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(Mo + N) codoped TiO2 for enhanced visible-light photoactivity
Applied Surface Science 257 (2011) 9355–9361
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(Mo + N) codoped TiO2 for enhanced visible-light photoactivity Hailin Liu a , Zhihong Lu b,∗ , ling Yue a , Jing Liu b , Zhanghua Gan b , Chang Shu a , Ting Zhang a , Jing Shi a,c , Rui Xiong a,c a b c
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, PR China School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China Hubei Key Laboratory on Organic and Polymeric Opto-electronic Materials, Wuhan 430072, PR China
a r t i c l e
i n f o
Article history: Received 26 April 2011 Received in revised form 18 May 2011 Accepted 19 May 2011 Available online 22 June 2011 Key words: (Mo + N) codoped TiO2 Photoactivity Sol–gel method First-principles band structure calculations
a b s t r a c t A series of Ti1−x Mox O2−y Ny samples were prepared by using sol–gel method and characterized by X-ray diffraction, transmission electron microscopy and UV–vis absorption spectroscopy. All Ti1−x Mox O2−y Ny samples are anatase phase. It is found that Mo, N mono-doping can increase visible light absorption, while (Mo + N) co-doping can greatly enhance absorption in whole visible region. Results of our first-principles band structure calculations reveal that (Mo + N)-doping, especially passivated co-doping can increase the up-limit of dopant concentration and create more impurity bands in the band gap of TiO2 , which leads to a greatly increase of its visible-light absorption without a decrease of its redox potential. It reveals that (Mo + N) co-doped TiO2 is promising for a photocatalyst with high photocalystic activity under visible light. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Today, the ever depleting energy and the ever deteriorating ecological environment are two severe problems faced by the international community and need to be solved urgently. The use of sunlight as a new energy source is one of the most promising ways for solving the problems not only because the sunlight offers tremendous energy but also its energy is green. To use the solar energy efficiently, good photocatalysts need to be found. A good photocatalyst should have strong photo-oxidizing potential and its photocatalytic reaction should be activated by visible light. In the past few decades, many efforts have been paid in developing materials with the good photocatalytic properties and a lot of potential materials have been found, including TiO2 , SrTiO3 , ␣-Fe2 O3 , WO3 , ZnO, and ZnS. Among them, TiO2 is thought to be the most promising one and has been extensively studied since the discovery of its photosensitation effect in 1972 [1]. As a photocatalyst, TiO2 has many attracting properties such as photo-oxidizing potential, high chemical stability, non-toxicity, and low cost, etc., which make it an ideal photocatalyst in many applications such as hydrogen production through photoelectrochemical (PEC) water splitting, water and air purification, dye-sensitized solar cells, self-cleaning surfaces and antibacterial agent, etc. However, there is a main drawback of TiO2 to be overcome before it can be widely used: its photocatalytic reaction can only be activated by ultraviolet light due to its wide
∗ Corresponding author. Tel.: +86 27 68754613; fax: +86 27 68752569. E-mail address: [email protected] (Z. Lu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.085
energy band gap (about 3.0–3.2 eV). As the ultraviolet light only takes a small proportion (about 5%) of the solar energy, the photocatalytic efficiency of pure TiO2 under sun light is very low. To use the solar energy effectively, it is of great significance to extend the absorption spectrum of TiO2 into the visible-light region, so that much higher proportion (45%) of the solar energy may be used. There are several methods to achieve visible-light absorption, such as dye sensitization, external surface modification, and band structure modification, among which band structure modification by ion doping is believed to be the most efficient approach. The dopant can be either transition metal cations (such as V [2], Cr [3], Fe [4], and Nb [5]) or anions (such as C [6], S [7], and nitrogen [8–12]). Ion doping usually introduces impurity energy levels within the energy band gap, which help to enhance the visible-light absorption. Normally, doping with transition metal will induce impurity levels near the conduction band, while doping with anions lead to impurity levels near the valence band. The impurity levels can locate shallowly or deep in the band gap depending on dopants. A deep impurity level usually acts as a recombination center and lead to a decrease of the photocatalytic activity of TiO2 , while a shallow impurity level tend to perform as a trapping center and enhance the photocatalytic activity [13]. The number of impurity band depends on the dopant concentration. Larger dopant concentration can introduce more impurity levels and thus can further increase the visible-light absorption of TiO2 . However, Ion doping can also cause the band gap narrowing if the high dopant concentration is so high that the impurity bands overlap with the valence band or the conduction band. Although it can enhance the visible-light absorption, band gap narrowing is not always desirable
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especially when it is leaded by the overlapping of impurity bands with the conduct band. The redox potential of TiO2 is related to the position of band edges with the reduce power determined by the conduct band minimum (CBM) and the oxidation power depending on valence band maximum (VBM). The overlapping of impurity bands with the conduct band will decrease the CBM and lead to a decrease of the reduce potential of TiO2 . One of most important applications of TiO2 is to act as photocatalyst in PEC water splitting process. Since the CBM of TiO2 is only slightly above the H2 /H2 O level, a decrease of CBM will lead to a loss of the photoreduction activity of TiO2 in the water splitting process. Whether or not the impurity bands overlap with the conduction band depends on the concentration of the doped transition metal cation: small amount doping induces localized impurity bands deep in the band gap, which will not change the CBM; heavy doping may cause the impurity bands overlap with the conduction band and decrease the CBM. The highest doping concentration for a transition metal dopant is determined by its solubility in TiO2 . However, for a transition metal element with fairly high solubility in TiO2 , the overlapping of impurity band with conduction band may happen before the dopant concentration reach the solubility limit. Therefore, to avoid the bands overlapping, another concentration limit of the dopant should be set. The up-limit for the concentration of the transition metal dopant is thus determined by the lower one of the two limits. Since very low dopant concentration will not lead to a significant increase of the visible-light absorption due to too small number of states in impurity band, to enhance the visible-light absorption as high as possible, while in the same time keep the photocatalysic activity of TiO2 , the best doping concentration for a transition metal element should be very close to and slightly lower than the up-limit. Recently, Devi and Murthy reported that doping with appropriate amount of Mo into TiO2 can extending the absorption spectrum into the visible light region and enhanced the photocatalystic activity of TiO2 under visible light irradiation [14]. However, monodoping with Mo can only induce impurity levels on the side of the conduction band and the dopant concentration is restricted by the up-limit of Mo. Since anion doping can induce extra impurity bands on the side of valence band, doping with a type of non-metal anion together with Mo may further increase the absorption in visiblelight region. Moreover, it was reported that the defect bands are passivated and will not be effective as carrier recombination centers when co-doping TiO2 with charge compensated cation–anion pair [15,16]. In present work, pure anatase phase TiO2 , N or Mo mono-doped TiO2 and (Mo + N) co-doped TiO2 nanoparticles were fabricated by using sol–gel method, the photo absorption properties of the nanoparticles were measured and discussed by combined theoretical and experimental results. It is found that greatly enhanced visiblelight absorption can be achieved by co-doping Mo and N into TiO2 .
