Synthesis and characterisations of Au-nanoparticle-doped TiO2

and CdO thin films

Aytaç Gültekin a, Gamze Karanfil a, Faruk Özel b, Mahmut Kus- b, Ridvan Say c,Savaş Sönmezoğlu d,n

a Karamanoğlu Mehmetbey University, Faculty of Engineering, Department of Energy Systems Engineering, 70200, Karaman, Turkeyb Selçuk University, Department of Chemical Engineering, Advanced Technology Research and Application Center, 42075, Konya, Turkeyc Anadolu University, Faculty of Science, Department of Chemistry, 26470, Eskişehir, Turkeyd Karamanoğlu Mehmetbey University, Faculty of Engineering, Department of Materials Science and Engineering, 70200, Karaman, Turkey

a r t i c l e i n f o

Article history:Received 5 November 2013Received in revised form19 December 2013Accepted 17 January 2014Available online 4 February 2014

Keywords:Thin filmsSol-gel growthMicrostructureOptical propertiesSurface properties

a b s t r a c t

In the present study, pure and gold nanoparticle (Au NP)-doped titanium dioxide (TiO2) and cadmiumoxide (CdO) thin filmwere prepared by the sol–gel method, and the effect of Au NP doping on the optical,structural and morphological properties of these thin films was investigated. The prepared thin filmswere characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet–visible–near infrared (UV–Vis–NIR) spectra. While the optical band increases from 3.62 to 3.73 for TiO2

thin films, it decreases from 2.20 to 1.55 for CdO thin films with increasing Au doping concentration.Analysis of XRD indicates that the intensities of peaks of the crystalline phase have increased with theincreasing Au NP concentrations in all thin films. SEM images demonstrate that the surface morphologiesof the samples were affected by the incorporation of Au NPs. Consequently, the most significant results ofthe present study are that the Au NPs can be used to modify the optical, structural and morphologicalproperties of TiO2 and CdO thin films.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Transparent conducting oxide (TCO) thin films have attractedconsiderable attention because of their low resistivity and highoptical transmittance and have been used extensively for a varietyof applications, including architectural windows, solar cells, heatreflectors, light transparent electrodes and thin-film photovoltaicand many other optoelectronic devices [1–5]. Among the TCOs,titanium dioxide (TiO2) and cadmium oxide (CdO) have beenwidely investigated. TiO2 has been attracting attention of research-ers worldwide, especially because of its unique properties such ashigh transparency in the visible range with a wide band gap, highrefractive index, high dielectric constant, ability to be easily dopedwith active ions, absence of toxicity, abundance in nature and goodchemical stability in adverse environment, etc [6–10]. In otherrespects, CdO has been used in solar cell applications because of itsoptical transmittance in the visible region of the solar spectrumand high electrical conductivity [11–13]. CdO is an n-type semi-conducting material with band-gap energy between 2.2 and 2.5 eV

and has low resistivity (10�2–10�4Ω cm) due to the defect ofoxygen vacancies and cadmium interstitials [14–16].

Many researchers have recently been focussed on enhancingphysical and chemical properties of TiO2 and CdO thin films withdifferent approaches. First, there has been a dramatic progress inthe development of cost-effective thin-film deposition techniques,especially in the field of optoelectronic technology [7]. TiO2 andCdO thin films have been prepared by various physical andchemical deposition techniques such as sol–gel [17], spray pyr-olysis [6], pulsed laser deposition [18], sputtering [8], plasmaoxidation [19], chemical bath deposition [20], etc. Among thesetechniques, the sol–gel technique has many advantages whencompared with the other methods, that is, it has a simple andcost-effective experimental arrangement and also low sinteringtemperature, high smoothness of the films, possibility of coatingover large-area substrates, reliability, controllability, reproducibil-ity, etc [21–25], s, the preparation of thin films containing NPs ofsemiconductors and metals is another one of the hot topics in thefield of materials science, as they show promise in electronic,photonic, chemical and magnetic applications [26,27]. Metal NPs,such as gold (Au), silver (Ag), tin (Sn), aluminium (Al), iron (Fe),etc., embedded in various transparent matrixes have been widelystudied as optical materials [28–32]. In fact, Au is among the mostinteresting metal, as it affects the non-linear optical properties of

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Journal of Physics and Chemistry of Solids

http://dx.doi.org/10.1016/j.jpcs.2014.01.0110022-3697 & 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ90 338 226 2210; fax: þ90 338 226 2213.E-mail address: svssonmezoglu@kmu.edu.tr (S. Sönmezoğlu).

