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Article

Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution

by
Mohammed Alsultan
1,*,
Shaymaa Al-Rubaye
2,
Amar Al-Keisy
3,*,
Gerhard F. Swiegers
4 and
Intisar Ghanim Taha
1
1
Department of Science, Collage of Basic Education, University of Mosul, Mosul 41002, Iraq
2
Department of Physics, College of Education for Pure Science, University of Babylon, Babylon 51002, Iraq
3
Nanotechnology and Advanced Materials Research Center, University of Technology, Baghdad 10066, Iraq
4
ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Wollongong, NSW 2522, Australia
*
Authors to whom correspondence should be addressed.
Colloids Interfaces 2022, 6(1), 1; https://doi.org/10.3390/colloids6010001
Submission received: 4 November 2021 / Revised: 3 December 2021 / Accepted: 14 December 2021 / Published: 22 December 2021
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Abstract

:
Tailoring conductive polymers with inorganic photocatalysts, which provide photoinduced electron-hole generation, have significantly enhanced composites leading to excellent photoelectrodes. In this work, MnFe2O4 nanoparticles prepared by a hydrothermal method were combined with polyaniline to prepare mixed (hybrid) slurries, which were cast onto flexible FTO to prepare photoelectrodes. The resulting photoelectrodes were characterized by XRD, FESEM, HRTEM and UV-VIS. The photoelectrochemical performance was investigated by linear sweep voltammetry and chronoamperometry. The photocurrent achieved by MnFe2O4/Polyaniline was 400 μA/cm2 at 0.8 V vs. Ag/AgCl in Na2SO4 (pH = 2) at 100 mW/cm2, while polyaniline alone achieved only 25 μA/cm2 under the same conditions. The best MnFe2O4/Polyaniline displayed an incident photon-to-current conversion efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) of 60% at 405 nm wavelength, and 0.17% at 0.8 V vs. Ag/AgCl, respectively. High and stable photoelectrochemical performance was achieved for more than 900 s in an acidic environment.

1. Introduction

Renewable and clean energy are, critically, important for future energy demand, to reduce reliance on fossil fuels that cause greenhouse gas emissions [1,2,3]. Several new techniques have been developed to address this issue. Among these, photoelectrochemical (PEC) water splitting is an attractive approach to clean, cheap and environmentally friendly energy [4,5,6]. This technique converts water to hydrogen (H2) and oxygen (O2) gases in the presence of photocatalysts illuminated with light. Several parameters influence both the oxygen evolution reaction (OER) and/or the hydrogen evolution reaction (HER) including: the reduction (oxidation) potential for each conduction (valence) band sites of the photocatalysts, and the charge separation and transfer, as well as the electrolyte used in the water splitting system, specifically its pH [7,8]. The main issues regarding the practical application of photocatalysts are stability and cost.
Spinel ferrite magnetic nanoparticles have been intensively studied in previous reports, showing multifunctional behavior and excellent photocatalytic properties due to a narrow band gap, low cost, magnetism and non-toxicity. Among these, MnFe2O4 has a bandgap of ~1.8 eV. However, even though nanotechnology has improved photocatalytic performance, a major drawback is high charge recombination [9,10,11,12].
It has been observed that photocatalysts combined with certain other materials may result in significantly enhanced water splitting photocatalytic activity and durability due to the features of these hybrids, including excellent electron transfer properties, permeability to water, suppression of electron (e−)–hole (h+) recombination, uncomplicated preparative method and high electrochemical stability under highly acidic conditions (a high efficiency hydrogen evolution reaction normally carried out under highly acidic conditions, as represented by the proton-reduction reaction) [13,14,15]. For example, the MnFe2O4 hybrid with reduced graphene oxide (rGO) more than doubled photocatalytic performance [16]. This heterostructure boosts the absorption of visible light, charge carriers and apparent quantum efficiency, and induces charge separation with a longer lifetime of interfacial charge carriers [17,18,19,20]. Conductive polymers have the added benefit of binding photocatalyst nanoparticles onto the photoelectrode in a conductive matrix, rather than using non-conducting binders which easily dissolve in electrolyte solution resulting in a non-stable photoelectrode.
Conductive polymers (CPs) such as poly 3,4-ethylenedioxythiophene (PEDOT) [21], polypyrrole (PPy) [20,22] and Polyaniline (PANi) [23] have been widely investigated as photocatalysts due to their conductivity, processability and stability. PANi hybrids with inorganic photocatalysts like TiO2 [24], PdS–CdS [25], Ni-ZnO [26], α-Fe2O3 [27] and NiMo [28], have demonstrated enhanced photoelectrochemical (PEC) performance due to synergy between the conductive polymer and inorganic photocatalyst. The conductive polymer plays a remarkable role in the photoelectrode, creating high charge carrier mobility, a strong interfacial interconnection between the spinel ferrite magnetic nanoparticles and the conductive polymer, thus enhancing charge transport as well as increasing stabilization of photoelectrode toward high acidic solution [29,30].
In this work, PANi@MnFe2O4 photoelectrode was successfully synthesized by a hydrothermal method (for further information, see the Experimental section in the Supporting Information), characterized and determined to be active for photoelectrochemical water-splitting, demonstrating a photocurrent density of 400 µA/cm2 at an externally applied bias of 0.8 V vs. Ag/AgCl. In contrast, pristine PANi displayed a very small photocurrent of 25 µA/cm2. The polyaniline in PANi@MnFe2O4 photoelectrode facilitates fast photoinduced charge transfer rather than photo-corrosion of the photoelectrode.

