^{a} Institute of Chemical Physics RAS
Kosygin st. 4, 117977 Moscow B-334, Russia
^{b} Institute of General & Inorganic Chemistry, Belarusian Academy of Sciences, Minsk 220072
^{c} M.Lomonosov Moscow State University, Department of Chemistry, Moscow 117234, Russia
Nanostructured oxidic semiconductors such as TiO_{2},
ZrO_{2}, etc., were widely used as photoelectrochemical electrodes and colloidal
photocatalysts for decomposition of toxic pollutants [1]. The importance of the hole-electron
recombination kinetics in determining the photocatalytic activity of colloidal semiconductors has
been discussed [2,3]. To improve photochemical properties of the photocatalysts, doping of their
surface with transition metal ions was usually applied [1-3]. An EPR study of TiO_{2}
aqueous colloids doped with Fe, V or Mo has been reported [4]. The authors observed inhibition
of hole-electron recombination by Fe^{3+} and V^{4+} dopants.
One of the most important characteristics of the doped semiconductor particles for better
understanding of their properties and action is knowledge about the structure and spatial
distribution of the dopant centers with measurement of the distances < r > between them
or their local concentration < C >. The EPR technique allows to obtain such data in the case
of paramagnetic centers (PC) in any diamagnetic matrix [5]. In our previous work [6] we have
presented some quantitative data for polycrystalline TiO_{2} electrodes doped with
V^{4+} and Cr^{3+} ions. The method for < C > and < r >
determination from magnitude of the dipole-dipole interaction in cases of random, cubic or pair
spatial distribution is well known and was proved experimentally in numerous works. The
problem of determination the parameters analogous to < C > and < r > for systems in
which PC were distributed on the surfaces of solid materials or in the interface is not solved yet.
In this paper we perform our recent results obtained for the nanosized TiO_{2}
(Degussa P 25, Hombicat-100 and nano-TiO_{2} prepared ourselves) and
ZrO_{2} particles contained different amount of V(4+) ions on the surface. For the
correct calculation of the corresponding parameters < C_{surf} > and <
r_{surf} > a special theoretical and computer analysis has been done.
Fig.1. EPR spectra of the P-25 particles after: 1 – 10 min, 2 – 6 days,3 –75 days of incubation in 0.15 cm^{3} of 0.65 M ascorbic acid in C_{2}H_{5}OH-H_{2}O = 3:2 solution. [V^{4+}] = 1.8 x10^{20} cm^{–3}; T = 77 K. |
Fig.1 presents typical EPR spectra of TiO_{2} (Degussa P25) particles at
different content of V^{4+} ions. It is evidently seen, that, as we have observed for the
doped matrix of polycrystalline TiO_{2} electrodes [6], there are two types of
V^{4+} dopants: a) 'isolated' centers with well resolved anisotropic EPR spectrum and
rather low concentration < C_{surf} > of them and b) 'aggregates' with a broad
singlet line in the EPR spectrum usual for PCs with strong spin-exchange interaction between
them and much shorter distances < r_{surf} > comparing with 'isolated' centers.
Similar results were obtained for all TiO_{2} and ZrO_{2} systems, but
appearance of the singlet line depended strongly on the vanadium content.
Figures 2 and 3 present changes in the low-field parts of the V^{4+} EPR spectra
for the samples prepared using Degussa P 25 and Hombicat-100 powders at various
V^{4+} contents (Fig. 2) or time after sample’s preparation (Fig. 3). The main
regularities of the structural features of V^{4+} centers in the TiO_{2} and
ZrO_{2} interface will be discussed in the paper.
