- Email: [email protected]
Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods
international journal of hydrogen energy 35 (2010) 8199–8205
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods Xuewen Wang a, Gang Liu a, Gao Qing Lu b,*, Hui-Ming Cheng a,** a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China b The University of Queensland, ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and Australian Institute of Bioengineering and Nanotechnology, Qld. 4072, Australia
article info
abstract
Article history:
Stability and efficiency are important to realize the practical applications of photocatalysts
Received 25 September 2009
for photocatalytic hydrogen evolution from water splitting. ZnO–CdS core–shell nanorods
Received in revised form
with a wide absorption range were designed and synthesized by a two-step route. The
17 December 2009
ZnO–CdS core–shell nanorods exhibit stable and high photocatalytic activity for water
Accepted 17 December 2009
splitting into hydrogen in the presence of S2 and SO2 3 as sacrificial reagents. Furthermore,
Available online 6 January 2010
the photocatalytic activity and stability of ZnO–CdS core–shell nanorods/RuO2 co-catalyst is superior to that of ZnO–CdS core–shell nanorods/Pt co-catalyst. The merits of stable ZnO
Keywords:
and CdS, core–shell and nanorod structures employed are considered to contribute to the
Water splitting
favorable photocatalytic hydrogen evolution of ZnO–CdS core–shell nanorods.
Photocatalysis
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
ZnO–CdS Z-scheme Core–shell
1.
Introduction
The photocatalytic water splitting over semiconductor photocatalysts to generate hydrogen has attracted increasing attention with sustainable energy and environment emerging as one of the top issues and challenges for humanity [1–10]. At the current stage, most photocatalysts suffer from low efficiency and/or serious instability for hydrogen evolution from water splitting under the irradiation of solar light. To solve the above issues, different hybrid semiconductor heterostructures [11–14] have been explored and designed due to their great flexibility in extending light response range and promoting the separation of photoinduced charge carriers in contrast to single phase photocatalysts [15–17]. Many efficient
heterostructures for photocatalytic water splitting are illuminated based on different electron transfer mechanisms [11,18–23]. For example, CdS/TiO2 heterostructures show a superior hydrogen evolution rate from water splitting to single CdS or TiO2 due to the efficient electron transfer from CdS conduction band (CB) to TiO2 CB and the wide light absorption of CdS itself [19]. To keep electron/hole with stronger redox capability on different reaction parts, the heterostructures based on indirect Z-scheme with redox mediators in solution or all solid state Z-scheme with vectorial electron transfer were also constructed [21–23]. Recently, we demonstrated that a ZnO–CdS heterostructure photocatalyst based on direct Z-scheme mechanism exhibits wide light response and high hydrogen evolution by prolonging lifetime
* Corresponding author. Tel.: þ61 7 33653735; fax: þ61 7 33656074. ** Corresponding author. Tel.: þ86 24 23971611; fax: þ86 24 23903126. E-mail addresses: [email protected] (G.Q. Lu), [email protected] (H.-M. Cheng). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.091
8200
international journal of hydrogen energy 35 (2010) 8199–8205
of charge carriers [11]. The key of the direct Z-scheme mechanism is that the holes with high oxidative power and electrons with high reductive power can be remained on different semiconductor parts by the direct recombination of low oxidative holes and low reductive electrons at the interface of heterostructures. For heterostructured photocatalysts, besides the mechanisms of charge carrier transfer, two more aspects can also play key roles in determining overall hydrogen evolution rate from photocatalytic water splitting, in particular for long-term applications. One is the stability of semiconductor units in heterostructures upon light irradiation. It is well known that both CdS prepared by traditional routes and ZnO as photocatalysts usually suffer from serious photocorrosion in longterm photocatalytic reactions [18]. The instability problem caused by photocorrosion also exists in our previously developed ZnO–CdS heterostructure photocatalysts [11]. However, the stability of CdS can be changed depending on preparation routes. For example, Jing et al. reported that CdS with steps on particle surface, prepared by a solid state reaction route, shows an ultra-high stability and activity in hydrogen evolution from water splitting [24]. Therefore, it is expected that the stability of ZnO–CdS heterostructures can be probably improved by employing different preparation routes.
The other is effective contacting among different semiconductor particles in heterostructures. The intimate and lasting contact can essentially promote the charge carrier transfer between different semiconductor particles in the heterostructures. In contrast to the heterostructures constructed with 0 dimensional (D) particles, the coupling of 1D semiconductor with 0D semiconductor has the obvious merit of forming more favorable interface combination, in particular when fine nanoparticles are coated on large sized rods. In addition, it is illuminated that 1D nanostrcutrue has a superior charge carrier transport capability to 0D structure by substantially lowering grain boundary recombination [25,26]. For example, single crystal (Cd0.8Zn0.2)S rods exhibit obvious advantage in facilitating charge carrier transfer and thus high photocatalytic hydrogen evolution [15]. Therefore, the hydrogen evolution of ZnO–CdS heterostructures is expected to be further improved by coupling rod-like morphology of ZnO with CdS nanoparticles. Here, we report ZnO–CdS core–shell hybrid nanorods as an efficient photocatalyst for hydrogen evolution from water splitting. In contrast to the previously developed ZnO–CdS heterostructures containing only ZnO/CdS nanoparticles prepared by wet chemistry route [11], ZnO–CdS core–shell nanorods show very stable hydrogen evolution in a long-term reaction under the irradiation of simulated solar light.
Fig. 1 – SEM images of (A) ZnO nanorods and (B) (ZnO)1–(CdS)0.2 core–shell nanorods, (C) TEM image of (ZnO)1–(CdS)0.2 core– shell nanorods and (D) high resolution TEM image of CdS particle coated on ZnO nanorods. The insets in (A) and (B) are the magnified SEM images of ZnO nanorods and (ZnO)1–(CdS)0.2 core–shell nanorods, respectively.
8201
A
CdS ZnO
Intensity / a.u
CdS
(ZnO)1-(CdS)0.2
ZnO
B
1.0
Absorbance / a.u
international journal of hydrogen energy 35 (2010) 8199–8205
0.8 106 105
0.6 0.4 0.2
103
104
102 101
CdS
ZnO
20
30
40
50
60
70
80
0.0 0 200
300
2 theta / degree
400
500
600
700
800
Wavelength / nm
Fig. 2 – (A) XRD patterns of reference ZnO, CdS and (ZnO)1–(CdS)0.2 core–shell nanorods; (B) UV–visible absorption spectra of ZnO, the ZnO–CdS core–shell nanorods ((ZnO)1–(CdS)x, x [ 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, are denoted as 101, 102, 103, 104, 105, 106, respectively), and CdS.
ZnO nanorods were synthesized by a modified hydrothermal route [27], where 0.1 M zinc acetate (Zn(CH3COO)2$2H2O) and 0.6 M hexamethylene tetramine (C6H12N4) dissolved into 50 mL deionized water was hydrothermally treated in a Teflonlined stainless autoclave with a volume of 80 mL at 95 C for 10 h. Appropriate ZnO nanorods from the above step suspended in 20 mL cadmium acetate (Cd(CH3COO)2$2H2O) solution were ultrasonically treated for 1 h. Then the suspension was recovered by evaporating at 80 C under constant magnetic stirring. Cd(CH3COO)2 in the recovered powder was converted to CdO by heating at 400 C for 1–3 h in air. To obtain ZnO–CdS core–shell nanorods, ZnO–CdO was heated in H2S atmosphere at a flux of 50 mL min1 at different temperature (300 C, 400 C and 500 C) for 0.5–2 h to sulfurize CdO to CdS [24]. Reference CdS was prepared by directly sulfurization of CdO from Cd(CH3COO)2 decomposing at 400 C. RuO2 loading on ZnO–CdS core–shell nanorods was conducted by impregnation using an tetrahydrofuran (THF) solution of triruthenium dodecacarbonyl (Ru3(CO)12) [28,29]. The ZnO–CdS core–shell nanorods was added into the THF solution containing a desirable amount of Ru3(CO)12 in an evaporating dish in a water bath at 60 C. The suspension was evaporated under constant stirring with a glass rod, and the resulting powder was collected and heated in air at 350 C for 1 h. Pt loading on ZnO–CdS core–shell nanorods was conducted by impregnation using an aqueous solution of H2PtCl6$6H2O. The ZnO–CdS nanorods was added into the aqueous solution containing a desirable amount of H2PtCl6$6H2O (1 mg mL1 Pt) in an evaporating dish and then irradiated by 300 W Xe lamp for 1 h. The suspension was evaporated at 60 C under constant stirring with a glass rod, and the resulting powder was collected and heated in air at 180 C for 1 h.
