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Hybrid Sol-Gel Combustion Synthesis of Nanoporous Anatase B. Mukherjee,† C. Karthik,†,‡ and N. Ravishankar*,†,‡ Materials Research Centre, Indian Institute of Science, Bangalore, India, and Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York ReceiVed: May 15, 2009; ReVised Manuscript ReceiVed: September 7, 2009
Nanoporous anatase with a thin interconnected filmlike morphology has been synthesized in a single step by coupling a nonhydrolytic condensation reaction of a Ti precursor with a hybrid sol-gel combustion reaction. The method combines the advantages of a conventional sol-gel method for the formation of porous structures with the high crystallinity of the products obtained by combustion methods to yield highly crystalline, phasepure nanoporous anatase. The generation of pores is initiated by the formation of reverse micelles in a polymeric polycondensation product, which expand during heating, leading to larger pores. A reaction scheme involving a complex formation and nonhydrolytic polycondensation reaction with ester elimination leads to the formation of an extended Ti-O-Ti network. The effect of process parameters, such as temperature and relative ratio of cosurfactants, on phase formation has been studied. The possibility of band gap engineering by controlled doping during synthesis and the possibility of attachment of molecular/nanoparticle sensitizers provide opportunities for easy preparation of photoanodes for solar cell applications. Introduction TiO2 is an extensively studied and technologically important wide band gap semiconductor widely used in batteries,1 for water splitting,2,3 photocatalysis and degradation of organic contaminants,4,5 photoinduced hydrophilicity,6-8 electrochromic windows,9,10 and electron injection solar cells.11 TiO2 is favored over ZnO in many applications despite their comparable band gap because of its higher chemical stability. Depending on the application, the surface properties, phase, and morphology need to be engineered to extract optimum performance. The anatase phase of TiO2 is photochemically more active compared with the brookite and the thermodynamically stable rutile form, and hence, a lot of focus has been dedicated toward synthesis of phase-pure anatase. For solar cell applications, TiO2 typically serves as the favored photoanode material (n-type semiconductor) in the form of an interpenetrating, three dimensionally continuous percolating matrix with the voids filled with the electrolyte or a p-type semiconductor.12 Anatase, having a band gap of 3.2 eV, absorbs light in the UV region of the solar spectrum that contributes to only 3-5% of the total solar radiation, and hence, there is a need to extend the spectral response to the visible region. This can be done either by engineering the effective band gap of TiO2 by doping with a transition-metal cation13-15 (Fe3+, Mo5+, Re4+, V4+, Rh3+) or an anion16-21 (N, S, C) or through sensitizing TiO2 by electronically coupling it with a narrow band gap semiconductor (quantum dot) or a dye molecule that absorbs in the visible region.12,22-24 This is achieved by sandwiching the low band gap material between the TiO2 and the p-type material. TiO2 with a nanoporous morphology25,26 is a prerequisite for preparing electrodes for solar cells as it renders a high surface area for adsorption of the sensitizers and channels for effective penetration of the hole conductor, mediator, or electrolyte and increases the scattering of incident radiation for better charge * To whom correspondence should be addressed. E-mail: nravi@ mrc.iisc.ernet.in. † Indian Institute of Science. ‡ Rensselaer Polytechnic Institute.
