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Biogenic N–P-codoped TiO2: Synthesis, characterization and photocatalytic properties
Bioresource Technology 101 (2010) 6829–6835
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Biogenic N–P-codoped TiO2: Synthesis, characterization and photocatalytic properties Ting Han, Tongxiang Fan *, Suk-Kwun Chow, Di Zhang State Key Lab. of Composites, Shanghai Jiaotong University, 200240 Shanghai, PR China
a r t i c l e Article history: Received 9 October Received in revised Accepted 20 March Available online 14
i n f o 2009 form 16 March 2010 2010 April 2010
Keywords: TiO2 Self-doping Biomass Biogenic Photocatalysis
a b s t r a c t Four typical kinds of crop seeds are studied as non-metallic bio-precursors to synthesize biogenic N–Pcodoped TiO2 (BNP-TiO2). The as-prepared BNP-TiO2 samples are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen-adsorption measurement and UV–Vis spectroscopy. The results show that BNP-TiO2 possesses single anatase phase with mesopore structures, and nitrogen and phosphorus contained in original crop seeds are self-doped into the lattice as anions and cations, respectively. Besides, BNP-TiO2 exhibits a strongly enhanced absorption in the UV–Vis light range and red shift of the absorption edge, implicating the highly efficient light-harvesting capacity and sensitization towards visible light. Furthermore, experiments of crystal violet degradation under Xe lamp irradiation indicate superior photocatalytic activity of BNP-TiO2, of which the degradation rate is almost three times that of common TiO2. Circled photocatalytic degradation also shows good photocatalytic stability of BNP-TiO2. This work may pave a new and facile pathway of utilizing discarded biomass to synthesize desirable element-doped metal oxides based on biomass precursors. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biomass, as a renewable resource, is ubiquitous and abundant on earth. It is estimated that world production of biomass is at 1.46E11 metric tons per year (Demirbas, 2001). Presently, the main methods for utilizing biomass are direct combusting for heat or electricity, converting to traditional solid, liquid and gas fuels for energy and producing fine chemicals (Yaman, 2004). However, large quantities of biomass are still underutilized because of some complicated conversion techniques and their high costs. Therefore, achieving effective and economic utilization of biomass is still a great challenge. In recent years, there has been great interest in biomorphic mineralization – a technique that employs nature living things as templates for mineralization to produce bio-templated materials with morphologies and structures resembling those of the templates (Fan et al., 2009). Besides, nature living things can also act as bio-precursors for mineralization, for they possess many desirable elements that can potentially be employed as chemical components of the material to be produced. For example, some plants accumulate metal elements such as Ni, Co, Cd and Au; most biomass contains abundant non-metallic elements such as C, H, O, N and P (Smith, 2004). Crop seeds, containing abundant N and P elements, can be employed as non-metallic precursors for mineralization to synthesize N- and P-codoped metal oxides. Crop seeds are composed of various chemical compositions, mainly starches, * Corresponding author. E-mail address: [email protected] (T. Fan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.03.107
proteins and fats, among which starches and proteins are hydrophilic colloids, favorable for the infiltration of metal-ion solution. Moreover, crop seeds contain abundant N and P elements in different forms, e.g. as storage proteins, enzymes, phytic acids, phospholipids, phosphoproteins and so forth. These N and P elements are expected to be retained and self-doped into metal oxides during mineralization of crop seeds into metal oxides. These characteristics make crop seeds appropriate biogenic non-metallic precursors for producing doped metal oxides. Photocatalysis is an important industrial process in wastewater treatment, heavy metal remediation, air purification, sterilization, etc. Titanium dioxide (TiO2) is the most widely used photocatalyst for its efficiency, low cost, and chemical stability (Gaya and Abdullah, 2008). However, the relatively large band gap of TiO2 (3.2 eV) allows TiO2 to absorb light only with wavelength less than 387 nm, which accounts for merely 5% of total solar energy reaching the earth’s surface. To sensitize TiO2 to visible light and enhance its photocatalytic activity, many efforts have been made, such as noble metal deposition (Sakthivel et al., 2004b), composite semiconductor (Bessekhouad et al., 2004), metal doping (Choi et al., 1994), rare earth doping (Xu et al., 2002), nonmetal doping (Asahi et al., 2001) and so forth. Among these, nonmetal doping has been reported to be an effective way by many researchers (Irie et al., 2003; Ohno et al., 2004; Wang et al., 2005; Yu et al., 2002; Zheng et al., 2008). Nonmetal doping could form a localized state slightly above the valence band in the band gap, causing band narrowing, and it could also reduce the formation energy of oxygen vacancies, which are energetically favorable for the enhancement of light