2.2.
Characterization
X-ray diffraction (XRD) patterns were recorded on a Rigaku ˚ ). Scanning diffractometer using Cu irradiation (l ¼ 1.54056 A
2.3.
Photocatalytic reactions
Water splitting reactions were carried out in a gas-closed circulation in vacuum [11]. 0.2 g photocatalyst powder was dispersed in a 300 mL aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3. The light source was a 300 W Xe lamp, and the light intensity reaching the surface of the reaction solution was 135 mW/cm2. The amount of H2 evolution was determined using a gas chromatography (Agilent Technologies: 6890N).
3.0 2.5 -1
2.1. Preparation of photocatalysts and loading of co-catalyst
electron microscopy (SEM) images were performed with Nova NanoSEM 430, and transmission electron microscopy (TEM) images were recorded on Tecnai F30. The UV–visible absorption spectra were measured with a UV–visible spectrophotometer (Jasco-V550). Chemical compositions and states of derived ZnO–CdS core–shell nanorods, ZnO and CdS were analyzed using X-ray photoelectron spectroscopy (Thermo Escalab 250, a monochromatic Al Ka X-ray source). All binding energies were referenced to the C 1s peak (284.6 eV) arising from adventitious carbon.
-1
Experimental section
H2 / mmol g h
2.
2.0 1.5 1.0 0.5 0.0
ZnO 101 102 103 104 105 106 CdS Photocatalysts
Fig. 3 – Comparison of the photocatalytic H2 evolution rate of CdS particles, ZnO nanorods, and the ZnO–CdS core– shell nanorods, where (ZnO)1–(CdS)x (x [ 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) are denoted as 101, 102, 103, 104, 105 and 106, respectively, prepared at 400 8C for 1 h in H2S atmosphere. Measurement conditions: 0.2 g sample, 300 mL aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3, and light source of 300 W Xe lamp.
8202
international journal of hydrogen energy 35 (2010) 8199–8205
B 2h
o
500
500 C
400 oC
400
300 oC
300 0.0
.5 0.5
Time / h
Temperature / oC
A
1h
0.5h .5 1.0 1.5 2.0 2.5 -1 -1 H2 / mmoll g h
3.0
2h
0.0
1h
0.5 h
0.5
1.0 1.5 2.0 2.5 -1 -1 H2 / mmol g h
3.0
Fig. 4 – Comparison of the amount of H2 evolution of the (ZnO)1–(CdS)0.2 nanorods prepared under H2S flow (A) at different temperature for 1 h and (B) at 400 8C for different time.
3.
Results and discussion
The starting ZnO shown in Fig. 1A has a rod-like morphology with an average diameter and length of ca. 800 nm and 3.5 mm, respectively. Compared to bare ZnO nanorods with very smooth surface, the surface of core–shell structured ZnO–CdS (Fig. 1B) is rough due to the coating of CdS particles around ZnO rods. TEM image (Fig. 1C) of ZnO–CdS nanorods further indicates that ZnO rods are uniformly coated with a layer of CdS particles, and the thickness of CdS shell is ca. 50 nm. The ˚ in Fig. 1D can be lattice fringes with the distance of 3.16 A assigned to (001) of hexagonal CdS. Fig. 2A shows XRD patterns of typical ZnO–CdS core–shell nanorods, and reference ZnO and CdS. All diffraction peaks can be indexed to hexagonal ZnO (Space group: P63mc) and CdS (Space group: P63mc) phases. Compared to the sharp peaks of CdS, the much broader diffraction peaks of CdS in the (ZnO)1–(CdS)0.2 core–shell nanorods suggest its relatively small particle size. It was illuminated that the photocatalytic water splitting capability of ZnO–CdS heterostructures drastically depends on the ratio of ZnO to CdS. Therefore, a series of (ZnO)x–(CdS)y core–shell nanorod samples (where the molar ratio x/y ranges from 10:1 to 10:6) were investigated. Their UV– visible absorption spectra are compared in Fig. 2B. Compared to reference ZnO, the optical absorption edges of the ZnO–CdS core–shell nanorods are extended into visible light range. It is found that the absorption edge of (ZnO)x–(CdS)y core–shell
B
6
-1 -1
4
2
0
6 5
H2 / mmol g h
-1
H2 / mmol g h
-1
A
nanorods is not obviously sensitive to the increased amount of CdS after the ratio of CdS to ZnO is larger than 0.2:1. Photocatalytic H2 evolution of (ZnO)1–(CdS)x core–shell nanorods (see Fig. 3) was conducted in an aqueous solution and S2 ions as sacrificial reagents under containing SO2 3 simulated solar light irradiation. The photocatalytic hydrogen evolution rate of (ZnO)1–(CdS)x core–shell nanorods varies with the ratio of ZnO to CdS. The photocatalytic H2 evolution of (ZnO)1–(CdS)x gradually decreases with the increase of CdS molar ratio from 0.2 to 0.6, though their optical absorption spectra are similar. The (ZnO)1–(CdS)0.2 core–shell nanorods exhibit the highest H2 evolution of 2.96 mmol h1 g1 among the (ZnO)1–(CdS)x photocatalysts, which is 34.4 times and 7.8 times higher that of ZnO nanorods prepared by the hydrothermal route and CdS prepared by the solid state route, respectively. Similar trend was also evidenced in the reported heterostructured ZnO–CdS nanoparticles prepared by a wet chemistry route [11]. This trend can be understood as follows: before x > 0.3 in (ZnO)1–(CdS)x heterostructured nanorods, the increased CdS particles can play double roles in enhancing hydrogen evolution rate by increasing visible light absorption and acting as reduction sites for hydrogen evolution. However, over coating of CdS particles on ZnO nanorods not only makes little increase in the absorbance but also substantially decreases the exposed ZnO surface as oxidation sites, As a consequence, direct Z-scheme mechanism will not work well due to the poor transfer of holes from ZnO nanorods to reactants.
4 3 2 1 0
Pt RuO2 Co-catalysts
0% 0.5% 1% 2% 3% ((ZnO)1-(CdS)0.2 with different contents of RuO2
Fig. 5 – (A) Hydrogen evolution comparison of (ZnO)1–(CdS)0.2 core–shell nanorods with loaded 1 wt% Pt and 1 wt% RuO2, respectively, and (B) effects of the content of RuO2 on photocatalytic H2 evolution of the (ZnO)1–(CdS)0.2 core–shell nanorods.
international journal of hydrogen energy 35 (2010) 8199–8205
60
H2 / mmol g
-1
50 40 30 20 10 0 0
5
10
15 20 Time / h
25
30
Fig. 6 – Time courses of water splitting over the (ZnO)1– (CdS)0.2 core–shell nanorods (solid square) and RuO2 (1 wt%)-loaded (ZnO)1–(CdS)0.2 core–shell nanorods (solid circle). The reaction was continued for 30 h, with evacuation every 10 h (solid vertical line).
