Nanostructured, Mn-doped, petal-like zinc oxide films for high-performance ammonia detection

Abstract

Nanostructured, Mn-doped, petal-like zinc oxide film composition for high-performance ammonia detection, comprising: a) zinc oxide (ZnO), formed from zinc acetate dihydrate as zinc precursor; b) Manganese (Mn 2+ ) dopant incorporated into the ZnO lattice via manganese(II) acetate tetrahydrate, with the Mn content in the range of 0.5 mol% to 5 mol% based on the zinc precursor; c) double distilled water as solvent and oxidising agent; d) a film morphology consisting of nanopetals and nanorods with an average particle size in the range of 23 to 59 nm; e) wherein the composition is deposited as a porous thin film by spray pyrolysis on a substrate preheated to 400°C at an atomization pressure of 30 psi and a spray rate of 2 ml / min; f) wherein the composition has high sensitivity, short response and recovery times and selectivity for ammonia gas at room temperature.

Description

The present invention relates to a nanostructured metal oxide-based sensor material. More specifically, it is a petal-like thin film composition of manganese-doped zinc oxide (Mn-ZnO) developed for high sensitivity and selective detection of ammonia gas at low concentrations.Ammonia (NH 3) is a widely used industrial chemical which is used in agriculture, refrigeration technology, pharmacy and as a cleaning agent. However, exposure to ammonia even at low concentrations can result in severe health risks including airway irritation, eye damage and long term lung problems. Therefore, the accurate and early detection of traces of ammonia is of decisive importance for safety in the workplace and protection of the environment. Conventional detection systems, such as electrochemical and catalytic sensors, often require high operating temperatures, frequent calibration, or lack of sensitivity at low concentrations, limiting their practical use in real-time and low power applications.To overcome these limitations, metal oxide semiconductor-based gas sensors have attracted attention because of their ease of manufacture, high sensitivity, and cost efficiency. Among them, zinc oxide (ZnO) excels due to its wide band gap, high electron mobility, and chemical stability. However, pure ZnO sensors typically require high temperatures for activation, suffer from slow response and recovery times, and exhibit limited selectivity to ammonia in the presence of other volatile organic compounds (VOCs). Moreover, the performance of undoped ZnO sensors is limited by low surface reactivity and insufficient adsorption / desorption dynamics under ambient conditions, hindering their use in room temperature detection scenarios.The present invention solves these problems by the introduction of a nanostructured manganese (Mn)-doped ZnO thin film composition prepared by spray pyrolysis. By precisely doping ZnO with Mn 2+- ions, the invention achieves a significant improvement in surface area, porosity, crystallinity and oxygen vacancy concentration. These structural and electronic modifications result in improved gas sensor performance including fast response and recovery at room temperature, high sensitivity at low ammonia concentrations (up to 15 ppm), and superior selectivity. The proposed composition not only overcomes the limitations of pure ZnO sensors, but also provides a low cost, scalable and energy efficient solution suitable for industrial safety, environmental monitoring and agricultural applications.The present invention relates to a nanostructured metal oxide-based sensor material. More specifically, it is a manganese-doped zinc oxide (Mn-ZnO) petal-like thin film composition developed for the high sensitivity and selective detection of ammonia gas at low concentrations.An object of the present disclosure is to enable high sensitivity ammonia detection at room temperature down to 15 ppm.Another object of the present disclosure is to use a low cost spray pyrolysis technique for scalable thin film deposition.Another object of the present disclosure is to achieve fast response and recovery times, particularly at Mn doping of 3% and 5%.Another object of the present disclosure is to achieve improved selectivity for ammonia gas over other VOCs.Another object of the present disclosure is to incorporate nanopetal nanorod morphology to increase porosity and surface area.Another object of the present disclosure is to demonstrate Mn-doping tunable optical and crystallographic properties.Another object of the present disclosure is to ensure structural stability