Growth Mechanism of TiO2 Nanotube Arrays by Etching Treatment and Their Photoelectric Property

SUN Qiong, YOU Di, ZANG Tao, WU Song-hao, HONG Yong， DONG Li-feng

(1. College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China; 2. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350116, China)*Corresponding Author, E-mail: sunqiong@qust.edu.cn

Abstract: In this research, densely aligned TiO2 nanorods are etched into nanotubes by hydrochloric acid, and the growth mechanism is also supposed. During the etching process, the dents appear from the top down and inside out along the growth direction of the nanorod, and finally form the tube structure. Actually, the nanotube with square hollow cross section consists of a mass of surrounded fine nanowires, which will be teared into independent ones at high temperature. When assembled into dye sensitized solar cells(DSSCs), the photoelectrical conversion efficiency of TiO2 nanotubes is much higher than that of nanorods, and the highest value(3.26%) is located at the etching temperature of 140 ℃. It could be deduced that the increased specific surface area and length shortening of the nanotube play the positive and negative effect on the photoelectrical property, respectively. Furthermore, the calcination effect on the structure and photovoltaic property of TiO2 nanotubes is also carried out. However, the fracture and aggregation of nanotubes could be observed after heating treatment, which therefore increase the difficulty of photo-induced carriers in directional transfer and also reduce the photoelectrical activity.

Key words: TiO2 nanotube; etching treatment; growth mechanism; photoelectric property

1 Introduction

As one of the promising alternatives to the conventional silicon solar cell, dye-sensitized solar cells (DSSCs) have attracted immense interests in scientific and industrial research owing to their low material and production cost and high photoelectrical conversion efficiency[1-3]. Some n-type semiconductor metal oxides such as TiO2, ZnO[4-6], Fe2O3[7]and WO3[8]are suitable for the photoanode materials. Due to the strong affinity to dye molecules and high chemical stability, TiO2based DSSCs have attracted the most attention and shown excellent performance in photoelectrical conversion[9]. As a result, special shaped TiO2nano-materials such as nanosphere, nanorod, nanotube, nanowire, and nanosheet,etc. have been already prepared from various ways[10-14].

Compared to other counterparts, the one-dimensional(1-D) TiO2with directional arrangement such as nanotube, nanorod or nanowire arrays owning precisely oriented nature and excellent electron percolation pathways can offer a direct electrical channel of photo-generated charge that suitable for electron transfer, thus has attracted plenty of attention and been widely employed in photoelectrochemical cells[15], photocatalytical treatments[13], as well as water-splitting reactions[16]. Among these 1-D architectures, the TiO2nanorod array(NRA) consist of mono-crystalline is studied firstly and mostly because of their superior chemical stability, excellent electron transport properties, structure controllability and low cost[17-19]. Various chemical techniques have been reported for preparing mono-crystallized TiO2NRAs, including chemical vapor deposition (CVD)[20-21], metal-organic chemical vapor deposition method(MOCVD)[22], hydrothermal method[23-24], chemical bath deposition[25], and so on. Since Liu[26]introduced a direct hydrothermal method for the growth of oriented, single-crystallized rutile TiO2nanorod film on FTO conductive substrates, many related studies of DSSCs have been developed focusing on thisin-situhydrothermal process[27]. However, owing to the relative low surface area of rutile TiO2NRAs, the photoelectrical conversion efficiency of DSSCs based on such photoanodes is significantly confined. Liuetal.[28]reported an anisotropic corrosion strategy for the transformation of TiO2nanorods into nanotubes with hydrochloric acid as the etching agent under hydrothermal condition. As a result, the surface area of received 1-D TiO2nanotubes was effectively enlarged. Lvetal.[29]reported the vertically aligned single-crystallized rutile TiO2NRAs with large internal surface area that prepared directly on FTO substrates by a facile two-step hydrothermal process, and a photoelectrical efficiency of 5.94% was finally achieved on the DSSC assembled with the etched TiO2NRAs, which was much higher than that with untreated NRAs(3%).

In this study, vertically aligned single-crystalline rutile TiO2nanotube films with large surface area were prepared from a modified two-step hydrothermal process, using concentrated hydrochloric acid as the etching agent. The perovskite CsSnI2.95F0.05was synthesized and utilized as the solid electrolyte for the fabrication of the all-solid-state dye-sensitized solar cell. Then the effects of the etching and calcination treatment on the photoelectric conversion efficiency were discussed. In addition, the growth mechanism of the TiO2nanotube was also assumed, and the effect of different additives during the second hydrothermal process was also clarified.

