Aqueous chemical growth of ZnO disks, rods, spindles and flowers: pH dependency and photoelectrochemical properties

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Solar Energy 85 (2011) 1119–1127 www.elsevier.com/locate/solener

Aqueous chemical growth of ZnO disks, rods, spindles and flowers: pH dependency and photoelectrochemical properties R.C. Pawar, J.S. Shaikh, A.A. Babar, P.M. Dhere, P.S. Patil ⇑ Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416004, M.S., India Received 22 December 2010; received in revised form 11 February 2011; accepted 6 March 2011 Available online 29 March 2011 Communicated by: Associate Editor Igor Tyukhov

Abstract Suitable morphology for fast electron transportation is a crucial requirement for photoelectrochemical (PEC) solar cells. Highly oriented and well defined zinc oxide (ZnO) nano/micro-scale structures were grown on the glass and FTO coated glass substrates. The grown nanostructures have been characterized by X-ray diffraction pattern (XRD), scanning electron microscope (SEM) and optical absorption techniques. XRD patterns confirm high crystalline quality of ZnO with hexagonal wurtzite structure. SEM micrographs show the formation of disk, rod, spindle and flower-like morphologies at different pH values ranging from 5 to 10. The PEC solar cell configuration of ZnO/0.5 M Na2SO4/graphite has been used to record the current voltage (I–V) and capacitance voltage (C–V) characteristics of the films. The junction ideality factor (nl), series and shunt resistance (Rs and Rsh), flat-band-potential (Vfb), donor concentration (ND), fill factor (FF) and efficiency (g) have been estimated. Energy band diagram of ZnO and Na2SO4 electrolyte has been constructed. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Zinc oxide; Photoelectrochemical cells; Capacitance–voltage; Flat-band potential

1. Introduction PEC cell is the most important application to obtain green energy (Pasquier et al., 2006). It exhibits number of advantages such as low cost, simple process, and large-scale production over the silicon solar cells. The most important feature of the PEC solar cell is that it can generate high photovoltage even with impure semiconductors (Dongre and Ramrakhiani, 2009; Hames et al., 2010). So far, porous TiO2 nanoparticles have been mostly used as a photoanode in PEC cells. However, it is difficult to grow TiO2 anisotropically to obtain hierarchical and ordered structures (Baxter and Aydil, 2006). ZnO favors formation of anisotropic structures, exhibits much higher electron mobility than TiO2 (155 cm2 V1 s1 vs. 105 cm2 V1 s1) (Dhas et al., 2008), Debye–Huckel screening length in ZnO is about ⇑ Corresponding author. Tel.: +91 231 2609230; fax: +91 231 2691533.

E-mail address: [email protected] (P.S. Patil). 0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.03.008

4 nm for a carrier concentration of 1018 cm3, higher exciton binding energy (60 meV), high breakdown strength, exciton stability and environmentally friendly material (Wang, 2009; Ahn et al., 2008; Pawar et al., 2010; Cheng et al., 2008). These properties of ZnO make it more desirable for the PEC application. It’s electrical and optical properties are strongly dependent on the shape, size and method used to synthesize ZnO nanostructure (Yang et al., 2002; Lu et al., 2010; Chen et al., 2010). The properties of ZnO can be controlled by tuning the surface morphology. Various architectures have been synthesized using physical and chemical methods that include nanowires, helixes, combs, belts, sheets, needles, rings, tubes etc. (Gao et al., 2005a, 2003; Kong and Wang, 2004; Kong et al., 2004). The techniques used to synthesize ZnO includes physical vapor deposition (PVD), chemical vapor deposition (CVD), molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), thermal evaporation have been used to grow various ZnO