collection in solar cells. However, porous TiO2 has a very low electron drift mobility27 (10-4-10-7 cm/(V s)) compared with single crystalline TiO2 and thus leads to a much slower interparticle charge transfer. This low drift mobility is limited by traps and grain boundaries and is detrimental to the overall efficiency of solar cells. Thus, an important goal is to obtain a microstructure with a minimal number of grain boundaries and isolated particles for optimal charge transport28 without sacrificing the benefits accrued due to the porous structure. Theoretical and experimental studies reveal the importance of morphology, porosity, and the degree of interconnection on the charge transport in such structures.29,30 A porosity of ∼50% is predicted to be ideal for solar cell applications. Apart from the morphology, the surface has to be engineered to minimize surface defect states31 by hybridizing with graphitic carbon or coating with Al2O3 or CaCO3 and maximizing the exposure of photoactive facets32 compared to thermodynamically stable2 facets. Templateless synthesis of porous materials is a very attractive strategy as it does not require postsynthesis treatments for template removal. Selective leaching of a biphasic material,33 reaction-limited aggregation of nanoparticles,34 or partial sintering of rod-shaped particles35 are some of the proposed strategies for synthesizing porous blocks. Evaporation-induced self-assembly36 (EISA) provides an ideal gridlike morphology with a continuous ordered network of anatase with a high surface area by condensation of a titanium precursor around selforganized organic templates in a gel phase, followed by burning of the template in the final step. Using this method, electrodes can be prepared directly on Si or fluorine-doped tin oxide (FTO) substrates. However, the serial nature of the process and the formation of very thin layers (∼300 nm per coating compared with ∼10 µm required for good light scattering and charge collection) are drawbacks of this method. Other routes for nanoporous TiO2 rely on the synthesis of nanoparticles of TiO2, followed by formation of a three-dimensional porous network of interconnected particles by bridging the particle through hydrolyzing a Ti precursor that acts as a glue between the particles. Here, we report a hybrid sol-gel combustion synthesis
10.1021/jp904563m CCC: $40.75 2009 American Chemical Society Published on Web 09/24/2009
Sol-Gel Combustion Synthesis of Nanoporous Anatase method to prepare large quantities of nanoporous TiO2 in a single step. Titanium tetraisopropoxide (TTIP) complexes with oleic acid to form a viscous Ti precursor that is stable against hydrolysis and condensation. On heating to 400 °C or higher, the precursor foams and burns to form TiO2 with a thin, flaky, nanoporous morphology. The evolution pathway and, hence, the final microstructure can be controlled by controlling the temperature of the flame by a suitable choice of the ratio of surfactants used. Recently developed methods have shown the possibility of preparing electrodes from powders without any high-temperature steps37 and can be exploited for realizing efficient solar cells on flexible substrates. The nanoporous morphology also provides opportunities to easily functionalize the surfaces and to coat it with light-absorbing materials (dyes or low band gap semiconductor quantum dots) to harness a wider region of the solar spectrum.
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Figure 1. Powder XRD pattern of the combustion synthesis of TiO2 (TTIP/oleic acid/oleylamine ) 1:2:1) that was annealed at 450 °C for 2 h indexed to the anatase phase, indicating phase purity.
