Ni-doped nano SnO2 semiconductor HCHO sensing material, and preparation method and application thereof

Abstract

The application discloses a kind of Ni doped nano SnO2 semiconductor HCHO sensing materials and preparation method and application thereof, and the preparation method of sensing material includes the following steps: step one, Sn salt-Ni salt solution is configured;Step two, using supercritical hydrothermal synthesis (CSHS) reaction device, Sn salt-Ni salt solution is contacted with supercritical water and crystallization is generated nano doped oxide, and reaction liquid is discharged and collected after cooling, and the crystalline material is separated, i.e. the Ni doped nano SnO2 semiconductor HCHO sensing material of the application.The Ni doped nano SnO2 semiconductor HCHO sensing material prepared by the method of the application has good structural consistency and excellent sensing performance.

Description

Technical Field

[0001] This invention relates to the field of metal oxide semiconductor gas sensing materials technology, specifically to a method and application for preparing Ni-doped nano-SnO2 semiconductor HCHO sensing materials using CSHS technology. Technical Background

[0002] HCHO is a colorless organic compound with a pungent odor. HCHO concentrations higher than 0.1 mg / m³... 3 However, prolonged exposure to HCHO can pose potential health risks, leading to considerable public concern about low concentrations of HCHO in indoor and other environments. Low-concentration HCHO sensors often become inactive due to poisoning in high-concentration HCHO environments. Furthermore, some factories may contain high concentrations of HCHO (1 mg / m³). 3 Given the potential leakage risk (as mentioned above), HCHO alarms also need to be addressed, as high concentrations of HCHO can cause coughing, wheezing, and even death in people exposed to the environment. Therefore, there is a need to develop an HCHO sensor capable of handling high concentration ranges; the market potential for such real-time monitoring sensing materials is enormous.

[0003] Metal-oxide-semiconductor (MOS) gas sensors have been widely used in environmental monitoring, pollution control, and safety early warning systems due to their excellent sensing characteristics, ease of integration, and significant cost-effectiveness, becoming an important technical means for detecting toxic and harmful gases. However, in real-time HCHO detection applications, this type of sensor suffers from limitations such as insufficient stability and low selectivity. Therefore, developing HCHO gas sensing materials with excellent stability and high selectivity has become a key issue and research focus in the field of ambient air detection technology. Summary of the Invention

[0004] To address the aforementioned technical problems in the existing technology, the present invention aims to provide a Ni-doped nano-SnO2 semiconductor HCHO sensing material, its preparation method, and its application.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A supercritical hydrothermal preparation method for Ni-doped nano-SnO2 semiconductor HCHO sensing material, characterized by comprising the following steps:

[0007] 1) Prepare an aqueous solution containing SnCl4 and NiCl2 precursors, denoted as Sn salt-Ni salt solution;

[0008] 2) Preparation is carried out using a CSHS reaction apparatus. The Sn salt-Ni salt solution and ultrapure water described in step 1) are transported separately by high-pressure pumps and premixed in a three-way mixer. Then, the Sn salt-Ni salt premixed solution and the high-pressure transported supercritical water at 450℃-550℃ are mixed again in a four-way mixer. After thorough mixing, the mixture is heated and kept at a certain temperature by an electric heating device, so that the SnCl4 / NiCl2 solution reacts rapidly upon contact with the supercritical water and crystallizes to form nano-doped oxides. The reaction solution is discharged, collected, and cooled to separate the crystals, thus obtaining the Ni-doped nano-SnO2 semiconductor HCHO sensing material.

[0009] Further, the supercritical hydrothermal preparation method of Ni-doped nano-SnO2 semiconductor HCHO sensing material as described in step 1) is characterized in that, in the Sn-Ni mixed solution prepared in step 1), the molar concentration of Sn salt is 0.01-0.03 mol / L, and the molar concentration of Ni salt is 0.1-5% of the molar concentration of Sn salt, preferably 1%.

