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General Synthesis and Optical Properties of Monodisperse Multifunctional Metal-Ion-Doped TiO2 Hollow Particles Bo Peng,†,‡ Xianwei Meng,† Fangqiong Tang,*,† Xiangling Ren,† Dong Chen,† and Jun Ren† Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing, 100049, China ReceiVed: July 22, 2009; ReVised Manuscript ReceiVed: October 9, 2009
Various, monodisperse, multifunctional metal-ion-doped TiO2 hollow particles are prepared by a general metalion-doped method. They show brilliant colors in the visible region and low reflectance in the ultraviolet range. For example, the reflectances of green Co-doped TiO2, red Fe-doped TiO2, green Co/Zn-doped TiO2, and sky blue Co/Al-doped TiO2 particles are 5.9, 4.8, 5.9, and 4.8%, respectively, and the infrared emissivities in the region of 8-14 µm are very high, 97.5, 93.7, 95.2, and 96.9%, respectively. These results show that the monodisperse hollow particles could be potential candidates as good pigments, ultraviolet prevention materials, and thermal conservation materials. In addition, Al-doped TiO2 and Al/Fe-doped TiO2 particles are also prepared by this strategy. Introduction Hollow nanostructures have attracted tremendous interest as a special class of materials compared to their solid counterparts, owing to their higher specific surface area, lower density, better permeation, and widespread potential applications in chemical reactors, catalysis, protection of biologically active agents, drug delivery, waste removal, and lightweight materials.1-7 A variety of chemical and physicochemical methods, such as spraydrying,8 lost-wax method,9 emulsion/interfacial polymerization strategies,10,11 self-assembly,12,13 sacrificial-core techniques,14-17 self-transformation strategy,18-21 and Ostwald ripening,22 have been employed for the manufacture of oxide hollow spheres. Recently, polymer beads have been demonstrated to be powerful templates for a variety of oxide hollow spheres. They are incorporated into core-shell composites either by controllable surface precipitation of inorganic precursors or by direct surface reactions utilizing specific function groups on the cores, and then the cores are removed by thermal or chemical methods to get hollow structure. Xia et al. prepared the TiO2 and SnO2 hollow spheres by templating the sol-gel precursor solutions on arrays of crystalline polystyrene spheres confined between two glass substrates.23,24 Caruso et al. have reported the layerby-layer self-assembly technique, whose basis is the electrostatic association between alternately deposited, oppositely charged species (e.g., small oxide nanoparticles or polyelectrolytes) and subsequently removal of the templates by calcinations or dissolution.3,25,26 Arche et al. demonstrated that a templating scheme based on monodisperse nonspherical hematite colloids provided a general route for preparation of nonspherical anatase TiO2 hollow or magnetic multifunctional core/shell particles.27 To our knowledge, however, combining the hollow property and other different properties is still a great challenge. There have been few works presenting general preparation of monodisperse multifunctional hollow particles. * Corresponding author: E-mail:
[email protected]. Phone: +8610-82543521. Fax: +86-10-62554670. † Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
Herein, we develop a general method to achieve tuning of the bandgap of the titania semiconductor by metal-ion doping to synthesize multifunctional hollow particles, including Codoped TiO2, Fe-doped TiO2, Co/Al-doped TiO2, and Co/Zndoped TiO2. They display brilliant colors and low reflectance in the ultraviolet range, indicating that they could be used as good pigments and excellent anti-UV materials. In addition, they could be potential candidates as energy efficient materials (e.g., thermal materials) due to their high infrared emissivity in the range 8-14 µm. The procedure offers some distinct advantages: (1) Appropriate selection of metal ions makes the method a general one for the fabrication of the multifunctional hollow composite. (2) No repeated absorption cycles for the metal ions are required, which reduces the number of processing steps and saves time. (3) Many kinds of metal ions could be absorbed on the surface of the templates simultaneously, which gives us opportunities to prepare hollow spheres with much more functions. (4) The multifunctional particles are lightweight due to their hollow structure and have a narrow size distribution. This would open up possibilities to extend the physical and chemical properties and the applications of oxides and probably bring new developments for hollow nanomaterials. Experimental Details 2. Materials and Methods. 