Experimental Section Synthesis of TiO2. Titanium(IV) isopropoxide (TTIP) (Aldrich) is mixed with oleic acid and oleylamine with different ratios to form a viscous yellowish-brown mixture that combusts with the generation of heat. The glass beaker containing the mixture is kept in a furnace preheated to 400 °C with the furnace door open to allow an unhindered supply of atmospheric oxygen. The mixture combusts with a flame, and hence, appropriate precautions and proper ventilation are required from a safety point of view. Alternately, the synthesis can also be carried out by partial polycondensation of the precursor in a solvothermal reactor at 280 °C for 3 h, followed by discarding the solventrich phase and combustion of the gelatinous product. This is a flameless modification of the formerly described method but takes more time and involves multiple steps. The mixture ignites within 2-3 min and burns with a yellow flame, leading to the formation of a foamy black product. The flame temperature is monitored using a thermocouple that is placed very close to this region in the furnace. The black product is ground in an agate mortar and annealed in air at different temperatures (400 °C for 2 h, 500 °C for 15 min, or 600 °C for 6 min) for complete removal of the carbonaceous material. Finally, the white powder is collected and characterized for phase and morphology. Characterization. All the samples were characterized using X-ray diffraction, transmission electron microscopy, and UV-vis spectrophotometry. X-ray powder diffraction patterns were recorded on a Philips X’PERT diffractometer equipped with an accelerator using Cu KR radiation. N2 adsorption analysis was carried out with a BELSORP Autosorp 1 at 77 K. Samples were outgassed at 300 °C for 3 h before the experiment. The specific surface area was obtained by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the absorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) was carried out using a FEI Tecnai F30 microscope equipped with EDS and EELS at 300 kV. TEM samples were prepared by drying a drop of the dispersion on a 200 mesh C-coated Cu grid. FT-IR studies were carried out with a PerkinElmer Spectrum RX-1 FTIR spectrometer in the 400-4000 cm-1 wavenumber range in transmission mode with resolution of 4 cm-1 and the number of scans of 64. Thermal analysis of the sample was performed in air (100 mL/min) with a simultaneous thermogravimetric analysisdifferential scanning calorimetry (TGA-DSC) instrument (model no. TA- SDTQ600). Results and Discussion As-synthesized TiO2 prepared by our method is black in color and was heat-treated in air to remove the carbonaceous
Figure 2. Powder XRD patterns showing phases formed after different postsynthesis heat treatments: (a) as-prepared, (b) heat-treated at 450 °C, (c) heat-treated at 500 °C, and (d) heat-treated at 600 °C. A, R, and B refer to anatase (101), rutile (110), and brookite (211) peaks, respectively.
impurities. The powder XRD pattern of combustion-synthesized TiO2 (TTIP/oleic acid/oleylamine ) 1:2:1) that was annealed at 450 °C for 2 h is shown in Figure 1. The pattern from the as-prepared and the heat-treated TiO2 under this condition could be indexed to the anatase phase of TiO2 with lattice constants a ) 3.785 Å and c ) 9.514 Å (JCPDS card no. 21-1272) without any detectable peak of the rutile or brookite. The grain size calculated from the (101) peak of the anatase using the Scherrer equation is 10 nm. A higher amount of oleic acid (TTIP/oleic acid/oleylamine ) 1:3:0, for instance) leads to an increase in the flame temperature, enabling formation of a small amount of brookite/rutile peak in the as-prepared sample (Figure 2a, for instance). The effect of the postsynthesis heat treatment temperature on the crystal phases formed is shown in Figure 2. A drastic decrease in the anatase-to-rutile transformation temperature is observed for the combustion-synthesized TiO2 with a discernible increase in the signature of the rutile phase in the powder XRD pattern for annealing temperatures as low as 450 °C. Thus, annealing treatments to remove residual organics was typically limited to 400 °C. Figure 3 shows the effect of surfactant concentrations on the evolution of phases in the assynthesized sample as well as after heat treatment at 450 °C for 2 h. It is clearly seen that, when the TTIP/oleic acid/ oleylamine ratio is 1:2:0 and 1:2:1, the as-prepared and heattreated samples contain exclusively anatase, whereas with an
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Figure 3. XRD patterns showing the evolution of phases for different TTIP/oleic acid/oleylamine ratios in the as-prepared samples and heattreated samples (marked H). All samples were heat-treated at 450 °C for 2 h. The formation of small amounts of rutile and brookite phases, as evidenced from the peaks above 27°, is seen with increasing amounts of oleic acid in the starting mixture.