Because the photocatalytic activity and stability of CdS greatly depend on the temperature and time of heat treatment in H2S atmosphere [24], we systematically optimize the heat treatment process of (ZnO)1–(CdS)0.2 core–shell nanorods in order to maximize the hydrogen evolution, as shown in Fig. 4. The treatment temperature varied from 300 C to 500 C. The morphology and optical absorption range of the nanorods treated at different temperatures are similar. However, the highest photocatalytic H2 evolution of the (ZnO)1–(CdS)0.2 nanorods, 2.96 mmol h1 g1, was obtained when the treatment temperature was at 400 C, which is 3.53 and 1.54 times higher that of the nanorods treated at 300 C and 500 C,
8203
respectively. The optimal treatment temperature of 400 C for the (ZnO)1–(CdS)0.2 nanorods is identical with the optimal preparation temperature for CdS reported by Jing et. al [24]. It was also found that the optimal treatment time for the (ZnO)1– (CdS)0.2 nanorods at 400 C is 1 h. The shorter treatment can not completely convert CdO to CdS in the heterostructured nanorods, while the longer treatment results in the formation of ZnS from ZnO. Co-catalyst can be equally important to the development of photocatalysts themselves in achieving high hydrogen evolution rate by substantially promoting the separation of photoinduced electrons and holes on photocatalyst surface. Various noble metal and transition oxides are often employed as co-catalysts to facilitate water splitting [1,17,30–33]. Particularly, Pt particles coated on photocatalysts are served as a H2 evolution co-catalyst for sulfide and oxide photocatalysts [20,33]. On the other hand, since Inoue et al. found that RuO2 coated on BaTi4O9 photocatalysts can remarkably improve the high photocatalytic activity in water splitting [34], increasing attention has focused on RuO2 co-catalyst prepared by decomposing Ru-based compounds. Therefore, Pt and RuO2 co-catalysts were applied on the surface of the (ZnO)1– (CdS)0.2 core–shell nanorods by impregnation. These co-catalyst particles were randomly dispersed on the photocatalyst surface and act as active sites for H2 reduction. Photocatalytic H2 evolution rates over the (ZnO)1–(CdS)0.2 core–shell nanorods with Pt or RuO2 particles loaded are shown in Fig. 5A. Compared to the Pt-loaded (ZnO)1–(CdS)0.2 core–shell nanorods, RuO2-loaded (ZnO)1–(CdS)0.2 core–shell nanorods exhibit higher photocatalytic H2 evolution rate. The result suggests that RuO2 can act as a more efficient co-catalyst for sulfide photocatalysts. H2 evolution over the (ZnO)1–(CdS)0.2 core–
Fig. 7 – High resolution XPS spectra of Zn 2p (A), O 1s (B), Cd 3d (C) and S 2p (D) in the (ZnO)1–(CdS)0.2 core–shell nanorods, ZnO nanorods and CdS particles.
8204
international journal of hydrogen energy 35 (2010) 8199–8205
Fig. 8 – Schematic of ZnO–CdS heterostructured nanoparticles (A) and core–shell nanorods (B).
shell nanorods with different contents of RuO2 co-catalyst is shown in Fig. 5B. The optimal loading content of RuO2 was found to be 1 wt%, which gives the highest H2 evolution of 6.18 mmol g1 h1 and is 2.1 times that of the (ZnO)1–(CdS)0.2 core– shell nanorods. This H2 evolution is also much higher than that of the previous Pt-loaded ZnO–CdS nanoparticles [11]. The superior photocatalytic activity of ZnO–CdS core–shell nanorods with RuO2 as co-catalyst to the nanorods with Pt as co-catalyst may be attributed to the more stable integration between RuO2 particles and nanorods as a result of high temperature oxidation transformation. Photocatalytic stability is a main bottleneck in the development of sulfide based photocatalysts for water splitting hydrogen evolution. To estimate the photocatalytic stability of the (ZnO)1–(CdS)0.2 core–shell nanorods, the time courses of photocatalytic H2 evolution over the (ZnO)1–(CdS)0.2 core–shell nanorods were conducted and are shown in Fig. 6. No noticeable degradation of photocatalytic H2 evolution was observed in repeated runs for the photocatalytic reaction of 30 h, which suggests a good anti-photocorrosion capability of our developed ZnO–CdS core–shell nanorods. The total H2 evolution after 30 h reaction was 87.2 mmol g1 and H2 evolution rate after the third circle can keep ca. 97% that of the first circle. For the RuO2-loaded (ZnO)1–(CdS)0.2 core–shell nanorods, the total H2 evolution reached up to 180.3 mmol g1 for 30 h. Importantly, its hydrogen evolution rate after 30 h reaction can retain ca. 96% of the initial rate. While only less than 50% of the initial hydrogen evolution rate for ZnO–CdS heterostructured nanoparticles prepared by a wet chemistry route [11] can be kept upon the irradiation of 30 h. The stable hydrogen evolution rate of ZnO–CdS core–shell nanorods upon irradiation makes it a potential candidate for practical photocatalysis application. It is drastically apparent from the above results that ZnO– CdS core–shell nanorods show much superior photocatalytic activity and stability to ZnO–CdS heterostructured nanoparticles. We can understand this issue from the following points: First, both CdS and ZnO employed in the current case have high crystallinity, which can substantially improve the stability of heterostructures upon irradiation. Their high crystallinity can be indicated from the well faceted ZnO nanorods and clear atomic lattices of CdS (See Fig. 1). Second, more intimate contact between ZnO nanorods and CdS nanoparticles in the (ZnO)1–(CdS)0.2 core–shell nanorods is realized compared to the ZnO–CdS heterstructured nanoparticles [11], which certainly promotes the photoinduced charge carrier transfer between them. In the preparation of the ZnO–CdS core–shell nanorods, CdS coating on the ZnO nanorods was prepared at 400 C in the H2S atmosphere, which makes CdS strongly adhere on the surface of ZnO
nanorods. To further detect the interaction between ZnO and CdS, X-ray photoelectron spectroscopy (XPS) was employed to determine the binding energies of Zn 2p, O 1s, Cd 3d and S 2p core electrons in ZnO–CdS core–shell nanorods, as shown in Fig. 7. Compared to the binding energies of Zn 2p3/2, Zn 2p1/2, O 1s and S 2p at 1020.2 eV, 1043.3 eV, 530.1 eV and 161.4 eV in the sole ZnO nanorods and CdS particles, their binding energies show an obvious shift to the high energy in the ZnO–CdS core– shell nanorods. On the contrary, the binding energy of Cd 3d in the ZnO–CdS core–shell nanorods shifts by 0.4 eV from 404.8 eV in the sole CdS to 404.4 eV. The binding energy shift clearly shows the formation of new chemical bonds, indicating the strong interaction between ZnO and CdS in the ZnO–CdS core– shell nanorods. Finally, the core–shell nanorod structure itself also plays an important role in improving photocatalytic H2 evolution rate. As shown in Fig. 8, compared to the agglomerated ZnO nanoparticles in ZnO–CdS heterostructured nanoparticles, the well dispersed ZnO nanorods have obvious merits of not only high transport capability within ZnO bulk but also avoiding the grain boundary recombination of photoinduced charge carriers. On the other hand, the efficient transfer of electrons and holes from ZnO and CdS in the core– shell nanorods to reactants can lower the photocorrosion of ZnO and CdS, to a large extent, and thus improve the stability of ZnO–CdS core–shell nanorods as photocatalysts.
4.
