Iron Pyrite/Titanium Dioxide Photoanode for Extended Near Infrared Light Harvesting in a Photoelectrochemical Cell.

The design of active and stable semiconducting composites with enhanced photoresponse from visible light to near infrared (NIR) is a key to improve solar energy harvesting for photolysis of water in photoelectrochemical cell. In this study, we prepared earth abundant semiconducting composites consisting of iron pyrite and Titanium oxide as a photoanode (FeS2/TiO2 photoanode) for photoelectrochemical applications. The detailed structure and atomic compositions of FeS2/TiO2 photoanode was characterized by high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), inductively coupled plasma with atomic emission spectroscopy (ICPAES) and Raman spectroscopy. Through the proper sulfurization treatment, the FeS2/TiO2 photoanode exhibited high photoresponse from visible light extended to near infrared range (900 nm) as well as stable durability test for 4 hours. We found that the critical factors to enhance the photoresponse are on the elimination of surface defect of FeS2 and on the enhancement of interface charge transfer between FeS2 and TiO2. Our overall results open a route for the design of sulfur-based binary compounds for photoelectrochemical applications.

Solar-induced water splitting by photoelectrochemical (PEC) cells provides an ideal solution to generate hydrogen energy, which is derived by electrochemical photolysis of H2O with semiconductors as photoanode and photocathode materials123. The effectiveness of photo-driven electrolysis processes showed strong dependency on the capability of absorbing UV, visible and infrared (UV-vis-NIR) light of semiconductors, as well as their ability to suppress the rapid combination of photogenerated electrons and holes45. Titanium dioxide (TiO2) has been considered to one of most attractive materials for PEC application because of its high photocatalytic activity and excellent chemical stability in the strong alkaline solution678. However, the absorption spectrum of TiO2 with large band gap (~3.2 eV) is only located on UV light (5% of sunlight), which cause less energy conversion efficiency. Recently, researchers have paid attention on finding the solutions to extend absorption range of TiO2 to visible light for enhancing light harvesting ability. An efficient method to narrow the band gap of TiO2 was utilizing chemical doping91011 or increasing of defect states1213 in TiO2 crystal structure. For example, a study indicated that the band gap of TiO2 was successfully reduced to 1.53 eV (absorption spectrum extend to ~810 nm) by introducing disorder in the surface layers of TiO2 through hydrogenation12. Although chemical doping TiO2 exhibited a great optical response to solar radiation, its absorption range in the visible and infrared remains insufficient9.

The way to extend light harvesting of TiO2 photoanode from visible and even near infrared (NIR) range is sensitizing lower band-gap chalcogenide semiconductors on TiO2, such as CdS1415, CdSe16, and PbS1718. The approaches have been widely applied in quantum-dot sensitized solar cells (QDSSCs)1920 and photoelectrochemical cell21. The advantages of these chalcogenides materials are their low band gaps (CdS~ 2.4 eV, CdSe~1.7 eV and PbS~1 eV) and efficient charge transfer from the chalcogenides to TiO2 due to their type II electronic band structure15. For examples, the N doping of TiO2 nanowires sensitized by CdSe as the photoanode in PEC resulted in photocurrents close to 3 mA·cm−2 22. Other reports have also highlighted the importance of the controlled deposition of the light-absorbing semiconductor (CdSe) on inverse opals of TiO2, resulting in photocurrents of 15.7 mA·cm−2 under AM 1.5 illumination23. However, Both Cd and Pb elements are considered to be quite toxic24. Therefore, searching low-cost and environmental-friendly materials as alternatives to toxic metal is crucial to make PEC more competitive for future commercial applications.

Earth-abundance and non-toxicity pyrite iron disulfide (FeS2) is a potential candidate to be applied for next-generation photovoltaic because it’s large optical absorption coefficient (>105 cm−1) and a narrow band gap of 0.95 eV2526. FeS2 has been predicted as showing the highest material availability among 23 existing semiconducting photovoltaic systems, which potentially lead to substantially lower costs than silicon24. Many recent studies indicated that FeS2 has been successfully applied in the photo-electronic devices with a photoresponse from near infrared (NIR) range272829. Previous reports have demonstrated the successful fabrications of pyrite NC-based polymer hybrid solar cell30 and photodiode devices3132 with a spectral response extended to near infrared (NIR) wavelengths. Also, we found that the catalytic activity of FeS2 nanocrystals (NCs) in dye-sensitized solar cell as a counter electrode showed comparable catalytic efficiency with traditional precious Pt electrode33. However, the photovoltaic devices based on the FeS2 materials are still lacking of photovoltaic response due to the highly conductive surface-related defects in pyrite3435. Although several recent reports indicated the FeS2 film was employed as a photoanode in PEC, the results showed the limited photoresponse in the visible light2836. Therefore, it is still a great challenge to explore a new PEC photoanode using FeS2 materials with enhanced photocurrent response and extended light response to near infrared (NIR) range.