increase in the amount of oleic acid, a small amount of brookite and rutile phases appear in the as-prepared sample. This may be due to a higher flame temperature arising from the increasing amount of oleic acid. Table 1 lists the phases formed for precursor compositions along with the average grain size of the different phases calculated using the Scherrer equation. Multiple Gaussian-fit peaks were used in each case to obtain the average grain size. In all cases, where there is an occurrence of peaks from multiple phases, the average grain size of rutile is more than that of the anatase phase, which indicates phase transformation, followed by grain growth. The composition of the surfactant mixture changes the flame temperature and thus has an effect on the phases formed with an increasing rutile/anatase ratio with an increase in the amount of oleic acid. The presence of oleylamine changes the combustion process to smoldering mode without any flame, and hence, the anatase phase becomes the only product, as seen in the case of the 1:2:1 TTIP/oleic acid/oleylamine mixture; hence, this ratio is used to promote the exclusive formation of the anatase phase. Figure 4a is a low-magnification bright field image clearly illustrating the interconnected porous morphology of the combustion-synthesized product. Images taken at various defocus values clearly show the Fresnel contrast from the void regions in the porous structure. A closer inspection reveals that the grain size varies from 10 to 20 nm and the pore diameter is of the order of 5-7 nm. Figure 4b shows the HREM image of a TiO2 crystallite with a lattice spacing of 3.54 Å, corresponding to the (101) plane of anatase. Electron energy loss spectroscopy was used to further characterize the product formed. The near-edge fine structure of the Ti L-core loss edge is shown in Figure 4c. TiO6 octahedron is the basic structural unit for both rutile and anatase. Due to arrangement and distortion of these octahedra, the local point group symmetry of the Ti atom is lowered from Oh to D2h and D2d, respectively in rutile and anatase.38 L-edge splitting to L2,3 arises due to splitting of the 2p spin orbit into 2p1/2 and 2p3/2 levels with 5.3 eV separation in both rutile and anatase.
Mukherjee et al. The L2,3 levels further split due to crystal-field splitting of d-orbitals of Ti. This changes the 2p3/2 3dσ peak and leads to a low-energy shoulder for rutile and a high-energy shoulder for anatase. Thus, the main peak separation between 2p3/2 3dπ and 2p3/2 3dσ is around 2.1 eV for anatase and ∼3 eV for rutile. The energy loss spectra taken from several portions of the sample for Ti L-core loss edge showed that this peak separation varies from 1.9 to 2.5 eV with the distribution centered around 2.3 eV, showing that we have phase-pure anatase. Figure 4d shows the energy loss spectrum in the O 1s core electron excitation region. A double-peak sharp structure near the edge can be observed, which corresponds to the transition to the O 2p-Ti 3d hybridized states of t2g-eg symmetry. The broad peaks in the 8-15 eV from the edge can be used to differentiate between anatase and rutile. Rutile has three peaks in this region that can be assigned to the transitions to Ti 4p states with b1u, b2u, b3u character hybridized with oxygen 2p states of the same character, whereas anatase has two peaks in the same region that arise due to transition to O 2p states hybridized with Ti 4p states of local symmetry b and e. The wide double-peak fine structure, as seen in the figure, further corroborates that the major phase is anatase. Figure 4e shows the low loss spectrum in the 0-35 eV region. The low-energy loss spectrum can also provide clear distinction between anatase and rutile where a prominent peak at ∼14 eV has been identified39-41 to be the characteristic of the rutile phase. In the low loss spectra collected from several regions of the sample, we did not observe any prominent peak corresponding to this value. This further confirms the anatase phase purity of the sample. Figure 5 shows the thermal studies indicating the formation mechanism of nanoporous TiO2. TTIP was complexed with oleic acid in a 1:2 molar ratio. To this a mixture 1.5 mL of oleylamine and 50 µL of water (reverse micelle) was added to make the final composition of TTIP/oleic acid/oleylamine ) 1:2:1. The mixture was then taken in a Teflon-lined solvothermal reactor and kept for 3 h at 270 °C for polycondensation, which resulted in separation of the reaction mixture into a yellowish white solid (polymer-rich phase with trapped reverse micelles) and a solvent-rich phase. The