2. Experimental

Aniline (99%), sulfuric acid, ammonium persulfate ((NH4)2S2O8) (99%), dimethylformamide (DMF) (99%) and hydrochloric acid were provided by Sigma-Aldrich (#242284, 248614, 227056) (Sydney, Australia). All chemicals were used without further purification. Fluorine-doped tin oxide (FTO) coated glass were purchased from Sigma Aldrich (thickness: 2 mm, sheet resistance: 15 Ω/cm2) and used as transparent conducting oxide substrate.

2.1. Preparation of MnFe2O4 and Polyaniline

Mn-ferrite nanoparticles (MnFe2O4) were prepared as described in the literature [31,32], by hydrothermal method. Firstly 1 mmol of MnCl2⋅4H2O and 2 mmol of FeCl3⋅6H2O were dissolved in distilled water, followed by dropwise addition of a diluted NaOH solution until the color became a brownish-black suspension and the pH was adjusted to 11. The above suspension was poured into a stainless-steel autoclave 100 mL, heated to 180 °C for 10 h, with the resulting product washed several times with ethanol and distilled water, and thereafter, dried overnight at 60 °C. PANi was synthesized via in situ chemical oxidation polymerization. (NH4)2S2O8 was used as an oxidant initiator to polymerize aniline with molar ratio 1:1 (NH4)2S2O8: aniline. Typically, 1 mol of (NH4)2S2O8 was dissolved in 10 mL of distilled water containing 0.2 M HCl, and 1 mol of aniline was added to above solution gradually with stirring. The stirring was continued over 12 h. The resulting polyaniline was washed in distilled water and ethanol and dried overnight.

2.2. Preparation of Polyaniline@ MnFe2O4 Catalyst

To prepare polyaniline@MnFe2O4, 1 g of polyaniline was dissolved in 5 mL of DMF. MnFe2O4 was then added to above solution in the ratio noted in Table 1 and mixed in a mortar for several minutes. The resulting Polyaniline@ MnFe2O4 mixture slurry was cast over flexible FTO, the sample dried overnight at 60 °C.

3. Characterization

Scanning Electron Microscopy (SEM); The morphology of catalyst polyaniline@MnFe2O4 film’s surface was examined using a JEOL 7500 SEM (Tokyo, Japan) with an accelerating voltage of 15 kV.
X-ray powder diffraction (XRD); XRD of polyaniline@MnFe2O4 was carried out on a Shimadzu XRD-6000 (Kyoto, Japan). The wavelength was 1.54 Å while the supplied voltage and current were kept at–40 kV and 25 mA, respectively.
Transmission Electron Microscope (TEM); TEM imaging of polyaniline@MnFe2O4 was investigated using a JEOL JEM-2010 (Tokyo, Japan). Sample powder was collected from FTO substrate and then sonicated in 3 mL ethanol. A drop of the suspension was placed on a TEM grid and then imaged.

