Synthesis, Characterization and Sonocatalytic Activity of Co/N/Er3+ : Y3Al5O12/TiO2 Films for the Degradation of Organic Dyes

The sonocatalytic degradation of organic dyes (C.I. 50040, C.I. Reactive Red 1, C.I. Acid Orange 7) catalysed by Co/N/Er3+ : Y3Al5O12/TiO2 films was studied. For the preparation of Co/N/Er3+ : Y3Al5O12/TiO2 films, the sol-gel coating process was used. The phase composition, morphology, precursor at different temperatures and emitting light properties of the calcined powders were analysed by X-ray diffraction (XRD), absorption spectra and upconversion emission spectra. The X-ray diffraction of powder samples of Co/N/Er3+ : Y3Al5O12/TiO2 took on anatase mine peaks and upconversion luminous agent, respectively. Analysis of absorption spectra of amorphous Co/N/Er3+ : Y3Al5O12/TiO2 showed that doping N stretching vibration peak of water or hydroxyl adsorption, Co2+ ion had very strong absorption in 1.0–1.7 μm wavelength range, the transition luminescence of Er3+ ions was just on Co2+ ions absorption band. The emission spectrum indicated that Co/N/Er3+ : Y3Al5O12/TiO2 could launch green 500–560 nm and red 650–700 nm, 525, 550 and 660 nm peaks corresponding to H11/2, S3/2 → I15/2 and H9/2 → I15/2 transition of Er3+. Doping Co and N enhanced the upconversion luminescence and absorption effect. Sonocatalytic degradation effect of organic dyes loading Co/N/Er3+ : Y3Al5O12/TiO2 was better when ultrasonic intensity was equal to 15 W cm–2. The degradation ratios of aqueous solutions of these three kinds of organic dyes by ultrasonic irradiation were obviously lower than by ultrasonic irradiation together with Co/N/Er3+ : Y3Al5O12/TiO2 films in the same conditions. Degradation kinetics of organic dyes by ultrasonic irradiation and by ultrasonic irradiation cooperating with Co/N/Er3+ : Y3Al5O12/TiO2 films followed the first-order reaction.


Introduction
Recently, photocatalytic degradation of organic pollutants assisted by TiO 2 has been extensively investigated because of its strong oxidation ability, low energy consumption, and simple operation. 1,2The discharge from the textile industries contains a lot of inorganic salts and surface-active agents.Inorganic salts and surfactants have complicated influence on the photocatalytic treatment processes of dye effluents. 3The research of E. Y. Bae et al. 4,5 manifested that the ultrasonic mineralization aided efficiently in opaque or low transparent degradation of dye effluents.K. Okitsu et al. 6,7 reported ultrasonic and solar photocatalytic decomposition of organic dye in aqueous solution.Owing to broadband gap of TiO 2 , only a small fraction (ultraviolet light) of sonoluminescence could be taken advantage of in sonocatalytic degradation.According to J. Wang et al. 8,9,10 the upconversion luminescence agent could transform visible light into ultraviolet light, which might activate the TiO 2 efficiently.Moreover, some semiconductor materials uniting some upconversion luminescence agent may improve the catalytic degradation efficiency for further expanding the spectral response range of ultrasound catalysis.
The preparation of Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 and the characterization of XRD are presented.This study details the ef-fects on crystal structure, spectra, degradation efficiency of organic dye in sonocatalytics of Co/N/Er 3+ : Y 3  equal to 1 : 9 : 90) were dissolved in HNO 3 , and excess HNO 3 was evaporated to obtain the lanthanide nitrates.Second, Al(NO 3 ) 3 (moles ratio of [Y(NO 3 ) 3 + Er(NO 3 ) 3 ] and Al(NO 3 ) 3 = 3 : 5) was added to the lanthanide nitrates, which were then N,N-dimethylformamide to produce Co/N/Er 3+ : Y 3 Al 5 O 12 solution.
In the preparation of composite TiO 2 gel, the molar ratio of diethanolamine : tetrabutyl titanate : ethanol : H 2 O : acetic acid : polyethylene glycol 3000 was equal to 2 : l : 20 : 2 : 0.3 : 0.002.First, tetrabutyl titanate and diethanolamine were added into ethanol and agitated for 1 hour with magnetic stirrer against white flocculent precipitation.Deionized water and acetic acid were then dripped in slowly and at the same time agitated for 10 minutes.Next, the above Co/N/Er 3+ : Y 3 Al 5 O 12 solution was dripped in and stirred at the same time, after which polyethylene glycol 3000 was added, and stirred for another 3 hours.Finally, Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 gel was prepared.
The glass slides were cleaned and dried with deionized water and ultrasonic wave.Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 gels were sprayed on glass slides by ultrasonic spraying equipment (Beijing Dongfang Jinrong ultrasonic electric Co. Ltd.).Spraying width was 20 mm, the flow rate was 10 ml min −1 .After the wet films had dried, they were put into the heating furnace for heat treatment, with 2 °C min −1 up to 120 °C, with 120 min insulation, in order to strengthen the porous thin films of Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 .

