Hydrothermal Etching Treatment to Rutile TiO2 Nanorod Arrays for Improving the Efficiency of CdS-Sensitized TiO2 Solar Cells
- Jingshu Wan†1,
- Rong Liu†1,
- Yuzhu Tong1,
- Shuhuang Chen1,
- Yunxia Hu1,
- Baoyuan Wang1Email author,
- Yang Xu1 and
- Hao Wang1Email author
© Wan et al. 2016
Received: 20 October 2015
Accepted: 5 January 2016
Published: 12 January 2016
Highly ordered TiO2 nanorod arrays (NRAs) were directly grown on an F:SnO2 (FTO) substrate without any seed layer by hydrothermal route. For a larger surface area, the second-step hydrothermal treatment in hydrochloric acid was carried out to the as-prepared TiO2 NRAs. The results showed that the center portion of the TiO2 nanorods were dissolved in the etching solution to form a nanocave at the initial etching process. As the etching time extended, the tip parts of the nanocave wall split into lots of nanowires with a reduced diameter, giving rise to a remarkable increase of specific surface area for the TiO2 NRAs. The TiO2 films after etching treatment were sensitized by CdS quantum dots (QDs) to fabricate quantum dot-sensitized solar cells (QDSSCs), which exhibited a significant improvement in the photocurrent density in comparison with that of the un-treated device, this mainly attributed to the enhancement of QD loading and diffused reflectance ability. Through modifying the etching TiO2 films with TiCl4, a relatively high power conversion efficiency (PCE) of 3.14 % was obtained after optimizing the etching time.
KeywordsQuantum dot-sensitized solar cells Cadmium sulfide TiO2 nanorod arrays Hydrothermal etching
Recently, quantum dot-sensitized solar cells (QDSSCs) have attracted much interesting research attributed to their unique advantages involving low cost and high theoretical conversion efficiency [1–3]. In typical configuration of QDSSCs, inorganic semiconductor quantum dots (QDs) such as CdS [4–6], CdSe [7, 8], CdTe [9, 10], and PbS  were usually used as sensitizer and exhibited huge advantages over organic dyes, such as low cost, high molar extinction coefficient, size-dependent band gap, and multi-exciton generation effect [12–14]. In addition, TiO2 semiconductor as the most successful photoanode material was served as a scaffold layer to adsorb QDs and a medium layer to transport a photo-generated electron. Therefore, the specific surface area and the electron mobility of TiO2 photoanode play a key role on the photovoltaic performance of QDSSCs. The electron mobility is defined as the drift velocity of electrons under the driving force of an extra electrical field. Hendry et al. had demonstrated that electron mobility is strongly dependent on the material morphology in nanostructured polar materials due to local field effects . In order to speed up the electron mobility and decrease the possibility of photo-generated charge recombination, 1D nanostructures such as nanotubes , nanorods [17, 18], and nanowires  were utilized as photoanode for QDSSCs, which supplied direct electrical pathways and facilitate electron transportation; this was considered as a powerful strategy to reduce the electron–hole recombination which abundantly existed in TiO2 nanoparticle-based solar cell. Among these 1D architectures, researchers had paid much attention to the rutile TiO2 nanorod arrays (NRAs) due to their superior electrical transport performance, excellent chemical stability, high refraction index, and cheap product cost [20–22]. However, it has a vital shortcoming, i.e., small surface area which results in poor QD loading. Thus, the QDSSCs fabricated from the rutile TiO2 NRAs exhibited poor photovoltaic performance. A lot of methods have been reported for enlarging the specific surface area of the rutile TiO2 NRAs. For example, Lv et al. developed a feasible etching treatment to TiO2 NRAs in hydrochloric acid solution under hydrothermal condition, which induced the compact TiO2 nanorods split into lots of small nanowires; thus, this method significantly improves the surface area of the TiO2 films and in turn, allowed superior dye-loading capacity of the TiO2 photoanode, a highest power conversion efficiency (PCE) of 7.91 % was achieved from the DSSCs assembled by the etching TiO2 films [23, 24]. From that on, the etching treatment was considered as a powerful strategy for enlarging the inner surface area of the TiO2 NRAs. Yuan et al. synthesized long single-crystalline rutile TiO2 NRAs with a high surface area by combining a mild hydrothermal method with a chemical etching route, and the DSSCs constructed by 7-h-etched TiO2 NRAs exhibited a PCE of 4.69 % . Chen’s group fabricated an ultralong TiO2 NRAs (17.6 μm) with a large inner surface area by using a hydrothermal method and post-etched with hydrochloric acid at high temperature. Such TiO2 NRAs were utilized as photoanode for CdS/CdSe co-sensitized solar cells and reached a maximum value of 17.22 mA/cm2, yielding the highest PCE of 2.66 % . Huang et al. grown polycrystalline TiO2 NRAs on FTO substrate and in situ converted NRAs into nanotube arrays (NTAs) by hydrothermal etching. After conversion, more CdSe QDs can be filled in the NTAs, so the PCE of QDSSCs increases by 60 %. QDSSCs with half-etched TiO2 nanotubes achieved the best conversion efficiency of 2.44 % .