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absorption and photocatalytic activity (Di Valentin et al., 2005; Livraghi et al., 2006). Generally, titania doping is achieved through introducing extra sources during sol–gel (Wang and Ying, 1999) or vapor-phase synthesis (Irie et al., 2003). For nitrogen doping, the N-precursors can be classified into two categories: organic and inorganic. The former includes urea (Nosaka et al., 2005), triethylamine (Burda et al., 2003) and other organic compounds with amine groups (Sakthivel et al., 2004a); the latter includes ammonia and other inorganic compounds containing NHþ 4 or NO3 (Ihara et al., 2003; Livraghi et al., 2008; Sato et al., 2005). For phosphorus (Shi et al., 2006) doping, the primary sources are inorganic PO3 4 and PO3 2 (Lin et al., 2005). In this paper, we employed crop seeds as non-metallic precursors to synthesize TiO2. N and P contained in original crop seeds were expected to be simultaneously self-doped into TiO2 during mineralization of the seeds, resulting in biogenic N–P-codoped TiO2, hereafter referred to as BNP-TiO2. The presence and states of N and P are identified by X-ray photoelectron spectroscopy (XPS) and the porous structures of BNP-TiO2 are studied by nitrogen-adsorption measurement. Moreover, light-harvesting and photocatalytic properties of BNP-TiO2, compared with that of common TiO2, are evaluated by UV–Vis absorption spectra and degradation of crystal violet solution under irradiation of Xe lamp. And the photocatalytic stability of BNP-TiO2 is evaluated by circled photocatalytic degradation experiments. 2. Methods 2.1. Synthetic procedure Four kinds of crop seeds, namely Oryza sativa L., Oryza sativa L. var. glutinosa Matsum., Glycine max(L.) Merr. and Sesamum indicum Linn., were employed as bio-precursors to synthesize biogenic N– P-codoped TiO2. Seeds of O. sativa L. and O. sativa L. var. glutinosa Matsum. mainly contain starches. Starches in the former seed are mixture of amyloses and amylopectins, while starches in the latter seed are all amylopectins. Seeds of G. max(L.) Merr. and S. indicum Linn. mainly contain proteins and fats, respectively. Crop seeds were cleaned with pure water and the testae were removed manually. Since sesame coats were too small and thin to remove, they were retained. The coatless crop seeds were treated with 5% HCl solution for 12 h to get rid of K, Ca and S ions. After being rinsed with pure water, the as-treated crop seeds were immersed in 5% TiCl3 solution under vacuum for 24 h. Since starches and proteins are hydrophilic colloids, containing many functional groups such as hydroxyls and carboxyls, Ti ions were easily introduced into the seeds. The impregnated and inflated crop seeds then underwent a graded drying process in an aerated oven at 25 °C, 40 °C, 60 °C, 80 °C and 105 °C successively, with each temperature for 2 h. Finally, the as-treated samples were calcined in air under 500 °C for 2 h, with a ramping rate of 1 °C/min. For comparison, common TiO2 was synthesized by common sol–gel method (Brinker and Scherer, 1990; Su et al., 2004): hydrolyzing the sol–gel precursor and calcining under the same conditions with BNP-TiO2. A 1.5% tetrabutyl titanate ethanol solution accompanied with acetylacetone (1.8:120:0.09 by volume) was employed as the sol–gel precursor.
son-Hall method (Hall, 1949; Williamson and Hall, 1953), expressed as follows: bi(cos hi)/k = K/D + 4e(sin hi)/k, where bi is the integral breadth (in radius 2h) of the ith Bragg reflection peak positioned at 2hi; K is a constant with the value of 0.89; k = 0.154 nm is the wavelength of the X-rays; D is the average size of the crystallites and e is the microstrain, whose distribution is isotropic. D and e were calculated through a least-squares fit. The presence and states of N and P in BNP-TiO2 were identified by XPS, performed on a Thermo ESCALAB 250 spectrometer with monochromatized Al Ka X-ray (hm = 1486.6 eV) at a pass energy of 20 eV. All the binding energies were calibrated by using the contaminant carbon (C1S = 284.4 eV) as a reference. The porous structures of BNP-TiO2 were studied by nitrogenadsorption measurement, operated at 77 K on a Micromeritics ASAP 2010 adsorption analyzer. All the BNP-TiO2 samples were degassed at 200 °C and 106 Torr for 5 h prior to the measurements. The pore-size distribution curve was derived from the desorption branch of the nitrogen isotherm by the Barrett–Joyner–Halenda (BJH) method, and the surface areas were calculated through the Brunauer Emmett Teller (BET) equation. The light-harvesting capacities of BNP-TiO2 were characterized by UV–Vis absorption spectra, recorded on a Varian Cary UV–VisNIR spectrophotometer in the spectral range of 200–800 nm. A BaSiO4 plate was used as the basic line for the spectra. The same quality for all the BNP-TiO2 samples was taken during the measurement. To evaluate the photocatalytic activities of BNP-TiO2, degradation experiments of crystal violet nonahydrate (C25H30ClN39H2O) in aqueous solution was carried out in the presence of BNP-TiO2 and common TiO2 as follows. 0.1 g corresponding TiO2 sample was dispersed in 100 mL of 105 mol/L crystal violet solution. The TiO2 aqueous suspension was stirred in dark for 1 h to reach its adsorption equilibrium and then it was irradiated under a 500 W Xe lamp, whose emission spectrum is the nearest to the solar spectrum and which is thus the most widely used light source of solar simulator. Degradation was monitored by taking sample solutions at 0, 4, 8, 12, 18, 25, 35, 45, 60 min, respectively during the irradiation. These sample solutions were centrifuged with 3000 r/min for 3 min. And then the supernatants were tested in sequence using a 25 Lambda UV–Vis spectrometer to get the absorption spectra. The rate of degradation was assumed to obey pseudo-first-order kinetics (Pillai et al., 2007) and the degradation rate constant, k, was obtained according to the following equation: ln(A0/A) = kt, where A0 is the initial absorbance, namely, the characteristic absorbance peak of original crystal violet; A is the characteristic absorbance peak of crystal violet after a t time degradation. To evaluate the photocatalytic stability of BNP-TiO2, three circles of crystal violet photocatalytic degradation were carried out in the presence of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. The circled degradation was monitored by taking sample solutions at 0, 5, 15, 25, 35, 45, 60 min, respectively during the photocatalytic reaction. After each degradation circle, the degraded solution with BNP-TiO2 photocatalyst suspended in it was centrifuged with 3000 r/min for 3 min. Then the supernatant was drawn out with a dropper and pure water was added to the remaining precipitate for washing. The suspension of BNP-TiO2 and pure water was centrifuged to get the clean BNP-TiO2 photocatalyst. The obtained wet BNP-TiO2 was dried at 25 °C for the next degradation circle.
