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Cadmium sulfide quantum dots sensitized tin dioxide–titanium dioxide heterojunction for efficient photoelectrochemical hydrogen production
Journal of Power Sources 269 (2014) 866e872
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Cadmium sulfide quantum dots sensitized tin dioxideetitanium dioxide heterojunction for efficient photoelectrochemical hydrogen production Xiaodong Li a, b, Zemin Zhang c, Lulu Chen c, Zhongping Liu a, b, Jianli Cheng a, b, Wei Ni a, b, Erqing Xie c, *, Bin Wang a, b, * a b c
Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, PR China Sichuan Research Center of New Materials, Mianyang 621000, Sichuan, PR China School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, Gansu, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SnO2eTiO2eCdS heterojunction serves as model architecture for PEC hydrogen production. A high efficiency of 2.18% at a low bias of 0.316 V vs. RHE. The influence of primary cell effect on PEC hydrogen production is investigated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2014 Accepted 11 July 2014 Available online 19 July 2014
CdS quantum dots (QDs)-sensitized branched TiO2/SnO2 heterojunction (BeSnO2 NFeCdS) with suitable combination of band gap and band alignment constitutes a promising architecture for photoanode for H2 generation. This novel structure combines the conflicting advantageous features of slow interfacial electron recombination, long electron life time, fast electron transport and visible light absorption. Remarkable photocurrent density of 3.40 mA cm2 at zero bias (vs. standard calomel electrode) has been obtained in a three electrode configuration, more than two times as large as that of TiO2eCdS photoanode. The BeSnO2 NFeCdS yields a high maximum applied bias photon-to-current efficiency (ABPE) of 2.18% at an applied bias of ~0.316 V vs. reversible hydrogen electrode (RHE), indicating excellent hydrogen generation performance at low bias. Moreover, on the basis of experimental results, we ascribe the remarkable “dark current/voltage” to the effect of primary cell. The influence of the primary cell on PEC hydrogen production is discussed. © 2014 Elsevier B.V. All rights reserved.
Keywords: Photoelectrochemical hydrogen production Tin dioxide Titanium dioxide Cadmium sulfide quantum dot Heterojunction Primary cell
1. Introduction
* Corresponding authors. Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (E. Xie), [email protected] (B. Wang). http://dx.doi.org/10.1016/j.jpowsour.2014.07.060 0378-7753/© 2014 Elsevier B.V. All rights reserved.
Solar energy is the only ultimate renewable energy resource to fulfill the global needs. Unfortunately, the inherent seasonal and daily variability of solar energy hinders its massive exploitation. Hydrogen energy is widely accepted as a desirable alternative to the existing oil based technologies due to its high energy density,
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simple and clean conversion to power or heat with no carbon emissions, and with water as the primary by-product. Consequently, the storage of solar energy in chemical bonds of H2 is one of the most promising strategies to develop a solar-based global energetic model. Of the various solar hydrogen production technologies, photoelectrochemical (PEC) water splitting provides the simplest solar to fuel conversion scheme by simultaneously capturing and storing solar energy [1e3]. Since the pioneering studies of Boddy in 1968 [4] and Honda and Fujishima in 1972 [5], titanium dioxide (TiO2) has been extensively investigated as one of the most attractive materials in this area due to its favorable band-edge position and high photoactivity coupled with excellent photocorrosion resistance, chemical stability and low cost [6e9]. However, the two obstacles are (1) limited solar-light harvesting because of its large band gap (e.g., ~3.2 eV for anatase; ~3.0 eV for rutile) and (2) fast recombination of electron hole pairs due to the slower electron mobility of TiO2 as compared with other materials such as SnO2. In general, two main strategies have been developed to increase the light harvesting in visible range, including doping with metal (Fe, Sn, etc. [10,11]) or nonmetal (C, N, etc. [12e15]) elements to narrow the band gap of TiO2; and sensitizing with narrow-bandgap semiconductor quantum dots (QDs) (CdS [7,16,17], CdSe [18], CdTe [19], etc.) or noble metals (Ag, Au NPs, etc. [15,20e22]). With regard to the second obstacle, recent work has focused on alternative materials with high electron mobility and their novel nanostructure with unique properties, including ZnO [23], WO3 [24], a-Fe2O3 [25], SnO2 [26], and Ta3N5 [27] etc.. Of the various materials, SnO2 is a good alternative of TiO2 in PEC applications due to its higher electron mobility (~100e200 cm2 V1 S1) [28] than that of TiO2 (~0.1e1.0 cm2 V1 S1) [29]. However, it has received less attention in PEC hydrogen production because the conduction band edge of SnO2 is lower than the reduction potential of water. As reported previously, SnO2eTiO2 heterojunctions were fabricated to integrate the advantages of both materials and shown an enhanced performance as compared to that of TiO2 nanostructures in applications of dye-sensitized solar cells (DSSCs) [30,31]. Due to the higher isoelectric points of TiO2 (~5e5.5) than that of SnO2 (~4e4.4) [30,32], the SnO2eTiO2 heterojunction could create a surface dipole layer toward SnO2, which accelerates the forward electrons transfer and suppress the back recombination. Considering the comparability of energetics of DSSCs and PEC hydrogen production [33], SnO2eTiO2 heterojunction could be extended to PEC hydrogen production and an improved hydrogen production performance could be expected. The main limiting factor for SnO2eTiO2 heterojunction is the large band gap of the two materials, which defines its light absorption only in the UV range. Therefore, in this work, we report a designed work electrode of CdS QDs-sensitized branched TiO2/ SnO2 heterojunction (BeSnO2 NFeCdS) with suitable combination of band gap and band alignment for efficient PEC hydrogen production. By using a facile solution heteroepitaxial growth method, cone-like TiO2 branches were grown on SnO2 NF network densely and uniformly. In this novel structure, the CdS QDs absorbs visible light and generates electronehole pairs; TiO2/SnO2 heterojunction accelerates the forward injection of electrons and suppresses the back recombination; SnO2 NF network works as the efficient charge transport path. The novel structure combined with 1 M KOH electrolyte solution containing reducing agent give rise to an increase of both output photovoltage and photocurrent when configured as a PEC hydrogen production system. Moreover, it was found that the BeSnO2 NFeCdS combined with 1 M KOH electrolyte solution containing reducing agent generated considerable voltage and current spontaneously in dark, which was also observed in many previous PEC experiments [34e36]. On the basis of our experiments, this phenomenon is associated with conversion of chemical
