CdS/CdSe co-sensitized 3D SnO₂/TiO₂ sea urchin-like nanotube arrays as an efficient photoanode for photoelectrochemical hydrogen generation

Abstract

In this paper, we report a novel electrode of 3D ordered CdS/CdSe co-sensitized sea urchin-like SnO₂/TiO₂ nanotube arrays for photoelectrochemical hydrogen production. This novel photoanode presents excellent PEC performance, yielding a maximum photocurrent density of ∼6 mA cm⁻² at 1.4 V vs. RHE under light illumination AM 1.5 (100 mW cm⁻²), which is due to the 3D structural effect of the SnO₂/TiO₂ sea urchin nanotubes, stepwise cascade band alignment between SnO₂/TiO₂ and CdS/CdSe as well as the improvement of light harvesting in the visible region.

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The utilization of photoelectrochemical (PEC) cells that convert solar energy into hydrogen fuel is a promising way to address the world's increasing energy demand as well as the reduction of the CO₂ emission.^1–4 The PEC water splitting processes consist of four main steps, i.e., light absorption, charge separation, charge transport and the charge recombination process.⁵ As such, an ideal photoelectrode should process high specific surface area, excellent charge transport and light harvesting ability. Since the innovative report on photocatalysis of water splitting using TiO₂–Pt system,⁶ great effort has been devoted to design novel electrodes with tailored nanostructures in order to boost the efficiency.^7–11 For instance, 1D nanowires,¹² nanotubes,¹³ 3D branched nanowires^14,15 and 3D urchin structured photoelectrodes have been explored.^16,17 Especially, 3D sea urchin structures that combine both 3D hollow spheres and 1D building blocks are particularly attractive due to their high specific surface area and strong light scattering ability.^16–19 SnO₂ with wide band gap (3.6 eV) and high electron mobility (100–200 cm² V⁻¹ S⁻¹) in contrast to TiO₂ (∼0.1–1 cm² V⁻¹ S⁻¹) is a promising candidate for optoelectronic devices. Moreover, the type II band alignment between SnO₂ and TiO₂ makes the SnO₂/TiO₂ composite to be very attractive in solar cells,^20–22 photocatalysis²³ and water splitting application.^24,25 Besides, coupling metal oxide semiconductor with narrow band gap sensitiser such as CdS^26,27 and/or CdSe^28–31 is an effective approach to improve the conversion efficiency by improving the light harvesting ability in the visible region.

Herein, we report an innovative electrode of 3D SnO₂/TiO₂ sea urchin-like nanotube arrays by combination of ZnO sea urchin structures as templates and atomic layer deposition of SnO₂ and TiO₂ for the first time. The hollow sea urchin nanotube structures might be more beneficial for quantum dots loading and electrolyte infiltration in contrast to that of nanowires structures. CdS and CdSe quantum dots are further used as co-sensitizer for extending the visible light absorption edge. PEC measurements demonstrate that CdS/CdSe co-sensitized 3D SnO₂/TiO₂ sea urchin-like nanotubes electrode presents the best performance in contrast to that of SnO₂/TiO₂ sea urchin-like nanotubes and only CdS sensitized one.

The 3D SnO₂/TiO₂ sea urchin like nanotubes arrays were fabricated by using the SnO₂ hollow spheres/ZnO nanorods sea urchins as sacrificial templates and subsequent ALD deposition of SnO₂ and TiO₂ layer. The fabrication process is illustrated in Fig. S1† and the typical SEM images of SnO₂/ZnO urchins are presented in Fig. S2.† It can be seen that the ZnO nanorods are radially grown on the surfaces of each microsphere, forming a periodical sea urchin pattern in a large area. After the ALD deposition of SnO₂ and TiO₂ and removal of ZnO templates, the SnO₂/TiO₂ nanotube sea urchin-like arrays were obtained. As shown in Fig. 1a, perfectly patterned sea urchin arrays are still reserved. From a high-magnification view in Fig. 1b, the overall diameter of individual SnO₂/TiO₂ nanotube sea urchin is about 5 μm. The SEM images of SnO₂/TiO₂ sea urchin-like nanotubes after CdS quantum dots sensitization and CdS/CdSe co-sensitization are displayed in Fig. 1c–f, respectively, it can be observed the quantum dots layer are uniformly coated on the nanotubes surfaces, without influencing the periodical characteristics. Besides, no obvious nanoparticles aggregation was observed. Further EDX measurement (Fig. S3†) confirms the chemical elements in CdS/CdSe sensitized SnO₂/TiO₂ sea urchin nanotube arrays.