2.2. Characterization
3. Results and discussion
The crystal phase of the BNP-TiO2 samples was examined by Xray diffraction (XRD) on a Bruker-AXS X-ray diffractometer system with Cu Ka radiation at 40 kV and 100 mA. The spectra were recorded in the 2h range from 20° to 90° with a scanning step of 0.02 °/s. The average crystalline sizes were calculated by William-
3.1. Composition and structure characterization of BNP-TiO2 XRD patterns of BNP-TiO2 and common TiO2 are shown in Fig. 1. It shows that all the BNP-TiO2 samples possess single anatase phase, except for BNP-TiO2 from seeds of S. indicum Linn. with
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Fig. 1. XRD patterns of BNP-TiO2 and common TiO2.
minor SiO2 phase, which is passed from the seed coats containing Si element. For common TiO2, both anatase and rutile phases are
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detected. It is possible that the presence of crop seed bio-precursors retards the phase transformation of anatase to rutile. The anatase phase is preferred over rutile, for anatase shows a greater photocatalytic activity (Linsebigler et al., 1995). The average crystalline sizes of BNP-TiO2, calculated by Williamson-Hall method, are 8.4 nm, 10.6 nm, 9.0 nm and 9.1 nm for BNP-TiO2 from seeds of O. sativa L., O. sativa L. var. glutinosa Matsum., G. max(L.) Merr. and S. indicum Linn., respectively. These smaller grain sizes are usually associated with higher specific surfaces (Carp et al., 2004; Tanaka et al., 1991). XPS tests identified the presence of doped N and P in TiO2 network. The whole XPS survey of BNP-TiO2 from seeds of G. max(L.) Merr. (Fig. 2a) demonstrates that N and P exist in the oxides and that their atomic contents are 1.49 at.% and 4.3 at.%. Mineral elements in crop seeds like K, Ca, S, Mg and others are not detected, implicating that the treatment with dilute HCl effectively got rid of them. The high-resolution scanning of N1s displayed in Fig. 2b shows two peaks, at 399.8 eV and 401.6 eV. The former N1s peak is attributed to N anion in O–Ti–N bonds, where the oxygen sites in O–Ti–O bonds are partly substituted by nitrogen atoms (Chen and Burda, 2004; Sathish et al., 2005). The latter peak at 401.6 eV was assigned to molecularly chemisorbed nitrogen species (Irie et al., 2003). As for P doping, there is only one peak at 133.7 eV and no peaks around 128.6 eV in the high-resolution XPS spectra of P2p (Fig. 2c), suggesting that P ions only exist as cations in a pentavalent oxidation state (P5+) (Shi et al., 2006), not as anions
Fig. 2. XPS patterns of BNP-TiO2 from seeds of G. max(L.) Merr. (a) the whole survey; (b) high-resolution spectra of N1s; (c) high-resolution spectra of P2p; and (d) highresolution spectra of O1s.
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Fig. 3. Nitrogen adsorption results of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. (a) adsorption–desorption isotherms and (b) pore-size distribution curves.
in a tervalent deoxidation state (P3). It is generally believed that P2p peaks at a relatively higher binding energy (133.4–133.8 eV) are ascribed to the result that P5+ replace part of Ti4+ in TiO2 lattice, forming Ti–O–P bonds, instead of P3 replacing O2 or P5+ in the form of PO3 4 , supported by many studies using XPS and Fourier transform infrared (FTIR) methods (Lin et al., 2007a,b; Shi et al., 2006; Yu et al., 2003; Zheng et al., 2008). However, the mechanism of P doping has not been fully revealed, which left an issue worthy of further research. Fig. 2d shows the spectra of O1s high-resolution scanning. The broad O1s region can be fitted by three peaks, namely, Ti–O bonds, P–O bonds and C–O bonds (Chen et al., 2008). The other three kinds of BNP-TiO2 samples exhibit similar XPS patterns. Their atomic contents of N and P are 1.22 at.% and 2.72 at.% (from O. sativa L. seeds), 0.82 at.% and 1.81 at.% (from O. sativa L. var. glutinosa Matsum. seeds) and 0.79 at.% and 2.7 at.% (from S. indicum Linn. seeds), respectively. The contents of doped N and P in the BNP-TiO2 from G. max(L.) Merr. seeds are significantly higher than those in the other three because seeds of leguminous plants have a high content of N and P. With a high content of iodine, Laminariaceae plants could also be explored as I-precursors for producing I-doped TiO2, as it has been proved that the photocatalytic properties of TiO2 are enhanced after doping with I (Hong et al., 2005). These suggest not only that N and P from crop seeds could be self-doped into BNP-TiO2, but also that the contents and even types of dopants could be adjusted by selecting different kinds of crop seeds or other biomass as precursors. The porous structures of BNP-TiO2 at nanoscale are characterized by nitrogen-adsorption measurements. Fig. 3a displays the nitrogen adsorption–desorption isotherms of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. The other three kinds of BNPTiO2 samples exhibit similar isotherms. According to the IUPAC classification (Sing et al., 1985), this is a type IV isotherm, which is given by absorbents with mesopore structures (Sing, 1998). The pore sizes at nanoscale are distributed mostly between 2 nm and 40 nm (Fig. 3b), with an average pore width of 14.7 nm. The BET surface area is 55.9 m2 g1. The mesopore structure and high surface area of BNP-TiO2 photocatalyst could offer more absorption and reaction sites for the photocatalytic reaction, favorable for enhancing photocatalytic performance.
absorbance of BNP-TiO2 and common TiO2. Compared with common TiO2, BNP-TiO2 shows two prominent features. One is the enhancement of overall absorbance intensities. The average absorbance intensities within visible range (the bar graph in Fig. 4) increase by 93%, 89%, 160%, and 112% for BNP-TiO2 from seeds of O. sativa L., O. sativa L. var. glutinosa Matsum., G. max(L.) Merr. and S. indicum Linn., respectively. Among four kinds of BNP-TiO2, BNP-TiO2 from G. max(L.) Merr. seeds has the highest absorbance in the visible light region, while BNP-TiO2 from S. indicum Linn. seeds has the highest absorbance in the ultraviolet light region. BNP-TiO2 from O. sativa L. var. glutinosa Matsum. seeds has fairly high absorbance both in the visible and ultraviolet light region. The second feature is the red shift of band gap absorption edges towards longer wavelengths, implicating band gap narrowing. This is caused by doping, which forms a localized state slightly above the
3.2. Light-harvesting and photocatalytic properties of BNP-TiO2 The light-harvesting properties of the BNP-TiO2 are characterized by UV–Vis absorption spectra. Fig. 4 displays the UV–Vis
Fig. 4. UV–Vis absorption spectra of BNP-TiO2 and common TiO2, with a bar graph of the average absorbance intensities within the visible range.