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energy into electrical energy and should be interpreted as the effect of primary cell. The influence of the primary cell on PEC hydrogen production is also discussed. 2. Experimental The details of the preparation of branched TiO2/SnO2 heterostructure (BeSnO2 NF) are described in our previous work [37]. CdS QDs were deposited on BeSnO2 NF by the sequential chemical bath deposition (S-CBD) method. Typically, the BeSnO2 NF films on FTO substrate were successively dipped into Cd(NO3)2$4H2O aqueous solution (0.05 M) for 4 min, deionized water for 1 min, Na2S$9H2O aqueous solution (0.05 M) for 4 min and deionized water for 1 min. This procedure was performed for one S-CBD cycle. The desired deposition of CdS QDs was achieved after several cycles and sintered at 400 C for 30 min in air. In our experiment we obtained the maximum photocurrent for ten S-CBD cycles, as shown in Fig. S1, Supporting information. The TiO2 nanowires (NWs) for control experiment were synthesized on the seeded-FTO substrate by using the hydrothermal method reported previously [6]. Deposition of the CdS QDs was achieved by the above mentioned S-CBD method with ten S-CBD cycles. The morphology and microstructure of the BeSnO2 NFeCdS were examined using a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai F30). Elemental analysis was performed on an energy-dispersive X-ray (EDX) spectroscopy attached to the TEM. X-Ray diffraction (XRD, Philips, X'pert pro, CuKa, 0.154056 nm) was employed to characterize the structural properties of the samples. Raman scattering spectra were carried out on a JobineYvon LabRam HR80 spectrometer (with a 532 nm line of Torus 50 mW diode-pumped solid-state laser) under the backscattering geometry at room temperature. Photoluminescence (PL) measurements were carried out on a JobineYvon LabRam HR80 spectrometer with a 325 nm line of 50 mW diode-pumped solid state laser at room temperature. The primary cell and PEC cell performances were measured using a standard three-electrode configuration, with the BeSnO2 NFeCdS and TiO2 NWeCdS electrodes as working electrodes, saturated calomel electrode (SCE) as reference electrode, and platinum foil as counter electrode. The electrolyte was a mixed aqueous solution of 1 M KOH (Rionlon Bohua (Tianjin) Pharmaceutical & Chemical Co., Ltd.) and 0.18 M Na2S (Shanghai Tongya chemical Technology Development Co., Ltd.) with 1 M KOH as control electrolyte. The pH of the electrolytes is 13.81 for the mixed solution and 13.85 for 1 M KOH. The illumination was generated by a 500 W Xenon lamp equipped with an AM1.5G filter, and the incident light density was calibrated by a standard Si solar cell to 100 mW cm2. The electrochemical measurements were performed on an Electrochemical Workstation (RST5200, Zhengzhou Shiruisi Instrument Technology Co., Ltd, China). 3. Results and discussion 3.1. Characterization of BeSnO2 NFeCdS The morphology and structure characterization of the BeSnO2 NFeCdS are shown in Fig. 1. The SnO2 NFs, ~52 nm in diameter, were prepared by electrospinning followed by sintering of the pristine nanofibers. Subsequently the NFs were coated on fluorinedoped tin oxide (FTO) glass by drop drying method to form the SnO2 NF network photoelectrode [37]. The TiO2 branches were grown on SnO2 NF network by a solution heteroepitaxial growth method [6]. As shown in Fig. 1a, the TiO2 branches radially grow on SnO2 NF network and display a branched feature. To achieve visible
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Fig. 1. FE-SEM images of (a) BeSnO2 NF and (b) BeSnO2 NFeCdS film on FTO substrate. (c) TEM image of a single BeSnO2 NFeCdS. (d) HR-TEM image taken from the area marked in Fig. 1c. (e) EDX spectrum of BeSnO2 NFeCdS.
light absorption, the BeSnO2 NF were decorated with numerous CdS QDs on the surface of the TiO2 branches by a previously reported sequential chemical bath deposition (S-CBD) method [17], as illustrated in Fig. 1b. By using this facial method, CdS QDs with an average size of ~5e10 nm were coated over the surface of TiO2 branches uniformly (Fig. 1c), which is crucial to obtaining good PEC properties. Further insight into the structural information was obtained by HRTEM taken from the TiO2/CdS interface, Fig. 1d. The resolved spacings between the two parallel neighboring fringes are 0.249 and 0.336 nm, corresponding to the [101] plane of rutile TiO2 and [002] plane of hexagonal CdS. Room temperature Raman scattering spectra further confirm the structure information of TiO2 branches and CdS QDs. As shown in Fig. 2, the BeSnO2 NF shows three distinct peaks at 239, 438 and 604 cm1, which should be assigned accordingly to B1g, Eg and A1g mode of rutile TiO2, respectively [38]. For the case of BeSnO2 NFeCdS, one additional peak corresponding to the 1LO optical
phonons [39] of CdS appears at 304 cm-1, which further confirms the presence of CdS QDs over large area. Here, no Raman peak characteristic of SnO2 NF is observed. The rutile structure of the SnO2 NF is demonstrated by XRD spectra, as illustrated in Fig. S2. It is noteworthy that the PL spectroscopy gives another key property of the BeSnO2 NF structure. The PL spectrum recorded from the SnO2 NF shows a strong visible emission ranging from 400 to 900 nm (Fig. 3), which is attributed to the recombination of electrons in the defects energy level of oxygen vacancies and tin interstitials in the band gap with photoexcited holes in the valence band (VB) [40]. Whereas in SnO2/TiO2 heterojunction, the photoinduced holes in the VB of SnO2 migrate to TiO2 due to the surface dipole layer created by the coreeshell structure, which prevents the radiation recombination of the electronehole pairs and results in a quenching of the PL emission (Fig. 3). These results indicate increase of the lifetime of the electronehole pairs in SnO2/TiO2 heterojunction, a desired advantageous feature for PEC applications.
