| It is much of importance to develop various advanced gas sensors because of their application in wide fields such as environmental monitoring,medical diagnostics,public safety,where the design and synthesis of the gas-sensing materials with high sensitivity,high selectivity,high stability,and low detection limits is particularly critical.In this paper,a series of ordered large-pore mesoporous chromium oxides with high specific surface area and ultrathin framework and their counterparts fuctionalized by noble metals were synthesized by a hard template method and their gas-sensing properties were also investigated.A series of ordered mesoporous chromium oxides were synthesized by using ordered mesoporous silica KIT-6 as template and chromium nitrate nonahydrate as precursor.By changing the hydrothermal aging temperature(40,80,100,130 and 140?),the pore size(5.6,6.1,7.6,8.8 and 10.1 nm)and interconnectivity of KIT-6 are gradually increased and the framework thickness of the corresponding ordered mesoporous chromium oxides increased from 5 to 9 nm,accompanied by the decrease of the specific surface area from 168 to 45 m2/g and the pore size from 11 to 3 nm.It is hard for chromium nitrate precursors to penetrate into the mesochannels of KIT-6 with small pore of 5.6 nm and thus bulk chromium oxide easily formed outside of KIT-6 when excessive chromium nitrate precursors were used(the loading amount of 60%),resulting in the decrease of specific surface area(from 168 to 146 m2/g).No bulk chromium oxide appears in the resultant product if decreading the loading amout of chromium nitrate precursors to 20%.Further repeating the filling step once and twice,the generation of bulk chromium oxide would result in the decrease of the specific surface area from 119 to 61 m2/g.The gas-sensing testing results shown that ordered mesoporous chromium oxide is sentive to toluene and their response increased with reducing the framework thickness and increasing the specific surface area,whose response and recovery time increased significantly with decreasing the pore size.Ordered large-pore mesoporous chromium oxide with both high specific surface area and ultrathin framework exhibited a highest response toward toluene,hose response(Rgas/Rair)toward 1 ppm of toluene at the working temperature of 270? is up to 3.2 and the detection limit was lower than 2 ppb.A series of ordered mesoporous chromium oxides functionalized by noble metal(Ag,Au,Pt,Pd)were synthesized by using KIT-6 as template,both chromium nitrate nonahydrate and precious metal salt as mixed precursors.The ordered mesoporous chromium oxide functionalized by 5 wt%of Ag still maintains a high specific surface area(128 m2/g)and an ordered large-pore mesoporous(11 nm)structure.The resultant product exhibited enhanced response toward acetone and good selectivity,whose response(15.6)is 3.5 times higher than that(4.4)of unfunctionalized mesoporous chromium oxide toward 1 ppm acetone at the working temperature of 210?.The response and recovery time(222 and 365 s)was faster than that of unfunctionalized mesoporous chromium oxide(238,610 s).The ordered mesoporous chromium oxide functionalized by 2.5 wt%of Au exhibited an enhanced response toward ethanol and good selectivity,whose response(7.9)to 1 ppm ethanol is 3.3 times higher than that of unmodified mesoporous chromium oxide(2.4)at the working temperature of 190?.The ordered mesoporous chromium oxide functionalized by 2.5 wt%of Pt exhibited enhanced response toward acetone and good selectivity at working temperature of 210 0C,whose response(13.9)to 1 ppm acetone is 3.1 times higher than that of unfunctionalized mesoporous chromium oxide(4.4).Its response and recovery time(236,393 s)was shorter than that of unfunctionalized chromium oxide(238 and 610 s).Pd functionalization leads to a significant decrease of mesoporous chromium oxide in gas-sensing response. |
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