Comparative analysis of photocatalytic activity of aqueous colloidal solutions of ReVO4:Eu3+(Re = La, Gd, Y), CePO4:Tb, CeO2 and C60

https://doi.org/10.1016/j.jphotochem.2015.05.019Get rights and content

Highlights

  • Photocatalytic activity of aqueous-colloidal solutions of orthovanadates, orthophosphates, cerium dioxide and fullerenes was investigated.

  • Nanoparticles of rare earth orthovanadates and fullerenes induce formation of radicals in water.

  • Nanocrystals of cerium dioxide inactivate radicals in water playing a role of radical sponge.

Abstract

Photocatalytic activity of aqueous colloidal solutions of nanoparticles of different nature at UV-irradiation was determined. 2 nm GdYVO4:Eu, 8 × 25 nm GdVO4:Eu, 6 × 40 nm LaVO4:Eu, 2 and 9 nm CeO2, 5 × 10 nm CePO4:Tb and 62 nm С60 nanoparticles were used. Rare earth orthovanadate nanoparticles as well as fullerenes induce radical formation in water. It was shown that intensity of photocatalytic process increases with increase of the linear sizes of orthovanadate nanocrystals. Rare earth orthophosphate nanoparticles do not participate in photocatalytic processes at all. Cerium dioxide nanocrystals inactivate radicals in the water playing a role of a radical sponge.

Introduction

Nowadays a lot of attention is paid to the photocatalytic processes taking place in the presence of inorganic nanoparticles (NPs) in water solutions [1], [2]. Especially interesting is the possible practical application of nanoparticles in biology and medicine. On the one hand, photosensitive nanomaterials under the action of an external light source are able to active generation of free radicals providing the necessary condition for their application in photodynamic therapy [3], [4], [5]. On the other hand, nanoparticles are widely used as a base for sunscreens [6], [7]. The sunscreens are developed for protection of human skin from sunburns and harmful impact of UV, i.e. collagen damage, DNA mutations and dysfunctions of immune system following sometimes by cancer deceases. The sunscreen action of nanoparticles is based on the physical principles of reflection, absorption and Rayleigh scattering of UV radiation or its conversion to the radiation in the different spectral range. The necessary conditions for effective photoprotectors are the following: absorption of UV and neutralization of radicals and active oxygen formed at irradiation of water and biological molecules. As the photosensitive inorganic materials semiconductors (for instance, SrWO4, TiO2, YVO4 and CeO2 [8], [9], [10], [11]) are usually used.

The photocatalytic properties of semiconductor materials are determined by the features of their electronic structure. The photocatalytic activity depends on the ability of the catalyst to create electron–hole pairs generating thereby free radicals able to take part in the secondary reactions.

An evaluation of an ability of some photosensitive material to posses the properties of the heterogenic photocatalyst requires consideration of peculiarities of its band structure. Bandgap of an effective photocatalyst must, on the one hand, be narrow enough to provide an effective transfer of electrons from the valence band to the conduction band and, on the other hand, be wide enough to prevent an instant recombination of the free charge carriers. Average lifetime of free charge carriers in the common crystal photosensitive materials is (30 ÷ 100) × 10−12 s for free electrons and (10 ÷ 250) × 10−9 s for holes [12], [13].

Electron–hole pair exists until the moment of recombination or until capture of either electron or hole by an alien agent—for instance, by molecule of oxygen, water or different substance adsorbed on the surface of NP.

Absorption of a photon by NP leads to the formation of free electrons (e) and free holes (h+) that either recombine instantly or migrate through the material and are able to localization on the defects of the crystal structure:NP +   NP(eCB + h+VB)where CB means conduction band, VB—valence band.

In the aqueous media free charge carriers localized near the surface of nanoparticles can easily interact with water molecules forming thereby free radicals:H2Oads + h+VB  HOradical dotads + H+OHads + h+VB  HOradical dotadsO2ads + eCB  O2radical dot adsO2radical dot ads + H+  HO2radical dotadsO2radical dot ads + eCB + 2H+  H2O2ads

At UV irradiation hydrogen peroxide dissociates with formation of hydroxyl radicals:H2O2 +   2HOradical dotads

The hydrogen peroxide as well as dissolved oxygen is an effective scavenger of photogenerated electrons:H2O2 + e  H2O2radical dot  HOradical dot + OH

Presence of the ions with variable valence in the crystal lattice leads to the reversible process:Ce4+ + eCB  Ce3+Ce3+ + h+VB  Ce4+,

which can cause either migration of free charge carriers within the volume of the crystal or their interaction with the molecules or radicals adsorbed on the surface of NPs. These processes can be accompanied by both radical induction and radical neutralization near the surface of the crystal.

On the other hand, the absorption of UV photons by water molecules is accompanied by formation of the same radicals as the ones formed on the surface of semiconductor NPs (HOradical dot, Hradical dot, O2radical dot and HO2radical dot).

So, an irradiation of hydrosols of photoactive NPs is accompanied by the formation of free radicals with participation of free charge carriers as on the surface of the nanocrystals, so within the water. On the other hand, opposite process also takes place i.e. the recombination and neutralization of radicals as in the water, so on the surface of NPs.MiRiads+Rivolwhere Mi denotes molecules or ions (for instance, H2O, O2, H+), Riradical dotads, Riradical dotvol are radicals on the surface of nanocrystals or in the water.

Shift of the equilibrium to the one or another side depends on the ability of nanocrystals to generate or inactivate free radicals in the system.

For determination of the intensity of radical formation in the system, the organic compounds able to interaction with radicals with formation of the products of destruction were used. Usually, for these purposes dyes easily destructing under an action of free radicals are chosen. During interaction with the free radicals decrease of an optical density of characteristic dye bands in the absorption spectra is observed.

The task of this paper was an estimation of the degree of photoinduction and inhibition of free radical generation at UV irradiation in the water solutions of NPs with different nature and form-factor. In the work NPs of rare earth based orthovanadates and orthophosphates, cerium dioxide and С60 fullerenes perspective for biological and medical applications [14], [15], [16], [17] were used.

Section snippets

Materials

Lanthanide chlorides 99.9% and anhydrous sodium metavanadate (NaVO3, 96%) (Acros Organics company) were used without further purification. Sodium tripolyphosphate (Na5P3O10, 98%), sodium citrate (Na3C6H5O7, 99%), hexamethylenetetramine ((CH2)6N4, 99 %), H2O2, 35%, NH4OH, 25%, from Macrochem Co., Ltd. were used. The inorganic salts, toluene and isopropanol were commercial products of reagent grade. The fullerene C60 (>99.9% pure) supplied by Aldrich was used. Na3VO4 solution with pH value of 13

General characteristic of hydrosols

According to TEM results solid phase of colloidal solution consist of nanoparticles with the following average sizes: orthovanadates – 2, 8 × 25, 6 × 40 nm; cerium dioxide – 2 and 9 nm; orthophosphates – 5 × 10 nm; fullerenes – 62 nm (Fig. 2). In the table the composition and some characteristics of the solid phase of hydrosols are shown.

In the Fig. 3 absorption spectra of cerium dioxide and orthovanadate NPs corrected by Kubelka–Munk absorption function for direct transitions are shown. Values of the

Conclusions

In the present study we have demonstrated that at UV irradiation of hydrosols containing NPs of different nature processes of both activation and inhibition of free radical formation are observed. NPs of rare earth orthovanadates, as well as fullerenes induce formation of radicals in water. Also, using the example of orthovanadate NPs with different sizes it was shown that intensity of the photocatalytic process increases with increase of the linear sizes of NPs. Nanocrystals of rare earth

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