CdS Nanoparticle-Modified α-Fe2O3/TiO2 Nanorod Array Photoanode for Efficient Photoelectrochemical Water Oxidation

In this work, we demonstrate a facile successive ionic layer adsorption and reaction process accompanied by hydrothermal method to synthesize CdS nanoparticle-modified α-Fe2O3/TiO2 nanorod array for efficient photoelectrochemical (PEC) water oxidation. By integrating CdS/α-Fe2O3/TiO2 ternary system, light absorption ability of the photoanode can be effectively improved with an obviously broadened optical-response to visible light region, greatly facilitates the separation of photogenerated carriers, giving rise to the enhancement of PEC water oxidation performance. Importantly, for the designed abnormal type-II heterostructure between Fe2O3/TiO2, the conduction band position of Fe2O3 is higher than that of TiO2, the photogenerated electrons from Fe2O3 will rapidly recombine with the photogenerated holes from TiO2, thus leads to an efficient separation of photogenerated electrons from Fe2O3/holes from TiO2 at the Fe2O3/TiO2 interface, greatly improving the separation efficiency of photogenerated holes within Fe2O3 and enhances the photogenerated electron injection efficiency in TiO2. Working as the photoanodes of PEC water oxidation, CdS/α-Fe2O3/TiO2 heterostucture electrode exhibits improved photocurrent density of 0.62 mA cm− 2 at 1.23 V vs. reversible hydrogen electrode (RHE) in alkaline electrolyte, with an obviously negatively shifted onset potential of 80 mV. This work provides promising methods to enhance the PEC water oxidation performance of the TiO2-based heterostructure photoanodes. Electronic supplementary material The online version of this article (10.1186/s11671-017-2278-3) contains supplementary material, which is available to authorized users.


Background
To solve the severe problem of pollution and limited resources of fossil, photoelectrochemical (PEC) water splitting to produce hydrogen has been regarded as one of the most promising strategies for solar energy conversion. Since the first report on PEC water oxidation based on TiO 2 [1], TiO 2 has drawn much attention as the photoanode materials for PEC water oxidation, due to its stable PEC properties, strong optical response, and suitable energy band position [2,3]. However, the PEC performance of pristine TiO 2 photoanode is greatly confined by the slow water oxidation kinetics originated from the poor photogenerated carrier separation capability and insufficient light absorption ability [4,5].
Therefore, various strategies have been taken to improve the PEC water oxidation performance of pristine TiO 2, such as surface modification [6], quantum dot sensitization, and heterojunction construction [7,8]. One efficient method to improve the photogenerated carrier separation performance is to construct heterostructured photoanode. For instance, constructing heterojunction between TiO 2 and other metal oxide semiconductors with matched energy band structures (like Co 3 O 4 /TiO 2 [9] and ZnIn 2 S 4 /TiO 2 [10,11]) can effectively facilitate the separation of photogenerated electrons and holes; therefore, PEC water splitting performance of the pristine TiO 2 can be obviously enhanced. Among various metal oxide semiconductors, hematite (α-Fe 2 O 3 ) is regarded as a promising photoanode material because of the suitable band gap (~2.0 eV) for sunlight harvesting, excellent stability, and low cost [12]. In addition, the theoretical power conversion efficiency (PCE) of α-Fe 2 O 3 can reach 15.3%, with a photocurrent density of 12.6 mA cm − 2 at 1.23 V vs. reversible hydrogen electrode (RHE) under the standard sun irradiation [13]. Therefore, constructing α-Fe 2 O 3 /TiO 2 heterostructured photoanode cannot only enhance the carrier separation performance in TiO 2 but also effectively extend the light absorption range of TiO 2 . Meanwhile, according to some latest research, α-Fe 2 O 3 photoanodes suffer from short electron-hole pair lifetime and hole diffusion length (2-4 nm), which results in high recombination rate of photogenerated carriers, hindering the improvement of the PEC performance [12]. In that case, to further enhance the PEC water splitting performance of Fe 2 O 3 /TiO 2 photoanodes, some narrow band gap semiconductors, like CdS [14,15] and PbS [16], can be coupled to facilitate the separation of photogenerated carriers. Among them, CdS/Fe 2 O 3 /TiO 2 heterostructured photoanode is considered to be a promising choice with matched band gap and expanded light absorption range. Also, carrier transport process can be effectively improved because photogenerated carriers can be quickly separated at the interface of CdS/Fe 2 O 3 /TiO 2 , thereby greatly decreasing the carrier recombination rates.
What is more, in order to construct an advanced electrode for PEC water splitting system, the electrode materials should possess the characteristics like sufficient incident light capture capability and tunnels for charge transport. Comparing with general planar photoanodes, one dimensional (1D) nanorod (NR) array photoanodes exhibit good incident light harvesting performance due to the enhanced multi-scattering processes [17], which would lead to an enhanced PEC water oxidation performance. Besides, it is reported that 1D NR array also exhibits excellent carrier transport performance since the photogenerated carriers can directly transport along the NR, thus direct carrier recombination at the crystal boundary can be effectively avoided [18]. Also, in order to further enlarge the surface area of such 1D NR arrays, which can bring more PEC reaction sites and enhance the PEC performance, 1D NR with branched nanostructures is expected [19]. Such integrated architecture offers a long optical path for effective light harvesting, short diffusion distance for excellent charge transport, and large surface area for fast interfacial charge collection, which is of great benefit for the enhancement of PEC performance. Hence, it would be of particular interest to design a CdS-modified Fe 2 O 3 /TiO 2 heterostructure NR array for PEC water oxidation.
Herein, we reported a facile successive ionic layer adsorption and reaction (SILAR)-hydrothermal method to synthesize CdS-modified Fe 2 O 3 /TiO 2 NR array for efficient PEC water oxidation. UV-vis study confirms the CdS/Fe 2 O 3 /TiO 2 NR array displays excellent optical response performance with an obvious broadened light absorption range. Improved charge transfer process and declined charge recombination rate can be evidenced by means of PL spectrum and EIS plots. Applied as the photoanode for PEC water oxidation, CdS/Fe 2 O 3 /TiO 2 NR array exhibits greatly enhanced photocurrent density of 0.62 mA cm − 2 (1.23 V vs. RHE) in alkaline electrolyte compared with pristine TiO 2 (0.32 mA cm − 2 at 1.23 V vs. RHE). It is believed that the synthesis route and the application of CdS/Fe 2 O 3 /TiO 2 NR array presently reported is of great importance and can be applied in other photovoltaic and photoelectronic devices.

