Low field magneto-tunable photocurrent in CoFe2O4 nanostructure films for enhanced photoelectrochemical properties

Efficient solar to hydrogen conversion using photoelectrochemical (PEC) process requires semiconducting photoelectrodes with advanced functionalities, while exhibiting high optical absorption and charge transport properties. Herein, we demonstrate magneto-tunable photocurrent in CoFe2O4 nanostructure film under low applied magnetic fields for efficient PEC properties. Photocurrent is enhanced from ~1.55 mA/cm2 to ~3.47 mA/cm2 upon the application of external magnetic field of 600 Oe leading to ~123% enhancement. This enhancement in the photocurrent is attributed to the reduction of optical bandgap and increase in the depletion width at CoFe2O4/electrolyte interface resulting in an enhanced generation and separation of the photoexcited charge carriers. The reduction of optical bandgap in the presence of magnetic field is correlated to the shifting of Co2+ ions from octahedral to tetrahedral sites which is supported by the Raman spectroscopy results. Electrochemical impedance spectroscopy results confirm a decrease in the charge transfer resistance at the CoFe2O4/electrolyte interface in the presence of magnetic field. This work evidences a coupling of photoexcitation properties with magnetic properties of a ferromagnetic-semiconductor and the effect can be termed as magnetophototronic effect.

Scientific RepORts | (2018) 8:6522 | DOI: 10.1038/s41598-018-24947-2 ferromagnetic-semiconductors and tuning of the PEC activity by the application of an external stimulus such as magnetic field can be of great interest. CoFe 2 O 4 , a ferromagnetic-semiconductor with n-type conductivity, can be a potential candidate for studying magnetic field effect induced tuning of the PEC properties owing to its high magnetostriction 21 , high rate of change of strain with magnetic field 22 , moderate saturation magnetization 23 and an optical bandgap in the visible light region 24,25 . CoFe 2 O 4 has inverse spinel structure where, Co 2+ ions occupy the octahedral sites and half of the Fe 3+ ions occupy the tetrahedral sites and remaining half of the Fe 3+ ions occupy the octahedral sites. However, due to a large amount of empty interstitial sites, a small fraction of the Co 2+ ions can also occupy the tetrahedral sites 22 . The electrical and optical properties of CoFe 2 O 4 can be tuned depending upon the relative distribution of metal ions (Co 2+ and Fe 3+ ) at the tetrahedral and octahedral sites 26,27 . It can also possess soft magnetic properties at nanoscale due to which low magnetic field will be required to tune the charge transport properties [28][29][30] . Herein, we report the growth of CoFe 2 O 4 films using hydrothermally synthesized CoFe 2 O 4 nanostructures on fluorine doped tin oxide (FTO) substrates and tuning of the PEC properties under low applied DC magnetic fields is demonstrated.

