Accurate Methods for Quantifying the Relative Ratio of Anatase and

J. Phys. Chem. C 2009, 113, 13703–13706

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Accurate Methods for Quantifying the Relative Ratio of Anatase and TiO2(B) Nanoparticles Thomas Beuvier, Mireille Richard-Plouet,* and Luc Brohan* Institut des Mate´riaux Jean Rouxel (IMN), UniVersite´ de Nantes, CNRS, 2, Rue de la Houssinie`re, BP32229 F44322 Nantes Cedex, France ReceiVed: April 23, 2009; ReVised Manuscript ReceiVed: May 25, 2009

Nanoparticles of TiO2 crystallizing as anatase and TiO2(B) phases with different ratios were synthesized by a three step treatment including an initial reflux process in highly alkaline medium. For the first time, the ratio of each variety has been quantified by analysing the galvanostatic curves during the lithium electrochemical insertion. The weight percentage of TiO2(B) reaches 95% (110 m2 g-1 specific surface area). In order to get quantitative information based on the ratios deduced from electrochemistry, the same TiO2 samples have also been investigated by Raman spectroscopy. This technique shows a sensitivity seven times higher for anatase than TiO2(B) detection compared to electrochemical method. The equation deduced from Raman refinement and electrochemical quantification permits, for the first time, to quantify the anatase/TiO2(B) ratio simply by using Raman spectroscopy. Introduction Among insertion compounds used as lithium-ion battery negative electrode, TiO2(B) has been investigated as a promising material for lithium storage.1 Compared to the titanate Li4Ti5O12 which has similar operating voltage (∼1.55 V vs. Li+/Li), TiO2(B) presents a higher electrochemical capacity up to 275 mAh g-1 (x ) 0.82 Li per Ti) for the first cycle2 against a theoretical specific capacity of 175 mAh g-1 expected for Li4Ti5O12.3-6 However, its power density is lower. To overcome this problem, nanoparticles of TiO2(B), which can improve the rate of insertion/deinsertion and hence the power density of the battery, were synthesized by various research teams2,7-10 reproducing Kasuga’s synthesis using hydrothermal reaction between NaOH and TiO2. Despite the effort made for obtaining pure TiO2(B), amount of anatase was always mixed to TiO2(B). Its origin may be related to unreacted initial TiO2 precursor or the partial transformation of TiO2(B) to anatase that can occur, depending on the annealing conditions. Recently, we reported that titanate nanotubes transform after ionic exchange and annealing into TiO2 anatase, whereas only titanate nanoribbons transform into TiO2(B) variety.11 These observations differ from the results of Armstrong et al12 who isolated nanotubes attributed to TiO2(B) when the titanate is prepared under autogenous conditions. However, their electrochemical measurement shows anatase mixed to TiO2(B), as proved by the insertion peaks at 1.5-1.6 V vs Li+/Li for the latter and the additional peak at 1.75 V, characteristic of anatase. Thus it is necessary to quantify the anatase/TiO2(B) ratio and to compare it to the proportion of each morphology of the sodium titanate, TiO2(B) and anatase precursors, and in particular if TiO2(B) nanotubes can really be isolated. Moreover the quantification is useful to understand and explain the energy and the power densities of TiO2, TiO2(B) exhibiting a pseudo-capacitive behavior,13 whereas anatase not. In this work, TiO2 mixtures with different anatase/TiO2(B) ratios were obtained under reflux reaction between NaOH and hydrated TiO2 as described in the experimental section. Three * Corresponding authors. (L.B.) 00 33 (0)2 40 37 39 35. 00 33 (0)2 40 37 39 95. [email protected]; (M.R.-P.) 00 33 (0)2 40 37 39 96. 00 33 (0)2 40 37 39 95. [email protected].

