Effect of Zn Doping in CuO Octahedral Crystals towards Structural, Optical, and Gas Sensing Properties

Abstract

Monodispersed CuO octahedral crystals were successfully synthesized using a low-temperature co-precipitation method. Zinc doping in CuO created surface defects that enhanced oxygen adsorption on the surface crucial for gas sensing applications. Pure and Zn-doped CuO sensor films were realized using the doctor blade method. The sensor films showed selective response towards a low concentration of NO₂ at a lower operating temperature of 150 °C. Doping with Zn causes the resistance of the sensor film to decrease due to the enhancement of charge carriers with an analogous improvement in the sensor response. The observed decrease in sensor resistance agreed well with the findings of the work function studies. Zinc doping resulted in an increase in work function by 180 meV which, after NO₂ exposure, was found to increase by a further 130 meV, attributed to the oxidizing behavior of the test gas.

1. Introduction

Cupric oxide (CuO) is the most widely studied oxide of all other copper oxides. Other copper suboxides, such as Cu₂O and Cu₂O₃, are not stable materials [1]. Copper oxide is a p-type semiconductor and has many notable properties like antibacterial capability, high stability, and ease of use. It has been utilized in numerous applications including photocatalysis, supercapacitors, lithium ion batteries, infrared photo detectors, biosensors, and gas sensors [2,3,4,5,6,7,8]. It is an ideal absorber material for solar cells, as its optical absorption edge lies in the range between 1.2 eV and 1.9 eV [9,10]. It is an excellent sensing material. For example, Yang et al. [7] synthesized CuO nanorods using hydrothermal method and investigated the gas sensing response towards organic vapors. Similarly, Wang et al. [8] reported reducing volatile organic sensing via sol-gel-synthesized CuO nanoparticles. It has been synthesized using numerous techniques such as hydrothermal, physical vapor deposition, sputtering, chemical vapor deposition, and co-precipitation methods [4,6,11]. Among these, the co-precipitation method using the water bath technique, in particular, has the benefit of low-temperature growth, low cost, and a simple process [12]. It has been demonstrated that by doping transition metals into the semiconductor matrix, electrical, optical, and functional properties can be improved [13,14,15,16]. Importantly, doping improves the gas sensing characteristics, as it provides more defect states followed by adsorption and desorption of oxygen species [17]. Among all transition metals, Zn causes more effective doping, as it has comparable ionic radii with Cu²⁺ and possesses the same oxidation states [18,19,20].

Nitrogen dioxide is one of most highly toxic gases and a major air pollutant in the environment. It is usually generated from industrial waste, automobile by-products, fossil fuel burning, cigarette smoke, and power plants. It is responsible for acid rain due to the fact of its interaction with water molecules present in the atmosphere [21,22]. When the concentration of NO₂ is higher than 20 ppm, it may lead to immediate distress and cause death. Hence, it is desirable to detect NO₂ at lower concentrations so that the timely preventive and precautionary measures can be taken to ensure better human health and environments.

Accordingly, in the present work, Zn-doped CuO octahedral crystals were synthesized using the water bath process. The Zn-doping effects in CuO octahedral crystals towards structural, optical, and electrical properties were investigated in detail. The sensor films showed selective response towards low concentrations of NO₂ at lower operating temperatures of 150 °C. A sensing mechanism is proposed based on gas sensing and work function studies.

2. Experimental Procedure

2.1. Chemicals

Polyvinylpyrrolidone (PVP), copper sulfate pentahydrate (CuSO₄·5H₂O), zinc sulfate heptahydrate (ZnSO₄·7H₂O), dextrose (C₆H₁₂O₆), sodium carbonate (Na₂CO₃), and trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O) were purchased from Wako Pure Chemical Industries Limited, Japan. All the chemical reagents were of analytical reagent (AR) grade and used directly as received without further purification.