Journal of Physics and Chemistry of Solids 75 (2014) 775–781

the materials. Therefore, investigation of doping Au metal NPs andtheir influences on the physical properties of thin films is of greatinterest.

As is well known from the literature: i) no work providing adetailed explanation of the optical parameters of Au NP-dopedTiO2 thin films has been reported so far and ii) there are noprevious reports of Au NP-doped CdO thin films. The purpose ofthis study is to synthesise TiO2 and CdO thin films with Au NP-doping material using the sol–gel method, and to investigate theeffects of Au NPs doping on optical, structural and morphologicalproperties of these thin films.

2. Experimental Procedure

2.1. Synthesis

The Au NPs were prepared in a two-phase water/toluenesystem using the Brust method [33,34]. Briefly, NPs were preparedby the dropwise addition of 0.01 M aqueous NaBH4 solution in anequal volume of ammoniacal aqueous 0.5�10�3 M HAuCl4 (pH ca.7.8) washed thoroughly several times with water and toluene anddried under nitrogen, followed by redispersion in ethanol.A transmission electron microscopy (TEM) image of Au NPs usinga FEITecnaiTM G2 Sprit transmission electron microscope (20–120 kV)is given in Fig. 1. The shape of the NPs is close to spherical,aggregated and with an average size of approximately 17 nm.

In order to prepare a TiO2 solution, first, 2.4 ml titaniumtetraisopropoxide ((Ti (OC3H7)4, Merck) was added in 25 ml ethylalcohol (C2H6O, Merck) and the solution was kept in a magneticmixture for 1 h. Then, 5 ml glacial acetic acid (CH3COOH, Merck)and 25 ml ethyl alcohol were added in the solution and stirringwas continued for 1 h. As the final step, 1.5 ml triethylamine((C2H5)3N, Merck) was added in the solution, and the final solutionwas subjected to the magnetic mixture for 1 h. Furthermore, inorder to obtain the Au NP-doped TiO2 solution, Au NPs were addedinto the pure TiO2 solution (Au/Ti¼1/10), and the solutions werestirred with a magnetic stirrer for 2 additional h. Finally, the pureand Au NP-doped TiO2 solutions were kept at room temperaturefor 1 day before deposition.

In the typical preparation procedure of a CdO solution, first,1.19 ml cadmium acetate (Cd(CH3COO)2þ2 H2O, Merck) wasadded to 50 ml methanol solvent (CH3OH, Merck) and stirringwas continued for 1 h. Then, 1 ml glycerol (C3H8O3, Merck) and1 ml C8H15N were added in the solution, and after the additivecomponent was added, it was stirred with a magnetic stirrer for1 h. Moreover, in an effort to synthesise the Au NP-doped CdOsolution, Au NPs were added into the pure CdO solution (Au/Cd¼1/10), and the solutions were kept in the magnetic stirrer for2 additional hours. Lastly, the pure and Au NP-doped CdO solutionswere also aged at room temperature for 1 day before deposition.

2.2. Deposition

Microscope glass slides were used as the substrates for thinfilms. Prior to the deposition, the glass slides were sequentiallycleaned in an ultrasonic bath with acetone and ethanol. Finally,they were rinsed with distilled water and dried. After the abovetreatment, a spin-coating process was applied to cover solutionson the glass substrates. The spinning process was performed usinga Holmarc Spin Coating Unit and the coating was done by rapidlydepositing approximately 0.6 mL of solution onto a glass substratespun in air at 6000 rpm for 30 s. In order to obtain as-depositedthin films, five spin-coated layers were obtained for each sub-strate. After each spinning process, samples were subjected torepeated annealing processes at a temperature of 300 1C for a 5-min

period and finally were post-annealed in an oven at a temperatureof 500 1C for 1 h.

2.3. Characterisations

X-ray diffraction (XRD) patterns of thin films were determinedby a Bruker D8 Advance X-ray diffractometer with CuKα radiation.Besides, the morphologic characterisations of all the thin filmswere performed by Zeiss LS-10 scanning electron microscopy(SEM). For the optical characterisation, the transmittance andreflectance spectra were also recorded from 300 to 1500 nmwavelength with a data interval of 1 nm using Jasco ModelV-570 type ultraviolet–visible–near infrared (UV–Vis–NIR) spec-trophotometer at room temperature. Furthermore, the thicknessesof these thin films were measured by using Woolam Vase M2000ellipsometer.

