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J. Phys. Chem. C 2009, 113, 9210–9217
Polyol-Mediated Synthesis of Ultrafine TiO2 Nanocrystals and Tailored Physiochemical Properties by Ni Doping Yanwen Wang and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China
Sa Li†,‡ and Puru Jena† Physics Department, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2000, and College of Physical Science and Technology, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: March 15, 2009; ReVised Manuscript ReceiVed: April 18, 2009
A polyol-mediated synthetic method has been developed to prepare ultrafine anatase TiO2 nanocrystals of about 2-5 nm in size, which, when connected, form a network with porous structure. The physiochemical properties of these nanocrystals are tuned by doping with Ni. High-resolution X-ray photoelectron spectroscopy analysis revealed that Ni incorporates into the TiO2 framework to form a Ti-O-Ni chain. Nitrogen adsorption measurements further showed that Ni doping can greatly enhance the surface area of these anatase TiO2 nanocrystals from 143 to 266 m2/g. Using UV-vis diffuse reflectance spectroscopy analysis, we found that the Ni doping reduced the band gap from 3.08 to 2.73 eV and permitted these TiO2 nanocrystals to successfully absorb light in the visible region. First principles band structure calculations were carried out to study the electronic origin of the nickel-induced optical absorption. The photocatalytic activities of the samples were tested through degradation of NO under typical indoor air flow and simulated solar light environment. Nidoped TiO2 was found to exhibit much higher photocatalytic activity than its undoped counterpart and P25. The enhancement of photocatalytic activity of Ni-doped TiO2 is attributed to the larger surface area and the band gap narrowing tuned by nickel doping. 1. Introduction Titanium oxide (TiO2) is one of the most efficient semiconductor photocatalysts. Because of its strong oxidizing power, nontoxicity, high photochemical corrosive resistance, and low cost, it is extensively used not only in environmental applications but also for degradation and complete mineralization of toxic pollutants in water, soil, and air.1-4 However due to the wide band gap (3.2 eV for anatase phase) and low quantum efficiency, arising from the easy recombination between photoelectrons and holes, full potential of TiO2 has not been achieved. Since the wide band gap permits TiO2 to absorb light only in the ultraviolet which forms a small fraction of the solar light (3-5%), there is a need to develop TiO2 based materials that can absorb light over a broad range of wavelengths. Doping metal elements such as Fe, Cr, Co, Mo, W, and V is one of the common ways used to extend light absorption in TiO2 to the visible region and hence reduce the recombination of photoinduced electrons and holes.5-10 It has been hypothesized that metal dopants in TiO2 act as temporary traps of the photogenerated charge carriers and inhibit their recombination during migration from inside of the material to the surface. Anpo and co-workers suggested that the doped metal ions isomorphicaly replace Ti4+ ions in TiO2 and decrease the band gap so that visible light11 can be absorbed. Ni2+ has been found to be an * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn. Phone/Fax: +86-27-6786 7535. † Virginia Commonwealth University. ‡ College of Physical Science and Technology, Central China Normal University.
efficient dopant for improving the photocatalytic activity of certain semiconductor photocatalysts for hydrogen evolution from water.12-14 However, few reports are published on TiO2 photocatalyst doped with Ni for photo-oxidative degradation of pollutants.15,16 Moreover, all these reported Ni-doped TiO2 photocatalysts were prepared with traditional sol-gel methods. The fast rate of hydrolysis in the sol-gel method is unfavorable for the formation of a homogeneous metal-oxygen-metal network in gels, and the subsequent calcinations process may result in uncontrollable crystal growth. The polyol process has recently been found to be quite useful for direct synthesis of nanocrystalline oxide and chalcogenide materials.17,18 The background of this method is the precipitation of a solid while heating sufficient precursors in a multivalent and high-boiling alcohol (e.g., diethylene glycol with boiling point of 246 °C). The alcohol itself acts as a stabilizer, limiting particle growth and prohibiting agglomeration. Due to the high temperatures which can be applied (>150 °C) for these highboiling alcohols, highly crystalline oxides are often formed. Moreover, the synthesis is quite easy to perform. Besides oxides, a variety of materials including sulfides, phosphates, as well as elemental metals have been produced with polyol processes.19 In this paper, we employ a polyol process to synthesize ultrafine TiO2 nanocrystals and use nickel doping to tune their optical and electronic properties. The tuning with nickel doping is systematically studied with experimental and theoretical methods. The Ni-doped TiO2 samples exhibit much higher photocatalytic activity than undoped TiO2 toward the degradation of
10.1021/jp902306h CCC: $40.75 2009 American Chemical Society Published on Web 05/04/2009
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Figure 1. XRD patterns of the as-prepared samples.