yellowish-white solid was centrifuged out, and one part of it was subjected to the TGA experiment while another part was directly put in a furnace at 400 °C for combustion. The TGA experiment was performed in three heating steps: (i) heating from room temperature to 400 at 10 °C /min, (ii) isothermal heating at 400 °C for 120 min, and (iii) heating from 400 to 600 °C at 5 °C/min. The TGA plot in Figure 5a shows three major weight loss regimes. The first one, peaking around 260 °C, as seen in the DTG plot, can be assigned to the removal of free organic molecules (oleic acid and oleylamine), resulting in a 17% weight loss; the second event at ∼330 °C can be assigned to the degradation of the tightly bound organics with TiO2 contributing to a further 17% weight loss, while the final part starting around 340 °C is due to removal of carbonaceous material after combustion that results in a loss of another 27% of the material. After the isothermal heating at 400 °C, the heating step from 400 to 600 °C results in only a 0.4% weight loss, which indicates complete removal of almost all the carbonaceous material in the isothermal heating step. The weight loss versus time plot shows that, within the first 25 min of the isothermal heating, a 15% weight loss is observed (carbonaceous impurities) and over the rest of the heating process, the observed weight loss was only 2%, indicating that the carbonaceous materials can be removed efficiently at 400 °C. Figure 5b shows the DTA plot where the first weight loss has a very weak exothermic peak, whereas the second weight
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TABLE 1: Effect of Surfactant Ratios on the Phase Formation of TiO2 heat treatment condition
precursor composition (TTIP/oleic acid/oleylamine)
flame temperature (°C)a
TO-14
1:4:0
690
as-prepared
TO-14H
1:4:0
690
450 °C, 2 h
TO-13
1:3:0
550
as-prepared
TO-13H1
1:3:0
550
450 °C, 2 h
TO-13H2
1:3:0
550
500 °C, 15 min
TO-13H3
1:3:0
550
600 °C, 6 min
TO-121 TO-121H TO-12
1:2:1 1:2:1 1:2:0
no flame no flame 480
sample
as-prepared 450 °C, 2 h
phases formed anatase (A) rutile (R) brookite (B) anatase (A) rutile (R) brookite (B) anatase (A) rutile (R) brookite (B) anatase (A) rutile (R) anatase (A) rutile (R) anatase (A) rutile (R) anatase (A) anatase (A) anatase (A)
average grain size (Dp in nm) A f 10 R f 20 B f 16 A f 12 R f 20 B A f 10 R f 26 B f 15 A f 13 R A f 17 R f 29 A f 15 R f 34 Af5 Af8 Af8
a
The flame temperature under the synthesis condition is indicated in each case (in degrees rounded to the nearest ten). The formation of the rutile and brookite phases is favored when the flame temperature is higher.
Figure 4. (a) Low-magnification TEM image of TiO2 showing the nanoporous morphology (b) High-resolution image of a TiO2 crystallite with the lattice fringes corresponding to the (101) plane of anatase. (c) Near-edge fine structure of the Ti L-core loss region showing the characteristic splitting, corresponding to the anatase phase. (d) O 1s core electron excitation EELS spectrum. The presence of two broad peaks 8-15 eV away from the O 1s edge is a characteristic feature of the anatase phase. (e) Low-energy loss spectra of TiO2. The absence of a characteristic prominent rutile peak confirms the phase purity of anatase.
loss at ∼320 °C corresponds to a very strong exothermic reaction, which may involve bond breaking in the Ti-oleic acid complex. This also hints that the first weight loss corresponds to removal of free organics through degradation instead of just
evaporation that would have shown an endothermic peak. TEM investigation of the sample after direct combustion shows similar porous morphology and pore size distribution as that of sample TO121 (see the Supporting Information). As the solvent-rich
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Figure 5. (a) TGA plot along with DTG clearly shows three different weight loss regimes with two strong DTG peaks at ∼260 and 330 °C, corresponding to a high rate of weight loss. (b) The DTA plot shows only a very weak endothermic peak between 200 and 250 °C and a very strong exothermic peak at 330 °C, resulting from degradation of the organics. The sharp rise at 400 °C is due to the isothermal step for 120 min. Two very weak exothermic peaks are also observed at around 475 and 550 °C. Plot (c) shows the weight loss over time. A weight loss of about 34% within the first 32 min accounts for the first two DTG peaks. The second hump is due to the 7% weight loss between 340 and 400 °C before the isothermal process starts. During the isothermal treatment at 400 °C, a further 20% weight loss has been incurred.