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nanostructures (Pan et al., 2001; Choi et al., 2001; Zhang et al., 2006). However, these methods require high temperature, expensive substrates, high cost, tedious procedures, sophisticated equipments and rigorous experimental conditions. It is necessary to prepare high quality ZnO thin films at relatively low temperature and cost for effective application and commercialization. Aqueous chemical method is the best option due to its simplicity, low processing temperature, cost-effectiveness, ability to tune surface morphology, non-hazardous nature, reproducibility and suitability to deposit large area thin films (Law et al., 2005; Baruah and Dutta, 2009a,b). Up to now nanorods, nanowhiskers, nanocolumns, nanoflowers and nanowires were grown on precoated ZnO seeded substrate using chemical method (Hu et al., 2009; Wang et al., 2004). The morphology of ZnO obtained from aqueous solution is strongly dependent on experimental conditions such as deposition temperature and time, precursor concentration, pH of growth solution and substrate used for deposition (Gao et al., 2005b; Duan et al., 2006; Finnegan et al., 2008). The pH of solution is a crucial parameter to modulate the ZnO surface structure. Few studies have been reported on the pH dependent growth of ZnO by wet chemical route. Vernardou et al. investigated pH effect on the shape of the ZnO nanostructures leading to a modification of the morphology from rod-to-flower-like structures (Vernardou et al., 2007). Samaele et al. observed the blue shift in band gap energy as the pH of solution changes from acidic to basic (Samaele et al., 2010). Baruah and Dutta have demonstrated faster growth of ZnO nanorods in basic medium than that of the acidic medium (Baruah and Dutta, 2009). In the present work, vivid ZnO morphologies such as nanodisk, nanorods, spindles and nanoflowers are obtained merely by adjusting the pH of solution. The grown morphologies are further explored in the PEC cells. 2. Experimental details Ultrasonically cleaned sodalime glass and FTO coated glass were used as substrates. The seed solution was prepared in an absolute ethanol with 0.05 M zinc acetate (Zn(CH3COO)22H2O) and 0.05 M diethanolamine (HN(CH2CH2OH)2, DEA). The substrates were dip coated for 10 s in a seed solution and then kept at room temperature over night for drying. The dried films were annealed at 400 °C for 5 min in air to yield a uniform ZnO seed layer on the substrate. The seeded substrate was further placed vertically in 200 ml solution of 0.05 M zinc acetate and 0.05 M hexamethylenetetramine (HMTA) and refluxed at 95 ± 3 °C for 5 h. The pH of growth solution was varied from 5 to 10 using HCl and ammonia, respectively. The refluxed films were washed with distilled water to get rid of loosely adhered material and then air dried at room temperature for further characterization. The pH of growth solution was monitored by a Trimeter (Jenway, 3510, India). The structural and morphological

characterizations of deposited films were examined by ˚ ) and XRD (Philips, model 3710 with Cr target k = 2.29 A SEM (JEOL, model JSM 6360). The UV–vis absorption spectra were obtained on a (Shimazdu UV 1800 model) spectrophotometer. 2.1. Fabrication of PEC cell PEC measurements were performed in a two-electrode configuration cell. The film (average area 1.0 cm2) and graphite rod (average area 1.0 cm2) were employed as the working and counter electrodes, respectively. The distance between the photoelectrode and counter electrode was 0.5 cm. The redox electrolyte was an aqueous solution of 0.5 M Na2SO4. Sunlux 500/250 V UV lamp was used for illuminating the electrode with a light intensity of 5 mW/ cm2. I–V curves were monitored and recorded in dark as well as under illumination using a computerized Keithley Model 4200 semiconductor characterization system. The log (I) against V plots were used to calculate the junction ideality factor in light. C–V characteristics were measured using a LCR (Applab Model 4912) at 1 kHz with respect to saturated calomel electrode (SCE) as a reference electrode. The Vfb is estimated by extrapolating the C2 vs. V (with respect to SCE) plot to the voltage axis. The films deposited at varies pH viz. 5, 7, 8.5 and 10 are denoted as Z5, Z7, Z8.5 and Z10, respectively. 3. Results and discussion The optical absorption spectra are recorded over 300– 1000 nm. The variations of optical absorbance with wavelength (k) for Z5, Z7, Z8.5 and Z10 samples are shown in Fig. 1a. It is clearly seen that the absorbance decreases with an increase in wavelength, and a sharp decrease in absorbance near the band edge (390 nm) indicating the better crystallinity of the samples. The optical band gap energy (Eg) can be determined by the absorption coefficient (a) hmE n and photon energy (hm) as follows, a ¼ ao ð hm g Þ where ao is a constant, and the exponent n could have the values of 1/2, 3/2, 2 and 3, depending on the type of electronic transition in k-space. For the allowed direct interband transition in a material, the best fit of ahm as a function of hm is obtained with n = 1/2. The value of Eg can be obtained by extrapolation of the linear region of the plot to zero absorption (ahm = 0) (Fig. 1b). From Table 1, it is seen that Eg value varies from 3.03 to 3.27 eV with respect to the morphology. The change in Eg is attributed to the crystallite size of grown structure, and morphologies of the samples (Samaele et al., 2010). The structural changes and identification of phases were studied with the help of XRD technique. XRD patterns of Z5, Z7, Z8.5 and Z10 samples are illustrated in Fig. 2(a–d). It is observed that the Z5, Z7, and Z10 samples exhibit a single peak along (002) plane close to 52.11° indicating perfectly c-axis oriented ZnO nanostructures. However Z8.5 sample exhibits peaks along (101), (002), (011), and (110)