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valence band (Di Valentin et al., 2005). The enhanced absorbance and red shifts indicates more photo-generated electrons and holes
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participating in the photocatalytic reactions could be excited by photons with lower energies, leading to high efficiency in
Fig. 5. Photocatalytic degradation studies of BNP-TiO2 and common TiO2. (a) Changes in the UV–Vis absorption spectra of crystal violet as a function of irradiation time in the presence of BNP-TiO2 from O. sativa L. seeds. (b) Changes in the UV–Vis absorption spectra of crystal violet as a function of irradiation time in the presence of common TiO2. Insets in a and b are corresponding color variations over irradiation time. (c) Kinetic study of degradation of crystal violet solution in the presence of BNP-TiO2 and common TiO2 under Xe lamp irradiation, with a bar graph of the degradation rates.
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Fig. 6. Circled photocatalytic degradation studies of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. (a) Circle 1; (b) circle 2; (c) circle 3; and (d) kinetic study of circled degradation.
photo-induced redox reaction of absorbed substances (Hashimoto et al., 2005). Photocatalytic degradation studies of crystal violet solution are presented in Fig. 5. As shown in Fig. 5a, the characteristic peak of crystal violet at 590 nm decreased dramatically and disappeared after irradiation for 60 min in the presence of BNP-TiO2, indicating that crystal violet was completely degraded by BNP-TiO2. While in Fig. 5b, in the presence of common TiO2, crystal violet was not fully degraded after the same time of irradiation. As it is illustrated in the insets of Fig. 5a and b, in the presence of BNP-TiO2, crystal violet solution shaded from dark purple to colorless and transparent solution in 60 min, while in the presence of common TiO2, the solution color varied slightly. To evaluate the degradation rates quantitatively, we introduce pseudo-firstorder kinetics to process the absorbance data during the degradation. The degradation rate could be obtained by fitting curves in Fig. 5c. Details of the method are provided in Methods section. The degradation rates of BNPTiO2 and common TiO2 are shown in the bar graph in Fig. 5c. The average degradation rate of all the four BNP-TiO2 specimens is 0.050 min1, almost three times that of the common TiO2 specimen, which was 0.017 min1. Besides crystal violet, other dyes such as methylene blue, methyl orange and rhodamine 6G have been degraded by BNP-TiO2. Here space lacks for a detailed description of them.
The results of circled photocatalytic degradation of crystal violet are shown in Fig. 6. The degradation rates of circle 1, 2 and 3 are 0.0593, 0.0589, 0.0587, respectively. Here it becomes evident that the repeated use of BNP-TiO2 does not reduce its photocatalytic activity, demonstrating the good photocatalytic stability of BNP-TiO2. Compared with common TiO2, the as-obtained BNP-TiO2 are endowed with excellent photocatalytic properties, in accordance with UV–Vis absorption spectra, under both ultraviolet and visible-light irradiation. This is a result of the synergy of their structures and components. The mesopore structure could offer more absorption and reaction sites for the photocatalytic reaction, and the successful co-doping of N and P could expand the wavelength absorption range and enhance the UV–Vis light absorption as well as the photocatalytic activity.
4. Conclusions The present work employed crop seeds, as prototypes of biomass, to synthesize N–P-codoped TiO2 with mesopore structures. The light-harvesting and photocatalytic properties of BNP-TiO2 are much enhanced, compared with common TiO2. Our method not only realizes mineralization of crop seeds but also achieves N
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and P contained in original crop seeds being simultaneously selfdoped into TiO2 lattice. Besides TiO2, the self-doping method could also be applied to other metal oxides like ZnO, CdO, In2O3, CeO2, etc. This work may provide a facile way for further extensive study of making full use of discarded biomass and converting the abundant resources into attractive functional materials. Acknowledgements This work is supported by National Natural Science Foundation of China (No. 50972090), Dawn Program of Shanghai Education Commission (No. 08SG15) and Shanghai Rising-star Program (No. 10Qh1401300). References Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 293, 269–271. Bessekhouad, Y., Robert, D., Weber, J., 2004. Bi2S3/TiO2 and CdS/TiO2 heterojunctions as an available configuration for photocatalytic degradation of organic pollutant. J. Photochem. Photobiol. A – Chem. 163, 569–580. Brinker, C., Scherer, G., 1990. Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing. Academic Press, San Diego. Burda, C., Lou, Y., Chen, X., Samia, A., Stout, J., Gole, J., 2003. Enhanced nitrogen doping in TiO2 nanoparticles. Nano Lett. 3, 1049–1051. Carp, O., Huisman, C., Reller, A., 2004. Photoinduced reactivity of titanium dioxide. Prog. Solid State Chem. 32, 33–177. Chen, X., Burda, C., 2004. Photoelectron spectroscopic investigation of nitrogendoped titania nanoparticles. J. Phys. Chem. B 108, 15446–15449. Chen, X., Zhang, X., Su, Y., Lei, L., 2008. Preparation of visible-light responsive P–Fcodoped TiO2 nanotubes. Appl. Surf. Sci. 254, 6693–6696. Choi, W.Y., Termin, A., Hoffmann, M.R., 1994. The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics. J. Phys. Chem. 98, 13669–13679. Demirbas, A., 2001. Biomass resource facilities and biomass conversion processing for fuels and chemicals. Energy Conv. Manag. 42, 1357–1378. Di Valentin, C., Pacchioni, G., Selloni, A., Livraghi, S., Giamello, E., 2005. Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations. J. Phys. Chem. B 109, 11414–11419. Fan, T., Chow, S., Zhang, D., 2009. Biomorphic mineralization: from biology to materials. Prog. Mater. Sci. 54, 542–659. Gaya, U., Abdullah, A., 2008. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems. J. Photochem. Photobiol. C – Photochem. Rev. 9, 1–12. Hall, W.H., 1949. X-ray line broadening in metals. Proc. Phys. Soc. A62, 741–743. Hashimoto, K., Irie, H., Fujishima, A., 2005. TiO2 photocatalysis: a historical overview and future prospects. Jpn. J. Appl. Phys. 44, 8269–8285. Hong, X., Wang, Z., Cai, W., Lu, F., Zhang, J., Yang, Y., Ma, N., Liu, Y., 2005. Visiblelight-activated nanoparticle photocatalyst of iodine-doped titanium dioxide. Chem. Mater. 