MARí Bernabé SINGH Krishan-Chander MOLLAR Miguel MOYA Mónica RANA Ravi

(1Departament de Física Aplicada-IDF,Universitat Politècnica de València,Camí de Vera s/n,46022,València,Spain; 2Department of Chemistry,Maharshi Dayanand University,Rohtak 124001,Haryana,India)

Growth Mechanism and Morphology of ZnO/eosin-Y Hybrid Thin Films

MARí Bernabé1,*SINGH Krishan-Chander2MOLLAR Miguel1MOYA Mónica1RANA Ravi2

(1Departament de Física Aplicada-IDF,Universitat Politècnica de València,Camí de Vera s/n,46022,València,Spain;2Department of Chemistry,Maharshi Dayanand University,Rohtak 124001,Haryana,India)

Thin hybrid films of ZnO/eosin-Y were prepared by electrodeposition at-0.8 and-0.9 V in aqueous and non-aqueous baths at temperatures ranging from 40 to 90°C with dye concentrations of 100 and 400μmol·L-1.The films were characterized by X-ray diffraction(XRD),scanning electron microscopy (SEM),energy-dispersive X-ray analysis(EDX),and absorption spectroscopy.The films prepared in a non-aqueous bath were non-porous and did not adsorb dye molecules on their surface.However,the films grown in aqueous media were porous in nature and adsorbed dye during the deposition of ZnO. Preferential growth of the film along the(002)face was observed,and the highest crystallinity was achieved when the film was deposited at 60°C.The maximum absorption was achieved for the films grown at 60 to 70°C,a deposition potential of-0.9 V,and a dye concentration of 100μmol·L-1.

Electrodeposition;ZnO/eosin-Y;Thin crystalline hybrid film;Hybrid film; Porous film;Growth mechanism

1 Introduction

Zinc oxide is an important material which is being used in various electrical,chemical,and optical applications1-4,such as piezoelectricand electro-acoustic transducers,5,6sensors for oxidizing and reducing gases,7ultraviolet(UV)lasers,8-10and transparent conducting electrodes.11Generally,thin films of zinc oxide are prepared by various techniques like chemical vapour deposition,12vacuum deposition,sputtering,sol-gel method,13spray pyrolysis,14and electrochemical deposition.15The gas phase deposition methods,as the first three methods quoted above give single phased high purity crystalline films.But these methods are not cost effective due to costly instruments and high power consumption.The last three preparation methods form polycrystalline porous films.The extremely high porosity of ZnO films leads toa comparatively large internal surface area and it is well known that larger surface area makes the material more sensitive and efficient in applications like catalysis,gas sensors,and photovoltaic cells.In gas sensors,the reactive molecules interact with the surface states of the sensing material and a charge transfer changes the conductivity of the bulk material.Recently hybrid films of zinc oxide with an organic dye prepared by electrochemical deposition method are found to have high porosity if the dye molecules are removed from the film by some adequate technique.The re-adsorption of dye on the ZnO film is thought as an alternative material fordye sensitized solar cells(DSSCs).It has been reported that ZnO performs particularly well when sensitized with organic dyes such as eosin-Y,16-19mercurochrome,20,21or rose bengal.22The performance of these cells depends on the structure and porosity of film and the nature of binding of dye molecules on ZnO surface.Originally the DSSCs were developed by Gr?tzel et al.23-25with porous TiO2nanocrystals deposited on a transparent conductive electrode.A DSSC consists of a dye modified photo-anode,an electrolyte solution,and a counter electrode in a sandwich configuration using a polymer separator and sealant.When a photo anode is illuminated with sunlight,the dye absorbs the light and becomes excited which injects an electron to the conduction band of the semiconductor. Simultaneously,the oxidized dye is reduced by the electron donor in electrolyte and returns to ground state.Electrons in conduction band of semiconductor are collected at the counter electrode and flow through the external circuit.The performance of ZnO/dye sensitized solar cells is still far below the TiO2/dye cell.But a lot of activity has been witnessed in recent years to enhance the performance of ZnO/dye solar cells by varying electrodeposition conditions like current density,tem-perature,and nature of dye and electrolyte and their concentrations,etc.

