Photoelectrochemical properties of Ti-doped hematite nanosheet arrays decorated with CdS nanoparticles

Abstract

Hematite (α-Fe₂O₃), with a relatively narrow bandgap (2.0–2.2 eV), is well-suited for potential application as a photoanode in photoelectrochemical (PEC) cells. Unfortunately, it suffers from severe bulk carrier recombination and low conductivity. This study provides a way to overcome these shortages by constructing a novel electrode system comprised of vertically aligned Ti-doped hematite nanosheet arrays decorated with cadmium sulfide nanoparticles (Ti-Fe₂O₃/CdS). Ti doping improves the conductivity of hematite and simultaneously extends the spectral responsive range. The incorporation of CdS nanoparticles further facilitates the charge separation and transfer process. Subsequently, the fabricated Ti-Fe₂O₃/CdS electrode achieves 6-fold enhancement of photocurrent density with respect to pristine Fe₂O₃ and excellent operation stability. Meanwhile, an obvious negative shift of photocurrent onset potential by 500 mV is observed.

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1. Introduction

Photoelectrochemical (PEC) water splitting using semiconductor electrodes offers a promising, but challenging approach to harvest solar energy in the form of storable chemical energy.^1–4 Hematite (α-Fe₂O₃), an n-type semiconductor material, has demonstrated its potential application in photocatalysis and photoelectrochemistry due to its proper optical band gap (2.0–2.2 eV), excellent chemical stability, abundance on the earth, and low cost.^5–8 Nevertheless, the poor conductivity restricted by the short diffusion length of holes (2–4 nm) in hematite results in severe bulk carrier recombination and thus largely restricts its practical applications.^9–13 One feasible method for alleviating these limitations is to build host-guest Fe₂O₃-based heterostructures. For instance, Fe₂O₃/Fe₂TiO₅,^14,15 Fe₂O₃/TiO₂,¹⁶ Fe₂O₃/NiO,¹⁷ Fe₂O₃/g-C₃N₄,¹⁸ Fe₂O₃/ZnFe₂O₄ (ref. 19) heterostructures have been reported to display significantly improved separation efficiency of photogenerated electron–hole pairs and enhanced PEC properties compared to the pristine hematite. To date, various nanostructures have been extensively investigated as host materials. Among the nanostructures employed, vertically aligned nanosheet arrays on the conducting substrate have been highlighted as the most desirable scaffold nanostructures due to their multiple features, including efficient electron transportation, enhanced light harvesting and enlarged surface area for loading guest materials, compared to their bulk counterparts, which undoubtedly maximized synergistic effect of host and guest materials.²⁰ However, because hematite has intrinsically poor conductivity, the PEC performance of Fe₂O₃ is still poor. Available experimental evidence demonstrates that incorporation of titanium could improve the electrical conductivity of hematite mainly by changing carrier density,^21,22 electronic structure²³ and/or crystal size.²⁴ In this study, Ti-doped hematite nanosheet arrays were used as host scaffold material.

On the other hand, cadmium sulfide (CdS) is considered to be an ideal guest material on hematite nanosheet arrays due to its relatively narrow band gap of 2.4 eV that allows efficient light absorption²⁵ and excellent match of the energy band positions with those of Fe₂O₃ for PEC water splitting. Such thermodynamically favored state enables photogenerated electrons to transfer from the conduction band of CdS to that of Fe₂O₃, thus promoting charge separation at the interface of Fe₂O₃ and CdS. Moreover, CdS can be inexpensively synthesized. Previous reports have suggested that the Fe₂O₃/CdS heterostructures could be used as high-performance solar-driven photocatalyst for pollutant degradation and PEC detection of chemical species.^26–29 Nonetheless, to the best of our knowledge, there is no relevant report that combines CdS with Ti-doped Fe₂O₃ nanosheet arrays as photoanode materials to develop PEC cells.

In this work, vertically aligned Ti-doped hematite nanosheet arrays are demonstrated as conducting host scaffolds to load CdS nanoparticles to form a heterostructure composite anode for efficient PEC water splitting. The optimal PEC activity of Ti-Fe₂O₃/CdS nanosheet arrays as a photoanode has been systematically analyzed. This study not only unveils these novel Ti-Fe₂O₃/CdS nanostructures' great potential as photoanodes for the PEC water splitting, but also offers a feasible strategy to boost the PEC performance based on structural design.

