Theoretical and Experimental Chemistry, Vol. 49, No. 3, July, 2013 (Russian Original Vol. 49, No. 3, May-June, 2013)

EFFECT OF THE METHOD OF PREPARATION OF ZnO/CdS AND

TiO2/CdS FILM NANOHETEROSTRUCTURES ON THEIR

PHOTOELECTROCHEMICAL PROPERTIES

UDC 544.526.2,544.174A. V. Kozytskiy,1

A. L. Stroyuk,1

S. Ya. Kuchmy,1

E. A. Streltsov,2

N. A. Skorik,3

and V. O. Moskalyuk3

A study was carried out on the photoelectric properties of ITO/TiO2/CdS and ITO/ZnO/CdS nanostructurized

films obtained by the SILAR method and the photocatalytic reduction of sulfur by ethanol in the presence of

Cd(II) salts. The nanostructures obtained by the latter method display much higher photocurrent density (by a

factor of 2 in the case of systems derived from ZnO and a factor of 5 in systems derived from TiO2) upon

irradiation with white light (� > 400 nm) in aqueous solutions of sodium sulfide than the heterostructures

obtained by the traditional SILAR method.

Key words: photoelectrochemistry, sensitization, heterostructures, photochemical precipitation, titanium

dioxide, zinc oxide, cadmium sulfide.

In recent decades, photoelectrochemical systems for the conversion of solar energy featuring nanocrystalline wide

band-gap semiconductors such as TiO2

and ZnO and organic dyes or metal complexes known as third-generation solar cells

[1-4] serve as an alternative to traditional silicon photovoltaic systems due to their low cost, possible flexible tuning of the

spectral and electronic properties of the cell components as well as rather high light conversion efficiency reaching 11-12% [2,

4, 5]. On the other hand, the output of solar cells using dyes is limited by their tendency to undergo destructive photochemical

side-reactions and their relatively narrow absorption band width, which requires the use of a set of sensitizers with different

light sensitivity ranges [2, 3, 5]. Hence, in recent years, there has been increasing interest in inorganic photoelectrochemical

systems, in which nanoparticles (NP) of narrow band-gap semiconductors, as a rule, cadmium chalcogenides CdX and, less

frequently, PbX (X = S, Se, Te) [1-3, 5, 6], serve as the wide band-gap photoanode sensitizer. Among the advantages of such

systems are the relatively high photostability of the metal chalcogenide NP under conditions of closed photoelectrochemical

systems, broad absorption bands permitting the generation of charge carriers by the action of visible light, and, in the case of

PbX NP, multiexciton generation [3, 5] such that the charge carrier generation efficiency may exceed 100%.

The most common approaches to the deposition of CdX NP on oxide photoanodes involve binding of prepared NP to

the TiO2

or ZnO surface using bridging molecules, precipitation from chemical baths, and successive ionic layer adsorption

and reaction of Cd(II) and X(II) ions (SILAR) [1, 4-6]. Recent work has shown that the precipitation of cadmium or lead

0040-5760/13/4903-0165 ©2013 Springer Science+Business Media New York 165

________

1L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, Prospekt Nauky, 31, Kyiv

03028, Ukraine. E-mail: stroyuk@inphyschem-nas.kiev.ua.

2Belorussian State University, Ul. Leningradskaya, 14, Minsk 220030, Republic of Belarus. E-mail: StreltEA@bsu.by.

3NanoMedTech, Vul. Gor’kogo, 68, Kyiv 03150, Ukraine. E-mail: skoryk@nanomedtech.kiev.ua.

___________________________________________________________________________________________________

Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 49, No. 3, pp. 153-158, May-June, 2013.

Original article submitted May 28, 2013.

chalcogenides in the photocatalytic reduction of various chalcogen sources such as S8, Se, Se(IV) with TiO

2[6-12] and ZnO

[10, 13] may be an alternative to the now traditional methods for the formation of titanium and zinc oxide nanocomposites with

cadmium and lead chalcogenides. On the other hand, there have been only a few reports of the use of such an approach for

obtaining visible-light-sensitive photoelectrochemical systems derived from TiO2

[6, 8, 9] and no such reports for systems

derived from ZnO. Hence, we undertook an attempt to obtain TiO2/CdS and ZnO/CdS film nanoheterostructures by the

photochemical precipitation of cadmium sulfide NP as well as by the SILAR method and compared the photoelectrochemical

properties of these nanostructures.

EXPERIMENTAL

In this work, we used HCl, NaOH, KCl, zinc, and sulfur obtained from Khimlaborreaktiv as well as titanium

tetraisopropoxide, gelatin, CdCl2, Cd(ClO

4)2, Zn(NO

3)2, Na

2S, and polyvinylpyrrolidone (M

w= 360,000 g/mol) along with

glass plates with a deposited layer of indium-tin-oxides (ITO) with specific resistance 70 �/cm2

obtained from Sigma Aldrich.