2. Experiment 2.1. Sample preparation 2.1.1. Pure TiO2 nanoparticles and Ti1−x Mox O2 nanoparticles The pure TiO2 and Ti1−x Mox O2 nano-particles with anatase structure were fabricated by using sol–gel method. To fabricate pure TiO2 , 10 ml Tetra-n-butyl titanate (Ti(C4 H9 O)4 ), 100 ml pure alcohol(C2 H5 OH) and 2 ml hydrochloric acid with a concentration of 2 mol/l solvents were used. For the fabrication of Ti1−x Mox O2 nano-particles, an amount of MoCl5 powder was solved in 2 ml hydrochloric acid and the amount depended on the atomic ratio of Ti and Mo. During the fabrication, 10 ml Tetra-n-butyl titanate was first carefully diluted by adding drop-wise to 2/3 of the pure alcohol (about 66 ml) with stirring
until a clear solution was obtained; secondly, 2 ml hydrochloric acid (for pure TiO2 fabrication) or hydrochloric acid plus MoCl5 solvent (for Ti1−x Mox O2 fabrication) was drop-wisely added to the obtained solution, stirring was performed during the adding process and kept for 30 min after the process until the mixing was completed; thirdly, the resulting solution was further diluted by drop-wisely adding the rest 1/3 pure alcohol in it with stirring during and after the adding process; finally, the solution was settled down to obtain the hydroxide of Titanium gel. After being washed several times to remove chloride ions, the gel was dried in an oven for 10 h at a temperature of 60 ◦ C. The oven dried TiO2 or Ti1−x Mox O2 was then grounded into fine powder. The heat treatment of the powder was performed in an oven. The temperature was increased slowly from room temperature to 500 ◦ C, kept at 500 ◦ C for 2 h and then air cooled to room temperature. The resulting powder was then grounded for 10 min in a mortar followed by a heat treatment for 2 h at 500 ◦ C 2.1.2. TiO2−y Ny and Ti1−x Mox O2−y Ny nano-particles To prepare TiO2−y Ny and Ti1−x Mox O2−y Ny , TiO2 and Ti1−x Mox O2 nano-particles were annealed in an ammonia atmosphere at 450 ◦ C. During the annealing, a mixed gas NH3 /N2 with the volume ratio of NH3 and N2 equal to 1:3 was introduced to flow over the particles. The N dopant concentration was adjusted by the controlling of the annealing time. Two TiO2−y Ny samples were fabricated, one was annealed for half an hour, and the other was annealed for 3 h. In the following part of the paper, these two samples will be named SN-0.5 and SN-3, respectively. Ti1−x Mox O2−y Ny was prepared by annealing Ti1−x Mox O2 for 3 h. Therefore, the N concentration of Ti1−x Mox O2−y Ny may be close to that of SN-3. 3. Sample characterization The crystal structures of the samples were characterized by Xray diffraction with Bruker AXS D8 Advance diffractometer and verified by TEM. The absorption spectra were measured by using UV-probe spectrophotometer. The morphology was observed by TEM. 3.1. Computation method The band structure calculations were carried out using the Vienna ab initio simulation package (VASP) [17,18]. The frozen-core projector-augmented wave (PAW) method within general gradient approximate (GGA) was used. To get reasonable band gap, the onsite effective U parameters proposed by Anisimov et al. [19] and Solovyev and Dederichs [20] were adopted for Ti 3d electrons and Mo 4d electrons, respectively. In the calculations, the lattice parameters of the primitive unit ˚ c/a = 3.51 u = 0.208. The of pure TiO2 were used with a = 3.785 A, changes of lattice parameters by doping were neglected. Although in experiment, the dopants may take the substitutional sides or the interstitial sides or both and produce oxygen vacancies in the same time, in our calculations, the dopants were considered to only take the substitutional sides without producing any oxygen vacancy. Three kinds of supercell were used to reach different dopant concentrations: 1) supercell with 192 atoms, as a 4 × 4 × 1 repetition of the primitive anatase unit cell; 2) supercell with 48 atoms, as a 2 × 2 × 1 repetition of primitive anatase unit cell; 3) primitive anatase unit cell. For mono-doping (Ti1−x Mox O2 or TiO2−y Ny ) case, only one Mo/N atom was introduced to substitute for Ti/O atom in each supercell. Therefore, the corresponding x and y values are around 0.0156, 0.0625 and 0.25, respectively. Since a N atom can accept one more electron than a O atom and a Mo atom can donate 2 more electrons than a Ti atom, the substitution of N on O site and
H. Liu et al. / Applied Surface Science 257 (2011) 9355–9361
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Fig. 1. Position of substitutional N anion in non-passivated doping (a) and passivated doping.
Ti0.95Mo0.05O2-yNy Ti0.95Mo0.05O2 Ti0.985Mo0.015O2
Fig. 4. Absorption spectra of TiO2 and N-doped TiO2.
Ti0.991Mo0.009O2
SN- 3 SN- 0.5 (101)
20
(200) (103) (004) (112)
30
40
(105)
50
TiO
2 (211) (204) (116) (220) (215)
60
70
80
2θ (degree) Fig. 2. X-ray diffraction spectra of samples.
the substitution of Mo on Ti site act as a single acceptor and a double donor, respectively. In Mo and N co-doped TiO2 , substitutions of one Mo atom for Ti and one N atom for O in the supercell leads to non-passivated doping in which one 4d electron of Mo atom is not compensated; when doping with one Mo atom and two N atoms in the supercell, the doping is passivated. In the study of Mo and N codoped TiO2 (Ti1−x Mox O2−y Ny ), both passivated and non-passivated cases were considered and the calculations were based on the 48atom supercells. Moreover, as to the substitution sites of N atoms, we found that the system with the substitutions of N for O around the Mo atom is energetically favorable. Therefore, in this study, we only consider substitutions of N for O bonding to the Mo atom as shown in Fig. 1. Projector-augmented wave potentials with cut-off energy of 400 eV and high symmetry for integration in the Brillouin zone
were used in calculations, 45, 729 and 2025 k-points were used for calculations with different supercells. The density of states (DOS) was calculated using the tetrahedron method with Blöhl corrections. 4. Result and discussion Fig. 2 shows the XRD spectra of the prepared nano-particles with x = 0, 0.009, 0.015 and 0.048. All peaks belong to anatase phase of TiO2 , no other phases are detected. The particle sizes of the samples were evaluated by using Sherrer formula and shown in Table 1. The particle sizes of TiO2 , TiO2−y Ny and Ti1−x Mox O2 with x < 0.015 are similar, and in a range of 23–28 nm, while the particle sizes of Ti1−x Mox O2 and Ti1−x Mox O2−y Ny with x = 0.05 are much smaller. It seems that the doping of Mo atom prevents the increase of particle size. The crystal structure and grain size of each sample were also evaluated by TEM. It is found that TEM gives similar results as those obtained by XRD. Let’s take sample SN-3 for example. The TEM image of sample SN-3 is shown in Fig. 3. By analyzing the diffraction pattern of sample SN-3 (Fig. 3b), it is found that all the diffraction rings belong to anatase phase of TiO2 , no TiN phase or rutile phase of TiO2 can be detected. The bright field image (Fig. 3a) shows that the nano-particles have an average grain size about 20 nm, which is in accord with the value obtained by XRD. The UV–vis absorption spectra of the N-doped samples are shown in Fig. 4. As expected, pure TiO2 only absorbs light in UV range ( < 400 nm), no apparent absorption happens in visible range. Compared with the absorption of pure TiO2 , the ultraviolet
Fig. 3. TEM images of sample SN-3: a) bright field image, b) diffraction.
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Table 1 Particle size of samples evaluated by Sherrer formula. Sample
TiO2
Size (nm)
TiO2−y Ny
28.5
Ti1−x Mox O2 SN-3
X = 0.009
X = 0.015
X = 0.048
X = 0.048
25.2
23.5
25.4
28.6
13.0
14.5
absorption of sample SN-0.5 does not show apparent difference and almost no red-shift of the absorption edge is observed; however, the absorption in visible region is quite different from that of pure TiO2 : noticeable optical absorption is observed in the whole visible region. As to sample SN-3, a red-shift of the absorption edge is observed; besides this, a significant absorption shoulder is observed in a wavelength range of 410–500 nm, which is in good agreement with the results reported by Asahi et al. [8]. Actually, this kind of absorption shoulder is also found in the absorption spectrum of sample SN-0.5, although much weaker. Although the N dopant concentration is not known, it is reasonable to regard that the long-time annealed sample SN-3 has higher N concentration than the short-time annealed sample SN0.5. Therefore, the difference of absorption spectra of these two samples may be due to the difference of N dopant concentration. The absorption spectra of Mo-doped and (Mo + N) co-doped TiO2 are shown in Fig. 5. For easy comparison, the absorption spectrum of pure TiO2 is also included. It is found that, the doping of Mo not only leads to the red-shift of the absorption edge, but also causes significant absorption in visible-light region; when x < 0.015, the red-shift of the absorption edge is small, while as x increases to 0.048, the red-shift of the absorption edge becomes apparent. The absorption spectrum of (Mo + N) co-doped TiO2 shows that (Mo + N) co-doping not only leads to a red-shift of absorption edge but also significantly enhances the absorption of visible light; The absorption of Mo–N co-doping samples in visible region is much higher than those single doped samples and goes through the whole visible region. The energy band gap corresponds to the absorption limit and can be roughly evaluated by using following equation: Eg =
1240 (eV) th
(1)
where th represents the absorption limit of the semiconductor. th can be extracted from the absorption spectrum in the following way: carry out the first derivative of absorbance with respect to wavelength near the absorption edge and find out the point at which the derivative spectrum reaches its minimum value – this point is actually the reflection point of the absorption curve. The 90
Ti0.95Mo0.05O2-yNy
Absorbance (%)
75
Ti0.95Mo0.05O2 Ti0.985Mo0.015O2
60
Ti0.991Mo0.009O2 45
tangent line of the absorption curve at the reflection point intersects with the x-axis on which absorbance reaches 0 and gives th . Carefully studying the absorption spectra of Mo-doped TiO2 samples, we can find that in visible region, the absorptions are not monotonically decreased, there are broad absorption peaks. The maximum position (m ) is used to represent each peak. The absorption limit th , the energy band gap Eg , the peak position of visible-light absorption (p ) and its corresponding energy difference (l ) of each sample are shown in Table 2. 5. Theoretical results To better understand the absorption spectra, we calculated the density of states of TiO2−y Ny , Ti1−x Mox O2 and Ti1−x Mox O2−y Ny . The DOS of Ti1−x Mox O2 and Ti1−x Mox O2−y Ny is aligned by referencing to the core levels of the atom farthest from the doping atom so that the positions of conduct edges and impurity levels can be easily compared. 5.1.1. Nitrogen-doped TiO2 The calculated total density of states (TDOS) and partial density of states (PDOS) of TiO2−y Ny are shown in Fig. 6. Three kinds of doping level were considered: 1) low doping level with y = 0.0156; 2) medium doping level with y = 0.0625; and 3) heavy doping level with y = 0.25. For easy comparison, the results of pure TiO2 are also shown. For pure TiO2 (y = 0), the energy band gap is about 3.00 eV, which is closed to the experimental result; the valence band edge of TiO2 is mainly determined by O 2p states, while the conduction band edge is of Ti 3d character predominantly; The Fermi level is at the top of the valence band. Therefore, the absorption edge corresponds to the electron transition from p states to d states. The neutral 2p orbital energy of nitrogen is 2.0 eV higher than O 2p orbital energy [20]. Therefore, the acceptor levels induced by N should be higher than the O 2p levels. As expected, when looking at the total density of states (TDOS) of N-doped TiO2 , extra peaks appear at the top of the valence band and these peaks are found to be mainly consisted of 2p states of N when compared with the PDOS of N. From the PDOS of the N atom, the degenerated p states split to two separated peaks in local crystal field, one is occupied and the other one is half occupied. As we know, a N atom can accept one more electron than a O atom, only two of the three p levels are filled when it replaces a O atom, the third one is only half filled Table 2 Absorption limit, band gap, visible absorption peak and corresponding energy difference of samples.