Preparation of CdS/Fe 2 O 3 /TiO 2 NR Heterostructured Photoanode Synthesis of TiO 2 NR Array
To synthesize TiO 2 NR array on the FTO glass, the FTO was cut into rectangle and ultrasonically cleaned with deionized water, acetone, and ethanol, successively. Then, the FTO was put into the autoclave containing a mixed solution of deionized water (20 ml), hydrochloric acid (20 ml), and titanium isopropoxide (1.1 ml) and baked at 160°C for 6 h. After the reaction, the FTO was washed with deionized water and ethanol for several times and then was annealed in air at 450°C for 0.5 h.

Synthesis of Fe 2 O 3 /TiO 2 NR Array
To grow α-Fe 2 O 3 on TiO 2 NR, as obtained TiO 2 NR array was put into a mixed solution of FeCl 3 (15 ml, 0.1 M) and NaNO 3 (15 ml, 0.5 M) and then transferred to the autoclave. Heating at 100°C for 2 h, the autoclave was cooled to room temperature and the FTO substrate was washed with deionized water and ethanol for several times. Finally, the FTO substrate was annealed in air at 450°C for 1 h.

Synthesis of CdS/Fe 2 O 3 /TiO 2 NR
The obtained α-Fe 2 O 3 /TiO 2 NR array was pretreated with an ethanol solution of mercaptopropinioc acid (MPA, 0.3 M) overnight at 50°C and then washed with ethanol to remove the excess MPA. In order to deposit CdS layer, a facile successive ionic layer adsorption and reaction (SILAR) method is applied. Pretreated NR array was successively immersed into four different solutions for 30 s, including Cd(NO 3 ) 2 ·4H 2 O (ethanol, 0.1 M), pure ethanol, Na 2 S·9H 2 O (methanol, 0.2 M) and pure methanol, respectively. The SILAR process was repeated for five times and then the substrate was washed with methanol to remove the extra CdS.