In the present work ZnO/eosin-Y hybrid thin films in aqueous and non-aqueous media at various temperatures and voltages were electrodeposited on indium tin oxide(ITO)substrates. The eosin-Y was added in different amounts in the electrolytic bath composed of ZnCl2and KCl for aqueous medium and ZnClO4and KClO4were dissolved in dimethyl sulphoxide (DMSO)for non-aqueous medium.The structures of films were characterized by XRD and the morphology was examined by SEM.EDX analysis was used to determine the exact composition of the films.The loading of dye in the film was estimated from the absorption spectra.

2 Experimental

High purity p.a.ACS reagent ZnCl2,ZnClO4,KCl,KClO4, DMSO,and eosin-Y dye from Aldrich chemicals were taken as starting materials.The experimental setup used to prepare ZnO/ eosin-Y thin films of various thicknesses by cathodic electrodeposition technique consisted of a computer-controlled potentiostat/galvanostat(EcochimiePGSTAT302)and aclassical three-electrode electrochemical cell filled with a solution containing 0.1 mol·L-1KClO4as supporting electrolyte and dissolved oxygen in DMSO solution,and 5.0×10-3mol·L-1ZnClO4was used as precursor for the non-aqueous bath.For aqueous bath 5.0×10-3mol·L-1ZnCl2and 0.1 mol·L-1KCl were dissolved in oxygen-containing water.A glass coated with ITO substrate with sheet resistance of 10 Ω·□-1,previously cleaned in an ultrasonic acetone bath for 15 min and then rinsed in distilled water and dried,was the working electrode.Pt and Ag/ AgCl electrodes were used as counter electrode and reference electrode,respectively.The deposition potential was fixed at-0.8 or-0.9 V and the solution temperature was varied from 40 to 90°C by a thermostat.Transparent thin films with smooth surfaces were obtained when using DMSO in contrast with the nano-columnar structure appearing when water was used as solvent.26After deposition,the films were rinsed with DMSO or distilled water.

The structural characterization of the hybrid films was performed by high-resolution XRD using a Rigaku Ultima IV diffractometer through θ-2θ scans with copper anticathode(Cu Kα,0.154 nm).The morphology of the films was studied by SEM using JEOL-JSM6300 scanning electron microscope operating at 10 kV.The chemical composition was determined by EDX.Optical transmittance measurements were performed by means of a deuterium-halogen lamp Ocean Optics DT-MINI-2-GS in association with an Ocean Optics HR4000 spectrometer optimized for the UV-Vis(visible)range.

3 Results and discussion

3.1 Crystallinity

XRD patterns of ZnO films prepared without and with 100 μmol·L-1dye at-0.8 V of deposition potential at 40,50,60, 70,80,and 90°C for all samples are shown in Fig.1.Pure ZnO films show reflexions corresponding to the wurtzite structure, however,with strong(002)but weak(100)and(101)reflexions(Fig.1(a)).The intensity of(002)peak increases with the rise of deposition temperature and it maximizes at 70°C.This has been reported before for electrodeposited ZnO films.27,28This means that the ZnO crystals grow preferentially with their c-axes perpendicular to the substrate.The reason is an intrinsic anisotropy in the growth rate of ZnO crystals due to differences in the dissolution rates of different ZnO surfaces.29,30The polar Zn-terminated(002)face of ZnO is known to dissolve more slowly than the non-polar surfaces perpendicular to this face.It seems that electrodeposition at the non-polar surface is further enhanced with the rise of temperature up to 60°C.After this the temperature becomes so high that the rate of deposition and dissolution of polar and non-polar surfaces of ZnO become almost equal.Therefore the intensity of the peak corresponding to(002)face becomes comparable with the intensities corresponding to faces(100)or(101).It may be observed in Fig.1 (b)that this crystallographic orientation of the ZnO can be further enhanced in electrodeposited ZnO/eosin-Y films,probably caused by preferential adsorption of eosin-Y on the(002)face, since eosin-Y is known to catalyze the electrodeposition of ZnO.31However,this catalytic effect of dye seems to maximize itself at 60°C and simultaneously it suppress the growth of film along(100)and(101)faces.So no peaks corresponding to the(100)and(101)faces are observed for the hybrid ZnO/ eosin-Y films compared to the“pure ZnO”films(Fig.1(a,b)). A few researchers27,28observed the small growth of film in the presence of dye along the(100)and(101)faces,however,they performed the experiments at room temperature with different ionic strength of the electrolytic bath from that of ours.Almost identical XRD patterns as shown in Fig.1(b)were obtained for the films grown at-0.9 V with 100 and 400μmol·L-1dye at different temperatures.Therefore they are not shown separately.

