords: Hydroxyapatite modified Ag–TiO2 catalyst Photocatalytic activity Acetone Visible light
a b s t r a c t In order to get a kind of materials with enhanced adsorption and photocatalytic performance, hydroxyapatite modified Ag–TiO2 powders (Ag–TiO2 –HAP) were prepared by a facile wet chemical strategy. The powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis absorption spectroscopy, N2 adsorption–desorption measurement (BET), photoluminescence spectroscopy (PL), etc. The photocatalytic activities were evaluated by photocatalytic oxidation decomposition of acetone in air under visible-light illumination. The results showed that the coupled system indicated a highest photocatalytic activity and photochemical stability under visible-light irradiation than all the other catalysts. The intensively improved visible-light-induced photocatalytic activity of the Ag–TiO2 –HAP hybrids could be attributed to its strong absorption in the visible-light region, low recombination rate of the electron–hole pair and large BET specific surface area. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The application of photocatalysis as a remedy to the environmental problems has been increased tremendously in recent years. As one of the most popular photocatalysts, nanosized TiO2 has long been investigated for photocatalytic degradation of organic pollutants, photocatalytic dissociation of water, and solar energy conversion due to its biological and chemical inertness, strong oxidizing power, nontoxicity and long-term stability against photo and chemical corrosion [1–3], etc. However, the low quantum yields and the lack of visible-light utilization hinder its practical application. To solve these problems, much effort has been made to enhance the photocatalytic efficiency and visible-light utilization of TiO2 by impurity doping, sensitization, and metallization [4–10], etc. Among these techniques, the doping with noble metal has been proved as an effective method to improve the photocatalytic efficiency of TiO2 -based photo catalyst [11–13]. Some investigations indicated that slightly depositing noble metals (e.g. Ag or Au) on TiO2 surface can capture the photo-induced electrons or holes, eliminate the recombination of electron–hole pairs, and also extend the light response of TiO2 to the visible-light region.
∗ Corresponding author at: School of Physics Science and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education , Wuhan University, Wuhan 430072, China. Tel.: +86 27 68754613; fax: +86 27 68752569. E-mail address: [email protected] (J.H. Wei). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.022
In addition to the low quantum yields and the lack of visiblelight utilization, the low adsorption capacity of TiO2 is another barrier to hinder its practical application. For example, due to the lack of action to attract substances, TiO2 can only decompose substances that happened to be come into contact with it, and this decomposing action fails to work when there is no impinging light. In contrast, hydroxyapatite (Ca10 (PO4 )6 (OH)2 , HAP) is known to have excellent adsorption capacity as well as biocompatibility [14,15]. However, since HAP is capable of only adsorption rather than decomposition, saturation will be reached over time, and the adsorption action weakens as well. Recently, Gholami et al. [16–19] found that silver–hydroxyapatite–titania hybrid can destruct bacteria under visible light and in the dark. However, to the best of our knowledge, little work has been carried out on the degradation of air pollutant under visible light by such hybrid until now. The volatile air pollutants such as acetone, formaldehyde and other have been the subject of numerous complaints regarding health disorders, such as leukemia, nausea, headache and fatigue. In order to improve indoor air quality, these volatile organic compounds must be eliminated. In the study, in order to get a kind of materials with enhanced adsorption and photocatalytic performance, hydroxyapatite modified Ag–TiO2 powders (Ag–TiO2 –HAP) were prepared by a facile wet chemical strategy. The photocatalytic activities were evaluated by photocatalytic oxidation decomposition of gaseous acetone under visible-light illumination. The resultant Ag–TiO2 –HAP photocatalyst showed interesting photocatalytic activity, suggesting that the Ag–TiO2 –HAP photocatalyst system could be a potential environmental catalyst system.
