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Polyhedron 19 (2000) 2337–2344

Preparation and properties of Zn�Cd films from non-aqueouscolloids

Galo Cardenas a,*, Rodrigo Segura a, Juan Morales b, Hugo Soto b, Carlos A. Lima b

a Departamento de Polımeros, Facultad de Ciencias Quımicas, Uni6ersidad de Concepcion, Casilla 160-C, Concepcion, Chileb Departamento de Fısica, Facultad de Ciencias Fısicas y Matematicas, Uni6ersidad de Concepcion, Casilla 160-C, Concepcion, Chile

Received 1 October 1999; accepted 10 July 2000

Abstract

Bimetallic films of Zn�Cd have been prepared from colloids using several organic solvents: acetone, 2-butanone, THF,2-propanol and 2-methoxyethanol. The colloids were prepared by simultaneous codeposition of metals and solvents at 77 K. Thebimetallic films and solids were obtained by evaporation under vacuum at room temperature (r.t.) A fully chemical characteriza-tion of bimetallic films and solids was carried out by different techniques such as elemental analysis, FT-IR spectroscopy,conductivity and thermal analysis. In the IR spectra for all solvents, except for ketones, the common absorption bands appearindicating no alteration upon incorporation of metal particles. The carboxylic band disappearance in ketones may be due to somekind of interaction with metallic particle surfaces. Thermograms showed that bimetallic solids present a decompositiontemperature over 400°C. The electric conductivity studies indicated that our bimetallic solids are semiconductors. © 2000 ElsevierScience B.V. All rights reserved.

Keywords: Colloids; Bimetallic films; FT-IR; Conductivity; Thermal analysis

1. Introduction

Nanoclusters with semiconductor behavior have beenstudied extensively [1–10]. The quantum size effectshifts the band gap of many semiconductors into thevisible range and allows the possibility of nanoclusterapplications in light-emitting diodes. These practicalapplications of semiconductor nanoclusters have beenhindered by surface problems due to non-radiative re-combination, lowering the quantum yield of emission.

Another method of increasing the quantum yield ofemission was reported more recently [11]. It involvesthe addition of an impurity to a quantum dot, produc-ing a doped nanocrystalline material. Wang [12] re-ported a cluster of mixed semiconductors ZnxMn1−xSbeing a magnetic semiconductor quantum dot. Man-ganese doped ZnS clusters have been studied by Bhar-gava [11,13,14] and Gallagher [15]. Sizes ranging from35 to 75 A, were obtained and have shown photolu-

minescence quantum yields around 18%. Cohen [16]has synthesized semiconductor nanoclusters with con-trolled size and narrow size distributions using blockcopolymer films prepared by ring-opening methatesispolymerization. The presence of a pendant whole trans-port group [17] and an electron-transport group canprovide, with an electrical access, to nanoclusters fordevice applications. Photoluminescence emission spec-trum for a polymer film containing Mn-doped ZnSnanoclusters was reported [18]. The intensity of emis-sion at 586 nm (Mn emission) was obtained as afunction of the excitation wavelength. When the inci-dent radiation decreases below 330 nm, ZnS nanoclus-ters begin to absorb light, and an increase in themanganese intensity is observed. The excitation of ZnSaround 330 nm results in the emission at 586 nm. Thisproves an energy transfer from ZnS to Mn, whichindicates that Mn is doped in the ZnS cluster.

Furthermore, CdS and Zn�Cd sulfide are useful asphotoconductors of visible and infrared radiation [19].These semiconductors are important to make moreavailable photochemical and photovoltaic cells. Themost promising photocell designs use their films of

* Corresponding author. Tel.: +56-41-204-256; fax: +56-41-245-974.

E-mail address: gcardena@udec.cl (G. Cardenas).

0277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S 0277 -5387 (00 )00569 -6

G. Cardenas et al. / Polyhedron 19 (2000) 2337–23442338

photosensitizers interfaced with CdS in which the crys-tal lattice parameters of the two films are molded[20–22].

Herein, we report the results of the synthesis ofzinc–cadmium films prepared from the evaporation ofthe solvents from colloidal dispersions prepared bychemical liquid deposition at 77 K. Films containingresidual solvent of acetone, 2-butanone, THF, 2-propanol and 2-methoxyethanol are reported.

2. Experimental

2.1. Preparation of films

The films were prepared by slowly dripping the corre-sponding colloidal dispersions onto a substrate. To

Fig. 2. The micrograph and histogram of Cd�acetone colloids. Mag-nification=100 K. Mean size: m=113, s=10.66 A, .