2.1. Chemicals. Tetra-n-butyl titanate (TBT) and acetonitrile were purchased from Sigma and used without further purification. Potassium persulfate (KPS), FeCl3 · 6H2O, CoCl2 · 6H2O, Zn(Ac)2 · 2H2O, Al(NO3)3 · 9H2O, polyethylene glycol (PEG 10000), styrene, ethanol, methylene blue (MB) trihydrate, and ammonia were all supplied by the Beijing Chemical Reagent Company. Styrene was purified by distillation under reduced pressure. Ethanol was dehydrated by molecule sieves. 2.2. Synthesis of Anionic PS Spheres. Anionic PS spheres used as core materials were prepared by emulsifier-free, emulsion polymerization using KPS as the anionic initiator. Typically, under gentle stirring, 9 mL of styrene was added at room temperature to 100 mL of deionized water which was purged with nitrogen before the reaction. After 0.3 g of KPS
10.1021/jp906937e CCC: $40.75 2009 American Chemical Society Published on Web 11/02/2009
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TABLE 1: Experimental Conditions for the Formation of Monodisperse Multifunctional Hollow Metal-Ion-Doped TiO2 Particles sample Co/Al-doped TiO2 Co/Zn-doped TiO2 Co-doped TiO2 Fe-doped TiO2 Al-doped TiO2 Al/Fe-doped TiO2
metal salt 0.6 0.6 0.8 0.4 1.2 0.2 1.2 0.8 0.4
mmol mmol mmol mmol mmol mmol mmol mmol mmol
CoCl2 · 6H2O Al(NO3)3 · 9H2O CoCl2 · 6H2O Zn(Ac)2 · 2H2O CoCl2 · 6H2O FeCl3 · 6H2O Al(NO3)3 · 9H2O FeCl3 · 6H2O Al(NO3)3 · 9H2O
annealing temperature (°C) 900 600 600 600 600 600
was added, the temperature was increased gradually to 70 °C, and the mixture was stirred for 24 h at 70 °C. The resulting PS spheres were recovered by centrifugation and washed three times with ethanol. The sample was redispersed in ethanol for subsequent experiments. 2.3. Fabrication of the PS/TiO2 Hybrid Particles. The coating reaction was processed in the mixed solvent of ethanol and acetonitrile by hydrolyzing TBT in the presence of ammonia, as described in our previous report:28,29 0.2 mg/mL of PS spheres were dispersed in the mixed solvent of ethanol/ acetonitrile (3/1 v/v) and then mixed with 0.3 mL of ammonia. Finally, the mixed solvent of ethanol/acetonitrile (3/1 v/v) containing 0.65 mL of TBT was added to the above suspension under stirring. After reacting for 1 h, the obtained particles were cleaned by three cycles of centrifugation and then dispersed in ethanol. 2.4. Preparation of Multifunctional Hollow Particles. The monodisperse multifunctional metal-ion-doped TiO2 hollow particles were prepared by using the Pechini sol-gel process. The requisite metal salts were FeCl3 · 6H2O, CoCl2 · 6H2O, Zn(Ac)2 · 2H2O, and Al(NO3)3 · 9H2O. The experimental conditions are shown in Table 1. In a typical procedure, 1.2 mmol of CoCl2 · 6H2O was first dissolved in a water-ethanol (1/7 v/v) solution containing citric acid which was two times as much as the metal ions in amount of substance. Then, a certain amount of the PEG (10000) was added. The solution was stirred for 2 h and then the above PS/TiO2 hybrid spheres (0.04 g) were added. After stirring for another 4 h, the particles were separated by centrifugation. The samples were dried at 60 °C for 2 h and then annealed at 500 °C for 4 h with a heating rate of 5 °C/ min. Finally, the preheated samples were annealed at 600 °C for 2 h with a heating rate of 2 °C/min. For Co/Al-doped TiO2 hollow particles, the samples were annealed at 900 °C for 2 h with a heating rate of 1 °C/min after annealing at 600 °C. 2.5. Catalytic Activity Measurements. The photocatalytic activity of the samples was evaluated by photodegradation of MB in an aqueous solution at room temperature under ultraviolet irradiation. In the process, the catalyst (45 mg) was suspended in a fresh dye aqueous solution (for the UV-induced reaction: C0 ) 1.0 × 10-5 M). The suspension was stirred in the dark for 20 min to allow an adsorption-desorption equilibrium of MB dye. Then, oxygen was bubbled into the reactor. After 20 min, the solution was illuminated while stirring. At a certain interval, a certain amount of sample (3 mL for the UV-induced reaction) was drawn from the system, centrifuged, and then the absorption spectrum at 664 nm of the dye was monitored. The UV source was a 100 W mercury lamp (λ > 330 nm). The photoreactor was placed on a magnetic stirrer to ensure homogeneous mixing during irradiation. 2.6. Characterization. A transmission electron microscope (TEM, JEOL-200CX) and a scanning electron microscope