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2. Experimental 2.1. Preparation of photocatalysts Ti(OC4 H9 )4 was chosen as a precursor of Ti, which is less reactive than titanium chloride and titanium isopropoxide. Firstly, 3 ml diethanolamide was dissolved in 10 ml ethanol, afterwards, Ti(OC4 H9 )4 was added drop wise into the above solution under vigorous stirring, then, definite amount of AgNO3 dissolved by ethanol was added into the mixture. The molar ratio of AgNO3 to Ti(OC4 H9 )4 was controlled to be 5%. During the synthesis, the mixtures were stirred at 400–500 rmp for 2 h at room temperature. Whereafter, 0.1 g of hydroxyapatite was dispersed in 5 ml n-hexane and was added into the solution. The mixed solution was continuously stirred until the gel was formed. Finally, the gel was dried at 110 ◦ C for half-hour, heat treated at 400 ◦ C for 4 h, and then ground to obtain the Ag/TiO2 /HAP nanoparticles. For comparison, undoped TiO2 was prepared with the same method by the absence of AgNO3 . P25 powder (composing of anatase and rutile, is widely studied and well known to have good photocatalytic activity) was obtained from the Degussa Company. 2.2. Characterization The crystalline phases of the catalysts were evaluated by X-ray diffraction (XRD D8-Advance Cu K␣, = 1.54184 nm) with a scanning range of 20–80◦ . The surface compositions of the catalysts were evaluated by X-ray photoelectron spectra (XPS, ESCA-3400). The Brunauer–Emmett–Teller (BET) surface area (SBET ) of the powders was analyzed by nitrogen adsorption in an ASAP2020 surface area and porosity analyzer (Micromeritics). All the samples were degassed at 180 ◦ C prior to nitrogen adsorption measurements. UV–visible diffused reflectance spectra of samples were obtained for the dry-pressed disk samples using a UV–visible spectrophotometer (UV–vis, VARIAN Cary-5000), BaSO4 was used as a reflectance standard in a UV–visible diffuse reflectance experiment. 2.3. Analysis of hydroxyl radicals (• OH) The analysis of • OH radical’s formation on the sample surface was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule, which readily reacted with • OH radicals to produce highly fluorescent product, The method relies on the PL signal at 425 nm of 2-hydroxyterephthalic acid, the PL intensity of 2-hydroxyterephtalic acid is proportional to the amount of • OH produced on the surface of TiO2 [20,21]. Experimental procedures were as follows: at ambient temperature, 0.1 g of powder sample was dispersed in a 20-ml of the 5 × 10−4 M terephthalic acid aqueous solution with a concentration of 2 × 10−3 M NaOH in a dish with a diameter of about 9.0 cm. A 125-W high-pressure Hg lamp (10 cm above the dishes) was used as a light source. The integrated visible-light intensity measured with a visible-light radiometer (Model: FZ-A, made in Photoelectric Instrument Factory of Beijing Normal University) was 2.9 ± 0.1 mW/cm2 , the wavelength range was 400–1000 nm. PL spectra of the generated 2-hydroxyterephthalic acid were measured on a Hitachi F-4600 fluorescence spectrophotometer. After irradiation every 10 min, the reaction solution was filtrated to measure the increase in the PL intensity at 425 nm. 2.4. Adsorption/photocatalytic activity measurements Photocatalytic activity was evaluated by the removal of acetone in air with initial equilibrium concentration (2500 ppm) in an 8-l reactor at room temperature (20 ◦ C). The weight of photocatalysts used for each experiment was kept at 0.5 g. A 125-W
Fig. 1. XRD patterns of (a) HAP, (b) TiO2 , (c) Ag–TiO2 and (d) Ag–TiO2 –HAP (A, anatase; R, rutile; H, hydroxyapatite).