Fig. 1. The electron micrograph and histogram of Zn�acetone col-loids. Magnification=100 K. Mean size: m=216, s=27.70 A, .

increase the speed of solvent evaporation a vacuum of10−3 Torr was used over 4 h.

2.2. Thermogra6imetric analysis

A Perkin–Elmer model TGA-7 thermogravimetricsystem with a microprocessor-driven temperature con-trol unit and a TA data station, was used. The mass ofthe samples was generally in the range of 2–3 mg. Thesample pan was placed in the balance system equipmentand the temperature was raised from 25 to 550°C at aheating rate of 10°C min−1. The weight of the samplepan was recorded continuously as a function of thetemperature.

G. Cardenas et al. / Polyhedron 19 (2000) 2337–2344 2339

Fig. 3. The micrograph and histogram of Zn�Cd�acetone colloids.Magnification=100 K. Mean size: m=83.50, s=10.89 A, .

Fourier transform infrared spectrometer. KBr pelletswere prepared for all the films with 2% film concentra-tion. A total of 128 scans were accumulated for eachspectrum.

2.4. Elemental analysis

The active solids were analyzed in order to determinetheir elemental composition (C, H, O).

The Zn and Cd were determined by atomic absorp-tion using a Perkin–Elmer 3100 model.

2.5. Infrared spectroscopy

The IR spectra were performed using a NicoletMagna 5PC Fourier transform infrared spectrometer.The samples were prepared in a KBr pellet 2% w/w) at15 000 lbs. The sample was carried out between 400and 4000 cm−1 with 128 scans.2.6. Conducti6ity

The electrical conductivity measurements were car-ried out using 120 mg sample and making a pellet at16 600 psi.

The electrical resistance was measured using a RCLFluke PM 6304 bridge and from the area and samplethickness the resistance was transformed to electricalresistivity, independent parameter of the sample size[23].

3. Results and discussion

The films and solids were prepared by solvent evapo-ration from the colloidal dispersions, using a flaskconnected to the vacuum line. After solvent evapora-tion the flask with the active solid is filled up withnitrogen gas to avoid the metal oxidation. The finepowder is later on used for complete characterization.

2.3. Infrared spectra

IR spectra were obtained using a Nicolet Magna 5PC

G. Cardenas et al. / Polyhedron 19 (2000) 2337–23442340

Table 1Elemental analysis of Zn�Cd solids

%Cd (w/w) %C (w/w) %H (w/w) %O (w/w)Solvent %Zn (w/w)

47.1 4.64THF 1.328.6 18.36THF 24.4 39.0 6.08 2.03 28.49

20.12-Methoxyethanol 75.760.9 0.6232.0 0.222-Methoxyethanol 6.26

27.72-Propanol 64.5 5.52 1.07 1.2130.42-Propanol 50.7 2.40 0.88 15.62

68.4 1.7429.0 0.522-Butanol 0.3455.4 1.922-Butanol 0.8532.0 9.8349.4 3.2334.0 0.992-Butanone 12.38

27.92-Butanone 47.8 2.65 0.98 20.6757.2 1.24Acetone 0.5034.4 6.66

0 0.8375.8 0.34Acetone 23.0343.0Acetone 31.4 2.40 1.20 22.00

Acetone 30.0 36.6 1.45 0.66 31.2986.9 0.924.1 0.37Acetone 7.71

0Acetone 87.6 1.35 0.46 10.59

Table 2Elemental analysis: mole percentages

%Cd molesSolvent %C moles%Zn moles %H moles %O moles

11.39THF 10.5011.89 35.05 31.186.91 10.087.43 40.11THF 35.46

12.542-Methoxyethanol 27.4632.01 3.052-Methoxyethanol 12.9028.92 23.1222.12 17.7216.33 40.922-Propanol 2.92

15.682-Propanol 15.21 6.74 29.44 32.9325.582-Butanol 35.09 8.35 29.75 1.23

18.96 6.1518.82 32.442-Butanol 23.6317.422-Butanone 14.73 9.01 32.91 25.9312.792-Butanone 12.74 6.61 29.14 38.72

24.82 5.0325.66 24.19Acetone 20.30Acetone 0.0038.58 2.30 11.22 47.90

7.55 5.4017.76 32.16Acetone 37.1413.05Acetone 9.26 3.43 18.63 55.63

43.89 4.35Acetone 20.843.56 27.3638.77 5.590.00 22.70Acetone 32.93

In order to distinguish the differences between thecolloids and the bimetallic colloids we are measuringthe particle size of the colloid, Cd colloid and Zn�Cdcolloid prepared with acetone. Fig. 1 shows theZn�acetone colloid with particle size, m=216 A, butFig. 2 exhibits Cd�acetone with a particle size of m=113 A, . Fig. 3 shows the particle size of the alloy(m=83.5 A, ) being smaller than the independent metalcolloids.