2. Experimental section

2.1. Synthesis of samples

Fabrication of the Fe₂O₃ nanosheet arrays is realized by electrodeposition followed by annealing at elevated temperature, using fluorine-doped tin-oxide (FTO) substrate, platinum electrode and saturated Ag/AgCl electrode as working electrode, counter electrode and reference electrode, respectively. The electrodeposition was carried out at 0.644 V versus Ag/AgCl at 70 °C for 10 min in aqueous solution containing 0.010 M (NH₄)₂Fe(SO₄)₂ and 0.040 M CH₃COOK. After electrodeposition, the films were rinsed with deionized water and heated at 500 °C for 1 h in N₂. Titanium treatment was performed by drop-casting tetrabutyl titanate solution onto the as-prepared Fe₂O₃ films prior to thermal treatment in 750 °C for 10 min.³⁰ CdS nanoparticles were directly grown on the obtained Fe₂O₃ and Ti-Fe₂O₃ by putting the films in aqueous solution containing 2 mM Cd(Ac)₂ and 2 mM thiourea, followed by sealing in an autoclave at 140 °C for 2 h. The obtained CdS-coated photoanodes were dried and sintered at 400 °C for 30 min in N₂.

2.2. Characterization

The as-synthesized samples were characterized by X-ray diffractometer (XRD, X' Pert MPD Pro), X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo), field emission scanning electron microscopy (FE-SEM, Merlin Compact, Carl Zeiss) and UV-vis diffuse reflectance spectroscopy (DRS, Hitachi U3010).

2.3. PEC measurements

PEC measurements were carried out on an electrochemical workstation (CHI660C, Chenghua, Shanghai) with a three-electrode configuration in aqueous solution containing 0.50 M Na₂S and 0.50 M Na₂SO₃. The fabricated films on FTO, a platinum electrode and a saturated Ag/AgCl electrode served as working electrode, counter electrode and reference electrode, respectively. AM 1.5G solar simulator (Oriel model 91192) was used as the light source with an output intensity of 100 mW cm⁻². The area of the working electrode exposed to the electrolyte was fixed at 0.25 cm² by nonconductive epoxy coating. The linear sweep voltammograms were swept from −1.1 to 0.6 V versus Ag/AgCl both in the dark and under irradiation. A potential of 0.2 V versus Ag/AgCl was applied for the current density–time transient response testing. Mott–Schottky analysis was conducted from −1.0 to 0.2 V versus Ag/AgCl under dark condition and the electrochemical impedance spectra (EIS) were recorded in a frequency range of 0.01 to 10⁵ Hz at open circuit voltages.

3. Results and discussion

XRD patterns of the samples are depicted in Fig. 1. The diffraction peaks of Fe₂O₃ can be well-indexed to the rhombohedral phase (JCPDS 33-0664). Compared to Fe₂O₃, the (104) and (110) peaks of Ti-Fe₂O₃ slightly shift to lower angles (Fig. 1B), indicating the lattice expansion of Fe₂O₃ when introducing Ti atoms.^31,32 This result confirms that Ti atoms are successfully embedded in the lattice of hematite. In addition, no peaks corresponding to TiO₂ are observed in the Ti-Fe₂O₃ sample, suggesting that Ti atoms are embedded in the lattice of hematite rather than forming a new phase. When the CdS nanoparticles are introduced onto hematite nanosheet surface, a new peak at 2θ = 41.6° can be observed in the patterns of the Fe₂O₃/CdS and Ti-Fe₂O₃/CdS samples, corresponding to (111) plane of CdS (JCPDS 43-0985), which confirms that CdS nanoparticles have been successfully synthesized. In contrast to strong and sharp diffraction peaks, the broad diffraction peaks of CdS in this work suggest that small CdS crystals could be obtained.

Fig. 1 (A) XRD patterns of (a) Fe₂O₃, (b) Ti-Fe₂O₃, (c) Fe₂O₃/CdS, and (d) Ti-Fe₂O₃/CdS nanosheet arrays. (B) Magnification of XRD patterns of Ti-Fe₂O₃ and Fe₂O₃ in 25–45°.