[0010] Further, the supercritical hydrothermal preparation method of Ni-doped nano-SnO2 semiconductor HCHO sensing material as described in step 1) is characterized in that the Sn salt is SnCl4 and the Ni salt is NiCl2.

[0011] Furthermore, the supercritical hydrothermal preparation method of the Ni-doped nano-SnO2 semiconductor HCHO sensing material as described in step 2) is characterized in that the specific operation of step 2) includes the following process:

[0012] Three high-pressure transfer pumps for liquid chromatography (HPLC) pump ultrapure water, precursor solution, and ultrapure water at specific flow rates at room temperature, respectively, applying an initial pressure of 22-25 MPa to each of the three lines. The first line of ultrapure water, delivered at room temperature and high pressure, merges into a three-way valve to undergo initial mixing with the second line of Sn-Ni salt solution, also delivered at room temperature and high pressure. Simultaneously, the third line of ultrapure water, delivered at room temperature and high pressure, flows through a first electric heating device and is heated to 450-550°C, reaching a supercritical state. The Sn-Ni salt solution and supercritical water then merge into the... The mixture is mixed twice in a four-way mixer. After thorough mixing, it flows into a second set of electric heating devices, where the heating temperature is maintained at 450℃–550℃. The SnCl4 / NiCl2 solution is brought into contact with supercritical water for more than 2 seconds to react. The Sn salt-Ni salt solution reacts rapidly with the supercritical water and crystallizes to form nano-doped oxides. The reaction liquid is collected after flowing out of the second set of electric heating devices, cooled, and filtered to finally obtain the Ni-doped nano-SnO2 semiconductor HCHO sensing material.

[0013] Further, as described in step 2), the supercritical hydrothermal preparation method of a Ni-doped nano-SnO2 semiconductor HCHO sensing material is characterized in that the volumetric flow rate ratio of the second ambient temperature high pressure Sn salt-Ni salt solution to the first ambient temperature high pressure ultrapure water is 1:1.8-2.5, preferably 1:2; and the volumetric flow rate ratio of the second ambient temperature high pressure SnCl4 / NiCl2 solution to the third ambient temperature high pressure ultrapure water is 1:0.8-1.2, preferably 1:1.

[0014] Further, as described in step 2), the supercritical hydrothermal preparation method of Ni-doped nano-SnO2 semiconductor HCHO sensing material is characterized in that, after the reaction is completed, the nano suspension is collected and allowed to settle for 12–24 h. After centrifugation and washing with ultrapure water for 4–6 times, the mixture is frozen for 4 h and freeze-dried for 24 h. The powder is then removed, ground, and placed in a glass container. The air inside the container is purged with argon gas, and the container is sealed with tape to preserve the powder, thereby obtaining nano-SnO2 semiconductor HCHO sensing material powders with different Ni doping contents.

[0015] Furthermore, an HCHO sensor is prepared, characterized in that the HCHO sensor comprises a substrate, a heating plate, interdigitated electrodes and a gas-sensitive material layer stacked sequentially, wherein the gas-sensitive material layer comprises a Ni-doped nano-SnO2 semiconductor HCHO sensing material as described in this invention.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) CSHS technology is well-suited for preparing various nano-metal oxides due to its ability to precisely control reaction parameters such as reaction temperature, reaction pressure, reaction time, and pH. Under high temperature and pressure, materials often form in their most stable state, resulting in highly uniform physicochemical properties in each batch, including microstructure, particle size, and crystal planes. Because of these similar microscopic properties, these materials exhibit excellent stability in resistance and response values ​​in macroscopic sensing performance, and the resulting sensors have relatively long lifespans, making them highly valuable for gas detection. Furthermore, the clever introduction of trace amounts of Ni during synthesis to dope and modify the nano-SnO2 semiconductor material, along with adjustments to the testing temperature, can significantly improve the HCHO sensing performance of the SnO2 semiconductor material, giving it a better application prospect in air quality control.