Conclusions
ZnO–CdS core–shell nanorods were synthesized by a two-step route and exhibit highly stable and efficient photocatalytic hydrogen evolution. By loading RuO2 co-catalyst, the hydrogen evolution of the (ZnO)1–(CdS)0.2 core–shell nanorods reaches 6.18 mmol g1 h1 in the presence of S2 and SO2 3 as sacrificial reagents under simulated solar irradiation. No deactivation was observed after 30 h photocatalytic water splitting reaction. Stable CdS and ZnO crystals, core–shell structure, strong interaction in the interfaces of ZnO and CdS and RuO2 co-catalyst are attributed to contribute to the high and stable photocatalyitc H2 evolution in a long-term reaction.
Acknowledgements The work was supported by the External Cooperation Program of Chinese Academy of Sciences (Grant No.GJHZ200815), Major Basic Research Program, Ministry of Science and Technology China (No. 2009CB220001), the Solar Energy Program of
international journal of hydrogen energy 35 (2010) 8199–8205
Chinese Academy of Sciences, the IMR SYNL-T.S. Keˆ Research Fellowship, and the Australian Research Council under its Centers of Excellence Program.
references
[1] Maeda K, Teramura K, Lu DL, Takata T, Saito N, Inoue Y, et al. Photocatalyst releasing hydrogen from water – enhancing catalytic performance holds promise for hydrogen production by water splitting in sunlight. Nature 2006; 440(7082):295. [2] Osterloh FE. Inorganic materials as catalysts for photochemical splitting of water. Chem Mater 2008;20(1):35–54. [3] Ashokkumar M. An overview on semiconductor particulate systems for photoproduction of hydrogen. Int J Hydrogen Energy 1998;23(6):427–38. [4] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38(1):253–78. [5] Kudo A. Development of photocatalyst materials for water splitting. Int J Hydrogen Energy 2006;31(2):197–202. [6] Lei Z, Ma G, Liu M, You W, Yan H, Wu G, et al. Sulfursubstituted and zinc-doped In(OH)3: a new class of catalyst for photocatalytic H2 production from water under visible light illumination. J Catal 2006;237(2):322–9. [7] Xing C, Zhang Y, Yan W, Guo LJ. Band structure-controlled solid solution of Cd1xZnxS photocatalystfor hydrogen production by water splitting. Int J Hydrogen Energy 2006; 31(14):2018–24. [8] Koca A, Sahin M. Photocatalytic hydrogen production by direct sun light from sulfide/sulfite solution. Int J Hydrogen Energy 2002;27(4):363–7. [9] Zou ZG, Ye JH, Sayama K, Arakawa H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001;414(6864):625–7. [10] Chen D, Ye JH. Photocatalytic H2 evolution under visible light irradiation on AgIn5S8 photocatalyst. J Phys Chem Solids 2007;68(12):2317–20. [11] Wang XW, Liu G, Chen ZG, Li F, Wang LZ, Lu GQ, et al. Enhanced photocatalytic hydrogen evolution by prolonging the lifetime of carriers in ZnO/CdS heterostructures. Chem Commun 2009;(23):3452–4. [12] Tsuji I, Kato H, Kudo A. Visible-light-induced H2 evolution from an aqueous solution containing sulfide and sulfite over a ZnS–CuInS2–AgInS2 solid-solution photocatalyst. Angew Chem Int Ed 2005;44(23):3565–8. [13] Zhang J, Xu Q, Feng Z, Li M, Li C. Importance of the relationship between surface phases and photocatalytic activity of TiO2. Angew Chem Int Ed 2008;47(9):1766–9. [14] Abe R, Takata T, Sugihara H, Domen K. Photocatalytic overall water splitting under visible light by TaON and WO3 with an IO3/I shuttle redox mediator. Chem Commun 2005; 30:3829–31. [15] Wang XW, Liu G, Chen ZG, Li F, Lu GQ, Cheng HM. Efficient and stable photocatalytic H2 evolution from water splitting by (Cd0.8Zn0.2)S nanorods. Electrochem Commun 2009; 11(6):1174–8. [16] Hara M, Hitoki G, Takata T, Kondo JN, Kobayashi H, Domen K. TaON and Ta3N5 as new visible light driven photocatalysts. Catal Today 2003;78(1–4):555–60. [17] Zong X, Yan HJ, Wu GP, Ma GJ, Wen FY, Wang L, et al. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as co-catalyst under visible light irradiation. J Am Chem Soc 2008;130(23):7176–7.
8205
[18] Reber JF, Meier K. Photochemical hydrogen-production with platinized suspensions of cadmium-sulfide and cadmium zinc-sulfide modified by silver sulfide. J Phys Chem 1986; 90(5):824–34. [19] Jang JS, Kim HG, Joshi UA, Jang JW, Lee JS. Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation. Int J Hydrogen Energy 2008;33(21):5975–80. [20] Sayama K, Mukasa K, Abe R, Abe Y, Arakawa H. Stoichiometric water splitting into H2 and O2 using a mixture of two different photocatalysts and an IO3/I shuttle redox mediator under visible light irradiation. Chem Commun 2001;23:2416–7. [21] Sayama K, Mukasa K, Abe R, Abe Y, Arakawa H. A new photocatalytic water splitting system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis. J Photochem Photobiol A 2002;148(1–3):71–7. [22] Higashi M, Abe R, Ishikawa A, Takata T, Ohtani B, Domen K. Z-scheme overall water splitting on modified-TaON photocatalysts under visible light (lambda < 500 nm). Chem Lett 2008;37(2):138–9. [23] Tada H, Mitsui T, Kiyonaga T, Akita T, Tanaka K. All-solidstate Z-scheme in CdS–Au–TiO2 three-component nanojunction system. Nature Mater 2006;5(10):782–6. [24] Jing DW, Guo LJ. A novel method for the preparation of a highly stable and active CdS photocatalyst with a special surface nanostructure. J Phys Chem B 2006;110(23):11139–45. [25] Morales AM, Lieber CM. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 1998;279(5348):208–11. [26] Duan XF, Huang Y, Cui Y, Wang JF, Lieber CM. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 2001;409(6816): 66–9. [27] Liu B, Zeng HC. Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm. J Am Chem Soc 2003;125(15): 4430–1. [28] Sato J, Saito N, Yamada Y, Maeda K, Takata T, Kondo JN, et al. RuO2-loaded beta-Ge3N4 as a non-oxide photocatalyst for overall water splitting. J Am Chem Soc 2005;127(12):4150–1. [29] Teramura K, Maeda K, Saito T, Takata T, Saito N, Inoue Y, et al. Characterization of ruthenium oxide nanocluster as a co-catalyst with (Ga1xZnx)(N1xOx) for photocatalytic overall water splitting. J Phys Chem B 2005;109(46): 21915–21. [30] Maeda K, Teramura K, Saito N, Inoue Y, Domen K. Improvement of photocatalytic activity of (Ga1xZnx)(N1xOx) solid solution for overall water splitting by co-loading Cr and another transition metal. J Catal 2006;243(2):303–8. [31] Maeda K, Teramura K, Lu DL, Saito N, Inoue Y, Domen K. Noble-metal/Cr2O3 core/shell nanoparticles as a co-catalyst for photocatalytic overall water splitting. Angew Chem Int Ed 2006;45(46):7806–9. [32] Kato H, Asakura K, Kudo A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003;125(10):3082–9. [33] Tsuji I, Kato H, Kobayashi H, Kudo A. Photocatalytic H2 evolution reaction from aqueous solutions over band structure-controlled (AgIn)(x)Zn2(1x)S2 solid solution photocatalysts with visible-light response and their surface nanostructures. J Am Chem Soc 2004;126(41):13406–13. [34] Inoue Y, Niiyama T, Asai Y, Sato K. Stable photocatalytic activity of BaTi4O9 combined with ruthenium oxide for decomposition of water. J Chem Soc Chem Commun 1992; 7:579–80.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods Xuewen Wang a, Gang Liu a, Gao Qing Lu b,*, Hui-Ming Cheng a,** a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China b The University of Queensland, ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and Australian Institute of Bioengineering and Nanotechnology, Qld. 4072, Australia
article info
abstract
Article history:
Stability and efficiency are important to realize the practical applications of photocatalysts
Received 25 September 2009
for photocatalytic hydrogen evolution from water splitting. ZnO–CdS core–shell nanorods
Received in revised form
with a wide absorption range were designed and synthesized by a two-step route. The
17 December 2009
ZnO–CdS core–shell nanorods exhibit stable and high photocatalytic activity for water
Accepted 17 December 2009
splitting into hydrogen in the presence of S2 and SO2 3 as sacrificial reagents. Furthermore,
Available online 6 January 2010
the photocatalytic activity and stability of ZnO–CdS core–shell nanorods/RuO2 co-catalyst is superior to that of ZnO–CdS core–shell nanorods/Pt co-catalyst. The merits of stable ZnO
Keywords:
and CdS, core–shell and nanorod structures employed are considered to contribute to the
Water splitting
favorable photocatalytic hydrogen evolution of ZnO–CdS core–shell nanorods.