Fig. 2. Raman scattering spectra of BeSnO2 NF and BeSnO2 NFeCdS.
Fig. 3. PL spectra of SnO2 NF and BeSnO2 NF.
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(RHE). Here, SCE is converted to RHE according to the equation ERHE ¼ ESCE þ 0.244 þ 0.059 * pH [3]. In general, an applied bias of ~0.4e1.4 V vs. RHE is needed to generate efficient Solar-to-Hydrogen (STH) efficiency since the limiting voltage for water splitting is 1.23 V [6,7,11,13,17,42e45]. High applied biases negate the advantage of using light. Therefore, increasing of the output voltage of the PEC water splitting system and decreasing of the applied bias are important issues in this area. The applied bias photon-to-current efficiency (ABPE) is an important characteristic of the PEC water splitting system, which can be calculated from Equation (1) [6]:
. ABPE ¼ Jph ð1:23 Vb Þ Plight Fig. 4. Absorption spectra of the BeSnO2 NF and BeSnO2 NFeCdS nanostructure on FTO substrate.
For further analysis in optical properties, the light absorbance analysis was conducted with BeSnO2 NF and BeSnO2 NFeCdS nanostructure. As illustrated in Fig. 4, the BeSnO2 NF only absorbs the UV light with correlation to optical bandgap of rutile SnO2 (3.6 eV) and TiO2 (3.0 eV). Upon sensitization with CdS QDs, the absorption range can be extended to 540 nm, which correlates with the CdS bandgap value (~2.4 eV). 3.2. PEC performance of BeSnO2 NFeCdS In order to evaluate the performance of this novel branched heterostructure, linear sweep voltammograms (Fig. 5) for different structures were measured using a standard three-electrode configuration in different electrolyte solutions. In 1 M KOH solution, the Jsc of BeSnO2 NF increases more than seven times, to 0.092 mA cm2 at a potential of 0 V vs. SCE, as compared to that of SnO2 NF (0.012 mA cm2), which is due to the acceleration of the forward electrons injection and suppression of the back recombination by the SnO2eTiO2 coreeshell structure and the higher photocatalytic activity of the cone-like TiO2 branches with exposed [001] facets [6,41]. Upon sensitization with CdS QDs, the Jsc increases sharply to ~1.04 mA cm2. As shown in Fig. 5, after adding 0.18 M Na2S into 1 M KOH aqueous electrolyte, BeSnO2 NFeCdS yields a high Jsc and Voc of 3.40 mA cm2 and 1.13 V vs. SCE, respectively. Sweeping the potential anodically, the JV curve of the BeSnO2 NFeCdS exhibits a steeper increase in current with respect to potential and achieves a prompt saturation of the photocurrent density at 0.3 V vs. reversible hydrogen electrode
(1)
where Jph is the photocurrent density at the measurement applied bias, Vb is the applied bias vs. RHE, and Plight is the incident light intensity (100 mW cm2) of the solar simulator as measured by a calibrated Si reference cell. As shown in Fig. 5b, the BeSnO2 NFeCdS exhibits a maximum ABPE of ~ 0.42% at 0.542 V vs. RHE in 1 M KOH. This ABPE value is lower than the PEC water splitting system based on commonly used TiO2 nanowire arrays, while the applied bias is superior to the values in those works (Table 1) [6,17]. Moreover, by adding the reducing agent (0.18 M Na2S) into the electrolyte, the BeSnO2 NFeCdS photoanode yields a high maximum ABPE of 2.18% at a low applied bias of 0.316 V vs. RHE, indicating this unique PEC water splitting system is able to generate more photocurrent at low bias. It is noteworthy that this applied bias is among the lowest values in the present studies (Table 1). The photoresponse of the BeSnO2 NFeCdS over time was measured at ~0.00 V vs. SCE for current response (Jet) and at opencircuit condition for voltage response (Vet) with chopped illumination, as shown in Fig. 6 and inset of Fig. 7. It is noticed that a dark current and voltage of ~0.5e0.75 mA cm2 and 0.57 V could be observed during the off illumination cycles, respectively. When the light was tuned on, the Jsc and Voc jumped to a higher value of ~3.0 mA cm2 and 1.05 V vs. SCE, respectively. The sharp spike/dip in the photocurrent during the on/off illumination cycles indicates (1) rapidly transferring of the photogenerated electrons from CdS QDs to TiO2/SnO2 heterostructure, and (2) fast transport of photogenerated electrons in SnO2 NF network. To double check the advancement of the band alignment of SnO2/TiO2/CdS, we compared the performance of BeSnO2 NFeCdS to that of CdS sensitized single crystalline TiO2 nanowire arrays (TiO2 NWeCdS). The morphology and structure of the TiO2
Fig. 5. Linear sweep voltammetry measurements of (a) BeSnO2 NFeCdS, BeSnO2 NF, SnO2 NF and TiO2 NWeCdS. (b) ABPE as a function of applied potential.
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Table 1 Comparison of photoelectrochemical performance of BeSnO2 NFeCdS and TiO2 NWeCdS photoanodes obtained in this study with the photoanodes from the literature. Photoanode
Photocurrent [mW cm2]/Vb
ABPEmax [%]/Vmax [V vs. RHE]
Electrolyte
Ref.