(SEM, Hitachi 4300) were used to observe the morphology of the particles. The high resolution transmission electron microscopy (HRTEM) images were performed on a JEM 2100F Electron Microscope coupled with an energy-dispersive X-ray (EDX) spectrometer with an accelerating voltage of 200 kV. The X-ray powder diffraction (XRD) measurements were taken with a Japan Regaku D/max γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54 Å) irradiated with a scanning rate of 0.02 deg/s. Ultraviolet and visible absorption (UV-vis) and diffuse reflectance spectra were recorded at room temperature with a Cary 5000 (Varian) spectrophotometer equipped with an integrated sphere. An IR-2 infrared radiometer (Shanghai Institute of Technology Physics, Chinese Academy of Science) was employed to measure the infrared emissivity. Results and Discussion In general, the selective absorption of light is the origin of the brilliant and pure colors of the solids, which is related to electronic interband transition. Therefore, to develop new inorganic pigments, a concept that different colors could be obtained by rationally designing the width of the bandgap has been proposed.30 Therefore, we focus on the option for narrowing the bandgap of titania by employing metal elements to tune its electronic structure. Typically, the sol-gel method results in nanoscale hollow particles with bright color, which are suspended in a high boiling point alcohol (e.g., ethylene glycol).31 Such suspensions containing Co-doped TiO2 (green), Fe-doped TiO2 (red), Co/Al-doped TiO2 (sky blue), and Co/ Zn-doped TiO2 (green) monodisperse hollow particles are pictured in Figure 1, and they show brilliant color, which indicates that the hollow particles can be used as pigments. The TEM and SEM images of Co-doped TiO2, Fe-doped TiO2, Co/Zn-doped TiO2, and Co/Al-doped TiO2 nanocomposites are shown in Figure 2 and prove that all samples exhibit very similar hollow structures with uniform size and shape. The size of all of the samples is about 300 nm, and the uniform intact shells are about 35 nm thick. In the reaction system, the chelate complexes of metal ions react with PEG to form polyesters with suitable viscosity, which coat on the surfaces of the PS/TiO2 particles. This implies that the templates determine the size and shape of the final products. However, we can find that the size and shape of holes in the shell derived from calcination under the same conditions are different, which may be attributed to the effect of the metal ions on the transformation of titania nanocrystals. The shells of Co-doped TiO2 and Co/Zn-doped TiO2 all consist of large nonuniform particles, but the shells of Co/Zn-doped TiO2 are more compact than that of Co-doped TiO2. The holes of Fe-doped TiO2 are uniform and compact, which is attributed to uniform small particles constituting the shells. For the Co/Al-doped TiO2 hollow particles, the particles constituting the shells are very large and nonuniform, which results in the big holes in the shells. The insets in Figure 2A, D, F, and G show the detailed structures, respectively. Figure 3 shows the UV-vis absorption spectra of metal-iondoped, pure TiO2 hollow particles. Interestingly, in contrast to the pure TiO2 obtained at a temperature of 600 °C (Figure 3a), the spectra of all metal-ion-doped TiO2 obtained at 600 °C (Figure 3b, d, and e) show red shift and the absorption edge shifts to a longer wavelength. Additional high visible absorption bands appear. The spectrum of the Co/Al-doped TiO2 sample (900 °C) (Figure 3c) also shows a red shift in contrast to the pure TiO2 prepared under the same conditions (Figure 3f). These
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Figure 1. Suspension of monodisperse hollow particles in ethylene glycol: (A) hollow Co-doped TiO2 particles (green); (B) hollow Fe-doped TiO2 particles (red); (C) hollow Co/Al-doped TiO2 particles (sky blue); (D) hollow Co/Zn-doped TiO2 particles (green).
