Optical, structural, and photoelectrochemical

J Solid State Electrochem DOI 10.1007/s10008-016-3190-y

ORIGINAL PAPER

Optical, structural, and photoelectrochemical properties of nanostructured ln-doped ZnO via electrodepositing method Abdellah Henni 1,2 & Abdallah Merrouche 1 & Laid Telli 1 & Amina Karar 3 & Fabian I. Ezema 4 & Hichem Haffar 1

Received: 23 September 2015 / Revised: 17 March 2016 / Accepted: 21 March 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Indium-doped zinc oxide nanorods were electrochemically deposited at low temperature on ITO substrates. The synthesized ZnO-arrayed layers were investigated by using X-ray diffraction, scanning electron microscopy, UV– vis transmittance, electrochemical impedance spectroscopy, and photocurrent spectroscopy. X-ray diffraction analysis demonstrates that the electrodeposited films are crystalline and present the hexagonal Würtzite ZnO phase with preferential (002) orientation. The ZnO films obtained forms aligned hexagonal nanorods, and depending on the increasing In concentration, the surface morphologies of the films are changed. The ln-doped ZnO nanorods (NRs) are well-aligned with the c-axis being perpendicular to the substrates when the ln concentration was between 0 and 2 at.%. of In, the grown films with In contents up to 4 at.%, changes in the optical band gap from 3.31 to 3.39 eV, and the blue shift in the band gap energy was attributed to the Burstein–Moss effect. The effect of In concentration on the photocurrent generated by films shows that the obtained thin films can be used as a photovoltaic material. Changes in the photocurrent response and the electronic disorder were also discussed in the light of In doping. It was found that the carrier density of IZO thin films varied

* Abdellah Henni [email protected]

1

Laboratoire des Matériaux Inorganiques, Université Mohamed Boudiaf, M’Sila 28000, Algeria

2

Université de Kasdi Merbeh, Ouargla 30000, Algeria

3

Laboratoire d’Energétique et d’Electrochimie du Solide, Université Ferhat Abbas, Sétif 1, 19000, Algeria

4

Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria

between 1.06 × 1018 and 1.88 × 1018 cm−3 when the In concentration was between 0 and 4 at.%. Keywords Electrodeposition . In-doped ZnO . Thin films . Photocurrent . Semiconductors

Introduction There exists a great interest in zinc oxide (ZnO) materials because of their usefulness in a wide range of high technology applications, resource availability, non-toxicity, and very high thermal/chemical stability [1]. The concerns in ZnO are fed by its prospects in flat panel displays, solar cells, and commercial significance in other different optoelectronic devices [2–6] through its direct band gap width being equal to 3.37 eV and its large exciton binding energy of 60 meV. Therefore, these properties of ZnO thin films can be modified by doping elements such as Al, Ga, Mg, Cd, In, or Ca [7–9]. The modifications are intended to search for new materials that are transparent conducting oxide (TCO) to serve as a replacement for ITO electrodes for applications in optoelectronics since In, an essential component of ITO, is costly as well as scare [8]. Among the considered replacement for TCOs, indium-doped zinc oxide (IZO) stands tall due to its high structural stability at high temperatures, with excellent electrical and optical properties [10, 11]. Various techniques have been used to deposit IZO which include radio frequency (RF) sputtering, pulsed laser deposition, spraying pyrolysis, sol–gel, chemical vapor deposition (CVD), atomic layer deposition (ALD), and electrodeposition [12–20]. The content of In in the ZnO thin films affects the room temperature resistivity as well as the optical properties as noted by several authors [21, 22]. It is reported that indium is the

J Solid State Electrochem

Experimental Electrodeposition of ZnO has been made in a threeelectrode cell, using the method based on the reduction of hydrogen peroxide (H2O2). The electrodes consisted in a saturated calomel electrode (SCE +0.241 V vs. SHE) as reference, and the Pt spiral wire and ITO (active surface area = 1 cm 2) were used as counter and working electrodes, respectively. Before the electrodeposition, the ITO glass was successively cleaned in an ultrasonic bath using acetone, ethanol, and ultrahigh purity water. The working electrode was fixed vertically equidistant from the counter and reference electrodes. Electrodeposition has been carried out without stirring at about 65 °C in a potentiostatic mode at −1.0 V vs. SCE during 40 min, with a computer-controlled potentiostat/galvanostat (Autolab PGSTAT 302 N) [29]. The initial aqueous solution containing was 0.1 M KCl

i

b

t

Fig. 1 Growth curves at constant applied potential of −1.0 V vs. SCE for pure and In-doped ZnO on ITO. a 0, b 0.5, c 1, d 2, and e 4 at.% of In. T = 65 °C

IZO (4%) IZO (2% ) IZO (1%)