This is good evidence for the presence of the bimetal-lic clusters.

3.1. Elemental analysis

Table 1 summarizes the w/w percentage of each elementforming the solids, which were obtained by solventevaporation from the colloidal dispersions.

The elemental analysis shows the incorporation be-tween 1–10% w/w of organic materials assuming only theC, H results.

In order to observe a better ration between the elements,the weight percentages were transformed in mole percent-age. Table 2 summarizes the values.

It is observed that in most of the cases the initial molerelationship of the metal is constant (50% Zn, 50% Cd)and for acetone the change in the concentration calculatedinitially is also shown in Table 2.

In general, we can see an excess of oxygen moles withrespect to carbon, e.g. for THF (C4H8O) for each four car-bon moles one mole oxygen should exist, and in this casewe can observe that the oxygen molar is three times greaterthan carbon. This could be due to the presence of Zn andCd oxides on the surface of the particles which could bedue to the high oxidation potential of these metals.

G. Cardenas et al. / Polyhedron 19 (2000) 2337–2344 2341

These solids are very reactive to moisture and canabsorb oxygen and carbon dioxide very rapidly, whichalso produces severe modification in the analysis.

3.2. FT-IR spectroscopy

The FT-IR spectra obtained for Zn�Cd solids showabsorption bands even though the solids exhibit a lowamount of solvent incorporated. The most relevantbands in each spectrum are summarized in Table 3[24].

Most of the solvents were incorporated to the fineparticles without structural changes. The acetoneshows a weak C�O stretching, mainly due to planarcarbonyl interaction with the particle surfaces asshown here:

Following this statement, the carbonyl band shoulddisappear for ketones. Besides, this interaction canexplain the colloid stability for ketones, due to theformation of a bond with the surface, producing asteric barrier.

Figs. 4–6 show the IR spectra of some Zn�Cdsolids.

It is observed in most of the spectrum bands near3400 cm−1 which corresponds to nOH. Most of thefilms get humidity easily during the KBr pellet prepa-ration [24]. Similar IR results were obtained for Cdfilms prepared with the same solvents [25].

Also, a band at 450 cm−1 has been found in all thebimetallic solids. This absorption is due to the zincand cadmium oxides [26] on the surface of the films.The C�H stretching has disappeared in the Zn�acetonesolid films after the oxidation process.

3.3. Thermogra6imetric analysis

The decomposition temperatures of the Zn�Cdsolids are summarized in Table 4.

It was observed that most of the solids show onedecomposition stage over 400°C. This fact allows us toconfirm the presence of a very stable bimetallic alloy.

The solids that exhibit two decomposition stages arethose in which microanalysis indicates a greateramount of organic material. As an example, THFfilms exhibit carbon percentages, 4.6 and 6.1, but 2-

Table 3IR bands of Zn�Cd solids

Solvent n (cm−1) Assignment Observation

THF 3392.66 (m) symmetric O�H, humiditystretchingasymmetric C�H2961.17 (w)stretching

1577.27 (s) H�O�H,scissoringhumidity

1395.01 (m) C�Otorsion1350.73 (m) torsion C�H

C�Oasymmetric1090.87 (m)stretchingrocking811.05 (w) C�H

O�H, humiditysymmetric2-Butanol 3417.0 (m)stretching

C�Hasymmetric2915.93 (w)stretching

2852.38 (w) symmetric C�Hstretchingscissoring1565.80 (w) H�O�H,

humidity1393.37 (m) torsion C�H

C�O1084.76 (s) asymmetricstretching

1022.03 (s) symmetric C�Cstretching

799.23 (s) rocking C�H

Acetone O�H, humidity3436.02 (w) symmetricstretching

2961.79 (m) asymmetric C�Hstretching

1750.00 (w) symmetric C�Ostretching

1550.00 (w) scissoring H�O�H,humidity

1406.10 (m) torsion C�HC�O1089.96 (s) asymmetric

stretchingC�C1023.24 (s) symmetric

stretchingrocking C�H800.83 (s)

Table 4Decomposition temperatures of Zn�Cd solids

Solvent TD (°C)

THF 276.43/502.60 a

230.70/461.70 aTHF2-Methoxyethanol 441.63/523.76 a

507.90 a2-Methoxyethanol294.86/505.332-Propanol432.532-Propanol486.902-Butanol493.432-Butanol442.562-Butanone

2-Butanone 435.00449.10Acetone390.00Acetone

Acetone 517.23413.63Acetone513.03Acetone482.00Acetone

a The thermogram exhibits two decomposition temperatures.

