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TiO2 nanorod arrays decorated with exfoliated WS2 nanosheets for enhanced photoelectrochemical water oxidation
Journal of Colloid and Interface Science 545 (2019) 282–288
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Regular Article
TiO2 nanorod arrays decorated with exfoliated WS2 nanosheets for enhanced photoelectrochemical water oxidation Yuxi Pi a,1, Bo Liu a,1, Zhen Li a, Yukun Zhu b, Yang Li a, Fengbao Zhang a, Guoliang Zhang a, Wenchao Peng a,⇑, Xiaobin Fan a,⇑ a State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300354, China b State Key Laboratory of Bio-fibers and Eco-textiles, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China
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Article history: Received 10 October 2018 Revised 11 March 2019 Accepted 12 March 2019 Available online 14 March 2019 Keywords: Photoelectrochemical water oxidation Exfoliation WS2 nanosheets TiO2 nanorod arrays Visible light
a b s t r a c t A novel three-dimensional (3D) photoanode consisting of TiO2 nanorod arrays (TiO2 NAs) coated by exfoliated WS2 nanosheets was fabricated for enhanced photoelectrochemical water oxidation. Mixed phase WS2 nanosheets with 1 T percentage of 55% were exfoliated by the lithium insertion, which were then coated on the top of TiO2 NAs by a drop-casting method. By optimizing the loading amount of WS2, a maximum photocurrent of 1.8 mA/cm2 could be obtained at +1.8 V vs. RHE under AM 1.5 irradiation (100 mW/cm2), which is 2.3 times higher compared to the pure TiO2 NAs (0.8 mA/cm2). The enhanced photo-activity should be attributed to the presence of the mixed phase WS2 nanosheets, which have excellent charge transport ability and can accept photogenerated holes for water oxidation. Ó 2019 Published by Elsevier Inc.
1. Introduction Photoelectrochemical (PEC) water splitting has been regarded as a powerful tool to utilize solar energy since 1970 [1–5]. TiO2, as one of the most significant semiconductor materials, has been ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Peng), [email protected] (X. Fan). 1 The authors contribute equal to this manuscript. https://doi.org/10.1016/j.jcis.2019.03.041 0021-9797/Ó 2019 Published by Elsevier Inc.
widely applied as photocatalysts and photoelectrodes due to its superior photocatalytic performance, excellent chemical stability, easy availability, and nontoxicity [6–10]. Compared to the TiO2 nanoparticles, one-dimensional (1D) ordered semiconductor nanostructures, such as TiO2 [11–13], ZnO [14,15], and Fe2O3 [16] based nanotube arrays, could offer enhanced charge transfer rate, increased active sites and improved ion diffusion ability. However, the solar conversion efficiency of pure TiO2 nanostructure under simulated solar illumination is still low due to its wide band gap
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(3.2 eV) and rapid recombination rate of photogenerated electron–hole pairs. In order to address these obstacles, various strategies have been developed to improve its photocatalytic activity, such as foreign atoms doping [17], heterojunctions construction with other semiconductors [18,19], or quantum dots deposition [20]. To develop more TiO2-based composites with high efficiencies is still a challenging task for the utilization of solar energy. Transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) and tungsten disulphide (WS2), have been widely used as photocatalytic cocatalysts for semiconductors due to their unique structural and electronic properties [21,22]. Their catalytic properties can be tailored according to the crystalline structure, the number and stacking sequence of their nanosheets [23,24]. Generally, there are two main phases (octahedral 1 T phase and trigonal prismatic 2H phase) existed for TMDs, and each phase has different physical properties [25]. For example, 2H-MoS2 is semiconducting, whereas 1 T-MoS2 is metallic [26]. The percentage of metallic 1 T phase in TMDs will greatly affect their co-catalytic activity in PEC water splitting. In this study, a novel three-dimensional (3D) binder-free photoanode was fabricated by coating exfoliated WS2 nanosheets on the surface of TiO2 NAs. The exfoliated WS2 nanosheets were in the mixed phase of 2H- and 1 T-after the lithium intercalation from bulk WS2. By optimizing the loading amount of WS2, greatly enhanced activity could be obtained compared to the TiO2 NAs. The mechanism during the PEC water oxidation was also proposed to explain the improved performance. 2. Experimental section 2.1. Preparation of the seed solution The FTO glass substrate was cut into small pieces (1 cm 6 cm), and cleaned with acetone, ethanol and deionized water, respectively. A solution containing 100 mL ethanol and 10 mL titanium (IV) butoxide was prepared as seed solution for the growth of the TiO2 nanorods. Subsequently, each substrate was spin coated with 0.4 mL of the seed solution over the conductive side of the glass. The coated FTO was then annealed at 500 °C for 30 min in air. 2.2. Synthesis of TiO2 nanorod arrays TiO2 nanorod arrays (TiO2 NAs) were synthesized by a simple hydrothermal approach. Typically, 30 mL deionized (DI) water was mixed with 30 mL concentrated hydrochloric acid in a 100 mL Teflon-lined stainless steel autoclave. The mixture was stirred at atmosphere conditions for 10 min. 1 mL titanium butoxide was then added into the above mentioned solution with vigorously stirring for 20 min. One piece of FTO substrate was then placed at an angle against the wall of the Teflon-liner with the conducting side facing down, which can avoid the possible influence of particle deposition. The autoclave was then sealed and kept in an oven at 180 °C for 6 h to facilitate the growth of the nanorods. After the autoclave was cooled to room temperature, the samples were taken out and rinsed with DI water and ethanol, respectively. They were then annealed at 500 °C for 30 min in nitrogen atmosphere to obtain the TiO2 NAs electrodes.
