Photoelectrochemical Production of Hydrogen in Aqueous Suspensions Nanoparticles Composites of CdS/ZnS

doi:10.4236/msa.2011.211217

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Materials Science s a nd Applications, 2011, 2, 1631-1638

doi:10.4236/msa.2011.211217 Published Online November 2011 (http://www.SciRP.org/journal/msa)

1631

Photoelectrochemical Production of Hydrogen in

Aqueous Suspensions Nanoparticles Composites of

CdS/ZnS

Kasem K. Kasem*, Melissa Dahn, Nida Zia, Aubrey Finney

Science, Mathematics & Information Department, School of Arts & Science, Indiana University Kokomo, Kokomo, USA.

Email: *[email protected]

Received July 20th, 2011; revised September 2nd, 2011; accepted September 19th, 2011.

ABSTRACT

Aqueous solutions of mixed CdS/ZnS semiconductor (SC) nanoparticle suspensions in phosphate buffers containing 10

mM [Fe(CN)6]4– were used for photochemical production of hydrogen via hydrated electron intermediates. CdS was

doped with varying percentages of ZnS to expand the absorption range of the composite to the UV region. Results show

that maximum generation of hydrated electrons by [Fe(CN)6]4– occurs at pH 6. Furthermore, native CdS amorphous

nanoparticles give the greatest photocurrent. Studies also show that, in phosphate buffer, the steady state photocurrent

was directly proportional to the CdS content in the mixture of CdS/ZnS. The aqueous nano-systems sustained their sta-

bility as indicated by the reproducibility of their photocatalytic activities. Solar radiated assemblies of CdS/ZnS/

[Fe(CN)6]4– sustained cyclic systems for continuous hydrogen production.

Keywords: Photolysis, Nanoparticles, Hydrogen, Suspensions, Cadmium Sulfide

1. Introduction

Traditional chemical or electrochemical methods for pro-

duction of hydrogen tend to be complicated and energy

consuming process. Some of these methods are not even

environmentally safe. Novel methods that avoidthe dis-

advantages of traditional methods are in demand. The use

of solar radiation in photolysis of water, can be one of

these novel methods that can meet this demand. Eviden-

ces that some semiconductors (SC) mediate hydrogen pro-

duction during photolysis of water werereported by sev-

eral researchers [1-8]. Nano-sized metal chalcogenides

such as sulfides, selenides, and tellurides with a specific

band gap that can serve this purpose are being produced

following a variety of procedures [9-14]. The photocur-

rent obtained using such nano-sized assemblies is often

low because fast charge recombination limits photocur-

rent generation. When composite semiconductors are used

it is possible to improve the efficiency of charge separa-

tion through charge rectification. This can take place by

modifying either the surface of the base semiconductor

or its composition with inorganic or organic semicon-

ductors.

In some studies [11] nano-size semiconductors were

used because of their larger surface area and their ability

to carry out all the reactions that were previously associ-

ated with thin solid films of semiconductor electrodes.

TiO2 (Titania) is known as a photo catalysis agent for a

wide range of substances [15]. Nano-sized TiO2 pos-

sesses enhanced photocatalytic activity and solar energy

conversion [16,17]. However, some disadvantages limit

the efficient use of TiO2. For example, the high band gap

energy (3.2 eV) of TiO2 requires UV radiation for photo-

activation, limiting its application in the visible light

range, and charge carrier recombination (e−/h+) occurs wi-

thin nanoseconds, limiting its photocatalytic activity [18].

The spatial separation between oxidative and reductive

sites is very small and increases the occurrence of back re-

actions.

CdS has been intensively studied [19-23] due to ad-

vantages such as its band-gap energy in the visible region

Eg = 2.5 eV and its relatively simple fabrication process.