Sonocatalytic degradation of the organic dye
The experimental apparatus is shown in Fig. 1.The reactor with a built-in trough was placed on a magnetic stirrer.The glass slides with Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 thin films were inserted into the trough.The glass slides had tiny holes so that dye solutions could pass through them to distribute homogeneously in the degradation process.The ultrasonic probe (model of ultrasonic generator: 88-1, frequency:    Fig. 5 shows the degradation ratios of these three kinds of dye when ultrasonic intensities were 5, 10, 15, and 20 W cm −2 respectively.At 120 minutes, the degradation ratios (γ t /γ 0 ) of C.  and 4.15 %, respectively, when ultrasonic intensities were 5, 10, 15, and 20 W cm −2 respectively.Apparently, the degradation ratios ((γ t /γ 0 )) of these three kinds of dye decreased with the increasing of ultrasonic irradiation time and ultrasonic intensity.The decrease in the value of (γ t /γ 0 ) was more obvious when the ultrasonic intensity increased from 5 to 15 W cm −2 than when the ultrasonic intensity increased from 15 to 20 W cm −2 .The reason might be that, when the ultrasonic intensity became largely excessive, the cavitation bubbles became too big in the negative phase of the sound wave to collapse sufficiently so that available ultrasonic energy decreased, thus forming a sound shield.Furthermore, when ultrasonic intensity was 20 W cm −2 , the ultrasonic generator could overheat and the ultrasonic probe surface could corrode significantly for cavitation.Therefore, the sound intensity, 15 W cm −2 , was suggested in the ultrasonic degradation experiments.degradation ratios all increased with the increase in time of ultrasonic irradiation.Secondly, it can be noted that the degradation ratios of C.I. 50040, C.I. Reactive Red 1, and C.I. Acid Orange 7 only under ultrasonic radiation were obviously lower than those under ultrasonic radiation and loading catalysts of Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 films.This might be because ultrasonic cavitation could have produced free radicals ·OH, which could have oxidized the dye molecules, but the dye molecular structure was too stable to be degraded easily only under ultrasonic radiation.Co/N/Er 3+ : Y 3 Al 5 O 12 /TiO 2 films had very large specific surface and thus relatively more catalytic active points.These contributed to the change of the charged properties and space configuration of dye molecules adsorbed by catalyst surface, which could improve the adsorption efficiency of surface active points, that is, make the surface radicals (⋅OH) oxidize the dye molecules easier.This corresponds to the bond (-N=N-) as the most active site for oxidative attack in dye molecule of C.I. 50040, C.I. Reactive Red 1, and C.I. Acid Orange 7.
Research on the sonocatalytic reaction kinetics was also conducted.The reaction rate constant and reaction order were determined by differential method.

The reaction rate equation was
where, υ t is the reaction rate at time t, c t is the reaction concentration at time t, k is the reaction rate constant, and superscript n is the reaction order.
Taking logarithm on both sides of Eq. ( 1), we could obtain the following equation According to Eq. ( 2), and using the reaction concentration and reaction rate measured in degradation tests, the reaction rate constant and reaction order were evaluated as shown in Table 2.The sonocatalytic degradation reactions all followed the first-order kinetics.

FigsFig. 6 -Fig. 5 -
Figs. 6a, 6b, and 6c illustrate the degradation ratios of the aqueous solutions of C.I. 50040, C.I. Reactive Red 1, and C.I. Acid Orange 7 with the time, respectively.Firstly, it can be seen that, for these three kinds of dye solutions, the