Fig. 1 SEM images of (a and b) 3D SnO₂/TiO₂ sea urchin-like nanotubes; CdS sensitized (c and d) and CdS/CdSe co-sensitized (e and f) 3D sea SnO₂/TiO₂ urchin-like nanotubes.

The microstructures of the SnO₂/TiO₂ urchin-like nanotubes and quantum dots sensitized samples were further characterized by transmission electron microscopy (TEM) and HRTEM. The typical TEM image of SnO₂/TiO₂ nanotubes in Fig. 2a clearly demonstrates the core/shell hybrid nanotube morphology. From the TEM image (inset of Fig. 2a), it can be seen that the tube wall thickness of SnO₂ and TiO₂ are ∼10 nm and ∼5 nm, respectively. The HRTEM image of SnO₂/TiO₂ nanotube in Fig. 2b indicates that the SnO₂ is crystalline. The measured lattice 0.24 nm is corresponding to the (200) plane of rutile phase of SnO₂. The TEM and HRTEM image in Fig. 2c and d represent the structure of CdS sensitized SnO₂/TiO₂ urchin-like nanotubes, indicating that the CdS layer is distributed uniformly on the surface of the SnO₂/TiO₂ nanotube. As shown in Fig. 2d, a clear interface between CdS and TiO₂ can be observed. The measured lattices 0.34 nm is corresponding to the (002) plane of CdS. The thickness of CdS layer is about 8 nm. Fig. 2e and f show the TEM and HR-TEM image of the of CdS/CdSe co-sensitized SnO₂/TiO₂ urchin-like nanotubes, suggesting that CdSe nanoparticles were deposited on the surface successfully. As shown in Fig. 2f, the measured lattice of 0.34 nm and 0.22 nm are corresponding to the (002) plane of CdS and the (110) plane of CdSe.

Fig. 2 TEM and HR-TEM images of (a and b) 3D SnO₂/TiO₂ sea urchin-like nanotubes, inset of (a) is individual SnO₂/TiO₂ nanotube; CdS sensitized (c and d) and CdS/CdSe co-sensitized (e and f) 3D SnO₂/TiO₂ sea urchin-like nanotubes.

To further investigate the composition and chemical environment of the as-fabricated CdS/CdSe co-sensitized SnO₂/TiO₂ urchin-like nanotubes, X-ray photoelectron spectroscopy (XPS) was performed. The high-resolution XPS spectra in Fig. 3a shows the O peaks that were divided into three peaks with binding energy at 530.0, 530.4 and 533.3 eV, which may be ascribed to the Sn–O in SnO₂,³² Ti–O in TiO₂,³³ and hydroxyl groups, respectively. The two peaks in Fig. 3b were corresponding to the Ti 2p_3/2 and Ti 2p_1/2 with binding energies of 458.8 and 464.7 eV.³³ The spectra of Sn 3d in Fig. 3c shows two peaks of Sn 3d_3/2 and Sn 3d_5/2 with binding energies centered at 486.6 and 495.1 eV, respectively.³² As shown in Fig. 3d, the Cd 3d_5/2 peak centered at 405.1 eV and the Cd 3d_3/2 peak centered at 411.8 eV.³³ The two peaks in Fig. 3e were corresponding to the S 2p_3/2 and S 2p_1/2 with binding energies of 160.1 and 161.5 eV, indicating that the valence state of S is −2.³⁴ The two peaks centered at 53.9 and 54.8 eV in Fig. 3f are associated with the Se 3d_5/2 and Se 3d_3/2, which may be ascribed to the Cd–Se bonds.³⁵

Fig. 3 XPS spectra collected from CdS/CdSe co-sensitized 3D SnO₂/TiO₂ sea urchin-like nanotubes. (a) O 1s; (b) Ti 2p; (c) Sn 3d; (d) Cd 3d; (e) S 2p; (f) Se 3d.

