Alpha bismuth trioxide (α-Bi2O3) is an environmentally-friendly material . Bi2O3 is a semiconductor with various physical and chemical properties including high refractive index, good photoconductive response, and high oxygen-ion conductivity. The Eg value of the α Bi2O3 is 2.85 eV . There are some polymorphs of bismuth trioxide including α-Bi2O3, β-Bi2O3, γ-Bi2O3, δ-Bi2O3, ε-Bi2O3, and ω-Bi2O3 that are stable based on temperature. The stable crystal phase in low temperature is monoclinic α-Bi2O3 [3, 4, 5]. Bi2O3 has been studied extensively, and it has different applications in electrical ceramics, fuel cells, gas sensors, superconductors, optical coatings, supercapacitors, energy storage, and photocatalysts . Furthermore, several methods have reported the fabrication of Bi2O3 such as solution [7,8], solution combustion , solvothermal , hydrothermal , laser ablation , microwave , sol–gel , pyrolysis , thermal decomposition [16,17], electrodeposition , thermal oxidation , chemical vapor deposition , green synthesis , and solid state with air annealing . Solid state method is a useful and scalable route which can prepare nanomaterials with high purity and large amount without using any liquid solvent . Additionally, there are several studies reporting the sensing application of nanomaterials [24-27].
The present work reports the facile, one-step, and low-temperature solid state fabrication of Bi2O3 nanomaterials. To our knowledge, there is no report about the preparation of the nanomaterials under the present conditions. The crystallographic data of the fabricated nanomaterials are obtained by Rietveld analysis. Moreover, the characterization, morphology as well as optical, magnetic, and electrochemical properties of the synthesized samples are investigated by XRPD, FESEM, FTIR, UV-Vis and VSM analyses. The main goal of the present work is the synthesis of new classes of Bi2O3 nanomaterial by the low-temperature solid state route. Besides, this paper describes the application of Bi2O3 nanomaterials as a CO gas sensor. Our results suggest that the obtained material has remarkable sensitivity as CO gas sensor.
MATERIALS AND METHODS
All of the used raw materials such as Bi5O(OH)9(NO3)4 were purchased from Sigma Aldrich Company, were of analytical grade (99%) and used without further purification. The characterization and identification of the crystal-phase type fabricated nanopowders was conducted by X-ray powder diffractometer D5000 (Siemens AG, Munich, Germany) using CuKα radiation with the deviation of ±0.02°. FullProf software was employed to study the Rietveld analysis. The morphology and elemental analysis of the obtained materials were examined by a Philips XL30 scanning electron microscope (Philips, Amsterdam, Netherlands). A Tensor 27 spectrometer (Bruker Corporation, Germany) with the deviation of ±2 cm-1 was utilized to record the FTIR spectrum. Recording the absorption spectra and calculation of the band gap energies were carried out by Analytik Jena Specord 40 (Analytik Jena AG Analytical Instrumentation, Jena, Germany) apparatus with the deviation of ±2 nm. In addition, the magnetic property of the samples was studied by a vibrating sample magnetometer (VSM, Model 7400- LakeShore) with the deviation of ±0.001 H(Oe). The gas sensing experiments were conducted in a home-made testing system (a cylindrical stainless steel chamber with a volume of 300 mL) which is called sensing analysis system in research institute of petroleum industry (SAS-RIPI).
The crystallite size data of the fabricated Bi2O3 nanomaterials is calculated by the Scherrer equation.
The parameter X-ray density (ρx) is calculated by the following formula:
SSA is calculated by ρxrd and D data according to the below equation :
Preparation of Bi2O3 nanomaterial
Pure and doped Bi2O3 nanomaterials were synthesized by a one-step solid state method involving thermal decomposition of bismuth nitrate raw material in normal atmospheric condition. In this case, 0.5 g (0.34 mmol) of Bi5O(OH)9(NO3)4 (MW=1462 g mol-1) was ground in a ceramic crucible and heated in one step at 400 ˚C for 24 h in a preheated electric furnace. Afterwards, the sample was cooled down to room temperature in the furnace.
Preparation and evaluation of the sensor
To fabricate gas sensor, the prepared Bi2O3 powder was ground into tiny-flour, and then blended with TX100 with the weight ratio of 3:1 to form a homogeneous paste. Then, the pastes were coated on alumina substrates (1mm×1 mm). Gold shoulder electrodes were coated on the surface substrates by the sputtering technique, followed by drying at room temperature. Before testing, the sensors were aged at 400 ºC for 12 h to improve repeatability and stability.
All sensors were pre-heated at different operating temperatures for 30 min. When the resistance reached a constant value, the test chamber was opened to let the gas in. As the air and target gas flowed through the test chamber, the corresponding resistances of the sensor in air (Ra) and target gas (Rg) were measured. In this paper, the sensitivity of the gas response (S) was defined as the ratio of sensor’s resistance in air to that in target gas (S = Ra/Rg). The response and recovery times were defined as the time required for a change in response to reach 90 % of the equilibrium value after injecting and removing the detected gas, respectively. During the test, the operating temperature range was set at 200-350 ºC, and the relative humidity was 40 %.
