ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2012, том 46, № 1, с. 3-11
ОБЗОР
AN OVERVIEW OF COMMONLY USED SEMICONDUCTOR NANOPARTICLES
IN PHOTOCATALYSIS © 2012 Shipra Mital Gupta and Manoj Tripathi
University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University Sector 16C Dwarka, Delhi 110075, India E-mail: shipra.mital@gmail.com Поступила в редакцию 22.06.2011 г. В окончательном виде 01.08.2011 г.
The depletion of non-renewable resources and rise in global warming has caused great concern to humankind. With a view to use renewable source of energy and to eliminate hazardous chemical compounds from air, soil, and water, photocatalysis utilizing solar energy is becoming a rapidly expanding technology. Semiconductor nanoparticles have the ability to undergo photoinduced electron transfer to an adsorbed particle governed by the band energy positions of the semiconductor and the redox potential of the adsorbate. A brief overview of some metal oxides and sulphides that can act as sensitizers for light-induced redox processes due to their electronic structure is presented here.
Major problems associated with industrial revolution are the depletion of non-renewable resources, rise in global warming and the generation of harmful wastes which cause pollution of the environmental and damage to life forms. Semiconductor photocatalysis is intended to be both supplementary and complementary to the more conventional approaches for the destruction of hazardous chemical wastes. A human being has been aware of the influence of solar radiation on matter and the environment from early times. But it is mainly during the last century that a systematic understanding of photochemical and photophysical processes has been developed.
Photocatalysis implies the acceleration of a photo-reaction by the presence of a catalyst. The overall process of semiconductor-sensitized photoreactions can be summarized as follows:
A + D-
semiconductor
->A- + D
Light
Activation of the semiconductor photocatalyst is achieved by the absorption of a photon of light corresponding to the bandgap energy, which results in the excitation of electrons in the valence band to the conduction band leaving behind holes in the valence band [1].
The primary criteria for a photocatalysts to be efficient is that the different interfacial electron processes involving electron and hole must compete effectively with the major deactivation processes involving electron-hole recombination. Moreover, the usage of a semiconductor as a photocatalyst depends upon its ease to produce and use, cost effectiveness, photosta-bility, non-toxicity for human beings as well as for environment, effective activation by solar light and ability to catalyze the reaction effectively [2].
To carry out the redox reaction, the donor level of valence band of semiconductor must be more positive than redox potential of reactant vs NHE. The lower level of conduction band must be more negative than the redox potential of given reactant vs NHE. In other words, the redox potential of the e-/h+ couple must lie within the bandgap domain of the photocatalyst. The lower edge of the conduction band and upper edge of the valence band along with the bandgap in electron volts for some of the common photocatalysts used are presented in Fig. 1 [3]. The energy scale is indicated in electron volts using either the normal hydrogen electrode (NHE) or the vacuum level as a reference. It presents internal and not free energy. The free energy of an electron-hole pair is smaller than the bandgap energy due to the translational entropy of the electrons and holes in the conduction and valence band, respectively. On the right side the standard potentials of several redox couples are presented against the standard hydrogen electrode potential. Depending on the semiconductor and pH value, the valence band holes are powerful oxidants (+1 to +3.5 V vs NHE), while conduction band electrons are good reductant (0.5 to - 1.5 V vs NHE) [4].
PbS
Lead sulphide belongs to IV—VI compound semiconductor materials [5]. It has a narrow bandgap of 0.41—0.78 eV and large exciton Bohr radius of 20 nm which leads to extensive quantum size effects [6, 7]. Due to its unique photoconductive properties, PbS is applied as an infrared detector and for mid-infrared lasers. PbS has promising photosensitive properties and is a good photocatalyst [8]. The drawback of PbS
q _ (vacuum)
E * (NHE)
-3.Q -3.5 -4.Q
-4.5 -5.Q -5.5 -6.Q -6.5 -7.Q -7.5 -8.Q
-—1.5 --1.Q —Q.5
- -q
-+Q.5 -+1.Q + 1.5 -+2.Q +2.5 -+3.Q +3.5
GaAs (n, P)
EC
GaP (n, P) GaAsP (n, P)
SiC (n, P)
AE= 1.4 eV
2.25 eV
2Л5 eV
CdS CdSe (П) Zn°
(n)
1.7
eV
2.5 eV
(n)
3.2 eV
W°3 —(n)—Sn°2-(n)
Ti°2 (n)2
3.2 eV
3.8 eV
3.2 eV
-3.Q
eV
Eu
2+/3+
H2/H+
[Fe(CN)6]
Fe2+/Fe3+
3-/4-
Ru(bipy)
2+/3+
Ce
4+/3+
Band positions of several semiconductors in contact with an aqueous electrolyte at pH 1.