Fig.1 XRD patterns of(a)pure ZnO films,(b)ZnO/eosin-Y hybrid films electrodeposited in aqueous bath at various temperatures with 100μmol·L-1dye in the bath

3.2 Morphology and dye loading of films

Fig.2(a,d)shows the SEM micrographs of two electrodeposited ZnO samples in water and DMSO,respectively,at-0.9 V and at 90°C((Fig.2(a))and 60°C((Fig.2(d)).Aqueous solvent produces perfectly defined columns in their hexagonal structure mainly oriented along the c-axis(Fig.2(a)).Sample grown from DMSO solution(Fig.2(d))shows smooth surface morphology.This suggests that the DMSO solution favours the deposition of a more continuous film where columns can not be appreciated.In DMSO the formation of stable clusters of critical nuclei during the growth of the film on the ITO substrate takes place,which generates a large number of nucleation centres that form smoother surface layers.27Similar types of smooth films were also obtained by the addition of dye.The experiments of film growth at different temperatures with different concentrations of dye always produced films of smooth surface.Adsorption of dye in the films was always found to be negligible by EDX analysis.The continuity of the films is not suitable for the construction of ZnO/eosin-Y photo electrode for DSSCs.

It has been observed that the eosin-Y molecules were reduced during electrodeposition32and the freshly prepared thin films in our experiments at-0.9 V were colourless and transparent.The films regenerate the red colour as eosin-Y being reoxidized by oxygen in air.In this paper,all the characterizations of the films were done after full reoxidation of eosin-Y. However,the films grown in DMSO at-0.8 and-0.9 V remain colourless and transparent and do not generate colour,further confirming the absence of dye in the films.

In Fig.2(b,c)the morphologies of the films grown at-0.9 V and 85°C with 100 and 400μmol·L-1dye in aqueous bath are presented,respectively.Both films possess a granular and porous character and consist of round-shaped particles.The particle size for the films made in 100μmol·L-1dye is about 500 nm in diameter and the particle size is almost double that in the film grown in 400μmol·L-1dye.These large particles are seemed to be further composed of numerous internal nanoparticles of various sizes.However,the films deposited at-0.8 V and 60°C with 100 and 400μmol·L-1dye exhibit a quite different structure,where the clear edges of particles are absent and the films show a compact and smooth granular morphology with some cracks on the film surface(as shown in Fig.3(a, b)).In Fig.3(c,d)the SEM images of films deposited at-0.8 V and 80°C with 100 and 400μmol·L-1dye are presented.Here again continuous surfaces with granular morphology are obtained.However,the size of the granulars is larger when dye concentration is 400μmol·L-1.

Fig.2 SEM micrographs of(a)pure ZnO films grown at T=90°C,V=-0.9 V in aqueous bath,(b)ZnO/eosin-Y films grown in aqueous bath at T=85°C,V=-0.9 V,with 100μmol·L-1dye,(c)ZnO/eosin-Y films grown in aqueous bath at T=85°C,V=-0.9 V,with 400μmol·L-1dye,(d)ZnO/eosin-Y films grown in non-aqueous medium at T=60°C,V=-0.9 V,with 100μmol·L-1dye

Fig.3 SEM micrographs of ZnO/eosin-Y hybrid films grown in aqueous bath under different conditions(a)T=60°C,V=-0.8 V,100μmol·L-1dye;(b)T=60°C,V=-0.8 V,400μmol·L-1dye; (c)T=80°C,V=-0.8 V,100μmol·L-1dye;(d)T=80°C,V=-0.8 V,400μmol·L-1dye

The molar ratio of eosin-Y/ZnO in the films prepared at different temperatures and at-0.8 and-0.9 V deposition potential with 100 and 400μmol·L-1dye in the electrolytic bath are presented in Fig.4.The same amount of charge of one column was passed for the electrodeposition of each film.This ratio is derived from the atomic ratio of Br/Zn in the film which was determined from the EDX analysis.As there are four atoms of Br in each molecule of eosin-Y,therefore the atomic ratios of Br/Zn determined from EDX were divided by four to get the molar ratio of eosin-Y/ZnO.It may be observed from the Fig.4 that the ratio in each case increases almost linearly with the increase of deposition temperature of hybrid film.The loading of dye at-0.8 V with 400μmol·L-1dye in electrolytic bath is almost equal to that of films deposited at-0.9 V with 100μmol· L-1dye at each temperature.The slope of line corresponding to-0.9 V with 400μmol·L-1dye is almost double that of other lines,showing that rate of loading is much higher in the films prepared under these conditions.