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solution in hexanes) with stirred for 48 h at 66 °C in a 50 mL Schlenk flask. The intercalated samples were obtained by centrifugation. The precipitates were then washed three times with hexane to remove excess lithium and organic residues. Standard air-free techniques were employed by using a Schlenk line under the protection of inert gas during the intercalation process. Exfoliation was achieved by immediately sonicating the freshly intercalated samples in water (200–300 mL) for about 30 min, and the exfoliated samples were purified by centrifugation, washed three times with DI water. 2.4. Synthesis of TiO2 NAs/WS2 composite Firstly, the TiO2 NAs grows on the surface of FTO glass using hydrothermal method as described in Section 2.2. The final TiO2 NAs/WS2 composites were then synthesized by a drop-casting method. Typically, desired amount of exfoliated WS2 nanosheets dispersions were deposited on the TiO2 NAs by drop-casting method and dried naturally at room temperature. Using this method, nearly no WS2 will be lost during the deposition process, and the deposition efficiency is 100%. The loading amount of WS2 nanosheets was determined to be 0.1 mg/cm2 (Fig. S1), and all TiO2 NAs/WS2 photoanodes used in the experiments were prepared using this optimized ratio. 2.5. Characterization The samples were characterized by X-ray diffraction (XRD) (Bruker-Nonius D8 FOCUS diffractometer), transmission electron microscope (TEM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) (FEI NOVA NanoSEM 430), Raman (Renishaw inVia reflex, Raman spectrometer with 532 nm laser excitation), diffusion reflectance UV–visible (UV–vis) spectra (Unico UV-2800) and X-ray photoelectron spectroscopy (XPS) (PHI5000VersaProbe). 2.6. Photoelectrochemcial water oxidation tests Photoelectrochemical water oxidation were carried out on an electrochemical workstation (CHI 660E, Chenhua) under AM 1.5 G illumination (100 mW/cm2) provided by a solar simulator (Microsolar300, PerfectLight). The as-prepared TiO2 NAs based samples were used directly as working electrodes with an active area of 1 cm2, and the illuminated light will first reach the TiO2 NAs side (Scheme 2a). The counter electrode and the reference electrode were platinum foil and Ag/AgCl (saturated KCl), respectively. 0.5 M Na2SO4 aqueous solution purged with N2 was used as the electrolyte during the measurement. The electrochemical impedance spectroscopy (EIS) for each photoanodes were recorded at corresponding open circuit potential, with the frequency ranging from 100 kHz to 0.01 Hz and the modulation amplitude of 10 mV. The measured potentials vs. Ag/AgCl reference electrode were converted to reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE ¼ EAg=AgCl þ 0:059 pH þ 0:1976), where EAg/AgCl is the applied potential vs. Ag/AgCl and 0.1976 V is the standard potential of the Ag/AgCl reference electrode at 25 °C. 3. Results and discussion
2.3. Exfoliation of WS2 nanosheets The WS2 nanosheets were exfoliated by sonication-assisted lithium intercalation according to our previous studies with slight modification [27]. Bulk WS2 was bought from Aladdin, CAS: 1213809-9. In a typical experiment, 1.125 g bulk WS2 powders were dispersed in the n-BuLi/hexane solution (15 mL, n-Butyllithium, 1.6 M
The overall synthetic procedure of TiO2 NAs/WS2 photoanode is illustrated in Scheme 1. The TiO2 NAs grow firstly on the surface of FTO glass using hydrothermal method. The WS2 nanosheets were then exfoliated by sonication-assisted lithium intercalation according to our previous studies with slight modification. The final TiO2 NAs/WS2 composites were synthesized by a simple
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Scheme 1. The fabrication process of the TiO2 NAs/WS2 photoanode.
drop-casting method. The morphologies and microstructures of the samples were firstly characterized by scanning electron microscopy (SEM). As shown in Fig. 1a, the bulk WS2 possesses an average diameter of 5 lm and a thickness of 200 nm. After lithium insertion, the bulk structure could be exfoliated into thinner nanosheets with fewer layers (Fig. 1b). As shown in Fig. 1c, highly ordered TiO2 nanorods arrays (TiO2 NAs) are vertically aligned on the FTO substrate after the hydrothermal process. The average diameter of the nanorods is 200 nm, and the mean length is about 3 lm (inset of Fig. 1c). The surface of these nanorods is smooth with a rectangular cross section. In the case of TiO2 NAs/ WS2 composite (Fig. 1d), the exfoliated WS2 nanosheets are uniformly deposited on the top of TiO2 NAs. The uniform distribution of WS2 on TiO2 NAs can also be confirmed by the energy dispersive X-ray (EDX) mapping of Ti, O, W and S elements (Fig. S2). No obvious difference can be found in the cross-section view of TiO2 NAs/ WS2 composite (inset of Fig. 1d) compared to TiO2 NAs. This result could prove that the WS2 layer on the top surface is very thin to be observed under SEM. Moreover, it is hard to find any WS2 in the sidewall of the TiO2 NAs, which is due to the large lateral sizes of the WS2 nanosheets and the small space between individual TiO2 nanorods. Typical transmission electron microscope (TEM) and high resolution TEM (HRTEM) images for bulk WS2 and exfoliated WS2 nanosheets are shown in Fig. 1e–h. As can be seen, bulk WS2 is thick multi-layers (Fig. 1e), while exfoliated WS2 is loosely restacked with translucent features (Fig. 1g). Interestingly, bulk