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Fig. 1. (a) UV–vis absorption spectra of Z5, Z7, Z8.5 and Z10 films. (b) Optical bandgap spectra of the Z5, Z7, Z8.5 and Z10 films in the energy range between 1.5 and 4 eV.

Table 1 The values of solar cell parameters for different morphologies. pH

Morpho-logy

Isc (lA/cm2)

Voc (mV)

Im (lA/cm2)

Vm (mV)

FF

g (%)

Eg (eV)

Vfb (V)

Rs (KO)

Rsh (KO)

n

5 7 8.5 10

Disk Nanorod Spindle Nano-flower

82 129 41 199

352 318 376 335

53 72 29 128

176 184 228 140

0.33 0.31 0.43 0.27

0.19 0.25 0.13 0.36

3.15 3.09 3.27 3.03

0.78 0.72 0.95 0.74

0.259 0.134 0.821 0.108

5.23 3.36 6.31 3.33

1.6 1.8 1.3 2.4

planes indicating polycrystalline phase of ZnO nanostructure. The comparison of observed XRD patterns with the standard JCPDS data (76-0704) confirms the formation of ZnO having a hexagonal wurtzite crystal structure. The average crystallite size is estimated using Scherrer’s formula (Baeza et al., 2006): t¼

0:9k bCOShB

ð1Þ

where b is FWHM (in radians) of the diffracted peak 2hB, hB ˚ (Cr Ka). The calculated is the Bragg’s angle, and k = 2.29 A crystallite sizes are found to be in between 25 and 45 nm. The morphologies of the various nanostructures grown under the different reaction conditions are examined using SEM. The nanodisks (NDs), nanorods (NRs), spindles (SPs) and nanoflowers (NFs) like nanostructures are obtained at different pH values. The morphology transformation from NDs to NFs is observed with the pH variation of solution. Thermodynamically stable phase for ZnO is zincite with the wurtzite crystal structure. It can be described as a number of alternating planes composed of tetrahedrally coordinated oxygen anions and zinc cations stacked alternately along the c-axis. The oppositely charged ions produce negatively charges (0001)-O and positively charges (0001)-Zn surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis as well as a variance in surface energy. There are