[0018] (2) The supercritical hydrothermal preparation method of the Ni-doped nano-SnO2 semiconductor HCHO sensing material of the present invention not only represents a technological breakthrough but also demonstrates excellent performance in practical applications. Experimental verification shows that the Ni-doped nano-SnO2 material of the present invention is particularly suitable for high concentrations of HCHO (1 mg / m³). 3 In the above tests, the response value is high.

[0019] Another major highlight of CSHS technology is its full utilization of the unique properties of supercritical water, enabling continuous 24-hour production. This translates to a significant increase in production efficiency, meeting the demands of modern industry for high-efficiency manufacturing. From an economic perspective, CSHS technology also boasts relatively low production costs, which is invaluable for promoting its industrialization and achieving large-scale production. More importantly, this preparation method does not use any acids, alkalis, or organic ligands as precipitants or pH adjusters, making it green and energy-efficient. Furthermore, due to precise control of reaction conditions and continuous production capabilities, both the quality and yield of the materials are significantly improved simultaneously, which is crucial for meeting market demands and driving industrialization.

[0020] In summary, by introducing CSHS technology and cleverly applying it to the preparation of nano-SnO2 semiconductor HCHO sensing materials, this invention not only achieves controllability, efficiency, and greening of the preparation process, but also significantly improves the sensing performance and application value of the materials. This injects new vitality into the development of materials science and sensor technology, and provides more reliable technical support for fields such as environmental monitoring and air quality control. Attached Figure Description

[0021] Figure 1 Schematic diagram of the CSHS device.

[0022] Figure 2 Response curve of 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material to 8ppm HCHO at 50℃–200℃ in Example 1; Test conditions: HCHO concentration: 8ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 3L / min, reaction temperature 50℃–200℃.

[0023] Figure 3 Response recovery curves of 0.1–5% Ni-doped nano-SnO2 semiconductor HCHO sensing materials at 175°C to 8ppm HCHO; Test conditions: HCHO concentration: 8ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 3L / min, reaction temperature 175°C.

[0024] Figure 4The sensing response of the 0.1–5% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Examples 1-2 to 8 ppm HCHO at 175°C; test conditions: HCHO concentration: 8 ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 3 L / min, reaction temperature 175°C.

[0025] Figure 5 Example 1: Sensing response recovery curves of 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material at 175°C to 1–8 ppm HCHO; Test conditions: HCHO concentration: 8 ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 1 L / min, reaction temperature 175°C.

[0026] Figure 6 The stability test results of the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Example 1 in response to 8ppm HCHO at 175℃; test conditions: HCHO concentration: 8ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 3L / min, reaction temperature 175℃.

[0027] Figure 7 Example 1: Selective response of 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material to five test gases at 175°C; Test conditions: gas concentration: 8ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 3L / min, reaction temperature 175°C.

[0028] Figure 8 The sensing response values ​​of the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Example 1 and the 1% antimony (Sb)-doped nano-SnO2 semiconductor HCHO sensing material in Example 3 to 5-20ppm HCHO were obtained. The test conditions were: HCHO concentration: 5-20ppm, O2 concentration: 20%, N2 as supplementary gas, total flow rate: 3L / min, and reaction temperature: 175℃.

[0029] Figure 9 XRD patterns of nano-SnO2 semiconductor HCHO sensing materials with different Ni doping contents in Examples 1 and 2.

[0030] The Ni-doped nano-SnO2 semiconductor HCHO sensing material prepared in this invention is an N-type semiconductor. For an N-type semiconductor, the sensitivity S is defined as the ratio of the resistance value (Ra) of the gas sensing element in clean air to the resistance value (Rg) in the target gas, i.e., S = Ra / Rg.

[0031] Ra is the resistance value of the device after it stabilizes in clean air, and Rg is the resistance value of the device after it stabilizes in the gas to be tested; response time is the time required for the device resistance value to change by 90% during the response phase, and recovery time is the time required for the device resistance value to change by 90% during the recovery phase. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Rather, this invention covers any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the invention as defined by the claims. Furthermore, to provide the public with a better understanding of this invention, certain specific details are described in detail below. Those skilled in the art will fully understand this invention even without these detailed descriptions.