WS2 only shows 2H-phase which is arranged in the shape of honeycomb (inset of Fig. 1f), while both 2H and 1 T phase can be observed in exfoliated WS2 nanosheets (inset of Fig. 1h). The crystal structures of the obtained samples were examined by X-ray diffraction (XRD) (Fig. 2a). The XRD pattern of the pure TiO2 NAs on the FTO substrate agrees well with that of rutile phase (JCPDS card No. 76-1940) [18]. As for the bulk WS2, all the diffraction peaks can be well indexed to the hexagonal WS2 phase (JCPDS card No. 08-0237) [28]. The XRD pattern of exfoliated WS2 nanosheets shows two broad peaks at 14.3° and 32.4°, corresponding to the (0 0 2) and (1 0 0) plane. The decreased (1 0 0) peak and the disappeared peaks of (0 0 4) and (0 0 6) arising from the symmetry of the vertical crystal planes indicate that the nanosheets have been exfoliated along the (0 0 2) axis [29]. For the spectra of TiO2 NAs/WS2, no obvious peaks for WS2 could be detected in these composite due to the weak reflection of the low percentage WS2 (0.1 mg/cm2). The peak intensity of (0 0 2) plane for TiO2 NAs decreased obviously in the composite, attributing to the covered WS2 affected the response intensity of TiO2 NAs. Raman spectra of the samples were also carried out to determine their phase compositions. As shown in Fig. 2b, bulk WS2 shows characteristic in-plane E12g and out-of-plane A1g modes of 2H-WS2. After exfoliation, additional J1, J2, and J3 in the lower fre-
quency regions appears, indicating the presence of the superlattice structure of 1 T-WS2 [21]. Notably, red-shift occurred in their A1g vibrational mode, further conforming the exfoliation of the bulk WS2 [28,30]. Raman spectra of TiO2 NAs at 143 cm1, 235 cm1, 445 cm1 and 608 cm1 correspond to the B1g, multi-photon process, Eg and A1g modes of rutile TiO2 (Fig. 2c), respectively [31]. After the TiO2 NAs were coated with exfoliated WS2 nanosheets, the Raman peaks of exfoliated WS2 nanosheets could also be well observed. While peaks of TiO2 at 608 cm1 shifts to lower frequency slightly, indicating that an increased tensile strain occurred in the coated WS2 nanosheets and intimate interactions exist between WS2 and TiO2 [32]. The UV–vis diffuse reflectance spectra (DRS) of bulk WS2 and exfoliated WS2 nanosheets are shown in Fig. S3. Corresponding Tauc plot reveals that the optical band gap of the bulk WS2 and the mixed phase exfoliated WS2 nanosheets are 1.34 and 1.50 eV, respectively. The UV–vis DRS of the TiO2 NAs based samples are shown in Fig. 2d. The absorption intensity of the composite is increased, and the absorption edge shifts to longer wavelength compared to the bare TiO2 NAs, indicating the improved ability for the utilization of solar energy. X-ray photoelectron spectroscopy (XPS) analysis was also performed for the obtained composites. All of the XPS data have been corrected using the C 1s as a reference, whose binding energy is at 284.8 eV (Fig. S2b). Two predominantly peaks of W 4f7/2 and W 4f5/2 at 32.8 eV and 35.0 eV corresponding to the 2H phase can be observed for bulk WS2 (Fig. 3a) [33]. The presence of a weak W 5p3/2 peak at 38.2 eV should be attributed to the slightly oxidation of WS2. After the lithium intercalation, the two peaks of W 4f shifts to lower energies (Fig. 3b). Two new peaks at 31.9 eV and 34.0 eV could be separated out, corresponding to the 1 T phase WS2 [25]. The 1 T component of the exfoliated WS2 is calculated to be 55% according to Fig. 3b. The same conclusion could be summarized from the XPS spectra of sulfur element (Fig. 3d and e). The detailed XPS information of the TiO2 NAs/ WS2 composite is shown in Fig. S4. Fig. 3c and f shows the XPS spectra comparison between bare TiO2 NAs and the TiO2 NAs/ WS2 composite. It could be observed that the binding energy of the reference C 1s peaks keep the same (Fig. S2b), while the W 4f7/2 and W 4f5/2 peaks of TiO2 NAs/WS2 composite shift to lower binding energy comparing to exfoliated WS2 nanosheets. Similar results could also be found in the S 2p spectra (Fig. 3f), suggesting the electronic interaction present between WS2 nanosheets and TiO2 NAs [12,34]. PEC water oxidation were then carried out to evaluate the photo-activity of the fabricated photoanodes. Fig. 4a shows the linear sweep voltammograms (LSV) of the samples under chopped AM 1.5 G illumination (100 mW/cm2). TiO2 NAs/WS2 composite displays a maximum photocurrent of 1.8 mA/cm2 at +1.8 V vs. RHE, which is 2.3-fold of pure TiO2 NAs (0.8 mA/cm2). Similar
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Fig. 1. SEM images of samples: (a) bulk WS2, (b) exfoliated WS2 nanosheets, top- and cross-sectional views of (c) TiO2 NAs and (d) TiO2 NAs/WS2 composite. Typical TEM and HRTEM images of the bulk WS2 (e and f) and exfoliated WS2 nanosheets (g and h).
Fig. 2. (a) XRD patterns of TiO2 NAs, bulk WS2, exfoliated WS2 nanosheets, and TiO2 NAs/WS2 composite. Raman spectra for (b) bulk WS2 and exfoliated WS2 nanosheets, (c) TiO2 NAs and TiO2 NAs/WS2 composite. (d) UV–vis spectra of TiO2 NAs and TiO2 NAs/WS2 composite and the corresponding plot of transformed Kubelka–Munk function versus photon energy (insert).
results could also be obtained under irradiation (Fig. 4b). Note that current density of TiO2 NAs/WS2 (dark) shows a tendency to increase for the potential of 1.7–1.8 V (vs. RHE). The exfoliated WS2 here is in the mixed phase of 2H- and 1 T-with good conductivity. Therefore, with the bias potential increasing to 1.7–1.8 V (vs.
