Photoelectrochemical cell (PEC) produces hydrogen through splitting the water using renewable sources (i.e., the sun)    . Photoelectrochemical (PEC) cells have been used to convert solar energy to hydrogen gas by splitting water into hydrogen and oxygen, hence offering clean and renewable energy  . Moreover, photoelectrochemical (PEC) has attracted attention since Honda and Fujishima utilized the first application of titanium dioxide (TiO2) in 1972  . Nevertheless, the large bandgap of TiO2 (3.1 - 3.3 eV) impedes the absorption of visible light, and limits the solar-to-hydrogen efficiency to 2.2%  . So, it is necessary to use material that has small bandgap and easy to harvest energy from sunlight (visible light 53%)  . Iron oxide, bismuth vanadate, tungsten oxide, and tantalum nitride are the examples of low band gap semiconducting materials  . The α-Fe2O3 is one of the most attractive photo-anode materials with efficiency of 16% to convert solar-to-hydrogen   -  . The α-Fe2O3 has been applied for photoelectrochemical applications due to low bandgap (2.1 - 2.2 eV), low cost, high chemical stability, nontoxicity, and abundance in nature  . However, α-Fe2O3 also has several drawbacks such as shorter hole diffusion length, low conductivity, shorter life time of photoexcitation and deprived reaction kinetics of oxygen evolution in photoelectrochemical applications  . The doping with several metallic ions such as zinc  , titanium   , molybdenum  , aluminum  , platinum  , silicon    , graphene   , and cadmium sulfide  have shown improved PEC performance. The zinc and aluminum doped α-Fe2O3 have shown enhanced photoelectrochemical properties compared to α-Fe2O3 nanostructures    .
Recently, two-dimensional (2D) dichalcogenide material “molybdenum disulfide (MoS2)” with bandgap of 1.8 eV has been used as n-and p-types structures for photoelectrochemical studies  . The MoS2 shows stimulating photocatalytic activity due to its bonding, chemical composition, doping, and nanoparticles growth on various film matrices, and has been used for hydrogen production in nanocluster structures      . Besides, MoS2 has shown different applications in photocatalyst, phototransistors and sensors applications  . It is understood that MoS2 could help to play an important role as the charge transfer with slow recombination of electron-hole pairs created due to photoenergy with the charge transfer rate between surface and electrons  .
Under this work, MoS2 particles were used to promote electron transport properties of the α-Fe2O3 nanomaterial by doping and homogenous structure due to MoS2-α-Fe2O3 nanomaterials. The doping of MoS2 particles varied by 0.1%, 0.2%, 0.5%, 1%, 2% and 5% in α-Fe2O3. The MoS2-α-Fe2O3 nanomaterials were characterized using X-ray diffraction, SEM, FTIR, Raman spectroscopy, particle analyzer, and UV-vis techniques. The cyclic voltammetry (CV) and impedance measurements were utilized to understand the electrochemical electrode/electrolyte interface and photoelectrochemical properties of MoS2-α-Fe2O3 based nanostructures for water splitting applications.
2. Experimental Details
The materials iron chloride (FeCl3), aluminum chloride (AlCl3), sodium hydroxide (NaOH), MoS2, and ammonium hydroxide (NH4OH) were purchased from Sigma-Aldrich. The fluorine tin oxide (FTO) coated glass with resistance of ~10 Ω was also procured from Sigma-Aldrich. The centrifuged containers were purchased to clean the synthesized nanomaterials from the solution.
2.2. Experimental Procedure
α-Fe2O3 and MoS2-α-Fe2O3 were synthesized by a sol-gel technique as shown in Equation (1).