Photoelectrocatalytic Activity

Linear sweep voltammetry (LSV). The photocatalyst, polyaniline@MnFe2O4, performed as a working electrode in the photoelectrochemical cell (PEC) while Pt mesh and saturated Ag/AgCl were used as counter and reference electrodes, respectively, with a scan rate of 5 mV s−1 over the range 0–0.8 V vs. Ag/AgCl. The electrolyte was an aqueous solution of Na2SO4 with pH adjusted to 2 using HCl. LSV was carried out using potentiostat module DY2300 (Digi-Ivy Inc., Austin, TX, USA). The PEC was exposed to a 400 W halogen lamp kept 20 cm away from the PEC. The light intensity was measured using a Hamamatsu (S1223) photodiode (Iwata City, Japan).
Chronoamperometry; The reproducibility, stability and durability of photocatalyst polyaniline@MnFe2O4 were examined with and without light illumination. The photocatalyst polyaniline@MnFe2O4 acted as the working electrode. Pt mesh and Ag/AgCl acted as counter and reference electrodes, respectively. The electrolyte was an aqueous solution of Na2SO4 with pH adjusted to 1 using HCl. Typically, an external bias of 0.8 V vs. Ag/AgCl was applied and the light was switched on/off every 60 s.
IPCE as a function of the wavelength at 0.8 V vs. Ag/AgCl and ABPE as a function of applied potential were investigated using a 10 W LED light with different wavelengths to evaluate the photoelectrode. The formula to calculate IPCE and ABPE followed previously studies [33,34,35].
UV-VIS Spectroscopy; Visible spectra of polyaniline and polyaniline@MnFe2O4 over 300–900 nm were obtained using a SHIMADZU UV-1800 (Kyoto, Japan).

4. Results and Discussion

The crystallinity of MnFe2O4 nanoparticles and PANi were investigated by XRD. As can be seen in Figure 1a the diffraction peaks of MnFe2O4 appeared at crystal planes of 220, 311, 222, 400, 422, 511, 440 and 533 corresponding to 2θ = 30.51°, 35.8°, 43.4°, 54.3°, 57.4° and 62.8°, respectively. It was confirmed that MnFe2O4 had high crystallinity with phase matching cubic spinel structure (JCPDS No. 73–1964) [36,37,38]. PANi was amorphous with the characteristic wide peak at 20–30°. PANi@MnFe2O4 displayed a combination of MnFe2O4 and PANi diffraction patterns, confirming its successful synthesis [39,40,41]. Further investigation using FTIR spectroscopy in Figure 1b confirmed the successful synthesis. MnFe2O4 displayed unique featured absorption bands in the 400–600 cm−1 range, two peaks at 534 cm−1 and 644 cm−1 related to the intrinsic stretching vibration of the metal-oxygen bond at the octahedral and tetrahedral sites, respectively, of spinel MnFe2O4 [16,42]. For PANi, the peaks at around 1570 cm−1 and 1421 cm−1 originate from stretching vibration of C=C in quinoid and benzenoid rings, respectively. The C-N and C=N stretching modes of the benzenoid ring appear at 1300 cm−1 and 1128 cm−1, respectively [43,44,45]. Most of the characteristic peaks of PANi and MnFe2O4 were observed in PANi@MnFe2O4 confirming successful synthesis of the composite.
SEM and HRTEM investigations were used to evaluate morphology and crystal structure. SEM of MnFe2O4 in Figure 2a shows that MnFe2O4 forms connected nano-spherical, pearl necklace-like morphology with a diameter of about 10–20 nm. Higher ratios of MnFe2O4 cause the Mn2Fe2O4/PANi to become unstable and rough as shown in Figure S1.
Crystallinity was further investigated by HRTEM and SAED. The images shown in Figure 2b,c displayed lattice parameters of interplanar crystal, the d-space of 0.257 nm can be assigned to (311) plane and also SAED shows the most brightness cycle line belongs to (311) while others assigned to (220) and (400). These results were consistent with XRD results which showed high intensity at peak (311). The surface of PANi@MnFe2O4 photoelectrode is shown in Figure 2d; it is characterized by high roughness formed during casting on FTO. PANi and MnFe2O4 each show broad visible-near infrared absorbance (Figure 3a). Thus the combination of PANi and MnFe2O4 results in a wide range of activity between 300–820 nm [46] as a result of higher light harvesting [47]. The bandgap energy has been calculated from the Tauc relation [48], with results shown in Figure 3b. It has observed values of 1.4 eV, 1.25 eV and 1.1 eV for PANi, MnFe2O4 and PANi@MnFe2O4, respectively.
The bandgap of the heterojunction was reduced by 0.15 eV leading to higher light harvesting compared with the other samples, thus enhancing the photoelectrochemical activity. The PANi@MnFe2O4 interface or conductive polymer/metal oxide interface can be considered to involve a strong interaction and a weak interaction. In the strong interaction, a chemical reaction has occurred at the interface region leading to reforming of chemical bonds, where some sites contain oxygen or metal vacancies and non-symmetry structures. The weak interaction is less effective and can lead to the creation of a defect state within the interface region, which can form additional occupied or unoccupied states within the bandgap. Thus, for this reason, there is an extended bandgap by 0.15 eV.