2 Experiment

2.1 Synthesis of TiO2 Nanotubes

The TiO2nanorod array films were prepared directly on transparent conductive fluorine-doped tin oxide(FTO) substrates by a modified hydrothermal method[26]. In a typical synthesis process, 15 mL of deionized(DI) water was mixed with 15 mL of concentrated hydrochloric acid(36.0%-38.0%) in a Teflon-lined stainless steel autoclave. The mixture was stirred for 10 min under ambient conditions, and then 0.6 mL of butyl titanate was added to the mixed solution and stirred for another 10 min. A piece of FTO substrate(4 cm×2.5 cm) was placed at an angle against the wall of the Teflon-liner with the conducting side facing down. Hydrothermal synthesis was conducted at 150 ℃ for 4 h in an oven, and the autoclave was then cooled to room temperature in air.

The TiO2nanotubes were synthesized by a second hydrothermal process with hydrochloric acid as the etching agent[29]. The as-prepared film on FTO substrate was immersed into the autoclave containing 15 mL DI water and 15 mL concentrated hydrochloric acid(36.0%-38.0%). The chemical etching reaction was conducted by hydrothermal method at 120-200 ℃ for 3 h. The received sample was rinsed with DI water extensively and dried in air. Moreover, the sintering treatment was set at 200-500 ℃ for 3 h, respectively.

2.2 Characterization

Morphological information of the samples was observed by field-emission scanning electron microscope(FESEM, JEOL JSM-6700F) and transmission electron microscope(TEM, JEOL JEM-2100). The crystal structure was examined by X-ray diffraction (XRD, Rigaku, D/MAX-2500/PC) using Cu Kα as X-ray radiation source(40 kV, 100 mA) and scanning from 20° to 90°.

2.3 Fabrication of DSSCs

DSSCs were fabricated with the TiO2film grown on FTO as the photoanode. The photoanode was immersed in a 0.3 mmol/L ethanol solution of cis-bis (isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(Ⅱ) bis-tetrabutylammo-nium dye (N719) for 12 h in the dark. After the adsorption equilibrium reached, the samples were taken out and rinsed with deionized water to remove surface dissociative dye. The platinum-coated FTO substrate was used as the counter electrode, which was prepared by thermal decomposition of H2PtCl6solution in isopropanol sintered at 400 ℃ for 20 min. The perovskite CsSnI2.95F0.05dissolved in N,N-dimethylformamide(DMF) was filled into the space between the two electrodes as the electrolyte, which was prepared according to our previous report[30]. Until the DMF was totally evaporated at room temperature, the all-solid-state DSSC was ready.

2.4 Photovoltaic Measurement

Photocurrent-voltage(J-V) curves of DSSCs were tested on a CHI760D electrochemical working station. The illumination was provided by a solar simulator(150 W, Newport 96000) with air mass 1.5 global filter(AM 1.5G, Newport 81094), which was calibrated by a high power thermal detector(Physcience Opto-Electronics, Beijing, LP-3A). The light power density was set as 100 mW/cm2to simulate the sun illumination. The active surface area of cells in our test is around 0.5 cm2with circular shading mask. For each sample, the best result among nine parallel sample cells was recorded.

3 Results and Discussion

3.1 Effect of Etching Temperature

The typical FESEM images of the TiO2nanorods and nanotubes are shown in Fig.1. It can be seen that densely aligned TiO2nanorod grew uniformly on the FTO substrate after 4 h hydrothermal reaction at 150 ℃(Fig.1(a)). The nanorods were tetragonal in shape with square top facets and little space exists between adjacent TiO2nanorods. When etched at 120 ℃ for 3 h, the top of the nanorods began to cave and became rough(Fig.1(b)). Many bulges appeared at the top of each individual TiO2nanorod and apparent space between adjacent bulges can be observed. While the etching temperature increased to 140 ℃, the nanorods converted thoroughly into tubular structure with square cross section (Fig.1(c)). Furthermore, it can be found that the tube wall was assembled by a mass of surrounded fine TiO2nanowires. While etched at 180 ℃, the wall of nanotubes became thinner and some of TiO2nanotubes were even teared into independent nanowires (Fig.1(d)). After etching at 200 ℃, most of the nanotubes were totally destroyed and gathered into nanowire clusters (Fig.1(e)). The effect of etching temperature on the length of nanotubes was also studied and summarized in Fig.1(f). From 120 ℃ to 200 ℃, the average lengths decreased significantly from 1.59 μm to 1.00 μm, which were all much lower than that of untreated TiO2nanorods(1.81 μm). Moreover, from the FESEM images, the nanotube wall also became thinner and thinner with the etching temperatures. As a result, the etching step greatly destroyed the solid TiO2nanorods, which happened from the top down and inside out by the driving force. Meanwhile, the nanorods also broke from the centre of the top during the corrosion process.

Fig.1 FESEM images of untreated TiO2nanorods(a) and TiO2nanotubes etched at 120 ℃(b), 140 ℃(c), 180 ℃(d) and 200 ℃(e), respectively. (f) Average length of TiO2nanorods and nanotubes etched at different temperatures.

