Crystal Morphology of Anatase Titania Nanocrystals Used in Dye

Chemistry, Huaqiao UniVersity, Quanzhou 362021, Fujian, China. ReceiVed March 10, 2007; ReVised Manuscript ReceiVed September 10, 2007. ABSTRACT: ...
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CRYSTAL GROWTH & DESIGN

Crystal Morphology of Anatase Titania Nanocrystals Used in Dye-Sensitized Solar Cells Jihuai Wu,* Sancun Hao, Jianming Lin, Miaoliang Huang, Yunfang Huang, Zhang Lan, and Pinjiang Li

2008 VOL. 8, NO. 1 247–252

The Key Lab for Functional Materials of Fujian Higher Education, Institute of Materials Physical Chemistry, Huaqiao UniVersity, Quanzhou 362021, Fujian, China ReceiVed March 10, 2007; ReVised Manuscript ReceiVed September 10, 2007

ABSTRACT: Anatase TiO2 nanocrystals were synthesized by hydrolysis of titanium isopropoxide followed by crystal growth under hydrothermal conditions in an acidic or basic environment. The crystal phase, crystal shape, and surface orientation of the TiO2 nanocrystals, as well as the photovoltaic performance of DSSCs, was studied by XRD, TEM, HRTEM, FTIR, and DSC-TG. It was found that anatase TiO2 nanocrystals prepared in tetramethylammonium hydroxide contained more {101} faces and presented a bipyramidal rod-like shape in comparison with the anatase TiO2 nanocrystal with a tetragonal ball-like shape prepared in nitric acid. The anatase-to-rutile phase transition temperature was largely affected by the crystal shape and surface orientation; it was about 950 °C for rod-like anatase and 700 °C for ball-like anatase. A 6.2% photoelectric conversion efficiency was obtained from the dye-sensitized solar cell (DSSC) with rod-like anatase, and a 5.4% the efficiency was obtained from the DSSC with ball-like anatase, which means that the anatase TiO2 nanocrystal prepared in tetramethylammonium hydroxide should be preferred for DSSCs.

1. Introduction Since the prototype of dye-sensitized solar cells (DSSCs) was first reported by O’Regan and Gratzel in 1991,1 DSSCs based on TiO2 nanocrystals have become an attractive alternative to traditional photovoltaic devices due to their high efficiency, ease of fabrication, and low production costs.2–4 The high light-toelectrical energy conversion efficiency achieved with dyesensitized solar cells may be attributed to the unique TiO2 nanocrystal porous film, which provides rapid electron transportation, a large surface area for adsorption of dye molecules, and electrical contact with the redox electrolyte. The TiO2 nanocrystal porous film is one of the puzzling components of the dye-sensitized solar cell. Many efforts have been aimed at the preparation and function of the TiO2 porous film.5–19 The ordered TiO2 film used in DSSCs was first prepared in a basic environment by Burnside et al. in 1998.20 The results show the shape and surface state of nanocrystal TiO2 particles are largely affected by the condition of the TiO2 colloid, and different TiO2 pillars could form by controlling the hydrothermal conditions. Zaban et al. reported a hydrothermal synthesis of TiO2 colloids in acidic solutions21 in which it is concluded that the TiO2 nanocrystal shows different growth mechanisms in a basic environment, which may result in different shapes of the TiO2 nanocrystal formed. Hore et al. compared the influence of acid/base conditions employed in the synthesis of TiO2 nanoparticles on the performance of DSSCs22 and found that the performance of DSSCs is affected by the preparation conditions of TiO2 nanoparticles. However, how to control the conditions to give TiO2 nanocrystals with a definite crystal shape and surface orientation to meet the requirements of DSSCs is still a crucial problem. In addition, the influence of crystal shape on the thermal stability of TiO2 nanocrystals and the photoelectric performance of DSSCs is rarely discussed. In this paper, two typical environmentssacidic solution and basic solutionswere chosen, and anatase TiO2 nanocrystals were * Author to whom correspondence should be addressed. Phone: +86-59522693899. Fax: +86-595-22693999. E-mail: [email protected].

prepared by a sol-hydrothermal method. The influences of preparation conditions on the crystal phase, crystal shape, and surface orientation of the TiO2 nanocrystals, as well as the photovoltaic performance of DSSCs, were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectrometer (FTIR), and differential scanning calorimetry thermogravimetric analysis (DSC-TG). It was found that the rod-like shaped anatase TiO2 prepared by the hydrothermal method from a tetramethylammonium hydroxide solution showed more of the {101} crystal face, better thermal stability, and better photoelectric performance of the DSSC than the ball-like shaped anatase prepared from a nitric acid solution.

