Enhanced Structural, Optical, and Gas Sensing Properties of Zn-Doped In 2 O 3 Nanomaterial Synthesized via Sol-Gel Technique

doi:10.21203/rs.3.rs-4719984/v1

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Enhanced Structural, Optical, and Gas Sensing Properties of Zn-Doped In 2 O 3 Nanomaterial Synthesized via Sol-Gel Technique

https://doi.org/10.21203/rs.3.rs-4719984/v1

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The effect of zinc doping on the structural, optical, and gas sensing properties of indium oxide (In2O3) nanomaterial was investigated. We used a simple sol-gel approach to create In₂O₃ nanomaterial with different zinc doping percentages (1, 3, 5, 7, and 9 moles %). Thermogravimetric analysis (TGA) was used to investigate the samples' thermal behaviour. The powders were analysed using X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Visible spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). The XRD investigation revealed that the produced powders were polycrystalline in nature, with a cubic structure and a favoured orientation (222). TEM scans revealed the presence of spherical particles. As the zinc doping concentration grew, the nanomaterial optical band gap dropped. The Zn-doped In₂O₃ nanomaterial was investigated for hydrogen sulfide (H₂S) gas sensing applications. The gas detecting characteristics were tested at different operating temperatures and H2S concentrations. When compared to several other solid-state gas sensors, the Zn-doped In₂O₃ nanomaterial displayed dramatically increased sensitivity to H₂S gas and shorter reaction times. Overall, the Zn-doped In₂O₃ sensors have high sensitivity and fast response.

Zinc Doping

Indium Oxide Nanomaterial

Sol-Gel Method

Gas Sensing Properties

Optical Band Gap

To synthesize Zn-doped In₂O₃ nanomaterial with varying zinc concentrations (1, 3, 5, 7, and 9 moles %) using the sol-gel method.
To characterize the structural properties of the synthesized Zn-doped In2O3 nanomaterial using X-ray diffraction (XRD) and transmission electron microscopy (TEM).
To analyze the optical properties of the Zn-doped In₂O₃ nanomaterial through UV-Visible spectroscopy and investigate the relationship between zinc doping levels and optical band gap variations.
To assess the thermal stability of the Zn-doped In₂O₃ nanomaterial using thermogravimetric analysis (TGA).
To evaluate the gas sensing performance of Zn-doped In₂O₃ nanomaterial for hydrogen sulfide (H2S) detection, focusing on sensitivity, response time, and operating temperature.

Indium oxide (In₂O₃) is a notable wide band gap semiconductor material with strong electrical conductivity in its non-stoichiometric state [1]. This material finds numerous uses in microelectronics, optoelectronics, and gas sensors [2]. Nanostructure research opens the door to new applications in a variety of domains, including chemical, electrical, catalytic, electrochemical, and photonics [3]. In₂O₃ is especially interesting for gas sensing applications, as nanoscale indium oxide has superior sensing properties [4]. The technical uses of In₂O₃ have sparked tremendous interest in developing synthesis processes capable of producing In₂O₃ nanoparticles with regulated particle sizes [5]. However, producing nanocrystalline In₂O₃ from affordable, non-toxic, and environmentally benign precursors remains a difficulty [6]. Several approaches have been used to create indium oxide nanostructures, including high vacuum thermal evaporation, sol-gel, chemical vapor deposition, glass crystallization, hydro/solvothermal procedures, and microemulsion-mediated hydrothermal synthesis [7]. These methods produce indium oxide particles of various sizes and morphologies [8]. Among these approaches, the sol-gel method appears to be particularly promising for manufacturing metal oxide nanoparticles [9]. It has various advantages over other synthesis methods, including ease of preparation, high purity, accurate stoichiometric control, homogeneity, ease of doping, and the capacity to create thin film coatings, fibers, porous powders, and self-supported bulk structures [10]. Glass crystallization is the process by which indium-containing glass is melted and then carefully cooled to produce indium oxide nanocrystals inside the glass matrix [11]. This method can yield highly crystallin nanoparticles with distinct forms [12]. High temperature and high pressure are used in hydro/solvothermal techniques to encourage the nucleation and development of indium oxide nanoparticles in a solvent [13]. Particles with controlled shape and excellent homogeneity can be produced using these techniques [14].

Previously, researchers used the sol-gel approach to synthesize ZnO nanoparticles with a hexagonal wurtzite structure and particle sizes ranging from 35 to 54 nm [15]. It was reported that the size-controlled synthesis of SnO2 nanoparticles using the sol-gel method resulted in titanium dioxide nanopowder with particle sizes between 12 and 20 nm [16]. This sol-gel produced compound outperformed reference materials in terms of photocatalytic reactivity [17]. Furthermore, Mn-doped ZnO nanoparticles with different doping concentrations (1, 2, 3, 4, 5 at%) were synthesized using the sol-gel method [18]. Structural study revealed that both undoped and Mn-doped ZnO nanoparticles had a hexagonal wurtzite structure, and their optical and magnetic characteristics varied with doping concentration [19–20].