Table 1 shows the amount of chemicals used for the synthesis of MoS2-α-Fe2O3. Different concentrations of FeCl3 with MoS2 were prepared in 500 ml round bottom flasks. NaOH solution was added to the resulting solution and stirred with a magnet for an hour. A condenser was connected to the round bottom flask, which allowed chemical reaction to proceed at 90˚C - 100˚C. The reaction was terminated after 24 hours, and the solution was cooled at room temperature. The synthesized material was separated using a centrifuge and continuous cleaning with water and initially left drying in room temperature. The synthesized materials containing different ratios of α-Fe2O3 to MoS2 in MoS2-α-Fe2O3 were obtained. Figure 1 shows the photographs of the MoS2-α-Fe2O3 materials synthesized using various percentage of MoS2 to α-Fe2O3. The immediate doping of 0.1% MoS2 changes the color of α-Fe2O3, whereas the dark red color can be visualized with the increase of MoS2 percentage in α-Fe2O3. The MoS2-α-Fe2O3 at various ratio were dried at various temperatures (100˚C, 200˚C, 300˚C, 400˚C,
Table 1. The amount of chemical used for synthesis of MoS2-composite α-hematite.
Figure 1. The synthesized α-hematite (α-Fe2O3) and MoS2-α-Fe2O3 composite materials.
and 500˚C). In each case, the temperature was maintained in furnace for one hour. The materials were collected by cooling at room temperature and stored in a tight bottle for characterization as well preparation of electrodes for electrochemical and photochemical studies.
2.3. The Film Formation of Substrate
The MoS2-α-Fe2O3 was prepared at different concentrations by mixing with acetic acid to obtain the homogenous solution to cast film on various substrates. 500 mg of MoS2-α-Fe2O3 (0.1%, 0.2%, 0.5%, 1%, 2%, and 5%) was grinded and then mixed into 10 ml acetic acid in a container, and left for 10 hours. Later, homogenous colloidal solution containing MoS2-α-Fe2O3 with acetic acid were used to make films on quartz, silicon, and fluorine tin oxide (FTO) coated glass plates. The films were cured at different temperatures (100˚C, 200˚C, 300˚C, 400˚C, and 500˚C) for an hour. It has been observed that the nanomaterials treated at 100˚C to 200˚C could still have the water molecules. However, the temperature at around 300˚C allowed to have a solid material. The nanomaterials were further treated to 400˚C and 500˚C. The XRD, SEM, cyclic voltammetry, and UV-vis characterizations were performed in room temperature in cooled samples which were heated till 500˚C of MoS2-α-Fe2O3 films. We had observed passivation, change in structure and morphology in the samples treated at 300˚C, 400˚C and 500˚C. However, the results are presented for the samples treated at 500˚C due to their enhance photocurrent.
3. Results and Discussions
3.1. UV-Vis Studies
Figure 2 shows the UV-vis spectra of α-Fe2O3, MoS2 and α-Fe2O3-MoS2-prepared at a different ratio of MoS2 to α-Fe2O3. An UV-Vis Spectrometer Jasco V-530 was used to measure the absorption spectra on various samples deposited on glass plates. Figure 2(a) shows the UV-vis absorption at around 550 nm for the
(a) (b) (c) (d)
Figure 2. UV-vis absorption spectra of MoS2 with α-hematite nanocomposite.
pristine α-Fe2O3 similar to shown in literature. Figure 2(b) shows the characteristics absorption bands 388, 453, 618 and 679 nm for the MoS2 nanomaterial film on glass plates. Figures 2(c)-(f) shows the UV-vis absorption spectra for MoS2 doped in different percentage (0.1%, 0.2%, 1% and 5%) with α-Fe2O3 nanomaterial. Figure 2(c) shows the absorption bands at 282, 454, 463 nm. Figure 2(d) shows the absorption bands at 446 and 565 nm. The distinct peaks can be seen at 382, 461 and 570 nm. Figure 2(e) absorbs the UV-vis band at 382, 456 and 559 nm whereas Figure 2(f) shows the absorption band at 382, 459 and 572 nm. There is a blue shift as an increase of MoS2 in α-Fe2O3  . However, the band observed for 0.1% MoS2 doping is shifted at 572 nm in 5% MoS2 doping in α-Fe2O3 nanomaterial. Suchresults are consistent with the result shown of transition composite metal ions  . The UV-vis spectra of the composite hematite have been estimated to be 2.17 eV for the band at 572 nm.
