Improvement in the photoelectrochemical responses of PCBM/TiO2 electrode by electron irradiation

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.


Introduction
TiO 2 has been widely used for photocatalysts because of its good chemical-and photostabilities to convert photon energy to electrical and chemical energies [1]. However, due to its wide bandgap, the light absorption is limited only to the ultraviolet (UV) region of the solar spectrum. Hence, sensitizing TiO 2 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][3][4], dyesensitized solar cells [5][6][7], and photoelectrochemical (PEC) cells [8][9][10].
Along with this current research trends, combining TiO 2 with carbonaceous nanomaterials has attracted much interest, and studies on these materials are increasing exponentially these days [11]. For instance, high performance photocatalysts such as carbon nanotube-TiO 2 [12][13][14], fullerene-TiO 2 (C 60 -TiO 2 ) [15][16][17], and graphene-TiO 2 [18,19] composites have been introduced by several groups and have shown enhanced photocatalytic activities. Notably, C 60 has shown interesting effects when combined with TiO 2 : facilitating the separation of photo-generated charge carriers from TiO 2 to C 60 [15,16] or sensitizing TiO 2 to absorb visible light [17]. However, the band-edge position of C 60 is unfavorable for a sensitizer of TiO 2 because the lowest unoccupied molecular orbital (LUMO) level of C 60 is lower than the conduction band of TiO 2 [17]. From the viewpoint of energy levels, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is a better candidate than C 60 for the sensitization of TiO 2 . We expect that the photoexcited electrons of PCBM can be transferred to TiO 2 more efficiently because the LUMO level of PCBM is slightly higher than the conduction band of TiO 2 [20]. In our previous study, we have found that the bandedge positions as well as the bandgap of PCBM can be tuned by electron irradiation at different fluences [21]. 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 [26] 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 [27] or by varying the conjugation linkers in organic dyes [28].
Based on our previous findings, we present here a novel approach to improve the PEC performance of PCBM/TiO 2 electrodes using electron beam irradiation. The photocurrent density and open-circuit potential of PCBM/TiO 2 were respectively improved by 90% and 36% by electron irradiation. The effects of the electron irradiation on the PEC performances of PCBM/TiO 2 were systematically analyzed in this study. Figure 1 shows the schematic representation of the preparation of PCBM/TiO 2 electrode and subsequent electron irradiation. The as-received TiO 2 nanoparticle paste (DSL 18NR-T, Dyesol Industries Pty Ltd., Queanbeyan, New South Wales, Australia) was deposited on a fluorine-doped tin oxide (FTO) glass substrate (8 Ωm -2 , Dyesol) by a doctor blade technique. Before the deposition of TiO 2 paste, FTO glass substrates were cut by 1.0 × 2.5 cm 2 in dimension and were sonicated successively in acetone, isopropanol, ethanol, and deionized water for thorough cleaning and dried in N 2 gas stream. After the deposition of TiO 2 paste, subsequent annealing process was performed at 450°C for 30 min with a temperature increase rate of 1°C min -1 . After the annealing, TiO 2 nanoparticle film was formed. The as-prepared TiO 2 electrodes were immersed vertically in a chlorobenzene solution containing 1.5 mM PCBM for 5 h while stirring. PCBM solution was prepared by dissolving PCBM (99.5% purity, Nano-C, Inc., Westwood, MA, USA) powder into chlorobenzene (≥99.5% purity, Sigma-Aldrich, St. Luois, MO, USA) solvent. After the immersion, the electrodes were washed in pure chlorobenzene several times and dried at ambient condition. As a result, PCBM/TiO 2 electrodes, where a thin layer of PCBM was coated on the TiO 2 nanoparticle electrodes, were prepared. Coating process of PCBM was carried out in darkness. The irradiation of an electron beam on PCBM/TiO 2 electrodes was carried out at room temperature and in vacuum lower than 2 × 10 -5 Torr. An electron beam was generated from a thermionic electron gun with electron energy of 50 keV, and current density of the electron beam was 1.6 μA cm -2 . The electron fluence was varied by adjusting the irradiation time. PCBM/TiO 2 electrodes were irradiated by 1, 2, and 4 h which correspond to electron fluence of 3.6 × 10 16 , 7.2 × 10 16 , and 1.44 × 10 17 cm -2 , respectively. Diffuse reflectance UV-visible (VIS) spectra of electron-irradiated PCBM/TiO 2 powders were measured on a spectrometer (S-4100, SCINCO CO., LTD., Seoul, South Korea) by scratching the nanoparticle film off the FTO glass substrate.

