Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (5): 691-694

#### The article information

Zhen-hong Xiao, Dao-chuan Jiang, Han Xu, Jing-tian Zhou, Qi-zhong Zhang, Ping-wu Du, Zhen-lin Luo, Chen Gao

UV Laser Regulation of Surface Oxygen Vacancy of CoFe2O4 for Enhanced Oxygen Evolution Reaction

Chinese Journal of Chemical Physics, 2018, 31(5): 691-694

http://dx.doi.org/10.1063/1674-0068/31/cjcp1804058

### Article history

Received on: April 3, 2018
Accepted on: May 14, 2018
UV Laser Regulation of Surface Oxygen Vacancy of CoFe2O4 for Enhanced Oxygen Evolution Reaction
Zhen-hong Xiaoa,b, Dao-chuan Jiangb, Han Xua,b, Jing-tian Zhoua,b, Qi-zhong Zhanga,b, Ping-wu Dub, Zhen-lin Luoa,b, Chen Gaoa,b
Dated: Received on April 3, 2018; Accepted on May 14, 2018
a. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China;
b. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Zhen-lin Luo, E-mail: zlluo@ustc.edu.cn
Abstract: Oxygen evolution reaction is one of the key processes in the promising renewable energy technique of electrocatalytic water splitting. Developing high efficient oxygen evolution reaction (OER) catalysts requires determination of the optimal values of the descriptor parameters. Using spinel CoFe$_2$O$_4$ as the model catalyst, this work demonstrates that irradiation with pulsed UV laser can control the quantity of surface oxygen vacancy and thus modify the OER activity, in a volcano-shape evolution trend. This strategy sheds light on quantitatively investigation of the relationship between surface cation valence, anion vacancy, and physicochemical properties of transition-metal-based compounds.
Key words: Oxygen evolution reaction    Spinel oxide    Transition metal oxide    Laser irradiation    Oxygen vacancy
Ⅰ. INTRODUCTION

Electrochemically splitting water can store the electrical energy effectively into chemical bonds, and be regarded as one potential renewable and clean energy technique for replacing the burning of fossil fuel [1-3]. The water splitting process consists of two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The slow kinetics of the OER requires high overpotential, resulting in considerable energy loss [4-6]. In the past decades, many progresses have been made in developing active catalysts for the OER, and precious IrO$_2$ and RuO$_2$ are regarded as the most efficient candidates [7, 8]. Recently, considering the cost aspect for practical use, efforts have been devoted to exploring non-precious transition-metal-based catalysts for the OER [9, 10].

The most valuable merit of transition metal oxide is that the cations therein possess variable valence states, which leads to tunable and rich functionalities. Theoretically, Man et al. found that the computed thermodynamic overpotentials show a volcano relation to the adsorption free energies of different reaction intermediates, which indicates the free energy as a descriptor for the OER electrocatalysis [11]. Trasatti et al. investigated the OER potentials of different binary transition metal oxides, revealing a similar volcano trend between the potential and enthalpy, i.e., a better catalyst should have neither strong nor weak affinity for oxygen [12]. Theoretical and experimental investigations both show that these volcano-like behaviors are relevant to the electronic band structure of the catalysts. The band structure of outer shell electrons, say the e$_\textrm{g}$ occupancy, closely depends on the coordination between cations and oxygen anions [13]. Shao-Horn et al. reported that the overpotential of the OER in perovskite oxides exhibits a volcano relationship with the e$_\textrm{g}$ occupancy [14, 15]. Subsequently, Xu et al. proved that this e$_\textrm{g}$ theory is also applicable to the spinel family of materials. As exemplified in MnCo$_2$O$_4$, the ORR/OER activity show a volcano shape with the Mn valence state in octahedral sites [16]. On the other hand, various approaches had been utilized to regulate the e$_\textrm{g}$ filling state and thus the OER catalytic activity, such as thermo-treatment [17], plasma sculpturing [18], hydrogenated treatment, etc. [6, 19]. Very recently, pulse laser irradiation has exhibited its powerful ability in controlling the oxygen vacancy in VO$_2$ films [20]. Considering the benefits of quantitatively controllable pulse number, frequency, irradiation density of laser, this approach is adopted to treat the typical spinel CoFe$_2$O$_4$ in this work, to exhibit its regulation ability of oxygen vacancy and the OER catalytic activity.