RHE), a weak oxygen evolution reaction (OER) will occur, leading a small dark current. Based on the photocurrent information, the solar energy conversion efficiency (g) for these samples are also calculated. The highest g of the bare TiO2 NAs photoanode is 0.03% at +1.0 V (vs.
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Fig. 3. XPS spectra of W 4f and S 2p for bulk WS2 (a, d), exfoliated WS2 nanosheets (b, c, e, f), and TiO2 NAs/WS2 composite (c, f).
Fig. 4. (a) Variation of photocurrent density versus applied potential, (b) photocurrent density under chopped AM 1.5 G illumination (100 mW/cm2), The scan rate of LSV is 10 mV/s in (a and b), (c) solar energy conversion efficiency as a function of applied potential, (d) transient current densities at an external bias of 1.4 V (vs. RHE), (e) photoanode stability at 1.4 V (vs. RHE), and (f) EIS plots measured from TiO2 NAs and TiO2 NAs/WS2 composite under their corresponding open circuit potentials.
RHE) (Fig. 4c), while this value of the TiO2 NAs/WS2 photoanode was as large as 0.09% at +0.96 V (vs. RHE), 3 times higher compared to the pure TiO2 NAs photoanode. Fig. 4d shows the transient photocurrent curves obtained at a constant bias of +1.4 V (vs. RHE). Both the TiO2 NAs and TiO2 NAs/WS2 photoanodes show fast responses to the switching of the light on-off signal. To be specific, TiO2 NAs/WS2 photoanode shows a photocurrent density of 0.77 mA/cm2, which is significantly higher than that of TiO2 NAs (0.40 mA/cm2), fitting well with the result in Fig. 4a. Electrochemical measurements were also carried out for the bulk and exfoliated WS2 nanosheets, as shown in Fig. S5. Obviously, the exfoliated WS2 nanosheets are more active than bulk WS2 with larger photocurrent. However, the current of pure exfoliated WS2 is
still very small compared to the TiO2 NAs/WS2 composite. Therefore, we propose that exfoliated WS2 mainly plays a role of accelerating charge separation in the TiO2 NAs/WS2 composite, owing to its specific conduction band edge and the presence of metallic 1 T phase. In addition, the stability of TiO2 NAs and TiO2 NAs/WS2 photoanodes were also tested under continuous irradiation for 9 h. As shown in Fig. 4e, the TiO2 NAs/WS2 photoanode exhibits better stability comparing to the pure TiO2 NAs. A photocurrent of 0.79 mA/cm2 (>91% of the starting current) for the TiO2 NAs/ WS2 can be obtained after 9 h PEC test, while this percentage is 87% for the TiO2 NAs. To investigate the photogenerated charge transport behavior, the electrochemical impedance spectroscopy
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Acknowledgments This study is supported by the National Natural Science Funds (Nos. 21676198 and 21506158) and the Program of Introducing Talents of Discipline to Universities (No. B06006). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.03.041. Scheme 2. (a) Schematic illustration of the mechanism for the improvement of the PEC performance and (b) the energy band structure of the TiO2 NAs/WS2 composite.
(EIS) for the photoanodes is measured. The Nyquist plots show that the overall charge-transfer resistance (Rct) of the TiO2 NAs/WS2 electrode is smaller not only in the dark (Fig. S6) but also under light illumination (Fig. 4f) compared to TiO2 NAs. The resistance (Rct) of TiO2 NAs/WS2 electrode fitted by Z-View software is 1081 X, which is much smaller than that of TiO2 NAs (1811 X). The smaller Rct indicates the faster charge transfer of TiO2 NAs/ WS2 in the water oxidation process, leading to the improved PEC performance. Based on the above results, the mechanism for this PEC process was proposed for the explanation of the activity enhancement (Scheme 2) [35–37]. Considering the photocurrent from WS2 excitation is weak compared to the contribution of TiO2 NAs (Fig. S5), we propose that exfoliated WS2 mainly plays a role of accelerating charge separation in the TiO2 NAs/WS2 composite, owing to its specific conduction band edge and the presence of metallic IT phase. However, the excitation of WS2 and the interfacial electron transfer from WS2 to TiO2 may exist under a suitable bias-voltage, which is also the possible enhancement in photocurrent. With irradiation, electron-hole pair could be separated, and the photogenerated electrons could move into the conduction band (CB) of TiO2. The Fermi level of FTO is lower than the CB of TiO2, the photoexcited electrons could then migrate to the Pt counter electrode through FTO, which will reduce the water to generate H2 under the applied electric field. Meanwhile, the VB of exfoliated WS2 nanosheets is higher than that of TiO2, the photogenerated holes on the VB of TiO2 would flow to the VB of WS2 nanosheets for the oxidation of water [38]. The WS2 in Scheme 2 stands for the mixed phased WS2 rather than the 2H-WS2 alone. The mixed WS2 should also have the properties of semiconductor with CB and VB, but with different energy band positions when compared with pure 2H-WS2. It should be noted that the metallic 1 T ‘‘domains” in the plane has superior electric conductivity [25], which could play a role in accelerating the charge transfer process from TiO2 to WS2, thus increasing the charge separation efficiency and improve the PEC performance of TiO2 NAs greatly. 4. Conclusions We developed a facile method for the fabrication of a 3D TiO2 NAs/WS2 binder-free photoanode. To be specific, mixed phase WS2 nanosheets with 1 T percentage of 55% was exfoliated by the lithium insertion, which were then coated on the top of TiO2 photoanode by a drop-casting method. In addition, we used the exfoliated WS2 as co-catalyst for the modification of photoanode, indicating that WS2 can accept photo-generated holes for water oxidation. Therefore, the TiO2 NAs/WS2 photoanode with optimized WS2 loading displays significantly enhanced PEC activity compared to the pure TiO2 NAs. This efficient photoanode contains no noble metal and could be synthesized very facilely, thus has great potential for the real application.
