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.
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.
E
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:


 
21
2
1423
12
Hratemolemh
mole10cm3.610 s
2 96500molecm
ic
ca




 

(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]:

2
gd
E
EE

  (5)

 




 
22
1
EkT
pgi pgi
EkT EkT
EEEEEE e
ee




 

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
 
 
4
6
3
6
2FeCN2
2FeCN C.B
aq
eh
eHatthe


 

 

hv

(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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