Intensity (a.u)

most suitable element as the dopant of ZnO. Peng et al. [23] were prepared highly transparent conductive ZnO/ In thin films on quartz glass substrates by RF magnetron sputtering method, and the lowest resistivity obtained is 2.4 × 10−3 Ω cm when the substrate temperature is 250 °C. However, doping of In in ZnO thin films reveals various issues such as degradation of ZnO crystallinity. With increasing In doping from 0.2 to 2.0 wt% [24], the preferential growth of crystallites with the (002) plane perpendicular to the substrate diminished and the ordered arrangement of crystallites decreased. Illiberi et al. and Ben-Yaacov et al. reported that incorporation of In into ZnO first increased carrier density, but it later decreased with further increases in indium addition over 5.2 % of In / [In + Zn] [25] and 3.7 × 1019 cm−3 [26]. Excess In incorporation may induce the creation of additional defects in the ZnO films, which act as carrier traps [25]. In this study, the IZO films have been deposited by electrodeposition method. This method has advantages such as the low cost, low operation temperature, largescale deposition, and ease of mass production [27–29]. Moreover, the preparation of IZO via electrodeposition is also eco-friendly since there is no usage of toxic chemicals in the electrolytic bath. Here, we focus on the effect of indium incorporation on the electrochemical, morphologies, microstructures, and optical properties of ZnO nanostructures. Previous work has focused on the use of molecular oxygen, sulfate, or nitrate as precursors of hydroxide for electrodeposition of ZnO and doped ZnO [30–32]. This study investigates the influences of indium concentration on ZnO properties using the hydrogen peroxide as precursor.

IZO (0.5 %)

IZO (0 %)

10

20

30

40

2 (°)

50

60

70

Fig. 2 XRD patterns of the pure and In-doped ZnO nanostructured on ITO substrates


(a)

(b)

(d)

(e)

(c)

Fig. 3 Effect of In concentration on electrodeposited ZnO film morphology. a 0, b 0.5, c 1, d 2, and e 4 at.%

(Fluka, purity > 99.5 %), 5 × 10−3 M ZnCl2 (Fluka, purity > 98.0 %) and 5 × 10−3 H2O2 (Biochem, 30 % )[33], and was not stirred during the electrodeposition. The pH of electrolyte used in this deposition was in the range of 6 to 6.5. Different concentrations of the doping compound lnCl3 in the electrolyte are explored: 0, 0.5, 1, 2, and 4 at.%, respectively. For clarity, the doped proportion of In3+ is labeled as IZO (x%), where x = ln / (ln + Zn) (at.%). The IZO film/electrolyte capacitance and photocurrent have been performed in the electrochemical cell with the same device (Autolab PGSTAT 302 N). X-ray diffraction (XRD) measurements were performed by using a Bruker AXS D8 Advance X-ray diffractometer with CuKα irradiation at λ = 1.5406 Å. Scanning electron microscope (SEM) (JEOL JSM-7001 F) was applied for morphological study. The optical transmittance was acq u i r e d i n a S h i m a d z u ( U V- 1 8 0 0 ) U V – v i s Spectrophotometer using a clean ITO substrate as reference. Electrochemical impedance spectroscopy (EIS) and photocurrent spectroscopy (PCS) measurements were performed in an electrochemical cell with the IZO films

Fig. 4 The illustration of the growth process of In-doped ZnO using 4 at.% of In

placed at the bottom and exposing to the electrolyte an area of 1 cm2. Impedance versus potential measurements were done and represented as Mott–Schottky plots (1/C2 versus potential) to calculate the flat band potential (Efb) and estimate the carrier density (ND). Mott–Schottky curves have been drawn in the range of potentials values from −1.30 to 1.30 V vs. SCE at a frequency of 200 Hz. The photoelectrochemical measurements were recorded at fixed potentials between 0.2 and 0.8 V by illuminating the sample from the solution side in an intermittent mode using 20 s on/off cycles using an UV lamp.

Results and discussion Electrochemical deposition and Structural studies The mechanism of electrodepositing ZnO from an aqueous solution containing Zn2+ ions is the reduction of aperoxide hydrogen precursor at the interface of electrode and precursor solutions. The generated hydroxide ions then react chemically