Methods
After electron irradiation of PCBM/TiO 2 electrodes, a custom-made PEC cell was constructed to measure the PEC responses of electron-irradiated PCBM/TiO 2 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., Chuoku, Tokyo, Japan) was used as a supporting electrolyte after 30 min purging with N 2 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).   form a film on a FTO glass substrate. A uniform TiO 2 NP film was formed by annealing the as-deposited TiO 2 paste at 450°C for 30 min. The TiO 2 NP film was submerged in a PCBM solution for 5 h, and consequently, the TiO 2 NP film was coated with PCBM. Subsequently, the PCBM/TiO 2 electrodes were irradiated with an electron beam. The energy of the electron beam was 50 keV, and the electron fluence was changed by controlling the irradiation time. These electron-irradiated PCBM/TiO 2 films on FTO glass substrates were used as photo-anodes of PEC cells for water splitting. Figure 2 shows the field emission scanning electron microscopy (FESEM) images of the fabricated PCBM/TiO 2 film. TiO 2 NPs with the diameter of approximately 20 nm were deposited on a FTO glass substrate (see details in the 'Methods' section). As shown in the FESEM image, the TiO 2 NPs were well interconnected with one another, forming a rigid film that is strongly attached to the FTO glass substrate. The thickness of the TiO 2 NP film was approximately 16.5 μm.

Results and discussion
We observed that transparent TiO 2 NP film became slightly yellowish after the PCBM coating. The UV-VIS absorption spectra shown in Figure 3 more clearly characterize the optical properties of the TiO 2 NP films. When PCBM was coated on TiO 2 , visible light absorption of TiO 2 in the wavelength range of 390 to 800 nm was increased, while absorption of UV in the range of 300 to 360 nm was decreased. In addition, when PCBM/ TiO 2 was irradiated with an electron beam, the absorbance in both UV and visible light region decreased gradually as the electron fluence increased. In our previous work, we reported that the bandgap of electron-irradiated PCBM increased as the electron fluence was increased. The modification of the bandgap was attributed to the change in the molecular structure of PCBM by electron irradiation. From these facts, we could conclude that the effective bandgap of electron-irradiated PCBM/TiO 2 also increased as the electron fluence increased (Figure 4).
In order to investigate the band-tuning effect caused by the electron irradiation, we tried to characterize the PEC cell device performances using the electron-irradiated PCBM/TiO 2 electrodes. The measurement results of the PEC responses of bare TiO 2 , PCBM/TiO 2 , and electronirradiated PCBM/TiO 2 electrodes are listed on Table 1, and the typical current density-potential curves of the electrodes are shown in Figure 5. The saturated current density at 0 V vs. saturated calomel electrode under dark conditions of all the electrodes was less than 15 μA cm -2 . Under illumination of simulated solar light, bare TiO 2 nanoparticle electrode shows saturated photocurrent density (J ph ) of 176 μA cm -2 and open-circuit potential (E ocp ) of -0.85 V vs. SCE. After coating PCBM on TiO 2 nanoparticles, the PEC performance was improved: J ph and E ocp of PCBM/TiO 2 electrode increased to 234 μA cm -2 and -1.05 V vs. SCE, respectively. The improvement in J ph and E ocp is attributed to the increment of visible light absorption of PCBM compared to that of TiO 2 . After electron irradiation of PCBM/TiO 2 electrode at electron fluence of 3.6 × 10 16 cm -2 , J ph and E ocp increased from 234 to 306 μA cm -2 and -1.05 to -1.16 V vs. SCE, respectively. The PEC performance of PCBM/TiO 2 electrode was further improved through electron irradiation of increased electron fluence. Both J ph and E ocp of electron-irradiated PCBM/TiO 2 were increased with increasing the electron fluence. J ph increased to 333 μA cm -2 , and E ocp increased to -1.16 V vs. SCE at the electron fluence of 7.2 × 10 16 cm -2 .
The fact that the PEC performance of PCBM/TiO 2 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 TiO 2 , electron-hole pairs produced in PCBM can contribute to the increase in the photo-current of TiO 2 . However, the energy difference between the LUMO energy level of PCBM and the  conduction band edge minimum of pure TiO 2 is 0.2 eV, which might not be high enough for efficient electron transfer from PCBM to TiO 2 [29]. 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 TiO 2 [25]. This can explain why J ph of electron-irradiated PCBM/TiO 2 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 TiO 2 provides efficient charge separation of the photo-excited electron-hole pairs, thereby improving E ocp [30].
However, when the electron fluence was further increased to 1.44 × 10 17 cm -2 , the PEC performance of electron-irradiated PCBM/TiO 2 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 TiO 2 . 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/TiO 2 , and this suggests that there is an optimum electron fluence at which the PEC performance is maximized. In our experiments, J ph increased by approximately 90% and E ocp increased by approximately 36% compared to bare TiO 2 at an optimum electron fluence at 7.2 × 10 16 cm -2 .

Conclusions
Using the fact that the electronic band structure of PCBM can be modified by electron irradiation, PCBM/ TiO 2 electrodes were fabricated and tested in a PEC cell. We observed that electron irradiation on PCBM/ TiO 2 electrodes led to an increase in J ph by approximately 90% and E ocp 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.