In this study, the photoanode consisting of earth abundant FeS2 formed on TiO2 thin film (FeS2/TiO2) for PEC applications was successfully prepared. The structure of FeS2/TiO2 photoanode was carefully characterized by high resolution scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction and Raman microscopy. Also, the photocurrent response of the photoanode was measured under AM 1.5 illumination and NIR laser (808 nm) irradiation. We found that the photoresponse of the photoanode showed strong dependency on the sulfur deficiency and surface defect of FeS2. With proper sulfurization treatment, the surface defect of the FeS2/TiO2 photoanode was reduced, which optimized the photocurrent response of the photoanode.

Experimental Section

Fabrication of FeS2 NCs

In brief, FeCl2 (189 mg), 1,2-hexadecanediol (384 mg), octadecene (30 mL), and oleic acid (OA) (12 mL) were mixed and subsequently reacted under N2 gas at 100 °C for 1 h to form the Fe–olei cacid complex. Subsequently, oleylamine (OLA) (15 mL) solution of sulfur (576 mg) was quickly injected into the solution. The resulting solution was heated to 240 °C and maintained for 1 h. After the solution was cooled to room temperature, a large amount of methanol was added to precipitate as-grown FeS2 NCs, followed by centrifugation. To obtain the FeS2 NCs solution with high solubility for film depositions, the as-grown FeS2 NCs were further purified by washing with ethanol, ethanol/chloroform (10/1 vol.), and methanol/chloroform (10/1 vol.). Subsequently, the larger NCs and any residual side products from the NCs suspension were removed by addition of chloroform, followed by centrifugation at 3500 rpm for 10 min. The resulting FeS2 NCs solution with high solubility and purification can be obtained.

Fabrications of FeS2/TiO2 photoanode

First, the fluorine doped tin oxide (FTO) glass was cleaned sequentially by a neutral cleaner, water, acetone, and IPA, as the initial step. A compact layer of TiO2 was coated on the FTO substrate using a solution, consisting of titanium (IV) isopropoxide (TTIP, + 98%, 0.5 g) in 2-methoxy ethanol (1.5 g), not only to obtain a good mechanical contact between the FTO and the TiO2 film but also to isolate the contact between the FTO and the electrolyte. Another TTIP solution was then hydrolyzed to acquire the TiO2 in a media, containing 0.1 M HNO3, by adopting a sol-gel method. The thus obtained TiO2 solution was autoclaved through a hydrothermal process at 240 oC for 12 h. By concentrating the autoclaved solution to 13 wt%, a paste of nanocrystalline TiO2 was obtained. In order to prevent the paste from cracking and to control the pore size of TiO2, 15 wt% of PEG corresponding to the amount of TiO2 was added to the paste. The TiO2 layer as a photoanode on FTO for PEC was prepared through the following procedure. The TiO2 paste prepared above was coated on the FTO glass by using a doctor-blade method. The thus coated FTO glass was annealed at 450 oC for 30 min. After repeating such a coating and sintering, another layer of TiO2 containing light scattering particles of 300 nm was coated on the FTO glass, and the sintering was performed in the same way. A TiO2 with active area of 1.0 cm2 was dipped overnight in a solution, containing 0.3 mM FeS2 solution in cholorform to from as-grown FeS2/TiO2 photoanode on FTO. Finally, The as-grown FeS2/TiO2 bilayer was further annealed by sulfur vapor at 450o C for 3hr to form the FeS2/TiO2 photoanode.

Electrochemical Measurement

Photoelectrochemical cell measurement was was carried out in a solution containing 0.35 M Na2SO3 and 0.24 M Na2S (pH = 13) with a standard three-electrodes system controlled by a Autolab electrochemistry workstation. The FeS2/TiO2 photoanode was used as working electrode, graphite rod as counter electrode and Ag/AgCl as reference electrode. The reference was calibrated against and converted to reversible hydrogen electrode (RHE). A AM 1.5 irradiation (100 mW/cm2, Newport Inc.) and a NIR continuous laser (808 nm) was used as the light source. Linear sweep voltammetry was carried out at 1 mV/s for the polarization curves.