Figure 6. FTIR spectra of the Ti precursor and the as-synthesized TiO2 after heat treatment. The absence of carbonaceous impurities in the spectra from heat-treated TiO2 is consistent with the TGA/DTA results.
part is removed before combustion, the product in this case is obtained without a flame. Figure 6 shows FTIR spectra of the Ti precursor (TTIP/oleic acid/oleylamine ) 1:2:1) and TiO2 after heat treatment at 400 °C. Peaks at 1540 and 1461 cm-1 are asymmetric and symmetric stretching vibrations of COO- with a frequency difference of 79 cm-1, indicative of bidentate chelation by oleic acid with TTIP42,43 in the precursor. Peaks at 1730 and 1645 cm-1 can be attributed to CdO stretching of free ester and/or oleic acid, whereas the peaks at 3007, 2927, and 2856 cm-1 correspond to olefinic C-H stretching and asymmetric and symmetric aliphatic C-H stretching, respectively. The presence of oleylamine can
Mukherjee et al. be detected by N-H stretching at ∼ 3320 cm-1, N-H wagging mode from 600 to 900 cm-1, NH2 bending mode at 964 cm-1, and NH2 scissor mode at 1567 cm-1. The peaks in the fingerprint region below 1300 cm-1 cannot be uniquely assigned due to their broad nature. The doublet peaks at 2357 and 2335 cm-1 are due to atmospheric CO2 or from the instrument body as it was also observed in the single beam spectra of reference KBr pellets. In the combustion-synthesized TiO2 sample after 2 h of heat treatment at 400 °C, no prominent organic peak is noticed, which is in agreement with the TGA experiment. A broad band with a peak at ∼3400 cm-1 with continuous absorption up to 3000 cm-1 can be attributed to physisorbed water that is weakly hydrogen-bonded to a surface hydroxyl group of TiO2,44-46 and the band at ∼1620 cm-1 results from the bending vibration of water in dimeric or polymeric forms. No clear shoulder at ∼3200 cm-1 could be seen, which is due to strongly bound water molecules with cationic sites of the TiO2 surface. A broad band from below 800 cm-1 arises from vibrations of the Ti-O and Ti-O-Ti framework. Analysis of the surface area and porosity has been characterized by measuring the N2 adsorption-desorption isotherm, as seen in Figure 7. Figure 7a shows a typical type-IV isotherm,47 which is a characteristic of a well-developed mesoporous material. The specific surface area calculated by the BET method is 96 m2/g. The hysteresis loop appears due to the capillary condensation of N2 in the mesopores of the material. The shape of the observed hysteresis loop is like an H1-type in which both the adsorption and the desorption branches are nearly vertical and almost parallel over an appreciable range of gas uptake. This feature can be associated with a narrow and fairly regular distribution of pores. Pore size distribution obtained by the BJH method shows a narrow distribution with an average pore size of 4 nm, which is reflected in the H1-type hysteresis loop (Figure 7b). The material contains a 0.224 cm3/g pore volume, which translates to about a 47% porosity. Figure 8 is a survey XPS scan from combustion-synthesized TiO2 over the energy range of 0-1000 eV, showing strong peaks of Ti, C, and O. The presence of carbon impurity is common on TiO2 surfaces prepared by the combustion method due to incomplete removal of the pyrolytic products of the organics used. The inset (a) shows the Ti 2p core level spectra having a double-peak structure. Peaks at 464 and 458 eV correspond to Ti 2p3/2 and Ti 2p5/2, respectively. No shoulder or splitting of the peak corresponding to the presence of Ti3+ species is observed. Inset (b) shows O 1s core level spectra. The peak at 531 eV is Gaussian without any splitting and a small shoulder at higher energy indicating the presence of a very small amount of surface hydroxyl group. One of the most widely employed methods for producing porous structures is the sol-gel technique that uses surfactants to direct the formation of highly ordered pores.48 However, one of the major drawbacks is the formation of amorphous phases that require postannealing treatments for crystallization. Combustion synthesis, on the other hand, is a high-temperature technique widely used for the formation of highly crystalline products and metastable phases not accessible by other techniques.49 Here, we use a nonhydrolytic sol-gel50-54 combustion technique to obtain a porous and highly crystalline structure in a single step. The surfactants direct the formation of the pores in the initial stages with an increase in the pore size taking place due to the foaming action of the organics at high temperatures. The higher temperature is also responsible for the formation of highly crystalline phases. The ratio of the surfactants plays a crucial role in the flame temperature and thus on the phases
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Figure 7. (a) Type-IV nitrogen adsorption-desorption isotherm with a hysteresis loop, indicating the mesoporous nature of the TiO2. (b) Pore size distribution calculated from the adsorption branch using the BJH method. A narrow pore distribution peaked around a 4 nm pore size is obtained.