Photocatalysis
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
ZnO–CdS Z-scheme Core–shell
1.
Introduction
The photocatalytic water splitting over semiconductor photocatalysts to generate hydrogen has attracted increasing attention with sustainable energy and environment emerging as one of the top issues and challenges for humanity [1–10]. At the current stage, most photocatalysts suffer from low efficiency and/or serious instability for hydrogen evolution from water splitting under the irradiation of solar light. To solve the above issues, different hybrid semiconductor heterostructures [11–14] have been explored and designed due to their great flexibility in extending light response range and promoting the separation of photoinduced charge carriers in contrast to single phase photocatalysts [15–17]. Many efficient
heterostructures for photocatalytic water splitting are illuminated based on different electron transfer mechanisms [11,18–23]. For example, CdS/TiO2 heterostructures show a superior hydrogen evolution rate from water splitting to single CdS or TiO2 due to the efficient electron transfer from CdS conduction band (CB) to TiO2 CB and the wide light absorption of CdS itself [19]. To keep electron/hole with stronger redox capability on different reaction parts, the heterostructures based on indirect Z-scheme with redox mediators in solution or all solid state Z-scheme with vectorial electron transfer were also constructed [21–23]. Recently, we demonstrated that a ZnO–CdS heterostructure photocatalyst based on direct Z-scheme mechanism exhibits wide light response and high hydrogen evolution by prolonging lifetime
* Corresponding author. Tel.: þ61 7 33653735; fax: þ61 7 33656074. ** Corresponding author. Tel.: þ86 24 23971611; fax: þ86 24 23903126. E-mail addresses: [email protected] (G.Q. Lu), [email protected] (H.-M. Cheng). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.091
8200
international journal of hydrogen energy 35 (2010) 8199–8205
of charge carriers [11]. The key of the direct Z-scheme mechanism is that the holes with high oxidative power and electrons with high reductive power can be remained on different semiconductor parts by the direct recombination of low oxidative holes and low reductive electrons at the interface of heterostructures. For heterostructured photocatalysts, besides the mechanisms of charge carrier transfer, two more aspects can also play key roles in determining overall hydrogen evolution rate from photocatalytic water splitting, in particular for long-term applications. One is the stability of semiconductor units in heterostructures upon light irradiation. It is well known that both CdS prepared by traditional routes and ZnO as photocatalysts usually suffer from serious photocorrosion in longterm photocatalytic reactions [18]. The instability problem caused by photocorrosion also exists in our previously developed ZnO–CdS heterostructure photocatalysts [11]. However, the stability of CdS can be changed depending on preparation routes. For example, Jing et al. reported that CdS with steps on particle surface, prepared by a solid state reaction route, shows an ultra-high stability and activity in hydrogen evolution from water splitting [24]. Therefore, it is expected that the stability of ZnO–CdS heterostructures can be probably improved by employing different preparation routes.
The other is effective contacting among different semiconductor particles in heterostructures. The intimate and lasting contact can essentially promote the charge carrier transfer between different semiconductor particles in the heterostructures. In contrast to the heterostructures constructed with 0 dimensional (D) particles, the coupling of 1D semiconductor with 0D semiconductor has the obvious merit of forming more favorable interface combination, in particular when fine nanoparticles are coated on large sized rods. In addition, it is illuminated that 1D nanostrcutrue has a superior charge carrier transport capability to 0D structure by substantially lowering grain boundary recombination [25,26]. For example, single crystal (Cd0.8Zn0.2)S rods exhibit obvious advantage in facilitating charge carrier transfer and thus high photocatalytic hydrogen evolution [15]. Therefore, the hydrogen evolution of ZnO–CdS heterostructures is expected to be further improved by coupling rod-like morphology of ZnO with CdS nanoparticles. Here, we report ZnO–CdS core–shell hybrid nanorods as an efficient photocatalyst for hydrogen evolution from water splitting. In contrast to the previously developed ZnO–CdS heterostructures containing only ZnO/CdS nanoparticles prepared by wet chemistry route [11], ZnO–CdS core–shell nanorods show very stable hydrogen evolution in a long-term reaction under the irradiation of simulated solar light.
Fig. 1 – SEM images of (A) ZnO nanorods and (B) (ZnO)1–(CdS)0.2 core–shell nanorods, (C) TEM image of (ZnO)1–(CdS)0.2 core– shell nanorods and (D) high resolution TEM image of CdS particle coated on ZnO nanorods. The insets in (A) and (B) are the magnified SEM images of ZnO nanorods and (ZnO)1–(CdS)0.2 core–shell nanorods, respectively.
8201
A
CdS ZnO
Intensity / a.u
CdS
(ZnO)1-(CdS)0.2
ZnO
B
1.0
Absorbance / a.u
international journal of hydrogen energy 35 (2010) 8199–8205
0.8 106 105
0.6 0.4 0.2
103
104
102 101
CdS
ZnO
20
30
40
50
60
70
80
0.0 0 200
300
2 theta / degree
400
500
600
700
800
Wavelength / nm
Fig. 2 – (A) XRD patterns of reference ZnO, CdS and (ZnO)1–(CdS)0.2 core–shell nanorods; (B) UV–visible absorption spectra of ZnO, the ZnO–CdS core–shell nanorods ((ZnO)1–(CdS)x, x [ 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, are denoted as 101, 102, 103, 104, 105, 106, respectively), and CdS.
ZnO nanorods were synthesized by a modified hydrothermal route [27], where 0.1 M zinc acetate (Zn(CH3COO)2$2H2O) and 0.6 M hexamethylene tetramine (C6H12N4) dissolved into 50 mL deionized water was hydrothermally treated in a Teflonlined stainless autoclave with a volume of 80 mL at 95 C for 10 h. Appropriate ZnO nanorods from the above step suspended in 20 mL cadmium acetate (Cd(CH3COO)2$2H2O) solution were ultrasonically treated for 1 h. Then the suspension was recovered by evaporating at 80 C under constant magnetic stirring. Cd(CH3COO)2 in the recovered powder was converted to CdO by heating at 400 C for 1–3 h in air. To obtain ZnO–CdS core–shell nanorods, ZnO–CdO was heated in H2S atmosphere at a flux of 50 mL min1 at different temperature (300 C, 400 C and 500 C) for 0.5–2 h to sulfurize CdO to CdS [24]. Reference CdS was prepared by directly sulfurization of CdO from Cd(CH3COO)2 decomposing at 400 C. RuO2 loading on ZnO–CdS core–shell nanorods was conducted by impregnation using an tetrahydrofuran (THF) solution of triruthenium dodecacarbonyl (Ru3(CO)12) [28,29]. The ZnO–CdS core–shell nanorods was added into the THF solution containing a desirable amount of Ru3(CO)12 in an evaporating dish in a water bath at 60 C. The suspension was evaporated under constant stirring with a glass rod, and the resulting powder was collected and heated in air at 350 C for 1 h. Pt loading on ZnO–CdS core–shell nanorods was conducted by impregnation using an aqueous solution of H2PtCl6$6H2O. The ZnO–CdS nanorods was added into the aqueous solution containing a desirable amount of H2PtCl6$6H2O (1 mg mL1 Pt) in an evaporating dish and then irradiated by 300 W Xe lamp for 1 h. The suspension was evaporated at 60 C under constant stirring with a glass rod, and the resulting powder was collected and heated in air at 180 C for 1 h.