Branched TiO2 NW TiO2 inverse OpalseCdS TiO2 NWeCdS TiO2 NWeCdS Nb:SnO2 TiO2:Sn TiO2:C TiO2ePbS TiO2 NW SnO2/TiO2/CdS TiO2 NWeCdS
0.83/0.8 V vs. RHE 4.84/0 V vs. Ag/AgCl 3.98/0 V vs. Ag/AgCl 5.5/0 V vs. Pt 2.26/1.4 V vs. RHE 1.63/0.4 V vs. Ag/AgCl 0.89/0 V vs. Ag/AgCl 1.50/0.6 V vs. Ag/AgCl 2.60/0 V vs. Ag/AgCl 3.40/0 V vs. SCE 1.61/0 V vs. SCE
0.49/0.65 2.62/0.45 1.70/0.57 e e 1.05/0.56 0.55/0.65 1.15/0.67 1.05/0.613 2.18/0.316 1.02/0.377
1 M KOH 0.25 M Na2S þ 0.35 M Na2SO3 1 M KOH 0.24 M Na2S þ 0.35 M Na2SO3 1 M NaOH 1 M KOH 1 M KOH 0.1 M Na2S þ 0.054 M Na2SO3 þ 0.1 M Na2SO4 1 M KOH 0.18 M Na2S þ 1 M KOH 0.18 M Na2S þ 1 M KOH
[6] [7] [17] [42] [43] [11] [13] [44] [45] Present Present
NWeCdS is shown in Fig. S3, Supporting information. As shown in Fig. 5b and c, the TiO2 NWeCdS yields a photocurrent of 1.61 mA cm2 at 0.00 V vs. SCE, and a maximum ABPE of ~1.02% at 0.377 V vs. RHE, respectively, much lower than that of BeSnO2 NFeCdS. This result is expected because the electron life time of BeSnO2 NFeCdS is significantly longer than that of TiO2 NWeCdS (Fig. 7), indicating the interfacial electron combinations can be effectively suppressed by applying the SnO2eTiO2 coreeshell heterojunction. Here, the lifetime of the photogenerated electrons can be derived from the Voc decay curve by Equation (2) [46]
Fig. 6. Photocurrent response of BeSnO2 NFeCdS and TiO2 NWeCdS under 100 mW cm2 (AM1.5G) illumination with on and off cycles at short-circuit condition.
Fig. 7. The electron lifetime of BeSnO2 NFeCdS and TiO2 NWeCdS as a function of potential derived from the open circuit voltageedecay curve shown in the inset. The photovoltage responses of BeSnO2 NFeCdS and TiO2 NWeCdS shown in the inset are measured under 100 mW cm2 (AM1.5G) illumination with on and off cycles at opencircuit condition.
k T tn ¼ B e
dVoc 1 dt
(2)
where, kBT is the thermal energy, e is the elementary charge, and dVoc/dt is the first-order time derivative of the Voc. 3.3. The effect of primary cell It is noteworthy that the PEC cell based on BeSnO2 NFeCdS can generate considerable voltage and current spontaneously in dark with electrolyte containing reducing agent (soecalled “dark current/voltage”), as shown in Figs. 6 and 7, which is also observed in many previous works [34e36]. To investigate the origin of this “dark current/voltage”, the output voltage (at open-circuit condition, Voc) and current density (at short-circuit condition, Jsc) were recorded in dark. As shown in Fig. 8, the cell with BeSnO2 NFeCdS as anode, 0.18 M Na2S and 1 M KOH aqueous solution as electrolyte and platinum sheet as counter electrode shows a Voc of 0.67 V and a Jsc of 0.37 mA cm2, much higher than that in 1 M KOH aqueous solution. Moreover, the values of Voc and Jsc display Na2S concentration-dependent behavior, as illustrated in Fig. S4a and b, Supporting information. These phenomena are associated with spontaneously conversion of chemical energy into electrical energy, which should be the effect of primary cell. Energetics of operation of the primary cell with BeSnO2 NFeCdS as anode is shown in Fig. 9. In dark, the holes in CdS QDs oxides the strong reducing S2 and the electrons are transferred directly to the VB of CdS or TiO2, followed by transferring to VB of SnO2 or jumping to defect energy levels in the band gap of TiO2 and SnO2 driven by the thermal excitation. The electrons on the defects energy levels elevate the Fermi level (EF) of the electrode. Once reaching equilibrium, the primary cell generates a
Fig. 8. Current density and open-circuit voltage of the primary cell with BeSnO2 NFeCdS as anode.
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Acknowledgements This work was financially supported by the Science and Technology Planning Project of Sichuan Province, China (No. 2014JY0094), National Natural Science Foundation of China (No. 61176058), Startup Foundation of China Academy of Engineering Physics, Institute of Chemical Materials (KJCX201301 and KJCX201306) and National High-tech Research and Development Program (863 Program: No. 2013AA050905). Appendix A. Supplementary data Fig. 9. Energetics of operation of the primary cell with BeSnO2 NFeCdS as anode.
steady output of both Voc and Jsc, Fig. 8. The possible reactions taking place in this primary cell can be summarized as follows [34,47]:
S2 þ 2hþ ðVBÞ/S 3S þ 6OH /2S2 þ SO3 2 þ 3H2 O SO3 2 þ S/S2 O3 2 0:5S2 O3 2 þ 1:5H2 O þ 2hþ ðVBÞ/SO3 2 þ 3Hþ SO3 2 þ 2OH þ 2hþ ðVBÞ/SO4 2 þ H2 O whereas under light illumination, the electrons on the VB of TiO2 and SnO2 are not only driven by the thermal excitation but also by light excitation. Moreover, upon supra-bandgap illumination, electron hole pairs are generated in CdS, TiO2 and SnO2. Subsequently, electrons in the CB of the three semiconductors are transported toward the counter electrode by the external circuit, where the reduction of Hþ takes place; and the holes are transferred to the electrolyte. It is noteworthy that the BeSnO2 NFeCdS shows larger Voc and Jsc in dark than that of TiO2 NWeCdS. These results may result from the rich defects in SnO2 NF, which can bridge the gap between the VB electrons and the energy levels in the CB; and the suitable band alignment of SnO2/TiO2/CdS, which facilitates trapping of electrons in the electrode materials and longer electron lifetime (Fig. 4). Due to the compatibility of the energetics of primary cell and PEC cell, we believed that the accumulative effect of primary cell and PEC cell is probably the source of low applied bias of our hydrogen production system.