2. Experimental Section Materials. Titanium(IV) isopropoxide and 4-tert-butylpyridine (TBP) were purchased from Fluka and used as received. Tetramethylammonium hydroxide aqueous solution (25%) was from Shanghai Chemical Reagent Co. China and was used without further purification. cisBis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) [RuL2(NCS)2] was from Solaronix SA, Switzerland. Other reagents were from Shanghai Chemicals Co. Ltd., China. Conducting glass plate fluorine doped tin oxide overlayer (FTO glass), sheet resistance 8 Ω/square purchased from Hartford Glass Co., was used as a substrate on which to precipitate a TiO2 porous film and was cut into 2 × 1.5 cm2 sheets. Preparation of TiO2 Nanocrystal Colloids. Two kinds of TiO2 nanocrystal colloids were prepared by an optimal hydrothermal method based on refs 8 and 20–22. The preparation procedure included the hydrolysis of titanium isopropoxide in deionized water, followed by filtration, peptization, aging crystallization, and hydrothermal processing. The main difference between the two kinds of TiO2 nanocrystal colloids in the preparation process was the kind of solution environment used in the peptization and hydrothermal process. Namely, one was in nitric acid solution, and the other was in tetramethylammonium hydroxide solution. To achieve the same crystal particle size and crystalline phase for both cases, small changes in the concentration of the medium and the autoclaving temperature were allowed. Twenty-five milliliters of titanium(IV) isopropoxide {Ti[OCH(CH3)2]4, Fluka, 99%} was evenly dissolved in 25 mL of isopropanol and slowly

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added into 150 mL of deionized water with vigorous stirring, which produced a white precipitate in the solution immediately. After the suspension solution was filtered and washed with deionized water adequately, a purified white precipitate was obtained. Under vigorous stirring, the precipitate was transferred to 150 mL of 0.5 M tetramethylammonium hydroxide solution with pH ) 10.5 or 150 mL of 0.1 M nitric acid solution with pH ) 1 to allow a peptization at 95 °C for 12 h, respectively. A translucent or transparent titanate colloid was thus obtained. Finally, the resulting colloid solution was subjected to a hydrothermal crystallization in the temperature range 210-270 °C for 12 h in a titanium autoclave. The resulting TiO2 colloid was concentrated with a rotatory evaporator at 40 °C until a viscous slurry with a concentration of TiO2 of 20 wt % was obtained. Then, under agitation, the emulsifier polyglycol M-20000 (20 wt % of TiO2) was added, and a stable TiO2 nanocrystal colloid was thus obtained. Assembling of the Dye-Sensitized Solar Cell (DSSC). A TiO2 nanoporous film was prepared by modifying the procedure from the refs 23 and 24. A conducting glass substrate was cleaned ultrasonically in a deionized water bath, and the TiO2 colloid paste was spread over the substrate with a doctor blading technique using adhesive tape as a spacer. The substrate was sintered at 450 °C for 30 min in air, which resulted in a TiO2 porous film about 10-µm thick on the conducting glass substrate. The diameter of the TiO2 particles estimated by TEM images was about 20 nm. X-ray diffractograms showed that the TiO2 film mainly consisted of anatase. The TiO2 substrate was immersed into a 3 × 10-4 mM RuL2(NCS)2 dye in ethanol solution for 24 h, which caused a sufficient adsorption of the sensitized dye RuL2(NCS)2 on the TiO2 film. After the substrate was adequately washed with anhydrous alcohol and dried in moisturefree air, a dye-sensitized TiO2 electrode was obtained. A DSSC was assembled by filling an electrolyte solution (0.6 M tetrapropylammonium iodide, 0.1 M iodine, 0.1 M lithium iodide, 0.5 M 4-tertbutylpyridine (TBP) in acetonitrille) between the dye-sensitized TiO2 electrode and a platinized conducting glass electrode. The two electrodes were clipped together, and a cyanoacrylate adhesive was used as sealant to prevent the electrolyte solution from leaking. Measurement. The X-ray analysis was done with an X-ray diffractometer (BRUKER D8 Advance, Germany) using Cu KR radiation (λ ) 1.5405 Å). The TEM and HRTEM spectra were measured with a JEM-2010 instrument (JEOL Ltd., Japan). The UV–vis diffuse reflectance spectrum was measured by using a UV–vis 3100 spectrophotometer (Shimadzu Corporation, Japan) using BaSO4 as background. FTIR spectroscopy of samples in KBr pellets was performed using a Nicolet Impact 410 spectrometer. The crystallization behavior was monitored with a Netzsch STA449C TG-DSC instrument in the temperature range from room temperature to 1000 °C. The photovoltaic test of DSSCs was carried out by measuring the I-V character curves under irradiation of white light from a 100 W xenon arc lamp (XQ-500W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere. The fill factor (FF) and the overall lightto-electrical energy conversion efficiency (η) of DSSCs were calculated according to the following equations:25 η (%) )