3. Results and Discussion

3.1. Optical Characterisation

In order to understand optical behaviours of the thin films,some optical parameters such as transmittance (T), reflectance (R),extinction coefficient (k), refractive index (n), band gap (Eg),complex dielectric constant (ε), optical conductivity (sopt) anddissipation factor (ε2=ε1) were investigated. The measured trans-mittance spectra in the wavelength range 300–1500 nm areshown in Fig. 2a. As seen in Fig. 2a and Table 1, the transmissionvalues of pure TiO2 and Au NP-doped TiO2 thin films are 72.6% and92%, respectively, in the 600 nm wavelength. The increment intransparency is related to the structural and surface properties

Fig. 1. A TEM image of Au NPs.

A. Gültekin et al. / Journal of Physics and Chemistry of Solids 75 (2014) 775–781776

(such as better crystallinity, less surface irregularity, less surfaceroughness (surface scattering reduces the specular transmission,which in turn depends on the grain size and shape) and less defectdensity) of the film characteristics, because it is known that thechanges in transmittance depend on the material characteristics ofthe films [35]. On the contrary, a decline in the transmission valuesof pure CdO and Au NP-doped CdO thin films is observed as 75.7%and 4.8%, respectively, in the 600-nm wavelength as shown inFig. 2a. The decline is probably due to the disorder in the latticewith the increases in the localised states near to the bands [6]. Thereflectivity values of the thin film are given in Fig. 2b and Table 1;it is evident from the figure and the table that reflectivity of TiO2

thin films decreases with Au doping at λ¼ 600 nm, whereas itincreases with Au doping in CdO thin films at λ¼ 600 nm.

The extinction coefficient and the refractive index of the thinfilms are given by the formulae [36]:

k¼ αλ4π

and n¼ 1þR1�R

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

4Rð1�RÞ2

s�k2 ð1Þ

where k is the extinction coefficient and n is the refractive index.Figs. 2c and 2d report the extinction coefficient and refractiveindex dependence of wavelength, respectively. As seen in Fig. 2c,the extinction coefficient (k) value decreases with the scaling up of

the wavelength. This reduction depend on the notable normaldispersion behaviour of the films. Besides, the k value close to zerois in agreement with the fact that thin films are transparent in thevisible spectral region. However, it can be seen clearly from Fig. 2cthat the k values of Au NP-doped CdO thin films are high whencompared with others. This case confirms the transmission spec-tra. The value of n decreases with increase in λ as shown in Fig. 2dand it is similar with k. These observations verify the reduction inthe loss of light due to scattering and absorbance with increase inwavelength [7]. In addition, the extinction coefficient and therefractive index at wavelength λ¼ 600 nm for all films are given inTable 1. As seen from this table, it is found that both k and n of TiO2

thin film decrease with Au NP doping, while an increase is seen forCdO thin films with Au NP doping. For TiO2 thin films, this effect ofAu NP doping on n can be attributed to the decrease of bothpolarisability and film density [7].

The optical band gaps of these thin films were estimated fromTauc's relation [37]:

ðαhvÞ ¼ Aðhv�EgÞn ð2Þwhere (hv) is the photon energy, Eg is the optical band gap of thematerial, A is a constant and n¼ 1=2 for the direct band-gapmaterial. When ðαhvÞ2is plotted as a function of (hv), the linearportion of the curve is extrapolated to ðαhvÞ2 ¼ 0. Fig. 3 (a,b,c and

0

20

40

60

80

100

Tra

nsm

ittan

ce (%

)

Wavelenght (nm)300 600 900 1200 1500 300 600 900 1200 1500

0

20

40

60

80

100

Ref

lect

ance

(%)

Wavelenght (nm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Ext

inct

ion

coef

ficie

nt, k

Wavelenght (nm)300 600 900 1200 1500 300 600 900 1200 1500

1

2

3

4

5

6

Ref

ract

ive

inde

x, n

Wavelenght (nm)

Fig. 2. Optical properties of pure TiO2 (pink line), Au NP-doped TiO2 (green line), pure CdO (blue line) and Au NP-doped CdO (cyan line). (a) Optical transmission spectra(b) the reflectivity spectra (c) extinction coefficient and (d) refractive index. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

Table 1Optical parameters of pure and Au-doped TiO2 and CdO thin films calculated by UV–vis–NIR spectra.