4. Conclusion To summarize, we have demonstrated a novel nanoarchitectured electrode design for efficient PEC hydrogen production by suitable combination of band gap and band alignment of SnO2/TiO2/CdS. Combined with electrolyte solution of 1 M KOH containing reducing agent, the BeSnO2 NFeCdS yields a remarkable photocurrent density of 3.40 mW cm2 at 0.00 V vs. SCE and a high maximum ABPE of 2.18% at a low applied bias of 0.316 V vs. RHE, indicating excellent hydrogen generation performance at low bias. On the basis of experimental results, we ascribe the remarkable “dark current/voltage” to the effect of primary cell, which may probably the source of the low applied bias of the PEC hydrogen production system.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Cadmium sulfide quantum dots sensitized tin dioxideetitanium dioxide heterojunction for efficient photoelectrochemical hydrogen production Xiaodong Li a, b, Zemin Zhang c, Lulu Chen c, Zhongping Liu a, b, Jianli Cheng a, b, Wei Ni a, b, Erqing Xie c, *, Bin Wang a, b, * a b c
Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, PR China Sichuan Research Center of New Materials, Mianyang 621000, Sichuan, PR China School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, Gansu, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SnO2eTiO2eCdS heterojunction serves as model architecture for PEC hydrogen production. A high efficiency of 2.18% at a low bias of 0.316 V vs. RHE. The influence of primary cell effect on PEC hydrogen production is investigated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2014 Accepted 11 July 2014 Available online 19 July 2014
CdS quantum dots (QDs)-sensitized branched TiO2/SnO2 heterojunction (BeSnO2 NFeCdS) with suitable combination of band gap and band alignment constitutes a promising architecture for photoanode for H2 generation. This novel structure combines the conflicting advantageous features of slow interfacial electron recombination, long electron life time, fast electron transport and visible light absorption. Remarkable photocurrent density of 3.40 mA cm2 at zero bias (vs. standard calomel electrode) has been obtained in a three electrode configuration, more than two times as large as that of TiO2eCdS photoanode. The BeSnO2 NFeCdS yields a high maximum applied bias photon-to-current efficiency (ABPE) of 2.18% at an applied bias of ~0.316 V vs. reversible hydrogen electrode (RHE), indicating excellent hydrogen generation performance at low bias. Moreover, on the basis of experimental results, we ascribe the remarkable “dark current/voltage” to the effect of primary cell. The influence of the primary cell on PEC hydrogen production is discussed. © 2014 Elsevier B.V. All rights reserved.
Keywords: Photoelectrochemical hydrogen production Tin dioxide Titanium dioxide Cadmium sulfide quantum dot Heterojunction Primary cell
1. Introduction
* Corresponding authors. Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (E. Xie), [email protected] (B. Wang). http://dx.doi.org/10.1016/j.jpowsour.2014.07.060 0378-7753/© 2014 Elsevier B.V. All rights reserved.
Solar energy is the only ultimate renewable energy resource to fulfill the global needs. Unfortunately, the inherent seasonal and daily variability of solar energy hinders its massive exploitation. Hydrogen energy is widely accepted as a desirable alternative to the existing oil based technologies due to its high energy density,
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simple and clean conversion to power or heat with no carbon emissions, and with water as the primary by-product. Consequently, the storage of solar energy in chemical bonds of H2 is one of the most promising strategies to develop a solar-based global energetic model. Of the various solar hydrogen production technologies, photoelectrochemical (PEC) water splitting provides the simplest solar to fuel conversion scheme by simultaneously capturing and storing solar energy [1e3]. Since the pioneering studies of Boddy in 1968 [4] and Honda and Fujishima in 1972 [5], titanium dioxide (TiO2) has been extensively investigated as one of the most attractive materials in this area due to its favorable band-edge position and high photoactivity coupled with excellent photocorrosion resistance, chemical stability and low cost [6e9]. However, the two obstacles are (1) limited solar-light harvesting because of its large band gap (e.g., ~3.2 eV for anatase; ~3.0 eV for rutile) and (2) fast recombination of electron hole pairs due to the slower electron mobility of TiO2 as compared with other materials such as SnO2. In general, two main strategies have been developed to increase the light harvesting in visible range, including doping with metal (Fe, Sn, etc. [10,11]) or nonmetal (C, N, etc. [12e15]) elements to narrow the band gap of TiO2; and sensitizing with narrow-bandgap semiconductor quantum dots (QDs) (CdS [7,16,17], CdSe [18], CdTe [19], etc.) or noble metals (Ag, Au NPs, etc. [15,20e22]). With regard to the second obstacle, recent work has focused on alternative materials with high electron mobility and their novel nanostructure with unique properties, including ZnO [23], WO3 [24], a-Fe2O3 [25], SnO2 [26], and Ta3N5 [27] etc.. Of the various materials, SnO2 is a good alternative of TiO2 in PEC applications due to its higher electron mobility (~100e200 cm2 V1 S1) [28] than that of TiO2 (~0.1e1.0 cm2 V1 S1) [29]. However, it has received less attention in PEC hydrogen production because the conduction band edge of SnO2 is lower than the reduction potential of water. As reported previously, SnO2eTiO2 heterojunctions were fabricated to integrate the advantages of both materials and shown an enhanced performance as compared to that of TiO2 nanostructures in applications of dye-sensitized solar cells (DSSCs) [30,31]. Due to the higher isoelectric points of TiO2 (~5e5.5) than that of SnO2 (~4e4.4) [30,32], the SnO2eTiO2 heterojunction could create a surface dipole layer toward SnO2, which accelerates the forward electrons transfer and suppress the back recombination. Considering the comparability of energetics of DSSCs and PEC hydrogen production [33], SnO2eTiO2 heterojunction could be extended to PEC hydrogen production and an improved hydrogen production performance could be expected. The main limiting factor for SnO2eTiO2 heterojunction is the large band gap of the two materials, which defines its light absorption only in the UV range. Therefore, in this work, we report a designed work electrode of CdS QDs-sensitized branched TiO2/ SnO2 heterojunction (BeSnO2 NFeCdS) with suitable combination of band gap and band alignment for efficient PEC hydrogen production. By using a facile solution heteroepitaxial growth method, cone-like TiO2 branches were grown on SnO2 NF network densely and uniformly. In this novel structure, the CdS QDs absorbs visible light and generates electronehole pairs; TiO2/SnO2 heterojunction accelerates the forward injection of electrons and suppresses the back recombination; SnO2 NF network works as the efficient charge transport path. The novel structure combined with 1 M KOH electrolyte solution containing reducing agent give rise to an increase of both output photovoltage and photocurrent when configured as a PEC hydrogen production system. Moreover, it was found that the BeSnO2 NFeCdS combined with 1 M KOH electrolyte solution containing reducing agent generated considerable voltage and current spontaneously in dark, which was also observed in many previous PEC experiments [34e36]. On the basis of our experiments, this phenomenon is associated with conversion of chemical