CdS nano-particles often present novel properties and

have been widely used in solar cells [24], optoelectronics,

and microelectronics. CdS particles form a group of di-

rect band gap materials suitable for light emitting appli-

cations, and their luminescent properties have been stud-

ied extensively [25]. The small size of nanoparticles

leads to their larger surface area, which will increase the

Photoelectrochemical Production of Hydrogen in Aqueous Suspensions Nanoparticles Composites of CdS/ZnS

1632

number of surface specific active sites for both chemical

reactions and photon absorption. When particle size de-

creases below the Bohr radius of the first excitation state,

quantum size effects can occur due to the confinement of

charge carriers [26].

Although the stability conditions are not met by CdS

because its decomposition potential (Ep,d) is less than the

oxidation potential of water (22

OHO

), its reduction po-

tential is more negative than that of H+. Mixing CdS and

ZnS by co-deposition may push the limits of the CdS/

ZnS band structure to the level where the decomposition

potential is greater than that the decomposition of water.

Effective conversion of solar energy to chemical en-

ergy in hydrogen form, requires an inexpensive source

that contains hydrogen and has a large surface area. Aque-

ous colloidal nanoparticle suspensions meet these re-

quirements. The suspension media are equally as impor-

tant as the suspension particles in generating an efficient

photolysis process. Previous studies used semiconductor

particles as photoactive systems in heterogeneous charge

transfer processes at the particle/electrolyte interface [27,

28]. Hydrated electrons can play an important role in pho-

to-dissociation of water through this reaction:

[Fe(CN)6]4– + hν = [Fe(CN)6]3– + eaq (1)

eaq + eaq = H2 + 2OH– (2)

(aq– = H2O)

2[Fe(CN)6]4– + 2hν = 2[Fe(CN)6]3– + H2 + 2OH– (3)

The molecular orbital structure of hexacyano iron (II),

[Fe(CN)6]4–, allows electronic transitions under the photo

excitation condition and produces hydrated electrons that

react according to the above reaction. [Fe(CN)6]4– un-

dergoes oxidation to [Fe(CN)6]3–. The disadvantage of a

homogeneous process for hydrated electron production is

its irreversibility. However, such a disadvantage can be

overcome by the use of a semiconductor system which

acts as an electron donor and reduces [Fe(CN)6]3– back to

[Fe(CN)6]4–. Achieving such a goal will create the condi-

tions of reversible ergo-dynamics.

A means to expand CdS band gap for efficient capture

of UV-visible radiation is mixing it with ZnS which pos-

sesses a greater band gap than CdS.

In this paper we investigated the effect of alteringthe

band gap of the mixed CdS/ZnS composite by changing

the percentage of ZnS on the photocatalytic behavior of

composite suspensions in buffered ferrocyanide solutions

as the hydrated electron supplier. Conditions that maxi-

mize the production of hydrated electrons were also ex-

plored.

2. Experimental

Reagents: All reagents were of analytical grade. All so-

lutions were prepared using de-ionized water, unless other-

wise stated. CdS/ZnS composites were prepared as de-

scribed elsewhere [29].

Instrumentation: All electrochemical experiments were

carried out using a conventional three-electrode cell con-

sisting of Pt wire as a counter electrode, Ag/AgCl as a

reference electrode, and Pt gauze as an electron collector.

A BAS 100 W electrochemical analyzer (Bio-analytical

Co.) was used to perform the electrochemical studies.

Steady state reflectance spectra were recorded using a

Shimadzu UV-2101 PC. Branson Model 250 digital so-

nifier was used to break down powder to the desired na-

no-particle size.

Sample preparation: The colloidal nano-particle sus-

pensions were prepared by dispersing 50 mg of the se-

miconductors powder in 100 mL of the bufferusing high

energy sonicators. Preliminary studies with large particle

sizes (500 - 1000 nm) gave poor photo responses. We did

our studies on diverse collections of small-particles sizes

blow 200 nm. An Olympus BX-FLA60 reflected light flu-

orescence microscope using polarized light at a wave-

length range between 330 to 550 nm was used to make

sure that the size of the suspensions of colloidal nano-

particles did not exceed 200 nm.