As schematically presented in FIG. 1, CoFe$_2$O$_4$ is a typical transition-metal spinel oxide, in which oxygen atoms are in tetrahedral-coordinated and octahedral-coordinated form. Studies have shown that OER/ORR catalytic activities therein are governed by the e$_\textrm{g}$ filling of the active cations in octahedral sites [6]. Therefore, it is reasonable to hypothesize that the escaped oxygen anions under irradiation will leave vacancies in the octahedral sites, thus to change the e$_\textrm{g}$ filling of the cations and to tune the OER activity of the catalyst. In this work, CoFe$_2$O$_4$ films were fabricated and then irradiated by pulsed laser of UV light. Subsequently, XRD, SEM, XAS, XPS and OER measurements were performed to evaluate the effect of irradiation on the crystal structure, surface electronic structure, and catalytic activity.

 FIG. 1 Schematic of laser irradiation process on spinel CoFe$_2$O$_4$.
Ⅱ. EXPERIMENTS

The CoFe$_2$O$_4$ films were deposited on FTO glass by pulsed laser deposition (PLD) at 600 ℃ under an oxygen pressure of 20 Pa. During deposition, half surface of the substrate was covered by metal mask in order to set aside the conductive area for electrochemistry test. After deposition the films were cooled down to room temperature in an O$_2$ atmosphere. The films were put in a vacuum chamber and subsequently irradiated by pulsed laser with wavelength of 248 nm. For the laser usage during irradiation, the repetition rate is 1 Hz, the laser energy density fallen on the film is 0.35 J/cm$^2$ per pulse, and the exposure time is used to control the irradiation dosage. Surface morphology is evaluated using scanning electron microscopy (SEM). The crystal structure and growth quality of the films were examined by X-ray diffraction (XRD) on a four-circle diffractometer (Rigaku SmartLab Film Version, Cu K$\alpha$ radiation). The valence state of the elements in the CoFe$_2$O$_4$ films was characterized by X-ray absorption spectrum (XAS) on the magnetic circular dichroism station at Hefei light source and X-ray photoelectron spectrum (XPS) using monochromatic Al K$\alpha$ emission as the excitation source. To guarantee the comparability of the XAS/XPS results, we divided one film into five regions and then exposed these regions to the laser with different exposure time using metal mask, i.e., 0, 15, 35, 70, and 100 min. The catalysis activity was measured using electrochemical station (CHI760E, Shanghai Chen Hua Instrument Co., Ltd.) in a standard three-electrode system.

Ⅲ. RESULTS AND DISCUSSION

Typical XRD patterns of the as-grown CoFe$_2$O$_4$ films under general conditions and after-irradiation are exhibited in FIG. 2(a). And the inset photograph shows the optical image of the sample. Besides the strong reflections of the FTO substrate, the marked diffraction peaks at 30.10$^{\circ}$, 35.54$^{\circ}$, 43.20$^{\circ}$, 57.02$^{\circ}$, and 62.70$^{\circ}$ are well assigned to the (220), (311), (400), (511), and (440) reflections of CoFe$_2$O$_4$ (JCPDS No.22-1086), indicating the successful synthesis of polycrystalline CoFe$_2$O$_4$ film on FTO. Note that no obvious difference was observed in the XRD patterns of the as-prepared and irradiated films, indicating no significant difference in the bulk structure before and after UV irradiation. Similar results were found in the bulk composition information revealed by XAS spectra (FIG. S1 in supplementary materials). These results indicate that the crystal structure of the thin films and the element electronic structure (depth $\sim$10 nm) did not change. In contrast, obvious change of the surface morphology can be seen in the SEM images (FIG. 2 (b-e)). Clear stacking growth mode can be observed in the as-grown film. However, after irradiation, the sample surface becomes more and more flat and dense, with less terraces and cracks.

 FIG. 2 (a) Typical X-ray diffraction patterns of CoFe$_2$O$_4$/FTO films as-grown or after-irradiation. The JCPDS No.22-1086 is shown as reference. (b-e) SEM images of the samples irradiated for different time.