TiO2
Sample
30
TiO2 TiO2 -N
15
Ti1−x Mox O2
0 200
300
400
Ti1−x Mox O2−y Ny
SN-0.5
500
600
700
800 Ti1−x Mox O2−y Ny
Wavelength ( nm) a
Fig. 5. Absorption spectra of Mo-doped TiO2 and Mo + N co-doped smaples.
th (nm)
Eg (ev)
p (nm)
l (ev)
SN-0.5 SN-3 X = 0.009 X = 0.015 X = 0.048 X = 0.048
390 388 425 411 409 449 437
3.18 3.19 2.91 3.02 3.03 2.76 2.84
443a 466a 566 616 557 751
2.80 2.70 2.19 2.01 2.23 1.65
Note: for each TiO2 -N sample, the wavelength at the reflection point of the absorption shoulder was taken as its p.
H. Liu et al. / Applied Surface Science 257 (2011) 9355–9361
Fig. 6. Comparison of TDOSs and PDOSs of TiO2−y Ny with y = 0, 0.0156, 0.0625 and 0.25.
and has higher energy than the two fully filled levels. Therefore, the peak with lower energy is consisted of fully filled p levels and the one with higher energy is consisted of half filled p level. When the dopant concentration is very low (y = 0.0156), two peaks are completely separated from each other: the filled one is at the top of the valence band, and the half-filled one stays inside the band gap and spans across the Fermi level. The filled states of N have only small contribution in building up the top of valence band and do not change the band gap significantly. Therefore, low N-doping will not change the intrinsic band gap (the gap between the top of the valence band and the bottom of the conduction band) much but introduces an isolated impurity band near the valence band. The reason for no apparent red-shift in the absorption edge observed in absorption spectrum of our short-time annealed sample may be able to attribute to no significant change of intrinsic band gap due to low N dopant concentration. Since there are some localized N 2p energy levels inside the band gap, electrons can transit from these levels to d bands and lead to absorptions in visible region. According to our calculation, in low N-doped TiO2 , the localized impurity band is about 0.2 eV away from the top of valence band, and the gap between the impurity band and the conduction band is about 2.7 eV, which corresponds to a wavelength around 450 nm. When the concentration of N dopant increases to y = 0.0625, the modification effect of N 2p states on the top of valence band becomes apparent – the top of the main valence band is now consisted of filled N 2p states. In the meanwhile, the impurity band induced by N dopant is no longer really isolated from the main valence band -there is a small overlap between them. The gap between the main valence band and the conduction band is about 2.9 eV which is about 0.1 eV narrower than that of pure TiO2 . The distance between “isolated” band and the CBM is almost unchanged when compared with that of TiO2−y Ny with y = 0.0156. Since the DOS of the “isolated” impurity band becomes higher with the increase of y, the absorption corresponding to the electron transition from this impurity band to conduction band will be enhanced. As further increase the N dopant concentration to y = 0.25, the impurity band now overlaps with filled N 2p states, forming a continuous band as can be seen from the PDOS of N, and the top of the valence band is of the character of N 2p. The band gap is narrowed to about 2.75 eV. Therefore, heavy N-doping can significantly decrease the energy band gap of TiO2 .
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Fig. 7. Effect of concentration of Mo dopant on the band structure of TiO2. (The red dash lines represent the Fermi levels of different systems).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article)
When comparing the theoretical results to the experimental results, it is found that the absorption phenomena of the samples SN-0.5 and SN-3 can be very well explained by the band structures of TiO2−y Ny with y = 0.0156 and y = 0.0625, respectively: the redshift of absorption edge is lead by the band gap narrowing and the absorption shoulder at 400–500 nm may be attributed to the transition of electron from the localized impurity band to the conduction band; for SN-0.5 sample, the N dopant concentration is very low and the band gap is not apparently narrowed and the DOS of impurity band is very low, therefore no red-shift in band edge is observed and the 400–500 nm absorption is weak; for sample SN-3, noticeable optical absorption is observed in range of 400–500 nm; while in SN-3 sample, due to higher N concentration, band gap is narrowed by a small amount, and the DOS of impurity band becomes higher, therefore, red-shift of band edge is observed and the absorption peak at 400–500 nm becomes stronger; it is worth noticing that the 0.1 eV band gap narrowing is qualitatively agree with the experimental result of sample SN-3. Based on the analysis above, we may be able to conclude that SN-0.5 and SN-3 have low and medium N dopant concentration, respectively. It is noticed that both SN-0.5 and SN-3 have noticeable absorption of visible-light with wavelength >500 nm which cannot be explained by the calculated band structures. It is well known that ion doping always leads to some O vacancies and the impurity bands induced by O vacancies are usually deep in the gap. Therefore, the absorption of visible-light with longer wavelength may be due to the transition of electrons from impurity bands of O vacancies to the conduction band. 5.1.2. Mo-doped TiO2 To study the effect of the concentration of Mo dopant on the band structure of TiO2 , Ti1−x Mox O2 with three different Mo doping levels—low, medium and heavy were considered with x equal to 0.0156, 0.0625 and 0.25, respectively. The TDOSs of Ti1−x Mox O2 with different doping level are shown in Fig. 7 and the TDOS of un-doped TiO2 is also shown for comparison. Since the Mo 4d orbits have lower energy (1.2 eV lower) than
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Ti 3d [21], the impurity bands induced by Mo will locate below the bottom of the conduct band. As can be noticed from the TDOSs, doping of Mo leads to new peaks below or at the bottom of conduction band, these peaks are composed of Mo 4d states. When doping level is low (x = 0.0156), two peaks appear near the bottom of the conduction band and completely isolated. Since one Mo atom can donate 6 electrons, while one Ti atom can donate 4 electrons – two electrons less than Mo atom, when Mo atoms substitute for Ti atoms, each Mo atom has 2 extra 4d electrons left, and lead to one of the 4d states occupied. Therefore, in the TDOS, one of the peaks has lower energy and spans over the Fermi level, which formed by partially occupied Mo 4d states, the other has higher energy and locates near the bottom of the conduction band which is consisted of empty 4d states. The partially occupied and empty 4d peaks are 0.5 eV and 0.2 eV below the bottom of the conduction band respectively. The intrinsic band gap is about 2.96 eV, which is very close to that of the pure TiO2 (3.0 eV). So, low Mo-doping does not show significant effect in band gap narrowing. Since these two isolated impurity bands are fairly deep in the band gap, they may act as electron recombination center. Photo-excitation of the electrons from valence band to these two bands corresponds to optical absorptions in visible region with wavelengths of about 460 nm and 530 nm, respectively. These wavelengths are within the wavelength range spanned by the broad absorption peak of the sample with x = 0.015 observed in visible region. The broad absorption peak may be formed partially with the contribution of photo-excitation of electron from the valence band to these two impurity bands. The rest contributions come from the electron transitions related to the impurity bands induced by O vacancies. Since the intrinsic band gap does not change much by low Mo-doping, it is not surprising that no apparent band edge red-shift is observed in our Ti1−x Mox O2 sample with x = 0.015. As the Mo dopant concentration increases to x = 0.0625, the two impurity bands mix with each other with small density of state. It is found that intrinsic band gap is actually narrowed because the second impurity band overlaps with the bottom of the conduct band with very tiny DOS. Compared with TDOS of x = 0.0156, the two peaks shift to higher energy with first one about 2.77 eV away from the top of the valence band and the second one about 2.9 eV away from the valence band. The photo-excitation of electrons from the valence band to these two Mo-4d mid-band leads to the absorptions of visible light with wavelengths of 428 nm and 447 nm, which are close to 440 nm – the intrinsic absorption wavelength of sample Ti1−x Mox O2 with x = 0.048. The transition of electrons from the valence band to the impurity bands may be the main reason for the experimentally observed red-shift of the absorption edge. As the Mo concentration increases to x = 0.25, impurity bands shift to higher energy and mix with the conduction band. The band gap is thus narrowed to about 2.6 eV. How, since the band gap narrowing is led by the downshift of the bottom of the conduction band, it is not desirable for photocatalysis. Based on the analysis above, it is found that, when x ≥ 0.0625, the impurity band will overlap with the conduction band and lead to a decrease of band gap. However, this band gap narrowing should be avoided because it leads to a downshift of the conduction band edge which will cause the decrease of the photoreduction activity of TiO2 , even though the visible-light absorption can be enhanced. To maintain the photoreduction activity, and in the same time, keep visible light absorption fairly high, x should be smaller than and close to 0.0625. Since the solubility (>12 at.% [22]) of Mo in TiO2 is much higher than this value, the up-limit of Mo concentration in Mo mono-doped TiO2 is x = 0.0625. Compared among the DOS of Ti1−x Mox O2 with different x, it is found that the impurity bands shift to higher energy and become less localized with the increase of x. Since deep impurity levels usually act as recombination center, while shallow impurity lev-
Fig. 8. Comparison of the decomposed PDOSs of Ti1−x Mox O2 with (a) x = 0.0156, (b) x = 0.0625.