techniques were compared: X-ray diffraction, electrochemical measurement in lithium metal battery and Raman spectroscopy. Experimental Section TiO2 Synthesis. The titanium oxide hydrate precursor, TiO2, 1.4-2H2O is obtained by precipitation, starting from TiOCl2 solution (4.85 mol L-1 in HCl, Millenium) and an excess of 12.5 wt % NH3, aq (SDS). The white precipitate is rinsed with de-ionized water, filtered and finally dried at 70 °C in air. The adsorption/desorption isotherms performed, at 77 K, on the dried product show a very high surface area SLangmuir ) 550 m2 g-1 with high micropore density. [Due to the high micropore density, fitting the adsorption data with Langmuir’s equation gave better results than using the BET equation.] Sample 1 was obtained by dispersing 4 g of the titanium oxide precursor in 120 mL of concentrated (12 mol L-1) NaOH solution. The suspension was poured in a 300 mL capacity PTFE round-bottom flask equipped with a coil reflux condense. The flask was placed in a silicon oil bath and heated up to 120 °C (suspension temperature). The preparation was kept under vigorous stirring for 5 days. The suspension was then rinsed with de-ionized water. The solid is dried at 70 °C under air for more than 24 h before being stirred in the diluted acidic solution (0.1 mol L-1) allowing Na+ ion full exchange by H+. The product is finally annealed at 400 °C for 48 hours, under air. Sample 2 was obtained under the same conditions as sample 1 except that the time duration was fixed to 6 days and the final annealing at 400 °C was conducted for 3 h, only. Sample 3 was obtained by dispersing only 2 g of titanium oxide precursor in 120 mL of concentrated (12 mol L-1) NaOH solution. The reaction time was fixed to 7 days. The suspension was rinsed with de-ionized water and then directly stirred in diluted acidic solution (0.1 mol L-1) prior to annealing at 400 °C for 24 h, under air. The synthesis of pure micrometric TiO2(B) was performed according to the method earlier reported by Marchand et al.14 The starting material K2Ti4O9 was obtained by heating KNO3 (Merck, 99 %) and TiO2 (anatase, Prolabo, 99%) in the molar ratio 1:2 at 1000 °C for 6 h. The resulting solid is ground in an agate mortar and then ion-exchanged for three days in

10.1021/jp903755p CCC: $40.75  2009 American Chemical Society Published on Web 07/06/2009

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Beuvier et al.

Figure 1. (a) XRD patterns, (b) derivations of the galvanostatic charge/discharge, Iapp ) 7.0 mA g-1 (C/48 for all the samples), and (c) Raman spectra. PC100 refers to TiO2 anatase Millenium PC100. (1), (2), and (3) refer to samples 1, 2, and 3 (see experimental section). A and B refer to anatase and TiO2(B) phases, respectively.

HNO3 < 0.5 mol L-1 (10-3 mol K2Ti4O9 in 100 cm3 HNO3). After filtering, the powder was heated at 500 °C in air for 15 h. Characterization. X-ray diffraction (XRD) data were obtained using a Bru¨ker “D8 Advance” powder diffractometer operating in the Bragg-Brentano reflection geometry with a Cu anode as X-ray source, a focusing Ge(111) primary monochromator (selecting the Cu KR1 radiation, 1.5406 Å) and a 1-D position-sensitive detector (Vantec). The active area of the detector was limited to 3° in 2θ to improve its spatial resolution. The isothermal adsorption/desorption curves were recorded with an ASAP 2010 system by using nitrogen gas. The BET model was used to extract surface areas, except for the Ti oxide precursor. Electrochemical tests were obtained with a Mac-Pile equipment (BioLogic), using a two-electrode cell and metallic lithium as the counter electrode. Al-foil was cut off as a current collector whose effective electrode surface was 0.85 cm2 on which 2-3.5 mg of active material are deposited. The effective electrode was composed of TiO2, acetylene black and Poly(vinylidene fluoride) in the following weight ratio: 71: 25:6. The electrolyte used was a 1 mol L-1 LiPF6 solution in ethylene carbonate:diethyl carbonate (Merck), with a molar ratio of 1:1. Glass fiber paper was used as separator. The potentials mentioned in the following are referenced to the Li+/Li redox couple. C-rate was defined by C/n with n ) IA/QTh, where IA is the applied current (mA g-1) and QTh is the theoretical capacity for one lithium per TiO2 (i.e., QTh ) 335 mAh g-1). This calculated value corresponds to the filling of all the Ti4+ vacancies available in titanium dioxides, whatever the structure is considered. FT-Raman studies were performed on a Bruker RFS100 equipped with a 1064 nm laser as the incident light, which allows one to focus a sample area with a diameter less than 1 mm. The average power at the surface of the target was fixed to 340 mW. Results and Discussion X-ray Diffraction Investigations. The powder X-ray diagrams are shown in Figure 1a. All of the peaks can be assigned to the anatase and TiO2(B) phases. The XRD pattern of TiO2 anatase (Millennium PC100) is presented for comparison. This compound was chosen for its specific surface area close to those of our samples. A decreasing anatase/TiO2(B) ratio is observed from sample 1 to sample 3. The sample 3 has a such high proportion of TiO2(B) than the anatase part is hardly detected by X-ray diffraction. This shows for the first time that obtaining TiO2(B) in a high proportion is not limited to synthesis in

