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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.30 No.8 pp.399-405
DOI : https://doi.org/10.3740/MRSK.2020.30.8.399

# Effect of Al Doping on the Properties of ZnO Nanorods Synthesized by Hydrothermal Growth for Gas Sensor Applications

Vibha Srivastava, Eadi Sunil Babu, Soon–Ku Hong†
Department of Materials Science Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
Corresponding author E-Mail : soonku@cnu.ac.kr (S. K. Hong, Chungnam Nat‘l Univ.)
April 29, 2020 July 16, 2020 July 20, 2020

## Abstract

In the present investigation we show the effect of Al doping on the length, size, shape, morphology, and sensing property of ZnO nanorods. Effect of Al doping ultimately leads to tuning of electrical and optical properties of ZnO nanorods. Undoped and Al-doped well aligned ZnO nanorods are grown on sputtered ZnO/SiO2/Si (100) pre-grown seed layer substrates by hydrothermal method. The molar ratio of dopant (aluminium nitrate) in the solution, [Al/Zn], is varied from 0.1 % to 3 %. To extract structural and microstructural information we employ field emission scanning electron microscopy and X-ray diffraction techniques. The prepared ZnO nanorods show preferred orientation of ZnO <0001> and are well aligned vertically. The effects of Al doping on the electrical and optical properties are observed by Hall measurement and photoluminescence spectroscopy, respectively, at room temperature. We observe that the diameter and resistivity of the nanorods reach their lowest levels, the carrier concentration becomes high, and emission peak tends to approach the band edge emission of ZnO around 0.5% of Al doping. Sensing behavior of the grown ZnO nanorod samples is tested for H2 gas. The 0.5 mol% Al-doped sample shows highest sensitivity values of ~ 60% at 250 °C and ~ 50% at 220 °C.

## 초록

Chungnam National University
2019-0834-01

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Fabrication of optoelectronic components which can be integrated with conventional silicon devices is in huge demand, since silicon can be grown in larger size at lower cost.1-3) In general, position-controlled vertical arrays of One-dimensional (1D) nanostructure on Si substrates offer ideal geometry for their use as a functional component in Si-based electronics and photonic nanodevices.4-9) 1D zinc oxide (ZnO) nanomaterials have received broad attention due to its potential applications in optoelectronic devices and semiconducting gas sensors. For such applications of ZnO nanowires or nanorods, conductivity control by doping is an important issue. Although p-type doping of ZnO has not been achieved yet in a reproducible manner,10,11) efforts for n-type doping are underway to improve the properties of ZnO nanostructures by doping various chemical elements such as Ga,12-15) In,14) Sn,14,16) Mn,17) Mg,18) Bi,19) and Al.20-24)

Al doped wurtzite ZnO nanorods with c-axis oriented crystalline structure can be used for potential device in broadband ultra-violet (UV) photo-detectors with high tuneable wavelength resolution. As observed by B. E. Sernelius25) the optical bandgap widens in proportion of Al-doping concentration and K. Tominaga26) have shown substantial improvement in the electrical conductivity, charge carrier density, and the mobility in the Al-doped films. Highest mobility has been reported for Al-doping concentration of 2 ~ 3 % by J. Ma27) and S. Zafar.28)

Recently nanostructures of ZnO have been tried as gas sensors by many investigators. ZnO nanowires were used as oxygen sensors by Y. Chiou29) and ZnO nanorods coated with palladium were also used to detect H2.30) It is also known that in the case of doped ZnO gas sensors doping lowers operating temperature apart from enhancing the selectivity and response to gas. In general nanostructure of ZnO is preferred due to its large surface area which in-turn shows high sensor signals and fast responses to sensing gases.

