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Enhanced photocatalytic hydrogen evolution under visiblelight over Cd1LxZnxS solid solution with cubic zincblend phase

Lu Wang, Wenzhong Wang*, Meng Shang, Wenzong Yin, Songmei Sun, Ling Zhang

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, P.R. China

a r t i c l e i n f o

Article history:

Received 17 August 2009

Received in revised form

26 October 2009

Accepted 27 October 2009

Available online 13 November 2009

Keywords:

Solid solution

Photocatalyst

Visible light

H2

* Corresponding author. Tel.: þ86 21 5241 52E-mail address: wzwang@mail.sic.ac.cn (

0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.10.084

a b s t r a c t

A series of Cd1�xZnxS solid solutions were synthesized at 80 �C with the assistance of

sodium dodecylsulfate. The structures, optical properties and morphologies of the solid

solutions have been studied by X-ray diffraction, UV–vis diffuse reflectance spectroscopy,

and transmission electron microscopy. The photocatalytic H2 evolution over the solid

solutions under visible-light irradiation was investigated and the highest rate reached

2640 mmol h�1 g�1 even without any co-catalysts. The solid solution with optimum

performance exhibited cubic structure rather than previously-reported hexagonal one and

the possible reasons were discussed. Moreover, the effects of sacrificial reagents on the

photocatalytic H2 evolution were explored by using Na2S solution with different

concentration.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction irradiation that takes up w4% of solar energy, which limits the

As a potential answer to the global energy crisis and envi-

ronmental pollution, the application of hydrogen energy has

attracted great attention. Extensive work has been devoted to

the efficient production of hydrogen at low cost. Since

Fujishima and Honda firstly reported the decomposition of

water on illuminated TiO2 electrodes in 1972 [1], photo-

catalytic water splitting has been considered as a promising

strategy of converting solar energy into hydrogen energy.

Because of the advantages of photocatalytic H2 production,

such as pollution-free and low energy consumption,

researchers have made enormous efforts to improve the effi-

ciency of H2 production by modifying TiO2 nanostructures

[2,3], as well as developing new photocatalysts [4–6]. However,

most of the photocatalysts could only respond to UV

95; fax: þ86 21 5241 3122.W. Wang).sor T. Nejat Veziroglu. Pu

application of photocatalysts to a great extent.

Due to the suitable energy band corresponding to visible-

light absorption, chalcogenides are regarded as good candi-

dates for photocatalysts. Among them, CdS is the most widely

used sulfide owing to its superior physical properties

compared with other sulfide semiconductors [7–13]. The band

gap of CdS is relatively narrow and the absorption edge rea-

ches 510 nm. Thus, CdS exhibits wide absorption of visible

light. Meanwhile, CdS is favorable for reducing water due to

the sufficiently high flat-band potential [14]. In spite of the

tremendous advantages of CdS used as a photocatalyst, there

are still some shortcomings prohibiting the wide utilization of

this semiconductor, i.e. the indispensable deposition of

expensive noble metal on the surface of CdS to split water

efficiently [15,16]. Moreover, the photocorrsion of CdS,

blished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 9 – 2 520

involving the transformation of CdS into Cd2þ and S under

prolonged irradiation [11], has not yet been figured out. On the

other hand, ZnS is another metal sulfide that has been

extensively studied for water splitting. The band gap of ZnS is

3.66 eV, which is too large for visible light response [17–19].

Therefore, metal ions (i.e. Ni2þ,Pb2þ and Cu2þ) were doped

into ZnS in order to shift the absorption edge of ZnS into

visible light region [17,19,20]. However, the photocatalytic

efficiency of the doped ZnS under visible light has not been

satisfying yet.

As the solid solution of CdS and ZnS, Cd1�xZnxS possesses

tunable composition as well as band gap [21]. The newly

formed energy band in Cd1�xZnxS could respond to visible

light while not act as recombination centers. Furthermore, the

more negative reduction potential of the conduction band of

Cd1�xZnxS could lead to more efficient hydrogen generation

than that of cadmium sulfide. Though photocatalytic H2

production over Cd1�xZnxS was firstly reported as early as in

1986 [22], not until recent years has this solid solution been

widely studied. In 2006, Guo et al. prepared Cd1�xZnxS by

a simple coprecipitation method and optimized its perfor-

mance by tuning the band gap energy [21]. They also improved

the photocatalytic activity of Cd1�xZnxS by thermal sulfu-

ration [23]. Lately, Cd1�xZnxS doped with Cu2þ and Ni2þ were

studied, too [24,25]. These previous results from different

groups indicated that the solid solutions with hexagonal

phase exhibited the highest activity with the composition of

Cd0.8Zn0.2S [21,23,26].