3.2. XRD Studies
The crystalline structure of MoS2-α-Fe2O3was investigated by using Powder X-ray diffraction (XRD), model PANalytical X’Pert Pro MRD system, with Cu K α radiation (wavelength = 1.5442 Å) operated at 40 kV and 40 mA. Figure 3 shows X-ray diffraction curves for different percentage of MoS2 (0.1%, 0.2%, 0.5%, 1%, 2%, and 5%) to α-Fe2O3; α-Fe2O has polycrystalline structure as revealed from XRD pattern. The diffraction common peaks in MoS2-α-Fe2O3 nanocomposite at different percentage of MoS2 displays bands at 31.2˚, 33.2˚, 37.5˚, 40.9˚, 49.5˚, 54.1˚, 62.2˚, and 64.2˚ which can be indexed to (012), (104), (110), (113), (024), (116), (214), and (300) for crystal planes of hexagonal iron oxide  . It is clear from strong and sharp diffraction peaks that Fe2O3 is well crystallized
Figure 3. X-ray diffraction pattern of MoS2 with α-hematite nanocomposite.
in the synthesis process for all percentage of MoS2 in α-Fe2O3  . The peak at 54.1˚ is due to the presence of MoS2 in MoS2-α-Fe2O3-structure.
3.3. FTIR Studies
Perkin Elmer spectrum one was utilized to study FTIR spectroscopy of various samples of MoS2-α-Fe2O3-nanocomposite. The MoS2-α-Fe2O3-nanocomposite was mixed with KBr, the pellets were made using the hydraulic press, and the samples were measured using the transmission mode from 400 to 4000 cm−1. FTIR spectra of MoS2-α-Fe2O3 shows the change of percentage of MoS2 doping with α-Fe2O3 with Curve 1% to 5%, Curve 2% to 0.2%, Curve 3% to 2%, Curve 4 to 1%, Curve 5% to 0.5%, and Curve 6% to 0.1% of MoS2 in MoS2-α-Fe2O3 in shown in Figure 4.. The infrared bands of each MoS2 doping to α-Fe2O3 are shown in Table 2.
The hydroxyl (OH) groups in α-Fe2O3 is related to infrared band at 3414 cm−1. The band at 1642 cm−1 is due to n (OH) stretching. The band at 562 cm−1 is due to Fe-O vibration mode in Fe2O3. The band at 620 - 654 and 474 - 512 are related to the lattice defects in Fe2O3   . The infrared band at 474 - 512 cm−1 is due to stretching vibration depicting the presence of MoS2 in the MoS2-α-Fe2O3 structure. The doping of 0.1% to 5% of MoS2 shifts the infrared band from 512 cm−1 to 474 cm−1. The band at 474 cm−1 is the band observed for exfoliated MoS2 nanosheets revealing that maximum doping in MoS2-α-Fe2O3 structure  .
3.4. SEM Studies
The scanning electron microscopy (SEM) of various MoS2-α-Fe2O3 samples were measured using FE-SEM, S-800, Hitachi. Figure 5 shows SEM images of MoS2-α-Fe2O3 nanomaterials which consisted of different percentages from 0.1 to 5% MoS2 to Fe2O3 in MoS2-α-Fe2O3. SEM images reveals that the morphology
Table 2. The infrared bands of each MoS2 doping to α-Fe2O3.
Figure 4. FTIR spectra of MoS2 with α-hematite nanocomposite. Each curve MoS2 doping with Fe2O3 is given as: Curve 1 = 5% MoS2, Curve 2 = 0.2% MoS2-Fe2O3, and Curve 3 = 2% MoS2-Fe2O3, Curve 4 = 1% MoS2-Fe2O3, Curve 5 = 0.5% MoS2-Fe2O3 and Curve 6 = 0.1% MoS2-Fe2O3.
MoS2-α-Fe2O3 resembles blooming flower-like nanoparticles. The blooming flower-like morphology is a result of doping MoS2 with α-Fe2O3  . The images reveal that the size of the particle changes with the increase of MoS2 doping from 0.1% to 5% in MoS2-α-Fe2O3 nanomaterial. Besides, it is difficult to differentiate simple α-Fe2O3 nanoparticles from MoS2 nanosheets; this shows a strong interface formation between Fe2O3 and MoS2in MoS2-α-Fe2O3 nanomaterial  .