ZnO nanorods and nanowire have been prepared by any one of the following methods such as catalytic growth,31-34) high temperature physical evaporation, chemical vapor deposition,35) metal-organic vapor-phase epitaxy,36) molecular beam epitaxy,37) high temperature decomposition,38) hydrothermal growth (HG)39) process and various solution methods.40,41) Among these methods, HG is the simplest process that can be applied to large area substrate. M. N. Islam42) have reported that Al chemically binds with oxygen and the operating temperature of ZnO gas sensor can be lowered because Al doing reduces particle size of ZnO nano-structures.43)

In this study we have shown effect of Al doping concentration on the length, size, electrical properties, optical properties, and H2 gas sensitivity of ZnO nanorods grown by HG process. The prepared ZnO nanorods have been characterized by X-ray diffractometer (XRD) for extracting structural and microstructural information, and field emission scanning electron microscope (FE-SEM) for getting details of surface morphology. Electrical property has been observed by Hall measurements and optical property is measured by photoluminescence (PL). H2 gas sensing behavior of the Al doped ZnO nanorods is also tested.

## 2. Experimental Details

ZnO nanorods were grown on sputtered ZnO seed layer over SiO2/Si (100) by HG process. The aqueous solution was prepared by mixing equal molar ratio of zinc-nitrate hexahyderate [Zn (NO3)2.6H2O, 99.99 %, Aldrich] and hexamethylenetetramine (C6H12N4, Aldrich), and concentrations of the reactants were kept constant as 0.1 M, in which different quantity of aluminium nitrate nonahyderate [Al (NO3)3·9H2O, 99.997 %, Aldrich] was dissolved in a precursor solution. The molar ratio of Al reagent to Zn [Al/Zn] was varied from 0.1 % to 3 %. The substrate was suspended with a manner that the seed layer surface is facing downward in a solution, which is kept at constant temperature of 95 °C for 3 h. Thereafter, the substrates were removed and rinsed with deionised water and then dried with N2 gas at room temperature. The structural information was acquired by XRD (Rigaku) using Cu Kα (kα = 1.5418 Å) radiation and morphology by FE-SEM (HITACHI S-4800). Optical property was observed by PL at room temperature with excitation by a 325 nm line of a He-Cd laser. The resistivity and carrier concentration were analyzed by Hall measurement performed at room temperature using a magnetic field of 0.50 Tesla. Samples with a size of 0.5 mm × 0.5 mm with variable thickness and four gold electrodes were used to make physical contact at the four corners of the samples as done in typical Van der Pauw arrangement. The Ohmic behaviour of these electrodes was verified and the Hall voltage was kept higher in magnitude than the noise voltage. For sensitivity measurements, two gold wires were connected to the sample surface with a conductive silver paste (Aldrich). The current-voltage (I/V) measurement was done prior to sensitivity measurement for confirming the quality of Ohmic contacts between the sensing material and the electrodes. Dynamic sensor response measurements were performed against hydrogen concentrations in the test chamber. The sample with electrical contact was mounted inside the chamber whose electrical resistance is measured before and after the exposure of H2 using Keithley 2,400 source meter. The H2 gas response of the sample was determined at an operating temperature from 220 to 250 °C, at a fixed concentration of H2 (10sccm) gas in presence for N2 (500 sccm).

## 3. Results and Discussion

To get information on the effect of Al doping on the structure of ZnO nanorods we have performed XRD study. Fig. 1 is the XRD patterns showing (0002), (0004) peaks of ZnO and (004) peak of Si from the ZnO nanorods doped with Al concentration from 0.1 % to 3 %. The peaks at 2θ values of 34.41°, 72.73° correspond to (0002), (0004) peaks of ZnO and the peak at 2θ value of 69.22° represents Si (004) peak (PDF No: 36-1451, 89-1397, 89-0510 and ICDD No: 01-080-0074). The XRD patterns show the formation of hexagonal wurtzite ZnO whose space group is P63mc (a = 3.2535 Å and c = 5.2151 Å). The sharp diffraction peak of (0002) ZnO indicates that the nanorods are crystalline, highly oriented and well aligned to c-axis of wurtzite ZnO. The position of the observed peak does not change significantly with the increase in aluminium concentration. No other phase corresponding to Al or its compound is seen in the XRD pattern, also no other characteristic peaks were observed for other impurities such as Zn or Zn(OH)2.

