Quantum Dots and Photocatalysts
A photocatalyst has strong oxidizing power on a surface when exposed to light from the sun, fluorescent lights, or other sources, thus enabling environmental purification by removing organic compounds, bacteria, and other harmful matter on contact, as well as catalytic action that generates hydrogen and oxygen from water through the power of light. A substance that is moved by light to an excited state and that can transfer that energy to other substances is able to act as a catalytic material for a photocatalyst. Semiconductors are typical photocatalysts, with TiO2 and other metal oxide semiconductors being well-known examples.
Photocatalysts work on the following principles.
１．Exposure to ultraviolet rays, visible light, or other light
As an example of this, when titanium dioxide, a common photocatalytic material, is exposed to light (ultraviolet rays), electrons are expelled from its surface. The holes through which the electrons escape, called electron holes, have a positive charge.
２．Emergence of OH radicals
Electron holes have a strong oxidizing power and strip electrons from hydroxide ions (OH−) in water. The OH− ions from which electrons were stripped become very unstable OH radicals. OH groups from which electrons were stripped become activated OH groups by stripping electrons from chain organic compounds they come into contact with, such as odorous components in the air or compounds dissolved in water, and these OH groups themselves seek to become stable. Called hydroxyl radicals, these OH groups are understood to exhibit higher oxidizing power than chlorine and ozone.
３．Breaking up organic substances
With their strong oxidizing power, hydroxyl radicals strip electrons from nearby organic substances and seek to become stable. Chemical bonds are broken in organic substances that are deprived of electrons; the substances eventually become carbon dioxide and water, which are dispersed into the atmosphere. Titanium dioxide, a semiconductor, enters a high-energy state when it receives energy from light, and it releases electrons from the light-exposed surface. If the energy received at this time is high enough, electrons (e−) in the valence band will jump to the conduction band. The energy for this electron jump is obtained from the light. This light energy is considered to be the wavelength of the light; because of the height of the hurdle that the electron has to jump, the energy requires that the light have the wavelength of ultraviolet rays (380 nm or less) in the case of titanium dioxide.
E = hn n = c / λ ; accordingly, E = hc / λ
E is energy, h is the Planck constant, n is frequency, c is the speed of light, and λ is the wavelength
Here, E is 3.2 eV for titanium dioxide (3.2 eV = 3.2 × 1.6 × 10–19 J). Upon solving by substituting a known value (c = 3.0 × 108 m/s; h = 6.63 × 10−34 J•s), he required wavelength is about 380 nm, clearly indicating that the light required for the photocatalyst to work is ultraviolet light.
The main actions of photocatalysts include the following.
Air purification: Removal of airborne harmful substances such as NOx, SOx, and formaldehydes
・Deodorization: Decomposition of odors such as acetaldehyde, ammonia, and hydrogen sulfide
・Water purification: Decomposition and removal of volatile organic chlorine compounds such as tetrachlorethylene and trichlorethylene pollutants dissolved in water
・Sterilization: Cleaning of the environment through antibacterial action
・Antifouling: Prevention of fouling on window glass, exterior walls, etc.
The use of photocatalysts outdoors is due to the titanium dioxide photocatalysts' functional requirement for high-energy ultraviolet light contained in sunlight. Research and development is exploring new photocatalysts that respond to visible light and that exhibit functions even under fluorescent lamps or other indoor lighting, in order to discover new ways of using photocatalysts and expand the range of applications. Studies are focusing on titanium oxide doped with metals such as iron and copper, and on tungsten oxide in particular, as materials that exhibit a marked visible-light response.main actions of photocatalysts include the following.
In recent years, research has begun to apply not only these materials but also quantum dots to photocatalytic materials. Quantum dots show promise as new photocatalysts for their high specific surface area, high excited-state quantum yield, and controllability of oxidation/reduction levels. However, since the particle size of quantum dots is smaller than the Bohr radius of the excitons generated by photoirradiation, and as the excitons always exist in the same space, charge separation of the excited electrons and the holes is not spatially possible in the quantum dots. As this spatial charge separation is not possible, photocatalysts made from quantum dots exhibit a high probability of exciton recombination. When excitons recombine, generated excitons are not used as photocatalysts for the desired reaction, resulting in the problem of low catalytic activity. Accordingly, achieving quantum-dot-based high-reaction-efficiency photocatalysis through light irradiation requires a quantum dot composite photocatalyst that reduces the possibility of exciton recombination by spatially performing charge separation of the generated excited electrons and holes. For this and other reasons, research is exploring quantum dots that are composites of titanium oxide and tungsten oxide.
The full mechanism behind photocatalysis in quantum dots is not yet known but has been proposed. First, electrons in the valence band are excited by a conductor to form electron–hole pairs.
QDs + hn → QDs (e− CB + h+ VB)
・CB: Conduction Band
・VB: Valance Band
Next, the oxygen molecules present at the photocatalyst interface are changed into superoxide radicals (O2•−) and hydrogen peroxide radicals (•OOH) by the photoexcited electrons.
QDs (eCB−) + (O2)ads → QDs + (O2-•)ads
(O2•−)ads + H+ → HO2•
2HO2• → H2O2 + O2
H2O2 + e− → OH− + OH•
Conversely, the holes in the valence band directly oxidize organic matter adsorbed on the surface of the quantum dot, or indirectly turn organic matter inorganic through the hydroxyl radical OH• generated by the reaction between these holes and water molecules or chemically adsorbed OH−.
ZnS quantum dots is an example of quantum dots that have been considered as photocatalytic material. It has been reported that photocatalytic performance changes when these ZnS quantum dots are doped with other transition metals, such as manganese, nickel, or copper. In this way, the photocatalytic effect of quantum dots and the lifespan of excitons (electrons and holes) change according to their size and through doping with metals. It is thought that surface states generated by doping with different metals act as a trap, with effects including an extended lifespan for the excitons. In addition to such metal-doped ZnS quantum dots, types of quantum dots reportedly applied to photocatalysts include graphene quantum dots, carbon quantum dots, CdS, CdS/ZnS, Ag-sensitized BiWO6, SnS, and ZnS–AgInS2.