Micro-arc oxidation and alkali etching to produce nanoflaky TiO2

Micro-arc oxidation (MAO) technique is a relatively convenient and effective technique for producing micrometer scale porous crystalline anatase TiO2 over a metal titanium surface. This technique involves the anodically charging of a metal (similar to conventional anodic oxidation but with a higher level of discharge voltage) in a specific electrolyte to reach a critical value at which dielectric breakdown takes place, and initiates micro-arc discharge over the entire metal surface. The micro-arc discharge enables the rapid oxidation of the metal due to the effect of impact or tunneling ionization over the metal surface. The schematic MAO system to obtain TiO2 films is shown in Fig. 12. First attempt using MAO technique to grow microporous TiO2 over a Ti surface for applying as DSSC electrode has also demonstrated, with however limited photovoltaic efficiency due to unsatisfactory specific surface area. In responding to the demanding in high efficiency PV device, we have developed another two-step method for the Ti foil to grow nanoflaky TiO2. An idea is proposed in this study simply by using alkali etching to develop nanoflaky morphology
over the pre-micro-arc oxidized Ti (i. e. MAO-TiO2) as the ideal electron emitter (or TiO2 electrode). Such a nano featured TiO2 layer shall be able to exhibit very large specific surface area and capable of efficient dye absorbing and eventually high photovoltaic efficiency. The alkali etching began with the immersion the MAO treated titanium foil into a NaOH solution and soaking for 12 h to develop nano-featured TiO2. Later on, an alkali etching treatment followed by MAO was proposed to develop 3D-network nanostructural anatase TiO2 without annealing, with the accompanied photovoltaic efficiency substantially improved. In this work, a further detailed observation on the microstructural development of the nanostructural anatase TiO2 is carried out as a function of alkali bath concentration and post-heat treatment effect to the associated photovoltaic efficiency is correlated.


Fig. 12. Schematic diagram of micro-arc oxidation system to obtain MAO-TiO2.

Figure. 13 shows the surface and cross sectional morphology of the MAO formed titania layer as well as the alkali etched TiO2 layers obtained at different bath concentration. After MAO treatment, the titanium forms porous crystallinic anatase TiO2 layer (as identified and described elsewhere) with numerous micrometer scale holes as observed in Fig. 13(a). These holes are discharge channels induced by the electrical breakdown of the oxide layer during the MAO treatment. It is worth noting that the surface is roughened, which is based on the fact that an intensive microdischarge occurs at a high voltage; as a result (Fig. 13(a)), the coating itself appears to be a microscopically splashed surface under the strong discharge effect. The morphology of the specimens alkali etched at different NaOH concentration shown in Fig. 13(b)~(d) reveal that nanoflaky TiO2 can be developed through the alkali etching. The nano featured layer was developed over the MAO-TiO2 scaffold surface with free interspace and nanoflakes of about 50~100 nm in size. As can be seen from the figure, these nanoflakes uniformly distribute over the entire surface of the treated specimen. The results revealed that alkali solution concentration appear to be an important variable in nanostructural control. Moreover, the higher NaOH concentration leads to much bigger free interspace and deeper nanoflaky TiO2 layer as well as bigger nanoflake size. It is therefore out of question that the TiO2 layer reformed by the alkali etching can have higher specific surface area than the MAO-TiO2. Through the evaluation of a series of alkali-etched specimen at different NaOH concentrations, the size of the developed nanoflakes is found to be determined by the NaOH concentration. The morphological development of the nanoflakes is thought to be associated with the complicated dissolution and re-precipitation mechanism that involves the attack by hydroxyl groups and negatively charged HTiO3 ions formed on the surface. The HTiO3 ions are thought to be consequently attracted and dissolved by the positively charged ions in the NaOH solution. In our case, it is hypothetically proposed that the low-concentration NaOH solution gives rise to the diffusion control mode enabling charged ion exchange between the MAO specimen surface and the alkali solution, where a limited ion flux yields a low reaction rate that favors fine structure formation. Contrarily, the high NaOH bath concentration enables fast exchange of the charged ion species and fast structure formation (accompanied by the flakes grown in larger dimension and larger interspace). In addition, cracks occur to the nanoflaky TiO2 layer when NaOH bath concentration is increased. The results reflected in Fig. 13(c) and (d) indicate that the cracks began to form on the MAO specimen surface and grow with the increasing NaOH concentration.