two other most commonly observed surfaces ð 2 110Þ and  ð0110Þfor ZnO, which are non-polar and have lower surface energy than the (0001). Due to the three fastest growth directions along ð2110Þ ð0110Þ and ±(0001) planes, ZnO exhibits variety of nanostructures (Xiao et al., 2008; Wu et al., 2008). The relative growth rate of these planes will determine the final shape and size of structure. The ZnO NRs are grown from the bath containing equimolar aqueous solutions of zinc acetate and HMTA at 7 pH. Highly uniform and densely packed arrays of NRs (Fig. 3a) with an average 130 nm diameter are formed on the substrate. Inset shows the uniform coverage of NRs over the entire substrate. HMTA is a water soluble, non-ionic tetradentate cyclic tertiary amine and releases OH ions at elevated temperature to grow the ZnO NRs. It also acts a non-polar chelating agent and will attach preferentially to the nonpolar surface of zincite crystal and promotes vertical growth along (0002) direction (Zhang et al., 2005). The reaction mechanism is summarized as follows; ðCH2 Þ6 N4 þ 6H2 O ! 6HCHO þ 4NH3  NH3 þ H2 O $ NHþ 4 þ OH

Zn2þ þ 2OH ! ZnðOHÞ2 D

ZnðOHÞ2 ! ZnOð sÞ þ H2 O  90 C

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Fig. 2. XRD patterns of the ZnO films deposited at various pH of growth solution – (a) Z5, (b) Z7, (c) Z8.5 and (d) Z10.

Finally Zn(OH)2 transforms into ZnO via dehydration reaction at 90 °C. As the pH of the growth solution is adjusted from 7 to 5 using dilute HCl, the sporadically distributed hexagonal disk with an average 5 lm diameter are formed on the substrate (Fig. 3b). Inset shows the coverage of NDs on the entire substrate. Formation of disk like morphology is attributed to the strong adsorption of Cl ions on the (0001)-Zn positively charged surface, which reduces the charge density of polar surface. The Cl adsorption stabilizes (0001) surface and promote the growth along longitudinal direction (Zaera et al., 2007). Hence the disk like structure is formed under the acidic condition of growth solution. Further pH of solution is increased up to 8.5 using the aqueous ammonia. The spindle like nanostructure is formed on the substrate (Fig. 3c). The size of the spindles is in the range of 250–400 nm. Here, ammonia plays a crucial role in the formation of spindle like structure. It provides OH ions to form ZnO crystals and also binds metal ions in the solution. At the same time, HMTA supply OH ions into the solution. Hence the

concentration of OH ions causes formation of primary ZnO nanocrystallites in the solution. These crystallites rapidly assemble to give highly stable and larger aggregates. The secondary aggregates take the shape of spindle like nanostructures. The NFs like structure is grown at 10 pH of solution (Fig. 3d). Inset shows the magnified image of a single NF. These are composed of tapered rods with its root size of 500–700 nm and average tip size of 125–150 nm. Formation mechanism of NFs can be explained by precipitation of Zn(OH)2. At higher pH, Zn(OH)2 dissolves and forms Zn2+ and OH ions in the solution. Hence the concentration of OH ions exceeds the critical value in the solution which forms NFs like structure. Thus OH ions play a crucial role in the formation of NFs (Li et al., 2007). I–V characteristics of the PEC cell formed with n-ZnO samples are studied in the voltage range of ±0.8 V, in dark and under light illumination. In dark, I–V characteristics showed the rectifying behavior. Under illumination of UV light, curves are shifted to the fourth quadrant indicat-

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Fig. 3. SEM images of all the films deposited at various pH of growth solution – (a) Z7, (b) Z5, (c) Z8.5 and (d) Z10.

ing that the samples are a generator of electricity. The results are good agreement with the literature (Inamdar et al., 2008). Using a well known diode equation, junction ideality factor is calculated as: I ¼ I 0e

eV ln KT

ð2Þ

where I is the forward current in dark, I0 the reverse saturation current, V the applied forward bias voltage and n is the junction ideality factor. The estimated values of n are as given in Table 1. It is seen that the values of junction ideality factors are higher than the ideality value. Fig. 4(a– d) shows photovoltaic I–V curves for Z5, Z7, Z8.5 and Z10 samples. Inset shows the semilog plots of dark and photo I–V characteristics. The g is calculated using the following equation (Pawar et al., 2008): g¼