to the (101) plane of the anatase phase (JCPDS 84-1285), and the peaks corresponding to 27.3, 35.9, 41.1, and 54.1° are in good agreement with the (110), (101), (111), and (211) planes of the rutile (JCPDS 77-0443). The rutile TiO2 particles are the predominant species in all samples. The fraction of the rutile phase is determined from the relative XRD intensities corresponding to anatase (101) and rutile (110) reflections. Then, the mass fraction of rutile, XR, can be obtained by the following equation.37 Figure 2. TEM and SEM images of the samples: hollow Co-doped TiO2 particles (A and B); hollow Fe-doped TiO2 particles (C and D); hollow Co/Zn-doped TiO2 particles (E and F); hollow Co/Al-doped TiO2 particles (G and H).
Figure 3. UV-vis absorption spectra of samples: (a) pure TiO2 (600 °C); (b) Co/Zn-doped TiO2; (c) Co/Al-doped TiO2; (d) Co-doped TiO2; (e) Fe-doped TiO2; (f) pure TiO2 (900 °C).
reuslts indicate that the charge-transfer transition between the metal ions and TiO2 conduction or valence band happened and metal ions have been implanted in the TiO2.32-36 To verify the compositions of the obtained multifunctional hollow nanoparticles, the energy-dispersive X-ray (EDX) spectra of Co/Al-doped TiO2 hollow particles, Co/Zn-doped TiO2 hollow particles, Co-doped TiO2 hollow particles, and Fe-doped TiO2 hollow particles are recorded. O, Ti, Co, and Al peaks for doped TiO2 (Figure 4A); O, Ti, Zn, and Co peaks for Co/Zndoped TiO2 (Figure 4B); O, Ti, and Co peaks for Co-doped TiO2 (Figure 4C); and O, Ti, and Fe peaks for Fe-doped TiO2 (Figure 4D), respectively, are observed (silicon signal from the silicon substrate), indicating that the hollow metal-ion-doped TiO2 nanocomposites are synthesized successfully. The molar rate of metal ions (R ) Mn+:(Mn+ + Ti4+)) in a single metalion-doped TiO2 hollow particle is shown in Table 2, which indicates that the amount of metal-ion dopants is very large. The resulting powders are highly crystalline, which mainly consist of anatase and rutile phases. Figure 5 shows the typical XRD patterns of Co/Al-doped TiO2 (a), Co/Zn-doped TiO2 (b), Co-doped TiO2 (c), and Fe-doped TiO2 (d) samples together. The characteristic peaks corresponding to 25.3° can be assigned
XR ) [1 + 0.79(IA /IR)]-1 where IA and IR are integrated intensities of anatase (101) and rutile (110), respectively. The fraction of rutile in Co/Al-doped TiO2 (900 °C), Co/Zn-doped TiO2 (600 °C), Co-doped TiO2 (600 °C), and Fe-doped TiO2 (600 °C) hollow particles is 86.84, 52, 51.8, and 100%, respectively (Table 2). However, all of the detectable peaks of TiO2 hollow particles without any dopant obtained at a temperature of 600 °C can be indexed as the TiO2 with anatase structure and the XRD pattern of pure TiO2 hollow particles obtained at a temperature of 900 °C consists of rutile (see Figure SI-1 in the Supporting Information). This indicates that the substitution of metal ions for Ti has an effect on the A-R phase transition.38,39 The average crystal size of the sample has been estimated by Scherrer’s formula. The primary crystallite sizes calculated from the (110) peak of the XRD pattern are about 7.9, 9.7, 4.3, and 28.1 nm corresponding to Co/Zndoped TiO2 hollow particles, Co-doped TiO2 hollow particles, Fe-doped TiO2 hollow particles, and Co/Al-doped TiO2 hollow particles. However, the crystal size