ð2Þ

Assuming that ln/Zn ratio is in an appropriate range, it is expected that during annealing, In3+ ions diffuse into the ZnO lattice and some of the Zn2+ sites are replaced by In3+. From the point defect theory, In3+ ion substituted in the crystal lattice acts as an impurity and forms the point defect. Figure 1 shows the current transient recorded at −1.0 V vs SCE upon nanowire deposition on ITO substrate. At the beginning of each curve, a decrease in current appears which corresponds to the charge of the double layer and the time required for the formation of the first germs on the active sites of the surface. Next, the current increases under the effect of the increase of germs to a peak, and then it decreases to the limit current indicating the response of an electrochemical system under linear diffusional control. The curves confirm that the ECD ZnO are good electrical conductors, because the current density collected at the electrode is kept at about −0.9, −1.1, −1.3, −1.4, and −1.7 mA cm−2 after 1500 s of growth for pure ZnO and IZO (0.5, 1, 2, 4 at.%) baths, respectively. The curves confirm the improved electrocatalytic properties of ZnO in the presence of indium. The films obtained are adherent and homogeneous. The one advantage of this method is that a fully crystalline material can be directly obtained without the need for further heat treatment because the electrodeposition was carried out at 65 °C. Figure 2 presents the evolution of the X-ray diffractograms of the lZO films obtained after 40 min of electrodeposition. Only ITO peaks (JCPDS no. 41–1445) and ZnO peaks (JCPDS no. 36–1451) in the Würtzite form were detected, the XRD shows overexpression of the (002) plane suggesting that the crystalline grains are mainly oriented along the c-axis normal to the ITO substrate surface. Since no indium-related peaks were observed after electrodeposition in XRD patterns, the formation of In-based clusters on ZnO can be excluded within the detection limit. This means that indium does not change the Würtzite structure of ZnO.

(a) IZO (0%) IZO (0.5%) -2

In3þ þ 3OH − →lnðOH Þ3 →In2 O3

IZO (1%)

2

From the thermodynamical point of view, the theoretical electrochemical potential (Ecp) window for zinc oxide deposition from hydrogen peroxide is very large, since the standard potential for metallic zinc deposition is −0.76 V vs. SHE [34]. For the solution that contains In3+ ions, similar reaction will occur and the hydroxide decomposes to the corresponding oxide during deposition or during post-deposition annealing as in Eq. (2).

IZO (2%)

2

ð1Þ

Figure 3 shows the SEM images of pure and In-doped ZnO nanorods (NRs) grown on ITO substrates. According to our experimental results, the most important conditions of synthesis that contribute to the variations of ln-doped ZnO films’ morphology are related to In3+ at.% in aqueous solution baths. IZO obtained at In3+ ≤2 at.% forms aligned hexagonal nanorods with the c-axis preferred orientation perpendicular to the substrate. With the diameters ranging from 160 to 250 nm (Fig. 3a–d), these results are similar with other results [35]. The increase in In doping from 0.5 to 2 at.% leads to the increase of particle size and around-shape barsis obtained for the samples obtained at 4 at.%. The illustration for the growth

IZO (4%)

( h ) (eV cm )

Zn2þ þ 2OH − →ZnðOH Þ2 →ZnO þ H 2 O

Morphological studies

1.6

2.0

2.4

2.8

3.2

3.6

4.0

Energy (eV) 3.40

(b) 3.38

Eg (eV)

with the Zn2+ ions in the solution to form zinc hydroxide depositing at the cathode. The chemical reaction can be expressed as follows:

3.36 3.34 3.32 3.30

0

2

4

In concentration (at. %) Fig. 5 a (αhυ)2 vs. energy dependence for the determination of the optical band gap energy. b The band gap values obtained by extrapolating the linear part of the curves are also shown


24

15 12

where A is a constant and hυ is the photon energy. From Fig. 5a, it is clear that the optical absorption edge has a blue shift to the higher photon energy region with the increase in In doping. This blue shift is associated with the increase in carrier concentration corresponding to a shift of the Fermi level within the conduction band which is well known as the Burstein– Moss effect [37, 38]. Considering this effect, the variation of

9 6 3 0

a)

Efb

-3

-1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5

E (V vs. SCE) Fig. 6 Mott-Schottky plots of the ZnO layers deposited at different In concentrations recorded at 0.2 kHz

of spherical shape clusters is shown in Fig. 4. The morphological characteristics of nanorods, such as diameter, height, and density, are changed with the In concentration during electrodeposition of IZO that influence the formation of nucleation sites, the direction, and the rate of growth nanorods. In fact, the density of rods is an important characteristic of the films. It depends on the density of sites energetically favorable for the nucleation, in which the density of NRs varies significantly with In concentration. The mean density of NRs of ZnO layers obtained is of the order of 22 × 108 NRs/cm2.The nanorods density is 19 × 108 NRs/cm2 for arrays deposited with 2 at.% of In, which decreases when the In concentration increases.

Optical and electronic characterization The Tauc plots [36] shown in Fig. 5 were performed using the transmission data obtained from each sample of IZO. The extrapolation of theses plots with photon energy axis leads an estimation of the optical band gap energy (Eg) of the

Table 1 Displays effect of the ln doping concentration on carrier density, thickness, and optical properties IZO (at.%)

Thickness (nm)

ND (cm−3)

Eg (eV)

0 0.5 1 2 4

592 608 596 579 473

1.06 × 1018 1.26 × 1018 1.34 × 1018 1.60 × 1018 1.88 × 1018

3.31 3.34 3.36 3.37 3.39

(a) IZO (0%) (b) IZO (0.5%) (c) IZO (1%) . (d) IZO (2%) (e) IZO (4%)

Photocurrent (µA)

-2

10

-2

C /10 (F )

18

.