In order to clarify the effect of hydrochloric acid, the second hydrothermal process(140 ℃, 3 h) was also carried out in the presence of NaOH aqueous solution(pH=12) or pure deionized water, respectively. As a result, both of the products still kept in nanorod structures(Fig.2(a) and (b)), proving the etching effect of hydrochloric acid on TiO2nanorods during the hydrothermal reaction.

Fig.3 showed the XRD patterns of the TiO2nanotubes etched at different temperatures, as well as the untreated TiO2nanorods for comparison. It confirmed that either TiO2nanorods or nanotubes were both formed in rutile phase(PDF No.21-1276), meaning no phase transformation occurred during the etching process. Compared with the polycrystalline rutile TiO2, the (002) diffraction peaks of TiO2nanorod and nanotube arrays were significantly enhanced, while some diffraction peaks such as (110), (111) and (211) were absent. According to the references[26,29]for the preparation of TiO2nanorod and nanotube arrays and the HRTEM images (Fig.4(c) and (d), vide infra), the received TiO2nanorod or nanotube arrays were both formed in mono-crystal rutile phase and grown along [001] direction with the growth axis parallel to the substrate surface normal, which means that the nanorods and nanotubes are not only aligned but also monocrystalline throughout their length. As a result, the diffraction peaks of TiO2arrays in this research are not well corresponded with the standard XRD pattern of rutile TiO2(PDF No. 21-1276). In addition, the intensity of (101) diffraction peak kept nearly unchanged, while the (002) diffraction peak weakened gradually with etching temperatures, which indicated that the growth along [001] direction was destroyed by etching treatment. According to our previous work[30], in the first hydrothermal step, TiO2nanorods formed preferentially along [001] direction with the growth axis perpendicular to the FTO substrates. From the characterization of FESEM and XRD, it could be proved that the destruction to the nanorods by hydrochloric acid also occurred from [001] direction, and finally produced hollow and shortened nanotubes.

Fig.2 FESEM images of TiO2obtained from the second hydrothermal process in NaOH aqueous solution (pH=12)(a) and deionized water(b), respectively.

Fig.3 XRD patterns of TiO2nanorods and nanotubes etched at different temperatures

In addition, transmission electron microscope (TEM) and high-resolution TEM(HRTEM) were also carried out. Compared to TiO2nanorods (Fig.4(a)), a V-shaped hollow top appeared in TiO2nanotube(Fig.4(b)). It is confirmed that the nanotube owns a much higher specific surface area than that of the corresponding nanorod precursor. From HRTEM images, two orthogonal interplanar spacings of (0.32±0.01) nm and (0.29±0.01) nm could be found both from the nanorod and nanotube(Fig.4(c) and (d)), corresponding to thed-spacing of the rutile (110) and (001) planes respectively, which further proved that the long axis growth direction was along [001] direction. From the report, the growth rate of different crystal faces in rutile TiO2nanorods follows the order: (001)>(101)>(100)>(110)[31]. When the nanorods were treated by hydrochloric acid during the second hydrothermal process, the (001) planes should be thus preferentially etched[32]. The decreasing concentration of hydrochloric acid along the [001] direction in the confined space of the TiO2nanotube and the increased exposure to corrosion for the previously eroded side wall induced the formation of V-shaped structure[28].

Fig.4 TEM and HRTEM images of TiO2nanorods(a， c) and nanotubes(b， d)

Fig.5 displays the photocurrent density-voltage curves of the dye sensitized solar cells assembled with TiO2nanotubes etched at different temperatures, and the corresponding photovoltaic parameters including open-circuit voltages(VOC), short circuit current densities(JSC), conversion efficiencies(η) and fill factors(FF) are summarized in Tab.1. Once the nanorods were etched into nanotubes at 120 ℃ to 200 ℃, the photovoltaic property of the solar cell was improved first and then worsened, with the optimal photoelectrical conversion of 3.26% at 140 ℃. When the configuration of nanotube formed in TiO2, the highly expanded specific surface area was beneficial for the adsorption of dye and absorption of incident photons, and also supplied more pathways for the photo-induced charge transfer. Therefore, a solar cell with modified photoelectrical performance was obtained. However, the etching treatment would simultaneously destruct the orientated growth of TiO2arrays and thin the photoanode, especially at high temperature, which thus decreased the dye adsorption, the absorption to the incident irradiation, and also the yield of the photoelectron-hole pairs. Furthermore, the adhesion between the film and FTO substrate became weak as the growth destruction of TiO2, resulting in a higher resistance for the transfer of photogenerated carriers. In summary, the etching treatment to TiO2nanorod would make a positive contribution to the photoelectrical activity within a certain temperature range. When HCl was absent during the second hydrothermal process, it has been proved that the etching process couldn’t occur and thus the photoelectrical conversion kept nearly unchanged (1.65%) to TiO2nanorod (1.63%). Moreover, the addition of NaOH in the second hydrothermal process would greatly reduce the photoelectrical conversion (0.38%). According to our precious work[33], this decline is because more hydroxyl groups on the surface of TiO2can be produced during the alkaline hydrothermal treatment than the acidic condition, which can change the interface structures between TiO2and dye molecules, and finally decrease the photoelectrical activity.