17, 1548–1552. Ihara, T., Miyoshi, M., Iriyama, Y., Matsumoto, O., Sugihara, S., 2003. Visible-lightactive titanium oxide photocatalyst realized by an oxygen-deficient structure and by nitrogen doping. Appl. Catal. B – Environ. 42, 403–409. Irie, H., Watanabe, Y., Hashimoto, K., 2003a. Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst. Chem. Lett. 32, 772–773. Irie, H., Watanabe, Y., Hashimoto, K., 2003b. Nitrogen-concentration dependence on photocatalytic activity of TiO2xNx powders. J. Phys. Chem. B 107, 5483–5486. Lin, L., Lin, W., Zhu, Y.X., Zhao, B., Xie, Y.C., 2005. Phosphor-doped titania – a novel photocatalyst active in visible light. Chem. Lett. 34, 284–285. Lin, L., Lin, W., Xie, J.L., Zhu, Y.X., Zhao, B., Xie, Y.C., 2007a. Photocatalytic properties of phosphor-doped titania nanoparticles. Appl. Catal. B – Environ. 75, 52–58.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Biogenic N–P-codoped TiO2: Synthesis, characterization and photocatalytic properties Ting Han, Tongxiang Fan *, Suk-Kwun Chow, Di Zhang State Key Lab. of Composites, Shanghai Jiaotong University, 200240 Shanghai, PR China
a r t i c l e Article history: Received 9 October Received in revised Accepted 20 March Available online 14
i n f o 2009 form 16 March 2010 2010 April 2010
Keywords: TiO2 Self-doping Biomass Biogenic Photocatalysis
a b s t r a c t Four typical kinds of crop seeds are studied as non-metallic bio-precursors to synthesize biogenic N–Pcodoped TiO2 (BNP-TiO2). The as-prepared BNP-TiO2 samples are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen-adsorption measurement and UV–Vis spectroscopy. The results show that BNP-TiO2 possesses single anatase phase with mesopore structures, and nitrogen and phosphorus contained in original crop seeds are self-doped into the lattice as anions and cations, respectively. Besides, BNP-TiO2 exhibits a strongly enhanced absorption in the UV–Vis light range and red shift of the absorption edge, implicating the highly efficient light-harvesting capacity and sensitization towards visible light. Furthermore, experiments of crystal violet degradation under Xe lamp irradiation indicate superior photocatalytic activity of BNP-TiO2, of which the degradation rate is almost three times that of common TiO2. Circled photocatalytic degradation also shows good photocatalytic stability of BNP-TiO2. This work may pave a new and facile pathway of utilizing discarded biomass to synthesize desirable element-doped metal oxides based on biomass precursors. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biomass, as a renewable resource, is ubiquitous and abundant on earth. It is estimated that world production of biomass is at 1.46E11 metric tons per year (Demirbas, 2001). Presently, the main methods for utilizing biomass are direct combusting for heat or electricity, converting to traditional solid, liquid and gas fuels for energy and producing fine chemicals (Yaman, 2004). However, large quantities of biomass are still underutilized because of some complicated conversion techniques and their high costs. Therefore, achieving effective and economic utilization of biomass is still a great challenge. In recent years, there has been great interest in biomorphic mineralization – a technique that employs nature living things as templates for mineralization to produce bio-templated materials with morphologies and structures resembling those of the templates (Fan et al., 2009). Besides, nature living things can also act as bio-precursors for mineralization, for they possess many desirable elements that can potentially be employed as chemical components of the material to be produced. For example, some plants accumulate metal elements such as Ni, Co, Cd and Au; most biomass contains abundant non-metallic elements such as C, H, O, N and P (Smith, 2004). Crop seeds, containing abundant N and P elements, can be employed as non-metallic precursors for mineralization to synthesize N- and P-codoped metal oxides. Crop seeds are composed of various chemical compositions, mainly starches, * Corresponding author. E-mail address: [email protected] (T. Fan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.03.107
proteins and fats, among which starches and proteins are hydrophilic colloids, favorable for the infiltration of metal-ion solution. Moreover, crop seeds contain abundant N and P elements in different forms, e.g. as storage proteins, enzymes, phytic acids, phospholipids, phosphoproteins and so forth. These N and P elements are expected to be retained and self-doped into metal oxides during mineralization of crop seeds into metal oxides. These characteristics make crop seeds appropriate biogenic non-metallic precursors for producing doped metal oxides. Photocatalysis is an important industrial process in wastewater treatment, heavy metal remediation, air purification, sterilization, etc. Titanium dioxide (TiO2) is the most widely used photocatalyst for its efficiency, low cost, and chemical stability (Gaya and Abdullah, 2008). However, the relatively large band gap of TiO2 (3.2 eV) allows TiO2 to absorb light only with wavelength less than 387 nm, which accounts for merely 5% of total solar energy reaching the earth’s surface. To sensitize TiO2 to visible light and enhance its photocatalytic activity, many efforts have been made, such as noble metal deposition (Sakthivel et al., 2004b), composite semiconductor (Bessekhouad et al., 2004), metal doping (Choi et al., 1994), rare earth doping (Xu et al., 2002), nonmetal doping (Asahi et al., 2001) and so forth. Among these, nonmetal doping has been reported to be an effective way by many researchers (Irie et al., 2003; Ohno et al., 2004; Wang et al., 2005; Yu et al., 2002; Zheng et al., 2008). Nonmetal doping could form a localized state slightly above the valence band in the band gap, causing band narrowing, and it could also reduce the formation energy of oxygen vacancies, which are energetically favorable for the enhancement of light