Results and Discussion
X-ray diffraction pattern (Fig. 1a) and X-ray photoelectron spectra (Fig. S1, Supplementary Information) confirm the single phase formation of CoFe 2 O 4 nanostructure film. In the XRD data, in addition to the peaks originating from the FTO conducting substrate, all observed diffraction peaks matches well with the standard diffraction data (JCPDS-1086) corresponding to cubic crystal phase of CoFe 2 O 4 nanostructure film. Figure 1b shows the top view and cross sectional view (inset of Fig. 1b)   and counter electrodes, respectively and 0.1 M Na 2 S solution as an electrolyte (Fig. 2a). A tungsten halogen lamp with illumination intensity ~100 mW/cm 2 was used as a light source. In order to study the effect of external magnetic field on the photoanodic behavior of CoFe 2 O 4 films, magnetic field parallel to the film plane was applied using a permanent magnet assembly. Figure 2b shows the current-potential (J-V) characteristics of as prepared CoFe 2 O 4 photoanode under dark and light conditions also J-V curves of CoFe 2 O 4 photoanode under light conditions in the presence of magnetic fields of different strength. In the absence of magnetic field, the J-V curves show a significant photocurrent ~1.55 mA/cm 2 (at 1.9 V vs. RHE) in CoFe 2 O 4 film. However, when CoFe 2 O 4 film was subjected to an external magnetic field of 400 Oe, the photocurrent was found to be enhanced to ~2.14 mA/ cm 2 (at 1.9 V vs. RHE). A maximum enhancement in the photocurrent to ~3.47 mA/cm 2 (at 1.9 V vs. RHE) was observed with the increase in the magnetic field strength to 600 Oe. With further increase in the magnetic field, no significant improvement in the photocurrent was observed (Fig. S3, Supplementary Information). The observed magnetic field induced change in the photocurrent corresponds to ~123% enhancement in the photocurrent which is significantly higher compared to earlier published works on the magnetic field effect on the photocurrents 5,6 as well as other effects on the photocurrents (Table S1, Supplementary Information). Figure 2c shows the chronoamperometry results of CoFe 2 O 4 nanostructure films at a fixed potential of 1.23 V (vs. RHE) in the presence of magnetic fields of different strength. Chronoamperometry results also reveal the same trend in the photocurrent enhancement with magnetic fields. CoFe 2 O 4 nanostructure photoanode also shows good chemical stability (Fig. S4, Supplementary Information).
The observed enhancement in the photocurrent (J ph ) can be understood in terms of enhanced generation rate (G) and separation capability of photoinduced charge carriers which in turn depends upon the width of the depletion region (W) at the CoFe 2 O 4 /electrolyte interface and is given by the following relationship 5 ; where, A is a constant, α is the absorption coefficient, hv is the absorbed photon energy and E g is the optical bandgap.
An optical bandgap of ~1.65 eV is estimated when no magnetic field is applied to CoFe 2 O 4 nanostructure film. However, the optical band gap reduces to 1.55 eV and 1.43 eV when magnetic field of strengths 400 Oe and 600 Oe are applied, respectively. Figure 3b shows the variation of optical bandgap of CoFe 2 O 4 nanostructure film with increase in the magnetic field strength.
In CoFe 2 O 4 , crystal field splits d level into g xy xz yz 2 levels and the optical bandgap is due to d (e g level) to d (t 2g level) transitions. The energy width between e g and t 2g levels is higher at the octahedral sites (Δ o ) as compared to the tetrahedral sites (Δ t ) and is given as 27,34 ; Δ = Δ t o 4 9 . As, in CoFe 2 O 4 , Co 2+ ions can reside at the octahedral sites or at the tetrahedral sites and the optical bandgap is strongly dependent upon the relative population of Co 2+ ions at the octahedral and the tetrahedral sites. It is reported 27 that the shifting of Co 2+ ions from the octahedral sites towards the tetrahedral sites results in the decrease in the optical bandgap of CoFe 2 O 4 . Thus, in the present case, it is expected that under the effect of magnetic field the strain gets produced due to magnetostrictive properties of CoFe 2 O 4 . The presence of strain can shift some of the Co 2+ ions towards the tetrahedral sites from the octahedral sites (probably close to the surface region of CoFe 2 O 4 nanostructures where super-exchange interactions are supposed to be relatively weak compared to bulk) which in turn will result in the reduction of the optical bandgap of CoFe 2 O 4 . In order to probe the redistribution of Co 2+ ions at the tetrahedral and the octahedral sites in the presence of magnetic fields, we carried out Raman spectroscopy measurements, which is a powerful technique to probe the cationic distribution in spinel oxides 35 . Figure 3c shows the Raman spectra of CoFe 2 O 4 measured with 632 nm excitation wavelength under varying magnetic field strengths. The Raman peaks observed at positions 704, 632, 480 and 310 cm −1 correspond to optically active Raman modes (A 1g + E g + 3T 2g ) of CoFe 2 O 4 36,37 . The Raman peak at 632 cm −1 corresponds to Co 2+ ions at the tetrahedral sites and the Raman peak at 480 cm −1 corresponds to Co 2+ ions at the octahedral sites 35,38 .