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energy into electrical energy and should be interpreted as the effect of primary cell. The influence of the primary cell on PEC hydrogen production is also discussed. 2. Experimental The details of the preparation of branched TiO2/SnO2 heterostructure (BeSnO2 NF) are described in our previous work [37]. CdS QDs were deposited on BeSnO2 NF by the sequential chemical bath deposition (S-CBD) method. Typically, the BeSnO2 NF films on FTO substrate were successively dipped into Cd(NO3)2$4H2O aqueous solution (0.05 M) for 4 min, deionized water for 1 min, Na2S$9H2O aqueous solution (0.05 M) for 4 min and deionized water for 1 min. This procedure was performed for one S-CBD cycle. The desired deposition of CdS QDs was achieved after several cycles and sintered at 400 C for 30 min in air. In our experiment we obtained the maximum photocurrent for ten S-CBD cycles, as shown in Fig. S1, Supporting information. The TiO2 nanowires (NWs) for control experiment were synthesized on the seeded-FTO substrate by using the hydrothermal method reported previously [6]. Deposition of the CdS QDs was achieved by the above mentioned S-CBD method with ten S-CBD cycles. The morphology and microstructure of the BeSnO2 NFeCdS were examined using a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai F30). Elemental analysis was performed on an energy-dispersive X-ray (EDX) spectroscopy attached to the TEM. X-Ray diffraction (XRD, Philips, X'pert pro, CuKa, 0.154056 nm) was employed to characterize the structural properties of the samples. Raman scattering spectra were carried out on a JobineYvon LabRam HR80 spectrometer (with a 532 nm line of Torus 50 mW diode-pumped solid-state laser) under the backscattering geometry at room temperature. Photoluminescence (PL) measurements were carried out on a JobineYvon LabRam HR80 spectrometer with a 325 nm line of 50 mW diode-pumped solid state laser at room temperature. The primary cell and PEC cell performances were measured using a standard three-electrode configuration, with the BeSnO2 NFeCdS and TiO2 NWeCdS electrodes as working electrodes, saturated calomel electrode (SCE) as reference electrode, and platinum foil as counter electrode. The electrolyte was a mixed aqueous solution of 1 M KOH (Rionlon Bohua (Tianjin) Pharmaceutical & Chemical Co., Ltd.) and 0.18 M Na2S (Shanghai Tongya chemical Technology Development Co., Ltd.) with 1 M KOH as control electrolyte. The pH of the electrolytes is 13.81 for the mixed solution and 13.85 for 1 M KOH. The illumination was generated by a 500 W Xenon lamp equipped with an AM1.5G filter, and the incident light density was calibrated by a standard Si solar cell to 100 mW cm2. The electrochemical measurements were performed on an Electrochemical Workstation (RST5200, Zhengzhou Shiruisi Instrument Technology Co., Ltd, China). 3. Results and discussion 3.1. Characterization of BeSnO2 NFeCdS The morphology and structure characterization of the BeSnO2 NFeCdS are shown in Fig. 1. The SnO2 NFs, ~52 nm in diameter, were prepared by electrospinning followed by sintering of the pristine nanofibers. Subsequently the NFs were coated on fluorinedoped tin oxide (FTO) glass by drop drying method to form the SnO2 NF network photoelectrode [37]. The TiO2 branches were grown on SnO2 NF network by a solution heteroepitaxial growth method [6]. As shown in Fig. 1a, the TiO2 branches radially grow on SnO2 NF network and display a branched feature. To achieve visible
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Fig. 1. FE-SEM images of (a) BeSnO2 NF and (b) BeSnO2 NFeCdS film on FTO substrate. (c) TEM image of a single BeSnO2 NFeCdS. (d) HR-TEM image taken from the area marked in Fig. 1c. (e) EDX spectrum of BeSnO2 NFeCdS.
light absorption, the BeSnO2 NF were decorated with numerous CdS QDs on the surface of the TiO2 branches by a previously reported sequential chemical bath deposition (S-CBD) method [17], as illustrated in Fig. 1b. By using this facial method, CdS QDs with an average size of ~5e10 nm were coated over the surface of TiO2 branches uniformly (Fig. 1c), which is crucial to obtaining good PEC properties. Further insight into the structural information was obtained by HRTEM taken from the TiO2/CdS interface, Fig. 1d. The resolved spacings between the two parallel neighboring fringes are 0.249 and 0.336 nm, corresponding to the [101] plane of rutile TiO2 and [002] plane of hexagonal CdS. Room temperature Raman scattering spectra further confirm the structure information of TiO2 branches and CdS QDs. As shown in Fig. 2, the BeSnO2 NF shows three distinct peaks at 239, 438 and 604 cm1, which should be assigned accordingly to B1g, Eg and A1g mode of rutile TiO2, respectively [38]. For the case of BeSnO2 NFeCdS, one additional peak corresponding to the 1LO optical
phonons [39] of CdS appears at 304 cm-1, which further confirms the presence of CdS QDs over large area. Here, no Raman peak characteristic of SnO2 NF is observed. The rutile structure of the SnO2 NF is demonstrated by XRD spectra, as illustrated in Fig. S2. It is noteworthy that the PL spectroscopy gives another key property of the BeSnO2 NF structure. The PL spectrum recorded from the SnO2 NF shows a strong visible emission ranging from 400 to 900 nm (Fig. 3), which is attributed to the recombination of electrons in the defects energy level of oxygen vacancies and tin interstitials in the band gap with photoexcited holes in the valence band (VB) [40]. Whereas in SnO2/TiO2 heterojunction, the photoinduced holes in the VB of SnO2 migrate to TiO2 due to the surface dipole layer created by the coreeshell structure, which prevents the radiation recombination of the electronehole pairs and results in a quenching of the PL emission (Fig. 3). These results indicate increase of the lifetime of the electronehole pairs in SnO2/TiO2 heterojunction, a desired advantageous feature for PEC applications.
Fig. 2. Raman scattering spectra of BeSnO2 NF and BeSnO2 NFeCdS.