2.2. Synthesis Procedure

The preparation of CuO octahedral crystals was carried out using the water bath process [23]. Sixty milliliters of DI water solution containing 4.5 gram of PVP, 0.653 gram of Na₃C₆H₅O₇·2H₂O, 0.382 gram of Na₂CO₃, 0.757 gram of C₆H₁₂O₆, and 34 mM CuSO₄·5H₂O were prepared. The solution was mixed by vigorous stirring at 1000 rpm for 30 min at room temperature resulting in a light blue solution with a pH value of 10. The solution was kept at 70 °C in a thermostatic water bath for 2 h. After the reaction, brick red Cu₂O crystal precipitates were obtained. It was repeatedly washed with DI water and ethanol and collected by centrifugation followed by drying at 60 °C for 12 h. For synthesizing Zn-doped Cu₂O octahedral crystals, ZnSO₄·7H₂O was mixed in starting solution with different molarities. Pure and Zn-doped CuO films were prepared by coating as-synthesized samples on a cleaned glass substrate using the doctor blade coating method followed by annealing at 450 °C for 2 h in air. The film thickness of the prepared samples was measured using cross-sectional SEM analysis and was found to be ~30 µm for all the prepared samples. The samples were labeled as P for pristine CuO; and S1, S2, S3, S4, S5, and S6 for Zn-doped CuO samples with increasing Zn concentrations of 0.34 mM, 1.02 mM, 1.7 mM, 3.4 mM, 6.8 mM, and 17 mM, respectively.

2.3. Sample Characterizations

The XRD studies were carried out on a RINT2200 X-ray diffractometer (XRD) (Rigaku Corporation, Tokyo, Japan) with a Cu-Kα radiation source (λ = 1.54 Å). Raman measurements were performed using a JASCO NRS-7100 instrument (JASCO Corporation, Tokyo, Japan) with an incident green laser having an excitation wavelength of 532 nm. Photoluminescence (PL) studies were done at room temperature using JASCO-FP8600 instrument (JASCO Corporation, Tokyo, Japan). The morphology and the structural characterization were performed with a JEOL JSM-7001F field emission scanning electron microscopy (FESEM) (JEOL Limited, Tokyo, Japan) at an accelerating voltage of 15 kV. Compositional analysis and elemental mapping were performed using energy-dispersive X-ray spectroscopy (EDS) (EDAX APOLLO X) (AMETEK Co. Limited, Tokyo, Japan). Elemental qualitative analysis was performed by X-ray photoelectron spectroscopy (XPS) (Shimadzu Axis Ultra DLD) (Shimadzu Corporation, Kyoto, Japan). Crystallographic properties, such as defects and d-spacing, were measured by transmission electron microscope (TEM) (JEOL JEM 2100F) (JEOL Limited, Tokyo, Japan) with an accelerating voltage of 200 kV.

2.4. Gas Sensing Measurements

For gas sensing studies, electrode contacts were realized by depositing a 120 nm thick gold layer with 500 µm separation using a thermal evaporation method. Final electrical connections were taken with Pt wires and silver paint as shown schematically in Figure 10a. The gas sensing study was performed on a table-top static gas sensing setup as described elsewhere [24,25]. Sensor films were then fixed in an airtight gas sensing chamber. Test gas was injected in the chamber with the required concentration of test gas using a gas-tight syringe. The sensor response (S) was calculated from the response curves:

$$\mathrm{S}(\%)=\frac{{\mathrm{R}}_{\mathrm{a}}-{\mathrm{R}}_{\mathrm{g}}}{{\mathrm{R}}_{\mathrm{a}}}\times 100,\mathrm{for}\mathrm{oxidizing}\mathrm{gases}$$

(1)

$$\mathrm{S}(\%)=\frac{{\mathrm{R}}_{\mathrm{g}}-{\mathrm{R}}_{\mathrm{a}}}{{\mathrm{R}}_{\mathrm{a}}}\times 100,\mathrm{for}\mathrm{reducing}\mathrm{gases}$$

(2)

where R_a and R_g are resistances of the sensor film in air and test gas, respectively.

2.5. Work Function Measurements

For work function measurement, pure and Zn-doped CuO films were coated on conducting an indium–tin oxide (ITO) substrate using the doctor blade method. The contact potential difference (CPD) measurements of the prepared films were carried out by a scanning Kelvin probe technique (SKP) system using a surface photovoltage (SPV) module (SKP5050 with SPV020 module, KP Technologies Ltd., UK) as described elsewhere [26,27]. From the CPD, the work function (in meV) was measured:

$$\mathsf{\varphi }=5100-{\mathrm{CPD}}_{\mathrm{Au}}+{\mathrm{CPD}}_{\mathrm{sample}}\mathrm{meV},$$

(3)

where 5100 is the work function of gold in meV, CPD_Au is the CPD between the tip and the gold-coated reference sample, and CPD_sample is the CPD between the tip and the sample. All the measurements were performed at room temperature. The average value of CPD was measured by raster scanning of the tip across the sample surface of a 4 mm² area (scan). To elucidate the effect of gas exposure on sensor films, work function measurements were performed by exposing the sensor to 50 ppm of NO₂ at 150 °C with subsequent quenching to room temperature.

