J Mater Sci: Mater Electron (2015) 26:8877–8886 DOI 10.1007/s10854-015-3568-0

Influence of pH on structural, morphological and optical properties of chemically deposited nanocrystalline ZnO thin films A. Sales Amalraj1 • A. P. Dharani2 • P. Fermi Hilbert Inbaraj2 • V. Sivakumar2 G. Senguttuvan2

•

Received: 15 June 2015 / Accepted: 30 July 2015 / Published online: 8 August 2015 Ó Springer Science+Business Media New York 2015

Abstract In the present work, a simple and cost effective successive ionic layer adsorption and reaction method was adopted to grow highly oriented crystalline ZnO nanorods (NRs). The ZnO NRs were grown over glass substrates by varying the pH. The surface morphology studies show that the dimension and orientation of ZnO NRs are influenced by varying the pH conditions. The SEM analysis reveals that the ZnO NRs grow vertically with perfect wurtzite hexagonal shape with a diameter range from 300 nm to 1 lm at optimized pH concentration. The XRD patterns of the ZnO NRs exhibit high crystalline orientation of ZnO wurtzite structure with a preferential (101) plane orientation. Optical spectra recorded using UV–Vis spectrophotometer on ZnO rods show that the optical band gap decreases with increase in pH of the solution. This simple and integrated approach is expected to lead to a cost effective and convenient way towards large scale growth of ZnO rods with subsequent interest in nano-based biosensor applications in future.

1 Introduction Recently, extensive research has been devoted to grow various kinds of nanostructures, which not only broaden our knowledge on mesoscopic physics phenomena but also on fundamental theories about the effect of its dimension

& G. Senguttuvan [email protected] 1

Department of Physics, PSNA College of Engineering and Technology, Dindigul 624 622, India

2

Department of Physics, Anna University Chennai, BIT Campus, Tiruchirappalli 620 024, India

and size. Among various metal oxide semiconductor nanostructures, zinc oxide (ZnO) has been identified as a potential material for future device applications. With a wurtzite hexagonal phase, ZnO has a direct band gap of 3.37 eV and with larger exciton binding energy of 60 meV. Nowadays nanostructured ZnO has a wide range of technological applications including sensors [1], field emitters [2], optoelectronics [3], dye-sensitized solar cells [4] and in biological application [5]. Due to the development of research on the fabrication and application, more research results have verified that the morphology of films play a key role in the application of ZnO in specific fields. So far, a wide variety of ZnO special morphologies (complex structured films) have been grown using physical or chemical methods. Thus, methods for tailoring the morphology of ZnO nanostructured films according to human need will be quite demanding. Over the past few years, several methods have been developed for the growth of vertical ZnO nanorods. In the synthesis part, expensive methods like pulsed laser deposition [6], thermal evaporation [7], vapor transport deposition [8], molecular beam epitaxy (MBE) [9], plasma-enhanced chemical vapor deposition [10], metal organic vapor phase epitaxy [11], magnetron sputtering [12], electro deposition [13], hydrothermal method [14], etc. have been used to grow one-dimensional (1D) ZnO nanostructures. All these techniques either demand stringent reaction conditions such as high temperature and pressure, and hazardous chemicals or both. However, solution growth method can produce good nanostructures without using metal catalyst or templates that too with better crystal quality. There are reports on the growth of ZnO NRs by chemical route [15, 16]. Direct growth of vertical ZnO NRs on glass substrate is extremely difficult because glass is an amorphous solid and

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therefore has no lattice matching with ZnO. Such a difficulty is common for 1D ZnO NR growth in many substrates. Hence, almost all NR growth processes involve precoating the substrate with a ZnO seed layer prior to ZnO NRs growth. In recent years, much expensive methods such as atomic layer deposition (ALD), spin coating [17], sol gel method [18], pulsed laser deposition [19], electron beam evaporation [20], etc., have been used for seed layer growth. These methods again involve either high vacuum condition or use of hazardous chemicals or both. Therefore developing a simple synthesis method to grow ZnO NRs is a challenging task. In our work, an attempt is made to grow nanorods on successive ionic layer adsorption and reaction (SILAR) grown ZnO thin films. We believe that this method is a simple, cost effective and a convenient route to grow vertical ZnO nanorods compared to other growth processes. The as-prepared ZnO NRs were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) for their structural and surface morphology. Investigations on the optical quality of ZnO NRs by UV– Vis-spectroscope have also been made.