Fig. 13. Surface morphology (upper, with different magnification) and cross sectional morphology (lower) of the (a) MAO treated specimen, and alkali etched specimen in NaOH bath concentration of (b) 0.50 M (c) 1.25M and (d) 2.50 M, respectively.

Further exam of the detailed microstructure of nanoflakes by using transmission electron microscope (TEM) in high magnification bright field images taken from specimen with alkali etching at 40 °C for 12 h are shown in Fig. 14. It can be seen that the hair-like structure (corresponds to the nanoflaky structure as been observed in Fig. 13) exists over the TiO2 surface as shown in Fig. 14(a). Here, it clearly presents a 3D network fine structure. In addition, the hair-like structure grown from the inner wall of the pore as also observed in the Fig. 14(b) is again seen as a 3D network feature. These 3D nanoflakes led to a significant increase in specific surface area and presumably photovoltaic efficiency. It should also be noted that these pores and voids are opened to the alkali etched and their surfaces are also involved with the reforming process via dissolution and re-deposition. This means that the nanoflakes grow not only on the TiO2 surface but also grow deep into the inner surfaces, thereby significantly increase specific surface area, even though these nanoflakes unfortunately appear to be amorphous as identified by TEM selected area diffraction technique and described elsewhere.


Fig. 14. Bright field image of nanoflaky Ti02 grown from (a) the MAO-TiC>2 surface and (b) the inner pore of the MA0-Ti02.

The I-V curves of DSSCs assembled with the MA0-Ti02 and alkali etched Ti02 obtained at different concentrations are shown in Fig. 15.


Fig. 15. I-V characteristic of the DSSC device assembled using (a) MA0 treated specimen and alkali etched specimens at different Na0H bath concentration.

Photovoltaic efficiency of the assembled DSSC is substantially increased by alkali etching. Apparently, the remarkable increase in the Jsc and Voc of the cell assembled from alkali etched specimens appear to be contributed to by the nanoflaky surface structure, which possesses a markedly higher specific surface area than the MA0 layer. Note that the Jsc is significantly dropped for the DSSC using alkali etched Ti02 specimen prepared at 2.5 M Na0H. This is due to the cracks formed and distributed over the entire oxide layer leaving the I2+LiI electrolyte to directly contact with fresh metallic titanium plate. A close look at Fig. 13(b), (c) and (d), the DSSC assembled by the alkali etched specimen at 1.25 M Na0H solution performs the highest Jsc and Voc among the three alkali etched specimens. Good explanation is that this is a compromising of the effect of the enlarged specific surface area and the effect of crack formation caused by the alkali etching, i. e. the increased Na0H bath concentration not only results in the increased specific surface area but also the increased free interspace and even worse the crack formation. As revealed in Fig. 13(d), the cracks causing the discontinuity of path for charge carrier shall be the main reason for the significant decrease in photovoltaic efficiency of the assembled DSSC which employs specimen alkali etched at 0.50 M NaOH. By comparison, the DSSC assembled by using MAO scaffold presents a photovoltaic efficiency of 0.078%, while it present a highest photovoltaic efficiency of 0.329% (over four times increment) for the DSSC assembled by the alkali etched specimen. Through this simple and low cost alkali etching route, it is able to produce nano structural TiO2 electrode for photovoltaic DSSC.

