A.R. Boccaccini, 1. Zhitomirsky/ Current Opinion in Solid State and Materials Science 6(2002)251-260 properties of ceramic coatings could be improved by the 3. 2. Coatings for electronic, catalytic and optical use of additives [74, 75 pplications The interest in ELD for fuel cell applications stems from the need to decrease the thickness of the electrolyte layer lectrolytic deposition has aroused considerable interest and deposit intermediate layers preventing interfacial for the development of thin films of titania and complex electrode-electrolyte degradation at elevated temperatures titanates for applications in electronics [ 64, 66]. What is Thin films of yttria stabilized zirconia and Ce -Gd, o2 very important about this recent work is that a relatively were prepared by cathodic electrolytic deposition [76,77] low temperature is required for film crystallization, i.e. the It was shown that the critical thickness of electrolyti formation of a perovskite phase was observed at 500C deposits achievable without crack formation could be Ishikawa and Matsumoto [67] used alternative elec- increased using a cationic polyelectrolyte with inherent trolysis in(NH4)2TIO(C2 O4)2] solutions for electrodepo- binding properties [771 sition of TiO2 into porous substrates. The deposits showed high photocatalytic activity for the decomposition of 3.4. Biomedical applications acetaldehyde. Another important approach is based on anodic oxidative hydrolysis of TiCl,. This method was Considerable attention has been given to electrodeposi- applied recently to produce TiO, nanowires [68]. Well- tion of ceramic coatings for biomedical applications. The aligned TiO, nanowire arrays prepared by this method interest in electrolytic deposition for implant development could be useful for photoelectrochemical applications stems from the possibility of deposition on substrates of Pauporte and Lincot [**69]reported epitaxial growth of complex shape and the high purity of the deposits inc oxide films on single crystal Gan layers. Prepared Dinamani and Kamath prepared various phosphate materi films exhibited good optical and luminescence properties. als using cathodic electrodeposition [78]. Electrodeposition The important finding was the possibility of tuning of film of hydroxyapatite coatings in basic conditions was reported properties by changing the applied potential, bath com- by Manso and co-workers [79]. These hydroxyapatite position or by post-deposition thermal treatments. Of coatings exhibited good adhesion to the substrates. Elec particular importance is the recently demonstrated possi- trolytic ZrO, coatings were deposited on Co-Cr-Mo bility of epitaxial depos of Zno nanopillars onto implant alloys for hip prosthesis [801 Au(111), Au(1 10)and Au(100) single crystal substrates The problem of cracking in electrolytic zirconia deposits [701. It was suggested that these nanopillars could be used hich usually occurs upon drying has recently been s templates for molecular electronics and for data storage. addressed by the use of polymer additives [*81, **82]. It The increasing interest in applications of Zno films was shown [81 that poly(diallyldimethylammonium resulted in further development of electrochemical strate- chloride)(PDDA) acts as a binder, providing better gies for film deposition. Pauporte and Lincot ["71] demon- adhesion of zirconia deposits and preventing cracking. An strated for example cathodic electrodeposition of Zno important finding was that the amount of organic phase in dissolved hydrogen the oxygen pre the deposits could be changed by variation of PDDa cursor concentration in solutions. These results pave the way for There is also a growing interest in electrodeposition of ELd of thick hybrid bioactive organic-inorganic films dye-modified ZnO films [72]. An interesting observation is that the adsorption of the dyes modifies the crystal growth 3.5. Hybrid nanostructured films and surface morphology of Zno films A promising ELD approach has been exploited for Electrodeposition of various hybrid materials based on preparation of macroporous Zno films [*73]. In this oxides or hydroxides of Zr, Ce, Gd, Fe, Ni, Cr, Y, Cu, Co, method highly ordered macroporous Zno films were La and charged or neutral polymers was recently reported electrodeposited using polystyrene colloidal crystals as [81, 82]. It was shown that composition, nanostructure templates. These results pave the way for formation of and morphology of the films could be tailored by variation advanced films combining photonic, luminescent and of bath compositions, deposition parameters and mass piezoelectric properties. Similar strategies used transport conditions for organic and inorganic components for a number of other photonic crystals The microstructure of an optimized hybrid ceria-PDDA/ zirconia-PDDA laminate film on a graphite substrate 3.3. High-temperature coatings and solid oxide fuel cells prepared by method developed in Ref. [81], is shown in Fig. 4. The results obtained are significant for electrodepo- le mportant developments were reported in the field of sition of hybrid thick films. These nanocomposites not ctrodeposition of materials for high temperature applica- only combine the advantageous properties of organic and tions. Advanced ceramic coatings were deposited for inorganic components but also exhibit novel properties,256 A.R. Boccaccini, I. Zhitomirsky / Current Opinion in Solid State and Materials Science 6 (2002) 251–260 fundamentals and principles of