Characterization of Photocatalytic Activity

Figure 4 presents LSVs of A0, A-Fe1, A-Fe2 and A-Fe3, whose sample preparation ratios are shown in Table 1. The pure A0 PANi film produced a very low photocurrent of around 25 µA/cm2. In contrast, when the PANi contained MnFe2O4 nanoparticles, the photocurrent increased significantly. As can be seen in Figure 3, the photocurrent of A-Fe1 increased to 160 µA/cm2 (4-fold greater than PANi A0 film). The photocurrent increased by 120 µA/cm2 due to light illumination. In addition, it was observed that when the ratio of MnFe2O4 nanoparticles was increased in the PANi, the photocurrent rose due to the PEC photocatalyst activity. A-Fe3 showed the highest photocurrent response of around 400 µA/cm2 (around 12-fold higher than PANi A0 film).
The films A0, A-Fe1, A-Fe2 and A-Fe3 were further investigated for PEC activity through chronoamperometry under switching the light on/off every 60 s (Figure 5). As expected, the PANi film showed the lowest current density of 100 µA/cm2 in the dark and 125 µA/cm2 under light illumination. In contrast, the optimized film A-Fe3 showed the highest current of 120 µA/cm2 in the dark and 520 µA/cm2, marking a gain of more than 400 µA/cm2 due to light illumination. All of the above films demonstrated reproducible currents and durability over the time range that was used (0–200 s). It was also observed at the end of the test that the A-Fe3 film had lost only 10% of its initial photocurrent. Thus, the presence of MnFe2O4 strongly enhanced the PEC performance toward the OER. A-Fe3 and PANi A0 films were further examined for long-term stability and durability for more than 900 s (Figure 6).
The film A-Fe3 showed a constant photocurrent of 400 µA/cm2 during this test. In contrast, PANi A0 film showed a very low current density that started to degrade after just 200 s of PEC operation. PANi has been considered as a hole injecting layer in organic photovoltaic cells [49,50,51,52]. Therefore, the combination of PANi with n-type semiconductors like MnFe2O4 [53] resulted in improved photoinduced charge separation.
Recently, conductive polymers have been examined as a hole transport layer (HTL) for photocatalyst, solar cell and light-emitting-diode. Because PANi has high electric conductivity and is a p-type semiconductor, it has previously been demonstrated that PANi exhibits excellent hole extraction ability and, therefore, hole transport leading to reaction in a time shorter than the time needed for recombination of the electron-hole [54].
Finally, to confirm the high PEC activity of A-Fe3 film in acidic media (at pH 2), A-Fe3 films were used as working electrodes during the recording of chronoamperograms with aqueous media of different pH values. As can be seen in Figure 7, the film exhibited a very low photocurrent response at pH 14. However, when the pH was changed toward lower values, the photocurrent increased to higher values and peaking at pH 2, the acidic electrolyte enhances the proton transport rate and H+ represented by the proton-reduction reaction.
Further characterization was achieved with measurement of IPCE as a function of wavelength at 0.8 V vs. Ag/AgCl, and ABPE as a function of applied potential under halogen lamp illumination (Figure 8). A-Fe3 demonstrated a decrease in IPCE from 60% at 405 nm to 30% at 620 nm, and a decrease in ABPE from 0.17% at 0.8 V vs. Ag/AgCl to 0.01% at 0 V vs. Ag/AgCl. In comparison, A0 demonstrated lower IPCE and ABPE results, consistent with the above photocurrent results (Figure 8). The overall results indicated that PANi accelerated photoinduced charges in the conduction and valence bands of MnFe2O4 to the surface to react with water on a timescale shorter than recombination or reaction with a bulk photocatalyst, thus preventing photocorrosion. In acidic electrolyte, it may be considered that our photoelectrode is stable and efficient for water-splitting to hydrogen (H2) and oxygen (O2) gases in the present solar light. For comparison with previous work, the table below contains specific previous works that related to our work with common materials like ZnO and TiO2. Table 2 shows excellent results of our work compared with previous in similar conditions and good candidate photoelectrode for water-splitting to hydrogen.