Fig. 3. PL spectra of SnO2 NF and BeSnO2 NF.
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(RHE). Here, SCE is converted to RHE according to the equation ERHE ¼ ESCE þ 0.244 þ 0.059 * pH [3]. In general, an applied bias of ~0.4e1.4 V vs. RHE is needed to generate efficient Solar-to-Hydrogen (STH) efficiency since the limiting voltage for water splitting is 1.23 V [6,7,11,13,17,42e45]. High applied biases negate the advantage of using light. Therefore, increasing of the output voltage of the PEC water splitting system and decreasing of the applied bias are important issues in this area. The applied bias photon-to-current efficiency (ABPE) is an important characteristic of the PEC water splitting system, which can be calculated from Equation (1) [6]:
. ABPE ¼ Jph ð1:23 Vb Þ Plight Fig. 4. Absorption spectra of the BeSnO2 NF and BeSnO2 NFeCdS nanostructure on FTO substrate.
For further analysis in optical properties, the light absorbance analysis was conducted with BeSnO2 NF and BeSnO2 NFeCdS nanostructure. As illustrated in Fig. 4, the BeSnO2 NF only absorbs the UV light with correlation to optical bandgap of rutile SnO2 (3.6 eV) and TiO2 (3.0 eV). Upon sensitization with CdS QDs, the absorption range can be extended to 540 nm, which correlates with the CdS bandgap value (~2.4 eV). 3.2. PEC performance of BeSnO2 NFeCdS In order to evaluate the performance of this novel branched heterostructure, linear sweep voltammograms (Fig. 5) for different structures were measured using a standard three-electrode configuration in different electrolyte solutions. In 1 M KOH solution, the Jsc of BeSnO2 NF increases more than seven times, to 0.092 mA cm2 at a potential of 0 V vs. SCE, as compared to that of SnO2 NF (0.012 mA cm2), which is due to the acceleration of the forward electrons injection and suppression of the back recombination by the SnO2eTiO2 coreeshell structure and the higher photocatalytic activity of the cone-like TiO2 branches with exposed [001] facets [6,41]. Upon sensitization with CdS QDs, the Jsc increases sharply to ~1.04 mA cm2. As shown in Fig. 5, after adding 0.18 M Na2S into 1 M KOH aqueous electrolyte, BeSnO2 NFeCdS yields a high Jsc and Voc of 3.40 mA cm2 and 1.13 V vs. SCE, respectively. Sweeping the potential anodically, the JV curve of the BeSnO2 NFeCdS exhibits a steeper increase in current with respect to potential and achieves a prompt saturation of the photocurrent density at 0.3 V vs. reversible hydrogen electrode
(1)
where Jph is the photocurrent density at the measurement applied bias, Vb is the applied bias vs. RHE, and Plight is the incident light intensity (100 mW cm2) of the solar simulator as measured by a calibrated Si reference cell. As shown in Fig. 5b, the BeSnO2 NFeCdS exhibits a maximum ABPE of ~ 0.42% at 0.542 V vs. RHE in 1 M KOH. This ABPE value is lower than the PEC water splitting system based on commonly used TiO2 nanowire arrays, while the applied bias is superior to the values in those works (Table 1) [6,17]. Moreover, by adding the reducing agent (0.18 M Na2S) into the electrolyte, the BeSnO2 NFeCdS photoanode yields a high maximum ABPE of 2.18% at a low applied bias of 0.316 V vs. RHE, indicating this unique PEC water splitting system is able to generate more photocurrent at low bias. It is noteworthy that this applied bias is among the lowest values in the present studies (Table 1). The photoresponse of the BeSnO2 NFeCdS over time was measured at ~0.00 V vs. SCE for current response (Jet) and at opencircuit condition for voltage response (Vet) with chopped illumination, as shown in Fig. 6 and inset of Fig. 7. It is noticed that a dark current and voltage of ~0.5e0.75 mA cm2 and 0.57 V could be observed during the off illumination cycles, respectively. When the light was tuned on, the Jsc and Voc jumped to a higher value of ~3.0 mA cm2 and 1.05 V vs. SCE, respectively. The sharp spike/dip in the photocurrent during the on/off illumination cycles indicates (1) rapidly transferring of the photogenerated electrons from CdS QDs to TiO2/SnO2 heterostructure, and (2) fast transport of photogenerated electrons in SnO2 NF network. To double check the advancement of the band alignment of SnO2/TiO2/CdS, we compared the performance of BeSnO2 NFeCdS to that of CdS sensitized single crystalline TiO2 nanowire arrays (TiO2 NWeCdS). The morphology and structure of the TiO2
Fig. 5. Linear sweep voltammetry measurements of (a) BeSnO2 NFeCdS, BeSnO2 NF, SnO2 NF and TiO2 NWeCdS. (b) ABPE as a function of applied potential.
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Table 1 Comparison of photoelectrochemical performance of BeSnO2 NFeCdS and TiO2 NWeCdS photoanodes obtained in this study with the photoanodes from the literature. Photoanode
Photocurrent [mW cm2]/Vb
ABPEmax [%]/Vmax [V vs. RHE]
Electrolyte
Ref.
