Sensing Mechanism And PH Online Detection Of Superhydrophilic TiO₂ Sensitive Electrode For PHh Value In Cultural Substrates

Cultural substrate in facility agriculture is the basis for the growth of crops,and the pH value of cultural substrate is one of the most important production process information of facility agriculture production.It is an inevitable demand of facility agriculture production to achieve online pH detection,by which the precise regulation for crop growth could be carried out.It is also an inevitable trend to develop modern agriculture,which is more precise,digitalized and intelligent.This dissertation reviewed the research status of online pH detection in cultural substrate,including online detection methods,error analysis methods and error compensation methods.It is found that the huge measuring error and poor adaptability of compensation method in online pH detection for cultural substrate is caused by the complex internal components of cultural substrate and the changeable contact state between sensitive electrode and cultural substrate.In this dissertation,a novel super-hydrophilic sensitive electrode for online pH detection in cultural substrate was proposed based on the advanced technology for online detection of ion concentration in complex environments by using hydrophilic/super-hydrophilic materials.This dissertation aims at the sensing mechanism of sensitive electrode in culture substrate,the spread characteristics of solutions on super-hydrophilic surface in culture substrate and the application of online pH detection in the culture substrate after the feasibility of infiltration principle,sensing materials and fabrication process of the super-hydrophilic sensitive electrode was demonstrated.The main research work and achievements of this dissertation are as follows:（1）Sensing mechanism of TiO₂ sensitive electrode for pH detection in cultural substrate.Theoretically analysis was carried out for the dissociation equilibrium and migration process of H⁺on the surface of cultured substrate particles and in the substrate solution and showed that H⁺was dissociated from the particle surface into the solution,and then moved from the cultural substrate to the sensitive electrode by diffusion method.H⁺was then driven to the electrode surface by diffusion and electromigration when it was closed to the sensitive electrode.Theoretically analysis was also carried out for the evolution of the contact state and contact form between the sensitive electrode and the cultural substrate with the change of the water content.The pH sensing mechanism under different contact states was studied based on the"oxygen intercalation"theory.It was found that only the sensitive electrode region in the solid-liquid-solid contact form can produce pH response potential.In this case,the influence mechanism of the effective response area ratio of sensitive electrode on the online pH detection results was studied based on the electrode process dynamics theory.The research showed that the higher the effective response area ratio of sensitive electrode,the smaller the pH measurement error.（2）Spreading mechanism of cultivation substrate solution on the surface of super-hydrophilic sensitive electrode.Theoretically analysis was carried out for the spread characteristics of substrate solution on the surface of the sensitive electrode and showed that the spread width of droplet on the surface of the electrode was mainly affected by the contact angle of the sensitive electrode in the cultural substrate（θ_s）,which is mainly affected by the matric potential（S）and the contact angle of the sensitive electrode in the air（θ_r）.For the sensitive electrode that θ_s<5°,the spread width of droplet increases obviously.θ_s could be decreased by reducingθ_r,which would expand the droplet spreading width,increase the actual contact area between the sensitive electrode and the substrate solution and the effective sensing area ratio of the sensitive electrode.The simulated analysis was carried out for the spreading behavior of droplets on the sensitive electrode with different hydrophilicity and the spreading characteristics of solution in the pores of peat and vermiculite substrates based on the porous element model of peat and vermiculite in lattice Boltzmann method（LBM）.The simulated analysis showed that the super-hydrophilic sensitive electrode can promote the spread of substrate solution on the electrode surface in culture substrate,and the effective sensing area ratios of the super-hydrophilic sensitive electrode in peat and vermiculite substrates were more than 85.70%and 74.82%respectively.（3）Fabrication of super-hydrophilic TiO₂ sensitive electrode and its hydrophilicity and pH sensing performance.Theoretically analysis was carried out for the reaction mechanism and influencing factors of porous TiO₂ film through chemical etching method.The TiO₂ sensitive electrodes with different properties were prepared by different process parameters under the condition of 80℃.The influence mechanism of fabrication process parameters on the film compositions,surface morphology,contact Angle and pH response performance of the fabricated sensitive electrodes were analyzed by XRD,XPS,AFM,SEM,contact Angle measurement and electrochemical analysis.It was found that the sensitive electrode fabricated in 1 M NaOH for 120min had the best hydrophilicity（CA=5.46°）,the highest sensitivity（54.13 m V/pH）,a very short response time（no more than 7 s in the range of pH4～9.64）,a low drift rate（9.25 m V/h）and a low hysteresis（11.4 m V）.It also showed good ion selectivity,process repeatability,super hydrophilic maintenance and electrochemical stability.（4）Surface damage of super-hydrophilic sensitive electrode used in cultural substrate and its degradation in hydrophilicity and pH sensing performance.Theoretically analysis was carried out for the damage mechanism and the affecting factors of micro-structures on the sensitive electrode when it was inserted into the cultural substrate.The normal contact force on the electrode surface during the insertion was simulated analyzed by discrete element method（DEM）.The simulated analysis showed that the main stress area of the electrode was at the head of electrode in the range of 0～10mm.The normal contact force decreases gradually in the range of 10～30 mm and remains stable in the range of more than 30 mm.The experimental study was carried out for the influences of the damage degree and damage morphology on the hydrophilicity and pH response of the electrode.The experimental study showed that the damage degree of the electrode in the range of 0～9 mm is serious whereas the damage degree within 18～27 mm is almost no damage.The damage morphology of the electrode is mainly affected by the hardness of cultural substrate particles.The degradation in hydrophilicity and pH sensing performance of the electrode is mainly related to the damage degree.（5）Fabrication of the super-hydrophilic pH sensor and its application by online pH detection in cultural substrates.A super-hydrophilic pH sensor was designed and fabricated based on the super-hydrophilic TiO₂ sensitive electrode with the contact angle of 5.89°and a PDA modified Ag/Ag Cl reference electrode with the contact angle of 33.63°.The super-hydrophilic pH sensor has a sensitivity of 53.61 m V/pH,a response time of 20.4 s,a drift of 7.91 m V/h and a hysteresis of 10.7 m V.The sensor was tested in peat,vermiculite and perlite to compensate the measuring error in cultural substrate,and a comprehensive compensation model for various cultural substrates was established.Then the online pH measuring performance of the super-hydrophilic pH sensor and the effectiveness of the compensation model was verified in 4 typical mixed substrates and 6 types of actual cultural substrate in a facility agriculture.The results showed that the maximum average measuring error was-0.62 and decreased to 0.19 after compensation.The average response time was less than 22.4s.The measuring error range of the actual cultural substrates was-0.248 to 0.084 and decreased to-0.09 to 0.03 after compensation.