high-pressure Hg lamp with a cutoff filter ( ≥ 400 nm) as a visiblelight source were passed into the reactor for 1 h. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was conducted on line with a Photoacoustic IR Multigas Monitor (INNOVA Air Tech Instruments Model 1312). The acetone vapor was allowed to reach adsorption equilibrium with catalysts in the reactor prior to visible-light irradiation. The photocatalytic activity of the samples was quantitatively evaluated according to the equation: ln(C/C0 ) = −kt [21]. Here C0 and C represent the initial equilibrium concentration and reaction concentration of acetone, respectively, k represents the apparent reaction rate constant, and t represents reactive time. The photocatalytic oxidation of the acetone is based on the following reaction: CH3 COCH3 + 4O2 → 3CO2 + 3H2 O
(1)
The adsorption capacity of the catalysts was measured in the similar way to that of photocatalytic activity measurements. The only difference is that the adsorption process was carried out without light irradiation. 3. Results and discussion 3.1. XRD and XPS analysis XRD is used to investigate the phase structure and the phase composition of materials. Fig. 1 shows the XRD patterns of HAP, TiO2 , Ag–TiO2 , and Ag–TiO2 –HAP respectively. The crystallinity of the HAP (Fig. 1a) is confirmed by the reflections observed at 2 values of 25.8◦ , 31.7◦ , 32.17◦ , 33.0◦ , 39.8◦ , 46.6◦ and 49.5◦ (JCPDS cards No. 74-565). Both the anatase (at 2 values of 25.3◦ , 48.0◦ and 62.5◦ , JCPDS cards No. 21-1272) and rutile phases (at 2 values of 27.4◦ , 36.0◦ , 41.2◦ and 56.5◦ , JCPDS cards No. 21-1276) could be observed in pure TiO2 and its corresponding hybrids (Fig. 1b–d). Based on the equation given by Spurr and Myers [22], the relative contents of anatase phase were 65.77%, 72.23% and 73.43% for TiO2 , Ag–TiO2 , Ag–TiO2 –HAP, respectively. As a result, it is reasonable to suggest that a small amount of rutile phase was transformed into anatase TiO2 with the addition of additives. From Fig. 1c and d, it is also found the metallic Ag phase (at 2 values of 38.12◦ , JCPDS cards No. 89-3722) in the Ag-doped catalysts, while the peaks belongs to Ag are too less and weak to describe the nature of Ag in the catalysts. To further investigate the nature of the Ag and HAP species introduced into the hybrids, we performed X-ray photoelectron spectroscopy (XPS) measurements. The XPS full spectrum of
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Fig. 3. UV–vis adsorption spectra of different samples.
the isotherms of type IV [26], with a hysteresis loop at relative pressure (P/P0 ) between 0.4 and 1.0, indicating the presence of mesopores. The shape of the hysteresis loop for Ag–TiO2 –HAP is a type H3, associated with plate-like particles giving rise to narrow slit-shaped pores. The shape of the hysteresis loop for the other catalysts are of type H2, associated with the pores with narrow necks and wider bodies (ink-bottle pores) [27], these mesopores are usually from the agglomeration of primary crystallite. As shown in Fig. 4, the order of BET specific surface areas is as follows: Ag–TiO2 –HAP > Ag–TiO2 > P25 > TiO2 . It is widely accepted that photocatalysts with high specific surface areas and porous structures are beneficial to the enhancement of photocatalytic performance, as a large amount of adsorbed organic molecules promotes the reaction rate [28]. So, Ag–TiO2 –HAP hybrids are expected to exhibit improved photocatalytic performance. Fig. 2. XPS spectra of (a) different samples and (b) Ag peak.
Ag–TiO2 –HAP hybrids confirms the existence of Ti, O, Ag, Ca, and P elements on the surface of the sample, mirroring the successful introduction of Ag and HAP into Ag–TiO2 –HAP hybrids (Fig. 2a). The high resolution XPS spectrum of the Ag 3d region (Fig. 2b) shows that the presence of silver in the form of Ag 0 in Ag–TiO2 and Ag–TiO2 –HAP samples, for Ag 3d5/2 peak appears at a binding energy of 368.2 eV, and the splitting of the 3d doublet is 6 eV [23,24], these results agree well with those obtained by XRD analysis.