Branched TiO2 NW TiO2 inverse OpalseCdS TiO2 NWeCdS TiO2 NWeCdS Nb:SnO2 TiO2:Sn TiO2:C TiO2ePbS TiO2 NW SnO2/TiO2/CdS TiO2 NWeCdS
0.83/0.8 V vs. RHE 4.84/0 V vs. Ag/AgCl 3.98/0 V vs. Ag/AgCl 5.5/0 V vs. Pt 2.26/1.4 V vs. RHE 1.63/0.4 V vs. Ag/AgCl 0.89/0 V vs. Ag/AgCl 1.50/0.6 V vs. Ag/AgCl 2.60/0 V vs. Ag/AgCl 3.40/0 V vs. SCE 1.61/0 V vs. SCE
0.49/0.65 2.62/0.45 1.70/0.57 e e 1.05/0.56 0.55/0.65 1.15/0.67 1.05/0.613 2.18/0.316 1.02/0.377
1 M KOH 0.25 M Na2S þ 0.35 M Na2SO3 1 M KOH 0.24 M Na2S þ 0.35 M Na2SO3 1 M NaOH 1 M KOH 1 M KOH 0.1 M Na2S þ 0.054 M Na2SO3 þ 0.1 M Na2SO4 1 M KOH 0.18 M Na2S þ 1 M KOH 0.18 M Na2S þ 1 M KOH
[6] [7] [17] [42] [43] [11] [13] [44] [45] Present Present
NWeCdS is shown in Fig. S3, Supporting information. As shown in Fig. 5b and c, the TiO2 NWeCdS yields a photocurrent of 1.61 mA cm2 at 0.00 V vs. SCE, and a maximum ABPE of ~1.02% at 0.377 V vs. RHE, respectively, much lower than that of BeSnO2 NFeCdS. This result is expected because the electron life time of BeSnO2 NFeCdS is significantly longer than that of TiO2 NWeCdS (Fig. 7), indicating the interfacial electron combinations can be effectively suppressed by applying the SnO2eTiO2 coreeshell heterojunction. Here, the lifetime of the photogenerated electrons can be derived from the Voc decay curve by Equation (2) [46]
Fig. 6. Photocurrent response of BeSnO2 NFeCdS and TiO2 NWeCdS under 100 mW cm2 (AM1.5G) illumination with on and off cycles at short-circuit condition.
Fig. 7. The electron lifetime of BeSnO2 NFeCdS and TiO2 NWeCdS as a function of potential derived from the open circuit voltageedecay curve shown in the inset. The photovoltage responses of BeSnO2 NFeCdS and TiO2 NWeCdS shown in the inset are measured under 100 mW cm2 (AM1.5G) illumination with on and off cycles at opencircuit condition.
k T tn ¼ B e
dVoc 1 dt
(2)
where, kBT is the thermal energy, e is the elementary charge, and dVoc/dt is the first-order time derivative of the Voc. 3.3. The effect of primary cell It is noteworthy that the PEC cell based on BeSnO2 NFeCdS can generate considerable voltage and current spontaneously in dark with electrolyte containing reducing agent (soecalled “dark current/voltage”), as shown in Figs. 6 and 7, which is also observed in many previous works [34e36]. To investigate the origin of this “dark current/voltage”, the output voltage (at open-circuit condition, Voc) and current density (at short-circuit condition, Jsc) were recorded in dark. As shown in Fig. 8, the cell with BeSnO2 NFeCdS as anode, 0.18 M Na2S and 1 M KOH aqueous solution as electrolyte and platinum sheet as counter electrode shows a Voc of 0.67 V and a Jsc of 0.37 mA cm2, much higher than that in 1 M KOH aqueous solution. Moreover, the values of Voc and Jsc display Na2S concentration-dependent behavior, as illustrated in Fig. S4a and b, Supporting information. These phenomena are associated with spontaneously conversion of chemical energy into electrical energy, which should be the effect of primary cell. Energetics of operation of the primary cell with BeSnO2 NFeCdS as anode is shown in Fig. 9. In dark, the holes in CdS QDs oxides the strong reducing S2 and the electrons are transferred directly to the VB of CdS or TiO2, followed by transferring to VB of SnO2 or jumping to defect energy levels in the band gap of TiO2 and SnO2 driven by the thermal excitation. The electrons on the defects energy levels elevate the Fermi level (EF) of the electrode. Once reaching equilibrium, the primary cell generates a
Fig. 8. Current density and open-circuit voltage of the primary cell with BeSnO2 NFeCdS as anode.
X. Li et al. / Journal of Power Sources 269 (2014) 866e872
871
Acknowledgements This work was financially supported by the Science and Technology Planning Project of Sichuan Province, China (No. 2014JY0094), National Natural Science Foundation of China (No. 61176058), Startup Foundation of China Academy of Engineering Physics, Institute of Chemical Materials (KJCX201301 and KJCX201306) and National High-tech Research and Development Program (863 Program: No. 2013AA050905). Appendix A. Supplementary data Fig. 9. Energetics of operation of the primary cell with BeSnO2 NFeCdS as anode.
steady output of both Voc and Jsc, Fig. 8. The possible reactions taking place in this primary cell can be summarized as follows [34,47]:
S2 þ 2hþ ðVBÞ/S 3S þ 6OH /2S2 þ SO3 2 þ 3H2 O SO3 2 þ S/S2 O3 2 0:5S2 O3 2 þ 1:5H2 O þ 2hþ ðVBÞ/SO3 2 þ 3Hþ SO3 2 þ 2OH þ 2hþ ðVBÞ/SO4 2 þ H2 O whereas under light illumination, the electrons on the VB of TiO2 and SnO2 are not only driven by the thermal excitation but also by light excitation. Moreover, upon supra-bandgap illumination, electron hole pairs are generated in CdS, TiO2 and SnO2. Subsequently, electrons in the CB of the three semiconductors are transported toward the counter electrode by the external circuit, where the reduction of Hþ takes place; and the holes are transferred to the electrolyte. It is noteworthy that the BeSnO2 NFeCdS shows larger Voc and Jsc in dark than that of TiO2 NWeCdS. These results may result from the rich defects in SnO2 NF, which can bridge the gap between the VB electrons and the energy levels in the CB; and the suitable band alignment of SnO2/TiO2/CdS, which facilitates trapping of electrons in the electrode materials and longer electron lifetime (Fig. 4). Due to the compatibility of the energetics of primary cell and PEC cell, we believed that the accumulative effect of primary cell and PEC cell is probably the source of low applied bias of our hydrogen production system.
4. Conclusion To summarize, we have demonstrated a novel nanoarchitectured electrode design for efficient PEC hydrogen production by suitable combination of band gap and band alignment of SnO2/TiO2/CdS. Combined with electrolyte solution of 1 M KOH containing reducing agent, the BeSnO2 NFeCdS yields a remarkable photocurrent density of 3.40 mW cm2 at 0.00 V vs. SCE and a high maximum ABPE of 2.18% at a low applied bias of 0.316 V vs. RHE, indicating excellent hydrogen generation performance at low bias. On the basis of experimental results, we ascribe the remarkable “dark current/voltage” to the effect of primary cell, which may probably the source of the low applied bias of the PEC hydrogen production system.
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