and phase purity, which is confirmed by XRD and EDAX analysis.Another object of the present disclosure is to have a simple precursor system that uses readily available zinc and manganese salts.Another object of the present disclosure is to provide promising use in environmental monitoring, industrial safety and agriculture.The present invention relates generally to a novel nanostructured thin film composition for sensitive detection of ammonia gas at room temperature. More specifically, it is a manganese (Mn)-doped zinc oxide (ZnO)-petal-like film composition synthesized by a low cost spray pyrolysis method to achieve high sensitivity, fast response / recovery times, and excellent selectivity for the detection of ammonia (NH 3) at low concentrations.An embodiment of the present invention is that the composition comprises ZnO as a base semiconductor matrix doped with different concentrations of Mn (preferably 1%, 3%, and 5% by molar ratio) to improve its structural, optical, and sensory performance. The doping process results in significant morphological changes leading to the formation of uniform nanopetal and nanorods with increased porosity and reduced grain size. This microstructural tuning promotes increased surface area and adsorption sites, enabling improved gas sensing performance.In another embodiment of the invention, the characterization studies confirm that the Mn ions are successfully incorporated into the ZnO lattice, resulting in lattice widening, dislocation density reduction, and optical property change such as tunable band gap due to the sp-d exchange interaction and the Moss burstin effect. Evaluations of the gas sensor system show that the 5% Mn-doped ZnO film has superior sensitivity (~80% reaction) to 15 ppm ammonia with a fast reaction (15 seconds) and recovery (17 seconds) at room temperature. The variant doped with 3% Mn moreover exhibits a virtually linear reaction over a broader range (15-30 ppm) and is therefore suitable for quantitative detection.Another embodiment of the invention is that the unique composition of Mn-doped ZnO films enables selective, sensitive and efficient ammonia detection that exceeds conventional ZnO and other metal oxide-based sensors in low temperature environments.A further embodiment of the invention is thus the invention provides a promising solution for environmental monitoring, industrial safety and agricultural applications requiring real time, low-level ammonia detection.The present invention relates to a novel nanostructured manganese (Mn)-doped zinc oxide (ZnO) thin film composition specifically developed for sensitive and selective detection of ammonia gas at room temperature. The invention is synthesized using a low cost and scalable spray pyrolysis technique and involves precise doping of ZnO with Mn 2+- ions at different concentrations (0% to 5%), resulting in a unique petal-like and nanotube-like morphology with increased porosity and crystallinity. This structural adjustment increases the active surface area and the oxygen vacancies, which significantly improves the gas sensor performance. The resulting films have high sensitivity, rapid response and recovery times, and excellent selectivity to ammonia even at low concentrations such as 15 ppm. The invention overcomes the major limitations of conventional ZnO-based sensors and provides a practical solution for real-time ammonia detection at low temperatures in environmental, industrial and agricultural environments.EXAMPLE 1: COMPOSITIONA nanostructured Mn-doped petalic zinc oxide film composition for high performance ammonia detection comprising: a) zinc oxide (ZnO) formed from zinc acetate dihydrate as a zinc precursor; b) manganese (Mn 2+)- dopant incorporated into the ZnO lattice via manganese (II) acetate tetrahydrate, wherein the Mn content is in the range of 0. 5 mole % to 5 mole %, based on the zinc precursor; c) double distilled water as solvent and oxidant; d) a film morphology consisting of nanopetalens and nanorods having an average particle size in the range of 23 to 59 nm; e) wherein the composition is deposited as a porous thin film by spray pyrolysis on a substrate preheated to 400°C at a sputtering pressure of 30 psi and a spraying rate of 2 ml / min; f) wherein the composition has high sensitivity, short response and recovery times and selectivity for ammonia gas at room temperature.EXAMPLE 2: PreparationThe present composition relates to the production of Mn-doped zinc oxide (ZnO) films by spray pyrolysis methods for high performance gas sensor applications. The formulation