Herein, a series of Cd1�xZnxS solid solutions with cubic zinc

blend phase were successfully synthesized with the assis-

tance of sodium dodecylsulfate (SDS) as the surfactant and

thiacetamide (TAA) as the S source. The average crystallite

sizes of as-obtained photocatalysts are in the range of 3–6 nm.

The optimum composition is different from that of the solid

solution with hexagonal phase. In particular, even without

a co-catalyst, the photocatalytic H2 evolution rate, as high as

2640 mmol h�1 g�1, was obtained from Cd0.44Zn0.56S suspen-

sion containing SO32� and S2� as sacrificial reagents. The

possible reason for the optimum performance was also

discussed.

2. Experimental

2.1. Synthesis of the solid solutions

All the reagents were of analytical grade and used without

further purification. In a typical synthesis, different molar

ratios of Zn(CH3COOH)2 and Cd(CH3COOH)2 and appropriate

amount of SDS were dissolved in 100 ml of deionized water.

The solution was constantly stirred for 20 min at room

temperature. Afterwards, excessive TAA was added. The final

solution was heated at 80 �C for 5 h. The as-produced precip-

itate was washed with deionized water for several times, and

then dried at 80 �C overnight under vacuum.

2.2. Characterization

The X-ray diffraction (XRD) patterns were measured with a

D/Max 2250 V diffractometer (Rigaku, Japan) using Cu Ka

(l¼ 1.5406 A) radiation over the range of 20� � 2q � 80�. The

transmission electron microscopy (TEM) analyses were per-

formed on a JEOL JEM-2100F field emission electron micro-

scope with an accelerating voltage of 200 kV. The accurate

composition of the samples was obtained by using the Oxford

INCA energy dispersive X-ray spectrometer (EDS) attachment

of the JEOL JXA-8100 electron probe microanalyzer (EPMA).

The optical diffuse reflectance spectrum was conducted on

a UV-Vis spectrophotometer (Hitachi U-3010) using an inte-

grating sphere accessory. Nitrogen adsorption–desorption

measurements were conducted at 77.35 K on a Micromeritics

Tristar 3000 analyzer. The Brunauer-Emmett-Teller (BET)

surface area was estimated using adsorption data.

2.3. Photocatalytic test

Photocatalytic reactions were conducted in a gas-closed

circulation system. The photocatalyst powder (0.1 g) was

dispersed in an aqueous solution (200 ml) containing 0.35 M

Na2SO3 and 0.25 M Na2S as electron donors in a Pyrex cell with

a quartz window on the top. The photocatalysts were irradi-

ated by visible light from a 500 W Xe lamp with a cutoff filter

(l � 420 nm). The amount of evolved H2 was determined with

on-line gas chromatography equipped with a thermal

conductivity detector (TCD). Nitrogen was purged through the

cell before the reaction to remove oxygen.

3. Results and discussion

3.1. Characterization of photocatalysts

The XRD patterns of the solid solutions with different

compositions and the standard diffraction patterns of cubic

ZnS and CdS are depicted in Fig. 1A. Three diffraction peaks

are detected in all samples, corresponding to the (111), (220),

(311) planes of zinc-blende phase. The progressive broadening

of all three reflection peaks with increasing amount of ZnS is

observed, suggesting the systematic decrease in particle size.

Since for true solid solutions there holds the well-known

Vegard’s law, the lattice constant a for the cubic phased

Cd1�xZnxS is determined by using the reported method

(Table 1) [27]. Fig. 1B exhibits the dependence between the

composition x and the lattice constants of the solid solution.

The lattice constants of the solid solutions decrease linearly as

a function of increasing ratio of ZnS in the sample, which is

due to the smaller radius of Zn2þ (0.74 A) than that of Cd2þ

(0.97 A) [21].