NO under typical indoor air flow and simulated solar light environment. 2. Experimental Section 2.1. Synthesis of Pure TiO2 and Ni-Doped TiO2. All of the chemicals were of commercially available analytical grade and used without further purification. Typically, 50 mL of diethylene glycol (DEG, 99.9%, Aldrich) was first added to a 250 mL round bottomed flask with a reflux condenser. Five millimoles of titanium tetraisopropoxide (TTIP, Ti(OC3H7)4, 97%, Aldrich) and appropriate amounts of nickel chloride (NiCl2 · 6H2O, 99.9%, Aldrich) were then added into DEG by vigorous stirring. The resulting mixture was heated to 140 °C in a silicon oil bath. After the solution became clear, 2 mL of deionized water was added and the temperature was increased to 180 °C. After 2 h of heating at 180 °C, the resulting suspension was cooled to room temperature. The precipitation was collected by centrifugation. The product was washed with ethanol to remove DEG thoroughly and finally dried at 80 °C in an oven. During synthesis, the molar ratios of NiCl2 · 6H2O to TTIP were controlled to be 0, 0.1, 0.2, 0.3, and 0.5. The corresponding samples are denoted as TN0, TN0.1, TN0.2, TN0.3, and TN0.5, respectively. 2.2. Characterization. X-ray powder diffraction (XRD) patterns were obtained using a Philips MPD 18801 diffractometer using Cu KR radiation. X-ray photoelectron spectroscopy
(XPS) measurements were performed on a PHI Quantum 2000 XPS System with a monochromatic Al KR source and a charge neutralizer. All the binding energies were calibrated to the C 1s peak at 284.8 eV of the surface adventitious carbon. A transmission electron microscopy (TEM) study was carried out on a Philips CM-120 electron microscopy instrument. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped onto carbon-copper grids. High-resolution transmission electron microscopy (HRTEM) analysis was performed on a JEOL JSM-2010 microscope operating at 200 kV. The nitrogen adsorption and desorption isotherms at 77 K were measured using a Micrometrics ASAP2010 system after samples were vacuum-dried overnight at 180 °C. A PerkinElmer Lambda35 spectrometer UV-visible system was used to obtain the absorption spectra of the samples over a range of 200-600 nm. 2.3. Photocatalytic Activity Test. The photocatalytic activity experiments on the resulting samples for the oxidation of NO in air were performed at ambient temperature in a continuous flow reactor. The volume of the rectangular reactor which was made of stainless steel and covered with Saint-Glass was 27.3 L (13 cm × 70 cm × 30 cm). Three sample dishes containing the photocatalysts powders were placed on a single path in the reactor. A 300 W commercial tungsten halogen lamp (General Electric) was used to simulate solar light. The lamp was vertically placed outside the reactor above the three sample dishes. Four minifans were fixed around the lamp, and adequate distance was maintained between the lamp and the reactor to avoid temperature rise of the flow system. For the photocatalytic activity test, an aqueous suspension of the photocatalyst sample was coated onto three dishes with a diameter of 5.0 cm. The weight of the photocatalyst used for each experiment was kept at 0.3 g. The dishes containing the photocatalyst were pretreated at 70 °C for several hours until a water was completely removed from the suspension and then cooled to room temperature before use. The NO gas was photodegraded at ambient temperature. The NO gas was acquired from a compressed gas cylinder at a concentration of 48 ppm NO (N2 balance, BOC gas) with traceable National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental, Inc., model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender,
TABLE 1: Summary of Physicochemical Properties of the Resulting Samples lattice parameter (Å)
sample
crystalline size (nm)
a)b
c
SBET (m2/g)
av pore size (nm)
pore volume (cm3/g)
band gap (eV)
degradation of NO after 25 min (%)
TN0 TN0.1 TN0.2 TN0.3 TN0.5
5.4 4.5 4.1 4.2 4.3
3.7842 3.7728 3.7746 3.7741 3.7732
9.6516 9.4727 9.4667 9.4871 9.4876
141 253 266 225 189
2.6 2.5 2.5 2.5 2.6
0.08 0.16 0.18 0.14 0.10
3.08 2.97 2.85 2.77 2.73