To look into the morphological evolution of ZnO nanorods with the variation of Al doping we have performed FE-SEM observations. In Fig. 2, plan view and cross-sectional FE-SEM images of ZnO nanorods are shown. The morphology of ZnO nanorods has hexagonal shaped cross-section, which can be seen in a plan view image whereas cross-sectional image shows vertically well aligned nanorods. From plan view image we also observed that small holes seem to appear on top of the hexagonal surface of ZnO nanorods. The origin for appearance of such holes are not clear. As shown in the cross-sectional images, the diameters of nanorods negligibly changed from bottom to the top of each nanorod.

Variation in length and average diameter of ZnO nanorods is shown in Fig. 3(a) and 3(b). Fig. 3(c) shows histogram plots of plan view FE-SEM images. It can be seen clearly from these plots that the average diameter and length of nanorods decrease with an increase in Al doping concentration which tends to minimum value for the range of Al doping from 0.5 % to 1.5 %, and beyond 1.5 % concentration increase in diameter and length of ZnO nanorods are observed.

Fig. 4 shows PL spectra of the ZnO nanorods at room temperature, indicating UV and visible PL spectra. A strong and narrow UV emission band is observed around ~376 nm (~3.3 eV) and a broader emission band is located in the green-yellow part of the visible spectrum with an emission peaks around ~ 550 nm (2.26 eV) and orange-red emission around ~ 625 nm (1.98 eV). The origin of the near-UV luminescence is attributed to the excited electron emitted from the conduction band to the valence band. That is, the peak 376 nm is due to exciton emission from the conduction band to valence band, which is an intrinsic property of ZnO. The visible luminescence around 550 and 625 nm occur mainly due to intrinsic and extrinsic defects. The intrinsic defects are associated with deep level emissions such as oxygen vacancies, zinc interstitials, zinc vacancies, oxygen interstitials or oxygen antisites. According to previous reports,25) the origin of green-yellow emission around 350 nm is due to surface hydroxyl (OH) group or due to absorbed water, while the orange-red emission around 625 nm is probably due to oxygen interstitials.

Fig. 5 shows changes of resistivity and carrier concentration obtained by Hall measurements from 5 cm × 5 cm samples with ZnO nanorods. Comparing the undoped sample, the Al-doped samples showed decreased resistivity and increased carrier concentration. However, there was no big difference in Al-doped samples with Al mol%.

The H2 gas sensitivity of the samples has been tested at temperatures of 220 °C and 250 °C. The sensor response is defined as [(Ra − Rg)/Ra] × 100 %. Here, Ra is the resistance in ambient air and Rg is the resistance upon exposure to hydrogen. The hydrogen sensing mechanism may be explained as follows. At first, oxygen is chemisorbed on the ZnO surface when the film is heated in air. During the chemisorptions, atmospheric oxygen forms ionic species such as O2 and O which acquire electrons from the conduction band. The reaction kinetics is as follows

$O 2 ( gas ) ↔ O 2 ( adsorbed )$
(1)

$O 2 ( adsorbed ) + 2 e - ↔ O 2 − ( adsorbed )$
(2)

$O 2 − ( adsorbed ) ↔ 2 O 2 − ( adsorbed )$
(3)

If a sensor is placed in a hydrogen-containing gas mixture (air + Н2), Н2 molecules dissociate into atoms on the sensor surface. The atoms adsorbed on the surface are occupied by О ions. The corresponding surface reaction is described by the following equation:

$H + O − ↔ OH − → OH + e$
(4)

Here, ОН will be desorbed from the surface after preliminary neutralization, and the electron transits into the ZnO conduction band. If the neutralization rate of ОН groups is much higher than the desorption rate of neutral groups, hydrogen atom adsorption will result in an increase in the sensor conductivity (which increases conductivity and decreases resistivity by emitting the electrons).