of pure TiO2 hollow particles with anatase structure is around 27 nm, much larger than that of metal-ion-doped TiO2 hollow particles obtained at a temperature of 600 °C. The crystal size of rutile pure TiO2 hollow spheres is estimated to be about 73.9 nm, which is also much larger than that of Co/Al-doped TiO2 hollow particles. These results indicate that metal ions greatly influence TiO2 crystallization. It is noteworthy that the peaks corresponding to (311) and (220) crystal planes of CoAl2O4 (JCPDS 82-2246) in Co/ Al-doped TiO2 particles, (104) planes of CoTiO3 (JCPDS 771373) in Co/Zn-doped TiO2 particles, and Co-doped TiO2 particles and (104) planes of Fe2O3 (JCPDS 86-0550) in Fedoped TiO2 particles signed by arrows can be observed obviously in the XRD spectra, due to the high concentration of metal-ion dopants.40,41 Figure 6 shows the HRTEM images of the samples. In all samples, we can find the rutile structure. The interfering distance is measured to be 0.32 nm, which is assigned to the (110) plane of the rutile TiO2 (Figure 6a, c, d, and e). For Co-doped TiO2 and Co/Zn-doped TiO2 hollow particles, the lattice plane of (003) of CoTiO3 is found, whose interfering distance is 0.46 nm (Figure 6b and c). Interestingly, The d-spacing of the planes in the lattice image seen in the HRTEM photograph of Co/Zndoped TiO2 hollow particles (see Figure SI-2 in the Supporting
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Figure 4. EDX spectra of the samples: (A) hollow Co/Al-doped TiO2 particles; (B) hollow Co/Zn-doped TiO2 particles; (C) hollow Co-doped TiO2 particles; (D) monodisperse hollow Fe-doped TiO2 particles.
TABLE 2: Molar Rate of Metal Ions, Infrared Emissivity of the Obtained Multifunctional Monodisperse Hollow Particles in the Region of 8-14 µm, and Mass Fraction of Rutile of the SamplessHollow Co-Doped TiO2 Particles, Hollow Co/Zn-Doped TiO2 Particles, Hollow Co/Al-Doped TiO2 Particles, and Hollow Fe-Doped TiO2 Particles sample molar rate of metal ions XR (%) emissivity (%)
Co/Al-doped TiO2
Co/Zn-doped TiO2
Co-doped TiO2
Fe-doped TiO2
Co (4.2%) Al (17.4%) 86.84 96.9
Co (26%) Zn (1.5%) 52 95.2
Co (17.5%)
Fe (36.7%)
51.8 97.5
100 93.7
Figure 5. XRD profile of (a) hollow Co/Al-doped TiO2 particles, (b) hollow Co/Zn-doped TiO2 particles, (c) hollow Co-doped TiO2 particles, and (d) hollow Fe-doped TiO2 particles.
Information) is 0.28 nm and corresponds to the (220) plane of ZnCo2O4.42 The interfering distance is measured to be 0.46 and 0.26 nm in Figure 6d and f, respectively, corresponding to the (111) plane of CoAl2O4 and (104) plane of Fe2O3. These results also indicate that CoTiO3, CoAl2O4, and Fe2O3 are formed in the obtained metal-ion-doped TiO2 hollow particles, which is consistent with the conclusion by XRD. The CoTiO3, ZnCo2O4, CoAl2O4, and Fe2O3 contribute to the brilliant colors.31,43,44 As shown in Figure 7, the reflection spectra of the samples were measured. It can be seen that the peaks for Co/Al-doped TiO2, Co-doped TiO2, Co/Zn-doped TiO2, and Fe-doped TiO2 monodisperse hollow particles in the visible region are 490, 535, 540, and 715 nm, respectively. The reflection edges of most
Figure 6. HRTEM images of samples: Co-doped TiO2 hollow particles (A and B); Co/Zn-doped TiO2 hollow particles (C); Co/Al-doped TiO2 hollow particles (D); Fe-doped TiO2 hollow particles (E and F).