3.4. PL spectra PL emission spectra can be used to investigate the fate of photogenerated electron and holes in a semiconductor, since PL emission results from the recombination of free charge carriers. Fig. 5a shows the change of PL spectra with irradiation time for Ag–TiO2 –HAP samples in terephthalic acid solution. A gradual increase in PL intensity at about 425 nm is observed with increasing irradiation time, however, no PL increase is observed in the absence of visible light or Ag–TiO2 –HAP samples. This suggests that the fluores-
3.2. UV–vis diffuse reflectance spectra The light absorption characteristics of different samples were probed with UV–vis diffuse spectroscopy (Fig. 3). From the result, it can be seen that he HAP shows no obvious adsorption in the range of 250–800 nm. The undoped TiO2 and P25 show the characteristic spectra of TiO2 with its fundamental absorption sharp edge rising at 400 nm. An obvious absorption peak around 500 nm in the visible range was observed for the Ag–TiO2 nanoparticles, it is the characteristic of surface plasma absorption corresponding to Ag particles [25]. The adsorption spectrum of Ag–TiO2 –HAP are similar to that of Ag–TiO2 , which infers that both Ag–TiO2 and Ag–TiO2 –HAP can effectively extend light absorption to the range of visible light. 3.3. BET Fig. 4 shows the nitrogen adsorption–desorption isotherms and BET surface area of different catalysts. All the samples show
Fig. 4. N2 adsorption–desorption isotherms of (a) P25, (b) TiO2 , (c) Ag–TiO2 and (d) Ag–TiO2 –HAP.
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Fig. 6. Photocatalytic degradation of acetone by (a) TiO2 , (b) P25, (c) Ag–TiO2 and (d) Ag–TiO2 –HAP.
Fig. 5. Fluorescence spectral changes in 5 × 10−4 M NaOH solution of terephthalic acid for (a) Ag–TiO2 –HAP and (b) different catalysts.
cence is from the chemical reactions between terephthalic acid and ·OH formed at the Ag–TiO2 –HAP/water interface via photocatalytic reactions [29,30]. Fig. 5b shows the change of PL spectra with irradiation time for different samples. Usually, PL intensity is proportional to the amount of produced hydroxyl radicals. It can be easily seen that at a fixed time (60 min), the formation rate of OH radicals on the Ag–TiO2 nanoparticles is much faster than that of the undoped TiO2 and P25, this implies that the Ag–TiO2 nanoparticles have higher visible-light photocatalytic activity than undoped TiO2 and P25. Especially, the Ag–TiO2 –HAP sample has the strongest PL intensity, implying a highest photocatalytic activity. Then reason maybe that the Ag◦ adsorbed on the surfaces of TiO2 can attract the photogenerated electrons and then reduce the recombination rate of the photogenerated electrons and holes. Therefore, there exists large mounts of separated electrons and holes on the surface of the Ag–TiO2 and Ag–TiO2 –HAP samples, the excessive electrons and holes are accepted by surface adsorbed O2 and H2 O, and formed more •OH radicals to participate the photocatalytic reaction. What is more, the OH radicals generated on the surface of Ag–TiO2 –HAP samples more inclined to remain adsorbed on its surface for its much stronger adsorption ability than the other samples. 3.5. Adsorption and photocatalytic activity The dark adsorption study of acetone on supported catalysts is presented in Table 1. After 1 h in the dark, about 23% acetone were absorbed by HAP, 7% acetone were absorbed by Ag–TiO2 –HAP and
2% acetone were absorbed by Ag–TiO2 , but adsorption of acetone by the undoped TiO2 and P25 within the same time is approximately zero (Table 1). Another 1 h in the dark, the concentration of acetone change little, this means that adsorption equilibrium can be reached after 1 h in the dark. The kinetics of photocatalytic degradation of acetone and the apparent reaction rate constant k of different samples are shown in Fig. 6. The undoped TiO2 and P25 samples show poor photocatalytic activities in visible-light region, which can be assigned to the large band gap of the titanium dioxide (3.0 eV for rutile and 3.2 for anatase). The photocatalytic activity of Ag–TiO2 sample is much higher than that of the pure TiO2 and P25, its rate constant k reaches 0.018 min−1 . For the Ag–TiO2 –HAP composite, it shows a highest photocatalytic activity, its k value is 0.049 min−1 which is 2.72 times to that of Ag–TiO2 and 7 times to that of P25, respectively. It is well known that the mixing of an active oxidizing anatase phase with