14 25 37 36 13
TABLE 2: Summary of Binding Energies, Atomic Percentages of Ni and Molar Ratios of Ni to Ti in Various Samples E (eV) sample
Ni (2p3/2)
Sat (2p3/2)
Ni (2p1/2)
Sat (2p1/2)
Ti (2p3/2)
Ti (2p1/2)
O (1s)
final atomic percentages of Ni (%)
ratio of Ni to Ti
TN0 TN0.1 TN0.2 TN0.3 TN0.5
854.4 854.4 854.4 854.3
860.5 860.5 860.5 860.4
872.1 872.0 872.0 871.9
878.9 878.8 878.8 878.8
457.9 458.1 458.2 458.3 458.3
463.7 463.9 464.0 464.1 464.1
529.4 529.4 529.2 529.2 529.1
0.21 0.30 0.34 0.63
0.016 0.021 0.025 0.054
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and the flow rate was controlled at 4 L · min-1 by a mass flow controller. After the adsorption-desorption equilibria among water vapor, gases, and photocatalysts were achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments, model 42c), which monitors NO, NO2, and NOx with a sampling rate of 0.7 L · min-1. The conversion rate (%) of NO was defined according to the equation
NO removal (%) )
[NO]inlet - [NO]outlet × 100% [NO]inlet
where [NO]inlet represents the concentration of NO in the feeding stream and [NO]outlet is the concentration of NO in the outlet stream. The reaction of NO with air was negligible when performing a control experiment with or without light in the absence of photocatalyst. 3. Results and Discussion 3.1. XRD Patterns. Figure 1 shows the XRD patterns of as-prepared samples. All the samples are seen to be composed of anatase phase (JCPDS file No. 21-1272). No crystalline phase of nickel oxides was observed even when the original molar ratio of nickel to titanium was as high as 0.5 (TN0.5). When the Debye-Scherrer formula was applied on the anatase (101) diffraction peaks, all of the samples were found to have very small crystal sizes. The average crystallite sizes of the TN0, TN0.1, TN0.2, TN0.3, and TN0.5 samples were estimated to be 5.4, 4.5, 4.1, 4.2, and 4.3 nm, respectively. This indicated that the addition of Ni2+ could suppress the growth of TiO2 crystal size to some degree. Table 1 summarizes the estimated lattice parameters. The a and c lattice parameters for all the Ni-doped TiO2 samples showed a slight decrease compared to the undoped TiO2, suggesting that nickel doping could effectively alter the crystal structure. 3.2. XPS Spectra. The XPS measurement was carried out to check the chemical state of Ni in Ni-doped TiO2. No peak at binding energy of 200 eV corresponding to Cl- was observed on the XPS survey spectra of Ni-doped TiO2, ruling out the existence of Cl-. The binding energies, atomic percentages of Ni, and atomic ratios of Ni to Ti in the resulting samples are summarized in Table 2. Since all the high-resolution XPS spectra of Ni-doped TiO2 are similar, we chose TN0.2 to analyze in detail (Figure 2). Figure 2a shows the Ni 2p high-resolution XPS spectrum of TN0.2. The spectrum is characterized by a Ni 2p3/2 peak at 854.4 eV and a Ni 2p1/2 peak at 872.0 eV. The difference between these peaks, namely, 17.6 eV, suggests the existence of a Ni-O band.20 No evidence of other oxidation states of nickel was found. The appearance of satellite peaks (denoted as “Sat.”) implies the presence of a high-spin divalent state of Ni2+ in the sample, as expected for Ni ions substituted at the Ti sites.21 The XPS spectrum of Ti 2p in TN0.2 (Figure 2b) has two peaks at 458.2 and 464.0 eV corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. These peaks are slightly shifted toward higher energies compared to the values of the undoped TiO2 (TN0). This implies that the Ni2+ may be incorporated into the TiO2 lattice.22 Due to the difference in electronegativity (Ni ) 1.91 vs Ti ) 1.54) a trace amount of titanium ions may transform to higher valent state by releasing the plethoric electrons in order to achieve charge balance in the TiO2 lattice following the introduction of Ni2+. This would lead to the XPS peaks in Ni-doped TiO2 to shift toward higher energies and broaden. The binding energy of the lattice oxygen in the TN0.2 is located at 529.2 eV, slightly lower than 529.4 eV observed for the pure TiO2 (Figure 2c). This can be attributed to the
Figure 2. High-resolution XPS spectra of the samples TN0 and TN0.2.