J Mater Sci: Mater Electron (2015) 26:8877–8886

Fig. 1 Schematic illustration of SILAR growth process. a Formation of Zn–ammonia complex [Zn(NH3)4]2?, b Formation of Zn(OH)2, c DI water rinsing, d Formation of ZnO thin film layer and e ZnO nanorod growth by annealing

2 Experimental ZnO nanorods were grown on microslide glass substrates by SILAR using aqueous zinc–ammonia complex as cationic precursor and deionized water kept at 358 K as anionic precursor. The aqueous zinc–ammonia complex ion ([Zn(NH3)4]2?) was prepared by mixing zinc sulphate (ZnSO47H2O) and concentrated ammonium hydroxide (NH4OH), with Zn:NH3 molar ratio of 1:20 and Zn2? concentration of 0.125 mol/L. ZnO thin film was deposited on glass substrate by alternate dipping made in zinc ammonia complex and in hot water [21]. In brief, each deposition cycle consists of four steps: (1) Immersion of the substrate in [Zn (NH3)4]2?) solution for 10 s for complex adsorption; (2) Instant immersion of the withdrawn substrates in hot water (358 K) for 12 s to form solid ZnO layer; (3) Rinsing the substrate in deionized water to remove the unreacted species (4) Drying the substrate in air for 30 s before the start of the next deposition cycle. Surface modification of the films was carried out by altering the preparation conditions. Typically, 50 deposition cycles (rather than 40 and 30) were performed to probe the growth and orientation of the ZnO nanostructures. Figure 1a–e shows the schematic SILAR procedure for the deposition of ZnO nanostructured thin films and the corresponding growth process of nanorods. In this present work, ZnO nanostructured thin films were synthesized by SILAR method, using aqueous solution of zinc sulphate and ammonia at different pH value.

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2.1 Characterization For investigation of pH effect of zinc–ammonia complex solution, the films were annealed at 200 °C in an oxygen atmosphere for 30 min. For structural studies, a X’pert Pro ˚) (PANalytical) diffractometer using Cu Ka (k = 1.5405 A radiation with 2h of 30°–70° was used. Surface morphology was studied using Hitachi S-3000H model SEM. In order to study the optical properties of the deposited films, the absorption measurements were carried out using a Perkin-Elmer UV/VS Lambda 2S Spectrometer with a wavelength resolution better than ±0.3 nm at room temperature.

3 Result and discussion 3.1 Mechanism of film deposition The mechanism of ZnO films formation by the SILAR method can be explained as follows: full usage of the thermal decomposition of [Zn(NH3)4]2? was made in a neutral aqueous solution, which released ions of Zn2? into the solution that resulted in the formation of ZnO or Zn(OH)2 particles. Equations (1)–(4) illustrate the chemical reactions related to the process 2 ZnSO4 þ 2NH4 OH ! ZnðOHÞ2 þ 2NHþ 4 þ SO4

ð1Þ

J Mater Sci: Mater Electron (2015) 26:8877–8886

8879

960 840 720 600 480 360 8

12

(103)

(112)

(102)

(110)

(100) (002)

(a) (b) (c) (d) (e)

Intensity (a.u.)

Consequently, the ZnO crystal will serve as nuclei for further film growth.

11

Fig. 2 Thickness of ZnO thin films deposited at different pH

As-deposited Zn(OH)2 will transform to ZnO in an aqueous solution at temperature over 358 K: ð4Þ

10

pH Value

ð3Þ

ZnðOHÞ2 ðsÞ ! ZnOðsÞ þ H2 O

9

(101)

When the glass substrate is immersed in the above solution, these zinc complex ions get adsorbed on the substrate due to attractive force between ions in the solution and the surface of the substrate. These forces may be Van der Waals, cohesive forces or chemical attractive forces [22]. Then, the glass substrate is immersed in hot water and the [Zn(NH3)4]2? complex decomposes, forming Zn(OH)2:  2þ  ZnðNH3 Þ4 þ 4H2 O ! ZnðOHÞ2 ðsÞ þ 4NHþ 4 þ 2OH

1080

Thickness (nm)