Fig. 2 shows UV–vis diffuse reflectance spectra of the solid

solutions. The colors of corresponding samples are also

showed to make a comparison with the light absorption

property. The intense absorption bands with a steep edge are

observed, indicating that the visible light absorption is

ascribed to the intrinsic band gap transition rather than the

transition from the impurity levels. The absorption edges of

the solid solutions gradually blue shift with the increasing

amount of ZnS, which is in agreement with the results of the

XRD patterns and the gradually changed colors of the solid

solutions. These results clearly indicate that homogeneous

solid solution of ZnS and CdS was formed. Compared with

Fig. 1 – (A) XRD patterns of Cd1LxZnxS solid solutions with different x values: (a) 1, (b) 0.56, (c) 0.3, (d) 0.2, (e) 0.12, (f) 0; (B)

Variation of lattice parameter of Cd1LxZnxS photocatalytsts as function of x value.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 9 – 2 5 21

ZnS, the onsets of the absorption edges of the solid solutions

reach almost 550 nm in visible light region, which significantly

improves the light absorption in the photocatalytic reaction.

The band gaps, Eg, of the solid solutions are obtained by the

plot of (ahy)2 vs. hy (inset of Fig. 2). The results were listed in

Table 1.

Fig. 3 reveals the morphology and microstructure of the

solid solutions by taking the composition of Cd0.44Zn0.56S as

a typical sample. As shown in Fig. 3a, the TEM image of the

Cd0.44Zn0.56S sample reveals that they are composed of

nanoparticles with the size ranging from 50 nm to 150 nm.

The composition of the sample was analyzed by the energy

dispersive X-ray spectrometer (inset of Fig. 3a) and the atomic

ratio of Zn:Cd is confirmed to be 0.56:0.44 within the error

range. In addition, the TEM image with higher magnification

(Fig. 3b) also indicates that each of these nanoparticles is in

fact composed of even smaller primary nanocrystals with

crystallite size of 3–6 nm. This result matches well with the

average crystallite size derived from the XRD patterns

described above. The selected area electron diffraction (SAED)

pattern taken from an individual particle is shown in Fig. 3b

(inset). The presence of the ring pattern approves that the

nanoparticles are polycrystalline. Moreover, the three

diffraction rings match with (111), (220), (311) planes of zinc-

blende phase, respectively. Fig. 3c shows a typical high-reso-

lution transmission electron microscope (HRTEM) image of

a single nanoparticle. The lattice fringes with different

orientations demonstrate that the nanoparticle is composed

of randomly oriented nanocrystals with an average size of

Table 1 – Summary of Materials Properties of Cd1LxZnxSsolid solutions.

Photocatalysts Composition(x)

Latticeconstants

a (A)

BET(m2/g)

Bandgap (eV)

a 1 5.4001 47.76 3.54

b 0.56 5.5971 54.48 2.47

c 0.3 5.6941 69.02 2.36

d 0.2 5.7294 38.19 2.31

e 0.12 5.7653 25.04 2.24

f 0 5.8198 17.12 2.20

3–6 nm. A local image is selected and magnified in Fig. 3d. The

interplanar spacing is about 3.273 A, which is in good agree-

ment with the interplanar distance of the (111) plane of

Cd0.44Zn0.56S (3.271 A). In accordance with the peak-shift in

the XRD patterns, the value of the (111) of Cd0.44Zn0.56S is

a little bigger than that of cubic ZnS, which can be explained

due to the higher radius of Cd2þ than that of Zn2þ.

CdS, ZnS and the solid solution of Cd1�xZnxS were gener-

ally prepared at high temperature [9,18,26] or by thermal

treatment process [8,16,28], which often resulted in hexagonal

wurtzite structure. However, the cubic zinc blend structure

was realized in our studies when a facile, low-temperature

synthesis route was adopted. Because of the adoption of high

concentration anionic surfactant, SDS, the cation (Zn2þ and

Cd2þ) in the solution would easily attach to the SDS molecules.

Then, hydrolysis of the anions of weak acids CH3COO� turned

the solution to be alkaline. Under alkaline conditions, TAA

decomposed and provided sulfur source for Zn2þ and Cd2þ

ions thus solid solutions formed. Since the interaction

between the cation and the SDS molecules, a homogeneous

reaction happened.

Fig. 2 – UV–vis diffuse reflectance spectra of Cd1LxZnxS

solid solutions with different x values: (a) 1, (b) 0.56, (c) 0.3,

(d) 0.2, (e) 0.12, (f) 0; The underside is the corresponding

samples; The inset is the plot of (ahy)2 vs. hy.