els can enhance the photocatalystic activity [13], the up-shift of the Mo impurity band is important. To understand the formation of impurity bands and reason for up-shift of the impurity band with the Mo-concentration, the decomposed density of states of Mo 4d-orbits and O 2p-orbits was extracted and plotted in Fig. 8. For density of states of Mo-4d orbits, only dxy , dxz and dyz states are plotted because dx2 −y2 and dz2 are not found to make significant contributions to the formation of impurity bands. According to the decomposed PDOSs of Mo and O, at low dopant concentration (x = 0.0156), the dxy states of Mo are de-coupled with O 2p states and have lower energy than dxz and dyz states which couple with p of O. The deeper impurity band is composed of non-bonding dxy states, and the shallower impurity band is formed by the weak bonding between p of O and dyz , dxz of Mo. The two impurity bands are localized due the energy difference of them. As the Mo concentration increases, dxy states become to couple with O p , which leads to an energy increase of dxy states; dxy is now degenerated with dyz and dxz . The coupling between Mo t2g with O p also leads to a splitting of dxy , dyz and dxz to form two sub-bands. Therefore, the shift of impurity bands to higher energy is due to the increase of the coupling strength between Mo t2g and O p . 5.1.3. Mo and N co-doped TiO2 The TDOSs of (Mo + N) co-doped TiO2 are shown in Fig. 9. The N impurity introduces isolated impurity band(s) near the VBM, and the Mo dopant induces impurity bands near the CBM. Compared with Mo or N mono-doped TiO2 , the number of impurity states of (Mo + N) co-doped TiO2 is much larger. Therefore, (Mo + N) codoped TiO2 has much higher absorption in visible-light region as observed in experiment. For non-passivated co-doping, the impurity band induced by Mo is isolated from conduction band with tiny gap. As to the passivated co-doping, the impurity band by Mo is apparently separated from the conduction band. Compared to Mo mono-doping case, (Mo + N) co-doping helps to isolate the impurity band by Mo from the conduction band. As we discussed in previous section, for Mo mono-doped TiO2 , the up-limit of Mo concentration is x = 0.0625, which is set by the band overlapping and is much lower than the solubility limit of Mo. By co-doping Mo with N, this up-limit of Mo concentration can thus be raised to a higher value. On the other hand, (Mo + N) co-doping may increase the solubility limits of both Mo and N due to the synergistic effect of N3− /Mo6+ [23,24]. The
H. Liu et al. / Applied Surface Science 257 (2011) 9355–9361
a
x=0.0625
Based on first-principles band structure calculations, we propose that (Mo + N) co-doping, especially the passivated co-doping can increase the concentration up-limit of dopants and create more impurity bands in the band gap, which leads to a greatly increase of visible-light absorption. We suggest that (Mo + N) co-doped TiO2 is very promising as a photocatalyst with very high photocatalystic activity under visible light.
non-passivated
Densityof states (arb. units)
y=0.0625
b
x=0.0625
Acknowledgments passivated
y=0.125
-6
-4
-2
9361
0
2
4
Energy (ev) Fig. 9. TDOSs of (Mo + N) co-codoped TiO2 (Ti1−x Mox O2−y Ny ), a) non-passivated; b) passivated (The red dash lines represent the Fermi levels of the systems).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article)
enhancement of solubility limit is more important for N, because normally, the solubility of N alone in TiO2 is very low. Therefore, (Mo + N) co-doping can greatly enhance the visible-light absorption by increasing the up-limits of both Mo and N concentration. Moreover, (Mo + N) co-doping leads to a charge compensation among donors (Mo6+ ions) and acceptors (N3− ions), which is beneficial for the reduction in the number of carrier recombination centers and the enhancement of the photocatalystic activity. In case of passivated doping, since the charges of donors and acceptors are fully compensated, the impurity bands are passivated and less effective to be recombination centers due to the proposed equilibrium charge mechanism. Therefore, (Mo + N) co-doped TiO2 , especially the passivated co-doped TiO2 is very promising as a photocatalyst with very high photocatalystic activity under visible light. 6. Conclusions In summary, Mo or N mono-doped and (Mo + N) co-doped anatase TiO2 nanoparticles have been fabricated by sol–gel method. The photo absorption spectra show that Mo or N mono-doping can increase visible light absorption, while (Mo + N) co-doping can greatly enhance photo absorption in whole visible region.
The authors would like to thank the financial support from 973 Program (2009CB939705), Chinese National Foundation of Natural Science (no. 51001083 and no. 10974148), and Chinese National Science Fund for Talent Training in Basic Science (no. J0830310), Funding from Wuhan University (5082003) and Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials (Hubei University). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
A. Fujishima, K. Honda, Nature 283 (1972) 37. S. Klosek, D. Raftery, J. Phys. Chem. B 105 (2001) 2815. H. Kato, A. Kudo, J. Phys. Chem. B 106 (2002) 5029. K.T. Ranjit, B. Viswanathan, J. Photochem. Photobiol. A 108 (1997) 79. W. Li, Y. Wang, H. Lin, S. Ismat Shah, C.P. Huang, D.J. Doren, S.A. Rykov, J.G. Chen, M.A. Barteau, Appl. Phys. Lett. 83 (2003) 4143. X. Yang, C. Cao, K. Hohn, L. Erickson, R. Maghirang, D. Hamal, K. Klabunde, J. Catal. 252 (2007) 296. T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates, J. Phys. Chem. B 108 (2004) 6004. Z. Zhang, X. Wang, J. Long, Q. Gu, Z. Ding, X. Fu, J. Catal. 276 (2010) 201. C.D. Valetin, E. Finazzi, G. Pacchioni, A. Selloni, S. Livraghi, M.C. Paganini, E. Giamello, Chem. Phys. 339 (2007) 44. K.S. Han, J.W. Lee, Y.M. Kang, J.Y. Lee, J.K. Kang, Small 10 (2008) 1682. D. Jing, Y. Zhang, L. Guo, Chem. Phys. Lett. 415 (2005) 73. L.G. Devi, B.M. Murthy, Catal. Lett. 125 (2008) 320. K.-S. Ahn, Y. Yan, S. Shet, T. Deutsch, J. Tuner, M. Al-Jassim, Appl. Phys. Lett. 91 (2007) 231909. M.N. Hudam, Y. Yan, S.H. wei, M. Al-Jassim, Phys. Rev. B 78 (2008) 195204. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. V.I. Anisimov, J. Zaanen, O.K. Andersen, Phys. Rev. B 44 (1991) 943. I.V. Solovyev, P.H. Dederichs, Phys. Rev. B 50 (1994) 16861. Y. Gai, J. Li, S.S. Li, J.B. Xia, S.H. Wei, Phys. Rev. Lett. 102 (2009) 036402. A. Kubacka, G. ColÓn, M. Fenández-Garcíam, Catal. Today 143 (2008) 286. T. Ikeda, T. Tomonori, K. Eda, Y. Mizutani, H. Kato, A. Kudo, J. Phys. Chem. C 112 (2008) 1167. G. Liu, C. Sun, L. Cheng, Y. Jin, H. Lu, L. Wang, S.C. Smith, G. Liu, H. Cheng, J. Phys. Chem. C 113 (2009) 12317.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
(Mo + N) codoped TiO2 for enhanced visible-light photoactivity Hailin Liu a , Zhihong Lu b,∗ , ling Yue a , Jing Liu b , Zhanghua Gan b , Chang Shu a , Ting Zhang a , Jing Shi a,c , Rui Xiong a,c a b c
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, PR China School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China Hubei Key Laboratory on Organic and Polymeric Opto-electronic Materials, Wuhan 430072, PR China
a r t i c l e
i n f o
Article history: Received 26 April 2011 Received in revised form 18 May 2011 Accepted 19 May 2011 Available online 22 June 2011 Key words: (Mo + N) codoped TiO2 Photoactivity Sol–gel method First-principles band structure calculations