Absolute ethanol was obtained by heating a sample of this alcohol over calcium oxide, followed by distillation taking the

middle fraction.

Mesoporous TiO2

films on an ITO surface formed using 10-15-nm nanocrystals with 80-85% anatase modification and

a trace of rutile were obtained using the sol-gel approach [14]. Zinc oxide films with thickness 2-3 �m were formed on the ITO

surface by electrodeposition in accord with our previous procedure [15]. Electron microscopy showed that the ZnO films

consist of platelet aggregates of individual zinc oxide particles with diameter 20-40 nm. The ZnO microplatelets have porous

structure with mostly end-on orientation to the ITO surface.

Cadmium sulfide was deposited on the ITO/TiO2

and ITO/ZnO film surface using both SILAR and photochemical

deposition (PCD) [11-13]. In the SILAR method, the films were placed into an aqueous solution of 0.01 mol/L CdCl2,

removed, washed with distilled water, again immersed into aqueous 0.01 mol/L Na2S, again removed, and washed. The amount

of CdS was varied by multiple repetition of this procedure. The films were dried in the air at 70 °C. In the PCD method,

8�35-mm ITO/TiO2

and ITO/ZnO films were immersed in an ampule with 4.0 mL ethanol solution containing 5·10–4

mol/L S8

(relative to atomic sulfur) and 0.01 mol/L Cd(ClO4)2. The solution was flushed with argon for 15-20 min. The ampule was

sealed and irradiation with a DRSh-1000 high-pressure mercury lamp with � = 310-390 nm and intensity I = 25 mW/cm2. The

amount of CdS deposited in the focus of the transmitted light flux was varied by changing the irradiation time. After deposition,

the films were washed with ethanol and dried in the air at 70 °C. Then, film segments not containing CdS were insulated from

the electrolyte by a layer of chemically-resistant lacquer.

The photoelectrochemical properties of the ITO/TiO2/CdS films were studied in aqueous solutions containing

0.1 mol/L NaOH and 0.1 mol/L Na2S upon irradiation with white light using a LED lamp with I = 30 mW/cm

2. The

ITO/ZnO/CdS films were irradiated in aqueous solutions of 0.1 mol/L Na2S by a LED lamp with I = 1.5 mW/cm

2(� > 400 nm).

In both cases, we used a three-electrode scheme with a platinum auxiliary electrode and silver chloride reference electrode. The

absorption spectra of the films were taken on a Specord 210 spectrophotometer. The scanning electron microphotographs

(SEM) of the films were obtained on a Tescan Mira 3 microscope with accelerating voltage 10-20 kV. The energy dispersive

X-ray spectroscopy experiments were carried out on an 80-mm2

Oxford X-Max instrument.

RESULTS AND DISCUSSION

The ITO/ZnO and ITO/TiO2

films had absorption bands with a blurred edge at 360-380 nm (Fig. 1a, curves 1 and 2).

The films were transparent in the visible spectral region but a rather strong structureless halo was observed due to light

scattering.

The deposition of CdS NP on the surface of the titanium oxide and zinc oxide films leads to the formation of additional

absorption bands in the visible region (Fig. 1a). The intensity and position of the edge of these bands (�t) depend on the method

and condition of the deposition. Thus, �tof CdS nanoparticles deposited on the ITO/ZnO film surface by the SILAR method is

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shifted from 440 nm (2.81 eV) upon the triple repetition of the successive adsorption of Cd(II) and S(II) ions to 510 nm

(2.43 eV). In the case of 12-fold repetition of the SILAR procedure (Fig. 1a, curve 3 and insert), there is a further shift close to

the value characteristic for bulk crystal cubic cadmium sulfide (518 nm, 2.4 eV [16]).

The light absorption of ITO/ZnO/CdS films in the region of cadmium sulfide absorption band increases linearly with

the number N of SILAR cycles (Fig. 1b, curve 1). Evaluation of the mean diameter d of the deposited CdS NP using the

effective mass approximation [16] and the spectral data for the value of Eg

using the differential form of the Tauc equation [15,

16] (Fig. 1a, insert) showed that an increasing number of SILAR cycles from 3 to 12 leads to an increase in d from 5-6 to

15 nm (Fig. 1b, insert).