The gas sensitivity of the samples has been tested at temperatures of 220 °C and 250 °C at constant flow of H2 (10 sccm) and N2 (500 sccm) in to the chamber. To get reproducible and comparable data, prior to sensing test of all the samples were initially heated up to 350 °C in nitrogen atmosphere for one hour to get stable value of resistance. For all sensitivity measurement gas concentration is kept fixed for 3,000 sec durations. As shown in Fig. 6 at operating temperature 250 °C, response time was 1,200 sec and recovery time was less than 3,000 sec for 0.5 % Al doping, whereas at operating temperature 220 °C, response time was 2,000 sec and recovery time was less than 5,000 sec for 0.5 % Al doping. Observed recovery time and response time at 0.5 % Al doping is less than those at other doping concentrations. As shown in Fig. 6, the observed sensitivity for undoped ZnO nanorods were 38 % and 18 % at operating temperatures 250 °C and 220 °C, respectively. However, for ZnO nanorods doped with 0.5 % Al, the maximum sensitivity values of 60 % and 50 % at operating temperatures 250 °C and 220 °C, respectively, were observed. The highest sensitivity at 0.5 mol% Al doped samples is probably caused by the decrease in diameter of ZnO nanorods as shown in Fig. 2 and Fig. 3(c), because the reduction in diameter of nanorods leads to increase in surface area resulting in greater hydrogen adsorption. In fact, the mechanism of Al doping on the hydrogen sensing properties of ZnO looks very complicating. Al doping changes growth of ZnO nanorods, resistivity and carrier concentration etc. Also, adding the Al reagent to the precursor solution will change the acid–based characteristics of hydrothermal process. All these factors will affect overall properties of ZnO nanorods. However, by doping the ZnO nanorods by Al, hydrogen gas sensing properties were greatly improved compared to the undoped samples as shown in Fig. 6.

## 4. Conclusion

To show the effect of Al doping on the structure, morphology, electrical, optical properties of ZnO nanorods we have used XRD, FE-SEM, PL, Hall measurements, and sensitivity measurements to H2 gas. The reduction in the intensity of (0002) peak in XRD pattern, decrease in diameter of nanorods, enhancement in carrier concentration, and increase in sensitivity for H2 gas were observed around 0.5 % Al doping. XRD study shows that the Al doped ZnO nanorods showed preferred orientation of ZnO <0001> and good vertical alignment with slight deterioration in crystalline quality. Morphological estimation of Al doped ZnO nanorods by FE-SEM show that the length and diameter was seen to become minimum around 0.5 % Al doping which can be seen from FESEM images. PL measurements of Al doped ZnO nanorods at room temperature showed shifting of band edge emission peak closer to 376 nm which is well known band edge emission of ZnO around 0.5 % Al doping hence Al doping of ZnO nanorods can also cause bandgap tunability. Hall measurement do agree well with other studies since the observed carrier concentration found maximum and resistivity minimum around 0.5 % Al doping. Highest values of sensitivity ~ 60 % at 250 °C and ~ 50 % at 220 °C for H2 gas were observed for 0.5 % Al doped nanorods.

## Acknowledgments

This study was financially supported by research fund of Chungnam National University (2019-0834-01).

## Figure

XRD pattern of as grown ZnO nanorods doped with Al concentration from 0.1 to 0.3 %.

Plane view and cross-sectional FE-SEM images of Al doped ZnO nanorods.

Plots of variation in diameter (a) and length (b) of ZnO nanorods calculated form cross sectional view FE-SEM images, (c) histogram plot showing variation in ZnO nanorods diameters with respect to Al doping concentrations.

PL spectra of Al doped ZnO nanorods with change in Al doping concentration.

Plot of change in resistivity and carrier concentration versus Al doping concentrations as observed by Hall measurements.

Plot of variation in H2 gas sensitivities of ZnO nanorods with Al doping at operating temperatures of 250 °C and 220 °C.

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