3.5. Raman Spectroscopy
The Raman spectrum is measured which is also a rapid and nondestructive surface characterization technique to probe the vibrational properties of bonding of MoS2 to Fe2O3 in MoS2-α-Fe2O3 nanomaterial. Figure 6 shows the Raman spectra of MoS2-α-Fe2O3 film excited by 532 nm laser  . The Raman shift at 532 cm−1 resonates with the electronic transition in ring structures for aromatic clustering processes in sp2-dominated particles. The shift associated at 374 and 417 cm−1 are due to in-plane vibrational (E2g1) and the out-of-plane vibrational (A1g) modes. The enhanced MoS2 is indicative of energy difference between Raman shifts due to MoS2 content in MoS2-α-Fe2O3 nanomaterial.
Figure 6. Raman spectra of MoS2-α-Fe2O3 film sample and ITO substrate as various percentage of MoS2 as shown in figures.
3.6. Particle Analysis
The Zetasizer Nano particle analyzer range model was used to measure the average particle size of various MoS2-α-Fe2O3 samples. Initially, the MoS2-α-Fe2O3 nanomaterial was dispersed in water and ultra-sonicated to have aggregated free colloidal sample. Figure 7 shows the particle size of MoS2-α-Fe2O3 as a function of MoS2 doping in α-Fe2O3. The average particle size in liquid sample ranges from 459 nm (0.1%) to 825 nm for (5%) do pant of MoS2 respectively. Although these particles are small, there are few particles which are larger than 5 microns. These larger particles that can be detected through SEM measurement are a result of aggregation. The average size of particles is important for the fabrication of the electrodes from the particles. This information of nanomaterial dispersion of MoS2-α-Fe2O3 can be exploited for the electrode fabrication or other applications.
3.7. Electrochemical Studies
3.7.1. Cyclic Voltammetry
The electrochemical measurements on various MoS2-α-Fe2O3 electrodes were measured from electrochemical workstation (Volta lab). The electrochemical set-up was adopted similar to our earlier studies on hybrid films   . Figure 8 shows the cyclic voltammetry (CV) of 1% MoS2-α-Fe2O3 in 1M NaOH as working electrode, platinum (Pt) as counter and Ag/AgCl as reference electrode in three electrodes based electrochemical cell. The continuous increase of CV current was observed with an increase in function of scan rate. The presence of MoS2 ions induces the electrochemical properties and 1.3 V can be seen as oxidation potential of water that is less than the Aluminum-doped from our previous studies  .
The CV is shown in Figure 9 with application of light simulated for solar radiation. However, with the scan rate of 100 mV/sec, there was a maximum photocurrent absorbed for MoS2-α-Fe2O3 film. The diffusion coefficient was
Figure 7. The particle size measurement of MoS2-α-Fe2O3 nanocomposite materials as a function of MoS2 dopant.
Figure 8. The cyclic voltammetry of 1% MoS2 with Fe2O3 nanocomposite without light in 1 M NaOH in three electrodes where platinum as reference and Ag/AgCl as reference electrode.
Figure 9. The cyclic voltammetry of 1% MoS2 with Fe2O3 nanocomposite with light in 1 M NaOH in three electrodes where platinum as reference and Ag/AgCl as reference electrode.
calculated by using peak current for a reversible cyclic voltammetry is given by the Randles-Sevcik equation (Equation (2)).
n = number of electrons
A = electrode area (cm2)
C = concentration (mole/cm3)
D = diffusion coefficient (cm2/s)
ν = potential scan rate (V/s)
Ip = current.
The diffusion coefficient has been estimated to be 0.24 × 10−16 cm2/s.