V OC I SC FF  100 P input

ð3Þ

where Isc is the short circuit current and Voc is the open circuit voltage, Pinput is the input light energy, FF is the fill factor. The calculated efficiencies are given in the Table 1. The Rs and Rsh are estimated from slope of the I–V curves using relation (Yadav et al., 2008):



dI dV



dI dV

 ffi

1 Rs

ð4Þ

ffi

1 Rs h

ð5Þ

I¼0

 V ¼0

The obtained g, Rs and Rsh are given in Table 1. It is seen that the NFs exhibits higher power conversion efficiency as compared to other morphologies. This is attributed to the increase in surface area and fast electron transportation in NFs. The measurements of capacitance as a function of applied voltage provided useful information such as type of conductivity, depletion layer width and Vfb. The Vfb of a semiconductor gives information of the relative position of the Fermi levels in photoelectrode as well as the influence of electrolyte and charge transfer process across the junction. The Vfb is obtained by using Mott–Schottky (M–S) relation by standardizing with SCE (Djellal et al., 2008):   2 KT 2 C ¼ V  V fb  ð6Þ qeeo N d q

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Fig. 4. I–V characteristics for ZnO/Na2SO4 PEC cell in dark and under illumination – (a) Z5, (b) Z7, (c) Z8.5 and (d) Z10.

where eo is the permittivity of free space, e the permittivity of the semiconductor electrode, e the charge on the carriers, ND the donor concentration, T the temperature of operation 300 K, kB the Boltzmann’s constant, and CSC is the space charge capacitance. M–S plots are constructed using C2 against applied bias voltage (with respect to SCE) for all ZnO PEC cell (Fig. 5a–d). The positive slope of the M–S plot confirms n-type conductivity of the ZnO films. The intercept of the linear plot (1/C2 = 0) is taken as an electrode potential for semiconductor at which band bending is zero. This potential is the flat-band potential (Vfb). The Vfb values for Z5, Z7, Z8.5 and Z10 samples are given in Table 2. A slight change in Vfb is due to the variation in reverse saturation current. Inset shows the C–V plots (Fig. 5a–d) of samples. The values of ND are calculated from the slope of M–S plot (Table 2). It is observed that, the Z10 exhibits higher ND = 1.55  1017 m3 as compared with Z5, Z7, and Z8.5 samples. The density of states in conduction band (Nc) of semiconductor is given by relation:

NC ¼ 2

 3=2 2pme KT h2

ð7Þ

It is found to be 5.17  1024 m3, where me (0.35 mo) is the affective mass for ZnO (Ozgur et al., 2005). The Fermi level in semiconductor is related to the donor concentration (ND) and Nc by the relation (Sawant et al., 2007; Kavasoglu et al., 2008):   ðEC  EF Þ N D ¼ N C exp ð8Þ KT Therefore,

  ND EC  EF ¼ KT ln NC

ð9Þ

The EC level of ZnO is situated above EF by an amount 0.09 eV. The valence band edge must be below the conduction band edge by an amount equal to the band gap energy of ZnO (3.03 eV). All the parameters are calculated by using Eqs. (7)–(9) for Z5, Z7, Z8.5 and Z10 samples. The

R.C. Pawar et al. / Solar Energy 85 (2011) 1119–1127 2500

(a) 2000

5000

-2

1/C (µ F )

4000 3000 2000 1000

1500

0.035

0.030

0.025

0.020

2

0.020

Capacitance (µ F-2)

-2 2

(b)

0.040

Capacitance (µF-2)

6000

1/C (µ F )

1125

0.018

-0.66

-0.63

-0.60

-0.57

-0.54

Voltage (V)

1000

0.016

500

0.014

0.012 -0.72

0 -0.80

-0.68

-0.64

-0.60

Voltage (V)

-0.75

-0.70

-0.65

-0.60

0 -0.80

-0.55

-0.75

-0.70

7000

(c)

-0.55

(d)

6000

7000

5000

4000

4000

2

-2

5000

2

-2

1/C (µ F )