This study employs the sol-gel process to investigate the novel idea of employing zinc doping to improve the structural and optical properties of In₂O₃. The feasibility and scalability of the process are improved by the capacity to synthesis these nanoparticles at lower temperatures with shorter reaction times. Higher purity nanoparticles are produced as a result of the process's reduced possibility for contaminants due to the lack of surfactants. The effective use of these Zn-doped In₂O₃ nanoparticles in H₂S gas sensing creates new opportunities for cutting-edge sensor development. This work offers a useful starting point for further investigations into doped metal oxide nanoparticles for a range of uses. The use of a simple one-pot sol-gel approach to synthesize Zn-doped In₂O₃ nanoparticles is unique to this study. This method addresses the need to produce pure and homogenous nanoparticles with small particle sizes at low temperatures and with a short reaction time. Unlike prior experiments, this method does not use surfactants, making it a simpler and more environmentally friendly process. Furthermore, the produced Zn-doped In2O₃ nanoparticles show promise for H₂S gas sensing applications, demonstrating a unique use for these materials. This research is pioneering in its exploration of Zn doping in In₂O₃ nanoparticles using the sol-gel method, which has not been previously described.

Zn-doped and undoped In₂O₃ nanoparticles were created utilizing a straightforward sol-gel method. Ammonia (AR grade), double distilled water, and indium (III) nitrate hydrate (In(NO₃)₃, Aldrich metal basis 99.9%) were utilized as starting materials to create indium hydroxide for the manufacture of undoped In₂O₃ nanoparticles. Initially, a 0.4 M concentration of indium nitrate was produced as an aqueous solution. Doping is a basic technique that modifies the crystal composition of In₂O3 powders to change their characteristics and, in turn, thick film sensor performance. In order to produce a material with improved functionality, particular atoms or ions are purposefully added to the host lattice. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99% purity, Merck) was utilized as the dopant source in the synthesis procedure previously mentioned. Based on the intended Zn doping concentration, a 0.4 M aqueous solution of indium hydroxide was mixed with the necessary amount of zinc nitrate hexahydrate to achieve various molar percentages of Zn in In₂O3 (1%, 3%, 5%, 7%, and 9%). Utilizing a variety of methods, the resultant undoped and Zn-doped In2O3 powders were analyzed.

The Perkin Elmer TGA-4000 (USA) instrument was utilized to perform thermogravimetric analysis (TGA) in this investigation. Using Bruker D8 Advance equipment and Cu-Kα radiation (λ = 1.542 Å) throughout a Bragg angle range of 20° to 80°, X-ray diffraction (XRD) spectroscopy was used to investigate the crystallinity and crystal phases of the calcined powder. Using a JASCO V-670 spectrometer and UV-visible spectroscopy, the optical characteristics of In2O3 were examined. Using a JASCO-650 instrument, the powder samples' Fourier-transform infrared (FTIR) spectra were captured using the KBr pellet method. The Technai and Philips CM-200 instruments were used to obtain transmission electron microscopy (TEM), high-resolution TEM (HRTEM) pictures, and selected area electron diffraction (SAED) patterns at an accelerating voltage of 200 kV.

3.1 XRD analysis

The calcined powder's crystallinity and phases were investigated using X-ray diffraction (XRD) spectroscopy using a Bruker D8 Advance apparatus. Cu-Kα radiation (λ = 1.542 Å) was used over a Bragg angle range of 20° to 80°. The phase of the nanocrystalline particles was confirmed by XRD analysis. The Scherrer formula was used to calculate the size of each crystallite.

The X-ray diffraction (XRD) pattern of calcined In₂O₃ nanocrystalline particles is displayed in Fig. 1. The diffraction pattern closely matches cubic In₂O₃ (JCPDS card number 06-0416), indicating that high-quality In₂O₃ nanocrystalline particles were successfully synthesized without any X-ray-detectable impurities. A preferentially oriented crystal plane of (222) is present in the particles generated by the sol-gel method. There is a significant degree of crystallinity in the sample, as seen by the strong diffraction peaks. Using least squares refinement, the powder's lattice constants were found from X-ray diffraction (XRD) peaks by indexing it in a cubic space group (Ia-3). This revealed a lattice parameter of a = b = c = 10.117 Å. The creation of crystalline In2O3 particles with a body-centered cubic (BCC) structure is confirmed by the XRD pattern. Approximately 15 nm is the typical crystallite size, as determined by the Scherrer formula.