contains zinc acetate dihydrate [Zn(CH 3 COO) 2-2 H 2 O, 98%] as the zinc source and manganese(II) acetate tetrahydrate [(CH 3 COO) 2 Mn-4H 2 O, 99%] as the manganese dopant, both dissolved in bi-distilled water as the solvent and oxidant. A 0.1 M zinc acetate solution is prepared and stirred at room temperature for 30 minutes. The manganese doping of 0%, 1%, 3% and 5% is achieved by varying the ratio of dopant to zinc precursor. The glass substrates are cleaned by boiling with chromic acid, rinsing with distilled water, and ultrasonic cleaning with acetone to ensure a contamination-free deposition surface.The precursor solution is atomized and sprayed at a rate of 2 mL / min onto the substrates preheated to 400° C., operating a spray pyrolysis unit at a pressure of 30 psi. The nozzle is maintained at a fixed distance of 20 cm from the substrate to assure uniform droplet deposition and controlled film formation.With this method nanostructured ZnO and Mn doped ZnO films with petal-like and nano-rod-like morphology are produced. The method allows fine control over porosity, crystallinity, and dopant distribution, and is thus an effective, scalable approach to the production of thin film compositions with customized gas sensor properties.EXAMPLE 3: X-ray diffraction analysis (XRD):The crystal structure of the deposited undoped and Mn-doped ZnO films was analyzed by X-ray diffraction (XRD). The measurements were carried out with a Rigaku Ultima IV multifunction diffractometer equipped with Cu-Kα radiation (λ=0.15406 nm). This technique allowed identification of crystallographic phases, confirmation of the hexagonal wurtzite structure, and evaluation of the effects of Mn doping on lattice parameters, crystallinity, and preferred orientation of the films.The results of X-ray diffraction (XRD) analysis of both the undoped and Mn-doped ZnO thin films showed marked diffraction peaks corresponding to planes (100), (002), (101), (102), (110), (103), (200), (112) and (201). These peaks conform well to standard JCPDS card No. 79-0206 and confirm the formation of a hexagonal wurtzite crystal structure. The lack of secondary or impurity peaks suggests the successful incorporation of Mn 2+- ions into the ZnO lattice without the formation of unwanted phases. The intensity and sharpness of the peaks reflect the polycrystalline nature of the films. Up to a Mn doping of 3%, the films exhibited preferred orientation along the (002) plane, suggesting c-axis oriented growth.However, at 5% Mn doping, a marked change in preferred orientation to the (101) plane was observed. Interestingly, the intensity of the (101) peak decreased first at 1% Mn doping, but then increased at higher Mn concentrations, indicating improved crystallinity due to Mn incorporation. In addition, a slight shift of the (100) diffraction peak was observed at lower angles due to grating expansion. This shift results from the substitution of smaller Zn 2+- ions by larger Mn 2+- ions, which cause distortion while maintaining the basic wurtzite structure of ZnO.The crystallographic parameters of undoped and Mn doped zinc oxide (ZnO) films are summarized in the following results. For pure ZnO, the lattice parameters were measured with a=0.3241a=0.3241 nm and c=0.5195c=0.5195 nm, giving a c / a ratio of 1.603. The crystal domain size (D) was 13.15 nm and the unit cell volume (V) was 0.3798 Å 3. The strain (ε) was -0.002×10 -3, and the dislocation density (δ) was 66.15×10 14 m -2.For the 1% Mn-doped ZnO thin film, the lattice parameters were a=0.3245a=0.3245 nm and c=0.5197c=0.5197 nm, with a c / a ratio of 1.602. The domain size increased to 15.80 nm and the unit cell volume was 0.3799 Å 3. The strain was -0.126×10 -3, and the dislocation density decreased to 42.47×10 14 m -2.In the case of the 3% Mn doped ZnO thin film, the lattice parameters were a=0.3246a=0.3246 nm and c=0.5205c=0.5205 nm, resulting in a c / a ratio of 1.603. The size of the crystal domain further increased to 16.35 nm, and the volume of the unit cell was 0.3796 Å 3. The strain was 0.022×10 -3, and the dislocation density further decreased to 40.88×10 14 m -2.For the 5% Mn doped ZnO film, the lattice parameters were a=0.3248a=0.3248a=0.3248 nm and c=0.5207c=0.5207 nm, giving a c / a ratio of 1.602. The domain size was 17.19 nm and the unit cell volume was 0.3797 Å 3. The strain was 0.055×10 -3, and the dislocation density further decreased to 37.44×10 14 m -2.EXAMPLE 4: Field Emission Scanning Electron Microscopy (FEESEM) and Energy Dispersive X-ray Spectroscopy (EDAX):The surface morphology