The optical characteristics of the as-fabricated CdS and CdS/CdSe sensitized SnO₂/TiO₂ sea urchin-like nanotubes were studied by collecting the diffuse light reflectance spectrum with UV-vis spectrophotometer. As shown in Fig. 4, the CdS/CdSe co-sensitized SnO₂/TiO₂ sea urchin-like nanotubes exhibits the best light harvesting ability in the whole measured wavelength scale in contrast to that of SnO₂/TiO₂ and CdS sensitized one. The absorption edge is extending to ∼650 nm, corresponding to the bandgap of CdSe quantum dots, which is improving the visible light harvesting ability greatly.

Fig. 4 UV-vis diffuse reflectance spectrum of 3D SnO₂/TiO₂ urchin-like nanotubes, CdS sensitized and CdS/CdSe co-sensitized 3D SnO₂/TiO₂ urchin-like nanotubes.

The PEC water splitting performance was evaluated by measuring the photocurrent density with a three-electrode electrochemical cell. Fig. 5a presents a group of the linear sweep voltammogram curves collected from SnO₂/TiO₂, CdS sensitized and CdS/CdSe co-sensitized SnO₂/TiO₂ sea urchin-like nanotubes photoanodes measured in the dark and under light illumination (AM 1.5, 100 mW⁻²). It can be seen that all the three samples present very small current density in the dark, while under light illumination, pronounced photocurrent density were observed, especially for the CdS and CdS/CdSe co-sensitized samples. It is worthy noting that the CdS/CdSe co-sensitized SnO₂/TiO₂ sea urchin-like nanotubes photoelectrode shows the best PEC performance, achieving a maximum photocurrent density of ∼6 mA cm⁻² at 1.4 V vs. RHE, which is 15 times and 2 times larger than that of bare SnO₂/TiO₂ sea urchin-like nanotubes (0.4 mA cm⁻²) and CdS sensitized one (3 mA cm⁻²), respectively. The achieved photocurrent density value is also superior to most of the previous reported results in quantum dots sensitized photoelectrodes, such as CdS sensitized TiO₂/ZnO urchins (∼3.6 mA cm⁻²),¹³ CdS sensitized TiO₂ inverse opals (4.84 mA cm⁻²),⁴ ZnSe/CdS/CdSe triple-sensitized ZnO nanowire arrays (∼5.3 mA cm⁻²),³⁶ and CdS/CdSe sensitized hollow TiO₂ nanowire arrays (∼4 mA cm⁻²).³⁷ This might be attributed to the 3D structural effect of SnO₂/TiO₂ sea urchin-like nanotubes that provides large surface area for quantum dots loading as well as the synergistic effect of CdS and CdSe quantum dots which improves the visible light absorption and charge transfer process. Fig. 5b presents the corresponding transient i–t curves of the three photoelectrodes collected under chopped light illumination (30 s every light/dark cycle) at the bias of 0.9 V vs. RHE, demonstrating excellent photoresponse ability and fast photoswitching property. What's more, the CdS/CdSe co-sensitized SnO₂/TiO₂ sea urchin-like nanotubes photoelectrode showed very excellent photo-stability with little decay under continuous light illumination, as shown in Fig. 5c.

Fig. 5 (a) Linear sweep voltammograms of different photoanodes measured under light illumination (AM 1.5, 100 mW cm⁻²) and in the dark. (b) Transient i–t curves of different photoanodes at a fixed bias of 0.9 V vs. RHE under chopped light irradiation (on/off cycles of 30 s). (c) Light stability test at a fixed bias of 0.9 V vs. RHE under light illumination (AM 1.5, 100 mW cm⁻²). (d) EIS spectra of 3D SnO₂/TiO₂ sea urchin-like nanotubes, CdS sensitized and CdS/CdSe co-sensitized 3D SnO₂/TiO₂ sea urchin-like nanotubes.

