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| Fig. 1: Photoelectric cell. A photocatalyst is embedded in water, which is shined with sunlight. The bubbles represent the production of oxygen (front cell) and hydrogen (back cell). Source: Wikimedia Commons |
The most common way to produce hydrogen today is still via steam reforming from hydrocarbons (usually methane), which yields carbon monoxide for the water gas shift reaction. [1] In the context of a global energy crisis, the production of hydrogen using photocatalysis has received a lot of attention as an electromagnetic to chemical energy converter. Photocatalytic reactions are ordinarily classified into two categories: 'down-hill' reactions, in which the photon energy absorbed by the photocatalyst is used to induce thermodynamically favored reactions (such as the oxidation of organic compounds), and 'up-hill' reactions, in which photon energy is converted into chemical energy. [2] The splitting of water into hydrogen and oxygen gases is an example of an 'up-hill' reaction.
The separation between the conduction band and the valence band of a semiconductor determines its band gap. If light of energy greater than this band gap is irradiated onto the semiconductor, electrons and holes are respectively generated in its conduction and valence bands. The overall water splitting is achieved since electrons reduce water molecules whereas holes oxidize them, according to the equations [3]
In order for these reactions to occur, the redox potential of H+/H2 (0V) has to be less negative than the bottom level of the conduction band, whereas the redox potential of O2/H20 (1.23 V) has to be less positive than the top level of the valence band. [4] In other words, the semiconductor's band gap must be greater than 1.23 eV.
The most common way to assess a photocatalyst's efficiency is to calculate its external quantum yield (QY), defined as
| QY = | (# of photogenerated electrons) (# of incident photons) |
= | (I × hν) (e × P) |
where I is the electrical current, e is the fundamental charge of the electron, P is the total power of photons, h is Planck's constant and ν is the frequency of incident light. [5]
Since the energy of a photon is dependent on the wavelength of light, QY responds to the various wavelengths in the spectrum of light shining on the photocatalytic material.
Several factors limit the materials that can be used as photocatalysts, such as having a suitable thermodynamic potential for water splitting, a sufficiently narrow band-gap to be responsive to visible photons and stability against photocorrosion. [6] Because the minimum band-gap required to achieve water splitting is 1.23 eV at standard temperature and pressure and pH=0, the theoretical minimum light wavelength required is 1008 nm, which falls in the range of infrared light. The several efficiency challenges mentioned above, however, have traditionally resulted in low efficiencies for photocatalytic water splitting even in the visible light range. [7] Furthermore, most photocatalytic semiconductor materials have a band-gap whose energy falls into the ultra-violet range, requiring innovative strategies (e.g. dopants) to narrow the band-gap into the visible light range. [8]
One of the main challenges of photocatalysis for water splitting are defects in the semiconductor structure, which cause the undesirable recombination of electron-hole pairs. As a result, fewer than 10% of the incident photons are actually used for overall water splitting. [6]
Protons are not restricted to the surface of the semiconductor. Because of this delocalization, the production of hydrogen can be rather difficult, requiring the use of co-catalysts such as platinum and nickel(II) oxide. [7] These ordinary co-catalysts, however, also catalyze the backwards reaction (water synthesis). As a result, significant research has been made to develop new and more efficient co-catalysts. Potential candidates include nanoparticulate rhodium and chromium mixed oxide. [6]
Current research has focused on developing good photo-catalysts which operate in the visible-light range. Many new powdered photocatalysts for water splitting have been developed. For example, the photocatalyst NiO (0.2%wt)/NaTaO3:La(2%) with a 4.1 eV band-gap has been used to reached a quantum yield of 56% at a wavelength of 270nm. [4] NiO/NaTaO3:La is the most active for water splitting among tantalite photocatalysts. Time-resolved infrared absorption spectroscopy showed that the addition of dopant lanthanoid makes the lifetime of the photogenerated electrons longer. Fig. 1 shows H2 and O2 evolution in the form of bubbles under the irradiation of sunlight.
Research has focused on the use of materials which can act as electron collector and transporter to enhance the lifetime of the photogenerated charge carriers of the original photocatalytic materials. For example, recent studies have relied on the use of graphene nanosheets decorated with cadmium sulfide (CdS) clusters to improve visible-light photocatalysis, reaching a hydrogen gas production rate of 1.12 mmol per hour (about 4.87 times higher than that of pure CdS nanoparticles). [9]
An alternative approach for water splitting is to use different photocatalytic materials for each step, in a process called Z-scheme. An example of Z-scheme is water splitting using Ru/SrTiO3 and BiVO4 with Fe3+/Fe2+ redox couple as an electron relay. [4]
Producing hydrogen gas in an efficient and environmentally friendly way is a big challenge. Photocatalytic water splitting, although it has been researched for several decades, is still a promising route to achieving this goal. Recent research has focused in improving quantum yield in the visible light by narrowing the band-gap of photocatalytic materials, reducing defects in their semiconductor structures and finding new and more efficient co-catalysts. Current efficiencies, however, are still not enough to achieve production of hydrogen in an industrial scale by photocatalysis.
© Idel Waisberg. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[6] K. Maeda and K. Domen, "Photocatalytic Water Splitting: Recent Progress and Future Challenges", J. Phys. Chem. Lett. 1, 2655 (2010).
[7] J. Oudenhoven, F. Scheijen and M. Wolffs, "Fundamentals of Photocatalytic Water Splitting by Visible Light", Technische Universiteit Eindhoven, 21 Mar 04.
[8] A. Kudo and Y. Miseki, "Heterogeneous Photocatalyst Materials for Water Splitting," Chem. Soc. Rev., 38, 253 (2009).
[9] Q. Li et al., "Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets", J. Am. Chem. Soc. 133, 10878 (2011).
