3.1 Metal Oxides/Sulfides
33
MO (s) + CO (g) → M (s) + CO 2 (g)
(3.1)
CO 2 (g) + C (s) → 2CO (g)
(3.2)
For instance, metal oxide reduction by gaseous metal carbides can occur through
the reaction of carbon and metal vapor, as in (3.3) and (3.4):
2MO (s) + MC 2 (g) → 3M (s) + 2CO (g)
(3.3)
M (s) + 2C (s) → MC 2 (g)
(3.4)
Then, thermal reduction by H 2 O and H 2 gaseous intermediates follows, as in (3.5),
(3.6), and (3.7):
MO (s) + H 2 (g) → M (s) + H 2 O (g)
(3.5)
C (s) + H 2 O (g) → H 2 (g) + CO (g)
(3.6)
MO (s) + C (s) → M (s) + CO (g)
(3.7)
Cu 2 O nanostructures have received attention in different applications such as
energy storage, (Minami et al. 2013; Mittiga et al. 2006) water splitting, (Paracchino
et al. 2011) gas sensing, (Deng et al. 2012) solar energy conversion, and the photocatalytic degradation of pollutants (Zheng et al. 2009). The unique aligned nanowire
structure can afford inherently efficient architectures with increased surface area
for charge transfer processes, minimized hole and electron diffusion lengths, and
increased superficial area for charge transfer in charge collecting electrodes (Lewis
2007). Accordingly, vertically aligned Cu 2 O nanowires can observably diversify
and enhance the portfolio of Cu 2 O-based nanostructure applications (Wu et al.
2014). Many synthetic methods for obtaining Cu 2 O nanowires have been proposed.
Distributed single-crystal Cu 2 O nanowire arrays with a diameter of approximately
300 nm and a length of over 20 μm have been synthesized by hydrothermal processes
(Tan et al. 2007; Hacialioglu et al. 2012). High-density Cu 2 O nanowires (approximately 200 nm) have been synthesized using porous nanotemplates by electrochemical techniques; however, the length was limited to 4 μm due to diffusion-restricted
electrolyte transport in the confined nanopores. Cu 2 O nanowire arrays have been
fabricated by the facile carbothermal reduction of CuO nanowires. CuO nanowires
were first obtained by the thermal oxidation of copper foils. The oxidation of 26 μm
copper foils at 700 °C for 10 h converted all the substrate to phase-pure cupric
oxide. Two significant effects were observed when the transformation was initiated
at a temperature of 350 °C. First and foremost, the CuO → Cu 2 O reduction reaction occurs via the diffusion of O 2 under a reducing atmosphere provided by the
33
MO (s) + CO (g) → M (s) + CO 2 (g)
(3.1)
CO 2 (g) + C (s) → 2CO (g)
(3.2)
For instance, metal oxide reduction by gaseous metal carbides can occur through
the reaction of carbon and metal vapor, as in (3.3) and (3.4):
2MO (s) + MC 2 (g) → 3M (s) + 2CO (g)
(3.3)
M (s) + 2C (s) → MC 2 (g)
(3.4)
Then, thermal reduction by H 2 O and H 2 gaseous intermediates follows, as in (3.5),
(3.6), and (3.7):
MO (s) + H 2 (g) → M (s) + H 2 O (g)
(3.5)
C (s) + H 2 O (g) → H 2 (g) + CO (g)
(3.6)
MO (s) + C (s) → M (s) + CO (g)
(3.7)
Cu 2 O nanostructures have received attention in different applications such as
energy storage, (Minami et al. 2013; Mittiga et al. 2006) water splitting, (Paracchino
et al. 2011) gas sensing, (Deng et al. 2012) solar energy conversion, and the photocatalytic degradation of pollutants (Zheng et al. 2009). The unique aligned nanowire
structure can afford inherently efficient architectures with increased surface area
for charge transfer processes, minimized hole and electron diffusion lengths, and
increased superficial area for charge transfer in charge collecting electrodes (Lewis
2007). Accordingly, vertically aligned Cu 2 O nanowires can observably diversify
and enhance the portfolio of Cu 2 O-based nanostructure applications (Wu et al.
2014). Many synthetic methods for obtaining Cu 2 O nanowires have been proposed.
Distributed single-crystal Cu 2 O nanowire arrays with a diameter of approximately
300 nm and a length of over 20 μm have been synthesized by hydrothermal processes
(Tan et al. 2007; Hacialioglu et al. 2012). High-density Cu 2 O nanowires (approximately 200 nm) have been synthesized using porous nanotemplates by electrochemical techniques; however, the length was limited to 4 μm due to diffusion-restricted
electrolyte transport in the confined nanopores. Cu 2 O nanowire arrays have been
fabricated by the facile carbothermal reduction of CuO nanowires. CuO nanowires
were first obtained by the thermal oxidation of copper foils. The oxidation of 26 μm
copper foils at 700 °C for 10 h converted all the substrate to phase-pure cupric
oxide. Two significant effects were observed when the transformation was initiated
at a temperature of 350 °C. First and foremost, the CuO → Cu 2 O reduction reaction occurs via the diffusion of O 2 under a reducing atmosphere provided by the