Study On Surface Modification And Seawater Salinity Sensing Of Boron Doped Diamond Film Electrode

The measurement of seawater salinity with high accuracy is of great scientific and engineering significance for the development of marine science and technology.The key point to obtain a high performance salinity sensor lies on the development of an electrode with higher mechanical durability,better reproducibility and longer lifetime and can be used in the extreme environment.Thanks to the unique tetrahedral sp3 hybrid structure,boron-doped diamond film electrode(BDD)has many favorable characteristics derived from intrinsic diamond,such as excellent physical and chemical stability,outstanding mechanical durability and strong anti-pollution properties.Therefore,BDD electrode is expected to overcome the shortcomings of the poor stability of existing electrode and holds great potential to be an alternate in seawater salinity detection.In this paper,BDD electrode is for the first time introduced to the application of seawater salinity detection and through the methods of surface nanocrystallization,surface terminal modification and surface modification with gold nanoparticles(AuNPs)to improve its seawater salinity sensing performance.Effects of the three improve strategy on the microstructure,electrochemical behavior and seawater salinity sensing performance of BDD electrode is studied thoroughly.Firstly,boron-doped diamond nanorod forest(BDDNF)electrodes are synthesized using silicon nanowires as template via hot filament chemical vapor deposition(HFCVD).It is found that the double-layer capacitance(Cdl)is the key factor affecting seawater salinity sensing performance of BDD electrode.Benefit from the nano-forest structure,the Cdl of the BDDNF electrode is 4 orders of magnitude larger than that of the flat BDD electrode.Thanks to the enhanced Cdl resulted by the nanostructure,the response of BDDNF exhibits an enhancement by a factor of 1.5 compared to BDD at the salinity of 40‰.Moreover,in striking contrast to commercial platinum black electrode,the obtained BDDNF electrodes exhibit outstanding stability due to the increased amount of carbon oxygen functional groups on electrode surface and the excellent stability of diamond.The results suggest that BDDNF electrode overcomes the limitation of low Cdl of the planar electrode and show excellent response and ultra-high long-term stability in seawater salinity detection.Secondly,H-BDD and H-BDD-RIE electrodes with hydrogen terminals and O-BDD and O-BDD-RIE electrodes with oxygen terminals are prepared by plasma treatment and reactive ion etching(RIE).It has been found that BDD electrodes with oxygen termination show more sensitive response than which with hydrogen termination,especially for the O-BDD-RIE electrode that exhibit the remarkable high response even exceed the commercial platinum black electrodes.Compared with the hydrogen terminated BDD electrode,the outstanding performance of oxygen terminated BDD electrodes in seawater salinity can be ascribed to surface C-O functional groups,it could improve the ion/surface interactions by changing the interfacial dipole alignment,leading to enhanced Cdl,thus improving response.Meanwhile,it is found that the surface oxygen is the major factor for the improved Cdl rather than nanotip structure by comparing the Cdl of O-BDD-RIE and H-BDD-RIE electrodes.More impressively,both O-BDD and O-BDD-RIE electrodes exhibit great long-term stability and good reproducibility in seawater salinity detection confirmed by 90 days of testing.It is ascribed to the relatively stable existence of surface oxygen that is incorporated in the bridged form(C-O-C)of oxygen termined BDD electrode.Therefore,compared with hydrogen terminated BDD electrode,oxygen terminated BDD electrode has higher sensitivity and stability,thus promising potential application in seawater salinity detection.Finally,BDD-RIE/AuNPs electrodes are designed using HFCVD,RIE,electron beam evaporation and annealing under Ar atmosphere aiming to improve sensing performance by modification with AuNPs and pave the way for preparation high performance seawater salinity sensor based on diamond film electrode.It is found that the optimal preparation process parameters are as followings:the annealing temperature is 800?,the annealing time is 2 h,the thickness of Au coating is 20 A.The BDD-RIE/AuNPs composite film electrode prepared by this preparation process parameter has an inlay structure of stalactite-like pits and AuNPs.Electrochemical behavior analysis and salinity sensitivity tests show that the inlay structure endows the electrode with a greater Cdl,from which it has higher seawater salinity test sensitivity.Thanks to the contribution of AuNPs,inlay structure of stalactite-like pits and AuNPs and their stable existence form,the BDD-RIE/AuNPs electrode shows excellent stability in long-term monitoring.The preparation of BDD-RIE/AuNPs electrode not only further improves the sensing performance of seawater salinity,but also lays a foundation for the devices preparation.It provides electrode modification methods for the construction of high-performance seawater salinity sensor based on diamond film,which is of guiding significance.The three methods described in this paper are simple and highly effective,which will benefit the application of BDD electrode in seawater salinity detection.