a b s t r a c t A series of Ti1−x Mox O2−y Ny samples were prepared by using sol–gel method and characterized by X-ray diffraction, transmission electron microscopy and UV–vis absorption spectroscopy. All Ti1−x Mox O2−y Ny samples are anatase phase. It is found that Mo, N mono-doping can increase visible light absorption, while (Mo + N) co-doping can greatly enhance absorption in whole visible region. Results of our first-principles band structure calculations reveal that (Mo + N)-doping, especially passivated co-doping can increase the up-limit of dopant concentration and create more impurity bands in the band gap of TiO2 , which leads to a greatly increase of its visible-light absorption without a decrease of its redox potential. It reveals that (Mo + N) co-doped TiO2 is promising for a photocatalyst with high photocalystic activity under visible light. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Today, the ever depleting energy and the ever deteriorating ecological environment are two severe problems faced by the international community and need to be solved urgently. The use of sunlight as a new energy source is one of the most promising ways for solving the problems not only because the sunlight offers tremendous energy but also its energy is green. To use the solar energy efficiently, good photocatalysts need to be found. A good photocatalyst should have strong photo-oxidizing potential and its photocatalytic reaction should be activated by visible light. In the past few decades, many efforts have been paid in developing materials with the good photocatalytic properties and a lot of potential materials have been found, including TiO2 , SrTiO3 , ␣-Fe2 O3 , WO3 , ZnO, and ZnS. Among them, TiO2 is thought to be the most promising one and has been extensively studied since the discovery of its photosensitation effect in 1972 [1]. As a photocatalyst, TiO2 has many attracting properties such as photo-oxidizing potential, high chemical stability, non-toxicity, and low cost, etc., which make it an ideal photocatalyst in many applications such as hydrogen production through photoelectrochemical (PEC) water splitting, water and air purification, dye-sensitized solar cells, self-cleaning surfaces and antibacterial agent, etc. However, there is a main drawback of TiO2 to be overcome before it can be widely used: its photocatalytic reaction can only be activated by ultraviolet light due to its wide
∗ Corresponding author. Tel.: +86 27 68754613; fax: +86 27 68752569. E-mail address: [email protected] (Z. Lu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.085
energy band gap (about 3.0–3.2 eV). As the ultraviolet light only takes a small proportion (about 5%) of the solar energy, the photocatalytic efficiency of pure TiO2 under sun light is very low. To use the solar energy effectively, it is of great significance to extend the absorption spectrum of TiO2 into the visible-light region, so that much higher proportion (45%) of the solar energy may be used. There are several methods to achieve visible-light absorption, such as dye sensitization, external surface modification, and band structure modification, among which band structure modification by ion doping is believed to be the most efficient approach. The dopant can be either transition metal cations (such as V [2], Cr [3], Fe [4], and Nb [5]) or anions (such as C [6], S [7], and nitrogen [8–12]). Ion doping usually introduces impurity energy levels within the energy band gap, which help to enhance the visible-light absorption. Normally, doping with transition metal will induce impurity levels near the conduction band, while doping with anions lead to impurity levels near the valence band. The impurity levels can locate shallowly or deep in the band gap depending on dopants. A deep impurity level usually acts as a recombination center and lead to a decrease of the photocatalytic activity of TiO2 , while a shallow impurity level tend to perform as a trapping center and enhance the photocatalytic activity [13]. The number of impurity band depends on the dopant concentration. Larger dopant concentration can introduce more impurity levels and thus can further increase the visible-light absorption of TiO2 . However, Ion doping can also cause the band gap narrowing if the high dopant concentration is so high that the impurity bands overlap with the valence band or the conduction band. Although it can enhance the visible-light absorption, band gap narrowing is not always desirable
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especially when it is leaded by the overlapping of impurity bands with the conduct band. The redox potential of TiO2 is related to the position of band edges with the reduce power determined by the conduct band minimum (CBM) and the oxidation power depending on valence band maximum (VBM). The overlapping of impurity bands with the conduct band will decrease the CBM and lead to a decrease of the reduce potential of TiO2 . One of most important applications of TiO2 is to act as photocatalyst in PEC water splitting process. Since the CBM of TiO2 is only slightly above the H2 /H2 O level, a decrease of CBM will lead to a loss of the photoreduction activity of TiO2 in the water splitting process. Whether or not the impurity bands overlap with the conduction band depends on the concentration of the doped transition metal cation: small amount doping induces localized impurity bands deep in the band gap, which will not change the CBM; heavy doping may cause the impurity bands overlap with the conduction band and decrease the CBM. The highest doping concentration for a transition metal dopant is determined by its solubility in TiO2 . However, for a transition metal element with fairly high solubility in TiO2 , the overlapping of impurity band with conduction band may happen before the dopant concentration reach the solubility limit. Therefore, to avoid the bands overlapping, another concentration limit of the dopant should be set. The up-limit for the concentration of the transition metal dopant is thus determined by the lower one of the two limits. Since very low dopant concentration will not lead to a significant increase of the visible-light absorption due to too small number of states in impurity band, to enhance the visible-light absorption as high as possible, while in the same time keep the photocatalysic activity of TiO2 , the best doping concentration for a transition metal element should be very close to and slightly lower than the up-limit. Recently, Devi and Murthy reported that doping with appropriate amount of Mo into TiO2 can extending the absorption spectrum into the visible light region and enhanced the photocatalystic activity of TiO2 under visible light irradiation [14]. However, monodoping with Mo can only induce impurity levels on the side of the conduction band and the dopant concentration is restricted by the up-limit of Mo. Since anion doping can induce extra impurity bands on the side of valence band, doping with a type of non-metal anion together with Mo may further increase the absorption in visiblelight region. Moreover, it was reported that the defect bands are passivated and will not be effective as carrier recombination centers when co-doping TiO2 with charge compensated cation–anion pair [15,16]. In present work, pure anatase phase TiO2 , N or Mo mono-doped TiO2 and (Mo + N) co-doped TiO2 nanoparticles were fabricated by using sol–gel method, the photo absorption properties of the nanoparticles were measured and discussed by combined theoretical and experimental results. It is found that greatly enhanced visiblelight absorption can be achieved by co-doping Mo and N into TiO2 .