Photolysis cell: The electrolysis cell was a one com-

partment 120 mL Pyrex cell with a quartz window facing

the irradiation source. A 10-cm2 platinum gauze cylinder

was used as working electrode. Aqueous suspensions

were stirred with a magnetic stirrer during the measure-

ments. An Ag/AgCl/Cl– reference electrode was also fit-

ted into this compartment. A 10-cm2 platinum counter elec-

trode was housed in a glass cylinder sealed at one end

with a fine porosity glass frit. The pH was adjusted by

addition of either 1 M NaOH or 1 M H3PO4.

Irradiations were performed with a solar simulator 300

watt xenon lamp (Newport) with an IR filter. Light was

focused on the cell window using a metal cylinder with 5

cm diameter, and 15 cm length. The cell position was

adjusted to allow full illumination of the 100 mL suspen-

sion. Photolysis of [Fe(CN)6]4– generated hydrated elec-

trons and [Fe(CN)6]3–. The potential of the working elec-

trode was fixed at 100 mV more negative than the reduc-

tion potential of [Fe(CN)6]3– to guarantee full reduction

of ferricyanide. The current due to the reduction of

[Fe(CN)6]3– collected by the working electrode during

the photolysis process was a measure of photocurrent.

Photocurrent-time curves were obtained with a BAS 100

W Bioanalytical system. The measured photocurrent was

normalized to A·m–2·h–1 (ampere per square meter per

hour) of illumination. Because the measured photocur-

rent is a function of regeneration of ferrocyanide (the

solvated electron suppliers that generate hydrogen), the

measured photocurrent was normalized considering two

photons per one hydrogen molecule (according to Equa-

Photoelectrochemical Production of Hydrogen in Aqueous Suspensions Nanoparticles Composites of CdS/ZnS1633

tion (2)), and was used to calculate the number of moles

of hydrogen generated per square meter per hour of illu-

mination. Hydrogen was detected using HY-ALERTATM

500 (h2 scan California).

The following equation was used to calculate H2 rate:







1423

Hratemolemh

mole10cm3.610 s

2 96500molecm







 



(4)

where i = photo current, C/s., a = electrode surface area,

cm2, 3600 s corresponds to one hour, 10,000 correspon-

ding to one square meter, and 96,500 C/mole corresponds

to the Farad.

3. Results and Discussion

3.1. Energy Map of CdS/ZnS Composite

Figure 1 illustrates the diffusive reflectance absorption

spectra of CdS/ZnS nanoparticle composites. Determina-

tion of direct and indirect band gap of the studied mix-

tures was done using the following Tauc equations [30]:







 (5)









EkT

pgi pgi

EkT EkT

EEEEEE e







 



1

　 (6)

where



is an absorption coefficient, and Eg is the op-

tical band gap. Analysis of the data provided by Figure 1,

indicates that the studied CdS/ZnS mixtures possess a

direct and indirect band gap. Figure 2 shows that these

band gaps varied monotonically with the percent compo-

sition between that of ZnS and the smaller band gap of

CdS. The fact that the absorption of the mixture under-

went blue shift, indicates that the band gap of the mixture

is being expanded to absorbshorter wave length. The

relative location of the C.B (conduction band) of CdS as

Figure 1. Steady state reflectance spectra for CdS/ZnS

Composites. Inset SEM for the CdS/ZnS particles before so-

nication.

Figure 2. Changes in the direct and indirect band gap with

ZnS percentage in the composites.

a major compound in the mixture is about –0.4 V vs

NHE, which is more negative to the redox potential of

[Fe(CN)6]4–/3– than the reduction potential of H+. This

guarantees the reduction of [Fe(CN)6]3–. The band gap of

the mixture increases by increasing the percentage of

ZnS due to the fact that ZnS possesses a greater band gap

than CdS. The intensity of UV in the used solar simulator

is much less than that of VIS portion. Addition of ZnS

makes the mixture less efficient in the absorption of the

VIS light. The observed photochemical outcome and con-

sequently, hydrogen production of the prepared mixtures

(premonition) decreases by increasing ZnS in the mixture.