In the presence of magnetic field, a shift in the Raman peaks has been observed as compared to the position of the Raman peaks without magnetic field. A shift in the Raman peaks indicates the presence of strain in the CoFe 2 O 4 nanostructure film which in corroborated with earlier published reports in literature 39  = which will provide an estimate of the distribution of the Co 2+ ions at the tetrahedral and octahedral sites. The variation of peak intensity ratio with magnetic field is shown in Fig. 3d. It is In order to get further insight into the magneto-tunability of photocurrents, the effect of magnetic field on the junction capacitance (C) was investigated and is shown in Fig. 4a. From the capacitance-voltage (C-V) curves it is clear that C decreases in the presence of magnetic field (600 Oe) which signifies an increase in the depletion region width (W) according to the relation 5 ; where, ε represents the dielectric constant. An increase in the depletion region width results in an effective built-in potential in the depletion region which facilitates the separation of the photogenerated electron-hole pairs and suppresses their recombination rate. To confirm the enhanced separation of the photogenerated charge carriers leading to the enhancement in the photocurrent, we carried out electrochemical impedance spectroscopy (EIS) measurements with and without magnetic field under light irradiation. EIS measures the charge transfer kinetics at the photoelectrode/electrolyte interface. Figure 4b shows EIS Nyquist plots of CoFe 2 O 4 nanostructure film measured in the presence of magnetic field (600 Oe) and without magnetic field. A semi arc is obtained due to depletion capacitance of semiconductor and Helmholtz capacitance at the electrode surface 40,41 . The semi arc curves are simulated using an equivalent circuit model (shown in the inset of Fig. 4b) with Z-View software and matched with experimental observations 42,43 . The solid lines are the simulted curves. Table 1 shows the estimated values of the parameters R ct , R s and C sc from the fitting of EIS Nyquist plots. The diameter of the semi arc gives the value of charge transfer resistance (R ct ) at the electrode/electrolyte interface. It is evident that the charge transfer resistance at the CoFe 2 O 4 /electrolyte interface is smaller in the presence of magnetic field compared to when measured without magnetic field. The decrease in the R ct value confirms the enhanced separation capability of the photogenerated charge carriers resulting in an enhancement in the photocurrent in the presence of magnetic field. We have also performed PEC measurements using Na 2 SO 4 as an electrolyte and the results are shown in Fig. S6 of the Supplementary Information. The results show that PEC properties are enhanced with the application of an external magnetic field using Na 2 SO 4 electrolyte also. In literature, charge carrier separation using electric field polarization and piezophototronic effect have been reported [44][45][46] however, there is no report on the enhancement in the charge separation efficiency in the presence of magnetic field for PEC applications.

Conclusions
To conclude, tuning of photoelectrochemical properties of CoFe 2 O 4 nanostructure film under low external magnetic fields has been demonstrated. It is shown that photocurrent of CoFe 2 O 4 nanostructure film can be enhanced  Photoelectrochemical measurements. Photoelectrochemical measurements were performed using a (Zahner Zennium, PP211) potentiostat with a three electrode cell assembly. Nanostructure film of CoFe 2 O 4 was used as a photoanode, platinum wire as the counter electrode and Ag/AgCl (in sat. KCl, 3.6 M) was used as the reference electrode. A 0.1 M Na 2 S solution was used as an electrolyte solution. A tungsten halogen lamp of intensity ~100 mW/cm 2 was used as a light source. Current-potential measurements were performed with a slew rate of 10 mV/s. Electrochemical impedance spectroscopy measurements were performed in the frequency range of 100 mHz to100 kHz. Voltage-capacitance measurements were performed at 1 kHz with an AC disturbance of 10 mV.