variation in electronegativities of the Ti and Ni elements. These observations confirm the formation of Ni-O-Ti complex in the Ni-doped TiO2.23 The shoulder peaks located at 531.4 eV in Ni-doped and undoped TiO2 indicate a great amount of surface hydroxyl groups or chemisorbed water molecules.24 These surface hydroxyl groups can play an important role in the photocatalytic reactions since the photoinduced holes can attack the surface hydroxyl groups and yield surface-bound OH radicals with high oxidation capability.25 3.3. TEM Images. The size and the morphology of TN0 and TN0.2 were characterized by transition electron microscopy.
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Figure 3. TEM (a, b) and HRTEM (c, d) images of TN0 and TN0.2.
Panels a and b of Figure 3 show the TEM images of TN0 and TN0.2. Both the samples are agglomerates of monodispersed nanoparticles of 2-5 nm in size, consistent with those calculated from the Scherer equation. The difference between the particle sizes of undoped and doped TiO2 is quite small, in agreement with XRD results. High-resolution TEM images provide further insight into the structure and crystallinity of the products (panels c and d of Figure 3). The insets in panels c and d of Figure 3 show well-crystallized structures with lattice fringes of about 0.35 nm, corresponding to the (101) planes of anatase TiO2. No crystalline nickel oxide phase was observed during the highresolution TEM analysis. 3.4. Nitrogen Adsorption. Figure 4 presents the nitrogen adsorption-desorption isotherms of the samples. All the isotherms have a well-defined step at intermediate partial pressures (0.3 < P/P0 < 0.8), characteristic of type IV isotherms with an H1 hysteresis loop. This adsorption should arise from the capillary condensation of N2 inside the mesopores. We believe that the mesopores are formed by the agglomeration of nanoparticles, as revealed in TEM images (Figure 3a, b). Table 1 summarizes the surface area (SBET), pore volume (Vp), and pore diameter (dp) of different samples calculated by the BJH method based on the N2 adsorption-desorption isotherms. It is found that all the doped TiO2 samples have larger surface area than pure TiO2. The ratio of Ni to Ti slightly influences the average pore size of the samples, but it significantly changes the surface area and pore volume. When the ratio of Ni to Ti is increased from 0 to 0.2, the BET surface area increases from
Figure 4. Nitrogen adsorption-desorption isotherms of the resulting samples.
143 to 266 m2/g dramatically. TN0.2 possesses the highest surface area and porosity among all the samples studied. This could be attributed to the formation of the Ni-O-Ti band during the synthesis of Ni-doped TiO2, which might inhibit the growth of the TiO2 crystals. However, when the ratio of Ni to Ti is further increased to 0.5, the surface area declines to 181 m2/g. This is possible because of the slightly larger crystal size of TN0.5 than that of TN0.2 according to XRD analysis (Table 1). The nitrogen
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Figure 6. The supercell geometry of NixTi1-xO2 (x ) 0.0625) anatase structure. The red, blue, and black colored atoms correspond to O, Ti, and Ni, respectively. Figure 5. UV-vis diffuse reflectance spectra of the resulting samples.