Fig.2. Low-field lines of the EPR spectra of the Hombicat-100 particles with [V^{4+}] content: 1 – 1.6 ×10^{19} cm^{–3}, 2 – 4.0 ×10^{19} cm^{–3}, 3 – 1.2 ×10^{20} cm^{-3} after 15 days of incubation in 0.15 cm^{3} of 0.75 M ascorbic acid; 4 –0.01M VO^{2+} in C_{2}H_{5}OH-H_{2}O = 3:2 solution; T = 77 K. | Fig.3. Low-field lines of the EPR spectra of the Hombicat-100 ( 1-3 ) and P-25 ( 4,5 ) particles after: 1 – 10 min, 2, 4 – 25 h, 3, 5 –15 days incubation in 0.15 cm^{3} of 0.75 M ascorbic acid. [V^{4+}] = 4.0 ×10^{19} cm^{–3} ( 1 – 3 ) and 2.0 ×10^{19} cm^{–3} ( 4 , 5 ); 6 –0.01M VO^{2+} in C_{2}H_{5}OH-H_{2}O = 3:2 solution; T = 77 K. |
One can see from these figures that there exist up to three types of surface V^{4+} centers with slightly different spectra and spin-Hamiltonian parameters. Analysis of these changes showed that in time V^{4+} ions from “aggregates” are transformed mainly to the “c” type of the “isolated” centers (Fig. 2, 3), though these “c” ions are still anchored to the surface, and not dissolved in a liquid phase. An attempt to attribute species “a-c” has been done, as well as to characterize quantitatively the peculiarities of spatial distribution of the V^{4+} ions on the oxidic surfaces.
Fig. 4. Fourier transformed dipole frequency spectra for 1-, 2- and 3-dimentional distributions of PCs. |
For the correct calculation of corresponding parameters < C _{surf}> and <
r_{surf}> for the studied systems, a special computer analysis has been carried out.
It was shown (Fig. 4) that there existing the essential difference in the EPR-line shape for three
cases of 3-, 2- and 1- dimensional systems. The obtained experimental results will be discussed
in the paper in detail and will be compared with structural data known for the TiO_{2}
and ZrO_{2} crystalline materials doped with V^{4+} inside the bulk matrix.
It was theoretically shown that there existing the essential difference in determination of
the structural parameters for three-, two- and especially one-dimensional systems [7, 8]. The
following equation performs the relation between T _{2 D } and n_{D} :
T_{2 D}^{-1}= 4.64 n_{D}^{3 / D}g^{2}ħ ,
where T_{2D} is the transversal relaxation time of electron spin, g is an electron magnetogyric ratio, n_{D} is D -dimensional density of paramagnetic centers and D is the dimension of the PC’s spatial localization [8].
Thus, comparing the structural properties of metal-doped oxidic
semiconductors one can conclude that:
1) a tendency to form the metal ion aggregates with high local concentration < C_{surf}>
of the doping ions is typical for V^{4+} species on the surface
of TiO_{2} and ZrO_{2} nanoparticles, similar to what has been observed
for a matrix of V^{4+}-doped polycrystalline TiO_{2} systems studied in [6].
2) Relative concentrations of different V^{4+} species of the 'isolated' and
'aggregated' V^{4+} ions depend on vanadium content in the sample and the nature of the carrier.
Various types of V^{4+}-doped TiO_{2} nanoparticles, as well as of ZrO_{2} ones, presented different peculiarities of the structure and spatial distribution of metal centres on their surface, that should be taken in consideration for explanation of photocatalytic and photoelectrochemical behaviour of such nanocrystalline systems.
Acknowledgements:
The authors are grateful to the RFBR (grant No.00-03-81168) and INTAS (Grant No.
00-0863) for financial support. We are thankful to Prof. A.Kh. Vorob’ev for providing us his
PC EPR-programs and to Prof. M.Ya. Melnikov for valuable discussions.
References
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[2] | Kalyanasundaram K., Gratzel M., Pellhzetti E., Coord. Chem. Rev., 69, 57 (1985). |
[3] | Martin S.T., Morrison C.L., Hoffmann M.R., J. Phys. Chem., 98, 13695 (1994). |
[4] | Grätzel M., Howe R.F., J. Phys. Chem., 94, 2566 1990). |
[5] | Abragam A., The principles of nuclear magnetism. Clarendon Press, Oxford, 1961. |
[6] | Kokorin A.I., Arakelian V.M., Arutyunian V.M., Proc. of 13-th QUANTSOL, Kirchberg, Austria, 2001, p. 13-15. The same , Russian Chem. Bull., 2002, in press. |
[7] | Dzeparov F.S., Lundin A.A., Khazanovich T.N., Russion JETP, 92, 342 (1987). |
[8] | Atsarkin V.A., Vasneva G.A., Demidov V.V., et al., JETP Letters, 72, 530 (2000). |
email: kokorin@chph.ras.ru