2.2.
Characterization
X-ray diffraction (XRD) patterns were recorded on a Rigaku ˚ ). Scanning diffractometer using Cu irradiation (l ¼ 1.54056 A
2.3.
Photocatalytic reactions
Water splitting reactions were carried out in a gas-closed circulation in vacuum [11]. 0.2 g photocatalyst powder was dispersed in a 300 mL aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3. The light source was a 300 W Xe lamp, and the light intensity reaching the surface of the reaction solution was 135 mW/cm2. The amount of H2 evolution was determined using a gas chromatography (Agilent Technologies: 6890N).
3.0 2.5 -1
2.1. Preparation of photocatalysts and loading of co-catalyst
electron microscopy (SEM) images were performed with Nova NanoSEM 430, and transmission electron microscopy (TEM) images were recorded on Tecnai F30. The UV–visible absorption spectra were measured with a UV–visible spectrophotometer (Jasco-V550). Chemical compositions and states of derived ZnO–CdS core–shell nanorods, ZnO and CdS were analyzed using X-ray photoelectron spectroscopy (Thermo Escalab 250, a monochromatic Al Ka X-ray source). All binding energies were referenced to the C 1s peak (284.6 eV) arising from adventitious carbon.
-1
Experimental section
H2 / mmol g h
2.
2.0 1.5 1.0 0.5 0.0
ZnO 101 102 103 104 105 106 CdS Photocatalysts
Fig. 3 – Comparison of the photocatalytic H2 evolution rate of CdS particles, ZnO nanorods, and the ZnO–CdS core– shell nanorods, where (ZnO)1–(CdS)x (x [ 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) are denoted as 101, 102, 103, 104, 105 and 106, respectively, prepared at 400 8C for 1 h in H2S atmosphere. Measurement conditions: 0.2 g sample, 300 mL aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3, and light source of 300 W Xe lamp.
8202
international journal of hydrogen energy 35 (2010) 8199–8205
B 2h
o
500
500 C
400 oC
400
300 oC
300 0.0
.5 0.5
Time / h
Temperature / oC
A
1h
0.5h .5 1.0 1.5 2.0 2.5 -1 -1 H2 / mmoll g h
3.0
2h
0.0
1h
0.5 h
0.5
1.0 1.5 2.0 2.5 -1 -1 H2 / mmol g h
3.0
Fig. 4 – Comparison of the amount of H2 evolution of the (ZnO)1–(CdS)0.2 nanorods prepared under H2S flow (A) at different temperature for 1 h and (B) at 400 8C for different time.
3.
Results and discussion
The starting ZnO shown in Fig. 1A has a rod-like morphology with an average diameter and length of ca. 800 nm and 3.5 mm, respectively. Compared to bare ZnO nanorods with very smooth surface, the surface of core–shell structured ZnO–CdS (Fig. 1B) is rough due to the coating of CdS particles around ZnO rods. TEM image (Fig. 1C) of ZnO–CdS nanorods further indicates that ZnO rods are uniformly coated with a layer of CdS particles, and the thickness of CdS shell is ca. 50 nm. The ˚ in Fig. 1D can be lattice fringes with the distance of 3.16 A assigned to (001) of hexagonal CdS. Fig. 2A shows XRD patterns of typical ZnO–CdS core–shell nanorods, and reference ZnO and CdS. All diffraction peaks can be indexed to hexagonal ZnO (Space group: P63mc) and CdS (Space group: P63mc) phases. Compared to the sharp peaks of CdS, the much broader diffraction peaks of CdS in the (ZnO)1–(CdS)0.2 core–shell nanorods suggest its relatively small particle size. It was illuminated that the photocatalytic water splitting capability of ZnO–CdS heterostructures drastically depends on the ratio of ZnO to CdS. Therefore, a series of (ZnO)x–(CdS)y core–shell nanorod samples (where the molar ratio x/y ranges from 10:1 to 10:6) were investigated. Their UV– visible absorption spectra are compared in Fig. 2B. Compared to reference ZnO, the optical absorption edges of the ZnO–CdS core–shell nanorods are extended into visible light range. It is found that the absorption edge of (ZnO)x–(CdS)y core–shell
B
6
-1 -1
4
2
0
6 5
H2 / mmol g h
-1
H2 / mmol g h
-1
A
nanorods is not obviously sensitive to the increased amount of CdS after the ratio of CdS to ZnO is larger than 0.2:1. Photocatalytic H2 evolution of (ZnO)1–(CdS)x core–shell nanorods (see Fig. 3) was conducted in an aqueous solution and S2 ions as sacrificial reagents under containing SO2 3 simulated solar light irradiation. The photocatalytic hydrogen evolution rate of (ZnO)1–(CdS)x core–shell nanorods varies with the ratio of ZnO to CdS. The photocatalytic H2 evolution of (ZnO)1–(CdS)x gradually decreases with the increase of CdS molar ratio from 0.2 to 0.6, though their optical absorption spectra are similar. The (ZnO)1–(CdS)0.2 core–shell nanorods exhibit the highest H2 evolution of 2.96 mmol h1 g1 among the (ZnO)1–(CdS)x photocatalysts, which is 34.4 times and 7.8 times higher that of ZnO nanorods prepared by the hydrothermal route and CdS prepared by the solid state route, respectively. Similar trend was also evidenced in the reported heterostructured ZnO–CdS nanoparticles prepared by a wet chemistry route [11]. This trend can be understood as follows: before x > 0.3 in (ZnO)1–(CdS)x heterostructured nanorods, the increased CdS particles can play double roles in enhancing hydrogen evolution rate by increasing visible light absorption and acting as reduction sites for hydrogen evolution. However, over coating of CdS particles on ZnO nanorods not only makes little increase in the absorbance but also substantially decreases the exposed ZnO surface as oxidation sites, As a consequence, direct Z-scheme mechanism will not work well due to the poor transfer of holes from ZnO nanorods to reactants.
4 3 2 1 0
Pt RuO2 Co-catalysts
0% 0.5% 1% 2% 3% ((ZnO)1-(CdS)0.2 with different contents of RuO2
Fig. 5 – (A) Hydrogen evolution comparison of (ZnO)1–(CdS)0.2 core–shell nanorods with loaded 1 wt% Pt and 1 wt% RuO2, respectively, and (B) effects of the content of RuO2 on photocatalytic H2 evolution of the (ZnO)1–(CdS)0.2 core–shell nanorods.
international journal of hydrogen energy 35 (2010) 8199–8205
60
H2 / mmol g
-1
50 40 30 20 10 0 0
5
10
15 20 Time / h
25
30
Fig. 6 – Time courses of water splitting over the (ZnO)1– (CdS)0.2 core–shell nanorods (solid square) and RuO2 (1 wt%)-loaded (ZnO)1–(CdS)0.2 core–shell nanorods (solid circle). The reaction was continued for 30 h, with evacuation every 10 h (solid vertical line).