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Regular Article
TiO2 nanorod arrays decorated with exfoliated WS2 nanosheets for enhanced photoelectrochemical water oxidation Yuxi Pi a,1, Bo Liu a,1, Zhen Li a, Yukun Zhu b, Yang Li a, Fengbao Zhang a, Guoliang Zhang a, Wenchao Peng a,⇑, Xiaobin Fan a,⇑ a State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300354, China b State Key Laboratory of Bio-fibers and Eco-textiles, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 October 2018 Revised 11 March 2019 Accepted 12 March 2019 Available online 14 March 2019 Keywords: Photoelectrochemical water oxidation Exfoliation WS2 nanosheets TiO2 nanorod arrays Visible light
a b s t r a c t A novel three-dimensional (3D) photoanode consisting of TiO2 nanorod arrays (TiO2 NAs) coated by exfoliated WS2 nanosheets was fabricated for enhanced photoelectrochemical water oxidation. Mixed phase WS2 nanosheets with 1 T percentage of 55% were exfoliated by the lithium insertion, which were then coated on the top of TiO2 NAs by a drop-casting method. By optimizing the loading amount of WS2, a maximum photocurrent of 1.8 mA/cm2 could be obtained at +1.8 V vs. RHE under AM 1.5 irradiation (100 mW/cm2), which is 2.3 times higher compared to the pure TiO2 NAs (0.8 mA/cm2). The enhanced photo-activity should be attributed to the presence of the mixed phase WS2 nanosheets, which have excellent charge transport ability and can accept photogenerated holes for water oxidation. Ó 2019 Published by Elsevier Inc.
1. Introduction Photoelectrochemical (PEC) water splitting has been regarded as a powerful tool to utilize solar energy since 1970 [1–5]. TiO2, as one of the most significant semiconductor materials, has been ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Peng), [email protected] (X. Fan). 1 The authors contribute equal to this manuscript. https://doi.org/10.1016/j.jcis.2019.03.041 0021-9797/Ó 2019 Published by Elsevier Inc.
widely applied as photocatalysts and photoelectrodes due to its superior photocatalytic performance, excellent chemical stability, easy availability, and nontoxicity [6–10]. Compared to the TiO2 nanoparticles, one-dimensional (1D) ordered semiconductor nanostructures, such as TiO2 [11–13], ZnO [14,15], and Fe2O3 [16] based nanotube arrays, could offer enhanced charge transfer rate, increased active sites and improved ion diffusion ability. However, the solar conversion efficiency of pure TiO2 nanostructure under simulated solar illumination is still low due to its wide band gap
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(3.2 eV) and rapid recombination rate of photogenerated electron–hole pairs. In order to address these obstacles, various strategies have been developed to improve its photocatalytic activity, such as foreign atoms doping [17], heterojunctions construction with other semiconductors [18,19], or quantum dots deposition [20]. To develop more TiO2-based composites with high efficiencies is still a challenging task for the utilization of solar energy. Transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) and tungsten disulphide (WS2), have been widely used as photocatalytic cocatalysts for semiconductors due to their unique structural and electronic properties [21,22]. Their catalytic properties can be tailored according to the crystalline structure, the number and stacking sequence of their nanosheets [23,24]. Generally, there are two main phases (octahedral 1 T phase and trigonal prismatic 2H phase) existed for TMDs, and each phase has different physical properties [25]. For example, 2H-MoS2 is semiconducting, whereas 1 T-MoS2 is metallic [26]. The percentage of metallic 1 T phase in TMDs will greatly affect their co-catalytic activity in PEC water splitting. In this study, a novel three-dimensional (3D) binder-free photoanode was fabricated by coating exfoliated WS2 nanosheets on the surface of TiO2 NAs. The exfoliated WS2 nanosheets were in the mixed phase of 2H- and 1 T-after the lithium intercalation from bulk WS2. By optimizing the loading amount of WS2, greatly enhanced activity could be obtained compared to the TiO2 NAs. The mechanism during the PEC water oxidation was also proposed to explain the improved performance. 2. Experimental section 2.1. Preparation of the seed solution The FTO glass substrate was cut into small pieces (1 cm 6 cm), and cleaned with acetone, ethanol and deionized water, respectively. A solution containing 100 mL ethanol and 10 mL titanium (IV) butoxide was prepared as seed solution for the growth of the TiO2 nanorods. Subsequently, each substrate was spin coated with 0.4 mL of the seed solution over the conductive side of the glass. The coated FTO was then annealed at 500 °C for 30 min in air. 2.2. Synthesis of TiO2 nanorod arrays TiO2 nanorod arrays (TiO2 NAs) were synthesized by a simple hydrothermal approach. Typically, 30 mL deionized (DI) water was mixed with 30 mL concentrated hydrochloric acid in a 100 mL Teflon-lined stainless steel autoclave. The mixture was stirred at atmosphere conditions for 10 min. 1 mL titanium butoxide was then added into the above mentioned solution with vigorously stirring for 20 min. One piece of FTO substrate was then placed at an angle against the wall of the Teflon-liner with the conducting side facing down, which can avoid the possible influence of particle deposition. The autoclave was then sealed and kept in an oven at 180 °C for 6 h to facilitate the growth of the nanorods. After the autoclave was cooled to room temperature, the samples were taken out and rinsed with DI water and ethanol, respectively. They were then annealed at 500 °C for 30 min in nitrogen atmosphere to obtain the TiO2 NAs electrodes.
