Improvement in the photoelectrochemical responses of PCBM/TiO2 electrode by electron irradiation
© Yoo et al; licensee Springer. 2012
Received: 21 October 2011
Accepted: 20 February 2012
Published: 20 February 2012
The photoelectrochemical (PEC) responses of electron-irradiated [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM)/TiO2 electrodes were evaluated in a PEC cell. By coating PCBM on TiO2 nanoparticle film, the light absorption of PCBM/TiO2 electrode has expanded to the visible light region and improved the PEC responses compared to bare TiO2 electrode. The PEC responses were further improved by irradiating an electron beam on PCBM/TiO2 electrodes. Compared to non-irradiated PCBM/TiO2 electrodes, electron irradiation increased the photocurrent density and the open-circuit potential of PEC cells by approximately 90% and approximately 36%, respectively at an optimum electron irradiation condition. The PEC responses are carefully evaluated correlating with the optical and electronic properties of electron-irradiated PCBM/TiO2 electrodes.
Keywordsphotoelectrochemical cell TiO2 electron irradiation PCBM band-edge tuning
TiO2 has been widely used for photocatalysts because of its good chemical- and photostabilities to convert photon energy to electrical and chemical energies . However, due to its wide bandgap, the light absorption is limited only to the ultraviolet (UV) region of the solar spectrum. Hence, sensitizing TiO2 with small bandgap semiconductors, such as quantum dots or organic dyes, has been extensively studied to harvest more photons in the visible light region of solar spectrum for the applications to quantum dot-sensitized solar cells [2–4], dye-sensitized solar cells [5–7], and photoelectrochemical (PEC) cells [8–10].
Along with this current research trends, combining TiO2 with carbonaceous nanomaterials has attracted much interest, and studies on these materials are increasing exponentially these days . For instance, high performance photocatalysts such as carbon nanotube-TiO2 [12–14], fullerene-TiO2 (C60-TiO2) [15–17], and graphene-TiO2 [18, 19] composites have been introduced by several groups and have shown enhanced photocatalytic activities. Notably, C60 has shown interesting effects when combined with TiO2: facilitating the separation of photo-generated charge carriers from TiO2 to C60 [15, 16] or sensitizing TiO2 to absorb visible light . However, the band-edge position of C60 is unfavorable for a sensitizer of TiO2 because the lowest unoccupied molecular orbital (LUMO) level of C60 is lower than the conduction band of TiO2 . From the viewpoint of energy levels, [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) is a better candidate than C60 for the sensitization of TiO2. We expect that the photo-excited electrons of PCBM can be transferred to TiO2 more efficiently because the LUMO level of PCBM is slightly higher than the conduction band of TiO2 . In our previous study, we have found that the band-edge positions as well as the bandgap of PCBM can be tuned by electron irradiation at different fluences . We believe that electron irradiation technique can be an alternative and unique method to modify the molecular structure and tune the bandgap [22, 23] compared to the conventional methods such as adjusting the particle size of quantum dots [24, 25] or modifying the molecular structure of the dyes  for larger light absorption. In addition to the bandgap, the band-edge positions can also be tuned by electron irradiation compared to the conventional methods such as ionic adsorption for specific quantum dots  or by varying the conjugation linkers in organic dyes .
Based on our previous findings, we present here a novel approach to improve the PEC performance of PCBM/TiO2 electrodes using electron beam irradiation. The photocurrent density and open-circuit potential of PCBM/TiO2 were respectively improved by 90% and 36% by electron irradiation. The effects of the electron irradiation on the PEC performances of PCBM/TiO2 were systematically analyzed in this study.
