Heterogeneous Catalysts. Группа авторов
electrolyte containing fluoride ions.
A number of industrially important metal oxide thin films have been prepared through anodization method such as alumina, silica, and titania. Many more academically interesting simple metal oxide films can be afforded by this method. Figure 3.4 shows scanning electron microscopy (SEM) images of metal oxides with anisotropic nanostructures obtained through anodization [20–22]. Some of the most important ones as relevant to catalytic applications include TiO2 and α‐Fe2O3 nanotubes, Nb2O5 nanorods, WO3 nanoflowers, and MoO3 truncated rhombohedra. These oxide materials are intrinsic semiconductors with or without nanostructured morphologies, but in the case of the former, their efficiencies as (photo)electrodes can be further augmented. The effect stems from (i) the increased ratio of surface area to volume for electrolyte contact and where reaction takes place, (ii) enhanced light absorption due to trapping of photons within pores, and (iii) the vectorial charge transport.
Figure 3.4 SEM images of the simple metal oxides obtained through anodization: (a) titanium dioxide (TiO2), (b) molybdenum trioxide (MoO3), and (c) tungsten trioxide (WO3).
Source: (a) Reprinted with permission from Yun et al. [20]. Copyright 2011, American Chemical Society. (c) Reprinted with permission from Ng et al. [21]. Copyright 2010, American Chemical Society.
The vectorial charge transport is an intriguing phenomenon that relies on the vertically oriented 1D array of the oxide semiconductor and hence deserves special mention here. Under photoexcitation, i.e. when the semiconductor photoanode is exposed to photons with energy equal to or greater than its band gap (see Chapters 11, 31, and 36 on the basics of photocatalysis), the generated photoelectrons would need to diffuse to the back of the electrode within its charge carrier lifetime, or they will recombine with the photoholes, hence the loss of photocharge for surface reaction. The photocharge transport can be described by the following equation:
where Lc is the diffusion length or distance traveled by the charge carrier (electrons or holes) before recombination, Dc is the diffusion coefficient of the charge carrier, and τc is the lifetime of the charge carrier. It should be noted that the Dc (and hence Lc) is different for both electron and hole even on the same semiconductor material. For a photoanode (or photocathode) that is composed of irregular‐shaped or randomly packed particles, the photoelectrons (or photoholes) undergo the “random walk motion” that are rarely the most straightforward path to the back of the electrode. With the creation of 1D array of the oxide semiconductor, the diffusion of the photoelectrons is restricted to the shortest vertical path to the back of the electrode. This enables a large fraction of photoelectrons (or photoholes in the case of photocathode) to be collected within their τc. At the same time, it is important to restrict the wall thickness/diameter to not more than twice the Lc of the photoholes (or photoelectrons) such that majority of them could diffuse to the semiconductor surface to catalyze the oxidation (or reduction) reaction. Owing to their advantages, aligned nanotube and nanorod arrays have been used for water photoelectrolysis and the reduction of CO2.
3.2.1.1 Pulse or Step Anodization
To control the growth of internal channel as mentioned above, a number of strategies external to anodization (dominant by lithography‐based method) have been explored. This includes electron beam lithography, focused ion beam lithography, colloid sphere lithography, and direct laser writing lithography. To search within electrochemical means, pulse or step anodization can offer alternative solutions. This approach is built on the understanding that the development of pore channel can be guided by surface texturization at the initial stage of anodization. Periodic nanostructures are known to grow along the internal channel under anodization. When pulse or step is introduced periodically during anodization, acid anions (such as fluoride ions) are intermittently transported or attracted to the layer near the top of pores. Therefore, compensation of such anions would be established from top to the channel bottom in a periodical manner, which results in the morphological variance of the formed anodic oxide thin film. In literature, it is reported that a low frequency voltage (period τ in minutes) permits the growth of bamboo‐type nanotubes and 2D nanolace sheets of alumina [23], while high frequency current oscillation (period τ in seconds) yields the irregular nanoporous morphology. [24] A more advanced anodization setup involving multiple steps has been explored to precisely control pore channels. Although not yet popular, approaches of changing the type of electrolyte (or concentration) after each step, modulating reaction temperature, and varying the applied voltage or current during anodization have also been explored [25–27]. Although overall aim is to achieve rapid reaction to shorten the catalyst preparation time (industrial's preference), an over‐reactive anodization with ultrafast growth rate can cause concerns, i.e. the physical breaking of nanostructures. Debris of nanotubes has been reported physically lying on the thin films due to the breakage. Unwanted delamination of metal oxide nanostructures from the substrate can happen. Therefore, stabilizing the anodization reaction at ultrafast metal oxide growth rate condition is one of the main aspects in improving the method. Depending on the selected catalytic reaction, amorphous or crystalline structure is desired. As‐anodization thin films usually possess lower degree of crystallinity. Additional crystallization process for the anodized thin film is sometimes needed to suit the targeted catalytic reactions.
3.2.2 Cathodic Electrodeposition
Cathodic electrodeposition is another well‐established electrochemical method in fabricating thin films of nanocrystalline metals, alloys, and composite catalysts, where instead of forming the desired oxide layer on the anode as demonstrated in the previous section of anodization method, the targeted materials are now deposited on the cathode through electroreduction of metal cations dissolved in the electrochemical bath. The electrochemical configuration is as shown in Figure 3.1, where precise control over the thickness and composition of the catalyst layer can be achieved either through the control of applied voltage (potentiostatic method) or current (galvanostatic method). It is a low‐temperature synthesis that is typically performed at ambient temperature.
The potentials needed to drive the reduction of metallic cations depend on the redox potential of such targeted metal. In electrodeposition of more than mono‐component, the element with positive reduction potentials will be reduced first as it can receive electrons relatively easier, although the reduction kinetics can be different. However, in principle, a convenient scanning of cathodic voltage over a window of voltage would be sufficient to identify the required potentials.
Electrodeposition is considered a strong method of coating because of their abrasion resistance, hardness, coating adhesion, and corrosion resistance. Furthermore, modulation of the abovementioned properties can be performed through a number of experimental variations including temperature of medium, precursors' concentration, pH of electrolyte, and current density. The higher thickness of coating can also be achieved by simply lengthening the duration of deposition. Because it follows the Faraday's law for electrolysis, a deposited metal (Met) started from its cations (Metz+) under constant current density (I) should follow this relationship:
(3.3)