XPS was employed to investigate the environment of Ti in the Fe₂O₃. As shown in Fig. 2A, Ti signal is detected in the Ti-Fe₂O₃ sample. The peaks are assigned to Ti 2p_1/2 and Ti 2p_3/2. The high resolution XPS spectrum of Ti 2p (Fig. 2B) reveals that the peak of Ti 2p_3/2 at binding energy of 458.4 eV shows a shift toward lower binding energy compared to the reported value of Ti⁴⁺ in pure TiO₂ (458.5 eV), which might be related to the creation of oxygen vacancies and Ti³⁺ defects. The Ti to Fe ratio in the Ti-Fe₂O₃ electrode is about 0.03 according to XPS analysis. Representative STEM mapping of Ti-Fe₂O₃ nanosheet shows the spatial distribution of Fe, O and Ti, confirming the uniform distribution of Ti in the Ti-Fe₂O₃ (Fig. 2C).

Fig. 2 XPS spectra of Ti-Fe₂O₃ (A) survey and (B) Ti 2p, (C) representative STEM mapping of the Ti-Fe₂O₃ nanosheets.

The morphological features of all the samples were characterized by FESEM (Fig. 3). As presented in Fig. 3A, Fe₂O₃ sample consists of vertically aligned nanosheet arrays with thickness varying from 50 to 100 nm. No additional deposits or obvious differences in morphology could be distinguished after Ti doping (Fig. 3B). Both Fe₂O₃ and Ti-Fe₂O₃ nanosheets display smooth surface. Deposition of CdS causes the surface roughening. Small CdS nanoparticles with sizes around 50 nm are compactly coated onto the surfaces of Fe₂O₃ and Ti-Fe₂O₃ nanosheets (Fig. 3C and D). A rough surface is favorable since it enlarges the surface area of nanosheets and leads to an improved charge transfer at the electrode/electrolyte interface.

Fig. 3 SEM images of (A) Fe₂O₃, (B) Ti-Fe₂O₃, (C) Fe₂O₃/CdS and (D) Ti-Fe₂O₃/CdS nanosheet arrays.

Because the optical absorption properties play an important role in PEC applications, the samples were also characterized by UV-vis diffuse reflectance spectroscopy. Fig. 4 shows the comparison of the DRS among Fe₂O₃, Ti-Fe₂O₃, Fe₂O₃/CdS and Ti-Fe₂O₃/CdS films. It is observed that all the samples respond to the visible light. Fe₂O₃ film exhibits a broad absorption with the absorption edge at approximately 620 nm. In comparison to the Fe₂O₃, Ti-Fe₂O₃ film exhibits much higher absorption intensity. Moreover, the absorption edge of the Ti-Fe₂O₃ obviously shifted to a longer wavelength, indicating the narrowing of the band gap occurs when introducing Ti atoms. This narrowing could be attributed to the creation of additional energy level which is more positive than the conduction band of Fe₂O₃ as a result of Ti doping. Such similar phenomenon is also found in other Ti-doped hematite materials.^33–35 After being deposited with CdS nanoparticles, both Fe₂O₃/CdS and Ti-Fe₂O₃/CdS show an enhanced absorption and extended absorption edge. The superior light absorption ability of the Ti-Fe₂O₃/CdS sample is also beneficial to the improvement of PEC response.

Fig. 4 UV-vis diffuse reflectance spectra of Fe₂O₃, Ti-Fe₂O₃, Fe₂O₃/CdS and Ti-Fe₂O₃/CdS samples.