Fig.5 Current density-potential(J-V) curves of the DSSCs assembled with TiO2nanorods and nanotubes etched at 140 ℃ and 200 ℃ under the irradiation of solar simulator

Tab.1 Photovoltaic parameters of DSSCs with TiO2nanorods and nanotubes etched at different temperatures

T/℃JSC/(mA·cm-2)VOC/VFF η/%Nanorod120140140?140??16018020011.2717.8320.7016.622.3619.3219.8014.020.500.530.530.510.610.580.570.510.290.270.290.190.260.290.280.351.632.593.261.650.383.203.192.48

*and**: The solar cells were fabricated with the nanorods that treated with deionized water and NaOH aqueous solution (pH=12) during the second hydrothermal process, respectively.

3.2 Effect of Calcination Treatment

The calcination treatment of TiO2was usually used to improve the crystallinity degree and also photoactivity in many reports[30,34]. In the following discussion, unless noted otherwise, the etching process was carried out at 140 ℃ for 3 h. When higher than 500 ℃, the FTO substrate would melt and bend, thus the highest temperature was set at 500 ℃. As the calcination temperature increased from 200 ℃ to 500 ℃, in FESEM images(Fig.6), TiO2nanotubes still attached to FTO substrate. However, the fracture of nanotubes could be observed from embedded figures, which resulted in the decreased average length. Meanwhile, the hollow part in nanotube became much smaller than that of unsintered sample, indicating that the surrounding nanowires tended to aggregating together during the heating process.

Fig.6 FESEM images of the top and cross-sectional views of TiO2nanotubes sintered at 200 ℃(a), 400 ℃(b) and 500 ℃(c) for 3 h.

According to XRD patterns(Fig.7), the diffraction peaks of sintered samples were still indexed to rutile TiO2(PDF No.21-1276). Furthermore, the intensity of all peaks kept nearly the same with the unsintered sample, which proved that although the tubular structure shrunken at high temperature, the crystallinity formed from the epitaxial growth on FTO during the hydrothermal process was relatively stable.

Fig.7 XRD patterns of TiO2nanotubes sintered at different temperatures. From bottom to top: unsintered, sintered at 200, 400, 500 ℃, respectively.

The photovoltaic properties of the solar cells containing sintered TiO2nanotubes were tested and summarized in Tab.2. After the sintering treatment, the values ofJSCandηboth decreased rapidly compared to the unsintered sample. According to the morphology and crystal characterizations in Fig.6 and 7, the most obvious change resulted from the sintering treatment was the break and aggregation of the nanotubes, which both raised the difficulty of the directional transfer of photo-generated electrons and finally decreased the photoelectrical conversion efficiencies.

Tab.2 Photovoltaic performance of DSSCs fabricated with sintered TiO2nanotubes

T/℃JSC /(mA·cm-2)VOC/VFFη/%Unsintered20030040050020.7010.139.516.056.010.530.620.610.640.650.290.260.260.370.333.261.641.521.441.29

4 Conclusion

In this research, the monocrystalline rutile TiO2nanotube arrays on FTO substrate were prepared with hydrochloric acid as the etching agent at different temperature. The morphology and crystal structure were tested by FESEM, XRD and HRTEM, and their effect on the photoelectrical performance of the DSSC that fabricated with the TiO2nanotube was also discussed. During the etching process, the transformation of the nanorod into nanotube gradually occurred from top down and inside out, accompanying with the shortening of the length and thinner of the nanotube wall simultaneously, while the higher the etching temperature, the clearer the nanotube structure. However, when etched at high temperature (over 200 ℃), the nanowires that surrounded connected to form the nanotubes were teared into independent ones. The enhanced surface area of the nanotube that generated from the etching treatment would contribute to the improvement of the photovoltaic property of the assembled DSSC, while the length shortening of the arrays was not favorable. As a result, the TiO2nanotube with the best performance in DSSC (ηwas 3.26%) was prepared at 140 ℃. In addition, the calcination effect on the structure and photoelectrical activity of TiO2nanotube was also studied. Unfortunately, both the broken and agglomeration of TiO2nanotubes occurred during the heating process, which thus dramatically decreased the photoelectrical conversion efficiency of the assembled DSSC.