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absorption and photocatalytic activity (Di Valentin et al., 2005; Livraghi et al., 2006). Generally, titania doping is achieved through introducing extra sources during sol–gel (Wang and Ying, 1999) or vapor-phase synthesis (Irie et al., 2003). For nitrogen doping, the N-precursors can be classified into two categories: organic and inorganic. The former includes urea (Nosaka et al., 2005), triethylamine (Burda et al., 2003) and other organic compounds with amine groups (Sakthivel et al., 2004a); the latter includes ammonia and other inorganic compounds containing NHþ 4 or NO3 (Ihara et al., 2003; Livraghi et al., 2008; Sato et al., 2005). For phosphorus (Shi et al., 2006) doping, the primary sources are inorganic PO3 4 and PO3 2 (Lin et al., 2005). In this paper, we employed crop seeds as non-metallic precursors to synthesize TiO2. N and P contained in original crop seeds were expected to be simultaneously self-doped into TiO2 during mineralization of the seeds, resulting in biogenic N–P-codoped TiO2, hereafter referred to as BNP-TiO2. The presence and states of N and P are identified by X-ray photoelectron spectroscopy (XPS) and the porous structures of BNP-TiO2 are studied by nitrogen-adsorption measurement. Moreover, light-harvesting and photocatalytic properties of BNP-TiO2, compared with that of common TiO2, are evaluated by UV–Vis absorption spectra and degradation of crystal violet solution under irradiation of Xe lamp. And the photocatalytic stability of BNP-TiO2 is evaluated by circled photocatalytic degradation experiments. 2. Methods 2.1. Synthetic procedure Four kinds of crop seeds, namely Oryza sativa L., Oryza sativa L. var. glutinosa Matsum., Glycine max(L.) Merr. and Sesamum indicum Linn., were employed as bio-precursors to synthesize biogenic N– P-codoped TiO2. Seeds of O. sativa L. and O. sativa L. var. glutinosa Matsum. mainly contain starches. Starches in the former seed are mixture of amyloses and amylopectins, while starches in the latter seed are all amylopectins. Seeds of G. max(L.) Merr. and S. indicum Linn. mainly contain proteins and fats, respectively. Crop seeds were cleaned with pure water and the testae were removed manually. Since sesame coats were too small and thin to remove, they were retained. The coatless crop seeds were treated with 5% HCl solution for 12 h to get rid of K, Ca and S ions. After being rinsed with pure water, the as-treated crop seeds were immersed in 5% TiCl3 solution under vacuum for 24 h. Since starches and proteins are hydrophilic colloids, containing many functional groups such as hydroxyls and carboxyls, Ti ions were easily introduced into the seeds. The impregnated and inflated crop seeds then underwent a graded drying process in an aerated oven at 25 °C, 40 °C, 60 °C, 80 °C and 105 °C successively, with each temperature for 2 h. Finally, the as-treated samples were calcined in air under 500 °C for 2 h, with a ramping rate of 1 °C/min. For comparison, common TiO2 was synthesized by common sol–gel method (Brinker and Scherer, 1990; Su et al., 2004): hydrolyzing the sol–gel precursor and calcining under the same conditions with BNP-TiO2. A 1.5% tetrabutyl titanate ethanol solution accompanied with acetylacetone (1.8:120:0.09 by volume) was employed as the sol–gel precursor.
son-Hall method (Hall, 1949; Williamson and Hall, 1953), expressed as follows: bi(cos hi)/k = K/D + 4e(sin hi)/k, where bi is the integral breadth (in radius 2h) of the ith Bragg reflection peak positioned at 2hi; K is a constant with the value of 0.89; k = 0.154 nm is the wavelength of the X-rays; D is the average size of the crystallites and e is the microstrain, whose distribution is isotropic. D and e were calculated through a least-squares fit. The presence and states of N and P in BNP-TiO2 were identified by XPS, performed on a Thermo ESCALAB 250 spectrometer with monochromatized Al Ka X-ray (hm = 1486.6 eV) at a pass energy of 20 eV. All the binding energies were calibrated by using the contaminant carbon (C1S = 284.4 eV) as a reference. The porous structures of BNP-TiO2 were studied by nitrogenadsorption measurement, operated at 77 K on a Micromeritics ASAP 2010 adsorption analyzer. All the BNP-TiO2 samples were degassed at 200 °C and 106 Torr for 5 h prior to the measurements. The pore-size distribution curve was derived from the desorption branch of the nitrogen isotherm by the Barrett–Joyner–Halenda (BJH) method, and the surface areas were calculated through the Brunauer Emmett Teller (BET) equation. The light-harvesting capacities of BNP-TiO2 were characterized by UV–Vis absorption spectra, recorded on a Varian Cary UV–VisNIR spectrophotometer in the spectral range of 200–800 nm. A BaSiO4 plate was used as the basic line for the spectra. The same quality for all the BNP-TiO2 samples was taken during the measurement. To evaluate the photocatalytic activities of BNP-TiO2, degradation experiments of crystal violet nonahydrate (C25H30ClN39H2O) in aqueous solution was carried out in the presence of BNP-TiO2 and common TiO2 as follows. 0.1 g corresponding TiO2 sample was dispersed in 100 mL of 105 mol/L crystal violet solution. The TiO2 aqueous suspension was stirred in dark for 1 h to reach its adsorption equilibrium and then it was irradiated under a 500 W Xe lamp, whose emission spectrum is the nearest to the solar spectrum and which is thus the most widely used light source of solar simulator. Degradation was monitored by taking sample solutions at 0, 4, 8, 12, 18, 25, 35, 45, 60 min, respectively during the irradiation. These sample solutions were centrifuged with 3000 r/min for 3 min. And then the supernatants were tested in sequence using a 25 Lambda UV–Vis spectrometer to get the absorption spectra. The rate of degradation was assumed to obey pseudo-first-order kinetics (Pillai et al., 2007) and the degradation rate constant, k, was obtained according to the following equation: ln(A0/A) = kt, where A0 is the initial absorbance, namely, the characteristic absorbance peak of original crystal violet; A is the characteristic absorbance peak of crystal violet after a t time degradation. To evaluate the photocatalytic stability of BNP-TiO2, three circles of crystal violet photocatalytic degradation were carried out in the presence of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. The circled degradation was monitored by taking sample solutions at 0, 5, 15, 25, 35, 45, 60 min, respectively during the photocatalytic reaction. After each degradation circle, the degraded solution with BNP-TiO2 photocatalyst suspended in it was centrifuged with 3000 r/min for 3 min. Then the supernatant was drawn out with a dropper and pure water was added to the remaining precipitate for washing. The suspension of BNP-TiO2 and pure water was centrifuged to get the clean BNP-TiO2 photocatalyst. The obtained wet BNP-TiO2 was dried at 25 °C for the next degradation circle.