In our work, the moderate etching treatment was introduced to short TiO2 NRAs (3.6 μm) and the CdS QDs prepared by successive ionic layer adsorption and reaction (SILAR) was used as the single sensitizer. We focused on the effect of etching time toward QDSSC performance and the underlying reason. Through optimizing the etching time, a PCE of 3.14 % was obtained after TiCl4 modifying under the illumination of 100 mW/cm2 AM 1.5G solar simulators, this is a relative good performance for CdS-sensitized TiO2 NRAs solar cells.
Hydrothermal Synthesis of TiO2 NRAs
Hydrothermal synthesis reported by Liu and Aydil was employed for the growth of highly aligned rutile TiO2 NRAs . In a typical synthesis, 8 ml deionized water (DI) and 8 ml hydrochloric acid (HCl) with 36.5–38 wt% concentration were mixed together and stirred for 5 min to achieve a homogeneous solution, then 220 μl of titanium butoxide were added into this solution as titanium precursor, followed by another stir until the titanium butoxide was completely dissolved in the hydrochloric acid solution. Finally, the resultant solution was poured into a Teflon-lined container sealed by a stainless steel autoclave (25-ml volume). A piece of FTO glass with a size of 2 cm × 2 cm was used as substrate and leaned on the wall of the reactor with an angle and the conductive side of the FTO substrate faced down. The reactor was transferred into an oven with a temperature at 150 °C maintained for 10 h. While waiting for hydrothermal synthesis to finish and the reactor to cool down to room temperature, the FTO substrate grown by TiO2 NRAs was taken out, carefully washed by DI water, and dried using N2 gas flow. Finally, the resultant TiO2 films were suffered from an annealing at 500 °C for 2 h under air condition.
Etching Treatment to the TiO2 NRAs
Seven milliliters of deionized water was mixed with 9 ml HCl to form an etching solution, then the mixture was transferred into a Teflon-lined container and the TiO2 NRAs synthesized by a hydrothermal method were immersed in the etching solution, and the Teflon-lined container was sealed by a stainless steel autoclave. The hydrothermal etching treatment was conducted at 150 °C for 4–6 h. After the autoclave cooled down, the TiO2 films on the FTO substrate was taken out from the reactor and immersed in DI water for 2 h to remove the residual acid.
TiCl4 Treatment to the Etching TiO2 Films
The etching TiO2 films were further modified by TiCl4 aqueous solution. In the typical treatment process, the etched TiO2 films were immersed in 0.3 M TiCl4 aqueous solution at 70 °C for 30 min. After modification, the samples were extensively rinsed with absolute ethanol and followed by 500 °C annealing treatment for 1 h in air.