Teflon-lined autoclave, as mainly reported, but can also be obtained under refluxing conditions. It is worth noting that the two (101) and (200) major peak positions of anatase are very close to the (110) and (020) peaks of TiO2(B), respectively. Therefore XRD may be a suitable technique to detect high amount of anatase (at least 10 % in volume) mixed to TiO2(B) as in the case of sample 1. Otherwise, XRD may no more become accurate enough, especially when the peak widths due to nanometric particle sizes complicate the quantification of each variety by Rietveld refinement. The nanometric size of our compounds is confirmed by the high BET specific surface area (100, 102, and 110 m2 g-1 from samples 1-3, respectively). Electrochemical Quantification. In order to quantify both phases, a more appropriate technique is the electrochemical measurement in lithium metal batteries. Whereas the potential of TiO2 anatase is well known for its flat discharge plateau at 1.75 V vs Li+/Li from Li0.05TiO2 to Li0.5TiO2,15,16 TiO2(B) presents a more particular behavior with formal potentials between 1.5 and 1.6 V vs Li+/Li.1,8,17 In Figure 1b are plotted the derivations of the galvanostatic charge/discharge for the three samples compared to TiO2 anatase (Millennium PC100). During the discharge, four peaks are detected: at 1.50 and 1.59 V attributed to TiO2(B), at 1.75 V due to anatase variety and a smaller one between 2.0 and 2.2 V (unexplained for the moment but apparently independent of the anatase/TiO2(B) ratio). The galvanostatic curves during the first reduction of TiO2 samples are shown in figure 2. First of all, the anatase contributions were extracted from the galvanostatic curve obtained for PC100. Two peaks were fitted by pseudo-voigt functions at 1.75 and 1.40 V with areas of 0.53 (bulk insertion) and 0.19 (surface contribution) Li+ per TiO2, respectively (Figure 2 PC100). The total insertion amount is 0.72 Li+ per TiO2 in PC100 anatase which is in agreement with the value usually reported.18,19 Therefore we further use this value for the quantification results of anatase. According to this fit, the peaks shape and their ratio areas are fixed to extract the anatase contribution, during the refinement of samples (1), (2), and (3) (Figure 2 (1), (2), and (3)). However, to evaluate the areas corresponding to each variety, six peaks were necessary to account for the experimental data, as shown in Figure 2. The lithium storage in TiO2(B) is determined through three Lorentzian functions, two of them are sharp (FWHM ) 0.035 V) and the third is larger (FWHM ) 0.44 V). The ratios IA/(IA + IB) where IA and IB are the associated areas to anatase and TiO2(B) respectively deduced from refinements of electrochemical measurement are 4.4%, 10.6%, and 39.4% for the samples (3), (2), and (1), respectively. Assuming

Ratio of Anatase and TiO2(B) Nanoparticles

Figure 2. Derivations of the galvanostatic discharge, Iapp ) 7.0 mA g-1 (C/48). Filled and hatched black areas refer to the anatase and the unknown contributions. Grey areas and grey lines refer to TiO2(B) part, red line: sum of the fitted contributions. x in dx/dU refers to the number of Li inserted.

Figure 3. IA/(IA + IB) obtained by electrochemical measurement (filled square) and by Raman (open square) as a function of the percentage of anatase. IA and IB refer to the areas of electrochemical contributions and to Raman peaks at 144 and 123 cm-1 for anatase and TiO2(B) phases, respectively. k are the sensitive factors for each technique.

that 0.72 Li+ per TiO2 anatase and 0.82 Li+ per TiO2(B), as usually experimentally measured,2 are inserted between 2.5 and 1.25 V at the first cycle, the anatase percentage deduced from electrochemical measurement as mentioned in figure 3 (black filled squares) reaches 5.0%, 12.0%, and 42.6% from samples 3 to 1 that is to say 95.0%, 88.0%, and 57.4% of TiO2(B), respectively. It is interesting to note that the relation which links y ) IA/(IA + IB) to the weight percentage of anatase (%anatase ) mA/(mA + mB)) can be written as follows y ) kE × %anatase /(1 + (kE - 1) × %anatase) (calculation details are available as Supporting Information) where kE is the number of lithium