The addition of excess ammonia solution reduces Zn2? ions by producing the complex ions of the type [Zn(NH3)n]2?; n = 4 is the most stable coordination number that avoids precipitation and makes the solution transparent. This process can be explained by the following reaction:  2þ ZnðOHÞ2 þ 4NHþ þ 2H2 O þ 2Hþ ð2Þ 4 ! ZnðNH3 Þ4

3.2 Structural analysis The film thickness was determined by the weight gain method using the formula m t¼ ð5Þ Aq where t is the thickness of the film, m is the weight gain, A is the area of the deposited film and q is the density of the film (5.6 g cm-3) [23]. The rate of film deposition of ZnO on glass substrates was quite high and the thickness of the films as seen in Fig. 2 increased linearly with the pH conditions. On the basis of the film thickness, the growth rate of dense ZnO films fabricated by SILAR was estimated to be 22–44 nm per cycle. The structural analysis of ZnO thin films was carried out by using XRD by varying the diffraction angle, 2h from 30° to 70°. Figure 3 shows the XRD pattern of ZnO films deposited on glass substrates at different pH conditions of 8, 9, 10, 11 and 12. XRD pattern of all the deposition samples are indexed to hexagonal ZnO, which is close agreement with the standard card (JCPDS 36-1451). The relative intensity of the ZnO nano-structures at the (101) peak has been drastically improved with increase in pH condition. This confirms that nanorods are much better aligned on a ZnO deposited glass substrate with well

10

20

30

40

50

60

70

80

2θ (Degree)

Fig. 3 XRD pattern for ZnO rods grown on glass substrate with pH of a 8, b 9, c 10, d 11 and e 12

defined and dominant peak at 2h = 36.2° corresponding to (101) orientation of the film. This further confirms the hexagonal wurtzite structured ZnO with good crystallinity. All other detected diffractions (hkl) peaks at 2h values of 31.4°, 34.4°, 47.5°, 56.6°, 62.8°, 67.96° and 72.56° corresponds to the following lattice planes: (100), (002), (102), (110), (103), and (112) respectively and are purely indexed to wurtzite phase of ZnO (JCPDS card No: 36-1451). No characteristic peaks of impurity phases such as Zn or Zn(OH)2 are observed, and no diffraction peaks except ZnO were found which indicates that only single-phase hexagonal ZnO is present. The relative intensity of the ZnO products at the (101) peak increases with increase in pH growth conditions from 8 to 12, Fig. 4 indicates that the ZnO nanorods grow in one dimension (1D) preferentially.

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the preferential growth of the films is related to pH of the precursor solution. From the width of the XRD peak broadening, the mean crystallite size was calculated using Scherer’s equation [26].

36.6

2θ (degree)

36.5

36.4

D¼

36.3

0:94k bcosh

where D is the crystallite size of the particle, k is shape factor taken to be 0.9, k is the X-ray wavelength, h is the diffraction angle and b is the full width at half maximum of the diffraction peak at 2h. Williamson–Hall equation was used to calculate the strain. The Williamson–Hall equation is expressed as follows [27].

36.2

36.1

36.0 8

9

10

11

12

bcosh ¼

pH value

Fig. 4 The variation in peak position along (101) plane as a function of pH of zinc–ammonia complex solution

The lattice parameters of five samples were calculated using the observed values of 2h and d-values for the hexagonal structure which is given by the relation [24]   2   2 1 4 h þ hk þ l2 l ¼ þ 2 ð6Þ d2 3 a2 c and the results are given in Table 1. The values for the c/a ratio have been computed and found to comparable with the standard values. In order to investigate the possibility of the preferential orientation of the samples, the texture coefficient TC (hkl) for all planes was calculated using the expression [25]. IðhklÞ I0ðhklÞ

TC ðhklÞ ¼ P 1 N

ð7Þ

IðhklÞ N I0 ðhklÞ

where I0ðhklÞ is the standard intensity of the (hkl) plane, I(hkl) is the observed intensity of the (hkl) plane and N is the number of diffraction peaks. From this definition, it is clear that when there is a higher deviation of TC of the unit, preferential orientation is higher too. The TC (hkl) values for various planes at different pH growth conditions are shown in Table 2. From these results, it can be inferred that

Table 1 The variation of lattice parameters of ZnO nanostructures on the effect of pH