sorption results reveal that the nickel doping could effectively tune the textural structure of TiO2 in this study. 3.5. Diffuse Reflectance UV-vis Spectroscopy. UV-vis diffuse reflectance spectra of undoped TiO2 and Ni-doped TiO2 are presented in Figure 5. Compared to the spectrum of undoped TiO2, the absorption edge in all the doped TiO2 materials shifted to the lower energy region in the spectra. Interestingly, such a red shift seems to depend linearly on the Ni content. It is wellknown that the origin of the visible spectra in the case of the metal-doped sample is due to the formation of a dopant energy level within the band gap of TiO2. The electronic transitions from the valence band to dopant level or from the dopant level to the conduction band can effectively red shift the band edge absorption threshold.9 Therefore, the observed red shift in band gap suggests that the optical absorption of the TiO2 nanocrystals could be successfully tuned from the UV to visible light region by nickel doping. The absorption band at 750 nm for the Nidoped TiO2 is due to the d-d transition of the Ni2+ ion in the TiO2 matrix.26 The estimated band gap energies are 3.08, 2.97, 2.85, 2.77, and 2.73 for the samples TN0, TN0.1, TN0.2, TN0.3, and TN0.5, respectively. The absorption extension of TiO2 photocatalyst to the visible region provides a possibility for enhancing its photocatalytic behavior for solar energy application. 3.6. Theoretical Calculations. We further utilized theoretical methods to study the electronic structure of NixTi1-xO2. For these calculations, we started with the anatase structure and constructed a 2 × 2 × 1 supercell. The Ti atom at (0.25, 0.375, 0.375) position was replaced by a Ni atom (Figure 6). The calculations were carried out using density functional theory (DFT)27 and generalized gradient approximation (GGA)28 for exchange and correlation potential. The relaxations in the lattice caused by the Ni atom, the total energy, and the electronic structure were calculated using the Vienna ab initio simulation package (VASP)29 and the projector augmented wave (PAW)30 method. The PAW potentials with the valence states 3d and 4s for Ti and Ni and 2s and 2p for O were used. High-precision calculations with a cutoff energy of 400 eV for the plane-wave basis were performed. The geometries of the above three supercells (ionic coordinates and c/a ratio) were optimized without any symmetry constraint. For sampling the irreducible wedge of the Brillouin zone, we used k-point grids of 6 × 6 × 4 for the geometry optimization. In all calculations, selfconsistency was achieved with a tolerance in the total energy of at least 0.5 meV. The calculated lattice constants for the anatase TiO2 primitive unit cell a ) 3.800 Å and c ) 9.471 Å
Figure 7. The calculated DOS of (top) a perfect TiO2 (48 atoms) anatase structure and (bottom) Ni-doped anatase TiO2.
agree very well with the corresponding experimental values of 3.785 and 9.514 Å, respectively. The calculated a and c lattice parameters for Ni-doped supercell are 7.585 and 9.454 Å, respectively. We see that the lattice parameters decrease in both directions. The Ni-O bond distance of 1.87 Å is shorter than the original Ti-O bond distance of 1.94 Å, thus causing the volume to decrease by 0.5%. The density of states (DOS) is plotted in Figure 7. In pristine TiO2, the lower valence band (LVB) is dominated by O 2s states. The upper valence band (UVB) is composed of O 2p states and Ti 3d states. The UVB has a calculated bandwidth of 4.8 eV and is separated from the valence band (VB) by a 1.9 eV band gap. Upon addition of Ni, Ni d states appear in the middle of the band gap. Ni d states also distribute at the UVB band and hybridize with O p states. This hybridization gives rise to a nonmagnetic ground state. These results are in agreement with earlier DFT calculations.31,32 The wider UVB in the Ni-doped system compared to that in intrinsic TiO2 suggests stronger hybridization between metal and oxygen atoms with the addition of Ni. The calculated band structures in the full irreducible Brillouin zone for TiO2 and Ni-doped TiO2 are presented in Figure 8. The doping effects are modeled by replacing titanium atom with one Ni atom. We can see both TiO2 and Ni-doped TiO2 are
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Figure 8. The calculated band structures of (a) a perfect TiO2 (48 atoms) anatase structure and (b) Ni-doped anatase TiO2.