Reprinted with permission from A. Hagfeldtt, M. Grätzel, Chem. Rev. 95, 49 (1995), copyright (1995), American Chemical Society [3].
is that it is insufficiently stable for catalysis, at least in aqueous media as it readily undergoes photoanodic corrosion [9]. It is suggested that the chemical modification provides more well-defined surface of Q-parti-cles of PbS [10, 11] and are suitable for investigation of effects on surface conditions of Q-particles on photo-induced charge transfers. Torimoto et al. [12] reported on photoinduced electron transfer on methyl viologen on 4-aminothiophenol modified Q-PbS particles. The chemical modification of Q-particle surfaces with organic reagents such as thiol compounds is useful as a means for preventing aggregation of Q-particles and allows the preparation of Q-particles with a relatively narrow size distribution.
The possibility of using PbS sensitized by nanoc-rystalline TiO2 for gas phase photocatalysis seems to be very attractive [8]. Vogel et al. [13] sensitized TiO2 by Q-PbS. However, a loss in efficiency due to photocorrosion of PbS in aqueous media under illumination with 460 nm light was demonstrated. Howe et al. [8] suggested the possibility of using such semiconductor sensitized by nanocrystalline TiO2 for gas-phase pho-tocatalysis.
Ratanatawanate et al. [14] prepared PbS quantum dots attached to TiO2 nanotubes on both the inside and outside surfaces of the nanotubes by using thiolac-tic acid, a bifunctional linker. The photocatalytic activity and stability of PbS/TiO2 nanotubes were evaluated for the photodegradation of both anionic and cat-ionic organic dyes. Functionalized TiO2 nanotubes were superior catalysts as compared with commercial P-25 catalyst for photodegradation. PbS quantum dots improved the photocatalytic activity of TiO2 because of multiple exciton generation and efficient spatial separation of photogenerated charge, preventing electron-hole recombination [15, 16] of cationic dyes. Additionally, the quantum dots enhance the activity and expand the usable portion of the solar spectrum.
CdSe
Among all semiconductor nanoparticles, metal selenides nanoparticles are the most studied due to their interesting properties and widely used as thermoelectric cooling materials, optical filters, optical recording materials, superionic materials, and biomedical label-
ing. Moreover, CdSe is one of the most attractive selenides among all selenides nanoparticles owing to its high photosensitivity, which has been widely used as photoconducting device [17, 18]. CdSe has a narrower bandgap making it sensitive to incident light in the visible spectrum.
The methods used for obtaining CdSe nanoparti-cles and nanocomposites have various disadvantages including expensive procedures, use of toxic precursors, in particular organometallic compounds in Cd(II) and Se(II), high temperatures and pressures or high-energy irradiation [19]. Li et al. [20] tried to minimize this limitation by synthesizing CdSe nanoparti-cles by a "green" route using starch as capping agent at room temperature and obtained nearly monodisperse CdSe nanoparticles. Zhu et al. [19] reported a photochemical method in synthesizing CdSe nanoparticles at room temperature by a very simple reaction between CdCl2 and Na2SeSO3 in the presence of acetic acid. This method avoided the need of high temperature, high pressure and inert atmosphere protection.
Kamat et al. [21] investigated photoinduced charge transfer events between CdSe semiconductor nanoc-rystals and an electron acceptor, methyl viologen (MV2+) by confining the reactants in an AOT/heptane reverse micelle. MV2+ interacted with the excited CdSe nanoparticles and quenched its emission effectively. The ultrafast electron transfer to MV2+, monitored from the exciton bleaching recovery of CdSe and the formation of MV+* radical was completed with an average rate constant of 2.25 x 1010 s-1. Under steady state irradiation at 450 nm the accumulation of MV+* was seen with a net quantum yield of 0.1. However, a major drawback with CdSe is its significant photocorrosion [22].
Aldana et al. [23] studied the photochemical instability of CdSe nanocrystals coated by hydrophilic thiols non-destructively and systematically in water. The results revealed that the photochemical instability of the nanocrystals actually included three distinguishable processes, namely, the photocatalytic oxidation of the thiol ligands on the surface of nanocrystals, the photooxidation of the nanocrystals, and the precipitation of the nanocrystals.
To overcome the above drawbacks, a sequential approach to make core/shell structures has often been used which alter the properties of the core. CdSe/ZnS was studied as a system to enhance photoluminescence stability and yield of CdSe as well as to lower the toxicity since ZnS is less toxic than CdSe [24-30]. Apart from ZnS, other semiconductors having larger bandgap and lower toxicity compared to the core CdSe have also been tried. For example, CdSe nanoc-rystals were overcoated with
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