S. No.

Parameters

kk þ 4sinh D

ð9Þ

Here Eq. (9) stands for uniform deformation model (UDM) where it is assumed that strain is uniform in all crystallographic directions. bcosh was plotted with respect to 4sinh for the peaks of ZnO with varied pH conditions. Strain is calculated from the slope of the fitted line. From the lattice parameters calculation it was observed that the strain might be due to the lattice shrinkage. The UDM analysis results are shown in Fig. 5a–e Furthermore, the intrinsic stress is calculated using the modified form of Williamson–Hall relation [28]. The generalized Hook’s law referred to the strain, keeping only the linear proportionality between the stress and strain, i.e., r = eY. Here, the stress is proportional to strain, with the constant of proportionality being the modulus of elasticity or Young’s modulus, denoted by Y. In this approach, the Williamson Hall equation is modified by substituting the value of strain (e) in Eq. (8); we get   kk 4rsinh bcosh ¼ þ ð10Þ D Y The above equation is known as Uniform Stress Deformation Model (USDM) where Y is the Young’s modulus for ZnO (*130 GPa), D is the crystallite size. The uniform stress can be calculated from the slope line plotted between 4sinh Y and bcosh as shown in Fig. 6. The USDM plots for ZnO NRs with varied pH growth conditions are shown in the Fig. 6a–e.

Standard value

Obtained values with different pH value 8

9

10

11

12

2.2973

2.327

2.3129

2.3299

2.3214

2.2959

0.3249

0.3291

0.3271

0.3295

0.3283

0.3247

3

˚) d (A ˚) a (A ˚) c (A

0.5206

0.5264

0.5228

0.5252

0.5240

0.5204

4

c/a

1.602

1.5995

1.5982

1.5939

1.5961

1.6027

1 2

123

ð8Þ

J Mater Sci: Mater Electron (2015) 26:8877–8886 Table 2 Texture coefficients of ZnO films

S. No.

8881

ZnO sample

Texture coefficient (100)

(002)

(101)

(102)

(110)

(103)

(112)

1

pH 8

1.03

1.15

0.45

1.37

1.06

–

–

2

pH 9

1.04

1.09

0.53

1.31

0.93

1.01

1.10

3

pH 10

1.73

1.19

0.93

1.14

0.98

0.41

0.62

4

pH 11

1.74

1.16

1.05

0.60

1.22

0.95

0.65

5

pH 12

1.81

1.20

1.14

1.25

1.02

0.82

0.91

0.020

(a)

0.014

0.016

(b)

0.012 0.010

cos

cos

0.012

0.008

0.008 0.006

0.004

0.004 1.0

1.2

1.4

1.6

1.8

2.0

2.2

1.0

1.2

1.4

4 sin

1.6

1.8

2.0

2.2

4 sin

0.016

(c)

0.012

(d)

0.014 0.010

cos

cos

0.012 0.010

0.008

0.008 0.006

0.006 0.004

0.004 1.0

1.2

1.4

1.6

1.8

2.0

1.0

2.2

1.2

1.4

1.6

1.8

2.0

2.2

4 sin

4 sin

(e)

0.010 0.009

cos

0.008 0.007 0.006 0.005 0.004 1.0

1.2

1.4

1.6

1.8

2.0

2.2

4 sin

Fig. 5 Williamson–Hall analysis of ZnO thin films deposited at pH of a 8, b 9, c 10, d 11 and e 12. Fit to the data, strain is extracted from the slope of the fit

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J Mater Sci: Mater Electron (2015) 26:8877–8886 0.020

0.016

(b)

(a) 0.014

0.016

0.010

0.012

cos

cos

0.012

0.008

0.008 0.006 0.004

0.004

0.002

8

9

10

11

12

13

4 sin / E (TPa)

14

15

16

8

17

9

10

-1

11

12

13

4 sin / E (TPa)

0.016

14

15

16

17

15

16

17

-1

0.014

(c)

(d)

0.014

0.012

0.012

cos

cos

0.010

0.010

0.008

0.008 0.006

0.006

0.004

0.004 8

9

10

11

12

13

14

4 sin / E (TPa)

0.010

15

16

17

8

9

10

-1

11

12

13

14

4 sin / E (TPa)

-1

(e)