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13705 ions inserted per TiO2 in anatase divided by the number of lithium ions inserted per TiO2 in TiO2(B), i.e., kE ) 0.72/0.82 ) 0.88. The kE factor can be interpreted as the sensitive factor for anatase detection by electrochemistry. Raman Sensitivity for Anatase Detection. With this quantification, we want to correlate the Raman areas of anatase and TiO2(B) main peaks to the relative proportions of both polytypes. TiO2 anatase exhibits six Raman active modes: the most intense at 144 cm-1 and the other minor ones at 197, 399, 514(×2), and 639 cm-1,20 whereas eighteen modes are Raman active for TiO2(B).21 According to the experiment, the most intense mode appears at 123 cm-1. Pure TiO2(B) (i.e., without anatase) was synthesized from potassium tetratitanate prepared by solid state reaction according to the method developed by Marchand et al.14 (see Experimental Section) This allowed us to calibrate the relative intensity of Raman peaks (Figure 4(a)). Between 80 and 216 cm-1, five vibration modes at 123, 145, 161, 172, and 196 cm-1 exhibit normalized areas of 1, 0.43, 0.14, 0.19, and 0.94, respectively, whereas anatase (PC100) exhibits two peaks at 144 cm-1 and 197 cm-1 with normalized areas of 1 and 0.02 (Figure 4e). By constraining the fitted areas to those values for both phases, we extracted the contributions of anatase and TiO2(B) for each compound using Lorentzian peaks (Figure 4b-d). Thus, the IA/(IA + IB) ratios, where IA and IB are the Raman peak areas of the 144 and 123 cm-1 peaks of anatase and TiO2(B) respectively, were plotted against the weight anatase percentage in Figure 3 (open squares). The shape of the curve can be fitted by considering that the scattering intensity of the 144 cm-1 peak linked to anatase and the 123 cm-1 peak assigned to TiO2(B) can be expressed as IA ) σΑmA and IB ) σΒmB where mA and mB are the relative mass of anatase and TiO2(B) respectively and σA and σB are the scattering factors of the 144 cm-1 peak of anatase and the 123 cm-1 peak of TiO2(B). Introducing these scattering factors to express IA/(IA + IB) as a function of %anatase ) mA/(mA + mB) leads to the relation y ) IA/(IA + IB) ) kR×%anatase/(1 + (kR - 1) × %anatase) where kR ) σΑ/σΒ. This relation is the same than the one encountered during electrochemical quantification. The experimental data of figure 3, (open squares) are fitted accordingly with a kR constant equal to 6.2, meaning that anatase is six times more efficiently detected than TiO2(B), by Raman spectroscopy. This high value explains why the presence of a small quantity of anatase mixed to TiO2(B) induces a strong reinforcement of the peak at 145 cm-1 on the Raman spectrum. This high sensitivity may originate from the few Raman active modes of anatase with respect to TiO2(B) due to their respective space groups (anatase: I41/amd, TiO2(B): C2/m). By comparing the k values obtained by electrochemistry and Raman spectroscopy, we conclude that the latter is seven (kR/kE) times more sensitive than the former. Thus Raman spectroscopy appears as a very sensitive technique for anatase detection and the experimental determination of the k value enables us to quantify

Figure 4. Raman spectra of five anatase/TiO2(B) ratios. (a) Pure TiO2(B), (b) sample 3, (c) sample 2, (d) sample 1, and (e) PC100. Light and dark gray areas are assigned to TiO2(B) and anatase vibration modes, respectively.