RESULTS AND DISCUSSIONS
The synthesized Bi2O3 sample was characterized by the X-ray diffraction method. The XRPD pattern is presented in Fig. 1. FullProf program employing profile matching with constant scale factors was used to perform the structural analysis of the as-fabricated sample. Red lines are the observed peaks intensities (Yobs) and the black ones are the calculated data (Ycalc). The blue line denotes the difference: Yobs - Ycalc. According to Fig. 1, the bars presented above the green line correspond to α-Bi2O3 with the crystal structure of monoclinic with space group of P121/c1 [1-5]. The miller indices are 24.55, 25.74, 26.75, 27.41, 32.98, 46.18 are (-102), (002), (-112), (101), (-211), and (-223) for the main peaks.
The crystallographic data of the obtained sample was calculated for monoclinic crystal system. The crystallite size (D) of the fabricated Bi2O3 nanomaterial is reported in Table 1. In the Scherrer equation, D is the crystalline sample’s entire diameter thickness; the X-ray diffraction wavelength is λ (0.154 nm); the Scherrer constant is K (0.9); B1/2 denotes the full width at half of its maximum intensity (FWHM) of the certain used peak, and θ is the half diffraction angle of the peak. In this formula to calculate ρx, M is the molecular weight of Bi2O3 (MW=466 gmol−1); N is the Avogadro number; Z is the number of formula unit per unit cell for Bi2O3 (Z=4) and a, b, c are lattice parameters. The specific surface area (SSA) per lattice volume is a physical property that can affect physical and chemical behavior.
Fig. 2 presents FESEM images of the as-prepared nanomaterial. As can be seen from the images, the morphology of the sample is particles. It is clear from the images that the homogeneity of particles morphology is high. Further, the particles have close sizes and most of the them have sizes smaller than 20 nm. However, there are some particles with larger sizes around 80-100 nm.
The magnetic property of the fabricated sample was investigated by VSM analysis to understand the magnetic behavior of the synthesized powder. The magnetic hysteresis (M-H) curve of the as-prepared Bi2O3 is shown in Figs. 3a and b. The plots reveal the evidence that the nanomaterials manifest ferromagnetic behavior at room temperature. According to Fig. 3a, the saturation magnetization (Ms) value is 0.01 emu/g. Furthermore, the data indicates that the compound keeps the magnetization in zero fields when the external magnetic field was applied. Remanence magnetization, Mr, is the magnetization strength in which the magnetization is retained when the external magnetic field is zero (H = 0) . By magnifying the loop (Fig. 3b), the data indicated that the most saturation remanence (Mr) is 0.008. The squareness ratio (Mrs = Mr/Ms) magnitude is calculated by the ratio of remanence and saturation magnetization. When a particle is distributed isotropically and uniformly magnetized without intergrain interactions, the material will have a squareness ratio below 0.5 which confirms the formation of multi-domain structure when the exchange coupling between adjacent grains takes place. In the present work, it was found that Mrs is 0.4. Therefore, the data demosntrate that there is no preferred direction in magnetization for the fabricated compound.
FTIR spectrum of the as-prepared nanomaterial is shown in Fig. 4. The general peaks for all samples are at about 512, 546, 620, 650, 770, 850, 1650, 2300, 2900 and 3400 cm-1. In general, metal oxides show absorption bands below 1000 cm−1. The peaks at 400 – 550 cm-1 are assigned to oxygen – metal – oxygen (O-M-O) vibrations. Additionally, the bands located at 546, 620, 650, 770 and 850 cm−1 are assigned to the stretching vibration modes of Bi–O bonds of BiO6 octahedron [30,31]. Moreover, the peak located at 646 cm−1 is attributed to Bi-O vibration [32,33], and the peaks at 1650 and 3400 cm-1 also correspond to the physically adsorbed H2O . The peaks at 2300 and 2900 cm-1 can be attributed to carbonate impurity vibrations .
UV-Vis absorption spectrum of the fabricated nanomaterial is shown in Fig. 5a. The Eg plot of the sample is also presented in Fig. 5b. According to the UV-Vis absorption data, it is found that the nanomaterial possesses a typical strong absorption edge at about 500 nm. The light absorption in the region suggesting a fantastic photoactive property under blue light irradiation. Besides, a wide absorption region from 330 – 430 nm exists in the absorption plot. The relation between the absorption coefficient and incident photon energy can be written as (αhν)2 = A(hν - Eg) for direct band gap energy. In this equation, A and Eg are a constant value and direct band gap energy, respectively . To measure the Eg value, the linear part of the curve to the energy axis is extrapolated. Fig. 6b indicates that the synthesized material shows a strong band structure at 2.30 eV.
Carbon monoxide sensing properties
To investigate the performance of bismuth oxide sensor for detecting carbon monoxide, the nanostructure of Bi2O3 nanomaterial was synthesized. According to Fig. 6, it can be seen that the Bi2O3 has good sensitivity to CO gas at low levels of carbon monoxide at 200 ºC.
The present work reported the facile solid-state synthesis of Bi2O3 nanomaterial. Rietveld analysis data confirmed the high purity of the obtained nanomaterial. FESEM images indicated that the homogeneity of particles morphology was high. In addition, the particles had close sizes and most of the particles had sizes smaller than 20 nm. It was found that the synthesized material showed strong band structure at 2.30 eV. The magnetic property data confirmed the ferromagnetic behavior of the synthesized materials. No preferred direction in the magnetization was found for the synthesized sample. Satisfactory CO gas sensing was confirmed by using Bi2O3 as sensor at 200 ºC sensing medium.
This work is funded by Sayyed Jamaleddin Asadabadi University Research Grant
CONFLICT OF INTEREST
The authors declare they have no conflict of interest for the present work.