.

.

.

.

.

.

t 100

b)

IZO (0%) IZO (0.5%) IZO (1%) IZO (2%) IZO (4%)

80

Photocurrent (µA)

21

corresponding sample. As we know, Eg is related to the absorption coefficient (α) as follows: ðαhυÞ2 ¼ A hυ−Eg ð3Þ

IZO (0%) IZO (0.5%) IZO (1%) IZO (2%) IZO (4%)

60

40

20

0

-0.2

0,0

0.2

0.4

0.6

E (V vs. SCE)

0.8

1.0

Fig. 7 a Photocurrent response of IZO samples; a 0, b 0.5, c 1, d 2, and e 4 at.% of In. b Photocurrent amplitude versus the applied potential for the same samples


the Eg with doping concentration is plotted in Fig. 5b. From the plot, it is observed that the band gap increases from a value of 3.31 eV for undoped ZnO to a value of 3.39 eV as the In doping concentration increases to 4 %. The IZO carrier concentration was estimated from Mott– Schottky (MS) measurements. This method is based on the Schottky barrier formation between the semiconductor material and an electrolyte. The technique involves measuring the capacitance of the space charge region (Csc) as a function of electrode potential under depletion condition and is based on the MS relation [39]: 1 2 kT E−E f b − ð4Þ ¼ q C 2SC qA2 N D εε0 In this equation, CSC represents the space charge capacitance, ε is the dielectric constant (8.5 for ZnO), ε0 is the permittivity of free space (8.85 × 10−14 F/cm), ND is the carrier concentration, Efb is the flat band potential, k is Boltzmann’s constant (1.38 × 10−23 J/K), T is the absolute temperature (298 K), and q is the elementary electron charge (1.6021 × 10−19 C). Figure 6 displays the Mott–Schottky curves obtained for the undoped and doped ZnO thin films with ln at different concentrations. According these curves, a linear relationship of C−2sc vs. E was observed. All the samples exhibited positive slopes, indicating n-type semiconductor characteristic. It is known from earlier reports that ZnO and doped ZnO films are n-type semiconductors [40, 41]. The potential at which the line intersects the potential axis gives the flat band potential (Efb), and the slope yields the carrier concentration (ND) of the sample. Thus, from Fig. 6, a Efb = 0.12 ± 0.02 V was obtained for all samples, and this values can be attributed to the observed changes in the IZO diameter and length within the In doping that would affect space charge layer width [42]. The carrier density fluctuates from 1.06 × 1018 to 1.88 × 1018 cm−3, and similar results have been observed when the aluminum is used as doping [43, 44]. Indium concentration is a major parameter that acts not only on the dimensions of the NRs but also on the donor density of the ZnO. The ND values found at different ln concentrations of IZO are shown in Table 1.

current. Meanwhile, we observe a positive photo-generated current which is a characteristic of an n-type semiconductor. The observed photocurrent could be explained by the accumulation of charges at the interface involving electrodes promoted by the injection of the minor photo carriers (h + *). The amplitude of the photocurrent increases steadily from undoped ZnO to the IZO film, going from 18 to 44 mA at 1.0 V. We have observed such behavior with Al-doped ZnO [43]. Nevertheless, an explanation could be related to the in photocurrents that increase with the donor density as mentioned before and the increase in electrical conductivity brought in by the Indium doping.

Conclusion An effective method for the electrodeposition of uniform, adherent, and crystalline n-type ZnO and IZO thin films on ITO substrates has been developed. The effects of indium concentration on IZO on the films structural, morphology, photocurrent, optical, and electrical properties are investigated. IZO exhibits aligned nanorods arrays synthesized directly on ITO substrates with diameters between 160 and 250 nm for sample obtained at In ≤ 2 at.%, but a spherical form is obtained for higher concentration of In, and IZO crystallizes in single phase with Würtzite structure. The microstructural and functional properties of the obtained IZO are strongly affected by the In concentration during the electrodeposition. The photoelectrochemical tests demonstrate clearly the ability of IZO nanostructured to generate significant photocurrents under UV light. Band gap measurements using Tauc plots from UV spectroscopy showed that the energy gap varies slightly with the In in range between 3.10 and 3.90 eV which agrees satisfactorily with report data. Acknowledgments We would like to thank Pr. Federico Rosei and Dr. Riad Nechache of Energy Materials Telecommunications center of INRS Research University in Quebec for their assistance in acquiring the SEM images.

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