Characterizations

High-resolution Transmission Electron Microscopy (TEM) (HR-TEM) images were obtained using a Philips Technai G2 (FEI-TEM) microscopy operating at 200 kV. X-ray Diffraction (XRD) measurements were performed by Bruker D8 tools advance, operating with Cu Kα radiation (λ = 1.5406 Å) generated at 40 keV and 40 mA. Scans were done at 0.01 S−1 for 2θ value between 20° and 60°. UV-Vis-NIR absorption spectra were obtained using a Cary 500 UV-Vis-NIR spectrophotometer. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to measure the atomic ratio of FeS2 NCs. The external quantum efficiencies (EQEs) were measured by using a Xe lamp in combination with a monochromator (Oriel Inc.). A UV filter was also used to avoid the overtones of the monochromator’s grating from illuminating the specimen.

Results and Discussion

The FeS2NCs were prepared using wet solution-phase chemical syntheses with some modifications according to previous reports3137. Figure 1 (a) demonstrated the UV–Vis–NIR absorption spectrum of FeS2 NC solution in chloroform. The absorption was extended to NIR wavelength ranging from 400 nm to 1300 nm. The inset of Fig. 1(a) showed the photograph image of the FeS2 NC solution, which was utilized to fabricate as grown FeS2/TiO2 photoanode by dip-coating process, as discussed in the experimental sections. Figure 1 (b) shows the high-resolution transmission electron microscopy (HR-TEM) image of FeS2 NCs with an average diameter of 15 nm. The inset of Fig. 1(b) showed the X-ray diffraction (XRD) pattern of the FeS2 NCs. The diffraction peaks were indexed to the (111), (200), (210), (211), (220), and (311) planes of pyrite cubic phase (JCPDS no. 42–1340). No other significant diffraction peak was observed in Fig. 1(b), indicating that the FeS2 materials on TiO2 film exhibited a single-phased pyrite structure.

Figure 1

(a) UV-Vis-NIR absorption spectrum of FeS2 NCs. The inset showed the photograph image of the FeS2 NCs solution. (b) TEM image of FeS2 NCs. The average sizes of the NCs are calculated to be ∼15 nm. The inset showed the x-ray diffraction (XRD) pattern of the FeS2 NCs.

For the fabrication of as grown FeS2/TiO2 photoanode, a paste of nanocrystalline TiO2 was first formed on the conductive FTO glass (TiO2/FTO) by the casting process. Then, FeS2 NC solution (80 mg/mL) was dip-coating onto TiO2/FTO substrates to form as-grown FeS2/TiO2 film on FTO substrate. Finally, to increase the crystallinity of FeS2 and reduce the interface connection, the as-grown FeS2/TiO2 film was sulfurized under sulfur vapor at a temperature of 450 oC to form the resulting FeS2/TiO2 photoanode.

To test photoresponse behavior of the FeS2/TiO2 photoanode, PEC device (Fig. 2(a)) were carried out using the FeS2/TiO2 photoanode, a Pt wire cathode, and a Ag/AgCl reference electrode in the alkaline electrolyte (pH = 13.5) with SO32−/S2O32− as sacrificial agent under simulated AM 1.5 illumination (100 mW/cm2) and NIR 808 nm laser (300 mW/cm2), respectively. The relevant energetics of each components obtained from related literature1531. The band gap of FeS2 is located around 4.0 to 4.95 eV versus vacuum energy, which is similar to our previous report31. The formation of the FeS2/TiO2 photoanode with a satisfied energy-level alignment was expected to assist charge separation of photogenerated carriers. Briefly, when incoming light excites free electrons and holes near the surface of the FeS2 electrode, the electrons and holes were separated from TiO2 as an electron acceptor layer. The electrons flowed through the TiO2 layer to the cathode electrode at the other side (Pt electrode) of the cell, where generated the hydrogen gas during water reduction reaction. The holes react with the sacrificial agent (SO32−/S2O32−) in the electrolyte which can suppress photocorrosion of metal sulfide materials38.