[0033] Example 1

[0034] A supercritical hydrothermal preparation method for Ni-doped nano-SnO2 semiconductor HCHO sensing material includes the following steps:

[0035] Step 1: Dissolve a certain amount of SnCl4 and NiCl2 in ultrapure water to prepare an aqueous solution with a SnCl4 molar concentration of 0.02 mol / L and a NiCl2 molar concentration of 0.0002 mol / L. Stir at 600 r / min for 30 min to obtain a SnCl4 / NiCl2 solution, where the molar concentration of NiCl2 is 1% of the molar concentration of SnCl4.

[0036] Step Two: Synthesis and preparation of 1% Ni-doped SnO2 semiconductor HCHO sensing material using a CSHS reaction apparatus. The CSHS reaction apparatus mainly consists of four parts: a liquid phase delivery system, a reaction system, a temperature control system, and a sample collection system, such as... Figure 1 As shown.

[0037] Liquid phase delivery system: It consists of three identical high-pressure delivery pumps for liquid chromatography and corresponding liquid phase pipelines and adapters, which are used to deliver ultrapure water, SnCl4 / NiCl2 solution and ultrapure water respectively.

[0038] The reaction system consists of two sets of electric heating devices, corresponding liquid phase pipelines, and adapters. The large electric heating device is used to heat the room-temperature, high-pressure ultrapure water to 450℃, bringing it to a supercritical state. The small electric heating device is used to insulate the straight pipe section at the mixer outlet, ensuring that the contact time between the SnCl4 / NiCl2 solution and the supercritical water at 450℃ reaches more than 2 seconds.

[0039] Temperature control system: controls two sets of electric heating devices to raise the temperature according to the program and stabilize the temperature above 450℃.

[0040] Sample collection system: It consists of a water cooling device, a filter, and a back pressure valve. The prepared nanomaterials flow out of the system as a suspension after being cooled and depressurized.

[0041] The process of preparing 1% Ni-doped SnO2 semiconductor HCHO sensing material using a CSHS device: Three high-pressure transfer pumps for liquid chromatography pump ultrapure water, SnCl4 / NiCl2 solution and ultrapure water at room temperature at 20 mL / min, 10 mL / min and 10 mL / min respectively, and apply an initial pressure of 23 MPa to the three pipelines. The internal pressure of the entire system is maintained by relying on the back pressure valve at the end of the system outlet. Room temperature and high pressure ultrapure water and room temperature and high pressure SnCl4 / NiCl2 solution are initially mixed in a three-way mixer. At the same time, the room temperature and high pressure ultrapure water flows through an electric heating device (large) and is heated to 450°C to reach the supercritical state. Then, the SnCl4 / NiCl2 solution and supercritical water are mixed in a four-way mixer for a second time. After thorough mixing, the mixture flows into an electric heating device (small), where the temperature is still maintained at 450°C. The SnCl4 / NiCl2 solution and supercritical water are brought into full contact at 450°C for more than 2 seconds to react. After contact with the supercritical water, the SnCl4 / NiCl2 solution reacts rapidly and crystallizes to form nano-oxide. The reaction liquid flows out of the electric heating device (small) and enters the sample collection system. After cooling, depressurization, and collection, a suspension of 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material is obtained.

[0042] Step 3: After the reaction, the nano suspension was collected and allowed to settle for 12 hours. After centrifugation and washing with ultrapure water for 6 times, the powder was frozen at -70°C for 4 hours and dried for 24 hours. The powder was then removed, ground, and placed in a glass container. The air inside the container was purged with argon gas, and the container was sealed with tape to preserve the powder. This yielded 1% Ni-doped nano SnO2 semiconductor HCHO sensing material powder.

[0043] Step 4: Weigh 100mg of the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material powder obtained in Step 3 using a balance. Measure 500μL of ultrapure water using a pipette and place it in a sample tube. Mix well. Use a pipette to draw 2.5μL of the mixture and drop it onto the gold-based interdigitated electrode sheet. The interdigitation spacing and interdigitation width are both 200μm. Dry the electrode sheet using a 70℃ heating stage and let it stand for 30min. After drying, the coating thickness is about 50μm, which is the electrode sheet that can be used to test the HCHO sensing performance, i.e., the gas sensor.