samples are somewhat steep, indicating brilliant colors of the hollow particles.45 Those results are consistent with the photos in Figure 1. Therefore, they could be used as the candidates for inorganic pigments. In nature, ultraviolet A (UV-A, 320-400 nm) and B (UVB, 290-320 nm) with great energy, especially UV-B, can result in large destructiveness, such as skin cancer, cataracts, and
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Figure 7. Reflection spectra of the samples: (a) hollow Co/Al-doped TiO2 particles; (b) hollow Co-doped TiO2 particles; (c) hollow Fe-doped TiO2 particles; (d) hollow Co/Zn-doped TiO2 particles.
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Figure 9. Photocatalytic degradation of MB on metal-ion-doped TiO2 hollow particles and P25.
Figure 8. Schematic photoexcitation (A) and recombination (B) of the electron and hole in metal-ion-doped TiO2 nanocrystals.
deterioration of materials.46 Thereby, it is of great significance to prepare materials with good UV prevention. What is important is that the multifunctional hollow particles have the property of low reflectance in the ultraviolet range shown in Figure 7. In the powder reflection spectra, low reflectance values indicate high absorption in the corresponding wavelength region. In the reflectance spectra, there is very little difference between Co-doped TiO2 and Co/Al-doped TiO2 hollow particles in the ultraviolet B region, showing the same range from 5.9 to 8.0%, but a clear difference is observed in the ultraviolet A region, especially in the region 380-400 nm. For the Co/Zn-doped TiO2 and Fe-doped TiO2 hollow particles, the reflectance in the range from 290 to 310 nm is the same, about 4.9%, but there is a very obvious difference in the region 310-400 nm. The reflectance of Co/Zn-doped TiO2 hollow particles increases sharply, but the increase of the reflectance for Fe-doped TiO2 hollow particles is much slower. Consequently, the absorption of long wavelength UV light for Fe-doped TiO2 particles is much more than that of Co/Zn-doped TiO2 particles. However, all in all, the properties of UV prevention of hollow doped TiO2 particles are excellent and we can use them as UV-blocking materials. It is well-known that the attenuation of the UV radiation in these materials is accomplished by bandgap absorption of light. An electron from a valence band is excited to the conduction band initiated by light absorption with energy equal to or greater than the band gap of the semiconductor (Figure 8A). Redox reaction occurs between these photoinduced electrons (or holes) and absorbed species. However, a lot of metal-ion dopants in the TiO2 nanocrystals work as recombination centers of the electrons and holes (Figure 8B), which results in the radiationless transfer of the absorbed photon energy from UV light and the decrease of activity of both photoformed electrons and holes.34,36,47 Figure 9 shows the photocatalytic activity of as-prepared metalion-doped TiO2 hollow particles under UV light, where C and C0 stand for the remnant and initial concentration of MB, respectively. The degradation rate of MB on different TiO2
Figure 10. TEM and SEM images of (A, B) hollow Al-doped TiO2 particles and (C, D) hollow Al/Fe-doped TiO2 particles.