After electron irradiation of PCBM/TiO2 electrodes, a custom-made PEC cell was constructed to measure the PEC responses of electron-irradiated PCBM/TiO2 electrodes, which act as photo-anodes of PEC cells. The PEC cell has a three-electrode configuration comprising a photo-anode, a Pt wire as a cathode, and a saturated calomel electrode (SCE) (0.242 V vs. NHE, BAS Inc., West Lafayette, IN, USA) as a reference electrode. An aqueous solution of 1 M NaOH (Junsei Chemical Co., Ltd., Chuo-ku, Tokyo, Japan) was used as a supporting electrolyte after 30 min purging with N2 gas. The PEC response of the electrodes was recorded on a potentiostat (Model SP-50, BioLogic, Claix, France) by sweeping the potential from -1.2 to 0.5 V (vs. SCE) at a sweep rate of 100 mV s-1. The photo-anodes were illuminated with a solar simulator (Model LS-150, Abet Technologies, Inc., Milford, CT, USA) equipped with AM 1.5 filter. The illumination power was estimated as 80 mW cm-2 at the photo-anode surface by a digital photometer (ILT1400-A, International Light Technologies, Inc., Peabody, MA, USA).
Results and discussion
Photoelectrochemical performance of various electrodes investigated
Jph (μA cm-2)
Eocp (V) vs. SCE
(3.6 × 1016 cm-2)
(7.2 × 1016 cm-2)
(1.44 × 1017 cm-2)
The fact that the PEC performance of PCBM/TiO2 electrode was improved by electron fluence is interesting because electron irradiation increases the bandgap of PCBM and accordingly decreases the light absorption. As verified in our previous work, the LUMO level of PCBM shifts upward to the vacuum energy level as electron fluence increases. Since the bandgap of PCBM is much lower than that of TiO2, electron-hole pairs produced in PCBM can contribute to the increase in the photo-current of TiO2. However, the energy difference between the LUMO energy level of PCBM and the conduction band edge minimum of pure TiO2 is 0.2 eV, which might not be high enough for efficient electron transfer from PCBM to TiO2 . Since LUMO energy level of PCBM is up-shifted by electron irradiation, electron-irradiated PCBM provides higher driving force of electron injection from PCBM to TiO2 . This can explain why Jph of electron-irradiated PCBM/TiO2 electrodes was increased by increasing the electron fluence. Moreover, the increase in the energy difference between the LUMO energy level of electron-irradiated PCBM and the conduction band edge minimum of TiO2 provides efficient charge separation of the photo-excited electron-hole pairs, thereby improving Eocp .
However, when the electron fluence was further increased to 1.44 × 1017 cm-2, the PEC performance of electron-irradiated PCBM/TiO2 became worse. As shown in Figure 4, the LUMO energy level of PCBM was constantly up-shifted toward the vacuum energy level as the electron fluence was increased. The up-shift in the LUMO energy level of electron-irradiated PCBM increases the driving force of electron injection from PCBM to TiO2. With the up-shift in the LUMO energy level, the bandgap of the electron-irradiated PCBM also increases with increasing the electron fluence. The increase in the bandgap reduces the light absorption of PCBM and consequently deteriorates the PEC performance. Therefore, electron irradiation induces the two contradictory effects on the PEC performance of the electron-irradiated PCBM/TiO2, and this suggests that there is an optimum electron fluence at which the PEC performance is maximized. In our experiments, Jph increased by approximately 90% and Eocp increased by approximately 36% compared to bare TiO2 at an optimum electron fluence at 7.2 × 1016 cm-2.
Using the fact that the electronic band structure of PCBM can be modified by electron irradiation, PCBM/TiO2 electrodes were fabricated and tested in a PEC cell. We observed that electron irradiation on PCBM/TiO2 electrodes led to an increase in Jph by approximately 90% and Eocp by approximately 36% at an optimum electron irradiation condition. These results show that electron irradiation approach can be a good tool to tune the bandgap and the band-edge positions of PCBM and provide an evidence that the approach is useful for PEC device application. We believe that the electron irradiation strategy can also control the electronic band structures of other organic semiconducting materials, and thus, this strategy can improve the performances of PEC and photocatalytic devices.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2011-0020764).
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