[0044] Example 2

[0045] The supercritical hydrothermal preparation of nano-SnO2 semiconductor HCHO sensing materials with Ni doping amounts of 0.1%, 0.5%, 3%, and 5% was carried out. The preparation method steps were repeated in Example 1, with the only difference being that "in the preparation of the SnCl4 / NiCl2 solution in step one, the molar concentration of NiCl2 in the solution was adjusted to 0.00002 mol / L, 0.0001 mol / L, 0.0006 mol / L, and 0.001 mol / L, so that the molar concentration of NiCl2 was 0.1%, 0.5%, 3%, and 5% of the molar concentration of SnCl4, respectively". All other conditions remained unchanged, and finally, SnO2 semiconductor HCHO sensing materials with Ni doping amounts of 0.1%, 0.5%, 3%, and 5% were obtained.

[0046] Example 3

[0047] The preparation method of 1% Sb-doped nano-SnO2 semiconductor HCHO sensing material is the same as that in Example 1, except that "in the preparation of SnCl4 / NiCl2 solution in step one, NiCl2 is replaced with SbCl3 of the same molar concentration", and the other conditions remain unchanged, and finally 1% Sb-doped nano-SnO2 semiconductor HCHO sensing material is obtained.

[0048] The HCHO sensing materials prepared in Examples 1-3 were used to fabricate gas sensors according to the method in step four of Example 1, and the relevant gas-sensing performance tests were performed on HCHO gas.

[0049] like Figure 2 As shown, the performance of the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material is significantly improved at 175℃, and is similar to that at 200℃. Considering the low temperature principle, it is believed that the optimal response performance for 8ppm HCHO is achieved at 175℃.

[0050] like Figure 3 As shown, the 0.1–5% Ni-doped nano-SnO2 semiconductor HCHO sensing material exhibits excellent HCHO sensing performance at 175°C, with both response and recovery times for 8ppm HCHO being less than 50s.

[0051] like Figure 4 As shown, the sensing response value of the 0.1–5% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Example 1 to 8ppm HCHO at 175°C shows that the performance is optimal when the Ni doping amount is 1%.

[0052] like Figure 5As shown, the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Example 1 exhibited excellent sensing performance. With the increase of HCHO gas concentration, the sensitivity of the sensing material in Example 1 increased significantly, and a good linear growth relationship was shown between sensitivity and concentration.

[0053] like Figure 6 As shown, the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Example 1 exhibited excellent stability, with a response value maintained at around 8.2, a response time of approximately 56 s, and a recovery time of approximately 215 s. After continuous testing, its response value to HCHO did not show significant fluctuations.

[0054] like Figure 7 As shown, the 1% Ni-doped nano-SnO2 semiconductor HCHO sensing material in Example 1 exhibits excellent gas selectivity.

[0055] like Figure 8 As shown, two SnO2 doped materials, 1% Ni and 1% Sb, were prepared and their performance was compared. The results showed that when tested with 5–20 ppm HCHO, the response value of the Ni-doped SnO2 material to HCHO was about 4 times that of the Sb-doped SnO2 material.

[0056] like Figure 9 As shown, the XRD patterns of the Ni-doped nano-SnO2 semiconductor HCHO sensing materials with different Ni contents in Examples 1 and 2 did not show any impurity peaks of other phases, which is consistent with the standard card. The space group is p42mm, and each peak can point to the tetragonal phase of SnO2. This shows that the crystal structure of the material prepared by supercritical hydrothermal technology is very uniform, and its crystal planes are stable (101) and (110) crystal planes, which is also related to its stable performance. According to the Scherrer formula, the average particle size of the prepared material is about 5nm.

