266
8 Porous Nanostructured Materials
Some metal foams obtained with the DHBT process can be oxidized electrochemically, hence obtaining a porous metal oxide structure. This metal-to-oxide transformation was demonstrated for electrodeposited porous Pb deposits [55], where the
oxidation was performed also with an electrochemical step after changing the Pb
bath to a Na 2 SO 4 solution. The oxidation process was shown to take place through
an initially formed PbSO 4 surface layer with a typical progressive nucleation and
growth mechanism. During the electrochemical oxidation process, the dual porosity
of the samples was retained, but the diameter of the nanowires increased, primarily
due to the difference in the densities of Pb and PbO 2 . The porous deposit was not
stable in air and underwent a colour change due to the formation of Pb 3 O 4 by the
synproportionation of the structure-forming Pb and the PbO 2 surface coating. In spite
of the ill-defined oxide composition, the oxide-coated Pb porous structures could be
used for galvanic exchange processes to deposit oxides that are not accessible for
direct deposition due to their small electrical conductivity. For example, a Co 3 O 4
surface layer on the Pb/Pb 3 O 4 dendrites was obtained by a spontaneous oxidation of
dissolved Co
2+ ions (Pb 3 O 4 + 3Co
2+
Co 3 O 4 + 3 Pb
2+ ). The resulting heterogeneous porous structure showed an outstanding activity for electrochemical oxygen
evolution reaction.
The significance of the above mentioned three-step process can be understood if
we compare the oxidation of porous Pb with the direct oxygen-templated deposition
of PbO 2 from Pb
2+ solution. As it was reported in several studies [56–59], the bubbleassisted direct anodic deposition of PbO 2 leads to porous structures but no secondlevel nanoscale porosity occurs in these deposits. Nevertheless, these deposits are
also suitable for galvanic exchange processes with metal ions (like Mn
2+ , Co
2+ and
Sn
2+ [59]) to obtain oxides whose direct deposition in a nanoporous self-supporting
form is yet to solve.
Since the dynamic bubble template method involves gas evolution, it is logical that
the change in the system pressure may modify the pattern forming with the assistance
of bubbles. The opportunity of changing the external pressure was exploited in the
usual process of copper foam deposition with the DHBT method [60]. Since the
change of the standard electrode potential of the redox systems (H
+ /H 2 and Cu
2+ /Cu)
involved in the DHBT process is negligible with the change of the pressure, the
volume of the gas evolved could be modified by keeping the amount of materials
produced constant. The variation of the pressure from 0.02 to 0.8 MPa resulted in
the decrease in the primary pore diameter from 70 to about 35 µm. The decrease
in the primary pore size was explained with the reduction of the frequency of the
bubble coalescence as the bubble size itself decreased due to the elevated pressure.
In contrast to the primary pore size, the secondary porosity did not change to an
appreciable level, and the copper dendrites formed at all pressures applied appeared
very much alike.
8 Porous Nanostructured Materials
Some metal foams obtained with the DHBT process can be oxidized electrochemically, hence obtaining a porous metal oxide structure. This metal-to-oxide transformation was demonstrated for electrodeposited porous Pb deposits [55], where the
oxidation was performed also with an electrochemical step after changing the Pb
bath to a Na 2 SO 4 solution. The oxidation process was shown to take place through
an initially formed PbSO 4 surface layer with a typical progressive nucleation and
growth mechanism. During the electrochemical oxidation process, the dual porosity
of the samples was retained, but the diameter of the nanowires increased, primarily
due to the difference in the densities of Pb and PbO 2 . The porous deposit was not
stable in air and underwent a colour change due to the formation of Pb 3 O 4 by the
synproportionation of the structure-forming Pb and the PbO 2 surface coating. In spite
of the ill-defined oxide composition, the oxide-coated Pb porous structures could be
used for galvanic exchange processes to deposit oxides that are not accessible for
direct deposition due to their small electrical conductivity. For example, a Co 3 O 4
surface layer on the Pb/Pb 3 O 4 dendrites was obtained by a spontaneous oxidation of
dissolved Co
2+ ions (Pb 3 O 4 + 3Co
2+
Co 3 O 4 + 3 Pb
2+ ). The resulting heterogeneous porous structure showed an outstanding activity for electrochemical oxygen
evolution reaction.
The significance of the above mentioned three-step process can be understood if
we compare the oxidation of porous Pb with the direct oxygen-templated deposition
of PbO 2 from Pb
2+ solution. As it was reported in several studies [56–59], the bubbleassisted direct anodic deposition of PbO 2 leads to porous structures but no secondlevel nanoscale porosity occurs in these deposits. Nevertheless, these deposits are
also suitable for galvanic exchange processes with metal ions (like Mn
2+ , Co
2+ and
Sn
2+ [59]) to obtain oxides whose direct deposition in a nanoporous self-supporting
form is yet to solve.
Since the dynamic bubble template method involves gas evolution, it is logical that
the change in the system pressure may modify the pattern forming with the assistance
of bubbles. The opportunity of changing the external pressure was exploited in the
usual process of copper foam deposition with the DHBT method [60]. Since the
change of the standard electrode potential of the redox systems (H
+ /H 2 and Cu
2+ /Cu)
involved in the DHBT process is negligible with the change of the pressure, the
volume of the gas evolved could be modified by keeping the amount of materials
produced constant. The variation of the pressure from 0.02 to 0.8 MPa resulted in
the decrease in the primary pore diameter from 70 to about 35 µm. The decrease
in the primary pore size was explained with the reduction of the frequency of the
bubble coalescence as the bubble size itself decreased due to the elevated pressure.
In contrast to the primary pore size, the secondary porosity did not change to an
appreciable level, and the copper dendrites formed at all pressures applied appeared
very much alike.