Although cobalt hydroxide
(Co(OH)2) has been attracting
attention in several applications, its photoelectrochemical property
has not yet been fully investigated. In this work, tuning the energy
band gap of Co(OH)2 nanosheets with silver atoms and enhancing
their electrical conductivity with silver nanoparticles were then
focused. A Ag-doped α-Co(OH)2 thin film was successfully
synthesized via an electrodeposition method. The
optical properties of the as-prepared materials were characterized
by UV–vis and fluorescence lifetime spectroscopies and further
confirmed by density functional theoretical calculation. It was found
that Ag atoms between adjacent layers of Co(OH)2 can reduce
its electronic band gap to 2.45 eV (α-Co(OH)2) as
compared to 2.85 eV of β-Co(OH)2. In terms of electrochemical
properties, silver nanoparticles (AgNPs) can enhance the electrical
conductivity of Co(OH)2 nanosheets, leading to faster charge
transfer reducing the internal resistance and significantly increasing
the overall charge storage performance. Interestingly, under light
illumination, Ag-doped α-Co(OH)2 exhibits ca. 0.8 times lower charge storage capacity as compared
to that under the dark condition. This is because the photoelectrons
can be recombined with the generated holes in the conduction band.
The charge storage mechanisms of Ag-doped α-Co(OH)2 operated under dark conditions and light irradiation were further
studied and confirmed using in situ electrochemical
X-ray absorption spectroscopy (XAS). Overall, the in situ XAS supports the electrochemical result. This finding may pave a
way to further develop photoactive advanced functional materials of
metal hydroxides and oxides.
A thin-film Ni 0.07 Co 2.93 O 4 p-type semiconductor finely tuned with better electronic conductivity and electrochemical activity than NiO and Co 3 O 4 is proposed as a new photoelectrode of single asymmetric hybrid energy conversion and storage cell (HECS). Photoactive Ni 0.07 Co 2.93 O 4 and N-doped reduced graphene oxide aerogel (N-rGO AE ) were used as positive and negative electrodes of the HECS. Under light illumination, the Ni 0.07 Co 2.93 O 4 can generate a hole (photogenerated carrier, Ni 0.07 Co 2.93 O 4 + ) at the valence band (VB) and a photoelectron at the conduction band (CB) via the photoelectric effect. During charging, the active hole (Ni 0.07 Co 2.93 O 4 + ) of the positive electrode can chemically react with OH − and H 2 O forming CoO(OH) and NiO(OH). This process can enhance the redox mechanism leading to the charge storage performance of the Ni 0.07 Co 2.93 O 4 electrode since OH − can easily react with the active Ni 0.07 Co 2.93 O 4 + hole having the lower energy level of the VB when compared with the electrode under the dark condition. The changes in the oxidation state of Ni and Co in Ni 0.07 Co 2.93 O 4 were fundamentally investigated by an in situ electrochemical X-ray absorption spectroscopy. The electrochemical performance of Ni 0.07 Co 2.93 O 4 //N-rGO AE HECS canbe enhanced under light illumination about 1.9-fold higher than that under dark condition. The HECS device may be useful for renewable energy applications.
Three-dimensional (3D) hierarchical
reduced graphene oxide (rGO)
aerogel with a finely tuned C/O ratio of 11.8 can be used for various
applications because of its high specific surface area (925.27 m2 g–1), specific pore volume of 6.46 cm3 g–1, and light weight as well as excellent
electrical conductivity and electrochemical property. The permselectivity
of the 3D rGO aerogels with different C/O ratios was investigated
using neutral, positive, and negative redox mediators. The neutral
and cationic redox mediators permselectively adsorb within the rGO
aerogels via π–π and noncovalent electrostatic
interactions, respectively. In contrast, the anionic redox mediator
does not adsorb within the 3D rGO aerogels due to the repulsion force.
The 3D rGO can be used as an absorbent for toxic dye removal such
as methylene blue (MB). The in situ electrochemical
spectroscopy suggests that the 3D rGO prefers to adsorb the reduced
form of MB. For lithium-storage capability, the 3D rGO aerogel is
ideal since it can be used as the high-energy anode of lithium batteries.
It can provide a reversible specific capacity of over 600 mAh g–1 at 0.5 A g–1 and over 1000 mAh
g–1 at 0.1 A g–1, which are much
higher than a theoretical maximum value (372 mAh g–1) of graphite.
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