2.2. Characterization
3. Results and discussion
The crystal phase of the BNP-TiO2 samples was examined by Xray diffraction (XRD) on a Bruker-AXS X-ray diffractometer system with Cu Ka radiation at 40 kV and 100 mA. The spectra were recorded in the 2h range from 20° to 90° with a scanning step of 0.02 °/s. The average crystalline sizes were calculated by William-
3.1. Composition and structure characterization of BNP-TiO2 XRD patterns of BNP-TiO2 and common TiO2 are shown in Fig. 1. It shows that all the BNP-TiO2 samples possess single anatase phase, except for BNP-TiO2 from seeds of S. indicum Linn. with
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Fig. 1. XRD patterns of BNP-TiO2 and common TiO2.
minor SiO2 phase, which is passed from the seed coats containing Si element. For common TiO2, both anatase and rutile phases are
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detected. It is possible that the presence of crop seed bio-precursors retards the phase transformation of anatase to rutile. The anatase phase is preferred over rutile, for anatase shows a greater photocatalytic activity (Linsebigler et al., 1995). The average crystalline sizes of BNP-TiO2, calculated by Williamson-Hall method, are 8.4 nm, 10.6 nm, 9.0 nm and 9.1 nm for BNP-TiO2 from seeds of O. sativa L., O. sativa L. var. glutinosa Matsum., G. max(L.) Merr. and S. indicum Linn., respectively. These smaller grain sizes are usually associated with higher specific surfaces (Carp et al., 2004; Tanaka et al., 1991). XPS tests identified the presence of doped N and P in TiO2 network. The whole XPS survey of BNP-TiO2 from seeds of G. max(L.) Merr. (Fig. 2a) demonstrates that N and P exist in the oxides and that their atomic contents are 1.49 at.% and 4.3 at.%. Mineral elements in crop seeds like K, Ca, S, Mg and others are not detected, implicating that the treatment with dilute HCl effectively got rid of them. The high-resolution scanning of N1s displayed in Fig. 2b shows two peaks, at 399.8 eV and 401.6 eV. The former N1s peak is attributed to N anion in O–Ti–N bonds, where the oxygen sites in O–Ti–O bonds are partly substituted by nitrogen atoms (Chen and Burda, 2004; Sathish et al., 2005). The latter peak at 401.6 eV was assigned to molecularly chemisorbed nitrogen species (Irie et al., 2003). As for P doping, there is only one peak at 133.7 eV and no peaks around 128.6 eV in the high-resolution XPS spectra of P2p (Fig. 2c), suggesting that P ions only exist as cations in a pentavalent oxidation state (P5+) (Shi et al., 2006), not as anions
Fig. 2. XPS patterns of BNP-TiO2 from seeds of G. max(L.) Merr. (a) the whole survey; (b) high-resolution spectra of N1s; (c) high-resolution spectra of P2p; and (d) highresolution spectra of O1s.
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Fig. 3. Nitrogen adsorption results of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. (a) adsorption–desorption isotherms and (b) pore-size distribution curves.
in a tervalent deoxidation state (P3). It is generally believed that P2p peaks at a relatively higher binding energy (133.4–133.8 eV) are ascribed to the result that P5+ replace part of Ti4+ in TiO2 lattice, forming Ti–O–P bonds, instead of P3 replacing O2 or P5+ in the form of PO3 4 , supported by many studies using XPS and Fourier transform infrared (FTIR) methods (Lin et al., 2007a,b; Shi et al., 2006; Yu et al., 2003; Zheng et al., 2008). However, the mechanism of P doping has not been fully revealed, which left an issue worthy of further research. Fig. 2d shows the spectra of O1s high-resolution scanning. The broad O1s region can be fitted by three peaks, namely, Ti–O bonds, P–O bonds and C–O bonds (Chen et al., 2008). The other three kinds of BNP-TiO2 samples exhibit similar XPS patterns. Their atomic contents of N and P are 1.22 at.% and 2.72 at.% (from O. sativa L. seeds), 0.82 at.% and 1.81 at.% (from O. sativa L. var. glutinosa Matsum. seeds) and 0.79 at.% and 2.7 at.% (from S. indicum Linn. seeds), respectively. The contents of doped N and P in the BNP-TiO2 from G. max(L.) Merr. seeds are significantly higher than those in the other three because seeds of leguminous plants have a high content of N and P. With a high content of iodine, Laminariaceae plants could also be explored as I-precursors for producing I-doped TiO2, as it has been proved that the photocatalytic properties of TiO2 are enhanced after doping with I (Hong et al., 2005). These suggest not only that N and P from crop seeds could be self-doped into BNP-TiO2, but also that the contents and even types of dopants could be adjusted by selecting different kinds of crop seeds or other biomass as precursors. The porous structures of BNP-TiO2 at nanoscale are characterized by nitrogen-adsorption measurements. Fig. 3a displays the nitrogen adsorption–desorption isotherms of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. The other three kinds of BNPTiO2 samples exhibit similar isotherms. According to the IUPAC classification (Sing et al., 1985), this is a type IV isotherm, which is given by absorbents with mesopore structures (Sing, 1998). The pore sizes at nanoscale are distributed mostly between 2 nm and 40 nm (Fig. 3b), with an average pore width of 14.7 nm. The BET surface area is 55.9 m2 g1. The mesopore structure and high surface area of BNP-TiO2 photocatalyst could offer more absorption and reaction sites for the photocatalytic reaction, favorable for enhancing photocatalytic performance.