5. Conclusions

MnFe2O4 nanoparticles were prepared by a hydrothermal method and then mixed with dissolved polyaniline to prepare slurries that were cast onto flexible FTO to prepare photoelectrodes. UV-VIS results showed extended absorption in the visible spectrum and near-infrared. Photoelectrochemical measurements revealed that photocurrent significantly increased from 25 to 400 μA/cm2 at 0.8 V vs. Ag/AgCl in Na2SO4 (pH = 2 adjusted with H2SO4) at 100 mW/cm2 with the addition of MnFe2O4 to Polyaniline. IPCE and ABPE measurements showed that A-Fe3 reached 60% at 405 nm and 0.17% at 0.8 V vs. Ag/AgCl, respectively. The results showed enhanced photoelectrochemical activity and stability of MnFe2O4/Polyaniline composites in an acidic environment.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/colloids6010001/s1, Figure S1: SEM image showing that higher ratios of causes the Mn2Fe2O4/PANi surface to become unstable and rough.

Author Contributions

Conceptualization, M.A. and A.A.-K.; methodology, M.A. and A.A.-K.; investigation, A.A.-K., S.A.-R., and I.G.T.; writing—original draft preparation, M.A. and A.A.-K.; writing—review and editing, G.F.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University of Technology, Baghdad, IRAQ.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available on reasonable request from the corresponding authors.

Acknowledgments

This work was supported by the University of Technology, Baghdad, IRAQ. We are grateful to Nanotechnology and Advanced Materials Research Centre, University of Technology, Baghdad, Iraq for investigations of all samples and materials and facilities.

Conflicts of Interest

The authors declare that they have no conflict of interest.

