Preparation Of Nano-porous BiVO₄ Electrodes For Photoelectrochemical Analysis

H2O2 is of great importance in many industries,but excessive H2O2 in environment causes many diseases.Meanwhile,Na NO2 has been widely used as a food additive as preservative,or for better color.However,ingested Na NO2 reacts with amine in human body,producing sodium nitrite.Sodium nitrite is a carcinogen,it can cause many health problems to human beings.Na NO2 can also be absorbed into blood rapidly and oxidize ferrous ion in the hemoglobin into ferric ion,making hemoglobin lose the ability to carry oxygen,which will lead to the shortage of oxygen.Therefore,it is of great importance to find an efficient way to detect H2O2 and Na NO2.Photoelectrochemical sensor combines optics with electricity.Because it has different excitation signal(light)and detection signal(electricity)and the disturbance between excitation signal and detection signal is relatively low,its sensitivity is higher than electrochemical sensor.Bi VO4 is a semiconductor with good optical properties because of its proper band gap(2.4-2.6 e V).It also has advantages such as easy synthesis,environmental friendly,low cost etc.Bi VO4 has been widely used as a photocatalyst for photocatalytic degradation of organic pollutants and photocatalytic water splitting.However,there are few reports about its application in photoelectrochemical detecting.In this study,we build a photoelectrochemical sensor to detect H2O2,and we also build two types of Bi VO4 electrodes with different morphologies to study the influence of morphology on detecting Na NO2.The main achievements are as follows.1.By electrodepositing Bi OI precursor and further temperature programmed thermal treatment,nano-porous Bi VO4 decorated FTO electrode was obtained.The prepared sample was characterized by SEM,XRD and UV-vis diffuse reflection spectroscopy.The obtained Bi VO4 are nanoparticles with good crystalline,which were distributed uniformly on the FTO substrate.This electrode was used to detect H2O2,and the result turns out that the electrode has good response to H2O2 under illumination.Experimental conditions such as examination voltage and electrode loading were optimized.Under optimum conditions,the linearity of the electrode towards detecting H2O2 is 9.54-764.8 ?mol L-1 while the linearly dependent coefficient is 0.9997 and the detection limit is as low as 1.59 ?mol L-1(S/N=3)with a sensitivity of 182.6 ?A(mmol L-1)-1 cm-2.While in dark environment,the electrode barely responds toward H2O2 while sensitivity is 0.110 n A(mmol L-1)-1 cm-2 with a linear range of 47.70-764.8 ?mol L-1.The result shows that light is very important in detecting H2O2.We also explored the Nyquist plots of Bi VO4 electrode in dark environment and under illumination.Under illumination,surface of the electrode has higher electron-hole separation and transportation efficiency,lower electron-hole recombination rate.The electrode also displays good anti-interference performance towards common interferences.2.We build two different morphology Bi VO4 electrodes by electrodepositing method and spin-coating method.The morphologies,structures and optical properties of the prepared electrodes were characterized by SEM,XRD and UV-Vis spectroscopy.The average particle size of the Bi VO4 electrode prepared by electrodepositing method is 200 nm,with plenty of pores in-between Bi VO4 particles.This kind of nano-porous structure tremendously increases the surface area between electrode and solution,hence enhances the amount of Na NO2 that is oxidized on the surface,and shortens the transportation distance of electrons and holes,minimizing the electron-hole recombination rate and aggrandizing photo-induced current.The average particle size of the Bi VO4 electrode prepared by spin-coating method is 500 nm,and the particle size is not uniform.Larger particles result in less total surface area of Bi VO4 electrode,reducing the amount of Na NO2 oxidized on the surface and increasing the transportation distance of electrons and holes,which will lead to higher recombination rate of electrons and holes inside the semiconductor.Moreover,the Nyquist plots of the electrodes reveal that the Bi VO4 electrode prepared by electrodepositing method presents higher electron-hole separation efficiency and electron transportation rate.The two electrodes present insignificant difference in crystallinity and optical absorbance properties.The two types of electrodes prepared were used to determine Na NO2,and the result shows that the Bi VO4 electrode prepared by electrodepositing method presents higher performance than the Bi VO4 electrode prepared by spin-coating method.We also optimized experiment conditions such as examination voltage and electrode loading.Under optimum conditions,the detection limit of the Bi VO4 electrode prepared by electrodepositing method reaches as low as 1.25 ?mol L-1,lower than 11.25 ?mol L-1 of the Bi VO4 electrode prepared by spin-coating method.The linear range of the Bi VO4 electrode prepared by electrodepositing method is 3.75-1051 ?mol L-1,while the Bi VO4 electrode prepared by spin-coating method is 51.25-1051 ?mol L-1.The two types of electrodes show disparate sensitivity.The sensitivity of the Bi VO4 electrode prepared by electrodepositing method reaches 200.8 ?A(mmol L-1)-1 cm-2,which is almost ten times of 18.00 ?A(mmol L-1)-1 cm-2 for the Bi VO4 electrode prepared by spin-coating method.The result illustrates that preparation method plays a very important role in photoelectrochemical determination of Na NO2.