of the thin films was examined by field emission scanning electron microscopy (FEESEM). This allowed detailed visualization of nanostructures such as nanopetals and nanorods, as well as estimation of their size and distribution. To supplement morphological analysis, investigations of the composition were performed by energy dispersive X-ray spectroscopy (EDAX) with a Pegasus EDS-EBSD system (JASCO-7310) to confirm the presence and uniform distribution of Mn in the ZnO matrix.The results show that the pure ZnO film has irregular nanometal structures of different sizes and larger rod-shaped structures with a diameter of 58.16 nm. These structures also have porosity and agglomeration. At 1% Mn doping, agglomerations remain, but the nanopetal and rods become more porous, with the diameter of the rods decreasing to 46.44 nm. At a Mn doping of 3%, the morphology becomes more uniform, with nanopetal and smaller rod diameters of 28.67 nm. The 5% Mn doped ZnO film has a further refinement, with sharpened uniform nanopetals and vertically elongated rods having a diameter of 23.48 nm, along with a significantly increased porosity and conspicuous voids.In terms of porosity, the Mn-doped ZnO films have a higher porosity compared to the pure ZnO films. The 3% and 5% Mn doped samples have especially prominent voids, with the 5% Mn doped sample having the greatest amount of voids and porosity. These voids, which are located between the petal and rod-shaped structures, are known to increase gas sensitivity.The results of energy dispersive x-ray spectroscopy (EDX) of 3% and 5% Mn-doped ZnO nanoplatelets show the elemental composition of the samples. The EDX spectra clearly show the presence of zinc (Zn), oxygen (O) and manganese (Mn) in the Mn-doped ZnO nanoplates, with a marked Mn peak indicating successful incorporation of manganese into the ZnO crystal structure. It is important that no additional peaks are observed in the spectra, confirming the high purity of the synthesized Mn-doped ZnO nanostructures.EXAMPLE 5: UV Visual Spectroscopy:The optical properties of the films were examined with a UV-Vis spectrophotometer. The absorption spectra were recorded to determine the band gap energy and evaluate the effects of Mn doping on the optical junctions. This analysis also suggested the light absorption behavior and the changes in the electronic structure caused by the doping.The results of the UV-Vis absorption spectra of Mn-doped ZnO thin films show a red shift of the absorption edge up to a Mn doping of 3%, which indicates a decrease in the band gap energy. At 5% Mn doping, a blue shift is observed, indicating an increase in band gap due to the Moss burstin effect. Analysis of the Tauc plot confirms this trend and shows band gap narrowing followed by magnification. These shifts are attributed to the sp-d exchange interactions and the filling of the conduction band by the incorporation of Mn 2+- ions.EXAMPLE 6: Gas Sensing Measurements:The gas sensor capability of the thin films produced was evaluated with a source electrometer. The measurements involved monitoring changes in resistance in response to various concentrations of ammonia gas under room temperature conditions. These tests provided important data on sensitivity, selectivity, response / recovery time, and detection limits that underlined the suitability of the Mn doped ZnO films for ammonia gas sensor applications.The response time, recovery time, and gas reactions to ammonia (NH 3) from undoped and Mn doped ZnO sensors at various ammonia concentrations were measured. For the undoped ZnO sensor, the response times were 22 seconds for 15 ppm, 20 seconds for 20 ppm, 20 seconds for 25 ppm, and 19 seconds for 30 ppm, with recovery times of 24, 37, 34, and 50 seconds, respectively. The response rates increased from 64% at 15 ppm to 74% at 30 ppm, indicating an improvement in gas sensitivity as the ammonia concentration increased.For the 1% Mn doped ZnO sensor, the response times were somewhat longer than for the undoped ZnO sensor, with values of 26 seconds for 15 ppm, 25 seconds for 20 ppm, 33 seconds for 25 ppm and 15 seconds for 30 ppm. Recovery times were 31, 26, 50 and 12 seconds respectively, with the percent reaction increasing from 39% at 15 ppm to 60% at 30 ppm.The 3% Mn doped ZnO sensor exhibited a faster response and recovery time than the 1% Mn doped sample. Reaction times were 18 seconds for 15 ppm, 21 seconds for 20 ppm, 17 seconds for 25 ppm and 19 seconds for 30 ppm, with recovery times of 11, 6, 12 and 10 seconds, respectively. The percentage