Fig.4 Molar ratio of eosin-Y/ZnO in the hybrid films prepared at different conditions(a)deposited at V=-0.9 V with 100μmol·L-1eosin-Y,(b)deposited at V=-0.8 V with 100μmol·L-1eosin-Y,(c)deposited at V=-0.9 V with 400μmol·L-1eosin-Y,(d)deposited at V=-0.8 V with 400μmol·L-1eosin-Y

It has been observed that the simply higher loading of dye in the porous ZnO film does not make it suitable for DSSCs.The ideal situation for the dye to be the most efficient is that it should form a unimolecular layer on the porous surface of ZnO film and the surface should be completely covered with dye molecules.When these films are exposed to sunlight each of the dye molecules should get excited to shift an electron to the conduction band of ZnO.This could easily be tested either by extracting the dye by KOH solution(if extraction is 100%,this means that all dye molecules are adsorbed on the surface of ZnO.These are not sandwiched between the ZnO layers)or by noting the intensity of the absorption spectra of dye in the film (high intensity of absorption means that the dye molecules are adsorbed on the surface and not inside the films).

3.3 Optical properties

Optical properties of the deposited films were studied by UV-Vis absorption spectra.Fig.5(a)shows the typical spectrum of ZnO/eosin-Y hybrid film prepared at 70°C,V=-0.8 V with 400μmol·L-1eosin-Y dye.Fig.5(c,d,e,f)show the absorption spectra of the ZnO/eosin-Y hybrid films prepared at various deposition potentials,temperatures,and dye concentrations in comparison with that from an aqueous solution of eosin-Y(Fig.5(b)).It can be seen that eosin-Y molecules are mostly in a monomer form in aqueous solution and a main absorption peak centred at ca 515 nm is observed.However,the absorption spectrum of eosin-Y molecules in the hybrid film shows a splitting into two bands at 494 and 525 nm.This is due to the close packing and a specific interaction of chromophores in the hybrid film and also due to the interaction between the dye molecules and ZnO matrix.32,33

According to the Beer-Lambert law,the amount of eosin-Y loaded in the hybrid films is proportional to the intensity of the absorption bands.But this relation will be true only if all the dye molecules in the film absorb the light.Those molecules that are sandwiched between the layers of ZnO or where the source radiations can not reach the dye molecules directly will not be able to absorb the radiations.Therefore the intensity of absorption spectra will represent the true concentration of dye, which can absorb the radiation and not the total amount of dye in the film.It shows that the intensity of the absorption bands is maximum for the films deposited at-0.9 V with 100μmol· L-1dye at 60°C(Fig.5(d)).The amount of dye loaded in the films prepared at-0.8 V with 400μmol·L-1dye in the bath is almost equal to that of films prepared under conditions of-0.9 V with 100μmol·L-1dye.But the intensity of absorption is less for the former.The loading of dye is much more for the films deposited at-0.9 V with 400μmol·L-1dye.But the absorption intensity is not very high(Fig.5(f)).Therefore,it may be concluded that the films prepared between 60 and 70°C with-0.9 V deposition voltage and with 400μmol·L-1dye are found to absorb maximum radiations.The morphologies of these films exhibit quite a porous structure and XRD peaks show the hexagonal crystallinity with preferential growth along the(002)face.

It is reported that the intermolecular attraction between chromophores leads to the formation of assemblies in an ordered arrangement.34Formation of eosin-Y assemblies(most possibly dimmers in this case)can be obliquely characterized by the appearance of a clear blue-shifted absorption centred at around 494 nm.It is believed that the band centred at about 525 nm can be mainly ascribed to the absorption of monomer eosin-Y. Therefore,the ratio between eosin-Y assemblies and monomer could be roughly estimated by the relative intensity of these two bands.It may be observed that the relative intensity of the band at 490 nm to that at 520 nm increased as the deposition potential shifted negatively,indicating that the interaction of chromophores was stronger and the tendency to form eosin-Y assemblies/dimers was much higher.