3.7.2. Chronoamperometry Study
We made an attempt to deposit MoS2-α-Fe2O3 film on ITO coated glass substrates uniformly using the homogenous paste obtained using acetic acid. The thickness of MoS2-α-Fe2O3 was around 30 µm. Figure 10(a) & Figure 10(b) shows the chronoamperometry study of two electrodes cell consisting of MoS2-α-Fe2O3 film as working and steel as counter in various concentrations (0.01 0.1, 1 M) of NaOH based electrolyte. The potential from −1000 mV to 1500 mV was applied, and the chronoamperometry photocurrent was studied. Figure 10(a) & Figure 10(b) shows the chronoamperometry photocurrent plot with t−1/2 for oxidation and reduction processes for MoS2-α-Fe2O3 film. The rise of photocurrent showed a linear relationship with t−1/2 due to excitation of light. The current transient was different from the excitation of light. The diffusion-controlled photocurrent is calculated using Cottrell equation in Equation (3)    .
n = the electron participating in the reaction
F = the faraday constant
A = the area of the electrode
i = the transient current
D = the diffusion coefficient
C = the concentration of the electrolyte
The D has been estimated to be 1.057 × 10−14 cm2/sec.
3.7.3. Impedance Study
Figure 10. The chronoamperometry photocurrent plot with t(s)-1/2 for oxidation and reduction processes for MoS2-α-Fe2O3 film.
Figure 11. Nyquist plot of MoS2-α-Fe2O3 film in 1 M HCl in photoelectrochemical cell without (a) and with (b) light irradiation.
set-up. The change in the impedance value has been observed for real and imaginary without light irradiation as shown in Figure 11(a) and Figure 11(b). The photocurrent is able to make process more conducting in presence of light.
3.7.4. Half Sweep Potential
Figure 12 shows the half sweep potential with and without light for both aluminum doped-α-Fe2O3and MoS2-α-Fe2O3. Our previous study on aluminum doping has shown the photocurrent to be 35 μA whereas for the same type of electrode for MoS2-α-Fe2O3 showed the current to be 150 μA. Schottky type current-voltage is experienced for both aluminum doped as well as MoS2-α-Fe2O3 based electrode in photoelectrochemical cell.
3.8. Schematic of MoS2-α-Fe2O3 Reaction Process
A schematic was drawn to understand the effect of MoS2 with α-Fe2O3. The schematic of hydrogen production using MoS2-composite α-Fe2O3 photocatalyst in 1 M NaOH is shown in Figure 13. The bandgap of MoS2 varies from 1.2 - 1.9 eV, whereas the band gap of α-Fe2O3 is 2.1 eV. It was estimated the bandgap of MoS2-composite α-Fe2O3 in range of 1.94 to 2.40 eV based on UV-vis measurements, which is well in the region of visible light. MoS2 doping also increased the conductivity of the samples. The schematic in Figure 12 shows the photogenerated electrons from conduction band of MoS2 is transferred to conduction band (CB) of α-Fe2O3 whereas holes from α-Fe2O3 e are transferred to valence band (VB) of MoS2. This enhances the photocatalytic activity of MoS2 composite with α-Fe2O3 in MoS2-α-Fe2O3 nanomaterial-based electrode.
The synthesized MoS2-α-Fe2O3 observed the shift in the band gap to 2.17 eV with MoS2 doping. There is a marked change in the band due to MoS2 doping in α-Fe2O3. The increase of MoS2 dominated the structure as marked from SEM measurements. The photocurrent can be clearly distinguishable with and without light irradiation through various electrochemical studies on MoS2-α-Fe2O3
Figure 12. Half sweep potential with and without light for Aluminum doped-α-Fe2O3 and MoS2-α-Fe2O3 film with and without light exposure.
Figure 13. The schematic of hydrogen production using MoS2-composite α-Fe2O3 photocatalyst in 1 M NaOH.
nanomaterial. The enhanced photocurrent is observed with MoS2 doping in MoS2-α-Fe2O3 nanomaterial. The MoS2-α-Fe2O3 nanomaterial thin film has the potential to produce hydrogen using a PEC water splitting process that could have renewable energy applications. Our future work is based on the use of MoS2-α-Fe2O3 as n-type in p-n photoelectrochemical studies for efficient water splitting applications.
The authors are grateful to Sina and Mike McCrory for their help in X-ray diffraction, EDS and SEM measurements.
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