VmaxJmax VocJscFF × 100% ) × 100% Pin Pin FF )

VmaxJmax VocJsc

(1) (2)

where JSC is the short-circuit current density (mA · cm-2), VOC is the open-circuit voltage (V), Pin is the incident light power, and Jmax (mA · cm-2) and Vmax (V) are the current density and voltage at the point of maximum power output on the J-V curves, respectively.

3. Results and Discussion Crystalline Phase and Morphology. Figure 1 presents the XRD patterns of samples. Before hydrothermal crystallization, the two kinds of TiO2 colloids coming from either nitric acid solution or tetramethylammonium hydroxide solution are mainly composed of an amorphous phase and a certain amount of anatase with a low crystallization degree [Figure 1 (I a) and (II h)]. But after hydrothermal crystallization, TiO2 from both

Figure 1. XRD patterns of TiO2 nanocrystals prepared by the hydrothermal method: (I) in nitric acid solution [(a) titania colloids before hydrothermal treatment; (b) hydrothermal crystallization for 14 h at 200 °C, (c) 220 °C, (d) 240 °C, (e) 250 °C, (f) 260 °C, and (g) 270 °C] or (II) in tetramethylammonium hydroxide solution [(h) titannia colloids before hydrothermal treatment; (i) hydrothermal crystallization for 14 h at 210 °C, (j) 230 °C, (k) 250 °C, and (l) 270 °C]. 1, anatase; b, rutile; [, brookite.

solutions under any hydrothermal temperatures is mainly composed of anatase phase, a small amount of rutile phase, and a minimum amount of brookite phase. According to the Scherrer formula, the average diameter of TiO2 particles along the [101] direction of anatase is calculated as about 15 nm in the two cases with the hydrothermal temperature at 230 °C. TEM and HRTEM bright field images of the TiO2 nanocrystals after a hydrothermal process are shown in Figure 2. TiO2 nanocrystals show different crystal shapes in the two cases: a tetragonal ball-like shaped TiO2 is obtained in nitric acid solution, and a bipyramid rod-like shaped TiO2 is obtained in tetramethylammonium hydroxide solution. According to the calculations by Penn and Zhang,26,27 the TiO2 nanocrystal with the bipyramid rod-like shape is more stable than the other shapes. All HRTEM images show the existence of anatase {101} crystal faces, which is consistent with the calculation by Oliver et al.28 showing that the {101} crystal faces of anatase have a lower surface energy and are expected to be more stable than the others. Anatase-to-Rutile Phase Transition. The stabilities for the different shapes of TiO2 nanocrystal are confirmed by their anatase-to-rutile phase transition temperature. Figure 3 shows the anatase-to-rutile transition curves for the two kinds of TiO2 nanocrystals. The percentage of rutile phase is calculated with the semiquantitative analysis program in Philips X′Pert HighScore using the JCPDS PDF-2 database29 according to the following formula: WR(%) )

1 × 100 1 + 0.8(IA ⁄ IR)

(3)

where WR is the weight percentage of rutile in the TiO2 sample and IA and IR are the intensities of the anatase {101} and rutile {110} peaks, respectively. It can be observed that the anatase-to-rutile phase transition is complete at 950 °C for the rod-like anatase prepared from

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Figure 2. TEM and HRTEM bright field images of TiO2 nanocrystals prepared by hydrothermal crystallizing at 240 °C (a, c, and e) in nitric acid solution or at 270 °C (b, d, and f) in tetramethylammonium hydroxide solution.