Claims

1. An application of an HCHO sensor, characterized in that, Used to detect HCHO in gas, where the concentration of HCHO in the gas to be tested is above 1 ppm; The HCHO sensor includes a substrate, a heating plate, interdigitated electrodes and a gas-sensitive material layer stacked sequentially, wherein the gas-sensitive material layer includes Ni-doped nano-SnO2 semiconductor HCHO sensing material. The supercritical hydrothermal preparation method of the Ni-doped nano-SnO2 semiconductor HCHO sensing material includes the following steps: 1) Prepare an aqueous solution containing tin (Sn) salt and Ni salt, denoted as Sn salt-Ni salt solution; 2) The Sn-Ni salt solution and ultrapure water described in step 1) are transported separately by high-pressure pumps and premixed in a three-way mixer. Then, the Sn-Ni salt premixed solution and the high-pressure transported supercritical water at 450℃–550℃ are mixed again in a four-way mixer. After thorough mixing, the mixture is heated and kept at a certain temperature by an electric heating device, so that the Sn-Ni salt solution reacts rapidly after contacting the supercritical water and crystallizes to form nano-doped oxides. The reaction solution is discharged, collected, and cooled to separate the crystals, thus obtaining the Ni-doped nano-SnO2 semiconductor HCHO sensing material. In step 1), the molar concentration of Sn salt in the prepared Sn-Ni mixed solution is 0.01-0.03 mol / L, and the molar concentration of Ni salt is 1% of the molar concentration of Sn salt.

2. The application as described in claim 1, characterized in that, The Sn salt is tin tetrachloride SnCl4, and the Ni salt is nickel dichloride NiCl2.

3. The application as described in claim 1, characterized in that, Step 2) includes the following specific operations: Three high-pressure transfer pumps for liquid chromatography (HPLC) pump ultrapure water, precursor solution, and ultrapure water at specific flow rates at room temperature, respectively, applying an initial pressure of 22-25 MPa to each pipeline. The first pipeline, with room-temperature, high-pressure ultrapure water, flows into a three-way connector for initial mixing with the second pipeline, with room-temperature, high-pressure Sn-Ni salt solution. Simultaneously, the third pipeline, with room-temperature, high-pressure ultrapure water, flows through a first electric heating device, where it is heated to 450-550°C, reaching a supercritical state. The Sn-Ni salt solution and supercritical water then flow into a four-way mixer for secondary mixing. After thorough mixing, the mixture flows into a second electric heating device, where the heating temperature is maintained at 450-550°C, ensuring sufficient contact between the Sn-Ni salt solution and the supercritical water. The reaction proceeds for more than s. After the Sn salt-Ni salt solution comes into contact with supercritical water, it reacts rapidly and crystallizes to form nano-doped oxides. The reaction liquid is collected after flowing out of the second set of electric heating devices, cooled, filtered, and finally the Ni-doped nano SnO2 semiconductor HCHO sensing material is obtained.

4. The application as described in claim 3, characterized in that, The volumetric flow rate ratio of the second ambient temperature and high pressure Sn-Ni salt solution to the first ambient temperature and high pressure ultrapure water is 1:1.8–2.5; the volumetric flow rate ratio of the second ambient temperature and high pressure Sn-Ni salt solution to the third ambient temperature and high pressure ultrapure water is 1:0.8–1.

2.

5. The application as described in claim 4, characterized in that, The volumetric flow rate ratio of the second ambient temperature high pressure Sn-Ni salt solution to the first ambient temperature high pressure ultrapure water is 1:2; the volumetric flow rate ratio of the second ambient temperature high pressure Sn-Ni salt solution to the third ambient temperature high pressure ultrapure water is 1:

1.

6. The application as described in claim 3, characterized in that, After the reaction was completed, the nano suspension was collected and allowed to settle for 12–24 h. After centrifugation and washing were repeated 4–6 times, the mixture was freeze-dried, the powder was taken out, ground, and placed in a glass container. The air inside the container was purged with argon gas, and the container was sealed with tape to preserve the powder, thus obtaining nano SnO2 semiconductor HCHO sensing material powders with different Ni doping contents.

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