2. Experiment 2.1. Sample preparation 2.1.1. Pure TiO2 nanoparticles and Ti1−x Mox O2 nanoparticles The pure TiO2 and Ti1−x Mox O2 nano-particles with anatase structure were fabricated by using sol–gel method. To fabricate pure TiO2 , 10 ml Tetra-n-butyl titanate (Ti(C4 H9 O)4 ), 100 ml pure alcohol(C2 H5 OH) and 2 ml hydrochloric acid with a concentration of 2 mol/l solvents were used. For the fabrication of Ti1−x Mox O2 nano-particles, an amount of MoCl5 powder was solved in 2 ml hydrochloric acid and the amount depended on the atomic ratio of Ti and Mo. During the fabrication, 10 ml Tetra-n-butyl titanate was first carefully diluted by adding drop-wise to 2/3 of the pure alcohol (about 66 ml) with stirring
until a clear solution was obtained; secondly, 2 ml hydrochloric acid (for pure TiO2 fabrication) or hydrochloric acid plus MoCl5 solvent (for Ti1−x Mox O2 fabrication) was drop-wisely added to the obtained solution, stirring was performed during the adding process and kept for 30 min after the process until the mixing was completed; thirdly, the resulting solution was further diluted by drop-wisely adding the rest 1/3 pure alcohol in it with stirring during and after the adding process; finally, the solution was settled down to obtain the hydroxide of Titanium gel. After being washed several times to remove chloride ions, the gel was dried in an oven for 10 h at a temperature of 60 ◦ C. The oven dried TiO2 or Ti1−x Mox O2 was then grounded into fine powder. The heat treatment of the powder was performed in an oven. The temperature was increased slowly from room temperature to 500 ◦ C, kept at 500 ◦ C for 2 h and then air cooled to room temperature. The resulting powder was then grounded for 10 min in a mortar followed by a heat treatment for 2 h at 500 ◦ C 2.1.2. TiO2−y Ny and Ti1−x Mox O2−y Ny nano-particles To prepare TiO2−y Ny and Ti1−x Mox O2−y Ny , TiO2 and Ti1−x Mox O2 nano-particles were annealed in an ammonia atmosphere at 450 ◦ C. During the annealing, a mixed gas NH3 /N2 with the volume ratio of NH3 and N2 equal to 1:3 was introduced to flow over the particles. The N dopant concentration was adjusted by the controlling of the annealing time. Two TiO2−y Ny samples were fabricated, one was annealed for half an hour, and the other was annealed for 3 h. In the following part of the paper, these two samples will be named SN-0.5 and SN-3, respectively. Ti1−x Mox O2−y Ny was prepared by annealing Ti1−x Mox O2 for 3 h. Therefore, the N concentration of Ti1−x Mox O2−y Ny may be close to that of SN-3. 3. Sample characterization The crystal structures of the samples were characterized by Xray diffraction with Bruker AXS D8 Advance diffractometer and verified by TEM. The absorption spectra were measured by using UV-probe spectrophotometer. The morphology was observed by TEM. 3.1. Computation method The band structure calculations were carried out using the Vienna ab initio simulation package (VASP) [17,18]. The frozen-core projector-augmented wave (PAW) method within general gradient approximate (GGA) was used. To get reasonable band gap, the onsite effective U parameters proposed by Anisimov et al. [19] and Solovyev and Dederichs [20] were adopted for Ti 3d electrons and Mo 4d electrons, respectively. In the calculations, the lattice parameters of the primitive unit ˚ c/a = 3.51 u = 0.208. The of pure TiO2 were used with a = 3.785 A, changes of lattice parameters by doping were neglected. Although in experiment, the dopants may take the substitutional sides or the interstitial sides or both and produce oxygen vacancies in the same time, in our calculations, the dopants were considered to only take the substitutional sides without producing any oxygen vacancy. Three kinds of supercell were used to reach different dopant concentrations: 1) supercell with 192 atoms, as a 4 × 4 × 1 repetition of the primitive anatase unit cell; 2) supercell with 48 atoms, as a 2 × 2 × 1 repetition of primitive anatase unit cell; 3) primitive anatase unit cell. For mono-doping (Ti1−x Mox O2 or TiO2−y Ny ) case, only one Mo/N atom was introduced to substitute for Ti/O atom in each supercell. Therefore, the corresponding x and y values are around 0.0156, 0.0625 and 0.25, respectively. Since a N atom can accept one more electron than a O atom and a Mo atom can donate 2 more electrons than a Ti atom, the substitution of N on O site and
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Fig. 1. Position of substitutional N anion in non-passivated doping (a) and passivated doping.
Ti0.95Mo0.05O2-yNy Ti0.95Mo0.05O2 Ti0.985Mo0.015O2
Fig. 4. Absorption spectra of TiO2 and N-doped TiO2.
Ti0.991Mo0.009O2
SN- 3 SN- 0.5 (101)
20
(200) (103) (004) (112)
30
40
(105)
50
TiO
2 (211) (204) (116) (220) (215)
60
70
80
2θ (degree) Fig. 2. X-ray diffraction spectra of samples.
the substitution of Mo on Ti site act as a single acceptor and a double donor, respectively. In Mo and N co-doped TiO2 , substitutions of one Mo atom for Ti and one N atom for O in the supercell leads to non-passivated doping in which one 4d electron of Mo atom is not compensated; when doping with one Mo atom and two N atoms in the supercell, the doping is passivated. In the study of Mo and N codoped TiO2 (Ti1−x Mox O2−y Ny ), both passivated and non-passivated cases were considered and the calculations were based on the 48atom supercells. Moreover, as to the substitution sites of N atoms, we found that the system with the substitutions of N for O around the Mo atom is energetically favorable. Therefore, in this study, we only consider substitutions of N for O bonding to the Mo atom as shown in Fig. 1. Projector-augmented wave potentials with cut-off energy of 400 eV and high symmetry for integration in the Brillouin zone
were used in calculations, 45, 729 and 2025 k-points were used for calculations with different supercells. The density of states (DOS) was calculated using the tetrahedron method with Blöhl corrections. 4. Result and discussion Fig. 2 shows the XRD spectra of the prepared nano-particles with x = 0, 0.009, 0.015 and 0.048. All peaks belong to anatase phase of TiO2 , no other phases are detected. The particle sizes of the samples were evaluated by using Sherrer formula and shown in Table 1. The particle sizes of TiO2 , TiO2−y Ny and Ti1−x Mox O2 with x < 0.015 are similar, and in a range of 23–28 nm, while the particle sizes of Ti1−x Mox O2 and Ti1−x Mox O2−y Ny with x = 0.05 are much smaller. It seems that the doping of Mo atom prevents the increase of particle size. The crystal structure and grain size of each sample were also evaluated by TEM. It is found that TEM gives similar results as those obtained by XRD. Let’s take sample SN-3 for example. The TEM image of sample SN-3 is shown in Fig. 3. By analyzing the diffraction pattern of sample SN-3 (Fig. 3b), it is found that all the diffraction rings belong to anatase phase of TiO2 , no TiN phase or rutile phase of TiO2 can be detected. The bright field image (Fig. 3a) shows that the nano-particles have an average grain size about 20 nm, which is in accord with the value obtained by XRD. The UV–vis absorption spectra of the N-doped samples are shown in Fig. 4. As expected, pure TiO2 only absorbs light in UV range ( < 400 nm), no apparent absorption happens in visible range. Compared with the absorption of pure TiO2 , the ultraviolet
Fig. 3. TEM images of sample SN-3: a) bright field image, b) diffraction.
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Table 1 Particle size of samples evaluated by Sherrer formula. Sample
TiO2
Size (nm)
TiO2−y Ny
28.5
Ti1−x Mox O2 SN-3
X = 0.009
X = 0.015
X = 0.048
X = 0.048
25.2
23.5
25.4
28.6
13.0
14.5
absorption of sample SN-0.5 does not show apparent difference and almost no red-shift of the absorption edge is observed; however, the absorption in visible region is quite different from that of pure TiO2 : noticeable optical absorption is observed in the whole visible region. As to sample SN-3, a red-shift of the absorption edge is observed; besides this, a significant absorption shoulder is observed in a wavelength range of 410–500 nm, which is in good agreement with the results reported by Asahi et al. [8]. Actually, this kind of absorption shoulder is also found in the absorption spectrum of sample SN-0.5, although much weaker. Although the N dopant concentration is not known, it is reasonable to regard that the long-time annealed sample SN-3 has higher N concentration than the short-time annealed sample SN0.5. Therefore, the difference of absorption spectra of these two samples may be due to the difference of N dopant concentration. The absorption spectra of Mo-doped and (Mo + N) co-doped TiO2 are shown in Fig. 5. For easy comparison, the absorption spectrum of pure TiO2 is also included. It is found that, the doping of Mo not only leads to the red-shift of the absorption edge, but also causes significant absorption in visible-light region; when x < 0.015, the red-shift of the absorption edge is small, while as x increases to 0.048, the red-shift of the absorption edge becomes apparent. The absorption spectrum of (Mo + N) co-doped TiO2 shows that (Mo + N) co-doping not only leads to a red-shift of absorption edge but also significantly enhances the absorption of visible light; The absorption of Mo–N co-doping samples in visible region is much higher than those single doped samples and goes through the whole visible region. The energy band gap corresponds to the absorption limit and can be roughly evaluated by using following equation: Eg =
1240 (eV) th
(1)
where th represents the absorption limit of the semiconductor. th can be extracted from the absorption spectrum in the following way: carry out the first derivative of absorbance with respect to wavelength near the absorption edge and find out the point at which the derivative spectrum reaches its minimum value – this point is actually the reflection point of the absorption curve. The 90
Ti0.95Mo0.05O2-yNy
Absorbance (%)
75
Ti0.95Mo0.05O2 Ti0.985Mo0.015O2
60
Ti0.991Mo0.009O2 45
tangent line of the absorption curve at the reflection point intersects with the x-axis on which absorbance reaches 0 and gives th . Carefully studying the absorption spectra of Mo-doped TiO2 samples, we can find that in visible region, the absorptions are not monotonically decreased, there are broad absorption peaks. The maximum position (m ) is used to represent each peak. The absorption limit th , the energy band gap Eg , the peak position of visible-light absorption (p ) and its corresponding energy difference (l ) of each sample are shown in Table 2. 5. Theoretical results To better understand the absorption spectra, we calculated the density of states of TiO2−y Ny , Ti1−x Mox O2 and Ti1−x Mox O2−y Ny . The DOS of Ti1−x Mox O2 and Ti1−x Mox O2−y Ny is aligned by referencing to the core levels of the atom farthest from the doping atom so that the positions of conduct edges and impurity levels can be easily compared. 5.1.1. Nitrogen-doped TiO2 The calculated total density of states (TDOS) and partial density of states (PDOS) of TiO2−y Ny are shown in Fig. 6. Three kinds of doping level were considered: 1) low doping level with y = 0.0156; 2) medium doping level with y = 0.0625; and 3) heavy doping level with y = 0.25. For easy comparison, the results of pure TiO2 are also shown. For pure TiO2 (y = 0), the energy band gap is about 3.00 eV, which is closed to the experimental result; the valence band edge of TiO2 is mainly determined by O 2p states, while the conduction band edge is of Ti 3d character predominantly; The Fermi level is at the top of the valence band. Therefore, the absorption edge corresponds to the electron transition from p states to d states. The neutral 2p orbital energy of nitrogen is 2.0 eV higher than O 2p orbital energy [20]. Therefore, the acceptor levels induced by N should be higher than the O 2p levels. As expected, when looking at the total density of states (TDOS) of N-doped TiO2 , extra peaks appear at the top of the valence band and these peaks are found to be mainly consisted of 2p states of N when compared with the PDOS of N. From the PDOS of the N atom, the degenerated p states split to two separated peaks in local crystal field, one is occupied and the other one is half occupied. As we know, a N atom can accept one more electron than a O atom, only two of the three p levels are filled when it replaces a O atom, the third one is only half filled Table 2 Absorption limit, band gap, visible absorption peak and corresponding energy difference of samples.