The photoelectrochemical performance of the as-prepared photoanodes was investigated using current density–potential (J–V) characteristic (Fig. 5A) and current density–time (J–t) transient response testing (Fig. 5B). Fig. 5A shows J–V curves of Fe₂O₃, Fe₂O₃/CdS, Ti-Fe₂O₃, and Ti-Fe₂O₃/CdS samples conducted both in the dark and under illumination. Because there is no significant distinction among the J–V curves recorded in the dark, only the dark current density curve of the Fe₂O₃ sample is presented as a representative. As illustrated in Fig. 5, the Fe₂O₃ electrode prepared in this work delivers considerably low photocurrent density less than 0.5 mA cm⁻² at 0.2 V versus Ag/AgCl. After coupling with CdS nanoparticles, the photocurrent density shows great enhancement in the whole potential range. Obviously, the introduction of CdS boosts the light absorption, as shown in Fig. 4, and the type-II band alignment between Fe₂O₃ and CdS benefits the separation of photogenerated electron–hole pairs. In addition, the CdS nanoparticles enlarge the surface area of the composite photoanode, which is beneficial to charge transfer at the electrode/electrolyte interface and thus the higher current density. It is also found that doping Fe₂O₃ with Ti leads to notable increase in photocurrent density. At 0.2 V versus Ag/AgCl, the photocurrent density of Ti-Fe₂O₃ reaches 1.5 mA cm⁻², which is more than 3-fold higher than that of pristine Fe₂O₃. The larger photocurrent can be attributed to the improvement of light harvesting and the enhancement of electrical conductivity derived from the incorporation of Ti dopants into hematite nanostructure. As a result, Ti-Fe₂O₃/CdS shows the maximum photocurrent density giving value of 2.7 mA cm⁻² at 0.2 V vs. Ag/AgCl, which is more than 6-fold higher than that of pristine Fe₂O₃ film and about 2- and 3-fold higher than that of Ti-Fe₂O₃ and Fe₂O₃/CdS. It is noteworthy that the onset potentials of the photocurrent shift cathodically for both Fe₂O₃/CdS and Ti-Fe₂O₃/CdS after CdS nanoparticles are attached, which would benefit the PEC water splitting at lower voltage. The negative shift of the onset potentials demonstrates more efficient interfacial charge transfer that avoid hole accumulating at the electrode surface and thus decreases surface charge recombination.³⁶ Beyond that, all the samples are prompt in generating reproducible response to light on–off cycles, indicating a quick charge transport process can be achieved (Fig. 5B). When exposing to 1800 s illumination, the Ti-Fe₂O₃/CdS electrode shows excellent stability with up to 94% retention of its initial photocurrent density.

Fig. 5 (A) J–V curves and (B) J–t curves of (a) Ti-Fe₂O₃/CdS, (b) Ti-Fe₂O₃, (c) Fe₂O₃/CdS and (d) Fe₂O₃ nanosheet arrays.

To understand the origin of the different PEC responses, Mott–Schottky (MS) plot and electrochemical impedance spectroscopy (EIS) measurements were carried out. The positive slopes in Fig. 6A indicate that the Fe₂O₃, Fe₂O₃/CdS, Ti-Fe₂O₃, and Ti-Fe₂O₃/CdS are all n-type semiconductors with electrons as majority carriers. The carrier density is linearly proportional to conductivity.³⁷ Enhanced conductivity would prolong the lifetime of the charge carriers and reduce the recombination of electron–hole pairs, thereby improving the PEC performance. Obtained from the Mott–Schottky equation,^38–40 the calculated electron densities of Fe₂O₃, Fe₂O₃/CdS, Ti-Fe₂O₃, and Ti-Fe₂O₃/CdS were 8.55 × 10¹⁹, 2.23 × 10²⁰, 3.74 × 10²⁰, 8.36 × 10²⁰, respectively. Apparently, the electron densities of the samples comply with the orders in PEC measurement. Ti-Fe₂O₃ films achieved higher carrier density than the pristine Fe₂O₃. This is direct evidence to support that the Ti dopant serves as electron donor to increase the electron density of Fe₂O₃. In addition, the electron density of Ti-Fe₂O₃/CdS is higher than that of Ti-Fe₂O₃. This is associated with the formation of a CdS enveloping layer which enhances charge carrier separation and transfer. The flatband potential at electrode/electrolyte interface were also estimated by the X-intercepts of the linear region in the Mott–Schottky plots. As displayed in Fig. 6A, deposition of CdS nanoparticles leads to negative shift of flatband potential, which is in good agreement with the onset potential in J–V curves. The negative shift of flatband potential and increased electron density of the Ti-Fe₂O₃/CdS electrode are favorable for the enhancement of PEC performance.

Fig. 6 (A) Mott–Schottky plots and (B) electrochemical impedance spectra (Nyquist plots) of Fe₂O₃, Ti-Fe₂O₃, Fe₂O₃/CdS and Ti-Fe₂O₃/CdS samples.