Because the photocatalytic activity and stability of CdS greatly depend on the temperature and time of heat treatment in H2S atmosphere [24], we systematically optimize the heat treatment process of (ZnO)1–(CdS)0.2 core–shell nanorods in order to maximize the hydrogen evolution, as shown in Fig. 4. The treatment temperature varied from 300 C to 500 C. The morphology and optical absorption range of the nanorods treated at different temperatures are similar. However, the highest photocatalytic H2 evolution of the (ZnO)1–(CdS)0.2 nanorods, 2.96 mmol h1 g1, was obtained when the treatment temperature was at 400 C, which is 3.53 and 1.54 times higher that of the nanorods treated at 300 C and 500 C,
8203
respectively. The optimal treatment temperature of 400 C for the (ZnO)1–(CdS)0.2 nanorods is identical with the optimal preparation temperature for CdS reported by Jing et. al [24]. It was also found that the optimal treatment time for the (ZnO)1– (CdS)0.2 nanorods at 400 C is 1 h. The shorter treatment can not completely convert CdO to CdS in the heterostructured nanorods, while the longer treatment results in the formation of ZnS from ZnO. Co-catalyst can be equally important to the development of photocatalysts themselves in achieving high hydrogen evolution rate by substantially promoting the separation of photoinduced electrons and holes on photocatalyst surface. Various noble metal and transition oxides are often employed as co-catalysts to facilitate water splitting [1,17,30–33]. Particularly, Pt particles coated on photocatalysts are served as a H2 evolution co-catalyst for sulfide and oxide photocatalysts [20,33]. On the other hand, since Inoue et al. found that RuO2 coated on BaTi4O9 photocatalysts can remarkably improve the high photocatalytic activity in water splitting [34], increasing attention has focused on RuO2 co-catalyst prepared by decomposing Ru-based compounds. Therefore, Pt and RuO2 co-catalysts were applied on the surface of the (ZnO)1– (CdS)0.2 core–shell nanorods by impregnation. These co-catalyst particles were randomly dispersed on the photocatalyst surface and act as active sites for H2 reduction. Photocatalytic H2 evolution rates over the (ZnO)1–(CdS)0.2 core–shell nanorods with Pt or RuO2 particles loaded are shown in Fig. 5A. Compared to the Pt-loaded (ZnO)1–(CdS)0.2 core–shell nanorods, RuO2-loaded (ZnO)1–(CdS)0.2 core–shell nanorods exhibit higher photocatalytic H2 evolution rate. The result suggests that RuO2 can act as a more efficient co-catalyst for sulfide photocatalysts. H2 evolution over the (ZnO)1–(CdS)0.2 core–
Fig. 7 – High resolution XPS spectra of Zn 2p (A), O 1s (B), Cd 3d (C) and S 2p (D) in the (ZnO)1–(CdS)0.2 core–shell nanorods, ZnO nanorods and CdS particles.
8204
international journal of hydrogen energy 35 (2010) 8199–8205
Fig. 8 – Schematic of ZnO–CdS heterostructured nanoparticles (A) and core–shell nanorods (B).
shell nanorods with different contents of RuO2 co-catalyst is shown in Fig. 5B. The optimal loading content of RuO2 was found to be 1 wt%, which gives the highest H2 evolution of 6.18 mmol g1 h1 and is 2.1 times that of the (ZnO)1–(CdS)0.2 core– shell nanorods. This H2 evolution is also much higher than that of the previous Pt-loaded ZnO–CdS nanoparticles [11]. The superior photocatalytic activity of ZnO–CdS core–shell nanorods with RuO2 as co-catalyst to the nanorods with Pt as co-catalyst may be attributed to the more stable integration between RuO2 particles and nanorods as a result of high temperature oxidation transformation. Photocatalytic stability is a main bottleneck in the development of sulfide based photocatalysts for water splitting hydrogen evolution. To estimate the photocatalytic stability of the (ZnO)1–(CdS)0.2 core–shell nanorods, the time courses of photocatalytic H2 evolution over the (ZnO)1–(CdS)0.2 core–shell nanorods were conducted and are shown in Fig. 6. No noticeable degradation of photocatalytic H2 evolution was observed in repeated runs for the photocatalytic reaction of 30 h, which suggests a good anti-photocorrosion capability of our developed ZnO–CdS core–shell nanorods. The total H2 evolution after 30 h reaction was 87.2 mmol g1 and H2 evolution rate after the third circle can keep ca. 97% that of the first circle. For the RuO2-loaded (ZnO)1–(CdS)0.2 core–shell nanorods, the total H2 evolution reached up to 180.3 mmol g1 for 30 h. Importantly, its hydrogen evolution rate after 30 h reaction can retain ca. 96% of the initial rate. While only less than 50% of the initial hydrogen evolution rate for ZnO–CdS heterostructured nanoparticles prepared by a wet chemistry route [11] can be kept upon the irradiation of 30 h. The stable hydrogen evolution rate of ZnO–CdS core–shell nanorods upon irradiation makes it a potential candidate for practical photocatalysis application. It is drastically apparent from the above results that ZnO– CdS core–shell nanorods show much superior photocatalytic activity and stability to ZnO–CdS heterostructured nanoparticles. We can understand this issue from the following points: First, both CdS and ZnO employed in the current case have high crystallinity, which can substantially improve the stability of heterostructures upon irradiation. Their high crystallinity can be indicated from the well faceted ZnO nanorods and clear atomic lattices of CdS (See Fig. 1). Second, more intimate contact between ZnO nanorods and CdS nanoparticles in the (ZnO)1–(CdS)0.2 core–shell nanorods is realized compared to the ZnO–CdS heterstructured nanoparticles [11], which certainly promotes the photoinduced charge carrier transfer between them. In the preparation of the ZnO–CdS core–shell nanorods, CdS coating on the ZnO nanorods was prepared at 400 C in the H2S atmosphere, which makes CdS strongly adhere on the surface of ZnO
nanorods. To further detect the interaction between ZnO and CdS, X-ray photoelectron spectroscopy (XPS) was employed to determine the binding energies of Zn 2p, O 1s, Cd 3d and S 2p core electrons in ZnO–CdS core–shell nanorods, as shown in Fig. 7. Compared to the binding energies of Zn 2p3/2, Zn 2p1/2, O 1s and S 2p at 1020.2 eV, 1043.3 eV, 530.1 eV and 161.4 eV in the sole ZnO nanorods and CdS particles, their binding energies show an obvious shift to the high energy in the ZnO–CdS core– shell nanorods. On the contrary, the binding energy of Cd 3d in the ZnO–CdS core–shell nanorods shifts by 0.4 eV from 404.8 eV in the sole CdS to 404.4 eV. The binding energy shift clearly shows the formation of new chemical bonds, indicating the strong interaction between ZnO and CdS in the ZnO–CdS core– shell nanorods. Finally, the core–shell nanorod structure itself also plays an important role in improving photocatalytic H2 evolution rate. As shown in Fig. 8, compared to the agglomerated ZnO nanoparticles in ZnO–CdS heterostructured nanoparticles, the well dispersed ZnO nanorods have obvious merits of not only high transport capability within ZnO bulk but also avoiding the grain boundary recombination of photoinduced charge carriers. On the other hand, the efficient transfer of electrons and holes from ZnO and CdS in the core– shell nanorods to reactants can lower the photocorrosion of ZnO and CdS, to a large extent, and thus improve the stability of ZnO–CdS core–shell nanorods as photocatalysts.
4.