3. Results and Discussion

3.1. Structural and Morphological Characterization

As shown in Figure 1, the XRD patterns for pure and Zn-doped CuO films indicated the pure phase formation of CuO without any impurity peaks related to Zn or its suboxides. This can be attributed to the good dispersion of Zn²⁺ ions on the sites of Cu²⁺ ions in the CuO matrix [20,28,29]. The XRD patterns could be well indexed with monoclinic CuO (JCPDS file No. 00-041-0254) with ($\overline{1}$ 1 1) and (1 1 1) major planes with a space group symmetry of C2/c. Further, the diffraction peak broadening indicates that the synthesized samples had a crystallite size in the nanometer range. Interestingly, the shift of diffraction angle (2θ) along major the ($\overline{1}$ 1 1) planes as a function of doping in Zn-doped CuO films were observed which suggests the substitution of Zn ions in place of Cu ions (Figure 2a). This is attributed to the tensile stress which results in the shift to a lower angle [19]. The d-spacing values for samples P, S1-S6 were calculated using Bragg’s law and found to be 0.252 ± 0.0001 nm which further suggests successful substitution of Zn ions on the sites of Cu ions in CuO lattice. The crystallite size was evaluated for ($\overline{1}$ 1 1) peak of all the samples using Scherrer formula [29]:

$$\mathrm{D}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\mathrm{Cos}\mathsf{\theta }},$$

(4)

where K = 0.9 is the shape factor, D is the average crystallite size, λ is the wavelength of the incident X-ray beam (0.15406 nm), β is the full width and half maxima value, and θ is the diffraction angle.

Further the effective strain in the films was calculated using Williamson–Hall (W–H) equation [30]:

$$\mathsf{\epsilon }=\frac{\mathsf{\beta }}{4\tan \mathsf{\theta }},$$

(5)

The Zn doping concentration’s dependence on average crystallite size and strain were calculated and are shown in Figure 2b. Crystallite size was found to be decreased, while strain was increased for high Zn doping. At lower Zn concentrations, crystallite size increased, and strain decreased which led to lattice distortion by anisotropic shrinkage of lattices due to the stress [14,19]. Maximum crystallite size and minimum strain were observed to be 17.6 nm and 6.4 × 10⁻³, respectively, for sample S1. Zinc ions provided the strain in the lattice which hindered the growth of the CuO crystal, resulting in the reduction in crystallite size. Strain was found due to the low activation energy, so ions could transfer from trap sites to nucleation sites resulting in smaller crystallite sizes. Further, the Zn dopant atomic percentages were estimated using EDS analysis and are shown in

Table 1. Elemental composition of pure and Zn-doped CuO films.

Sample	Composition (atomic %)
	Cu	O	Zn
P	68.64	31.36	-
S1	66.68	32.43	0.89
S2	64.21	34.44	1.35
S3	68.10	30.17	1.73
S4	68.06	29.95	1.99
S5	65.17	31.73	3.10
S6	52.12	41.19	6.69

.

The Raman spectra of pure and Zn-doped CuO films with different Zn concentrations are presented in Figure 3. The primitive cells of monoclinic CuO contain two molecular units, and therefore there are 12 vibrational modes: three acoustic modes (A_u + 2B_u), six infrared active modes (3A_u + 3B_u), and three Raman active modes (A_g + 2B_g) [29]. Due to the site symmetry, only oxygen atom displacements contribute to active Raman modes. The Raman spectra of pure CuO films showed Raman vibration at 294 cm⁻¹, 343 cm⁻¹, and 628 cm⁻¹ corresponding to A_g, B_1g, and B_2g, respectively. A slight change in the wave number could be assigned to the size and crystallinity effect [29,31]. No extra peak was observed after Zn doping which confirms that there was no new phase formation which is consistent with the XRD results. Figure 3 confirms the continuous red-shift and the peak broadening of the A_g phonon mode with an increase in the Zn dopant concentration. This is attributed to a reduction in particle size and defects due to the strain after doping [29,31,32,33]. A red shift in the Raman spectra confirms the tensile strain [34] which is the cause of a size reduction and is in accordance with the XRD results (Figure 2b). There were not many differences observed in XRD patterns and Raman spectra for the Zn-doped samples (S1 to S6); therefore, for further investigations, samples P, S3, and S6 are only considered.

The structural morphologies and sizes of Zn-doped CuO crystals deposited on glass substrate were investigated using FESEM and are depicted in Figure 4. The FESEM images show the uniform growth of monodispersed CuO octahedral crystals. From the SEM images, after doping with Zn, the shapes of the particles were converted from octahedral to diffused spherical structures with a reduction in the granular nature with an increase in Zn concentration owing to tensile strain (Figure 2 and Figure 3) [34].