Figure 2 depicts the X-ray diffraction (XRD) patterns of undoped and Zn-doped In₂O₃ samples at various Zn concentrations (1%, 3%, 5%, 7%, and 9% mole). The lattice constant decreased with increasing Zn concentration. The samples have a significant preferred orientation along the (222) direction, as well as lesser intensity reflections corresponding to the (211), (400), (440), and (622) planes, which is compatible with the JCPDS No. 06-0416 standard for In₂O₃ compounds. The strength of the (222) peak rises with Zn doping, indicating increased crystallinity. The high strength of the diffraction peaks in the XRD pattern indicates that the samples are highly crystalline. The lattice constants of undoped and Zn-doped In₂O₃ were determined to determine the effect of doped Zn ions.

In X-ray diffraction (XRD), the spacing between crystal planes (denoted as d) in a cubic crystal structure is related to the lattice constant (\(\:a\)) and Miller indices (hkl) by the following Eq. (1).

\(\:a=\frac{\sqrt{3}}{\text{s}\text{i}\text{n}{\theta\:}}\:\) λ (1)

Table 1

Lattice constant and crystallite size calculated for undoped and Zn doped In₂O₃
Sr. No	Material	Lattice constant \(\:\varvec{a}\:\)(Å)	Crystallite Size (nm)
1	Undoped In₂O₃	10.117	15
2	1% Zn doped	10.111	16.62
3	3% Zn doped	10.108	16.22
4	5% Zn doped	10.107	14.88
5	7% Zn doped	10.107	13.27
6	9% Zn doped	10.103	18.36

Table 1 shows the lattice constants and crystallite sizes determined for undoped and Zn-doped In2O3 samples. The X-ray diffraction (XRD) spectra show significant alterations in peak positions with increasing Zn doping. We attribute these movements to two possible causes. For starters, because doping occurred at the percentage level rather than the atomic level, complete substitution of lattice sites may not have occurred, resulting in material stress. This behavior is explained by the substituting Zn2 + ion's lesser ionic radius (0.074 nm) compared to the host ion, In3+ (0.081 nm).

3.2 TEM analysis

The TEM picture of calcined In2O3 nanocrystalline powder and the matching selected area electron diffraction (SAED) pattern are displayed together in Fig. 3. The particles have diameters that range from 17 to 23 nm, and their form appears to be primarily spherical. The crystalline cubic structure of the In₂O₃ sample is indicated by the patchy ring pattern seen in its SAED pattern in the (222) plane, which lacks any extra diffraction spots or rings from second phases. The X-ray diffraction pattern (JCPDS card number 06-0416) and the measured interplanar spacings (d, hkl)) from the SAED pattern in Fig. 4 match and are consistent with the cubic phase of In₂O₃ nanocrystalline material. Reflections like (211), (222), (400), (411), (332), (431), and (440) correspond to these measurements.

A high-resolution transmission electron microscopy (HRTEM) image of In₂O₃ nanoparticles is shown in Fig. 3. The image shows well-crystallized particles in the form of single crystals, as indicated by distinct lattice planes that sharply define the borders of the particles. The majority of the particles are spherical in shape and have a tendency to group together.

3.3 UV-vis spectrum analysis

UV-Visible spectroscopy was used to examine the optical characteristics of In2O3. A clear absorption peak at 293 nm, or 4.23 eV, can be seen in the UV-visible absorption spectra of In2O3.

Fitting the absorption data to the direct transition equation yielded the sample's direct band gap energy (Eg) as given by Eq. (2).

αhν = E_D (hν-E_g )^1/2 (2)

Where, α is absorption coefficient, hν is photon energy, E_g is direct band gap and E_D is constant. Plotting (αhν) ² as function of photon energy and extrapolating linear portion of curve to absorption equal to zero. The direct band gap (E_g) deduced from this plot was found to be 3.6 eV. This value of optical band gap exactly matches with reported value. Plotting the changes in the band gap of produced In₂O₃ doped with different mole percentages of Zn (1%, 3%, 5%, 7%, 9%) against incident photon energy (hv) using the sol-gel process is shown in Fig. 6. Eq. (2) was used to compute the optical band gap. The link between photon energy (hν) and (αhν)² is seen in Fig. 5.

3.4 FTIR analysis

In order to determine whether molecular species were present in undoped Indium Oxide (In₂O₃) and to see how Zn doping affected the vibrational modes of In₂O₃ nanoparticles, the study used FTIR spectroscopy. The Zn-doped In₂O₃ (Fig. 7) and undoped In₂O₃ (Fig. 8) FTIR transmission mode spectra were recorded. Peaks were found at 406, 428.96, 501.28, and 590.56 cm^− 1, which correspond to the cubic In2O3 In-O phonon vibration mode. While CO₂ absorption was responsible for the peak at 2347.07 cm^− 1, OH stretching was indicated by a large absorption band about 3447.50 cm^− 1. Absorption bands at 1340.52, 1507.72, and 1636.30 cm-1 were associated with nitrate groups and water bend deformation. Minor variations in the frequency or strength of these bands indicate susceptibility to varied In³⁺ distributions in interstitial locations and defects, such as oxygen vacancies. The findings verified complete phase development and the absence of organic intermediates in the samples. Zn-doped In₂O₃ (at 1, 3, 5, 7, and 9 mol %) showed a decrease in the intensity of In₂O₃ bands.