Material For λ¼ 600 nm d (nm) Eg (eV) ε1 N/mn (cm�3 g�1)

T (%) R (%) n k

Pure TiO2 72.62 28.08 3.25 0.115 132 3.62 10.2 2.9�10 [42]Au NP-doped TiO2 92 9.08 1.86 0.035 111 3.73 10.6 2.2�10 [42]Pure CdO 75.67 20.58 2.66 0.066 201 2.20 8.6 2.1�10 [42]Au NP-doped CdO 4.80 27.1 2.89 0.848 171 1.55 8.9 3.3�10 [42]

A. Gültekin et al. / Journal of Physics and Chemistry of Solids 75 (2014) 775–781 777

d) shows the dependence between ðαhvÞ2 and photon energy hvfor pure and Au NP-doped TiO2 and CdO thin films. The band-gapvalue of TiO2 thin films increases with Au NP doping as seen inTable 1. For Au-doped TiO2, compared to pure TiO2 thin films, thecontribution from Auþ ions on substitutional sites of Ti2þ ions andAu interstitial atoms determines the widening of the band gapcaused by an increase in the carrier concentration. This well-known Burstein–Moss effect is due to the Fermi level moving intothe conduction band. Because Au NP doping increases the carrierconcentration in the conduction band, the optical band-gap energyincreases [35,38]. On the other hand, the band-gap value of CdOthin film decreases with Au NP doping. The reduction in slope ofthe linear portion of the plot (Fig. 3d) observed in our presentwork suggests the introduction of defect states, explained in termsof stress relaxation mechanism due to enhanced thickness, withinthe band gap [39]. Thus, we interpret this shift due to the mergingof an impurity band into the conduction band, thereby shrinkingthe band gap. The formation of such impurity bands giving rise tonew donor electronic states just below the conduction band ispossible and this rises due to hybridisation between states of CdOthin films and Au NPs dopant [40]. It seems that such a formation

of donor levels compensates the Burstein–Moss effect and resultsin the narrowing of the effective band gap of Au NP-doped CdOthin films. Thus, an alteration of band gap also ensures that Au NPswere successfully doped into the TiO2 and CdO thin films.

The complex dielectric constant ε, that is the optical constantaccessible to physical interpretation, components of a material interms of optical constants kand n is given as [41]

ε1 ¼ n2�k2 ð3Þ

where ε1 is the real part and

ε2 ¼ 2nk¼ ε1ω2p

8π2c3τ

!λ3 ð4Þ

where ε2 is the imaginary part. Besides, ε1is the high-frequencydielectric constant, ωp is the plasma frequency, cis the velocity ofphoton and τis the optical relaxation time. The variation of ε1 andε2 with λ are shown in Figs. 4a and b, respectively. As seen fromthese figures, the variation of the real and imaginary dielectricconstants with wavelength is similar in that they decrease with

0

2

4

6

8

10

12

14

(ahv)2 (e

V/m

)2

hv (eV)

Pure TiO2

0

20

40

60

80

100

120

140

(ahv)2 (e

V/m

)2

hv (eV)

Au NPs doped TiO2

0.0

0.2

0.4

0.6

(ahv)2 (e

V/m

)2

hv (eV)

Pure CdO

3.0 3.2 3.4 3.6 3.8 3.0 3.2 3.4 3.6 3.8 4.0

0.8 1.2 1.6 2.0 2.4 2.8 3.2 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.00

8

16

24

32

40

48

56

(ahv)2 (e

V/m

)2

hv (eV)

Au NPs doped CdO

Fig. 3. The band-gap values of (a) Pure TiO2 (b) Au NPs doped TiO2 (c) Pure CdO and (d) Au NPs doped CdO.

4

8

12

16

20 Pure CdO Pure TiO2 Au NPs doped CdO Au NPs doped TiO2

Rea

l par

t of d

iele

ctri

c

Wavelenght(nm)300 600 900 1200 1500 300 600 900 1200 1500

0

2

4

6

8 Pure CdO Pure TiO2 Au NPs doped CdO Au NPs doped TiO2

Imag

inar

y pa

rt o

f die

lect

ric

Wavelenght (nm)

Fig. 4. Variation in (a) real part and (b) imaginary part with the wavelength.

A. Gültekin et al. / Journal of Physics and Chemistry of Solids 75 (2014) 775–781778

increasing wavelength and become constant at higher wave-lengths, except Au NP-doped CdO thin films.