This makes the data for indirect band gap displayed in

Figure 2 irrelevant. This is because the indirect band gap

of samples with greatest content of ZnS (50%) is less

than 2.3 eV (smaller than that of pure CdS). We con-

cluded that the samples used in this study acted with di-

rect band gap transitions. These studied nano particles

give similar photocurrent in aerated and de aerated buff-

ers containing [Fe(CN)6]4–. These results show no effect

of de-aeration process on the photolysis process. This

also indicates that the C.B did not shift to more negative

value (cathodically) to cover the reduction potential 31of

the dissolved oxygen (≈ –0.8 V vs NHE). Although in-

creasing ZnS did not increase the direct band gap of the

mixture with amount proportional to ZnS content (Fig-

ure 2), the decrease in photocurrent reflects that ZnS did

not causes catholic shift for the flatband potential of CdS.

These two observations support general conclusion that

the addition of ZnS may cause an upward shift in the

valence band.

3.2. Photolysis of Aqueous Solutions of

[Fe( CN)6]4– Effect of pH

One of the products of reaction 3 is OH–, which makes

this reaction pH dependant. This reaction product sug-

gests that an acid pH range would be suitable to shift its

Photoelectrochemical Production of Hydrogen in Aqueous Suspensions Nanoparticles Composites of CdS/ZnS

1634

equilibrium to favor of H2 production. However,

[Fe(CN)6]4– as a reactant is pH sensitive. We have found

that, at pH less than 6, [Fe(CN)6]4– will form a green

compound known as Berlin green or ferric ferrocyanide

[Fe(CN)6]3–. Photolysis reactions of aqueous [Fe(CN)6]4-

in phosphate buffer at different pH values were perfor-

med. The results are displayed in Figure 3, from which

we can notice a drop in photocurrent at pH greater than 6.

For these reasons, photolysis of aqueous [Fe(CN)6]4– in

the presence of CdS/ZnS mixture nano- particles took

place at pH 6. The observed change shown in Figure 3

can be attributed to the kinetics of reduction of

[Fe(CN)6]3– on the Pt electrode and the presence of some

Pt oxides formed at pH greater than 6 as indicated by

Pourbaix diagrams [32]. The kinetics of such reductions

at higher pH is beyond the scope of this article and it can

be a subject of future studies.

3.3. Photolysis of Aqueous [Fe(CN)6]4– in the

Presence of CdS/ZnS Suspensions

Aqueous suspensions of pure CdS in 0.2 M phosphate

buffer containing 10 mmole of [Fe(CN)6]4– at pH 6 were

subject to the photolysis process. The potential of the Pt

collector electrode kept constant at 0.000 V vs Ag/AgCl.

The results are displayed in Figures 4 and 5. The re-

corded photocurrent in Figure 4 is due to electrochemi-

cal reduction of [Fe(CN)6]3–. In the presence of illumi-

nated CdS, reduction of [Fe(CN)6]3– can take place by an

electrochemical and/or by a photochemical process. The

collector electrode records only the electrochemical pro-

cess. The amount of [Fe(CN)6]3– reduced by the photo-

chemical process can be estimated by the difference be-

tween the current recorded for the photolysis of

[Fe(CN)6]4– at pH 6 in the absence and in the presence of

SC according to the following equation:

Figure 3. Effect of pH on the recorded photocurrent during

the photolysis of [Fe(CN)6]4–.

Figure 4. Reproducibility of the photocurrent during the

photolysis of [Fe(CN)6]4– in presence of pure CdS nanopar-

ticle suspensions.

Figure 5. Photolysis of aqueous CdS/ZnS nanoparticles in

10 mM [Fe(CN)6]4–/Phosphate buffer (pH = 6).

Photo-reduction current Iphotored = Ired in absence of SC

– Ired in presence of SC (7)

It can be noticed that the recorded Ired (Equation (7)) in

the presence of SC (Figures 4 and 5) is less than that

recorded for 10 mmole of [Fe(CN)6]4– only (Figure 3).