0.009

cos

0.008 0.007 0.006 0.005 0.004 8

9

10

11

12

13

4 sin / E (TPa)

14

15

16

17

-1

Fig. 6 The modified form of Williamson–Hall analysis for ZnO thin films at pH of a 8, b 9, c 10, d 11 and e 12 assuming USDM. Fit to the data, stress is extracted from the slope of the fit

Additionally, the amount of defects in the films was determined by calculating the dislocation density (d) from the following formula [29]. d¼

1 D2

ð11Þ

where D is the crystallite size. The smaller value of dislocation densities and larger grain size is an indication of

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the better crystallization. Table 3 summarizes the geometric parameters of ZnO nano particle obtained from Scherrer’s formula, UDM and USDM results. 3.3 Surface morphological analysis Figure 7a–e shows the Scanning Electron Microscope images of the ZnO films deposited at different pH values of

J Mater Sci: Mater Electron (2015) 26:8877–8886 Table 3 Geometric parameters of the ZnO thin films using different pH values

S. No

8883

pH value

FWHM

D (nm)

e 9 10-3

r (GPa)

d 9 1012 (cm-2) 35.43

1

8

0.4945

16.80

14.39

1.8711

2

9

0.4606

18.05

10.43

1.3559

30.69

3

10

0.3588

23.14

9.48

1.2324

18.66

4

11

0.3310

25.11

8.42

1.0946

15.86

5

12

0.2664

31.19

5.19

0.6747

10.27

Fig. 7 SEM images of ZnO nanostructured films prepared at different pH values: a 8, b 9, c 10, d 11 and e 12

8, 9, 10, 11 and 12 respectively. The ZnO sample deposited at lower pH condition (pH 8) shows agglomeration of the ZnO products distributed all over the substrate as shown in Fig. 7a. Most of the deposited glass surface is covered by vertically oriented ZnO rods. The lengths and diameter of

the ZnO rods are less than 1 lm and 100–200 nm, respectively. There is no significant growth of ZnO nanorods below pH of 8 but there is evidence of initial nucleation which indicates the need for a minimum value of pH condition for the growth of nanorods. For pH of 9,

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Fig. 8 Photographic image of ZnO thin films prepared by different pH of a 8, b 9, c 10, d 11 and e 12 of zinc–ammonia complex solution

the morphological image shows a large distribution of randomly oriented vertical ZnO nanorods perpendicular to the substrate as observed in Fig. 7b. The distributed ZnO nanorods have hexagonal shape with a diameter of about 50–200 nm and a length of about *1.5 lm. These ZnO nanorods have relatively large length to diameter (L/D) ratios than the nanorods that are grown at lower pH growth condition. Further, increase in pH value to 10, results in a characteristic change of surface morphology of the vertical growth with perfect hexagonal shaped rods as shown in Fig. 7c. The diameters of these NRs are around 100–250 nm. This illustrates that it is possible to form oriented ZnO nanorod arrays via an in situ sample deposition process, similar to those formed via complex and complicated approaches. However, an increase in pH of the precursor to 11 (Fig. 7d) evidently shows a uniform and densely packed array of nanorods with a diameter of 200–350 nm and corresponding cross-sectional view clearly exhibits the nanorods grow almost perpendicularly from the ZnO deposited substrates with lengths of 1.3–2 lm. A significant increase in the density of the NRs is observed when the pH condition is increased from 10 to

Fig. 9 Compositional analysis spectra (EDAX) of ZnO thin film for pH of 11

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11. On further increase in higher pH growth condition, the surface morphology shows hexagonal ZnO platelets structure overlapping and grouping each other with diameter up to 500 nm in size as shown in Fig. 7e. At the very early stage of the growth, a large number of tiny hexagonal particles of ZnO would be formed uniformly on the glass substrate and would act as nuclei for the subsequent growth of ZnO rods, thereby facilitating large-scale-oriented growth. There was no further oriented growth of ZnO NRs observed, when the pH of precursor was increased beyond 12. These results indicate that the tailored ZnO nanorods with perfect hexagonal structures have been realized at a pH of 11. The optical images of the ZnO nanostructured thin films deposited at different pH values are shown in Fig. 8a–e. 3.4 Composition analysis Energy dispersive X-ray (EDAX) spectra for ZnO nanostructures are shown in Fig. 9. The characteristic peaks for Zn and O are clearly observed. This spectrum clearly indentified that the material synthesized was ZnO. No impurity peaks were observed, which is a clear indication of the purity of grown ZnO nanostructures. The corresponding weight and atomic percentage of elements were also measured. All the five samples show nearly the same atomic and weight percentages. Excess O may be due to the presence of adsorbed amount which is unavoidable in these studies. 3.5 Optical analysis Figure 10 shows the optical transmittance spectra of ZnO films prepared for different pH values. The transmittance for the film prepared at lower pH value was observed to be the highest (*95 % at 800 nm). The transmittance of the film is observed to decrease with increase in pH value. This