TiO2
Sample
30
TiO2 TiO2 -N
15
Ti1−x Mox O2
0 200
300
400
Ti1−x Mox O2−y Ny
SN-0.5
500
600
700
800 Ti1−x Mox O2−y Ny
Wavelength ( nm) a
Fig. 5. Absorption spectra of Mo-doped TiO2 and Mo + N co-doped smaples.
th (nm)
Eg (ev)
p (nm)
l (ev)
SN-0.5 SN-3 X = 0.009 X = 0.015 X = 0.048 X = 0.048
390 388 425 411 409 449 437
3.18 3.19 2.91 3.02 3.03 2.76 2.84
443a 466a 566 616 557 751
2.80 2.70 2.19 2.01 2.23 1.65
Note: for each TiO2 -N sample, the wavelength at the reflection point of the absorption shoulder was taken as its p.
H. Liu et al. / Applied Surface Science 257 (2011) 9355–9361
Fig. 6. Comparison of TDOSs and PDOSs of TiO2−y Ny with y = 0, 0.0156, 0.0625 and 0.25.
and has higher energy than the two fully filled levels. Therefore, the peak with lower energy is consisted of fully filled p levels and the one with higher energy is consisted of half filled p level. When the dopant concentration is very low (y = 0.0156), two peaks are completely separated from each other: the filled one is at the top of the valence band, and the half-filled one stays inside the band gap and spans across the Fermi level. The filled states of N have only small contribution in building up the top of valence band and do not change the band gap significantly. Therefore, low N-doping will not change the intrinsic band gap (the gap between the top of the valence band and the bottom of the conduction band) much but introduces an isolated impurity band near the valence band. The reason for no apparent red-shift in the absorption edge observed in absorption spectrum of our short-time annealed sample may be able to attribute to no significant change of intrinsic band gap due to low N dopant concentration. Since there are some localized N 2p energy levels inside the band gap, electrons can transit from these levels to d bands and lead to absorptions in visible region. According to our calculation, in low N-doped TiO2 , the localized impurity band is about 0.2 eV away from the top of valence band, and the gap between the impurity band and the conduction band is about 2.7 eV, which corresponds to a wavelength around 450 nm. When the concentration of N dopant increases to y = 0.0625, the modification effect of N 2p states on the top of valence band becomes apparent – the top of the main valence band is now consisted of filled N 2p states. In the meanwhile, the impurity band induced by N dopant is no longer really isolated from the main valence band -there is a small overlap between them. The gap between the main valence band and the conduction band is about 2.9 eV which is about 0.1 eV narrower than that of pure TiO2 . The distance between “isolated” band and the CBM is almost unchanged when compared with that of TiO2−y Ny with y = 0.0156. Since the DOS of the “isolated” impurity band becomes higher with the increase of y, the absorption corresponding to the electron transition from this impurity band to conduction band will be enhanced. As further increase the N dopant concentration to y = 0.25, the impurity band now overlaps with filled N 2p states, forming a continuous band as can be seen from the PDOS of N, and the top of the valence band is of the character of N 2p. The band gap is narrowed to about 2.75 eV. Therefore, heavy N-doping can significantly decrease the energy band gap of TiO2 .
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Fig. 7. Effect of concentration of Mo dopant on the band structure of TiO2. (The red dash lines represent the Fermi levels of different systems).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article)
When comparing the theoretical results to the experimental results, it is found that the absorption phenomena of the samples SN-0.5 and SN-3 can be very well explained by the band structures of TiO2−y Ny with y = 0.0156 and y = 0.0625, respectively: the redshift of absorption edge is lead by the band gap narrowing and the absorption shoulder at 400–500 nm may be attributed to the transition of electron from the localized impurity band to the conduction band; for SN-0.5 sample, the N dopant concentration is very low and the band gap is not apparently narrowed and the DOS of impurity band is very low, therefore no red-shift in band edge is observed and the 400–500 nm absorption is weak; for sample SN-3, noticeable optical absorption is observed in range of 400–500 nm; while in SN-3 sample, due to higher N concentration, band gap is narrowed by a small amount, and the DOS of impurity band becomes higher, therefore, red-shift of band edge is observed and the absorption peak at 400–500 nm becomes stronger; it is worth noticing that the 0.1 eV band gap narrowing is qualitatively agree with the experimental result of sample SN-3. Based on the analysis above, we may be able to conclude that SN-0.5 and SN-3 have low and medium N dopant concentration, respectively. It is noticed that both SN-0.5 and SN-3 have noticeable absorption of visible-light with wavelength >500 nm which cannot be explained by the calculated band structures. It is well known that ion doping always leads to some O vacancies and the impurity bands induced by O vacancies are usually deep in the gap. Therefore, the absorption of visible-light with longer wavelength may be due to the transition of electrons from impurity bands of O vacancies to the conduction band. 5.1.2. Mo-doped TiO2 To study the effect of the concentration of Mo dopant on the band structure of TiO2 , Ti1−x Mox O2 with three different Mo doping levels—low, medium and heavy were considered with x equal to 0.0156, 0.0625 and 0.25, respectively. The TDOSs of Ti1−x Mox O2 with different doping level are shown in Fig. 7 and the TDOS of un-doped TiO2 is also shown for comparison. Since the Mo 4d orbits have lower energy (1.2 eV lower) than
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Ti 3d [21], the impurity bands induced by Mo will locate below the bottom of the conduct band. As can be noticed from the TDOSs, doping of Mo leads to new peaks below or at the bottom of conduction band, these peaks are composed of Mo 4d states. When doping level is low (x = 0.0156), two peaks appear near the bottom of the conduction band and completely isolated. Since one Mo atom can donate 6 electrons, while one Ti atom can donate 4 electrons – two electrons less than Mo atom, when Mo atoms substitute for Ti atoms, each Mo atom has 2 extra 4d electrons left, and lead to one of the 4d states occupied. Therefore, in the TDOS, one of the peaks has lower energy and spans over the Fermi level, which formed by partially occupied Mo 4d states, the other has higher energy and locates near the bottom of the conduction band which is consisted of empty 4d states. The partially occupied and empty 4d peaks are 0.5 eV and 0.2 eV below the bottom of the conduction band respectively. The intrinsic band gap is about 2.96 eV, which is very close to that of the pure TiO2 (3.0 eV). So, low Mo-doping does not show significant effect in band gap narrowing. Since these two isolated impurity bands are fairly deep in the band gap, they may act as electron recombination center. Photo-excitation of the electrons from valence band to these two bands corresponds to optical absorptions in visible region with wavelengths of about 460 nm and 530 nm, respectively. These wavelengths are within the wavelength range spanned by the broad absorption peak of the sample with x = 0.015 observed in visible region. The broad absorption peak may be formed partially with the contribution of photo-excitation of electron from the valence band to these two impurity bands. The rest contributions come from the electron transitions related to the impurity bands induced by O vacancies. Since the intrinsic band gap does not change much by low Mo-doping, it is not surprising that no apparent band edge red-shift is observed in our Ti1−x Mox O2 sample with x = 0.015. As the Mo dopant concentration increases to x = 0.0625, the two impurity bands mix with each other with small density of state. It is found that intrinsic band gap is actually narrowed because the second impurity band overlaps with the bottom of the conduct band with very tiny DOS. Compared with TDOS of x = 0.0156, the two peaks shift to higher energy with first one about 2.77 eV away from the top of the valence band and the second one about 2.9 eV away from the valence band. The photo-excitation of electrons from the valence band to these two Mo-4d mid-band leads to the absorptions of visible light with wavelengths of 428 nm and 447 nm, which are close to 440 nm – the intrinsic absorption wavelength of sample Ti1−x Mox O2 with x = 0.048. The transition of electrons from the valence band to the impurity bands may be the main reason for the experimentally observed red-shift of the absorption edge. As the Mo concentration increases to x = 0.25, impurity bands shift to higher energy and mix with the conduction band. The band gap is thus narrowed to about 2.6 eV. How, since the band gap narrowing is led by the downshift of the bottom of the conduction band, it is not desirable for photocatalysis. Based on the analysis above, it is found that, when x ≥ 0.0625, the impurity band will overlap with the conduction band and lead to a decrease of band gap. However, this band gap narrowing should be avoided because it leads to a downshift of the conduction band edge which will cause the decrease of the photoreduction activity of TiO2 , even though the visible-light absorption can be enhanced. To maintain the photoreduction activity, and in the same time, keep visible light absorption fairly high, x should be smaller than and close to 0.0625. Since the solubility (>12 at.% [22]) of Mo in TiO2 is much higher than this value, the up-limit of Mo concentration in Mo mono-doped TiO2 is x = 0.0625. Compared among the DOS of Ti1−x Mox O2 with different x, it is found that the impurity bands shift to higher energy and become less localized with the increase of x. Since deep impurity levels usually act as recombination center, while shallow impurity lev-
Fig. 8. Comparison of the decomposed PDOSs of Ti1−x Mox O2 with (a) x = 0.0156, (b) x = 0.0625.