Surface-sensitive technique XPS was utilized to reveal the element valence state on the film surface, and the resulting O 1s spectra with varied exposure time are shown in FIG. 3 (Co 2p and Fe 2p XPS spectra are shown in FIG. S2 in supplementary materials). The O 1s curves were fitted using three peaks: O1 peak at 529.7 eV corresponds to the metal-oxygen bonds, O2 at a higher value of 531.4 eV stands for the surface-adsorbed hydroxyl groups, and O3 peak at 532 eV is attributed to the absorbed molecular water. For comparison, the spectra shown in FIG. 3(a) are normalized using the O1 peak. The sum of O2 and O3 could quantitatively reflect the "surface effective vacancy" which absorbs the reactants and thus affects the chemical reaction. In the XPS results, it is obvious that the relative ratio and position of the peak associated with such "surface effective vacancy" change a lot with the increasing irradiation time. The relative intensity of the merged O2-O3 peaks increases with the increasing exposure time until 70 min, and then decreases after that time. Meanwhile, the position of the merged O2-O3 peak initially shifts to a higher energy, and then to a lower energy. The quantity of "surface effective vacancy" presents a volcano shape, which could be ascribed to the competitive effects of vacancy-fabrication and surface-densification induced by UV irradiation.

 FIG. 3 (a) O 1s XPS spectra of the CoFe$_2$O$_4$ film with varied exposure time under the UV irradiation. (b) The fitting peaks of O 1s spectra.

To evaluate the effect of UV-light irradiation on the catalytic activity of CoFe$_2$O$_4$ towards oxygen evolution, the OER measurements were implemented in 1 mol/L NaOH. As shown in FIG. 4(a), the OER performance of the CoFe$_2$O$_4$ catalyst showed a decreasing overpotential and increasing current density under UV irradiation. The best catalytic activity was achieved after UV exposure for 50 min. At a current density of 10 mA/cm$^2$, the required potential values versus the exposure-time are plotted in FIG. 4(b). An obvious volcano shape is observed in this result. The applied potential for the OER reaches the lowest value of 1.69 V after 50 min UV irradiation, suggesting the best OER efficiency under this condition. The above electrochemical results indicate that UV-light irradiation can enhance the OER catalytic activity of CoFe$_2$O$_4$, which is consistent with the above XPS result. With the increasing exposure time under UV-irradiation, the surface effective vacancies increase, which in-turn changes the coordination environment and the filling of the e$_\textrm{g}$ electron of metal ions. The change in microstructure and electron energy state finally affects the macro-property of the OER.

 FIG. 4 For the CoFe$_2$O$_4$ films with varied exposure time to the UV laser, (a) the current-potential curves for the OER measurements (inset is the schematic of the OER electrochemical test, in which CE is counter electrode, WE is working electrode and RE is reference electrode) and (b) the OER potential values at 10 mA/cm$^2$ as a function of irradiation time.

It could be noticed that there is minor shift between the extremum values found in the XPS and OER measurements. It is partly because the specimens used for OER measurement is not the same one and therefore could not guarantee the same original state of the oxide vacancy. The sample with high comparability used for the above XPS characterization could not be utilized for the OER measurement due to the very limited area of the regions with different dosage of irradiation. So, there is a reasonable offset in the optimum irradiation time.

Supplementary materials: XAS spectra of the CoFe$_2$O$_4$ film with varied exposure time under the UV irradiation are given. Co 2p and Fe 2p XPS spectra are also shown.

Ⅳ. CONCLUSION

In summary, we proved that irradiation with 248 nm UV-light could enhanced the OER activity of CoFe$_2$O$_4$ films. The OER activity exhibits a volcano shape as a function of the irradiation time, consistent with the relative ratio of surface effective vacancy, which is a trade-off between the laser-induced increase of oxygen vacancy and the laser-induced decrease of surface area ratio. Since the irradiation source used in this work was a pulsed laser with merits of quantitatively controllable frequency and pulse number, this strategy provides possibility for quantitatively investigation of the relationship between surface cation valence, anion vacancy and physical/chemical properties of various transition metal based compounds.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Key Basic Research Program of China (2016YFA0300102), the National Natural Science Foundation of China (No.11675179, No.U1532142, and No.11434009) and the Fundamental Research Funds for the Central Universities. This work was partially carried out at the USTC center for Micro and Nanoscale Research and Fabrication. We thank the support from the magnetic circular dichroism endstation at Hefei light source.

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a. 中国科学技术大学国家同步辐射实验室，合肥 230029;
b. 中国科学技术大学材料科学与工程系，中国科学院能量转换材料重点实验室，合肥 230026