3.4 Growth mechanism of hybrid films

Fig.5 (a)Typical transmittance spectrum of ZnO/eosin-Y hybrid film prepared at 70°C,V=-0.9 V,400μmol·L-1dye;(b)absorption spectrum of pure eosin-Y dissolved in water;(c,d,e,f)absorption spectra of ZnO/eosin-Y hybrid films prepared at 60,70,80,and 90°C under various set of conditions:(c)100μmol·L-1dye,V=-0.8 V,(d)100μmol·L-1dye,V=-0.9 V, (e)400μmol·L-1dye,V=-0.8 V,(f)400μmol·L-1dye,V=-0.9 V

By analyzing the morphologies,optical properties,and the amount of eosin-Y presented in the hybrid films comprehensively,we can depict the primary growth mechanism of ZnO/ eosin-Y hybrid films via the electrochemical deposition.When a less negative potential(such as-0.8 V in the case)is applied, the rate of ZnO growth is slow.The hybrid film has a porous structure and continuous structure.The loading of dye in the ZnO film is less and most of the loaded eosin-Y molecules exist as adsorbed on the surface of ZnO crystals,but as the deposition temperature increases,the rate of deposition of ZnO and dye also increases.But the amount of dye adsorbed is still low as compared to the deposition rates under other conditions and some amount of dye molecules get inside the ZnO crystal layers or form self associated species(the extent of association can be judged from the intensity of absorption band at 494 nm in Fig.5)and becomes inaccessible to direct sunlight.Thus,the peaks with lower intensity are obtained in the absorption spectrum at high temperatures of 80 and 70°C(Fig.5(c)).When the dye concentration in the electrolytic bath is increased from 100 to 400μmol·L-1and deposition is carried at-0.8 V,the loading of dye becomes more than the former case and its loading increases with the increase of temperature.At high temperature again low-intensity peaks are obtained in the absorption spectra,showing the unavailability of some of the dye to radiations. If the deposition potential is increased to-0.9 V,and concentration of dye in solution is kept 400μmol·L-1,the rate of deposition,as well as the loading of dye increases in the film.Almost same amount is absorbed when deposition potential was-0.8 V with 400μmol·L-1dye.The reason for the increased loading at-0.9 V is due to the reduction of dye at this potential.The reduced dye forms a strong complex with the Zn2+ions.32Therefore,most of the surface of ZnO will be covered by dye molecules.But again as the temperature of deposition increases the rate of deposition becomes fast and some of the associated dye molecules are trapped in the ZnO layers.We get the low intensity of absorption peak at high temperature 80°C.But the intensity of absorption is maximum at 60 and 70°C.The large concentration of dye(400μmol·L-1)in electrolytic bath and electrodeposition athigh temperature with-0.9 V deposition potential causes deposition of large amount of dye inside as well as outside the ZnO layers(as can be seen from Fig.3(d)the large deposits of dye molecules(dark shades)on the continuous surface of the ZnO film).The most suitable conditions for the growth of porous film covered completely with unimolecular layer of dye seems to be electrodeposition at-0.9 V with 400μmol·L-1dye in electrolytic bath having temperature from 60 to 70°C.This is because the absorption spectrum of dye shows maximum intensity of absorption peaks.These conditions seem to be most suitable for preparing the DSSC electrode.

4 Conclusions

Thin hybrid porous films of ZnO/eosin-Y can be prepared by electrodeposition technique in aqueous bath of composition 5.0×10-3mol·L-1of ZnCl2+0.1 mol·L-1of KCl and 100μmol· L-1or 400μmol·L-1of eosin-Y at various temperatures ranging from 40 to 90°C.The films prepared in non aqueous bath are found to be non porous with continuous surface and these do not adsorb the dye on their surfaces.However,the films grown in the aqueous medium are found to be porous in nature and dye is adsorbed during the deposition of ZnO film.The preferential growth of film along the(002)face is observed and best crystallinity is reached when it is deposited at 60 and 70°C.The absorption of radiation is maximum for the films grown from 60 to 70°C and at-0.9 V deposition potential with 400μmol·L-1dye in the bath.These conditions seem to be most suitable for preparing the DSSC electrode.

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10.3866/PKU.WHXB201228251

August 8,2011;Revised:November 5,2011;Published on Web:November 8,2011.

*Corresponding author.Email:bmari@fis.upv.es.Tel:+34-963-877525.

The project was supported by the Spanish Government through MCINN Grant(MAT2009-14625-C03-03)and MEC Financial Fund (SAB2010-0019)for Singh,K.C.