熱處理效應對CdS/CdSe 共增感TiO2光電極在光分解水產氫效能的探討

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Research Express@NCKU - Articles Digest

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Research Express@NCKU Volume 23 Issue 7 - March 22, 2013 [ http://research.ncku.edu.tw/re/articles/e/20130322/3.html ]

Heat Annealing Effect on the Performance of CdS/CdSe

Sensitized TiO

₂

Photoelectrodes in Photochemical

Hydrogen Generation

Ching-Fa Chi, Shih-Yi Liau and Yuh-Lang Lee

*

Department of Chemical Engineering, College of Engineering, National Cheng Kung University [email protected]

Nanotechnology, 21, 025202 (2010)

I

ntroduction

Since the innovative discovery by Fujishima and Honda in 1972,1 development of semiconductor photocatalyst for efficient water splitting application has been focused in many studies. Nevertheless, the wide band gap of TiO₂ limits its photocatalytic properties

in the UV region. On the contract, low band gap semiconductor like CdS and CdSe, possess good light harvest property were applied in solar cell widely. In general, performance in energy conversion is not only determined by the light harvest range, but also by the charge transport rate in the sensitizer and at the TiO₂/sensitizer interface. In this work, CdS and CdSe were coupled with TiO₂ for extending the light harvest region, furthermore, heat annealing was utilized to eliminate structural defects present in an as-prepared material and enhance the performance of emiconductor-sensitized TiO₂ photoelectrodes for photo-electrochemical hydrogen generation. This strategy was shown to be effective not only for CdS- and CdSe-sensitized electrodes, but also for CdS/CdSe co-sensitized electrode.

Result

Heat annealing effect on the light absorption properties of the semiconductor-modified TiO₂ (TiO₂/CdS and TiO2/CdSe) electrodes as well as a bare TiO2 film, are evaluated by an UV-vis spectrometer. The absorption

spectra of these electrodes are shown in figure 1. The apparent colors of these electrodes were also shown in the inset of figure 1. The bare TiO₂ film only absorb the UV light at the wavelength below 420 nm. For the TiO2/CdS and TiO2/CdSe electrodes, heat annealing triggers a red-shift of the absorption spectrum and the

wavelength of absorption edge increases with increasing the annealing temperature.

To study the performance of these photoelectrodes at various wavelengths, experiments were conducted to obtain the incident photon to current conversion efficiencies (IPCE). These measurements were performed in a photoelectrochemical cell at an applied voltage of 0.5 V and carried out from the photocurrents monitored at

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Figure 1. UV-vis absorption spectra of the TiO2/CdS (a) and TiO2/CdSe (b) electrodes by heat annealing at

various temperatures. The inset shows the photos of the relative TiO₂ electrodes.

Figure 2. Incident photon to current conversion efficiencies (IPCE) for various electrodes measured at different excitation wavelengths. Heat annealing effect on TiO2/CdS and TiO2/CdSe electrodes(a). TiO2/CdS/CdSe

prepared with the different heat annealing process (b).

different excitation wavelengths. The result shown in figure 2(a) demonstrates that IPCE obtained for the 150 ℃ annealing CdS (TiO₂/CdS₁₅₀) and CdSe (TiO₂/CdSe₁₅₀) electrodes are about 52% and 21%, respectively. This result indicates that the electron-hole pairs excited in CdS can be separated and collected more efficiently than in CdSe, attributable to the higher conduction band edge of CdS with respect to that of CdSe. The TiO₂/CdS electrode only responses to the light with wavelength smaller than about 550 nm, while the absorption region of TiO₂/CdSe can be extended to about 700 nm. When the annealing temperature was increased to 300 ℃, IPCEs was increased to ca. 63 and 33%, respectively, for TiO₂/CdS₃₀₀ and TiO₂/CdSe₃₀₀ electrodes. This result indicates that the performances of both TiO₂/CdS and TiO₂/CdSe electrodes were improved by the heat annealing effect.

It had been shown that the performance of a QD-sensitized electrode can be greatly enhanced by the co-sensitization effect of CdS/CdSe.2 Since the heat annealing was proved to be effective for the single sensitizer systems, it was anticipated to be valid for the co-sensitized electrode. Various treating procedures were utilized to obtain an optimal one. For the first, electrodes with TiO₂/CdS/CdSe cascade structure were prepared first and

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then subjected to a heat annealing at 150 or 300 ℃. These electrodes were termed as (TiO₂/CdS/CdSe) ₁₅₀ and (TiO₂/CdS/CdSe) ₃₀₀, respectively. For the second, a two-steps annealing process was utilized. CdSe was deposited on a 300 ℃ annealed TiO₂/CdS electrode (TiO₂/CdS₃₀₀), followed by a second annealing at 150 or 300 ℃. These electrodes were denoted by TiO₂/CdS₃₀₀/CdSe₁₅₀ and TiO₂/CdS₃₀₀/CdSe₃₀₀, respectively. In general, the heat annealing effect on the absorption spectra of the co-sensitized electrodes is similar to that on the single-sensitizer systems (not shown here). Heat annealing triggers a slightly red-shift of the absorption spectrum and this effect is more significant for a higher annealing temperature.

Comparing TiO₂/CdS/CdSe with the CdSe-sensitized electrode prepared at the optimal annealing condition (TiO₂/CdSe₃₀₀), similar light absorption ranges but much higher IPCEs were obtained for the co-sensitized CdS/CdSe electrodes﹙figure 2(b)﹚. In the long-wavelength region (>500 nm) where CdS cannot be photoexcited, the IPCE increment of the co-sensitized electrodes clearly indicates the superior ability of the cascade structure in charge transport. Furthermore, the heat annealing effect on the charge transport in the co-sensitized electrodes can be observed by comparing the IPCEs of TiO2/CdS300/CdSe150 and (TiO2/CdS/CdSe) 150 electrodes. By using the two-steps annealing process, the IPCE of the TiO2/CdS300/CdSe150 electrode can

be increased up to 80%, which also attributed to a higher crystallization state of the semiconductor sensitizers, as well as a better connections at the TiO2/sensitizer and sensitizer/sensitizer interfaces.

Since the co-sensitized electrodes appear to have superior performances than the electrodes sensitized by single material, these electrodes were utilized for hydrogen generation. These experiments were performed in an electrochemical cell and operated at the applied potential of 0.5V (vs. OCP). The hydrogen evolved from Pt cathode was collected and analyzed using a gas chromatograph. Figure 3 shows the evolution amount of hydrogen and the corresponding generation rate as a function of time for the TiO₂/CdS₃₀₀/CdSe₁₅₀ and (TiO₂/CdS/CdSe)150 electrodes. In the first hour, the hydrogen evolution rates for the TiO₂/CdS₃₀₀/CdSe₁₅₀ electrode (201 μmol cm-2_h-1₎

Figure 3. Hydrogen evolution in photoelectrochemical cells using TiO₂/CdS/CdSe photoanodes annealing at various temperatures. The inset shows the hydrogen generation rates corresponding to the cumulative hydrogen

gas.

is higher than that of the (TiO₂/CdS/CdSe)₁₅₀ (174 μmol cm-2h-1), consistent with the IPCE measured result. Apparently, appropriate heat treatment has been proved to be an efficient process to increase the crystallinity of

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CdS and CdSe nanocrystallites deposited on mesoporous TiO2 films, improve the charge transport characteristics

of the semiconductor-sensitized TiO₂ films, and increase the performance the photoelectrodes.

Reference

1. Fujishima et. al Nature 238 (1972) 37-38. 2. Lee et. al. Adv. Funct. Mater. 19 (2009) 604-9