Fig. 3 – Microstructure analyses of the as-prepared Cd0.44Zn0.56S solid solution: (a) TEM image of low-magnification, (b) TEM

image of high-magnification, (c) HRTEM image, (d) the enlarged lattice fringes of the selected regions from c; The inset in

a and b are the EDS and the electron diffraction pattern, respectively.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 9 – 2 522

3.2. Photocatalytic activity

The wide absorption band resulting from the combination of Cd

and Zn is advantageous for utilizing the solar energy. Mean-

while, the high BET surface area together with the nanosized

crystalline of the as-prepared sample will increase the effective

reaction sites as well as shorten the diffuse distances of the

photogenerated carriers [18,21,29]. Fig. 4 shows the photo-

catalytic hydrogen evolution from the solid solutions. No H2 was

detected when ZnS alone was used as the catalyst even after 5 h

of visible-light irradiation, suggesting that ZnS is not active

under visible light for photocatalytic H2 evolution. On the other

hand, the activity of CdS was low, in which the H2 evolution rate

was about 80 mmol h�1 g�1. In particular, Cd0.44Zn0.56S showed

the highest photocatalytic activity, in which the H2 evolution

rate reached 2640 mmol h�1 g�1 even without any co-catalyst. As

revealed in Fig. 4, the photocatalytic H2 evolution rate of

Cd0.44Zn0.56S is nearly 33-fold higher than that of CdS, which

suggests that the dissolution of CdS and ZnS as well as this

particular composition is significant for the high rate of

hydrogen evolution. Moreover, the solid solutions exhibit

a gradually enhanced photocatalytic H2 rate as the Cd/Zn molar

ratio gets lower. The variation tendency is quite opposite to that

of the hexagonal wurtzite phased solid solutions [21,23].

There are several possible factors indispensable to

promote the photocatalytic H2 evolution rate effectively under

visible-light irradiation, i.e. the relatively wide visible light

absorption band, the sufficient high conduction band position,

the efficient separation and migration of charge carriers, as

well as the fast reduction of the adsorbed species by the

photogenerated electrons [30–32]. For the solid solutions of

Cd1�xZnxS, the conduction band potentials tend to be more

negative as x increases [21]. Although Cd0.44Zn0.56S possesses

less light absorption compared with other composition, the

more negative potential of the conduction band of

Cd0.44Zn0.56S would allow for more efficient hydrogen gener-

ation. Furthermore, the nanosized dimension of the sample

leads to the reduced recombination opportunities of the

photogenerated electron-hole pairs which could move effec-

tively to the surface [33]. In addition, the BET surface area

increases with the decrease of particle size, which was also

beneficial to absorb more light and increase active catalytic

sites. As shown in Table 1, Cd0.44Zn0.56S possesses a high BET

surface area, which is one of the important factors for the high

photocatalytic activity. This reveals that the solid solution of

Cd0.44Zn0.56S is favored in the photocatalytic reaction.

Although the zinc cadmium sulfide was studied before,

most of them showed the hexagonal wurtzite structure,

Fig. 4 – Photocatalytic H2 evolution by different samples

under visible light (l ‡ 420 nm), 0.1 g of catalyst in a 200 mL

aqueous solution containing 0.35 M Na2SO3 and 0.25 M

Na2S.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 9 – 2 5 23

and the highest activity was realized in the solid solution

with a high Cd/Zn ratio, for instance, 0.8/0.2 [21–23,26]. But

in our experiment, the solid solutions exhibited an

optimum composition in such a different Cd/Zn ratio. It

Fig. 5 – (a) Space configuration of ZnS4 tetrahedral (left) and the

directions of the tetrahedrals: the plane (111) for cubic zinc blen

structure (right).

has been considered that the unusual optimum composi-

tion is related to the different crystal structure. Bando et al.

reported that the cubic and hexagonal ZnS can be

described as stacking of the parallel arranged {ZnS4} tetra-

hedral by sharing the common corners [34,35], partially

ZnS4 tetrahedral would be replaced by CdS4 tetrahedral in

the solid solution of zinc cadmium sulfide (Fig. 5a). More-

over, the arrangement direction of the ZnS4/CdS4 is totally

different between the hexagonal and cubic type. In the

cubic zinc-blende structure, the ZnS4/CdS4 tetrahedral

arrange with the direction parallel to (111) [31], while the

plane is (001) in the hexagonal wurtzite structure [36], as

shown in Fig. 5b. The transfer properties of the electrons in

different planes are different, resulting in a different

optimum composition.