Furthermore, Figure 4 indicates that the recorded current

is reproducible at almost constant valuesindicating that

CdS particles maintained their stability against photo-

deteriorations. It can be noticed also from Figure 5, that

the steady state current (point a) for the reference (10

mmole of [Fe(CN)6]4– at pH 6) has very small plateau (ca

120 s) and is followed by a gradual decrease in the re-

corded current under dark condition (point b) while, in

the presence of CdS (Figure 4), the plateau of the peak

current is large (ca 1000 s), followed by a sharp drop

Photoelectrochemical Production of Hydrogen in Aqueous Suspensions Nanoparticles Composites of CdS/ZnS1635

under dark conditions. Such a large steady state current

plateau is an enhancing factor in photolysis processes in

suspension systems. The slope of line ab in Figure 5 is

smaller than the slope of line bc. This can be attributed to

the slow diffusion of [Fe(CN)6]3– to the collector elec-

trode.

The drop in the reduction current for the reference

system even under illumination can be explained by the

fact that at steady state peak current, the high concentra-

tion of [Fe(CN)6]4– captivated by the cylindrical mesh of

the Pt electrode creates a local concentration overvoltage.

This overvoltage reduces the diffusion of [Fe(CN)6]3–

formed in the bulk electrolyte. Under dark conditions no

more [Fe(CN)6]3– is formed and the reduction of remain-

ing [Fe(CN)6]3– is irreversible. This causes the greater

drop along the line bc. Similar reasoning can be used to

explain the observed results obtained for CdS/ZnS mix-

tures displayed in Figure 5. The larger plateau observed

with this suspension than with the reference sample in-

dicates that the suspension particles adsorb the reduction

product, eliminating the concentration overvoltage. This

allows a steady amount of [Fe(CN)6]3– to reach the col-

lector electrode. Figure 5 also indicates that as the ZnS

percentage increases, the photo-reduction current (Equa-

tion (5)) decreases. When pure ZnS nanoparticles were

used the measured photo-reduction current was the least

as shown in Figure 6.

3.4. Photolysis of Aqueous Solutions of

[Fe( CN)6]4– in Basic Media

Working at pH 9 using platinum as collector electrode

requires a potential of less than 0.4 V vs NHE, or 0.2 V

vs Ag/AgCl. The kinetics of reduction of [Fe(CN)6]3– are

slow at pH 9 as indicated by Figure 3. The reduction

currents reported for [Fe(CN)6]3– in the presence of the

Figure 6. Effect of CdS percentage on the photocurrent

during the photolysis of [Fe(CN)6]4– in presence of CdS/ZnS

nanoparticle suspensions.

studied suspensions at pH 9 were greater than those re-

ported in absence of these suspensions indicating that

more [Fe(CN)6]3– is produced in the presence of CdS/ZnS

mixtures through oxidation of [Fe(CN)6]4–.

Such results can be explained on the basis that the CdS

/ZnS particles acted as a n-type semiconductor at pH 9,

and the following reaction took place

 

 

2FeCN2

2FeCN C.B

eHatthe

















(8)

(The dot●, in equation 8, represent the nanoparticles of

CdS or CdS/ZnS). In reaction 8 formation of [Fe(CN)6]3–

is due to the action of both the direct light oxidation of

[Fe(CN)6]4– and the oxidation of n-type nanoparticles.

Because we focused our studies on homogeneous pro-

duction of hydrogen, the amount of hydrogen formed at

the C.B was not measured and therefore was not included

in the obtained results.

Because the Pt electrode was potentiosated at 0.00 V

vs Ag/AgCl (0.2 V vs NHE), the collector electrode main-

tained it immunity against corrosion [31,32]. Therefore

the increase of the reduction current is due to a photoly-

sis process rather than a catalytic process that can take

place if PtO2 existed [32]. This also would reflect the

dual nature of nanoparticle suspensions. Upon illumina-

tion, reactions 1 and 3 take place simultaneously. Ac-

cording to reaction 3, at pH 9 [Fe(CN)6]4– acted as hole

scavenger and oxidized to [Fe(CN)6]3–. This increased

the amount of [Fe(CN)6]3– reduced electrochemically.