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solution in hexanes) with stirred for 48 h at 66 °C in a 50 mL Schlenk flask. The intercalated samples were obtained by centrifugation. The precipitates were then washed three times with hexane to remove excess lithium and organic residues. Standard air-free techniques were employed by using a Schlenk line under the protection of inert gas during the intercalation process. Exfoliation was achieved by immediately sonicating the freshly intercalated samples in water (200–300 mL) for about 30 min, and the exfoliated samples were purified by centrifugation, washed three times with DI water. 2.4. Synthesis of TiO2 NAs/WS2 composite Firstly, the TiO2 NAs grows on the surface of FTO glass using hydrothermal method as described in Section 2.2. The final TiO2 NAs/WS2 composites were then synthesized by a drop-casting method. Typically, desired amount of exfoliated WS2 nanosheets dispersions were deposited on the TiO2 NAs by drop-casting method and dried naturally at room temperature. Using this method, nearly no WS2 will be lost during the deposition process, and the deposition efficiency is 100%. The loading amount of WS2 nanosheets was determined to be 0.1 mg/cm2 (Fig. S1), and all TiO2 NAs/WS2 photoanodes used in the experiments were prepared using this optimized ratio. 2.5. Characterization The samples were characterized by X-ray diffraction (XRD) (Bruker-Nonius D8 FOCUS diffractometer), transmission electron microscope (TEM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) (FEI NOVA NanoSEM 430), Raman (Renishaw inVia reflex, Raman spectrometer with 532 nm laser excitation), diffusion reflectance UV–visible (UV–vis) spectra (Unico UV-2800) and X-ray photoelectron spectroscopy (XPS) (PHI5000VersaProbe). 2.6. Photoelectrochemcial water oxidation tests Photoelectrochemical water oxidation were carried out on an electrochemical workstation (CHI 660E, Chenhua) under AM 1.5 G illumination (100 mW/cm2) provided by a solar simulator (Microsolar300, PerfectLight). The as-prepared TiO2 NAs based samples were used directly as working electrodes with an active area of 1 cm2, and the illuminated light will first reach the TiO2 NAs side (Scheme 2a). The counter electrode and the reference electrode were platinum foil and Ag/AgCl (saturated KCl), respectively. 0.5 M Na2SO4 aqueous solution purged with N2 was used as the electrolyte during the measurement. The electrochemical impedance spectroscopy (EIS) for each photoanodes were recorded at corresponding open circuit potential, with the frequency ranging from 100 kHz to 0.01 Hz and the modulation amplitude of 10 mV. The measured potentials vs. Ag/AgCl reference electrode were converted to reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE ¼ EAg=AgCl þ 0:059 pH þ 0:1976), where EAg/AgCl is the applied potential vs. Ag/AgCl and 0.1976 V is the standard potential of the Ag/AgCl reference electrode at 25 °C. 3. Results and discussion
2.3. Exfoliation of WS2 nanosheets The WS2 nanosheets were exfoliated by sonication-assisted lithium intercalation according to our previous studies with slight modification [27]. Bulk WS2 was bought from Aladdin, CAS: 1213809-9. In a typical experiment, 1.125 g bulk WS2 powders were dispersed in the n-BuLi/hexane solution (15 mL, n-Butyllithium, 1.6 M
The overall synthetic procedure of TiO2 NAs/WS2 photoanode is illustrated in Scheme 1. The TiO2 NAs grow firstly on the surface of FTO glass using hydrothermal method. The WS2 nanosheets were then exfoliated by sonication-assisted lithium intercalation according to our previous studies with slight modification. The final TiO2 NAs/WS2 composites were synthesized by a simple
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Scheme 1. The fabrication process of the TiO2 NAs/WS2 photoanode.
drop-casting method. The morphologies and microstructures of the samples were firstly characterized by scanning electron microscopy (SEM). As shown in Fig. 1a, the bulk WS2 possesses an average diameter of 5 lm and a thickness of 200 nm. After lithium insertion, the bulk structure could be exfoliated into thinner nanosheets with fewer layers (Fig. 1b). As shown in Fig. 1c, highly ordered TiO2 nanorods arrays (TiO2 NAs) are vertically aligned on the FTO substrate after the hydrothermal process. The average diameter of the nanorods is 200 nm, and the mean length is about 3 lm (inset of Fig. 1c). The surface of these nanorods is smooth with a rectangular cross section. In the case of TiO2 NAs/ WS2 composite (Fig. 1d), the exfoliated WS2 nanosheets are uniformly deposited on the top of TiO2 NAs. The uniform distribution of WS2 on TiO2 NAs can also be confirmed by the energy dispersive X-ray (EDX) mapping of Ti, O, W and S elements (Fig. S2). No obvious difference can be found in the cross-section view of TiO2 NAs/ WS2 composite (inset of Fig. 1d) compared to TiO2 NAs. This result could prove that the WS2 layer on the top surface is very thin to be observed under SEM. Moreover, it is hard to find any WS2 in the sidewall of the TiO2 NAs, which is due to the large lateral sizes of the WS2 nanosheets and the small space between individual TiO2 nanorods. Typical transmission electron microscope (TEM) and high resolution TEM (HRTEM) images for bulk WS2 and exfoliated WS2 nanosheets are shown in Fig. 1e–h. As can be seen, bulk WS2 is thick multi-layers (Fig. 1e), while exfoliated WS2 is loosely restacked with translucent features (Fig. 1g). Interestingly, bulk WS2 only shows 2H-phase which is arranged in the shape of honeycomb (inset of Fig. 1f), while both 2H and 1 T phase can be observed in