The chemical composition of pure and Zn-doped CuO films were analyzed using EDS. The elemental atomic percentages of the pure and Zn-doped CuO films are presented in Table 1 for all the prepared samples. The atomic percentage of Zn was found to increase as the doping concentration increased, but it was very less compared to the Zn molarity which was used in the starting solution. This was due to the different solubility limits and reaction kinetics of Zn compared to Cu. The sample S6 showed a higher atomic percentage of oxygen (~41%) species compared to other samples (~31%). The EDS elemental mapping showed the uniform distribution of Zn, Cu, and O atoms for sample S6 (Figure 5a). The EDS spectra for sample S6 showed peaks corresponding to Cu, Zn, and O elements (Figure 5b).

Transmission electron microscopy (TEM) analysis was performed to check the morphological changes, defects, and d-spacings for the samples. The TEM images for samples P, S1, and S6 showed a polycrystalline nature as shown in Figure 6. For the measurement of the d-spacing value, the major plane ($\overline{1}$ 1 1) in CuO crystals were confirmed using fast Fourier transformation (FFT) fitting. The d-spacings for samples P, S1, and S6 were found to be equal, i.e., 0.25 nm, as like the XRD results. There was no significant change in the d-spacing after Zn doping attributable to the doping of Zn ions inside the CuO lattice without disturbing the crystal structure [19]. Elemental mapping was recorded using EDS equipped with scanning transmission electron microscopy (STEM) at higher magnification to check atomic distribution in Zn-doped CuO particles. Figure 7 shows the STEM image and elemental distribution for sample S6. It is evident that Cu, Zn, and O atoms were distributed uniformly in the sample.

The XPS study was performed to confirm the formation of CuO and Zn doping in the octahedral crystals. The whole spectrum was calibrated by a C1s peak at 284.8 eV [35]. The Cu 2p spectra (Figure 8a) present two peaks corresponding to Cu 2p_1/2 and Cu 2p_3/2 at 952.4 eV and 932.6 eV, respectively, with two shake-up satellites at higher binding energy sides, shown with arrows, which are characteristics of Cu (II) in its oxide form [26,36]. After Zn doping, Cu 2p peaks were shifted to a higher binding energy side as compared with those of pure CuO particles. Higher electronegativity of Zn (1.372) as compared to that of Cu (1.336) caused lowering of the electron density around the central Cu ion by introducing oxygen and thereby results in a blue shift [37,38,39]. In the O 1s spectra, three major peaks in the range of 529 eV to 534 eV were assigned to the lattice oxygen (O_L), oxygen in the crystal matrix at the oxygen-deficient site (O_V), and surface adsorption oxygen (O_S), respectively [40]. As shown in Figure 8b, after Zn doping, the O1s peak also shifted to the higher binding energy side, like Cu 2p. For a higher concentration of Zn doping (i.e., in the case of sample S6), a separate oxygen peak was observed. It corresponded to the adsorbed oxygen species present on the surface [41]. The Zn 2p (Figure 8c), for the Zn-doped CuO films, was identified by the two characteristic peaks at 1045.1 eV and 1021.9 eV corresponding to 2p_1/2 and 2p_3/2, respectively [42,43].

3.2. Optical Studies

The influence of Zn doping on CuO films regarding the intrinsic and extrinsic defects were investigated by photoluminescence (PL) spectra recorded in the wavelength range of 380–520 nm and is shown in Figure 9. The spectrum showed emission peaks at 396.8 nm, 418.8 nm, 452.0 nm, 465.2 nm, and 479.2 nm for pristine CuO film. The PL peak between 450 and 475 nm is attributed to the intrinsic defects or surface states in CuO [44]. After Zn doping in CuO, the peak at 452 nm disappeared and emission intensity were found decreased with change in slope of PL curves. The decrease in intensity is attributed to the lattice distortions, defects and reduction in crystallinity [45]. The PL intensity of the Zn-doped CuO films was lower than that of the pure CuO crystals, indicating the suppression of electron–hole recombination [46]. This is attributed to the increment of charge carriers (holes) after Zn doping in CuO; this evident of the enhancement of the adsorbed oxygen species.