Figure 2

(a) Schematic illustration of the PEC device with a FeS2/TiO2 photoanode, and a passive Pt cathode, for light driven water splitting in aqueous solution. (b) The photocurrent–potential (I–V) responses of FeS2/TiO2 photoanode and pure TiO2 in the alkaline electrolyte (pH = 13.5) with SO32−/S2O32− as sacrificial agent under simulated AM 1.5 illumination (100 mW/cm2). (c) The photocurrent–potential (I–V) responses of FeS2/TiO2, and TiO2 photoanodes in the alkaline electrolyte (pH = 13.5) with SO32−/S2O32− as sacrificial agent under NIR laser (808 nm) illumination (300 mW/cm2). (d) Light chopping photocurrent measurements in a three electrode cell using FeS2/TiO2 photoanode as working electrode.

Figure 2 (b) displays the current–voltage (I–V) curves of the FeS2/TiO2 photoanode and pure TiO2 photoanode under darkness and AM1.5 simulated sunlight, respectively. The current of FeS2/TiO2 photoanode could be determined at −0.6 V versus Ag/AgCl under darkness at first, then the current rises slowly to 1 mA/cm2 at 0.15 V versus Ag/AgCl. The current of FeS2/TiO2 photoanode found at −0.6 V represented that FeS2 exhibited a catalytic activity for the sacrificial agent. When the FeS2/TiO2 electrode was illuminated under AM1.5 illumination, the current was increased and reached 2-fold of the dark current at 0.15 V. This result indicated that the FeS2/TiO2 photoanode exhibited a photoresponse under AM-1.5. In order to distinguish the photoresponse contribution from FeS2 and/or TiO2, the PEC devices were illuminated under the NIR laser (808 nm) with 300 mW/cm2 for the comparison. Figure 2 (c) showed the I−V characteristics of the device. The results showed that the anodic photocurrents of FeS2/TiO2 photoanode increased as the potential was around −0.61 V, and reached saturation (5.8 mA/cm2) when the potential was higher than −0.2 V (vs Ag/AgCl). The photocurrent of the pure TiO2 photoanode is negligible under NIR illumination, indicating that FeS2 is a major contributor to the observed photocurrent under NIR illumination. Figure 2 (d) demonstrated the current-time (i-t) characteristics of the FeS2/TiO2 photoanode under the on/off cycles of the NIR illumination at a constant bias of 0.1 V. The results indicated that the photocurrent of the FeS2/TiO2 photoanode reached saturation very fast, representing the less surface traps in the FeS2 film during sulfurization treatment.

Figure 3 (a) represented the dependence of photocurrent of the FeS2/TiO2 photoanode operated at a bias of 0.1 V as a function of incident power under excitation with an 808 nm laser. The photocurrent of the photoanode exhibited a linear increase with incident power, which may be attributed to efficient carrier transport and collection of the FeS2 thin film between the heterjunction of FeS2 and TiO2 layers. The photoconversion efficiency () curves for the FeS2/TiO2 photoanodes are presented in Fig. 3(b). The photoconversion efficiency are calculated using the following equation,

Figure 3

(a) The dependence of photocurrent of the FeS2/TiO2 photoanode operated at a bias of 0.1 V as a function of incident power under excitation with an 808 nm laser. (b) The photoconversion efficiency () curves for the FeS2/TiO2 photoanode.

where is photocurrent density, is standard state-reversible potential (i.e. 1.23 V vs. RHE), is the intensity of the incident light, and is the applied potential vs. RHE. At a bias of 0.7 V, the efficiency of FeS2/TiO2 photoanode reached~ 0.84% at NIR irradiation which was the highest efficiency in PEC measured to date for FeS2 materials2836.

To further quantify the PEC performance, incident photon to current conversion efficiency (IPCE) measurements (Fig. 4(a)) have been made to study the photoresponse of the FeS2/TiO2 photoanode from visible light to NIR. For the comparison, CdSe/TiO2 and PbS/TiO2 photoanodes were both fabricated in this work. Their detailed synthesis, characterization and device fabrications were described in the supporting information. We found that the FeS2/TiO2 photoanode and PbS/TiO2 photoanode showed higher photoresponse in the NIR region than that of as grown FeS2/TiO2 photoanode and CdSe/TiO2 photoanode. No photoresponse of the pure TiO2 photoanode was found from wavelength of 600 nm to 900 nm due to its large band gap of 3.7 eV. Moreover, the IPCE value of FeS2/TiO2 photoanode at illumination light from 600 nm to 900 nm is improved~ 2-fold in comparison with as-grown FeS2/TiO2. The stability of the FeS2/TiO2 photoanode was studied by a chronoamperometric (i–t) measurement (Fig. 4(b)). Under NIR light irradiation, the clearing of bubbles found at cathode electrode in our PEC device. The results of i–t measurement showed that the photocurrent of the FeS2/TiO2 photoanode remained stable over continuous operation for 4 hours under alkaline conditions at a bias of 0.1 V versus Ag/AgCl reference. The retention of both photoanodes exceeded 80%. In comparison with other metal sulfide case, our PbS/TiO2 photoanode was not stable and their retention is only 50% under operation of 4hr which is similar to other report39.