the process, have been protection of stainless steel against oxidation at high covered [*65]. temperatures [*74,75]. It was demonstrated that protective properties of ceramic coatings could be improved by the 3 .2. Coatings for electronic, catalytic and optical use of additives [*74,75]. applications The interest in ELD for fuel cell applications stems from the need to decrease the thickness of the electrolyte layer Electrolytic deposition has aroused considerable interest and deposit intermediate layers preventing interfacial for the development of thin films of titania and complex electrode–electrolyte degradation at elevated temperatures. titanates for applications in electronics [**64,*66]. What is Thin films of yttria stabilized zirconia and Ce Gd O 12x x 22y very important about this recent work is that a relatively were prepared by cathodic electrolytic deposition [76,77]. low temperature is required for film crystallization, i.e. the It was shown that the critical thickness of electrolytic formation of a perovskite phase was observed at 500 8C. deposits achievable without crack formation could be Ishikawa and Matsumoto [67] used alternative elec- increased using a cationic polyelectrolyte with inherent trolysis in (NH ) [TiO(C O ) ] solutions for electrodepo- binding properties [77]. 42 2 42 sition of TiO into porous substrates. The deposits showed 2 high photocatalytic activity for the decomposition of 3 .4. Biomedical applications acetaldehyde. Another important approach is based on anodic oxidative hydrolysis of TiCl . This method was Considerable attention has been given to electrodeposi- 3 applied recently to produce TiO nanowires [68]. Well- tion of ceramic coatings for biomedical applications. The 2 aligned TiO nanowire arrays prepared by this method interest in electrolytic deposition for implant development 2 could be useful for photoelectrochemical applications. stems from the possibility of deposition on substrates of Pauporte and Lincot [**69] reported epitaxial growth of complex shape and the high purity of the deposits. zinc oxide films on single crystal GaN layers. Prepared Dinamani and Kamath prepared various phosphate materi- films exhibited good optical and luminescence properties. als using cathodic electrodeposition [78]. Electrodeposition The important finding was the possibility of tuning of film of hydroxyapatite coatings in basic conditions was reported properties by changing the applied potential, bath com- by Manso and co-workers [79]. These hydroxyapatite position or by post-deposition thermal treatments. Of coatings exhibited good adhesion to the substrates. Elec￾particular importance is the recently demonstrated possi- trolytic ZrO coatings were deposited on Co–Cr–Mo 2 bility of epitaxial deposition of ZnO nanopillars onto implant alloys for hip prosthesis [80]. Au(111), Au(110) and Au(100) single crystal substrates The problem of cracking in electrolytic zirconia deposits [*70]. It was suggested that these nanopillars could be used which usually occurs upon drying has recently been as templates for molecular electronics and for data storage. addressed by the use of polymer additives [*81,**82]. It The increasing interest in applications of ZnO films was shown [*81] that poly(diallyldimethylammonium resulted in further development of electrochemical strate- chloride) (PDDA) acts as a binder, providing better gies for film deposition. Pauporte and Lincot [*71] demon- adhesion of zirconia deposits and preventing cracking. An strated for example cathodic electrodeposition of ZnO important finding was that the amount of organic phase in using dissolved hydrogen peroxide as the oxygen pre- the deposits could be changed by variation of PDDA cursor. concentration in solutions. These results pave the way for There is also a growing interest in electrodeposition of ELD of thick hybrid bioactive organic–inorganic films. dye-modified ZnO films [72]. An interesting observation is that the adsorption of the dyes modifies the crystal growth 3 .5. Hybrid nanostructured films and surface morphology of ZnO films. A promising ELD approach has been exploited for Electrodeposition of various hybrid materials based on preparation of macroporous ZnO films [**73]. In this oxides or hydroxides of Zr, Ce, Gd, Fe, Ni, Cr, Y, Cu, Co, method highly ordered macroporous ZnO films were La and charged or neutral polymers was recently reported electrodeposited using polystyrene colloidal crystals as [*81,**82]. It was shown that composition, nanostructure templates. These results pave the way for formation of and morphology of the films could be tailored by variation advanced films combining photonic, luminescent and of bath compositions, deposition parameters and mass piezoelectric properties. Similar strategies might be used transport conditions for organic and inorganic components. for a number of other photonic crystals. The microstructure of an optimized hybrid ceria–PDDA/ zirconia–PDDA laminate film on a graphite substrate, 3 .3. High-temperature coatings and solid oxide fuel cells prepared by method developed in Ref. [*81], is shown in Fig. 4. The results obtained are significant for electrodepo￾Important developments were reported in the field of sition of hybrid thick films. These nanocomposites not electrodeposition of materials for high temperature applica- only combine the advantageous properties of organic and tions. Advanced ceramic coatings were deposited for inorganic components but also exhibit novel properties