3.3. Gas Sensing Studies

Based on the previous characterizations, to know the effect of adsorbed oxygen and Zn doping, samples P and S6 were used for gas sensing investigation as other samples doped at lower concentration did not show significant changes in structural and electronic properties. All gas sensing measurements were performed at the optimum temperature of 150 °C [47,48,49,50]. Figure 10b shows the sensor response curves recorded for samples P and S6 towards 10 ppm NO₂ at an operating temperature of 150 °C. Sensor films were exposed with NO₂ gas for 10 sec in an airtight gas chamber, and the chamber was opened for recovery, as shown by the arrows in Figure 10, as gas in and gas out, respectively. The response time and recovery times were found to be ~10 s and ~20 mins, towards 10 ppm of NO₂ for the S6 sample. The base resistance of sensor film was found to decrease after Zn doping. This was due to the enhancement of adsorbed oxygen species on the sample surface resulting in more charge carriers, lowering the film resistance compared to pristine CuO film. The variations in sensor response as a function of NO₂ concentration were performed as shown in Figure 11. The sensor response for the Zn-doped sample was enhanced when compared to the pure sample. Both pure and Zn-doped CuO sensor films exhibited a linear response towards NO₂ up to 30 ppm. Sensor films responded to NO₂ at a low concentration of 1 ppm also. The resistance of samples P and S6 were found to be 93 and 38 kΩ before gas exposure and 81 and 28 kΩ after 1 ppm NO₂ exposure at 150 °C, respectively. The stability of the sensor film is an important criterion; accordingly, the base resistance of both sensor films was measured as a function time. The base resistance of sensor films at an operating temperature of 150 °C was measured over a period of 10 days as shown in Figure 12. The base resistances of the sensor films were found to be nearly constant which confirms its stability. To check the selectivity, sensor film was exposed to the different gases, namely, NH₃ (50 ppm), H₂S (20 ppm), CO (300 ppm), and C₂H₅OH (300 ppm). A selectivity histogram is shown in Figure 13. No response was observed for these gases, even at higher concentrations, which confirms the high selectivity of the sensor film towards NO₂.

To investigate the effect of doping and gas interaction towards work function (ϕ), Kelvin probe measurements were performed. Figure 14a shows the work function area map recorded for samples P and S6. The average work function values for samples P and S6 were recorded and found to be 5410 meV and 5590 meV, respectively. There was an increase in work function after Zn doping of 180 meV. This was due to the shift in Fermi levels towards the valance band after doping. The increase in charge carriers (holes) by the enhancement of adsorbed oxygen species contributed to the shifting of the Fermi level. After exposure of NO₂ gas on sample S6, the work function was further increased from 5590 meV to 5720 meV as shown in Figure 14b. This was due to the shift of the Fermi level towards the valence band, as NO₂ is an oxidizing gas. Similarly, a decrease in the film resistances were observed after doping and NO₂ exposure which confirms our results. Similar studies were performed in our previous report [26].

Figure 15 depicts the underlying gas sensing mechanism. An energy band diagram of semiconductor material is shown with different energy levels, vacuum level (E_vac), conduction band (E_C), valance band (E_V), and Fermi level (E_F). The CuO is a p-type semiconductor material, having holes as the majority of charge carriers. The Fermi level for this p-type material as nearer to valance band as shown in Figure 15a. After Zn doping, the adsorption of oxygen species on the sample surface increased as observed in the EDS and XPS studies. This was due to the excess electrons which are provided by Zn metal (Zn²⁺) to the surface oxygen atoms. This influences the adsorption of the oxygen species (O₂⁻) on the sample surface. It enhances the number of charge carriers (holes) in the sensor film, so the Fermi level shifts down towards the valance band as shown in Figure 15b. This results in a further increase of the work function and, hence, film base resistance decreases after doping. Nitrogen dioxide is an oxidizing gas, when it is exposed to sensor film, provides holes. When NO₂ gas is exposed to Zn-doped CuO (p-type) sensor films, the adsorbed O₂ are replaced with NO₂ and more holes are generated in the sensor film. Therefore, the Fermi level shifts down further towards valence band (Figure 15c), resulting in an increased work function and decreased base resistance of the sensor film. Similar results were observed in our work function studies.

4. Conclusions

Monodispersed, uniform CuO octahedral crystals were successfully fabricated using a water bath co-precipitation technique at low temperature. The effect of Zn doping towards structural, optoelectronic, and NO₂ sensing properties were investigated in detail. The Zn doping resulted in the decrease in the crystallite sizes with identical d-spacing, also confirmed using XRD and TEM studies. Gas sensing studies revealed that the Zn-doped sensor exhibited a highly sensitive and selective response towards NO₂ at 150 °C with a minimum detection limit of 1 ppm. Work function measurements further supported the sensing results.