The CdS nanoparticles deposited on the ITO/TiO2

film prepared by the three cycles of the SILAR method have

�t= 420 nm (2.95 eV, Fig. 1a, curve 5), which corresponds to d = 3.0 nm. As in the case of the ITO/ZnO system, an increasing

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Fig. 1. a) Absorption spectra of films of ITO/ZnO (1) and

ITO/TiO2

(2) as well as ITO/ZnO/CdS obtained by the

SILAR (3) and photochemical deposition methods (4),

ITO/TiO2/CdS obtained by the SILAR (5) and photochemical

deposittion methods (6) (SILAR cycles N = 12 (ZnO) and

N = 3 (TiO2), PCD time 120 min (ZnO) and 30 min (TiO

2);

insert: curve 3 in coordinates of the differential form of the

Tauc equation [15, 16]; b) dependence of the light absorption

of CdS NP (A) at � = 420 nm on the number N of SILAR

cycles (1) and PCD time (2) (insert: dependence of the mean

diameter of CdS NP d on N).

number of SILAR cycles leads to an increase in the light absorption of the film in the visible spectral range and a simultaneous

drop in Eg

to 2.45 eV (for N = 10) [14].

The photochemical deposition of cadmium sulfide on the film surface leads already in the initial step of the

photoprocess (first 20-40 min irradiation) to the formation of larger CdS NP: 5 nm in the case of ITO/TiO2

(�t= 475 nm, E

g=

2.61 eV, Fig. 1a, curve 6) and 8-10 nm for ITO/ZnO, the absorption edge for which is shifted at 120 min exposure to a value

characteristic for massive CdS (�t= 520 nm, E

g= 2.4 eV, Fig. 1a, curve 4). The light absorption of CdS and, thus, the amount of

this sulfide increase proportionally to the PCD time (in the range studied t < 120 min, Fig. 1b, curve 2). The value of Eg

approaches the value characteristic for bulk crystal CdS, which hinders the evaluation of the mean particle diameter using the

spectral data. The lack of a marked dependence of the mean diameter of the CdS NP on the duration of the photocatalytic

process was observed in our previous work on colloidal ZnO solutions [13]. This behavior is related to favorable conditions in

the ZnO/CdS nanostructures for the photoinduced transfer of a conduction band electron from CdS to ZnO and a valence band

hole from ZnO to CdS so that ZnO/CdS particles apparently become more efficient photocatalysts for the reduction of sulfur by

means of ethanol than ZnO NP. This leads to the predominant deposition of cadmium sulfide on CdS NP in the ZnO/CdS

heterostructure and the rapid growth in size of these heterostructures.

An SEM study of ITO/ZnO/CdS composite films showed that the deposition of cadmium sulfide NP both by the

photochemical and SILAR methods, on the whole, does not alter the structure of the zinc oxide films. Comparison of the

microphotographs of ITO/ZnO films before (Fig. 2a-c) and after deposition of CdS (Fig. 2d-f) indicates some increase in the

size of the crystallites in the microplatelets and increased roughness. Energy dispersive X-ray spectral analysis of the

ITO/ZnO/CdS prepared by the SILAR method showed repetition of the distribution of cadmium and sulfur atoms and, on the

whole, the profile of the ZnO microplatelets observed in the SEM photographs of the film segment studied (Fig. 2g-j). The

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Fig. 2. Scanning electron microphotographs of films of ITO/ZnO (a-c)

and ITO/ZnO/CdS heterostructures obtained by the SILAR method (d,

e, g) and PCD (f). Distribution of atoms of Zn (h), Cd (i), and S (j) on

heterostructures g. Scale: a, d) 5 �m, b, e) 500 nm, c, f) 200 nm.

energy dispersive X-ray spectral data indicate that the ratio of the amounts of cadmium and sulfur in the ITO/ZnO/CdS films is

1 : 1 regardless of the means of their preparation.

In comparing the photoelectrochemical properties of electrodes prepared by different methods of CdS deposition, we

used ITO/TiO2/CdS films, whose spectra are given in Fig. 1a (curves 5, 6) and also ITO/ZnO/CdS films obtained by nine-fold

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Fig. 3. a, b) Chronoamperograms and chronopotentiograms (insert) of

films of ITO/TiO2/CdS (a) and ITO/ZnO/CdS (b) obtained by the

SILAR (1) and PCD methods (2) (3) ITO/ZnO film without sensitizer),

c) dependence of the photocurrent density iph

in the photoexcitation of

ITO/ZnO/CdS films on the time of PCD for cadmium sulfide (1) and

number of SILAR cycles N (2) (insert: iph

/A ratio for ITO/ZnO/CdS

films prepared by the PCD and SILAR methods).

repetition of the SILAR procedure and PCD time 90 min. The latter films are characterized by virtually the same light

absorption at 420 nm, namely, 15-20% (Fig. 1b).