semiconductors with direct band gap at the Γ point. The GGA calculated direct band gap of 1.9 eV for TiO2 agrees with the band gap of 2.2 eV obtained by Wang et al.33 using the FPLAPW method, but is much underestimated compared with experimental band gap of 3.1 eV.34 When Ni is substituted, the band gap is reduced to 0.98 eV. We note that DFT systematically yields smaller band gaps compared to experiment. However, the reduction in the band gap due to Ni substitution is not expected to be sensitive to the use of DFT. If we scale the computed band gap in pure TiO2 with experiment, the corresponding band gap of 2.18 (3.1 - 1.9 + 0.98) eV in Nidoped TiO2 will lie in the visible range. This conclusion is consistent with the experimental results. The addition of Ni introduces acceptor states above the valence band and facilitates hole and electron formation. Electrons from the top of the valence band can be excited to these lower energy empty states, thus leaving holes in the valence band. Holes produced this way can react with H2O or OH- bound on the surface to produce a strong oxidant (e.g., hydroxyl radicals). Such a reaction can be used for efficient photocatalytic degradation of pollutants. On the other hand, the conduction band electrons may be absorbed by the dissolved oxygen species to generate superoxide anion radicals. They are highly reactive for oxidizing pollutants.35 The band gaps of all the Ni-doped TiO2 synthesized with the polyol method in this work are larger than the calculated value. The reasons may be as follows: On one hand, because the discontinuity in the exchange-correlation potential is not taken into account within the framework of DFT, theoretical values of the energy gap between unoccupied and occupied orbitals in semiconductors and insulators are underestimated compared with experimental values.36 3.7. Photocatalytic Activities. Nitrogen oxide is one of the most common gaseous pollutants found in the indoor environment with a concentration level in the range of 70-500 partsper-billion (ppb). The photocatalysis technique provides a promising solution for the removal of indoor air pollutants at low concentration. Figure 9 shows how the NO concentration changes as a function of irradiation time in the presence of various samples (TN0, TN0.1, TN0.2, TN0.3, and TN0.5 and P25) under simulated solar light irradiation. Prior to the simulated solar light irradiation, the adsorption/desorption equilibrium between the gas and photocatalysts was reached. When the lamp was turned on, the photocatalytic reaction of NO on the photocatalysts was initiated. It was found that about 4% of NO
Figure 9. Photocatalytic activity of the samples for the oxidation of NO. Residence time 3.72 min, humidity levels 2200 ppmv, 400 ppb NO.
was photolyzed in the absence of photocatalysts under simulated solar light irradiation. After 30 min of degradation under simulated solar light irradiation, 4, 14, 25, 37, 36, and 13% of NO were photocatalytically oxidized on the samples of P25, TN0, TN0.1, TN0.2, TN0.3, and TN0.5, respectively. Obviously, all these polyol synthesized samples exhibited much higher photocatalytic activities than P25. For the Ni-doped TiO2 photocatalysts, the photocatalytic activities increase with the Nidoping concentration when the original molar ratio of Ni and Ti is below 0.2. TN0.2 was found to be the best photocatalyst for the removal of NO. Ni doping became detrimental when the original nickel doping concentration was higher than 0.2. When the original molar ratio of Ni to Ti reached 0.5, the photocatalytic activity of Ni-doped TiO2 (TN0.5) was even slightly lower than that of undoped TiO2. Principally, when a semiconductor absorbs a photon with energy greater than or equal to the band gap energy, an electron would be promoted from the valence band to the conduction band, leaving a hole in the valence band. If this charge separation is effective, these electrons and holes can be used for efficient photocatalytic degradation of pollutants. Holes can react with the surface-bound H2O or OH- to produce a powerful oxidant such as hydroxyl radicals and therefore directly oxidize the pollutants.5,37 In our experiments, the oxidation reaction of NO
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Figure 10. Illustration of the possible formation mechanism of TiO2 and Ni-doped TiO2.
was believed to be initiated by •OH radicals. In the presence of O2, the OH radicals are formed as follows38
TiO2 + hν T h+VB + e-CB -
-
e -
CB
+ O2 f O2 +
-
O2 + 2H + e
CB
f H2O2
(1) (2) (3)
H2O2 + O2- f •OH + OH- + O2
(4)
h+ + H2O f •OH + H+
(5)
According to the above mechanism, increasing the number of active surface sites and surface charge carrier transfer rate in the photocatalysis system and inhibiting the undesirable electron-hole pair recombination enhances the photocatalytic activity by producing more hydroxyl radical groups, •OH, which is beneficial for the oxidation of NO. Particle size is an important parameter for catalysis in general since it directly affects the specific surface area of a catalyst. With a smaller particle size, the number of active surface sites increases, as so does the surface charge carrier transfer rate in photocatalysis.5 In our experiments, all the undoped and Ni-doped TiO2 samples synthesized by the polyol process have very small particle sizes and high BET surface areas, which increase the number of active surface sites, the surface charge carrier transfer rate, and the adsorption of reactant molecules. Meanwhile, a great amount of surface hydroxyl groups existing on the surface of TiO2 (as