Figure 8. XPS survey spectra of the porous TiO2, showing the presence of Ti, O, C, and Ag (from the conducting Ag paste). Inset (a) shows the 3d core level spectrum of Ti and inset (b) shows the O 1s core spectrum.
formed with a higher temperature flame stabilizing the rutile form over the anatase form of TiO2. Oleic acid has been extensively used for the synthesis of TiO2 nanorods by nonhydrolytic ester elimination.43,55,56 The reaction between TTIP and oleic acid proceeds through a nonhydrolytic condensation with ester elimination or a competitive hydrolysiscondensation reaction after slow in situ generation of ester and water molecules.42 The peaks at 1730 and 1645 cm-1 in the IR spectrum due to the CdO group of ester supports that the reaction possibly goes through the nonhydrolytic ester elimination pathway. Oleic acid forms a complex with TTIP to form an oxocarboxyalkoxide42 that is stable toward hydrolysis and undergoes polycondensation upon heating to form a Ti-O-Ti network. Oleylamine promotes crystallization,42 acts as a cosurfactant, forms reverse micelles, and also controls the flame temperature. These micelles expand during foaming, leading to an increase in the pore size from that achieved by using the surfactant, possibly due to an expansion of the in situ generated alcoholic core during heating and foaming. A schematic illustrating the reaction and the pathway for pore formation is presented in Figure 9. The sizes of the mesopores increase with an increased amount of oleic acid. At the same time, excess oleic acid means that the product stays longer in contact with the flame (>600 °C) and hence an increased possibility of forming rutile phase. A molar ratio of 1:2 (TTIP/oleic acid) was found to be ideal to yield pore sizes of 4 nm in the anatase phase. When the precursor/surfactant mixture is rapidly heated to an elevated temperature, a fast polycondensation process takes place and results in a rapid nucleation of nanoscale TiO2 along the walls of the surfactant. Upon complete removal of the organics, the nanoporous anatase network evolves with a pore diameter of ∼5 nm. A mechanistically similar method has been
reported in the literature to prepare ultralight alumina foams57 where an aluminum chloride-isopropyl ether complex (AlCl3 · Pri2O) was heated above the boiling point of PriCl, resulting in polycondensation and foaming due to instant formation of bubbles of PriCl and, finally, foam stabilization by gelation in the polymer-rich region. TGA results supporting the mechanism clearly revealed three steps, viz., (i) removal of free organics around 260 °C (responsible for foaming), (ii) oxidative degradation of strongly bound organic molecules from the gel to create stabilized TiO2 foam, and (iii) removal of carbonaceous material, leading to the final mesoporous oxide network. We have also observed large single-crystalline platelets with a large number of faceted voids in it. This is possibly due to a rapid sintering of the nanoporous structure in the flame, leading to the formation of large grains with faceted pores in them (Figure 10). This observation leads to an interesting possibility of formation of pores in large single-crystalline grains having high porosity as well as good electron transport properties comparable to that of single-crystalline anatase. However, more detailed investigation of the mechanism of formation of such structures will be required before they can be formed in a controlled manner. Although the phase-transformation temperature from anatase to rutile is above 900 °C for the bulk TiO2 phase,58 we observed the beginning of phase transformation at 450 °C. Annealing at 520 °C for 15 min converted more than 60% of the anatase to rutile, whereas at 600 °C for 6 min, more than 80% of the anatase is converted to the rutile phase. This lower phasetransformation temperature in TiO2 can be understood from the size-dependent phase stability59-62 in nanoparticulate titania. Earlier studies predict anatase as the most stable phase below 11 nm, brookite to be stable between 11 and 35 nm, and rutile to be stable above 35 nm, which is why low-temperature synthesis usually yields the anatase phase. Grain growth during thermal treatment initiates phase