els can enhance the photocatalystic activity [13], the up-shift of the Mo impurity band is important. To understand the formation of impurity bands and reason for up-shift of the impurity band with the Mo-concentration, the decomposed density of states of Mo 4d-orbits and O 2p-orbits was extracted and plotted in Fig. 8. For density of states of Mo-4d orbits, only dxy , dxz and dyz states are plotted because dx2 −y2 and dz2 are not found to make significant contributions to the formation of impurity bands. According to the decomposed PDOSs of Mo and O, at low dopant concentration (x = 0.0156), the dxy states of Mo are de-coupled with O 2p states and have lower energy than dxz and dyz states which couple with p of O. The deeper impurity band is composed of non-bonding dxy states, and the shallower impurity band is formed by the weak bonding between p of O and dyz , dxz of Mo. The two impurity bands are localized due the energy difference of them. As the Mo concentration increases, dxy states become to couple with O p , which leads to an energy increase of dxy states; dxy is now degenerated with dyz and dxz . The coupling between Mo t2g with O p also leads to a splitting of dxy , dyz and dxz to form two sub-bands. Therefore, the shift of impurity bands to higher energy is due to the increase of the coupling strength between Mo t2g and O p . 5.1.3. Mo and N co-doped TiO2 The TDOSs of (Mo + N) co-doped TiO2 are shown in Fig. 9. The N impurity introduces isolated impurity band(s) near the VBM, and the Mo dopant induces impurity bands near the CBM. Compared with Mo or N mono-doped TiO2 , the number of impurity states of (Mo + N) co-doped TiO2 is much larger. Therefore, (Mo + N) codoped TiO2 has much higher absorption in visible-light region as observed in experiment. For non-passivated co-doping, the impurity band induced by Mo is isolated from conduction band with tiny gap. As to the passivated co-doping, the impurity band by Mo is apparently separated from the conduction band. Compared to Mo mono-doping case, (Mo + N) co-doping helps to isolate the impurity band by Mo from the conduction band. As we discussed in previous section, for Mo mono-doped TiO2 , the up-limit of Mo concentration is x = 0.0625, which is set by the band overlapping and is much lower than the solubility limit of Mo. By co-doping Mo with N, this up-limit of Mo concentration can thus be raised to a higher value. On the other hand, (Mo + N) co-doping may increase the solubility limits of both Mo and N due to the synergistic effect of N3− /Mo6+ [23,24]. The
H. Liu et al. / Applied Surface Science 257 (2011) 9355–9361
a
x=0.0625
Based on first-principles band structure calculations, we propose that (Mo + N) co-doping, especially the passivated co-doping can increase the concentration up-limit of dopants and create more impurity bands in the band gap, which leads to a greatly increase of visible-light absorption. We suggest that (Mo + N) co-doped TiO2 is very promising as a photocatalyst with very high photocatalystic activity under visible light.
non-passivated
Densityof states (arb. units)
y=0.0625
b
x=0.0625
Acknowledgments passivated
y=0.125
-6
-4
-2
9361
0
2
4
Energy (ev) Fig. 9. TDOSs of (Mo + N) co-codoped TiO2 (Ti1−x Mox O2−y Ny ), a) non-passivated; b) passivated (The red dash lines represent the Fermi levels of the systems).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article)
enhancement of solubility limit is more important for N, because normally, the solubility of N alone in TiO2 is very low. Therefore, (Mo + N) co-doping can greatly enhance the visible-light absorption by increasing the up-limits of both Mo and N concentration. Moreover, (Mo + N) co-doping leads to a charge compensation among donors (Mo6+ ions) and acceptors (N3− ions), which is beneficial for the reduction in the number of carrier recombination centers and the enhancement of the photocatalystic activity. In case of passivated doping, since the charges of donors and acceptors are fully compensated, the impurity bands are passivated and less effective to be recombination centers due to the proposed equilibrium charge mechanism. Therefore, (Mo + N) co-doped TiO2 , especially the passivated co-doped TiO2 is very promising as a photocatalyst with very high photocatalystic activity under visible light. 6. Conclusions In summary, Mo or N mono-doped and (Mo + N) co-doped anatase TiO2 nanoparticles have been fabricated by sol–gel method. The photo absorption spectra show that Mo or N mono-doping can increase visible light absorption, while (Mo + N) co-doping can greatly enhance photo absorption in whole visible region.
The authors would like to thank the financial support from 973 Program (2009CB939705), Chinese National Foundation of Natural Science (no. 51001083 and no. 10974148), and Chinese National Science Fund for Talent Training in Basic Science (no. J0830310), Funding from Wuhan University (5082003) and Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials (Hubei University). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
A. Fujishima, K. Honda, Nature 283 (1972) 37. S. Klosek, D. Raftery, J. Phys. Chem. B 105 (2001) 2815. H. Kato, A. Kudo, J. Phys. Chem. B 106 (2002) 5029. K.T. Ranjit, B. Viswanathan, J. Photochem. Photobiol. A 108 (1997) 79. W. Li, Y. Wang, H. Lin, S. Ismat Shah, C.P. Huang, D.J. Doren, S.A. Rykov, J.G. Chen, M.A. Barteau, Appl. Phys. Lett. 83 (2003) 4143. X. Yang, C. Cao, K. Hohn, L. Erickson, R. Maghirang, D. Hamal, K. Klabunde, J. Catal. 252 (2007) 296. T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates, J. Phys. Chem. B 108 (2004) 6004. Z. Zhang, X. Wang, J. Long, Q. Gu, Z. Ding, X. Fu, J. Catal. 276 (2010) 201. C.D. Valetin, E. Finazzi, G. Pacchioni, A. Selloni, S. Livraghi, M.C. Paganini, E. Giamello, Chem. Phys. 339 (2007) 44. K.S. Han, J.W. Lee, Y.M. Kang, J.Y. Lee, J.K. Kang, Small 10 (2008) 1682. D. Jing, Y. Zhang, L. Guo, Chem. Phys. Lett. 415 (2005) 73. L.G. Devi, B.M. Murthy, Catal. Lett. 125 (2008) 320. K.-S. Ahn, Y. Yan, S. Shet, T. Deutsch, J. Tuner, M. Al-Jassim, Appl. Phys. Lett. 91 (2007) 231909. M.N. Hudam, Y. Yan, S.H. wei, M. Al-Jassim, Phys. Rev. B 78 (2008) 195204. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. V.I. Anisimov, J. Zaanen, O.K. Andersen, Phys. Rev. B 44 (1991) 943. I.V. Solovyev, P.H. Dederichs, Phys. Rev. B 50 (1994) 16861. Y. Gai, J. Li, S.S. Li, J.B. Xia, S.H. Wei, Phys. Rev. Lett. 102 (2009) 036402. A. Kubacka, G. ColÓn, M. Fenández-Garcíam, Catal. Today 143 (2008) 286. T. Ikeda, T. Tomonori, K. Eda, Y. Mizutani, H. Kato, A. Kudo, J. Phys. Chem. C 112 (2008) 1167. G. Liu, C. Sun, L. Cheng, Y. Jin, H. Lu, L. Wang, S.C. Smith, G. Liu, H. Cheng, J. Phys. Chem. C 113 (2009) 12317.