The effect of the sulfide and sulfite anions acted as sacri-

ficial reagents has been reported by different researchers. In

the absence of sacrificial reagents, a well-known anodic

photocorrosion reaction of sulfide in water would happen:

[11,37]

CdSþ hy/hþ þ e� (1)

CdSþ 2hþ/Cd2þ þ S (2)

2H2Oþ 2e�/H2 þ 2OH� (3)

replaced CdS4 tetrahedral (right); (b) different arrangement

d structure (left) and the plane for hexagonal wurtzite

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 9 – 2 524

Therefore, during the illumination on the sulfide suspen-

sions, hydrogen was evolved and the catalyst was consumed by

photoexcited holes. After SO32� and S2� were utilized, different

reactions occurred for the photoexcited holes as follows: [33]

SO2�3 þ 2OH� þ 2hþ/SO2�

4 þ 2Hþ (4-1)

2S2� þ 2hþ/S2�2 (4-2)

S2�2 þ SO2�

3 /S2O2�3 þ S2� (4-3)

SO2�3 þ S2� þ 2hþ/S2O2�

3 (4)

As shown in equations (4-1) and (4-2), photoexcited holes

react with SO32� and S2� to produce SO4

2� and S22�, respectively.

The production of S22� ions act as an optical filter and electron

acceptor capturing the photogenerated electrons, so prevent

the water from being reduced. But this process is efficiently

suppressed in the presence of SO32� ions, and the ions of S2�

and S2O32� are produced (4-3). The main anodic reaction

product, S2O32�, is colorless, avoiding the decrease of the light

absorption. Furthermore, S2O32� is inactive in the reduction

process, which has little negative effect on the reaction. The

main anodic reaction process could be represented by equa-

tion (4). These results reveal that the sacrificial reagents would

be converted into S2O32� as the photocatalytic reaction goes on,

leading to the decrease of photocatalytic activity.

Based on the significant role played by the sacrificial

reagents in the process of photocatalytic water splitting, Na2S

alone was used as the sacrificial reagent. The relationship

between the concentration of Na2S and the rate of H2 evolu-

tion was studied as well, as shown in Fig. 6. The highest rate of

H2 evolution was found when the concentration of Na2S was

0.3 M, and the rate is even higher than that in 0.25 M Na2S/

0.35 M Na2SO3. However, an obvious decrease of the rate of H2

evolution was observed when the Na2S was used alone, which

is due to the decrease of the S2� concentration as well as the

competition of S22�with the reduction of proton, revealing the

applicability of above model in our photocatalytic system.

Furthermore, the rate of H2 evolution was inversely

Fig. 6 – Photocatalytic H2 evolution rates of Cd0.44Zn0.56S in

aqueous solution with different Na2S concentration under

visible light (l ‡ 420 nm).

proportional to the concentration of Na2S in high concentra-

tion region, which is due to the increase of the pH value [33]. It

was reported that the photocatalytic activity of the sulfur

compounds would be greatly influenced by the shift of the

flat-band potential [11]. When Na2S alone was used as sacri-

ficial reagent, the increased concentration of S2� shifted the

flat-band potential to a less negative location than the redox

potential of Hþ/H2. As a result, the photocatalytic activity

of the solid solution decreased [14]. On the other hand, the

alkalinity of the suspension system become stronger with

the increase of the concentration of S2�, which will decrease

the driving force of reaction according to equation 3 and thus

is unfavorable for the photocatalytic reaction thermodynam-

ically [33]. As shown in Fig. 6, the appropriate concentration of

the Na2S could be ranged from 0.1 M to 0.3 M, suggesting the

appropriate concentration of 0.25 M Na2S/0.35 M Na2SO3.

4. Conclusion

A series of Cd1�xZnxS solid solutions with high visible-light-

induced photocatalytic activity of water splitting were

synthesized via a simple SDS-assisted method. The solid

solutions exhibited cubic zinc blend structure, and obvious

peak shifts have been found in the XRD patterns. The photo-

catalytic H2 evolution from aqueous solutions of sulfide and

sulfite were investigated and the solid solutions of

Cd0.44Zn0.56S showed the highest photocatalytic activity. The

high H2 evolution rate of this composition can be ascribed to

the energy band structure, the nanoscale crystallites, the large

specific surface areas etc. In addition, based on differences of

the crystal structure, the optimum composition was different

compared with the previously reported Cd1�xZnxS solid solu-

tions with hexagonal wurtzite structure. The effect of the

sacrificial reagents in our reaction system was also discussed.

Acknowledgement

We acknowledge the financial support from the National

Natural Science Foundation of China (50672117, 50732004),

National Basic Research Program of China (973 Program,

2007CB613305), and Solar Energy Project of Chinese Academy

of Sciences.

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