shown in XPS results) can play an important role in the photocatalytic reactions since the photoinduced holes can attack the surface hydroxyl groups and yield surface-bound OH radicals with high oxidation capability.25 Therefore, it is reasonable to find that the undoped anatase TiO2 showed enhanced photocatalytic activity than P25. The reasons for the higher activity of Ni-doped TiO2 than undoped TiO2 can be attributed to their larger surface area and extended light absorbance in the visible region. However, when the molar ratio of Ni to Ti was further increased to 0.5, excessive Ni incorporated in TiO2 may act as recombination center for photogenerated electron-hole pairs, resulting in a decrease of photocatalytic efficiency.14 Therefore, the nickel doping could effectively tune the physiochemical properties of TiO2, resulting in a different photocatalytic performance. 3.8. Possible Formation Processes of Ni-Doped TiO2 Nanocrystals. Polyol-mediated preparation of nanoscale oxides was carried out by dissolving a suitable metal precursor (e.g., acetate, alcoholate, halogenide) in DEG.39 In this work, when Ti(OC3H7)4 was added to DEG with subsequent heating, the titanium ions were chelated with DEG to form a complex compound. When no nucleophile reagent was added, the Ti chelated complex remained stable for at least 6 h at 180 °C with no precipitate formed. However, when an appropriate amount of deionized water was added, the OC3H7- ligand substituted with the OH- and the metal atoms remained chelated with DEG. When the mixture containing deionized water was
Synthesis of Ultrafine TiO2 Nanocrystals rapidly heated to 180 °C, precipitation was observed after a certain duration of heating, suggesting the hydrolysis and condensation of Ti-DEG complexes. TiO2 nanocrystals would finally form (route A in Figure 10). Therefore, the advantage of this method over a simple ion metathesis reaction is that the rate of nucleophilic substitution reaction with neutral molecules of water is much lower. The generation of crystal nuclei and the subsequent crystal growth of TiO2 could be easily controlled by adjusting water amount and heating temperature. Meanwhile, the surface chelating of DEG would also slow the crystal growth, providing the possibility for the manipulation of crystal growth. During the synthesis of Ni-doped TiO2, the Ni can also be chelated with DEG forming a complex compound like Ti-DEG (route B in Figure 10). Some of the Ni-DEG complexes could react with Ti-DEG complexes to form Ti-O-Ni bonds, resulting the incorporation of Ni in TiO2 crystals (route C in Figure 10). Since the Ni-DEG complexes were well-dispersed in Ti-DEG complexes solution, it is possible to make a homogeneous interaction between the two metal-DEC complexes, leading to a uniformly structured oxide with welldispersed dopants. When only Ni-DEG complexes were heated using the same procedure, no precipitation was observed, even the heating time was longer than 6 h. Therefore, we believe NiO could not be formed during polyol synthesis of Ni-doped TiO2. Only a small portion of Ni2+ was incorporated into the TiO2 lattice, leaving most of Ni2+ in the form of chelated complexes dissolved in the solution. This is why the final atomic ratios of Ni to Ti obtained with XPS analysis were much lower than the original molar ratios of NiCl2 · 6H2O to TTIP added into the precursor solutions. These polyol-synthesized TiO2 and Ni-doped TiO2 nanoparticles with high photocatalytic activities could be well dispersed in DEG to form a stable suspension, ready for coating on different substrates including walls, streets, trees, and so on.40 This point is very meaningful for indoor and outdoor air purification. 4. Conclusions In summary, ultrafine TiO2 nanocrystalline photocatalysts of 2-5 nm in size were synthesized by using a polyol-mediated synthetic method. Their physiochemical properties were then tuned by nickel doping. The Ni ions were incorporated into the framework of TiO2 to form a Ti-O-Ni chain. The results revealed that the nickel doping can effectively modify the crystal structure, enhance the surface area, and tune the optical absorption of TiO2 from UV to visible light region. Theoretical calculations were performed to reveal the electronic origin of optical absorption in the visible by nickel doping. The synthesized Ni-doped TiO2 exhibited much higher photocatalytic activities than the undoped counterpart. The promoting effect of Ni-doping on the photocatalytic activity of TiO2 can be attributed to the larger surface area and extension of light absorption to the visible region. This leads to great enhancement in quantum efficiency and higher photocatalytic activity. Acknowledgment. This work was supported by National Basic Research Program of China (973 Program) (Grant 2007CB613301), National Science Foundation of China (Grant 20777026), Program for New Century Excellent Talents in University (Grant NCET-07-0352), and the Scientific Research
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