3.5. Hydrogen Production in the Photolysis

Process

The results displayed in Figures 3, 4, and 5 indicate that

[Fe(CN)6]4– is an excellent supplier of hydrated electrons

that produce hydrogen according to reaction 3. The con-

tinuous presence of [Fe(CN)6]4– will insure the genera-

tion of hydrated electrons and, consequently, hydrogen.

Rates of hydrogen production via regeneration of

[Fe(CN)6]4– by photochemical reduction of [Fe(CN)6]3–

at semiconductor surfaces are listed in Table 1. The data

listed in this table indicate that pure CdS generated hy-

drogen at the greatest rate among the studied mixtures.

As the percentage of ZnS increases, the rate of hydrogen

production decreases. On the other hand, rates of hydro-

gen production via regeneration of [Fe(CN)6]4– by elec-

trochemical reduction of [Fe(CN)6]3– at a Pt electrode are

displayed in Figure 7. Contrary to the results listed in

Table 1, Figure 6 indicates that, as the percentage of

ZnS in the mixture increases, the hydrogen production

rate increases. This observation is consistent with fact that

Fe(CN) 6]3– is reduced by photolysis and electro chemical

Photoelectrochemical Production of Hydrogen in Aqueous Suspensions Nanoparticles Composites of CdS/ZnS

1636

Table 1. Hydrogen production rate during the photolysis

process of [Fe(CN)6]4– in the aqueous suspensions of the

studied nanoparticles.

Semiconductor compostion

(mass percent)

Steady state

H2 production, mole·h–1·m–2

CdS 100% 0.0676

CdS/ZnS 90:10 0.0546

CdS/ZnS 70:30 0.0341

CdS/ZnS 50:50 0.0316

Figure 7. H2 prodiced via electrochemical redcution of

[Fe(CN)6]4– in phospahe buffer at pH 6, in presence of (a)

CdS; (b) CdS/ZnS (9:1); (c) CdS/ZnS (7:3); (d) CdS/nS

(5:5).

processes. Electrochemical reduction occurs very readily

in presence of high percentages of ZnS. Reduction in the

presence of substances with no or low concentrations of

ZnS occur primarily through a photochemical process.

Because the amount of [Fe(CN)6]3– generated in reaction

3 is constant, when the product of one process increases,

the product of the other process decreases.

4. Conclusions

The low band gap of CdS (2.3 eV) generated the greatest

photochemical production of hydrogen compared with its

mixtures with ZnS. This indicates that the addition of

ZnS widened the band gap of the CdS/ZnS alloy, result-

ing in less absorption of the solar radiation. This widen-

ing of the band gap took place by a downward shift of

the valence band, and asmall upward shift in the conduc-

tion band of the CdS/ZnS alloy.Studies show that the

direct band gap of mixed CdS/ZnS materials varied mo-

notonically with the percent of CdS in the mixture. Pho-

tonic transitions that cause the photolysis process oc-

curred mainly by the direct band transition [33]. Maxi-

mum photochemical response during the photolysis of

[Fe(CN)6]4– was reported at pH 6. The amount of

[Fe(CN)6]3– reduced via the photochemical process de-

creased by increasing ZnS content in the mixture (Figure

6). The reproducibility of thephotocatalytic activities of

CdS/ZnS nanoparticles is an indication that these systems

sustained their stability under illumination conditions.

The problems related to heterogeneous production of hy-

drogen were eliminated, as hydrogen was generated via

homogenous photoreaction in these heterogeneous sus-

pensions. This process can be scaled up for large scale

hydrogen production. Using such a method in industry

makes the production process of hydrogen totally green.

This is because the homogenous production of hydrogen

by photolysis and heterogonous reduction of [Fe(CN)6]3–

to generate [Fe(CN)6]4– can take place by solar energy

powered potentiosate.

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