G. Cardenas et al. / Polyhedron 19 (2000) 2337–23442342

Fig. 4. FT-IR spectrum of (Zn�Cd)-2-butanol films in KBr pellet.

Fig. 5. FT-IR spectrum of (Zn�Cd)�acetone films in KBr pellet.

G. Cardenas et al. / Polyhedron 19 (2000) 2337–2344 2343

Fig. 6. FT-IR spectrum of (Zn�Cd)�THF films in KBr pellet.

propanol is 5.5% w/w. On the other hand, the othersolvents only contain between 0.6 and 3.2% w/w.

This phenomenon can be explained if we considerthat most of the organic solvents are physically ad-sorbed and not through a bond with the metal orbimetal particles. Another possibility is that solventsare occluded in the bimetal cluster particles. The elec-tron micrograph shows spherical particles. Under theseconsiderations we can conclude that the free solventsdecompose first before the other bonded with theZn�Cd films.

There are no tendencies in the decomposition temper-atures as a function of concentration, solvent and metalpercentage. For instance, the decomposition of thesystem is independent of the particle size or solventstructure, but is related to the amount and incorpora-tion mechanism of the organic solvents.

It is interesting to observe that Zn�acetone [26] andCd�acetone [25] films shown a TD at 390 and 482°C,respectively. However, the (Zn�Cd) acetone films exhib-ited a TD at 517 and 513°C, being different from theseparate metals.

From the decomposition curves we can see that inmost of the cases the solids are very stable without lossof weight until complete solid decomposition. Thisbehavior is an advantage because we can have a greatrange of working temperatures important for futureapplications.

It is interesting to notice the 2-methoxyethanol filmsin which (TD=507°C), 2-propanol (TD=505°C), 2-bu-tanol (TD=493°C) and acetone (TD=482°C) whichare higher than the Zn melting point are showing thepresence of the alloy. For THF a TD higher than 460°Cwas also observed.

There is no relationship between the amount ofweight loss during the decomposition and the amountof organic material obtained from the microanalysis.Most probable because the decomposition temperaturenear 500°C including the metal vaporization interfereswith the TGA analysis.

3.4. Conducti6ity

The conductivity measurements were carried out forthe (Zn�Cd) acetone solids. The study was performedto observe the influence of the metal percentage in thesolids.

The electrical conductivities of the Zn�Cd solids arein the same range as semiconductors. These conductivi-ties are lower than that for pure metals [27], 1.66×107

(s/m) for Zn and 1.38×107 (s/m) for Cd.The decrease in conductivity with respect to the

metals is due to the absence of a bulk structure inwhich the electrons can move with freedom, conductingelectricity. However, we are in the presence of isolatedmetal particles, separated by a dielectric material [27],

G. Cardenas et al. / Polyhedron 19 (2000) 2337–23442344

Fig. 7. Conductivity of (ZnxCd1−x)�acetone films versus Cd percent-age.

Acknowledgements

The authors would like to thank financial supportfrom CONICYT (Grant 1960621) and technical assis-tance from Direccion de Investigacion, Universidad deConcepcion.

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the organic solvent adsorbed on the surface of themetal clusters. Therefore, the conduction mechanismfor these materials is different from that of the puremetals due to the availability of electrons to conductelectricity in each particle confined in potential holesseparated by a greater energetic barrier (dielectric).Schmid has reported that the conduction for thesemetal clusters is of the tunnel effect type [28], wherethe electrons are excited to a higher energy level forelectrical conductivity.

Another effect under study is the change in con-ductivity as a function of the percentage of metalincorporated in the solids. As we can observe in Fig.7, there is an increase in the conductivity of thebimetallic solid with respect to the monometallics.

4. Conclusions

Metal films containing organometallic residues wereobtained. The IR analysis showed the presence of theorganic solvents in the films. These alloyed films oractive powders containing solvents showed higher ther-mal stability than pure metals.

These active bimetallic films exhibit semiconductorproperties which depend on the increase in the Cdcontent, keeping the measurements at r.t.

.