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the anatase variety by Raman spectroscopy simply according to the relation: %anatase ) y/(6.2 - 5.2y) where y ) IA/(IA + IB). According to our researches the presence of anatase originates from two possible sources. One of them is related to the presence, in the initial sodium titanate, of nanotubes which transform after ionic exchange and annealing in anatase.11 In our conditions, it has to be reminded that only nanoribbons transform into TiO2(B). The second origin is the TiO2(B) to anatase transformation at annealing temperatures higher than 500 °C22 or for long time thermal treatment, at lower temperature. Compared to the poor sensitivity of XRD experiments for detecting small amounts of anatase in anatase/TiO2(B) nanoparticle mixture, Raman spectroscopy appears as a smart and easy-handling solution for quantifying the ratio and its sensitivity towards both titanium dioxide has now been clearly established. Conclusion Nano-TiO2 has been synthesized by a chimie douce process. The analysis by XRD, electrochemical measurement and Raman spectroscopy reveal that both anatase and TiO2(B) varieties are formed which is characteristic of this synthesis process. Due to the nanometric particle size and strong overlapping of the two major peaks of anatase with those of TiO2(B), XRD is not the most appropriate technique to estimate the proportion of each variety. The quantification was then performed for the first time by analyzing the galvanostatic curves during the electrochemical insertion of Li+. The three samples exhibit TiO2 anatase ranging from 5.0% to 42.6% in weight. The weight percentage of TiO2(B) reaches 95% with a 110 m2 g-1 specific surface area. This quantification allowed us to experimentally establish the sensitivity of Raman spectroscopy towards anatase and TiO2(B). Raman spectra show a high sensitivity for anatase variety detection in a mixture of anatase/TiO2(B) notably due to its intense peak at 144 cm-1. A simple relation was deduced from area measurements which enables an easy quantification of the anatase/TiO2(B) ratio especially for the low percentage of anatase. This should be useful to find the key parameters to synthesize pure nano-TiO2(B) and finally to correlate the energy and the power densities with the proportion of each variety. Acknowledgment. The authors thank T. Brousse, O. Crosnier, and R. Marchand for micro-TiO2(B) synthesis and their

Beuvier et al. assistance in electrochemical measurements. T.B. thanks CNRS and Total for financial support. Supporting Information Available: Additional calculation details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Brohan, L.; Marchand, R. Solid State Ionics 1983, 9-10, 419. (2) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2004, 43, 2286. (3) Murphy, D. W.; Cava, R. J.; Zahurak, S. M.; Santoro, A. Solid State Ionics 1983, 9-10, 413. (4) Ferg, E.; Gummow, R. J.; De Kock, A.; Thackeray, M. M. J. Electrochem. Soc. 1994, 141, L147. (5) Colbow, K. M.; Dahn, J. R.; Haering, R. R. J. Power Sources 1989, 26, 397. (6) Brousse, T.; Fragnaud, P.; Marchand, R.; Schleich, D. M.; Bohnke, O.; West, K. J. Power Sources 1997, 68, 412. (7) Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Angew. Chem., Int. Ed. 2008, 47, 2930. (8) Zukalova´, M.; Kalba´cˇ, M.; Kavan, L.; Exnar, I.; Haeger, A.; Graetzel, M. Prog. Solid State Chem. 2005, 33, 253. (9) Jitputti, J.; Suzuki, Y.; Yoshikawa, S. Catal. Commun. 2008, 9, 1265. (10) Lan, Y.; Gao, X.; Zhu, H.; Zheng, Z.; Yan, T.; Wu, F.; Ringer, S. P.; Song, D. AdV. Funct. Mater. 2005, 15, 1310. (11) Beuvier, T.; Richard-Plouet, M.; Mancini-Le Granvalet, M.; Brohan, L. Chem. Mater., submitted. (12) Armstrong, G.; Armstrong, A. R.; Canales, J.; Bruce, P. G. Electrochem. Solid-State Lett. 2006, 9 (3), A139. (13) Zukalova´, M.; Kalba´cˇ, M.; Kavan, L.; Exnar, I.; Graetzel, M. Chem. Mater. 2005, 17, 1248. (14) Marchand, R.; Brohan, L.; Tournoux, M. Mater. Res. Bull. 1980, 15, 1129. (15) Hardwick, L. J.; Holzapfel, M.; Nova´k, P.; Dupont, L.; Baudrin, E. Electrochim. Acta 2007, 52, 5357. (16) Wagemaker, M.; Van de Krol, R.; Kentgens, A. P. M.; Van Well, A. A.; Mulder, F. M. J. Am. Chem. Soc. 2001, 123, 11454. (17) Kavan, L.; Rathousky, J.; Graetzel, M.; Shklover, V.; Zukal, A. J. Phys. Chem. B 2000, 104, 12012. (18) Cava, R. J.; Murphy, D. W.; Zahurak, S.; Santoro, A.; Roth, R. S. J. Solid State Chem. 1984, 53, 64. (19) Zachau-Christiansen, B.; West, K.; Jacobsen, T.; Atlung, S. Solid State Ionics 1988, 28-30, 1176. (20) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7 (6), 321. (21) Ben Yahia, M.; Lemoigno, F.; Beuvier, T.; Filhol, J.-S.; RichardPlouet, M.; Brohan, L.; Doublet, M.-L. J. Chem. Phys. 2009, 130, 204501. (22) Brohan, L.; Verbaere, A.; Tournoux, M. Mater. Res. Bull. 1982, 17, 355.

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