The variation of the dielectric constant with λ2 indicates thatsome interactions between photons and electrons are produced inthis wavelength range. When n2ck2 and ωτ{1, the dielectricconstant in Eq. 3 equals Spitzer–Fan model [42].

ε1 ¼ ε1� e2

4π4c2ε0

� �Nopt

mn

h

!λ2 ð5Þ

where ε0 is the free space dielectric constant and Nopt=mn

h is theratio of free optical carrier concentration (Nopt) to the free carriereffective mass (mn

h). The variation of the real electric constantswith λ2 is shown in Fig. 5 in which the slope of the curve givesNopt=mn

h ratio while the y-axis intercept for the linear part athigher wavelengths gives the value of ε1. Also, The obtainedvalues of Nopt=mn

h and ε1 are signified in Table 1. It is seen fromthe table clearly that the high-frequency dielectric constant ε1increases for TiO2 thin films with Au NP doping, while it decreasesfor CdO thin films.

Optical conductivity can be used to study the optical response.As seen in Eq. 6, in order to calculate the optical conductivity, sopt,the absorption coefficient, α, can be used [7]:

sopt ¼αnc4π

ð6Þ

where c is the velocity of light. The variation of the opticalconductivity with wavelength is given in Fig. 6a. As seen fromthe figure, at low wavelength (at high photon energies), a suddenincrease occurs in optical conductivity of the thin films except AuNP-doped CdO thin film. The increase is related with the highabsorbance of the thin films; therefore, this behaviour of the Au

NP-doped CdO thin film in optical conductivity is expectedbecause of its low absorbance.

In order to describe dielectric behaviour of the thin films, thatis a measure of the dielectric losses, dissipation factor is asignificant parameter. The dissipation factor can be calculated asfollows [43]:

tan δ¼ ε2ε1

ð7Þ

Fig. 6b gives the plot of the dissipation factor as a function ofwavelength. As clearly seen from the figure, the dissipation factordecreases with increasing wavelength.

3.2. Structural Characterisation

In order to investigate the structural properties of pure and AuNP-doped TiO2 and also pure and Au NP-doped CdO thin films, theXRD measurements were performed over the range 20–801 andspectra are given in Figs. 7a and b, respectively. It is seen fromFig. 7a that pure TiO2 thin filmwas the anatase-phase crystal planewith (1 0 1) reflection, while Au NP-doped TiO2 thin film alsoexhibited characteristic peaks of anatase crystal planes (1 0 1) and(2 0 0) of preferred orientations at 2θ ¼25.31 and 48.11, respec-tively. The relative peak intensity of these diffraction peaksincreases with Au NP doping and resulted in better crystallinity.The presence of several peaks in the XRD patterns shown in Fig.7brevealed that pure and Au NP-doped CdO thin films demonstratedpeaks of cubic-type structures with planes that exhibited peaks at2θ¼33.01, 38.31, 55.31, 65.91 and 69.31, which correspond to the(1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) reflections, respectively.The obtained diffraction peaks substantiate the polycrystallinenature of CdO thin films, whereas their intensities suggest a morerandomly distributed orientation of crystallites [11].

XRD is essential to determinate the crystal structure andcrystallinity, and to estimate the crystal size according to theScherrer equation [44,45]:

D¼ 0:9λβ cos θβ

ð8Þ

where D is the crystallite size, 0:9is a constant, λ is the wavelengthand β is the full width at half maximum of diffraction peakmeasured in radians and θ is the Bragg angle. The broadening ofa particular peak in a diffraction pattern associated with aparticular planar reflection from within the crystal unit cell ismeasured to determine the crystal size. The crystallite sizes ofpure and Au NP-doped TiO2 thin films are 19.87 and 24.32 nm,respectively. A comparison of other works on Au-doped TiO2 thinfilms shows that the crystallite sizes of Au-doped TiO2 increasewith the dopant concentration [46]. The calculated values indicatethat the crystallite size increases with Au NP doping in terms ofcrystal plane (1 0 1) of anatase structure. Furthermore, it is perceptible

25.012.50.00

10

20

30

40

ε 1=n

2 -k2

λ2*105(nm)2

Au NPs doped TiO2 Au NPs doped CdO Pure TiO2 Pure CdO

Fig. 5. The plot of the variation the real dielectric constants with λ2.