3.5 Thermal gravimetric analysis

The thermal behavior of the precursor material was investigated using thermal gravimetric analysis (TGA). The precursor employed was Indium Nitrate Hydrate (In(NO₃)₃ x H₂O), and 9.446 mg of the sample was placed in a quartz pan to reduce bed effects during decomposition. The sample was heated in air from 30 to 800°C at a rate of 15°C per minute. The purge gas was nitrogen, flowing at a rate of 20 ml per minute. Figure 9 illustrates the sample's thermal behavior. The thermogram shows that the weight loss process continues up to 400°C, with the complete disintegration of Indium Nitrate Hydrate and the creation of In₂O₃ nanophase powder. The weight loss is due to the removal of physisorbed and chemisorbed water from In (NO₃)₃ x H₂O. A weight loss of 10.04% was reported during the first step of thermal degradation (at about 150°C). The considerable weight loss observed between 150 and 250°C is most likely caused by the elimination of nitrate from the sample. The TGA curve shows essentially no weight loss between 400 and 800°C.

3.6 Application in gas sensing

The study was investigated the response of pure, 1, 3, 5, 7, and 9 mol% Zn-doped In₂O₃ thick film resistors (TFRs) to 100 ppm H₂S at various operating temperatures as shown in Fig. 10. Sensitivity peaked at 150°C for both undoped and 7 mol% Zn-doped In₂O₃, with the latter being the most sensitive. Selectivity experiments at 150°C against C₂H₅OH and NH₃ revealed that the 7 mol% Zn-doped sensor had superior H₂S selectivity as shown in Fig. 11. The calibration curve for this sensor showed a linear response to H₂S values of up to 200 ppm, followed by saturation. The 7 mol % Zn-doped sensor had a faster recovery time (~ 240 sec) compared to the undoped sensor, but the response time remained identical at ~ 7 sec. Table 2 shows the comparison between the sensing characteristics different materials and sensor performances.

Table 2

Comparison between the sensing characteristics different materials
Sr. No	Materials	H₂S (ppm)	Optimal Temp. (⁰C)	Sensitivity / Response (%)	References
1	In₂O₃	50	150	68	[16]
2	CO: In₂O₃	50	125	82	[17]
3	La: In₂O₃	50	125	86	[18]
4	In₂O₃	10	195	50.2	[19]
5	Gd: In₂O₃	10	195	52.3	[20]
6	Zn: In₂O₃	100	150	98.74	Present work

The sol-gel approach was effectively used to produce pure and zinc-doped indium oxide, yielding impurity-free BCC structures as confirmed by XRD measurements. The size of the crystallites reduced as Zn doping increased. FTIR investigation verified complete phase development, which is typical of cubic In₂O₃, and the optical band gap was consistent at 3.6 eV. The TEM and HRTEM examinations indicated a spherical distribution of nanocrystalline particles, with uniform spherical nanoparticles around 17 nm, indicating their potential for gas sensing applications. Zn doping greatly improved the H₂S sensing properties of In₂O₃ nanopowder, resulting in increased responsiveness, shorter response and recovery times, and lower operating temperatures. These findings indicate that Zn-doped In₂O₃ nanopowder is an excellent choice for H₂S sensors.

Contribution of the authors:

All authors contributed equally to the preparation of this manuscript. All authors reviewed the manuscript.

Conflict of Interest:

This research work has been done by all the listed authors with mutual interest. All the data used in this research work are cited in the manuscript. Therefore, no conflict of interest related to any person or agency for this manuscript.

Funding:

Not Applicable

Acknowledgement:

We are grateful to the Management of M.G. Vidyamandir and the principal of M.S.G. Arts, Science & Commerce College, Malegaon-Camp (Nashik).

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No competing interests reported.

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Enhanced Structural, Optical, and Gas Sensing Properties of Zn-Doped In 2 O 3 Nanomaterial Synthesized via Sol-Gel Technique

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Abstract

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Research Highlights

1. Introduction

2. Experimental details

3. Results and discussion

3.1 XRD analysis

3.2 TEM analysis

3.3 UV-vis spectrum analysis

3.4 FTIR analysis

3.5 Thermal gravimetric analysis

3.6 Application in gas sensing

4. Conclusions

Declarations

References

Additional Declarations

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