To understand the charge transfer mechanism of CdS/CdSe sensitized SnO₂/TiO₂ sea urchin-like nanotubes, electrochemical impedance spectroscopy (EIS) measurement was conducted at open-circuit condition under simulated light illumination. Fig. 5d presents the Nyquist plots of the collected EIS data, which can be fitted by the physical model developed in similar quantum dots sensitized heterostructures.^37,38 The semicircle in the high frequency (>10⁴ Hz) in the Nyquist plots corresponds to the charge transfer in TiO₂/FTO interface, while the second semicircle portion in the middle frequency range (1–10³ Hz) corresponds to the charge transfer resistance at the photoelectrode/electrolyte interface. Apparently, the CdS/CdSe sensitized SnO₂/TiO₂ urchin-like nanotubes photoanode shows the smallest charge transfer resistance in contrast to that of SnO₂/TiO₂ and CdS sensitized one, which suggests that the stepwise cascade band alignment between CdS/CdSe and SnO₂/TiO₂ (as illustrated in Fig. 6b) are facilitating the photo-induced charge separation and transfer process.

Fig. 6 (a) Scheme of the CdS/CdSe co-sensitized of SnO₂/TiO₂ sea urchin-like nanotube arrays. (b) Band alignment and charge transfer process between the CdS/CdSe and SnO₂/TiO₂.

The scheme of urchin-like nanotubes arrays and processes of electron transfer in the CdS/CdSe co-sensitized SnO₂/TiO₂ urchin-like nanotubes photoelectrode can be simplified in Fig. 6a and b. As shown in Fig. 6b, under light illumination, the semiconductor SnO₂/TiO₂ and QDs absorb the photons and generated electron–hole pairs. Compared with SnO₂ and TiO₂, CdS has a higher conduction band edge, CdSe has a higher valence band edge, which is advantageous to the charge transportation.³¹ The stepwise cascade band alignment between the CdS/CdSe and SnO₂/TiO₂ is facilitating the photo-induced carrier separation and transfer process, as a result in reduction in the electron-holes recombination process. The higher electron mobility of SnO₂ nanotubes would be beneficial for improving the charge collection efficiency.²⁴

Based on the above discussions, the excellent PEC performance for 3D CdS/CdSe co-sensitized TiO₂/SnO₂ electrode can be explained as follows. First, the 3D urchin-like TiO₂/SnO₂ electrode provided increased surface area for both CdS and CdS quantum dots loading, leading to an enhancement of light harvesting, as evidenced in Fig. 4. Second, the stepwise cascade band alignment between the CdS/CdSe and SnO₂/TiO₂ is beneficial for the charge separation and transfer process, result in improving the charge separation and collection efficiency.

Conclusions

In summary, we have fabricated a novel three-dimensional sea urchin-like SnO₂/TiO₂ nanotube arrays photoelectrode by ALD route using ZnO sea urchin as a sacrificial template. The 3D SnO₂/TiO₂ composite nanotubes design provides increased surface area for CdS and CdSe quantum dots loading and improved charge collection efficiency due to the higher electron mobility of SnO₂. The as-fabricated CdS/CdSe co-sensitized SnO₂/TiO₂ urchin-like nanotubes exhibited excellent photoelectrochemical performance, which is attributed to the improved light harvesting ability by CdS and CdSe and stepwise cascade band alignment between SnO₂/TiO₂ and CdS/CdSe that facilitates the charge separation and transfer processes. Our approaches may open up new opportunity for design of other multi-components semiconductor electrode for high-efficiency PEC water splitting application.