0

40

80

120

Opt

ical

con

duct

ivity

(sec

-1)*

1013

Wavelenght (nm)

Pure CdO Pure TiO2 Au NPs doped CdO Au NPs doped TiO2

300 600 900 1200 1500 300 600 900 1200 15000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Dis

sipa

tion

fact

or

Wavelenght (nm)

Pure CdO Pure TiO2 Au NPs doped CdO Au NPs doped TiO2

Fig. 6. The variation of (a) the optical conductivity and (b) the dissipation factor with wavelength.

A. Gültekin et al. / Journal of Physics and Chemistry of Solids 75 (2014) 775–781 779

from Fig. 7b that the diffraction peak from (1 1 1) emerges as thestrongest orientation for whole CdO thin films. The calculatedcrystallite sizes for crystal plane (1 1 1) are 23.92 and 28.75 nm forpure and Au NP-doped CdO thin films, respectively. All of the thinfilms give similar patterns without any peak having Au NPs. Theabsence of Au characteristic patterns can be explained by disper-sing Au NPs in the thin films uniformly. On the other hand, thepresence of Au content influences the crystallite sizes. The resultsplotted in Fig. 7 are probably due to the crystallinity of the samplesbeing improved. The crystallite sizes become larger with increase ofAu concentration owing to increased ionic radii and atomic weight ofAuþ as compared to Ti2þ and Cd2þ . The increment is in accordancewith results available in the literature [47].

3.3. Morphological Characterisation

It is known that the physical properties of TCO thin films, whichare important factors for optoelectronic device applications, areinfluenced by surface properties, and therefore the investigation ofthe surface morphology is very important. The morphologicalcharacterisation of the pure and Au NP-doped TiO2 and CdO thinfilms were performed by using SEM.

Figs. 8a–d show the SEM micrographs of pure and Au NP-dopedTiO2 and also pure and Au NP-doped CdO thin films, respectively. Itcan be obviously seen from SEM images that the grain size of TiO2

and CdO thin films increases with Au NP doping and it results in amore homogeneous surface morphology. It can be obviously observedthat the hole and pit, forming on the surface, have disappeared withincreasing Au NPs concentrations over the surface. A significantimprovement of surface properties has been explained by the dis-tribution of small and large crystallites located on top of the formerones. The smaller crystallites have a characteristic high defect density[48]. By contrast, in our study, Au NP doping leads to large crystalliteswith considerably reduced defect densities. In addition, the grain sizebecomes larger, indicating the results from XRD analysis, and a porousstructure (especially CdO thin films) is obtained as the dopingconcentration increases.

4. Conclusion

The influence of Au NP doping on the optical, morphological andstructural properties of TiO2 and CdO thin films has been investi-gated. When Au NPs are supplemented to transparent matrixes, the

Inte

nsity

(a.c

.)

2θ (degree)

Pure TiO2Au NPs doped TiO2(1

01)

(200

)

20 30 40 50 60 70 80 20 30 40 50 60 70 80

Pure CdOAu NPs doped CdO

Inte

nsity

(a.c

.)

2θ (degree)

(111

)

(200

)

(220

)

(311

)(2

22)

Fig. 7. X-ray diffraction patterns of (a) pure and Au NP-doped TiO2 thin films and (b) pure and Au NP-doped CdO thin films.

Fig. 8. SEM images of (a) Pure TiO2 (b) Au NP-doped TiO2 thin films (c) Pure CdO and (d) Au NP-doped CdO thin films.

A. Gültekin et al. / Journal of Physics and Chemistry of Solids 75 (2014) 775–781780

optical band gap of the TiO2 thin films increased from 3.62 to 3.73whereas it decreased from 2.20 to 1.55 for CdO thin films. Accordingto the SEM results for the surface morphology, the grain sizes ofthese thin films are influenced by the Au NP doping. TiO2 thin filmswere polycrystalline with a body-centred tetragonal structure in theanatase phase and CdO thin films were polycrystalline in naturewith cubic crystal structure. The crystallite sizes were calculated byusing the Debye–Scherrer method and they increase with Au NPincorporation for all thin films. The improved optical, morphologicaland structural properties achieved by Au NP supplementation hereare comparable to pure TiO2 and CdO thin films, suggesting that theAu NP-doped TiO2 and CdO thin films have potential applicationsfor the future of nanotechnology.

Acknowledgements

This work is partially supported by the Scientific ResearchCommission of Karamanoğlu Mehmetbey University (Project no.:23-M-11).

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