Materials Characterization
The phase structures were characterized by X-ray powder diffractometer (XRD) in a 2θ range of 20 to 80°. The morphology of the products was studied with field emission scanning electron microscopy (FE-SEM) attached energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) images were collected via Tecnai 20 U-Twin equipment. The absorption and photoluminescence (PL) spectra were tested with TU-1900 and Hitachi U-4100, respectively.

Photoelectrochemical Performance Characterization
The PEC water oxidation performance was characterized with CHI660E electrochemical station with a threeelectrode mode. The applied electrolyte was consisted of 1M NaOH. Before testing, the system was bubbled with argon for 30 min to remove the electrolyte dissolved gas. The linear sweep voltammograms (LSV) and chronoamperometric I-t curves were recorded under standard sunlight illuminations (100 mW cm − 2 ). Mott-Schottky plots were measured in the dark at an AC frequency of 1.0 kHz.
Hereafter, the electrode potential was converted into the RHE potential with the Nernst equation: where E RHE was the converted potential vs. RHE, E Ag/ AgCl was the measured potential vs. the Ag/AgCl electrode, and E o Ag/AgCl = 0.1976 V at 25°C.

Structure and Morphology Characterization
The phase structures of the synthesized products are characterized by the XRD patterns in Fig. 1. As shown in Fig. 1a Figure S1, Additional file 2: Figure S2.  The chemical composition and valence states of the CdS/Fe 2 O 3 /TiO 2 hybrid NRs are studied by XPS spectra. Figure 3a shows the survey spectra, the existence of Ti, Fe, O, Cd, and S elements are demonstrated. The appearance of element C is assigned to the carbon-based containment. For the Ti 2p XPS spectrum in Fig. 3b, these splitted two distinct peaks at 458.2 and 464.2 eV can be assigned to Ti 2 p 3/2 and 2 p 1/2 of TiO 2 [20]. The XPS spectrum of Fe 2p is shown in Fig. 3c. Two distinct peaks at 710.6 and 724.10 eV can be seen, which correspond well to Fe 2 p 3/2 and 2 p 1/2 peaks of α-Fe 2 O 3 [21].
The core level XPS spectrum of O 1s is shown in Fig.  3d, where the peak at 531.2 eV is attributed to the Ti-O bond between titanium and oxygen, and the peak at 531.9 eV can be attributed to the Fe-O bond between iron and oxygen [20,21]. Figure 3e shows XPS spectrum of Cd, which is attributed to the Cd 3d 5/2 at 405.2 eV. The XPS spectrum of S 2P is shown in Fig. 3f [22]. The center peak is splitted into two peaks of S 2p 1/2 and 2p 3/2 at 161.5 and 162.6 eV [22]. Figure 4a shows the absorption spectra of different photoelectrodes. TiO 2 shows a typical absorption band Photoluminescence (PL) spectrum is applied to study the influence of incorporation of CdS and Fe 2 O 3 in the CdS/Fe 2 O 3 /TiO 2 hybrid on photogenerated carriers' transport and recombination behavior. The lower the intensity of PL peak, the higher separation efficiency of photogenerated carrier pairs in the samples. Figure 4b shows the PL spectra of TiO 2 , Fe 2 O 3 /TiO 2 , and CdS/ Fe 2 O 3 /TiO 2 samples. It is obvious that Fe 2 O 3 /TiO 2 NR achieves lower carrier recombination rate than pristine TiO 2 , and CdS/Fe 2 O 3 /TiO 2 NR achieves the best carrier transport performance.
In order to further confirm this conclusion, the picosecond-resolved fluorescence transient plots are tested and shown as Additional file 3: Figure S3. The average lifetime τ is calculated according to τ = (B 1 τ 1 [2] + B 2 τ 2 [2])/ (B 1 τ 1 + B 2 τ 2 ) and the time constant of the fluorescence transients at 511 nm is listed in the Additional file 4: Table S1 [23]. It can be seen that after modifying pristine TiO 2 with Fe 2 O 3 , the photogenerated carrier lifetime is prolonged. Coupled with CdS, the carrier lifetime can be further enhanced. This result obviously demonstrates the charge separation performance can be effectively enhanced by forming CdS/Fe 2 O 3 /TiO 2 multi-junction.