Conclusions
ZnO–CdS core–shell nanorods were synthesized by a two-step route and exhibit highly stable and efficient photocatalytic hydrogen evolution. By loading RuO2 co-catalyst, the hydrogen evolution of the (ZnO)1–(CdS)0.2 core–shell nanorods reaches 6.18 mmol g1 h1 in the presence of S2 and SO2 3 as sacrificial reagents under simulated solar irradiation. No deactivation was observed after 30 h photocatalytic water splitting reaction. Stable CdS and ZnO crystals, core–shell structure, strong interaction in the interfaces of ZnO and CdS and RuO2 co-catalyst are attributed to contribute to the high and stable photocatalyitc H2 evolution in a long-term reaction.
Acknowledgements The work was supported by the External Cooperation Program of Chinese Academy of Sciences (Grant No.GJHZ200815), Major Basic Research Program, Ministry of Science and Technology China (No. 2009CB220001), the Solar Energy Program of
international journal of hydrogen energy 35 (2010) 8199–8205
Chinese Academy of Sciences, the IMR SYNL-T.S. Keˆ Research Fellowship, and the Australian Research Council under its Centers of Excellence Program.
references
[1] Maeda K, Teramura K, Lu DL, Takata T, Saito N, Inoue Y, et al. Photocatalyst releasing hydrogen from water – enhancing catalytic performance holds promise for hydrogen production by water splitting in sunlight. Nature 2006; 440(7082):295. [2] Osterloh FE. Inorganic materials as catalysts for photochemical splitting of water. Chem Mater 2008;20(1):35–54. [3] Ashokkumar M. An overview on semiconductor particulate systems for photoproduction of hydrogen. Int J Hydrogen Energy 1998;23(6):427–38. [4] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38(1):253–78. [5] Kudo A. Development of photocatalyst materials for water splitting. Int J Hydrogen Energy 2006;31(2):197–202. [6] Lei Z, Ma G, Liu M, You W, Yan H, Wu G, et al. Sulfursubstituted and zinc-doped In(OH)3: a new class of catalyst for photocatalytic H2 production from water under visible light illumination. J Catal 2006;237(2):322–9. [7] Xing C, Zhang Y, Yan W, Guo LJ. Band structure-controlled solid solution of Cd1xZnxS photocatalystfor hydrogen production by water splitting. Int J Hydrogen Energy 2006; 31(14):2018–24. [8] Koca A, Sahin M. Photocatalytic hydrogen production by direct sun light from sulfide/sulfite solution. Int J Hydrogen Energy 2002;27(4):363–7. [9] Zou ZG, Ye JH, Sayama K, Arakawa H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001;414(6864):625–7. [10] Chen D, Ye JH. Photocatalytic H2 evolution under visible light irradiation on AgIn5S8 photocatalyst. J Phys Chem Solids 2007;68(12):2317–20. [11] Wang XW, Liu G, Chen ZG, Li F, Wang LZ, Lu GQ, et al. Enhanced photocatalytic hydrogen evolution by prolonging the lifetime of carriers in ZnO/CdS heterostructures. Chem Commun 2009;(23):3452–4. [12] Tsuji I, Kato H, Kudo A. Visible-light-induced H2 evolution from an aqueous solution containing sulfide and sulfite over a ZnS–CuInS2–AgInS2 solid-solution photocatalyst. Angew Chem Int Ed 2005;44(23):3565–8. [13] Zhang J, Xu Q, Feng Z, Li M, Li C. Importance of the relationship between surface phases and photocatalytic activity of TiO2. Angew Chem Int Ed 2008;47(9):1766–9. [14] Abe R, Takata T, Sugihara H, Domen K. Photocatalytic overall water splitting under visible light by TaON and WO3 with an IO3/I shuttle redox mediator. Chem Commun 2005; 30:3829–31. [15] Wang XW, Liu G, Chen ZG, Li F, Lu GQ, Cheng HM. Efficient and stable photocatalytic H2 evolution from water splitting by (Cd0.8Zn0.2)S nanorods. Electrochem Commun 2009; 11(6):1174–8. [16] Hara M, Hitoki G, Takata T, Kondo JN, Kobayashi H, Domen K. TaON and Ta3N5 as new visible light driven photocatalysts. Catal Today 2003;78(1–4):555–60. [17] Zong X, Yan HJ, Wu GP, Ma GJ, Wen FY, Wang L, et al. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as co-catalyst under visible light irradiation. J Am Chem Soc 2008;130(23):7176–7.
8205
[18] Reber JF, Meier K. Photochemical hydrogen-production with platinized suspensions of cadmium-sulfide and cadmium zinc-sulfide modified by silver sulfide. J Phys Chem 1986; 90(5):824–34. [19] Jang JS, Kim HG, Joshi UA, Jang JW, Lee JS. Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation. Int J Hydrogen Energy 2008;33(21):5975–80. [20] Sayama K, Mukasa K, Abe R, Abe Y, Arakawa H. Stoichiometric water splitting into H2 and O2 using a mixture of two different photocatalysts and an IO3/I shuttle redox mediator under visible light irradiation. Chem Commun 2001;23:2416–7. [21] Sayama K, Mukasa K, Abe R, Abe Y, Arakawa H. A new photocatalytic water splitting system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis. J Photochem Photobiol A 2002;148(1–3):71–7. [22] Higashi M, Abe R, Ishikawa A, Takata T, Ohtani B, Domen K. Z-scheme overall water splitting on modified-TaON photocatalysts under visible light (lambda < 500 nm). Chem Lett 2008;37(2):138–9. [23] Tada H, Mitsui T, Kiyonaga T, Akita T, Tanaka K. All-solidstate Z-scheme in CdS–Au–TiO2 three-component nanojunction system. Nature Mater 2006;5(10):782–6. [24] Jing DW, Guo LJ. A novel method for the preparation of a highly stable and active CdS photocatalyst with a special surface nanostructure. J Phys Chem B 2006;110(23):11139–45. [25] Morales AM, Lieber CM. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 1998;279(5348):208–11. [26] Duan XF, Huang Y, Cui Y, Wang JF, Lieber CM. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 2001;409(6816): 66–9. [27] Liu B, Zeng HC. Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm. J Am Chem Soc 2003;125(15): 4430–1. [28] Sato J, Saito N, Yamada Y, Maeda K, Takata T, Kondo JN, et al. RuO2-loaded beta-Ge3N4 as a non-oxide photocatalyst for overall water splitting. J Am Chem Soc 2005;127(12):4150–1. [29] Teramura K, Maeda K, Saito T, Takata T, Saito N, Inoue Y, et al. Characterization of ruthenium oxide nanocluster as a co-catalyst with (Ga1xZnx)(N1xOx) for photocatalytic overall water splitting. J Phys Chem B 2005;109(46): 21915–21. [30] Maeda K, Teramura K, Saito N, Inoue Y, Domen K. Improvement of photocatalytic activity of (Ga1xZnx)(N1xOx) solid solution for overall water splitting by co-loading Cr and another transition metal. J Catal 2006;243(2):303–8. [31] Maeda K, Teramura K, Lu DL, Saito N, Inoue Y, Domen K. Noble-metal/Cr2O3 core/shell nanoparticles as a co-catalyst for photocatalytic overall water splitting. Angew Chem Int Ed 2006;45(46):7806–9. [32] Kato H, Asakura K, Kudo A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003;125(10):3082–9. [33] Tsuji I, Kato H, Kobayashi H, Kudo A. Photocatalytic H2 evolution reaction from aqueous solutions over band structure-controlled (AgIn)(x)Zn2(1x)S2 solid solution photocatalysts with visible-light response and their surface nanostructures. J Am Chem Soc 2004;126(41):13406–13. [34] Inoue Y, Niiyama T, Asai Y, Sato K. Stable photocatalytic activity of BaTi4O9 combined with ruthenium oxide for decomposition of water. J Chem Soc Chem Commun 1992; 7:579–80.