Deposition CdS QDs onto TiO2 Photoanode
The SILAR method was used for the preparation of CdS QDs on TiO2 films. Briefly, the TiO2 films treated by hydrochloric acid and TiCl4 were immersed in an aqueous solution of Cd(OOCH3)2 (0.3 M) for 2 min and rinsed with DI water, then immersed in 0.3 M Na2S aqueous solution for another 2 min, followed by another rinsing with DI water. Such a SILAR cycle was repeated several times to obtained the desired thickness of the CdS layer. The TiO2/CdS photoelectrodes were sintered at 380 °C for 1 h to improve the crystallinity of the CdS QDs
The field-emission scanning electron microscope (FESEM, JEOL JSM-7100F) and transmission electron microscopy (TEM, JEOL JEM 2010) were used for the morphological observation of the samples. The crystalline structure of the products was characterized by X-ray diffractometer (XRD, Brucker D8), the XRD patterns were collected from the samples grown on FTO substrates via θ–2θ scanning mode, and Cu Kα radiation (λ = 1.54060 Å) was used as the source operating at 45 kV and 40 mA. The light absorption and the diffused reflectance spectra were examined by UV-vis spectrophotometer (UV-3600, Shimadzu). The Pt films sputtered on an FTO glass substrate was served as the counter electrode and face to face bonded with the TiO2/CdS photoelectrode. Polysulfide electrolyte was applied as redox couples and injected into the free space between the two electrodes to complete the QDSSC fabrication. The photocurrent density versus voltage (J–V) curves of the cells were recorded by digital multi-meter (Keithley 2402) under the illumination of an AM 1.5G solar simulator (Newport, 100 mW/cm2), and the irradiated area of devices was defined to be 0.125 cm2 using a mask.
Results and Discussion
As the formula expressed, the reaction is reversible, and there are two competing reactions in this system. On the one hand, the TiO2 was dissolved in hydrochloric acid solution to produce the Ti (IV) complex. On the other hand, the Ti (IV) complex will hydrolyze into TiO2. However, during the etching treatment, the etching solution, contained 9 ml HCl and 7 ml DI water, will push the reaction along the dissolved direction. In addition, the surface stability and reactivity of the TiO2 nanorods are dominated by surface chemistry, which is critical for the equilibrium morphology . The surface energy of the rutile TiO2 nanorods follows sequences (110) < (100) < (101) < (001) [30, 31]. Generally, the facet with higher surface energy diminishes faster for minimization of the total surface energy. The (001) face corresponds to the core of the TiO2 nanorods, and the (110) face is the sidewall of the TiO2 nanorods. Thus, the (001) core of the TiO2 nanorods is etching faster than the (110) face of the sidewall. As a result, nanocaves appeared on the center portion of the TiO2 nanorods by hydrothermal selective etching of the core and the remaining sidewall of (110) face, which can be obviously detected from the inset of Fig. 1b. For the 5-h-etched sample, the inner diameter of the TiO2 nanocave continually enlarged. Interestingly, it can be found that the tip wall of the nanocave was divided into lots of small nanowires, and the amount and length of secondary nanowires increased as the etching time extended to 6 h. In order to better understand the etching treatment, the schematic diagrams of the etching process was presented in Additional file 1: Figure S1, images (a), (b), (c), and (d) correspond to the structure of the TiO2 films etching for 0, 4, 5, and 6 h, respectively. The inset defines the depth and the inner diameter of the nanocaves. As the Additional file 1: Figure S1 depicted, when the etching time was 4 h, the center portion of the nanorods was cut off by hydrochloric acid to form a nanocave. As the etching time was prolonged to 5 h, the upper part of the nanocave wall would split into small nanowires. Extending the etching time, the length and the amount of the secondary nanowires were continually increased.
Photovoltaic parameters obtained from the J–V curves of QDSSCs based on TiO2 photoanodes etched for different times
V oc (V)
J sc (mA/cm2)
Fill factor (%)
Photovoltaic parameters obtained from the J–V curves of QDSSCs based on TiCl4 treatment of TiO2 photoanodes etched for different times
V oc (V)
J sc (mA/cm2)
Fill factor (%)
In this study, a hydrothermal method was used to grow TiO2 NRAs on FTO substrate. For the sake of a large specific surface area, a facile hydrothermal etching was employed to the TiO2 NRAs. The relation between the etching time and the performance of TiO2 films had been comprehensively studied. The results showed that the etching treatment enlarged the gap space among the compact nanorods and hollowed out the center part of the nanorod to form a nanocave, and the wall of the nanocave split into lots of small nanowires; these changes in morphology lead to the improvement of the surface area. In addition, the hydrothermal etching in HCl solution did not damage the rutile crystal structure of the TiO2 nanorods and enhanced the diffused reflectance ability of photoanode. All these factors resulted in better photovoltaic performance for the QDSSCs made from the etching TiO2 films. Finally, through modifying with TiCl4, a relatively high PCE of 3.14 % is obtained after optimizing the etching time.
This work is supported in part by the National Natural Science Foundation of China (No. 51372075, 51502084).
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