exfoliated WS2 nanosheets (inset of Fig. 1h). The crystal structures of the obtained samples were examined by X-ray diffraction (XRD) (Fig. 2a). The XRD pattern of the pure TiO2 NAs on the FTO substrate agrees well with that of rutile phase (JCPDS card No. 76-1940) [18]. As for the bulk WS2, all the diffraction peaks can be well indexed to the hexagonal WS2 phase (JCPDS card No. 08-0237) [28]. The XRD pattern of exfoliated WS2 nanosheets shows two broad peaks at 14.3° and 32.4°, corresponding to the (0 0 2) and (1 0 0) plane. The decreased (1 0 0) peak and the disappeared peaks of (0 0 4) and (0 0 6) arising from the symmetry of the vertical crystal planes indicate that the nanosheets have been exfoliated along the (0 0 2) axis [29]. For the spectra of TiO2 NAs/WS2, no obvious peaks for WS2 could be detected in these composite due to the weak reflection of the low percentage WS2 (0.1 mg/cm2). The peak intensity of (0 0 2) plane for TiO2 NAs decreased obviously in the composite, attributing to the covered WS2 affected the response intensity of TiO2 NAs. Raman spectra of the samples were also carried out to determine their phase compositions. As shown in Fig. 2b, bulk WS2 shows characteristic in-plane E12g and out-of-plane A1g modes of 2H-WS2. After exfoliation, additional J1, J2, and J3 in the lower fre-
quency regions appears, indicating the presence of the superlattice structure of 1 T-WS2 [21]. Notably, red-shift occurred in their A1g vibrational mode, further conforming the exfoliation of the bulk WS2 [28,30]. Raman spectra of TiO2 NAs at 143 cm1, 235 cm1, 445 cm1 and 608 cm1 correspond to the B1g, multi-photon process, Eg and A1g modes of rutile TiO2 (Fig. 2c), respectively [31]. After the TiO2 NAs were coated with exfoliated WS2 nanosheets, the Raman peaks of exfoliated WS2 nanosheets could also be well observed. While peaks of TiO2 at 608 cm1 shifts to lower frequency slightly, indicating that an increased tensile strain occurred in the coated WS2 nanosheets and intimate interactions exist between WS2 and TiO2 [32]. The UV–vis diffuse reflectance spectra (DRS) of bulk WS2 and exfoliated WS2 nanosheets are shown in Fig. S3. Corresponding Tauc plot reveals that the optical band gap of the bulk WS2 and the mixed phase exfoliated WS2 nanosheets are 1.34 and 1.50 eV, respectively. The UV–vis DRS of the TiO2 NAs based samples are shown in Fig. 2d. The absorption intensity of the composite is increased, and the absorption edge shifts to longer wavelength compared to the bare TiO2 NAs, indicating the improved ability for the utilization of solar energy. X-ray photoelectron spectroscopy (XPS) analysis was also performed for the obtained composites. All of the XPS data have been corrected using the C 1s as a reference, whose binding energy is at 284.8 eV (Fig. S2b). Two predominantly peaks of W 4f7/2 and W 4f5/2 at 32.8 eV and 35.0 eV corresponding to the 2H phase can be observed for bulk WS2 (Fig. 3a) [33]. The presence of a weak W 5p3/2 peak at 38.2 eV should be attributed to the slightly oxidation of WS2. After the lithium intercalation, the two peaks of W 4f shifts to lower energies (Fig. 3b). Two new peaks at 31.9 eV and 34.0 eV could be separated out, corresponding to the 1 T phase WS2 [25]. The 1 T component of the exfoliated WS2 is calculated to be 55% according to Fig. 3b. The same conclusion could be summarized from the XPS spectra of sulfur element (Fig. 3d and e). The detailed XPS information of the TiO2 NAs/ WS2 composite is shown in Fig. S4. Fig. 3c and f shows the XPS spectra comparison between bare TiO2 NAs and the TiO2 NAs/ WS2 composite. It could be observed that the binding energy of the reference C 1s peaks keep the same (Fig. S2b), while the W 4f7/2 and W 4f5/2 peaks of TiO2 NAs/WS2 composite shift to lower binding energy comparing to exfoliated WS2 nanosheets. Similar results could also be found in the S 2p spectra (Fig. 3f), suggesting the electronic interaction present between WS2 nanosheets and TiO2 NAs [12,34]. PEC water oxidation were then carried out to evaluate the photo-activity of the fabricated photoanodes. Fig. 4a shows the linear sweep voltammograms (LSV) of the samples under chopped AM 1.5 G illumination (100 mW/cm2). TiO2 NAs/WS2 composite displays a maximum photocurrent of 1.8 mA/cm2 at +1.8 V vs. RHE, which is 2.3-fold of pure TiO2 NAs (0.8 mA/cm2). Similar
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Fig. 1. SEM images of samples: (a) bulk WS2, (b) exfoliated WS2 nanosheets, top- and cross-sectional views of (c) TiO2 NAs and (d) TiO2 NAs/WS2 composite. Typical TEM and HRTEM images of the bulk WS2 (e and f) and exfoliated WS2 nanosheets (g and h).
Fig. 2. (a) XRD patterns of TiO2 NAs, bulk WS2, exfoliated WS2 nanosheets, and TiO2 NAs/WS2 composite. Raman spectra for (b) bulk WS2 and exfoliated WS2 nanosheets, (c) TiO2 NAs and TiO2 NAs/WS2 composite. (d) UV–vis spectra of TiO2 NAs and TiO2 NAs/WS2 composite and the corresponding plot of transformed Kubelka–Munk function versus photon energy (insert).
results could also be obtained under irradiation (Fig. 4b). Note that current density of TiO2 NAs/WS2 (dark) shows a tendency to increase for the potential of 1.7–1.8 V (vs. RHE). The exfoliated WS2 here is in the mixed phase of 2H- and 1 T-with good conductivity. Therefore, with the bias potential increasing to 1.7–1.8 V (vs.
RHE), a weak oxygen evolution reaction (OER) will occur, leading a small dark current. Based on the photocurrent information, the solar energy conversion efficiency (g) for these samples are also calculated. The highest g of the bare TiO2 NAs photoanode is 0.03% at +1.0 V (vs.