absorbance of BNP-TiO2 and common TiO2. Compared with common TiO2, BNP-TiO2 shows two prominent features. One is the enhancement of overall absorbance intensities. The average absorbance intensities within visible range (the bar graph in Fig. 4) increase by 93%, 89%, 160%, and 112% for BNP-TiO2 from seeds of O. sativa L., O. sativa L. var. glutinosa Matsum., G. max(L.) Merr. and S. indicum Linn., respectively. Among four kinds of BNP-TiO2, BNP-TiO2 from G. max(L.) Merr. seeds has the highest absorbance in the visible light region, while BNP-TiO2 from S. indicum Linn. seeds has the highest absorbance in the ultraviolet light region. BNP-TiO2 from O. sativa L. var. glutinosa Matsum. seeds has fairly high absorbance both in the visible and ultraviolet light region. The second feature is the red shift of band gap absorption edges towards longer wavelengths, implicating band gap narrowing. This is caused by doping, which forms a localized state slightly above the
3.2. Light-harvesting and photocatalytic properties of BNP-TiO2 The light-harvesting properties of the BNP-TiO2 are characterized by UV–Vis absorption spectra. Fig. 4 displays the UV–Vis
Fig. 4. UV–Vis absorption spectra of BNP-TiO2 and common TiO2, with a bar graph of the average absorbance intensities within the visible range.
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valence band (Di Valentin et al., 2005). The enhanced absorbance and red shifts indicates more photo-generated electrons and holes
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participating in the photocatalytic reactions could be excited by photons with lower energies, leading to high efficiency in
Fig. 5. Photocatalytic degradation studies of BNP-TiO2 and common TiO2. (a) Changes in the UV–Vis absorption spectra of crystal violet as a function of irradiation time in the presence of BNP-TiO2 from O. sativa L. seeds. (b) Changes in the UV–Vis absorption spectra of crystal violet as a function of irradiation time in the presence of common TiO2. Insets in a and b are corresponding color variations over irradiation time. (c) Kinetic study of degradation of crystal violet solution in the presence of BNP-TiO2 and common TiO2 under Xe lamp irradiation, with a bar graph of the degradation rates.
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Fig. 6. Circled photocatalytic degradation studies of BNP-TiO2 from seeds of O. sativa L. var. glutinosa Matsum. (a) Circle 1; (b) circle 2; (c) circle 3; and (d) kinetic study of circled degradation.
photo-induced redox reaction of absorbed substances (Hashimoto et al., 2005). Photocatalytic degradation studies of crystal violet solution are presented in Fig. 5. As shown in Fig. 5a, the characteristic peak of crystal violet at 590 nm decreased dramatically and disappeared after irradiation for 60 min in the presence of BNP-TiO2, indicating that crystal violet was completely degraded by BNP-TiO2. While in Fig. 5b, in the presence of common TiO2, crystal violet was not fully degraded after the same time of irradiation. As it is illustrated in the insets of Fig. 5a and b, in the presence of BNP-TiO2, crystal violet solution shaded from dark purple to colorless and transparent solution in 60 min, while in the presence of common TiO2, the solution color varied slightly. To evaluate the degradation rates quantitatively, we introduce pseudo-firstorder kinetics to process the absorbance data during the degradation. The degradation rate could be obtained by fitting curves in Fig. 5c. Details of the method are provided in Methods section. The degradation rates of BNPTiO2 and common TiO2 are shown in the bar graph in Fig. 5c. The average degradation rate of all the four BNP-TiO2 specimens is 0.050 min1, almost three times that of the common TiO2 specimen, which was 0.017 min1. Besides crystal violet, other dyes such as methylene blue, methyl orange and rhodamine 6G have been degraded by BNP-TiO2. Here space lacks for a detailed description of them.
The results of circled photocatalytic degradation of crystal violet are shown in Fig. 6. The degradation rates of circle 1, 2 and 3 are 0.0593, 0.0589, 0.0587, respectively. Here it becomes evident that the repeated use of BNP-TiO2 does not reduce its photocatalytic activity, demonstrating the good photocatalytic stability of BNP-TiO2. Compared with common TiO2, the as-obtained BNP-TiO2 are endowed with excellent photocatalytic properties, in accordance with UV–Vis absorption spectra, under both ultraviolet and visible-light irradiation. This is a result of the synergy of their structures and components. The mesopore structure could offer more absorption and reaction sites for the photocatalytic reaction, and the successful co-doping of N and P could expand the wavelength absorption range and enhance the UV–Vis light absorption as well as the photocatalytic activity.
4. Conclusions The present work employed crop seeds, as prototypes of biomass, to synthesize N–P-codoped TiO2 with mesopore structures. The light-harvesting and photocatalytic properties of BNP-TiO2 are much enhanced, compared with common TiO2. Our method not only realizes mineralization of crop seeds but also achieves N
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