The possible carrier transport process is illustrated in Fig. 5. In the CdS/Fe 2 O 3 /TiO 2 ternary system, because both the conduction band position and valence band position of CdS are higher than that of Fe 2 O 3 , the photoinduced electrons in CdS will be transported to conduction band of Fe 2 O 3 , while the photoinduced holes in valence band in Fe 2 O 3 will be transported to CdS. For the designed abnormal type-II heterostructure between Fe 2 O 3 /TiO 2 , the conduction band position of Fe 2 O 3 is higher than that of TiO 2 . Under sunlight illumination, photoexcited electron-hole pairs will generate both in TiO 2 and Fe 2 O 3 . Photogenerated electrons in the conduction band of Fe 2 O 3 will immediately move to the valence band of TiO 2 to recombine with the photogenerated holes, thus greatly improving the separation efficiency of photogenerated holes within Fe 2 O 3 and enhances the photogenerated electron injection efficiency in TiO 2 [24,25]. It implies that the coupling of TiO 2 with Fe 2 O 3 and CdS can effectively reduce the recombination rate of the photogenerated carrier pairs. Meanwhile, the photogenerated electrons in TiO 2 move to the counter electrode where the reduction reaction takes place. So, the abnormal type-II heterostructure  Fig. 6b, it can be seen that the samples remain excellent stability and good optical-response property under chopped illumination. CdS/Fe 2 O 3 /TiO 2 NR sample maintains a photocurrent density of about 0.6 mA cm − 2 , which is in accordance with the LSV curves.
EIS measurement is performed under illumination and the Nyquist plots are shown in Fig. 7a and Additional file 5: Figure S4. They demonstrate that the Nyquist plots have two semicircles with a contact series resistance (R s ) on the FTO substrate. The small semicircle in the Nyquist plots is attributed to the charge transport resistance at the electrode/electrolyte interface, and the large semicircle represents the charge transfer resistance related to the electron transport/recombination within the photoanode materials. The sheet resistance (R s ) of the substrate, the charge transfer resistance of the counterelectrode (R ct1 ), and the charge transfer resistance (R ct2 ) were simulated by the Zview software and the corresponding data are shown in Additional file 6: Table S2. The fitted R s and R ct1 values for all samples are similar due to the same configuration and growing substrates are applied, while the R ct2 values show obviously variation of 1079.5, 880.6, and 679.5 Ω for TiO 2 , Fe 2 O 3 / TiO 2 , and CdS/Fe 2 O 3 /TiO 2 , respectively. It can be seen that after modifying TiO 2 with Fe 2 O 3 and CdS, the interfacial charge transfer kinetics are greatly enhanced.
The Mott-Schottky plots of the as obtained samples are listed in Fig. 7b. The slopes determined from the Mott-Schottky plots are used to estimate the carrier density according to the following equation [26]: where e 0 is the electronic charge, ε is the dielectric constant of the sample, ε 0 is the permittivity of the vacuum, Nd is the donor density, and V is the applied voltage. In general, relatively smaller the slope represents higher carrier density.
The flat band potential can be estimated by the following equation: The flat band potential (E fb ) is determined by taking the x intercept of a linear fit to the Mott-Schottky plot, 1/C 2 , as a function of applied potential (E). Additionally, a remarkable cathodic shift in the flat potential from 0.44 V for TiO 2 sample to 0.36 V for the CdS/Fe 2 O 3 / TiO 2 NR sample was observed. This suggests a larger accumulation of electrons in the heterojunction and reflects decreased charge recombination.
It should be noticed that the PEC water oxidation performance of as synthesized CdS/ Fe2O3/TiO2 sample is comparable to some related works. For instance, Sharma et al. reported Fe-TiO 2 /Zn-Fe 2 O 3 thin films with a performance of 0.262 mA cm − 2 at 0.95 V (vs. SCE) [27], while the FTO/Fe 2 O 3 /ZnFe 2 O 4 photoanode achieves a photocurrent density of 0.4 mA cm − 2 [28]. In addition, Fig. 6 a LSV curves of TiO 2 NR, Fe 2 O 3 /TiO 2 NR, and CdS/Fe 2 O 3 /TiO 2 NR samples illumination in 1M NaOH, b chronoamperometric I-t curves at a bias potential of 1.2 V under chopped illumination for the reported Fe 2 O 3 /TiO 2 nanotube photoanodes, a photocurrent density of 0.5 mA cm − 2 is achieved [29,30]. Comparing with the related works, it can be seen that obtained CdS/Fe 2 O 3 /TiO 2 photoanode does obtain outstanding and reliable PEC water splitting performance here.