The I-V characteristics of the DSSCs assembled by using specimens with MAO-TiO2 and nanoflaky TiO2 (as-etched and annealed at 400 °C), with their corresponding XRD patterns are shown in Fig. 16. The annealing work significantly improves the crystallinity of the nanoflakes and consequently photovoltaic efficiency can be dramatically increased for the device assembled with the specimen with nanoflaky TiO2. It was 0.329% for the specimen with the as-etched nanoflaky TiO2 and 2.194% for the specimen with annealed nanoflaky TiO2. Both are however greater than that of the MAO-TiO2 specimen only with 0.061%. By contrast, the Jsc and Voc of the solar cell assembled by alkali etched specimen are substantially higher. Apparently, the dramatic increase in Jsc and Voc of the cell assembled by alkali treated specimens is contributed by the nanoflaky surface structure, which possesses far higher specific surface area than MAO layer does. With this simple and low cost post­alkali etching demonstration, the photovoltaic efficiency of the DSSC using the MAO treated Ti foil as the back electrode can be significantly increased. The increased crystallinity provides higher dye-absorption for generating more electron-hole pairs and suppresses the electron loss due to the recombination of electron-hole pairs. Therefore, the DSSC assembled with Ti electrode which synthesized by MAO, treated by alkali etching and annealing, presents highest photovoltaic efficiency.


Fig. 16. The I-V characteristics of the DSSCs assembled by using specimens with MAO-TiO2 and nanoflaky TiO2 (as-etched and annealed at 400 °C), with their corresponding XRD patterns.

1.4 Summary

Here at last, a method to combine with micro-arc oxidation and post-alkali etching has succeeded in forming an 3D network nanoflaky anatase TiO2 layer on the surface of a Ti substrate. The nanoflaky TiO2 completely cover the upmost surface of the MAO TiO2 layer as well as the inner pores and voids, therefore provides very large surface area for dye absorption to increase the efficiency of the assembled DSSCs. Without post-annealing, maximum photovoltaic efficiency of 0.329% for the DSSC is achieved with the amorphous nanoflaky TiO2 layer alkali etched at 1.25 M NaOH. Post-annealing at 400 °C significantly enhances crystallinity of the nanoflaky TiO2 layer and ultimately photovoltaic efficiency 2.194% for the DSSC is achieved. The above results have shown that the method to combine with micro-arc oxidation and post-alkali etching was a potential and low-cost process for developing the nano featured TiO2 photoanode for obtaining high efficiency DSSC. However, post-annealing shall not be abandoned for additional windfall of the photovoltaic efficiency.

2. Conclusion

Here, we begin our conclusion by reviewing the results in previously described. We started developing nano featured TiO2 layer by using LPD method and found that it is capable of obtaining nanocluster TiO2 with unacceptable growth rate. Crystallinic anatase TiO2 layer can be obtained through suitable annealing treatment to achieve only 0.0056% photovoltaic efficiency. In the work of TiO2 nanowires growth on AIP-TiO2 template via hydrothermal route, an ultimate PV efficiency of 3.63% can be achieved by optimizing hydrothermal process condition and annealing treatment. A hydrothermal treatment time so long as 24 hours shall be required for achieving this, which however has shorter treatment time than the LPD process and a fair PV efficiency. In the work of preparing TiO2 nanotubes array by arc ion plating pre-deposit metal Ti layer on ITO glass followed by anodic oxidation, the key to successfully develop 10 micrometer long TiO2 nanotubes array lies in the strongly adhered Ti-layer which tolorates the electrolyte attack during anodic oxidation. Ultimate photovoltaic efficiency of 1.88% appears on the DSSC assembled from TiO2 nanotubes array which was annealed at 350 °C. Although the tube length and diameter is controllable, it is expected to exhibit higher photovoltaic efficiency by further reducing tube diameter for more specific surface area of the photoanode. At last, a DSSC assembled from the nanoflaky TiO2 prepared by using micro-arc oxidation and alkali etching was demonstrated. Ultimate photovoltaic efficiency 2.194% for the DSSC is achieved.

Results in these studies are remarkably consistent with what we expected. Cost saving and easy operation processes for obtaining TiO2 photoanode has been achieved. Despite the encouraging result of this study as the positive effect of nanostructural surface engineering, future research is required in a number of directions about chasing high efficiency DSSCs. However, a step further has been taken in the improved photovoltaic efficiency by nanostructural surface engineering and an opportunity for commercializing DSSC using low-cost process.