a comparatively inactive rutile phase could produce a kind of photocatalysts with unusually high activity [31,32]. According to XRD analysis, it was reasonable to conclude that a mixed effect between anatase and rutile would exist in all our prepared samples for the photocatalytic reactions. However, it should be noted that there is an obvious enhancement in the photoactivity of the TiO2 powder after modified by Ag and HAP. Therefore, it is suggested that in addition to the mixed effect of anatase and rutile phases, there must be some other factors enhancing the photocatalytic activity of the Ag–TiO2 –HAP. In a typical photocatalytic reaction, photogenerating electrons and holes are captured by O2 and H2 O absorbed by TiO2 forming superactive • OH, • O2 − and • OOH oxidants which decomposed acetone into CO2 and H2 O on the surface of TiO2 . Therefore, the effective separation of electron–hole pairs, or the enhanced photo degradation of acetone can be promoted by the increase in the concentration of surface hydroxyl groups and adsorbed molecular oxygen [33,34]. In this study, after modified by Ag and HAP, the SBET and the content of hydroxyl groups increase significantly. On the one hand, the catalysts with higher specific surface areas can provide greater reaction platform for organic pollutants decomposition. On the other hand, the increase of amount of hydroxyl not only increase the trapping sites for photogenerated holes, but also can increase the trapping sites for photogenerated electrons by adsorbing more molecule oxygen, resulting in more hydroxyl radicals to participate the photocatalytic reaction. As a result, Ag–TiO2 –HAP shows a highest photocatalytic activity than all the other catalysts.
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Table 1 Adsorptive ability of the catalysts for acetone. Sample
Initial concentration (%)
Concentration after 1 h in dark (%)
Concentration after 2 h in dark (%)
HAP P25 TiO2 Ag–TiO2 Ag–TiO2 –HAP
100 100 100 100 100
77 100 100 98 90
76 100 100 98 89
Ag–TiO2 and 7 times to that of P25, respectively. This research will give some new insights into the design and preparation of highly active photocatalytic nanomaterials. Acknowledgments We are grateful for the financial support from the National Program on key Basic Research Project (973 Grant No. 2009CB939700), the National Natural Science Foundation of China (No. 10974148) and the Open Project Program of Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (No. DWKF0806). References
Fig. 7. Circled photochemical experiments to observe the photostability of Ag–TiO2 and Ag–TiO2 –HAP photocatalysts.
3.6. Photochemical stability To evaluate the photochemical stability of the catalyst, the repeated experiments for the photodegradation of gaseous acetone were performed in Ag–TiO2 and Ag–TiO2 –HAP samples, and the results are shown in Fig. 7. After each photocatalytic reaction, the sample was left for reuse without further treatment. As shown in Fig. 7, 87.8% of acetone could be degraded when Ag–TiO2 sample is used for the first time. However, after six recycles, a significant decrease of photocatalytic activity for Ag–TiO2 samples is found, and only 71.7% of acetone is degraded within 60 min. In contrast, the reused Ag–TiO2 –HAP samples show less change in photocatalytic activity, 93.2% of acetone could be degraded when Ag–TiO2 –HAP sample is used for the first time and after six recycles, about 86.5% of acetone is degraded within 60 min. In other words, HAP modification cannot only improve the photocatalytic performance but also long-term stability of Ag-doped TiO2 nanocrystals. This result is significant from the viewpoint of practical application, as the enhanced photocatalytic activity and prevention of catalyst deactivation will lead to more cost-effective operation. 4. Conclusions HAP modified Ag–TiO2 powders exhibit a marked improved photocatalytic activity than that of the Ag–TiO2 , undoped TiO2 and P25 powders. After modified by HAP and Ag particles, the BET specific surface areas of the undoped TiO2 powders greatly increases and anatase content slightly increases. Meanwhile, the amount of hydroxyls on the surface of the TiO2 powders obviously increased. The Ag–TiO2 –HAP samples show a highest visible-light-induced photocatalytic activity and photochemical stability than all the other catalysts, its degradation rate constant is 2.72 times to that of
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