6000

-2

3000 2000

0.014

2000

0.013 0.012

1000

-0.80

-0.8

0.014 0.013

-0.66

-0.76

-0.72

-0.68

-0.64

0 -0.80

-0.60

Voltage (V)

-0.9

0.015

0.012

0.011

1000 0 -1.0

0.016

3000

Capacitance ( µ F-2)

0.015

Capacitance ( µ F )

1/C (µ F )

-0.60

Voltage (V)

Voltage (V)

8000

-0.65

-0.7

-0.64

-0.62

-0.60

-0.58

Voltage (V)

-0.75

-0.70

-0.65

-0.60

-0.55

-0.6

Voltage (V)

Voltage (V)

Fig. 5. M–S plots were measured with respect to SCE as a reference electrode for the PEC cell formed with ZnO thin films – (a) Z5, (b) Z7, (c) Z8.5 and (d) Z10.

Table 2 Various physical parameters of different morphologies calculated from M–S plots. Physical parameters

Z5

Z7

Z8.5

Z10

Electrolyte Ef, redox (V) Vfb (V) Donor concentration ND (m3) Density of states in conduction band NC (m3) Ec  Ef (eV) Band bending Vbb (eV) Barrier height, VB (eV) ˚) Depletion width (A

Na2SO4 0.23 0.78 0.92  1017 5.17  1024 0.104 1.01 0.77 102

Na2SO4 0.23 0.72 1.36  1017 5.17  1024 0.097 0.95 0.86 81

Na2SO4 0.23 0.95 0.71  1017 5.17  1024 0.110 1.18 1.06 126

Na2SO4 0.23 0.74 1.55  1017 5.17  1024 0.091 0.97 0.88 59

band diagram is drawn from the obtained values (Fig. 6a and b). The amount of equilibrium band bending (Vbb) at the semiconductor/electrolyte interface is obtained by the difference of the redox Fermi potential (Vf,redox) of the electrolyte, and Vfb of the semiconductor in that electrolyte (Sawant et al., 2010):

V bb ¼ V f ;redox  V fb ¼ 0:23  ð0:74Þ ¼ 0:97V

ð10Þ

The barrier height VB is calculated using: V B ¼ qeV bb þ ðEC  EF Þ

ð11Þ

VB is estimated to be 0.88 eV for flower-like structure. Based on these parameters, width of depletion layer is calculated by the relation:

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“Research Fellowship in Science for Meritorious students to promote quality research in universities” under the UGC-DSA–I programme. The authors are also thankful to Dr. K.Y. Rajpure for the valuable discussion. References

Fig. 6. Band diagram of ZnO flower structure and Redox energy levels – (a) Before contact and (b) after contact.

 1=2 V bb W ¼ 2eo es eN D

ð12Þ

All the physical parameters calculated from M–S plot are given in Table 2. The Voc is given by relation: V OC ¼ V fb þ

EF ;redox e

ð13Þ

However, the observed value of VOC is lower than the theoretical value. This is attributed to the electron-hole recombination in bulk, surface recombination at electron-hole traps and surface states (Chandra, 1985). 4. Conclusions ZnO samples with various interesting morphologies were synthesized by a facile aqueous chemical method at relatively low temperature. Samples having disk, rods and flower-like structure are single crystalline, while spindle like structure is polycrystalline in nature with wurtzite crystal structure. Interesting morphological transformations from rod-to-disk-to-spindle-to-flower have been observed merely by varying the pH of growth solution. PEC performance of ZnO samples with their different morphologies was investigated in 0.5 M Na2SO4 electrolyte. Morphology plays a decisive role in determining efficiency of the PEC cells; it is higher (g = 0.36%) for NFs like morphology, as a result of fast electron transportation and improvement in surface area. The energy band diagram of ZnO/Na2SO4 heterojunction has been proposed for NFs structure. Acknowledgements One of the authors Mr. R.C. Pawar is thankful to University Grants Commission, New Delhi, for awarding the

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