Experimental section

Fabrication of SnO₂/TiO₂ sea urchin-like nanotube arrays

The 3D SnO₂/TiO₂ sea urchin-like nanotube arrays were prepared by using ZnO urchin as a sacrificial template combined with atomic layer deposition (ALD) technique. In a typical process: first, the ZnO urchin arrays on FTO substrate were fabricated with a colloid spheres templates and hydrothermal growth route according to our previous reports.¹⁶ The ALD SnO₂/ZnO coated PS microspheres substrate was immersed into a 35 mL aqueous solution of equimolar zinc nitrate (Zn(NO₃)₂·6H₂O) (0.025 M) and hexamethylenetetramine (C₆H₁₂N₄) in an autoclave. The reaction was conducted at 95 °C for 3 h. Subsequently, the ZnO urchin arrays were deposited with 10 nm of SnO₂ and 5 nm of TiO₂ by atomic layer deposition system (Picosun SUNALE R-200). The tin tetrachloride, titanium tetrachloride and H₂O were used as Sn, Ti and O precursors, respectively. During the reaction, the reaction chamber was kept at 1.0 mbar with a steady N₂ gas at 200 sccm (cubic centimeter per minute) and the deposition temperature was 350 °C. Finally, the as-prepared films were immersed in NH₃·H₂O for 24 h to remove the ZnO templates, forming the 3D SnO₂/TiO₂ urchin-like nanotube arrays.

Fabrication of CdS/CdSe Co-sensitized SnO₂/TiO₂ sea urchin nanotube arrays

CdS and CdSe quantum dots were sensitized on the 3D SnO₂/TiO₂ sea urchin nanotube arrays using a successive ionic layer adsorption and reaction (SILAR) method.⁴ First, 3 layers of CdS quantum dots were assembled by immersing the substrates Cd(Ac)₂ ethanol solution (0.1 M) for 1 min, followed by immersing in the Na₂S methanol solution (0.1 M) for 1 min, rinsed with methanol and water respectively, then dried under N₂ stream. This procedure was repeated 3 times. Then CdSe quantum dots were deposited with the Cd(NO₃)₂ and Na₂SeSO₃ as the Cd and Se source, respectively. The Na₂SeSO₃ aqueous solution was prepared by stirring and refluxing 0.3 M Se and 0.6 M Na₂SO₃ at 70 °C until the solution is clear. The CdS sensitized samples were immersed in Cd(NO₃)₂ ethanol solution for 1 min, rinsed with ethanol, dried under N₂ stream, then kept immersing in the Na₂SeSO₃ aqueous solution at 50 °C for 30 min. This procedure was repeated 5 times for a desirable thickness.

Characterizations

The structures and morphology of as-obtained products were characterized by FEI Sirion200 emission scanning electron microscopy (FE-SEM) and JEM 2010F transmission electron microscope (TEM). The diffuse reflectance spectrum was collected by UV-vis spectrophotometer (Hitachi, U3900 H) with an integration sphere.

Photoelectrochemical measurements

The PEC properties were measured by a electrochemical workstation (CHI760D, CHI instrument) using three-electrode electrochemical cell, where Pt foil, Ag/AgCl electrode and the as-prepared products serve as the counter electrode, reference electrode and working electrode, respectively. A mixed aqueous solution that contains 0.25 M Na₂S and 0.35 M Na₂SO₃ was used as electrolyte. The PH value was around 12.5, and all the measurements were referred to the reversible hydrogen electrode (RHE) by the equation V_RHE = V_Ag/AgCl + 0.197 + 0.059pH. A 150 W Xe lamp (Zolix SS150) with an AM 1.5 G filter which was calibrated by a standard Si solar cell was used as the light source. Time-dependent photo-response tests were carried out at bias of 0.9 V vs. RHE under chopped light irradiation (30 s every light/dark cycle). The electrochemical impedance spectroscopy (EIS) was obtained at open-circuit condition under simulated light illumination in a three-electrode configuration with a frequency range from 0.1 Hz to 1000 kHz.

Acknowledgements

This work was financially supported by 973 Program (Grant no. 2013CB632701), the National Natural Science Foundation of China (Grant no. 51202163) and Doctor Programs Foundation of MOE (20120072120044).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Scheme of the fabrication process of CdS/CdSe sensitized SnO₂/TiO₂ urchin nanotubes arrays, SEM image of ZnO sea urchin structure and EDX spectrum of CdS/CdSe co-sensitized SnO₂/TiO₂ sea urchin-like nanotube arrays. See DOI: 10.1039/c6ra02176j

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