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Fig. 3. XPS spectra of W 4f and S 2p for bulk WS2 (a, d), exfoliated WS2 nanosheets (b, c, e, f), and TiO2 NAs/WS2 composite (c, f).
Fig. 4. (a) Variation of photocurrent density versus applied potential, (b) photocurrent density under chopped AM 1.5 G illumination (100 mW/cm2), The scan rate of LSV is 10 mV/s in (a and b), (c) solar energy conversion efficiency as a function of applied potential, (d) transient current densities at an external bias of 1.4 V (vs. RHE), (e) photoanode stability at 1.4 V (vs. RHE), and (f) EIS plots measured from TiO2 NAs and TiO2 NAs/WS2 composite under their corresponding open circuit potentials.
RHE) (Fig. 4c), while this value of the TiO2 NAs/WS2 photoanode was as large as 0.09% at +0.96 V (vs. RHE), 3 times higher compared to the pure TiO2 NAs photoanode. Fig. 4d shows the transient photocurrent curves obtained at a constant bias of +1.4 V (vs. RHE). Both the TiO2 NAs and TiO2 NAs/WS2 photoanodes show fast responses to the switching of the light on-off signal. To be specific, TiO2 NAs/WS2 photoanode shows a photocurrent density of 0.77 mA/cm2, which is significantly higher than that of TiO2 NAs (0.40 mA/cm2), fitting well with the result in Fig. 4a. Electrochemical measurements were also carried out for the bulk and exfoliated WS2 nanosheets, as shown in Fig. S5. Obviously, the exfoliated WS2 nanosheets are more active than bulk WS2 with larger photocurrent. However, the current of pure exfoliated WS2 is
still very small compared to the TiO2 NAs/WS2 composite. Therefore, we propose that exfoliated WS2 mainly plays a role of accelerating charge separation in the TiO2 NAs/WS2 composite, owing to its specific conduction band edge and the presence of metallic 1 T phase. In addition, the stability of TiO2 NAs and TiO2 NAs/WS2 photoanodes were also tested under continuous irradiation for 9 h. As shown in Fig. 4e, the TiO2 NAs/WS2 photoanode exhibits better stability comparing to the pure TiO2 NAs. A photocurrent of 0.79 mA/cm2 (>91% of the starting current) for the TiO2 NAs/ WS2 can be obtained after 9 h PEC test, while this percentage is 87% for the TiO2 NAs. To investigate the photogenerated charge transport behavior, the electrochemical impedance spectroscopy
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Acknowledgments This study is supported by the National Natural Science Funds (Nos. 21676198 and 21506158) and the Program of Introducing Talents of Discipline to Universities (No. B06006). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.03.041. Scheme 2. (a) Schematic illustration of the mechanism for the improvement of the PEC performance and (b) the energy band structure of the TiO2 NAs/WS2 composite.
(EIS) for the photoanodes is measured. The Nyquist plots show that the overall charge-transfer resistance (Rct) of the TiO2 NAs/WS2 electrode is smaller not only in the dark (Fig. S6) but also under light illumination (Fig. 4f) compared to TiO2 NAs. The resistance (Rct) of TiO2 NAs/WS2 electrode fitted by Z-View software is 1081 X, which is much smaller than that of TiO2 NAs (1811 X). The smaller Rct indicates the faster charge transfer of TiO2 NAs/ WS2 in the water oxidation process, leading to the improved PEC performance. Based on the above results, the mechanism for this PEC process was proposed for the explanation of the activity enhancement (Scheme 2) [35–37]. Considering the photocurrent from WS2 excitation is weak compared to the contribution of TiO2 NAs (Fig. S5), we propose that exfoliated WS2 mainly plays a role of accelerating charge separation in the TiO2 NAs/WS2 composite, owing to its specific conduction band edge and the presence of metallic IT phase. However, the excitation of WS2 and the interfacial electron transfer from WS2 to TiO2 may exist under a suitable bias-voltage, which is also the possible enhancement in photocurrent. With irradiation, electron-hole pair could be separated, and the photogenerated electrons could move into the conduction band (CB) of TiO2. The Fermi level of FTO is lower than the CB of TiO2, the photoexcited electrons could then migrate to the Pt counter electrode through FTO, which will reduce the water to generate H2 under the applied electric field. Meanwhile, the VB of exfoliated WS2 nanosheets is higher than that of TiO2, the photogenerated holes on the VB of TiO2 would flow to the VB of WS2 nanosheets for the oxidation of water [38]. The WS2 in Scheme 2 stands for the mixed phased WS2 rather than the 2H-WS2 alone. The mixed WS2 should also have the properties of semiconductor with CB and VB, but with different energy band positions when compared with pure 2H-WS2. It should be noted that the metallic 1 T ‘‘domains” in the plane has superior electric conductivity [25], which could play a role in accelerating the charge transfer process from TiO2 to WS2, thus increasing the charge separation efficiency and improve the PEC performance of TiO2 NAs greatly. 4. Conclusions We developed a facile method for the fabrication of a 3D TiO2 NAs/WS2 binder-free photoanode. To be specific, mixed phase WS2 nanosheets with 1 T percentage of 55% was exfoliated by the lithium insertion, which were then coated on the top of TiO2 photoanode by a drop-casting method. In addition, we used the exfoliated WS2 as co-catalyst for the modification of photoanode, indicating that WS2 can accept photo-generated holes for water oxidation. Therefore, the TiO2 NAs/WS2 photoanode with optimized WS2 loading displays significantly enhanced PEC activity compared to the pure TiO2 NAs. This efficient photoanode contains no noble metal and could be synthesized very facilely, thus has great potential for the real application.
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