EIS is used to characterize charge transfer properties at electrode/electrolyte interface (Fig. 6B). The radius of the arc in the EIS Nyquist plots reflects the interfacial charge transfer resistance.⁴¹ It is obvious that the Ti-Fe₂O₃/CdS electrode possesses the lowest interfacial charge transfer resistance, indicating the highest charge separation and transfer efficiency. The arc radius of Fe₂O₃/CdS is smaller than that of the pristine Fe₂O₃, proving that the decoration of CdS nanoparticles has accelerated the charge transfer process due to the type II band alignment between Fe₂O₃ and CdS. Moreover, the smaller interfacial charge transfer resistance of Ti-Fe₂O₃/CdS than that of Fe₂O₃/CdS implies that Ti dopant played an important role in promoting the charge separation and transfer process.

Based on the aforementioned analysis, the origin of the remarkable PEC performance of the Ti-Fe₂O₃/CdS electrode was proposed and displayed in Scheme 1. Employing vertically aligned nanosheet arrays of Fe₂O₃ offers advantages of efficient electron transportation, enhanced light harvesting and high surface area for the deposition of CdS. Ti doping into the hematite promotes the conductivity and simultaneously extends the spectral responsive range of hematite. The introduction of CdS nanoparticles leads to increasing surface roughness and larger surface area, which is not only conducive to light absorption, but also beneficial to accelerate the interfacial charge transfer. Furthermore, the matched energy band positions of CdS with those of Ti-Fe₂O₃ induce the formation of a Ti-Fe₂O₃/CdS heterojunction, which allows for the better separation of the photogenerated electron–hole pairs. Briefly, under illumination, both Ti-Fe₂O₃ and CdS can be photoexcited to generate electron–hole pairs. Due to the type II alignment between Ti-Fe₂O₃ and CdS, the excited electrons in the conduction band of CdS can easily transport to that of Ti-Fe₂O₃ and subsequently transfer to the Pt counter electrode under the assistance of applied voltage to produce hydrogen by reducing water (2H⁺ + 2e⁻ → H₂). That is, the Ti-Fe₂O₃ scaffold acts as electron trap to promote charge separation. Simultaneously, the holes remaining in the valence band of Ti-Fe₂O₃ can efficiently inject to those of CdS and then be consumed by the hole scavengers (S²⁻) at electrode/electrolyte interface. Thus, the photogenerated electron–hole pairs are effectively separated, and accordingly hindering their recombination. All the PEC measurements provide evidences that the Ti-Fe₂O₃/CdS electrode shows an enhanced PEC activity due to the synergistic effect of CdS, Ti dopant, and Fe₂O₃.

Scheme 1 Schematic of the Ti-Fe₂O₃/CdS photoanode.

4. Conclusion

In summary, a novel electrode comprised of vertically aligned Ti-doped hematite nanosheet arrays decorated with cadmium sulfide (CdS) nanoparticles (Ti-Fe₂O₃/CdS) was fabricated. The Ti-Fe₂O₃/CdS electrode was demonstrated as an effective photoanode for PEC water splitting. The introduction of Ti dopant into the hematite not only results in the improvement of conductivity but also contributes to the enhancement of spectral responsive range. Loading CdS nanoparticles onto the surface of Ti-Fe₂O₃ nanosheets leads to efficient separation of photogenerated electrons and holes. Compared to pristine hematite electrode, the Ti-Fe₂O₃/CdS electrode shows an increased photocurrent density to more than 6-fold at 0.2 V versus Ag/AgCl, and the onset potential of the photocurrent shifted cathodically by 500 mV. The Ti-Fe₂O₃/CdS electrode also displays high stability. The significant improvement of PEC performance of the Ti-Fe₂O₃/CdS film can be ascribed to enhanced light absorption and increased charge carrier density as well as effective separation and transfer of the photogenerated electron–hole pairs. The work demonstrates an efficient way to fabricate Fe₂O₃-based photoanodes with 3D nanostructures and high performance for photoelectrochemical water splitting.

Acknowledgements

This work was financially supported by Guangdong Natural Science Foundation (2014A030311039).

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