response rates were 31% at 15 ppm, 43% at 20 ppm, 72% at 25 ppm and 90% at 30 ppm, showing a marked improvement in sensitivity.For the 5% Mn doped ZnO sensor, the response times were shortest at 15 seconds for 15 ppm, 50 seconds for 20 ppm, 34 seconds for 25 ppm and 23 seconds for 30 ppm, with recovery times of 17, 20, 19 and 8 seconds, respectively. The response rates were 80% at 15 ppm, 84% at 20 ppm, 90% at 25 ppm and 92% at 30 ppm, indicating excellent gas sensitivity.As the ammonia gas concentration increased from 15 ppm to 30 ppm, more gas molecules changed with the sensor surface, resulting in an increase in sensitivity. The literature on Mn-doped ZnO nanostructures for ammonia gas sensors reveals that these sensors have different morphologies and function at different temperatures. Some sensors are more effective in detecting higher ammonia concentrations, while others function better at lower concentrations. Other metal-doped ZnO ammonia sensors, particularly those with a nano-rod morphology, show good results at higher concentrations and operate at 300°C, but have longer response and recovery times.In this work, Mn-doped ZnO ammonia sensors were made with a mixed nano-rod nano-plateau-like morphology to measure 15 ppm ammonia gas at room temperature, having fast response and recovery times, as well as high sensitivity of up to 80%. Among the two most efficient sensors, the 3% Mn-doped ZnO sensor exhibited significant responses to low concentration differences (15, 20, 25 and 30 ppm) with short recovery times. The 5% Mn-doped ZnO sensor showed the highest sensitivity and reached 80% at 15 ppm, with the response and recovery times at room temperature being remarkably low.Examples1. A nanostructured Mn-doped petalic zinc oxide film composition for high performance ammonia detection comprising: a) zinc oxide (ZnO) formed from zinc acetate dihydrate as a zinc precursor; b) manganese (Mn 2+)- dopant incorporated into the ZnO lattice via manganese (II) acetate tetrahydrate, wherein the Mn content is in the range of 0. 5 mole % to 5 mole %, based on the zinc precursor; c) double distilled water as solvent and oxidant; d) a film morphology consisting of nanopetalens and nanorods having an average particle size in the range of 23 to 59 nm; e) wherein the composition is deposited as a porous thin film by spray pyrolysis on a substrate preheated to 400°C at a sputtering pressure of 30 psi and a spraying rate of 2 ml / min; f) wherein the composition has high sensitivity, short response and recovery times and selectivity for ammonia gas at room temperature.

Claims

A nanostructured Mn-doped petalic zinc oxide film composition for high performance ammonia detection comprising: a) zinc oxide (ZnO) formed from zinc acetate dihydrate as a zinc precursor; b) manganese (Mn 2+)- dopant incorporated into the ZnO lattice via manganese (II) acetate tetrahydrate, wherein the Mn content is in the range of 0. 5 mole % to 5 mole %, based on the zinc precursor; c) double distilled water as solvent and oxidant; d) a film morphology consisting of nanopetalens and nanorods having an average particle size in the range of 23 to 59 nm; e) wherein the composition is deposited as a porous thin film by spray pyrolysis on a substrate preheated to 400°C at a sputtering pressure of 30 psi and a spraying rate of 2 ml / min; f) wherein the composition has high sensitivity, short response and recovery times and selectivity for ammonia gas at room temperature.The composition of claim 1, wherein the manganese dopant is present in an amount of 5 mole %, resulting in a sensor response of at least 80% at 15 ppm ammonia.The composition according to claim 1, wherein the resulting film has a polycrystalline hexagonal wurtzite structure confirmed by X-ray diffraction.The composition of claim 1, wherein doping the ZnO matrix with Mn 2+ results in lattice expansion and oxygen vacancies forming, which increases gas sensitivity.The composition of claim 1, wherein the morphology comprises vertically oriented nano-rods interspersed with petal-like nanostructures, which increases surface area and porosity.The composition of claim 1, wherein the crystallite size is between 13 nm and 18 nm, depending on the degree of Mn doping.The composition of claim 1, wherein the optical band gap is tunable between 3.1 eV and 3.3 eV due to the sp-d exchange interaction induced by Mn doping and the Moss Burstein effect.The composition of claim 1, wherein the film has a response time of 15 seconds and a recovery time of 17 seconds at room temperature for 15 ppm ammonia gas.