catalyst samples under UV irradiation follows the order: e > b > d > c > a. However, the C/C0 of P25, Fe-doped TiO2, Co/ Al-doped TiO2, Co-doped TiO2, and Co/Zn-doped TiO2 hollow particles after 2 h under UV light is 0.02, 0.76, 0.83, 0.90, and 0.96, respectively, which indicate that the photocatalytic activity of the obtained metal-ion-doped TiO2 hollow particles is much lower than commercial P25 and it is very poor. In other words, the metal-ion-doped TiO2 hollow particles show excellent UV prevention and poor photoactivity. It is very important for the application of the multifunctional particles in plastic and filters. Therefore, we suggest that the hollow particles will be suitable building blocks for the application in ultraviolet prevention, especially for organic materials. The infrared emissivities of the preformed hollow particles in the wavelength range 8-14 µm are further measured by IR-2 infrared radiometer at room temperature and recorded. The results are shown in Table 2. Surprisingly, they are very high. It is up to approximately 97.5% for Co-doped TiO2 particles. The smallest value for all samples is about 93.7%, which is also very large. Early researchers pointed out that the peak wavelength of the human body is around 9-10 µm. Moderate wavelength (6-14 µm) infrared rays are absorbed completely in the epidermal layers, resulting in the thermal sensation of pleasant warmth.48 Thereby, they can be used as building blocks for thermal materials, such as IR enhanced fabrics and indoor thermal coatings.49,50 As demonstrated above, our method uses metal salts as the starting materials; therefore, it is advantageous for the synthesis of metal-doped hollow particles. We can also prepare other metal-doped TiO2 hollow nanocomposites. Figure 10 shows the TEM and SEM images of Al-doped TiO2 hollow particles (Figure 10A and B) and Al/Fe-doped TiO2 hollow particles (Figure 10C and D). The size of all of the hollow particles is about 300 nm, and the shells are about 35 nm. The shells of
Metal-Ion-Doped TiO2 Hollow Particles Al-doped TiO2 and Al/Fe-doped TiO2 hollow particles are compact. The detailed structures are shown in the insets in Figure 7C and E, respectively. The EDX spectra show that O, Ti, and Al peaks for Al-doped TiO2 (see Figure SI-3A in the Supporting Information) and O, Ti, Fe, and Al peaks for Al/ Fe-doped TiO2 (see Figure SI-3B in the Supporting Information) are observed (silicon signal from the silicon substrate), indicating that we can successfully prepare various metal-doped TiO2 hollow nanocomposites. Conclusion Monodisperse, multifunctional metal-ion-doped TiO2 hollow particles have been successfully prepared by a general chemical method based on the tuning of the bandgap of titania by metalion doping, which uses polystyrene spheres as templates. Codoped TiO2 hollow particles, Fe-doped TiO2 hollow particles, Co/Al-doped TiO2 hollow particles, and Co/Zn-doped TiO2 hollow particles display brilliant colors in the visible range, low reflectance in the ultraviolet range, and high infrared emissivity during in the range 8-14 µm, indicating that they will be potential building blocks for the application in pigments, ultraviolet prevention, and energy conservation. Other metalion-doped TiO2 hollow nanocomposites, such as Al-doped TiO2 and Al/Fe doped TiO2 hollow particles are also prepared by this method. Acknowledgment. This work was financially supported by the National Hi-Tech 863 Programme (2009AA03Z302), National Science Foundation of China (60736001), and Beijing Natural Science Foundation (2093044). Supporting Information Available: Characterization details and the XRD pattern of the TiO2 hollow particles without any dopants. The HRTEM image of ZnCo2O4 in Co/Zn-doped TiO2 hollow particles. The EDX spectra of the samples: (A) monodisperse hollow Al-doped TiO2 particles, (B) hollow Al/Fedoped TiO2 particles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cochran, J. K. Curr. Opin. Solid State Mater. Sci. 1998, 3, 474. (2) Huang, H. Y.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (3) Caruso, F. Chem.sEur. J. 2000, 6, 413. (4) Caruso, F.; Shi, X. Y.; Caruso, R. A.; Susha, A. AdV. Mater. 2001, 13, 740. (5) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. AdV. Mater. 2000, 12, 693. (6) Cheng, X. J.; Chen, M.; Wu, L. M.; Gu, G. X. Langmuir 2006, 22, 3858–3863. (7) Liu, Z. Y.; Sun, D. D.; Guo, P.; Leckie, J. O. Chem.sEur. J. 2007, 13, 1851. (8) Lu, Y. F.; Fan, H. Y.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (9) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (10) Rana, R. K.; Mastai, Y.; Gedanken, A. AdV. Mater. 2002, 14, 1414.
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