transformation to the more stable phases. In nanomaterials, the transformation proceeds through interface nucleation at particle-particle contacts and, hence, the rate of transformation is dramatically increased due to availability of more nucleation sites. Quantitatively, this translates to a 7-order increase compared with micrometer-sized particles in the pre-exponential factor of the Arrhenius equation. From control experiments and the corresponding XRD and FTIR data, heating at 400 °C is seen to be a better option compared with heat treatment at 600 °C for a shorter time as, in the former case, anatase is the exclusive product. The presence of surface hydroxyl groups is another important aspect of TiO2 for its role in photocatalytic applications. Upon heating for the removal of the surfactants, a shift of the hydroxyl group peak toward higher
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Figure 9. Schematic illustration of the pore formation process. The first step is the formation of reverse micelles in the polymeric polycondensation product. These micelles expand during heating, leading to larger pores. A model reaction scheme showing oleic acid acting as a bidentate ligand to form a complex. This complex then takes part in a nonhydrolytic polycondensation reaction with ester elimination to form an extended Ti-O-Ti network.
tion of forming faceted cavities hints that proper optimization of the process could produce faceted pores in large singlecrystalline grains that can significantly increase the electronic properties without sacrificing the porous morphology. Conclusions
Figure 10. Bright field TEM image of the faceted cavities formed in anatase during the reaction.
energy is observed in the FTIR spectrum, indicative of removal of the absorbed water. Peaks for isolated hydroxyl groups are present after heating for short times, whereas these are completely removed after heating for longer times. The present method is mechanistically very similar to the EISA method for the generation of porosity due to the presence of heterogeneity in the reaction mixture; however, this method overcomes the slow self-assembly and condensation process at the price of pore order. When the thickness of ∼10 µm is required for the TiO2 layer, our method will be more efficient. In addition, combustion provides appropriate reaction conditions for selective doping and can be used to prepare doped anatase with the same porous structure and appropriately engineered band gap (see the Supporting Information) and surface structure. It is also possible to sensitize the nanoporous powder with quantum dots prior to making the photoelectrodes. The observa-
In conclusion, we have reported a single-step synthesis of nanoporous anatase powder with a high surface area of ∼96 m2/g and an optimal porosity of ∼47%. Nonhydrolytic condensation through ester elimination of the titanium precursor coupled with a sol-gel combustion-type63 reaction yields a thin interconnected porous filmlike morphology that can be easily made to a paste to form a coating of desired thickness. Oleylamine was used as a surfactant to limit the pore size to 5 nm, but pore size may be increased up to 30 nm64 using appropriate block copolymers as surfactants (see the Supporting Information). The present method relies on the preparation of mesoporous TiO2 in powder form in a single step that can be further processed to form photoelectrodes or photocatalytic membranes from a paste prepared from the powders. As recently new methods37 are available to prepare coatings on desired substrates at low temperature, this kind of paste provides an ideal way to prepare thick crack-free films with good lightabsorbing properties while maintaining the porous network for penetration by the dye, hole conductor, or high viscous redox mediator. Acknowledgment. Financial support from the Nanoscience and Technology Initiative, Department of Science and Technology, Government of India, is gratefully acknowledged. Supporting Information Available: TEM images and UV-vis absorption spectra of combustion-synthesized TiO2. This material is available free of charge via the Internet at http:// pubs.acs.org.
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