Figure 3. Percentage of rutile in the TiO2 sample at different thermal treatment temperatures for 12 h (measured by XRD).

the tetramethylammonium hydroxide solution and is complete at 700 °C for the ball-like anatase prepared from the nitric acid solution. A high phase transition temperature for the rod-like anatase means a better thermal stability than that for the balllike anatase. The different transition temperatures may be attributed to variance in either the crystal shape or the surface orientation of the two kinds of nanocrystals which come from different preparation conditions. The phase transition from anatase to rutile also can be estimated from the change of anatase average diameter along

Figure 4. Average diameters of the two shapes of anatase along the [101] direction vs the thermal treatment temperatures.

the [101] direction with the thermal treatment temperature. As Figure 4 shows, the average diameter of TiO2 prepared by the nitric acid route increases remarkably from about 14 to 40 nm when the thermal treatment temperature increases from 450 to 650 °C, but for the TiO2 prepared by the tetramethylammonium hydroxide route, the average diameter only increases from 14 to 21 nm, which means that the anatase prepared in tetramethylammonium hydroxide solution presents a better thermal stability than the one prepared in nitric acid solution does. In

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Figure 5. DSC-TG curve of the two TiO2 shapes under flowing argon. Figure 6. The anatase-to-rutile phase transition for two kinds of anatases.

addition, it is concluded that the transition from anatase to rutile markedly starts at 40 nm of the anatase average diameter along the [101] direction for the two kinds of anatase nanocrystals. According to the above, the phase transition of anatase-to-rutile is related tightly to the crystal shape and particle size. The phase transition behaviors of the two shapes of TiO2 are also investigated by DSC-TG. First, the samples of the two shapes of TiO2 were calcined at 450 °C for 1 h to remove any impurities, so as to eliminate or reduce confusion, and then they were naturally cooled down to room temperature. Subsequently, the phase transition behavior for the two shapes of TiO2 was monitored with a TG-DSC instrument in the temperature range from room temperature to 1000 °C in an argon flow with a heating rate of 10 °C/min. The results are displayed in Figure 5; the two shapes of TiO2 show different DSC behaviors. From Figure 5, an endothermic peak can be seen at 675 °C for ball-like TiO2 and an exothermic peak can be seen at 890 °C for rod-like TiO2. It is believed that two transitions occur during the heating process: one is the endothermic crystal growth of a TiO2 grain, and the other is the exothermic phase transition from anatase to rutile. The trends of DSC curves are determined by the two transitions together. When the temperature is low, the thermal effect may be mainly determined by the growth of the crystal grain, but at high temperature, it is mainly determined by the phase transition. The DSC results are approximately accordant with Figure 3, in that the phase transition is complete at 950 °C for the rod-like anatase and at 700 °C for the balllike anatase, and with Figure 4, in that the crystal grain obviously grows up at 560 °C. In addition, the TG curve shows a weight loss less than 1 wt % for the two shapes of TiO2; the difference in the DSC results may be mainly attributed to the influence of crystal shape on particles of the same size. Based on the above discussions, the phase transition for the two kinds of anatases can be illustrated in Figure 6. For the anatase prepared in tetramethylammonium hydroxide solution, the strong alkalinity makes a solution-precipitate mechanism highly activated,30 oxygenated surfaces are obtained, and elongated rod-like anatase is formed and grows rapidly along the [100] and [101] directions. However, in nitric acid solution, the anatase nanocrystal presents hydrogenated surfaces. A second coarsening mechanism may play a key role,31 which leads to the depression of the growth rate along the [101] direction: other stable crystal faces such as {103} and {221} grow rapidly, and ball-like anatase can be formed finally. From Figure 6, it can be seen that the crystal has to deform along the

Figure 7. FTIR spectra of two shapes of anatase nanocrystals. The samples were sintered at 550 °C for 2 h to remove any impurites.