A photopotential and photocurrent are observed upon irradiation of ITO/TiO2/CdS and ITO/ZnO/CdS films immersed

in aqueous 0.1 mol/L aqueous sodium sulfide at � > 400 nm. The photopotential �Eph

in the ITO/TiO2/CdS system (Fig. 3a,

insert) is approximately the same for films obtained by the SILAR (460 mV) and PCD methods (480 ± 5 mV). In the case of the

heterostructures with zinc oxide, �Eph

is 380 ± 5 and 480 ± 5 mV for films obtained by the SILAR and PCD methods,

respectively (Fig. 3b, insert).The lower photopotential in the case of ITO/ZnO/CdS films obtained by the SILAR method may

be attributed to partial sulfidization of the surface layer of the ZnO microplatelets upon the deposition of CdS to give CdxZn

1–xS

transition layers, which affect the structure of the ZnO-CdS phase boundary.

Figure 3a,b shows that sensitization of the TiO2

and ZnO films by the photochemical deposition of CdS NP permits

much more efficient conversion of light energy into electricity than when employing the SILAR method. Thus, the

photocurrent density upon photoexcitation of ITO/TiO2/CdS films prepared by the PCD method is five times greater than for

the same heterostructures obtained by the SILAR method (Fig. 3a, see sectors 1 and 2) even after taking account of the

significantly different light absorption of the films in the region � > 400 nm. A similar tendency is observed in the zinc oxide

systems: photoexcitation of ITO/ZnO/CdS films prepared by photodeposition of CdS NP gives twice the photocurrent density

than for systems obtained by the SILAR method (Fig. 3b, see sectors 1 and 2).

There is no photocurrent upon the photoexcitation of ITO/ZnO and ITO/TiO2

films lacking sensitizers (Fig. 3b,

curve 3), while �Eph

does not exceed 30-40 mV (Fig. 3b, insert, curve 3).

Hence, our results indicate that in ITO/TiO2/CdS and ITO/ZnO/CdS nanostructures formed by PCD, in which phase

transfer of electrons (holes) from the oxide photocatalyst nanocrystals to participants of the photoreaction and spatial

separation of the photogenerated electrons and holes between the components of such heterostructures play an important role.

After completion of PCD, there is retention of the much more favorable conditions for electron phototransfer in the direction

CdS � oxide � ITO and photocurrent generation than in analogous structures obtained by the SILAR method, i.e., without the

action of light. This conclusion is in accord with our previous work [12], in which laser pulse photolysis was used to show that

separation of the electrons and holes between the structure components appearing upon irradiation in the

photochemically-generated TiO2/CdS heterostructures and the formation of the initial products of this process (Ti(III) inTiO

2

NP and S��

in CdS NP) proceed more efficiently by an order of magnitude, while the reverse processes involving the

recombination of these intermediates proceed much more slowly than in the analogous composites prepared by deposition in

the dark from solutions containing the ammonia complex of Cd(II) and thiourea.

The photocurrent density iph

generated upon the irradiation of ITO/ZnO/CdS films increases directly proportional to

the amount of CdS in their composition, which, in turn, is a function of the duration of PCD (Fig. 3c, curve 1) or the number of

repetitions of the SILAR procedure (curve 2). An increase in the photocurrent density proportional to the amount of deposited

CdS was observed in our previous work for the ITO/TiO2/CdS system [14]. On the other hand, the ratio of the photocurrent

density iph

to the film light absorption A (Fig. 3c, insert), which characterizes the efficiency of the conversion of light energy

into electricity remains virtually constant for ITO/ZnO/CdS films prepared by the SILAR method with N = 3-12 and for

samples obtained by PCD with t = 20-90 min. Since an increase in the number of cadmium sulfide deposition cycles is

accompanied by a significant growth in the mean diameter of the CdS NP, we may conclude that, as in ITO/TiO2/CdS films

studied in our previous work [14], change in the size of the CdS particles has no significant effect on the value of iph

.

Thus, we have studied the photoelectrochemical properties of nanostructurized ITO/TiO2/CdS and ITO/ZnO/CdS

films obtained using two methods for formation of cadmium sulfide nanoparticles on the surface of titanium oxide and zinc

oxide, namely, the traditional method of successive adsorption and reaction of ions (SILAR) and photoinduced deposition due

to the photochemical reduction of sulfur by ethanol in the presence of cadmium salts. The nanostructures obtained by the

photochemical reduction method were found to have higher photocurrent density (by a factor of 2 for ZnO systems and by a

factor of 5 for TiO2

systems) upon irradiation with white light (� > 400 nm) in aqueous solutions of sodium sulfide than for

analogous films formed by the SILAR method. The photocurrent generation efficiency was found to be a function of the

amount of deposited cadmium sulfide and is virtually independent of the size of the CdS particles.

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This work was carried out with the support of the State Basic Research Fund of Ukraine (Projects F41.2/005 and

F54.3/007), Ukrainian President’s Grant to Science Doctors (Project F47/20), and the Belorussian Basic Research Fund

(Projects Kh11K-018 and Kh13K-023).

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