Figure 4

(a) Incident photon to current conversion efficiency of TiO2, CdSe/TiO2, as-grown FeS2/TiO2, FeS2/TiO2 and PbS/TiO2 photoanodes. (b) Stability test of the FeS2/TiO2 photoanode in the alkaline electrolyte (pH = 13.5) with SO32−/S2O32− as sacrificial agent under NIR laser (808 nm) illumination (300 mW/cm2).

To eliminate the surface defects of as-grown FeS2/TiO2 film, the sulfurization process was carried out to reduce the sulfur deficiency in the FeS2 film. The change of sulfur deficiency of FeS2 NCs before and after annealing was analyzed by inductively coupled plasma atomic emission spectroscopy (ICPAES). The ratio of Fe to S in the as-grown FeS2 thin film is 1:1.92 and the sulfur deficiency is approximately 4.5% higher than ~ 1.94–1.98 for the FeS2 film after sulfurization based on ICPAES measurement. Furthermore, in Raman spectra (Fig. 5(a)), we found that there are only three peaks found at 343, 379, and 430 cm−1 in the FeS2/TiO2 photoanode with sulfurization treatment, which are the characteristic active modes for pure pyrite corresponding to the S2 libration (Eg), S–S in-phase stretch (Ag), and coupled libration and stretch (Tg) modes, respectively. However, there is a few FeS phase (presence of Raman peaks at 210 and 280 cm−1) observed in the as-grown FeS2/TiO2 film. FeS phases could be caused by the sulfur deficiency on the surface of as-grown FeS2. Several previous reports indicated that there is less possibility to make the iron pyrite thin film as photoactive in photovoltaic device by using solution process because of lots of surface states and the sulfur vacancies40. The large short-circuit photocurrent densities have been only found from pyrite single crystals, which suffered from a low open-circuit voltage and low efficiency3140. In our work, the FeS2 sensitized on TiO2 photoanode for the PEC application was successfully fabricated by a simple solution process. Besides, TiO2 played an important role in enhancing charge transfer between the interface of FeS2 and TiO2 to improve the photo conversion efficiency of the FeS2/TiO2 photoanode. Figure 5(b) showed the dark current of TiO2/ as-grown FeS2 and TiO2/FeS2 devices. We found that TiO2/FeS2 device exhibited a better rectification ratio than that of TiO2/as-grown FeS2 device. Forward bias current was enhanced and reverse bias current was also reduced by an order, indicating that TiO2/FeS2 device with reducing the sulfur vacancies substantially improved pn junction behavior with a clearly rectifying current-voltage characteristic in comparison with TiO2/ as-grown FeS2 device. Therefore, overall results indicated that our FeS2/TiO2 photoanode have achieved a high photocurrent response extended from visible light to NIR range (900 nm) in PEC, leading to H2 generation successfully in the cathode electrode.

Figure 5

(a) Raman spectra of as-grown FeS2/TiO2 photoanode and FeS2/TiO2 photoanode. (b) The dark current of TiO2/ as-grown FeS2 and TiO2/FeS2 devices.

Conclusions

This study demonstrated that the FeS2/TiO2 photoanode composed of all earth-abundant elements exhibited high photo response from visible to NIR range for PEC hydrogen generation. The surface defect of FeS2 was found to be a critical factor to affect the photo response of FeS2/TiO2 photoanode in PEC application. The proper sulfurization was utilized to eliminate surface defect of FeS2 and to enhance the interface charge transfer between FeS2 and TiO2. We believed that this work demonstrated not only a breakthrough of using FeS2 as photoanode materials to generate hydrogen from the input of visible to NIR radiation but also a new approach for the design of sulfur-based binary compounds for photoelectrochemical applications.

Additional Information

How to cite this article: Wang, D.-Y. et al. Iron Pyrite/Titanium Dioxide Photoanode for Extended Near Infrared Light Harvesting in a Photoelectrochemical Cell. Sci. Rep. 6, 20397; doi: 10.1038/srep20397 (2016).