[101] direction to realize the transition from anatase to rutile. Due to the more stable {101} and {100} crystal faces in the rod-like anatase, the deformation is restrained, and the average diameter along the [101] direction increases slowly with the increase of temperature. However, for the ball-like anatase, the unstable crystal faces {100}, {103}, and {221} make the deformation easily, so the anatase-to-rutile transition temperature is lower. The above explanation is accordant with the calculated results of Oliver et al.28 A comparison of the FTIR spectra of ball-like titania and rod-like titania is made to identify the difference in surface orientation and structure between the two kinds of titanias, and the results are shown in Figure 7. The rod-like titania shows a similar IR absorption to that of the ball-like titania, which suggests that the two shapes of titania have a similar anatase crystalline phase, which is consistent with the XRD results. The Influence on the Performance of the DSSC. The crystal phase, morphology, and surface orientation of the TiO2 nanocrystals are important for dye-sensitized solar cells. The adsorptive capacity for the sensitizer, the electron transportation and recombination processes, and the reflectivity of the TiO2 film are all tightly related to the performance of the DSSC. To eliminate the influence of dye loading and light harvesting on the photoelectric performance of the DSSC, the UV–vis diffuse reflectance spectra of dye-sensitized TiO2 films with different anatase shapes were measured; only the dye-sensitized TiO2

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Figure 8. (a) UV–vis reflecting spectra of a dye-sensitized TiO2 electrode with different anatase shapes. (b) Voltage-current curves of DSSCs with two kinds of anatase nanocrystals. The electrolyte consists of 0.6 M tetrapropylammonium iodide, 0.1 M iodine, 0.1 M lithium iodide, and 0.5 M 4-tert-butylpyridine in acetonitrile; the incident light intensity is 100 mW · cm-2; and the active area of the DSSC is about 1 cm2.

electrodes with a similar reflectivity (shown in Figure 8a) were chosen to construct DSSCs. The DSSCs were fabricated32,33 by using the above two kinds of anatases, and their photovoltaic performances are presented in Figure 8b. These results show that the DSSC with the rod-like anatase produces a light-to-electrical energy conversion efficiency of 6.2%, and the DSSC with the balllike anatase obtains an efficiency of 5.4%. Obviously, the rod-like anatase is more preferable for DSSCs than the balllike anatase; this conclusion is accordant to the previous reports of Hore et al.22 The good performance of DSSCs fabricated with rod-like anatase may come from the crystal morphology and surface orientation of the anatase. For a similar crystal particle diameter along [101], the rod-like shaped TiO2 crystal has less surface tension and more defects and dislocations than the ball-like one has. Thereby, compared to the ball-like anatase, the rod-like anatase may have a few active spots22,30 which can capture the electrons injected from the dye photoexcited state to the anatase conducting band, reduce the annihilation of electrons, and increase the electron concentration of the photoanode; thus, the transportation quantum efficiency of the anatase film is enhanced. In addition, the contact resistance between TiO2 particles may be affected by the crystal shape of TiO2; as shown in Figure 8b, the DSSC consisting of ball-like anatase showed a lower open circuit voltage and closed circuit current than the one with rod-like anatase does, which indicates a greater Ohmic loss for ball-like anatase. This may also be ascribed to the better performance of rod-like anatase compared to ball-like anatase. Further experiments to clarify this mechanism are in progress.

orientation also affect the photoelectric performance of dyesensitized solar cells (DSSCs) based on anatase; a 6.2% lightto-electrical energy conversion efficiency is obtained from DSSCs with rod-like anatase, and a 5.4% efficiency is obtained from DSSCs with ball-like anatase, which means that the anatase TiO2 nanocrystal prepared in tetramethylammonium hydroxide should be preferred for DSSCs. Acknowledgment. The authors thank the National Natural Science Foundation of China (No. 50572030, No. 50372022) and the Nano Importance Scientific Special of Fujian Province, China (No. 2005HZ01-4), for joint support.

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4. Conclusions

(14)

Anatase TiO2 nanocrystals were synthesized by hydrolysis of titanium isopropoxide followed by crystal growth under hydrothermal conditions in an acid or basic environment. Anatase TiO2 nanocrystals prepared in tetramethylammonium hydroxide solution contain more {101} faces and present a bipyramid rod-like shape in comparison with the anatase TiO2 nanocrystals prepared in nitric acid solution with a tetragonal ball-like shape. The differences in crystal shape and surface orientation affect the anatase-to-rutile phase transition temperature; it is about 950 °C for rod-like anatase and 700 °C for ball-like anatase. The different anatase shape and surface

(15) (16) (17) (18) (19) (20) (21)

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