J Mater Sci: Mater Electron (2015) 26:8877–8886

8885

3.30

Band gap (eV)

3.25

3.20

3.15

3.10

3.05 8

9

10

11

12

pH Value

Fig. 10 Optical transmittance spectra of ZnO nanostructures as a function of wavelength prepared from different pH values a 8, b 9, c 10, d 11 and e 12

Fig. 12 The band gap energy of ZnO thin films changing with increasing the pH of the cationic precursor

a¼

n A hc  Eg hc

ð12Þ

where A is constant, Eg is the optical band gap and n assumes the values of 1/2, 2, 3/2 and 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively. The band gap energies of the films were determined by the extrapolation of the linear region on the energy axis (hm), shown in Fig. 11. The optical band gap values determined from Fig. 11 are plotted in Fig. 12 with the pH growth conditions. As can be seen from Fig. 12, the band gap energies decreased with increasing pH value of the precursor solution. The band gap decreases with increase in pH due to the increase of grain size [31]. This decrease in band gap causes a strong red shift in the optical spectra, due to agglomeration of the nanocrystallites into larger crystallites. Fig. 11 The (ahm)2 versus hm curves for the optical band gap determination of ZnO nanorods deposited at pH of a 8, b 9, c 10, d 11 and e 12 of zinc–ammonia complex solution

can be ascribed to the formation of larger particles on the surface of ZnO thin films with increase of pH value which causes scattering of light. The absorption coefficient (a) of the deposited ZnO films has been found to be in the order of 104 cm-1. The study of the absorption coefficient in the fundamental region and near the fundamental edge provides valuable information about the energy band structure of the material. To determine the energy band gap values, (ahm)2 versus hm plots were drawn, where a is the absorption coefficient and hm is the photon energy. The theory of inter band absorption shows that the edge of optical absorption coefficient a varies with the photon energy hm according to Ref. [30].

4 Conclusion In summary, highly oriented, densely packed and size controlled ZnO nanorods were obtained through a simple four step chemical processes by changing the pH of the cationic precursor solution. The XRD results conclude that the unit cell dimension for the films did not suffer significant modifications with respect to the pH value of the used precursor. On the other hand the crystallite size, microstrain, stress, dislocation density and texture coefficient values were influenced by the changes in the pH value of the precursor wherein it was observed that the microstrain and dislocation density values of the structures decreased whereas the crystallite size increased. This could be attributed to the enhancement in the lattice structure of

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ZnO films. The morphology of the ZnO rods confirms that the growth process is controlled by the pH of the reaction mixture and the optimum vertical growth occurs within the pH range of 8–12. It could beconcluded that this kind of morphologies provide for high surface area-dependent applications such as gas sensors, solar cells, solar energyhydrogen conversion devices, photo-electrochemical (PEC) hydrogen generation, etc. The compositional analysis spectra confirmed the presence of Zn and O with proper chemical stoichiometry. The growth of ZnO thin films with excellent crystallinity and surface uniformity was achieved under the optimized Ph condition of 12 ± 0.2 in the bath with a molar concentration of 0.125 M and 50 times dipping. The lower absorption and higher transmittance in the visible region observed in the pH range from 8 to 12 illustrates the good optical quality of the crystals with low scattering and absorption losses which leads to industrial application especially as a transparent electrode. The direct band gap value of the film was found to decrease with increasing pH value of the precursor solution. The film prepared at a pH of 12 shows maximum absorption intensity with more oxygen vacancies and interstitial oxygen defects. Furthermore, the soft chemical route employed is expected to speed up the preparation of high quality ZnO nano structures.

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