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Figure 1. X-ray Diffraction (a), and FTIR (b) of MnFe2O4, PANi@MnFe2O4 and PANi.
Figure 1. X-ray Diffraction (a), and FTIR (b) of MnFe2O4, PANi@MnFe2O4 and PANi.
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Figure 2. (a) SEM, (b) HRTEM and (c) SAED of MnFe2O4. (d) Top view SEM of PANi@MnFe2O4 photoelectrode surface.
Figure 2. (a) SEM, (b) HRTEM and (c) SAED of MnFe2O4. (d) Top view SEM of PANi@MnFe2O4 photoelectrode surface.
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Figure 3. Visible light absorbance (a) and bandgap (b) of PANi, MnFe2O4 and polyaniline@MnFe2O4.
Figure 3. Visible light absorbance (a) and bandgap (b) of PANi, MnFe2O4 and polyaniline@MnFe2O4.
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Figure 4. LSV of PANi A0, A-Fe1, A-Fe2 and A-Fe3 films. The film acted as the working electrode in the PEC while Ag/AgCl and Pt mesh acted as a reference and Counter electrode respectively; illuminated with light (100 W of halogen lamp). A solution of HCl adjusted pH to 2 was used in Na2SO4 electrolyte.
Figure 4. LSV of PANi A0, A-Fe1, A-Fe2 and A-Fe3 films. The film acted as the working electrode in the PEC while Ag/AgCl and Pt mesh acted as a reference and Counter electrode respectively; illuminated with light (100 W of halogen lamp). A solution of HCl adjusted pH to 2 was used in Na2SO4 electrolyte.
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Figure 5. Chronoamperograms over 200 s (at 0.80 V (vs. Ag/AgCl) in solution Na2SO4 adjusted to pH 2, with and without light illumination (100 W of halogen lamp), of flexible FTO slides coated with control thin films comprising of PANi A0, A-Fe1, A-Fe2 and A-Fe3.
Figure 5. Chronoamperograms over 200 s (at 0.80 V (vs. Ag/AgCl) in solution Na2SO4 adjusted to pH 2, with and without light illumination (100 W of halogen lamp), of flexible FTO slides coated with control thin films comprising of PANi A0, A-Fe1, A-Fe2 and A-Fe3.
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Figure 6. Chronoamperograms over 900 s (at 0.80 V (vs. Ag/AgCl) in the solution of HCl adjusted to pH 2, with light illumination (100 W of halogen lamp) of flexible FTO slides coated with control thin films comprising of PANi A0 and A-Fe3.
Figure 6. Chronoamperograms over 900 s (at 0.80 V (vs. Ag/AgCl) in the solution of HCl adjusted to pH 2, with light illumination (100 W of halogen lamp) of flexible FTO slides coated with control thin films comprising of PANi A0 and A-Fe3.
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Figure 7. Chronoamperograms (at 0.80 V (vs. Ag/AgCl), with light illumination (100 W halogen lamp) of flexible FTO slides coated with thin films comprising of A-Fe3 at different pH values.
Figure 7. Chronoamperograms (at 0.80 V (vs. Ag/AgCl), with light illumination (100 W halogen lamp) of flexible FTO slides coated with thin films comprising of A-Fe3 at different pH values.
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Figure 8. (a) IPCE and (b) ABPE of PANi A0, A-Fe1, A-Fe2 and A-Fe3.
Figure 8. (a) IPCE and (b) ABPE of PANi A0, A-Fe1, A-Fe2 and A-Fe3.
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Table
Table 1. Sample preparation ratios.
Table 1. Sample preparation ratios.
SamplePolyaniline Solution (µL)Mn2Fe2O4
(mg)
A00.250
A-Fe10.2513
A-Fe20.2526
A-Fe30.2552
Table
Table 2. Comparison of result with previous results.
Table 2. Comparison of result with previous results.
Photoelectrode/FTOElectrolyteMax. PhotocurrentRef.
PANI0.1 M Na2SO4/N280 μA/cm2 at 0.7 V[52]
PANI@ZnO0.1 M Na2SO425 μA/cm2 at 0.8 V[50]
PANi/TiO20.1 M NaCl50 μA/cm2 at 0.8 V[45]
Camphor sulfonic acid doped PANi-WO30.1 M Na2SO4200 μA/cm2 at 0.8 V[51]
TiO2/polyaniline0.1 M Na2SO480 μA/cm2 at 0.8 V[53]
PANI@ MnFe2O4H2SO4 and Na2SO4400 μA/cm2 at 0.8 VOur work
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Alsultan, M.; Al-Rubaye, S.; Al-Keisy, A.; Swiegers, G.F.; Taha, I.G. Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution. Colloids Interfaces 2022, 6, 1. https://doi.org/10.3390/colloids6010001

AMA Style

Alsultan M, Al-Rubaye S, Al-Keisy A, Swiegers GF, Taha IG. Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution. Colloids and Interfaces. 2022; 6(1):1. https://doi.org/10.3390/colloids6010001

Chicago/Turabian Style

Alsultan, Mohammed, Shaymaa Al-Rubaye, Amar Al-Keisy, Gerhard F. Swiegers, and Intisar Ghanim Taha. 2022. "Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution" Colloids and Interfaces 6, no. 1: 1. https://doi.org/10.3390/colloids6010001

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