Preparation And Performance Of C-doped And Fe,C-codoped WO₃ Sensing Materials

Chemo-resistive gas sensors have wide application prospect in the field of breath analysis due to their real-time analysis,eliminating the need for sample collection,and allowing the rapid feedback of results.Most of exhaled gases are products of metabolic processes.The change of the concentration of the exhaled gases can reflect physical condition,and some of which can be utilized as biomarkers of specific diseases.Acetone,as the metabolite of acetyl-CoA,is a promising biomarker of diabetes.Several metal oxides,such as γ-Fe₂O₃,TiO₂,ZnO and WO₃ have been used to detect acetone.WO₃ has different crystal phases,especially ε phase WO₃ is widely used to detect acetone.ε-WO₃ is a type of ferroelectric material,thus the interaction between ε-WO₃ and acetone is stronger than other breath gases,leading to a high sensitivity and selectivity to acetone.However,ε-WO₃ is not stable at operating temperature.Thus improving the stability of ε-WO₃ is great significance in the gas sensing investigation of WO₃ to acetone.In present study,ε phase WO₃ was stabilized by C doping to improve the acetone selecitivity;Fe dopant was used to increase the sensitivity based on ε phase WO₃.Principal results are as follows:1.Porous C-doped WO₃ hollow sphere materials have been synthesized by coupling carbon sphere templates with post-heat treatment method,and characterized by TG-DSC,XRD,Raman,XPS,SEM,TEM,and nitrogen sorption-desorption.The results of XRD and Raman indicate that materials are mainly composed of ε-WO₃.The XPS analysis suggests that carbon atoms are doped into the lattice of the WO₃.The results of SEM and TEM reveal that the synthesized WO₃ are spheres with the particle size in the range of 120-280 nm,the SEM of the sample polished using an argon ion beam shows the hollow structure;the test of BET shows that the calculated value of specific surface area is 22.0 m2·g-1,and the diameter of mesopores is about 6.2 nm.2.The gas sensing performance of synthesized C-doped WO₃ was systematically studied.The effect of the proportion of the added carbon spheres and calcination temperature is evaluated,the results show that the materials with the added carbon spheres in the proportion of 23 wt.% and sintered at 450 oC have a better gas sensing performance to acetone;the effect of operating temperature to sensing performance was also investigated and the optimal condition is around 300 oC;the sensitivity to different gases of C doped WO₃ is evaluated,the response to 0.9 ppm ethanol,methanol,toluene,NH₃,NO and CO is below 2,to 0.9 ppm acetone is 5.1,indicating the good selectivity to acetone;the effect of relative humidity to sensing performance was investigated,the results show that the response of C-doped WO₃ to 2 ppm acetone is 3.1,while that to 0.9 ppm acetone is 1.8 at 90% RH,72% response increase is still sufficient to separate diabetic patients from healthy persons;the test of long-time stability shows that the response of the synthesized materials aging at 350 oC for the duration of 3 months is 11.4 to 10 ppm acetone,maintaining almost constant values（original value S = 11.3）,indicating C doped WO₃ have a good stability.3.Fe,C co-doped WO₃ submicron spheres were synthesized by coupling carbon sphere templates with post-heat treatment method,and characterized by TG-DSC,XRD,Raman,XPS,SEM,TEM,PL and nitrogen sorption-desorption.The results of XRD and Raman indicate that materials are mainly composed of ε-WO₃;XPS results show that all the peaks of C 1s exhibit a positive shift,except that at 284.8 e V,and the peaks of Raman spectra are observed to shift to lower frequency,indicating that iron is doped into the lattice of the WO₃;The results of SEM and TEM reveal that the synthesized Fe,C-codoped WO₃ are porous spheres with the particle size in the range of 120-220 nm;in the PL spectrum of Fe,C-codoped WO₃,the increase in intensity of the peaks at the range of 440-490 nm and the emergence peak at ⁴40 nm indicate that the doping of Fe would generate oxygen vacancies in the lattice of WO₃.4.The acetone gas sensing performance of synthesized Fe,C-codoped WO₃ is studied in detail.The effect of the proportion of Fe dopant is investigated,results show that the materials with 0.445 mol% Fe have a highest response of 17.7 to 10 ppm acetone at the operating temperature of 300 oC;the effect of operating temperature to sensing performance is also studied and the optimal condition is around 300 oC;the sensitivity to different gases of the synthesized materials is evaluated,the response to ethanol,methanol,toluene,NH₃,NO and CO is below 2,to 0.9 ppm acetone is 7.3,indicating the good selectivity to acetone;the effect of relative humidity to sensing performance was investigated,the response of Fe,C-codoped WO₃ to 2 ppm acetone is 4.7,while that to 0.9 ppm a cetone is 3.2 at 90% RH,the response increase is sufficient to separate diabetic patients from healthy persons;the test of long-time stability shows that the response of the materials aging at 350 oC for the duration of 3 months is 17.6 to 10 ppm acetone,maintaining almost constant values（original value S = 17.7）,indicating Fe,C-codoped WO₃ materials have a good stability.