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Journal of Chemical Technology and Metallurgy, 50, 5, 2015, 606-612

NITRIC ACID ACTIVATION OF La-DOPED ZnO PHOTOCATALYST

FOR WATER DECONTAMINATION

Katya Milenova1, Alexander Eliyas1, Vladimir Blaskov2, Irina Stambolova2,

Sasho Vassilev3, Slavcho Rakovsky1, Nikoleta Kasabova4

1Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G.Bonchev Str, bl.11, 1113 Sofia, Bulgaria, E-mail: kmilenova@ic.bas.bg 2Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G.Bonchev Str, bl.11, 1113 Sofia, Bulgaria.3Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G.Bonchev Str, bl. 10, 1113 Sofia, Bulgaria.4University of Chemical Technology and Metallurgy, 8 Kliment Ohridski, 1756, Sofia, Bulgaria.

ABSTRACT

An original patented procedure was applied to obtain activated ZnO powder and then it was doped with 1.5 mass % La to improve the photocatalytic performance. The effect of a nitric acid treatment (index a.t.) of the La/ZnO investigated by comparing with the non-treated La/ZnO samples, manifests a higher photocatalytic efficiency of the former in the oxidative degradation of the model textile wastewater pollutant azo dye Reactive Black 5. The acidically treated La/ZnO powders have been obtained by impregnation in 0.1 M HNO3 acidic solution, followed by thermal treatment at 100oC, 350oC and 500oC. The materials have been characterized by the XRD, XPS and single point BET methods. The XRD analysis shows the presence of three crystallographic phases: wurtzite, Zn(NO3)(OH).H2O and 4ZnO(OH)2.Zn(NO3)2.H2O. The XPS data give evidence of N-atoms in ZnO after the acidic treatment. The best photocatalytic performance is shown by the La/ZnO a.t. dried at 100oC due to its highest nitrogen content.

Keywords: zinc oxide, La doping, photocatalytic activity, azo dye pollutant.

Received 12 February 2015Accepted 05 June 2015

INTRODUCTION

During the last several decades there has been a growing interest in the decontamination of air, water and land, due to the fact that it is one of the primary causes for various health problems, as well as, for possible changes in the global climate [1]. The uncontrolled release of wastewater, contaminated with textile coloring dyes, or waste waters originating from the paper, rubber and plastic industries is resulting in serious environmental pollution [2]. Photocatalysis is a method that has been proposed for quite some time. It is being applied to several environmental pollutants of air and water [3]. Ideally, from a green chemistry perspective, reactions

would be performed at ambient temperature and pressure [4]. Photocatalysis can be used to completely decompose nearly all organic molecules. The principal products are CO2 and H2O [5]. Recent studies have shown that the heterogeneous semiconductor photocatalysis can be an alternative method to the conventional ones, for the removal of dye pollutants from water [6]. Zinc oxide is n-type semiconductor due to its deviations from stoi-chiometric composition, following from the appearance of oxygen vacancies and zinc cation interstitials [7]. However, there have been few reports on the positive effect of La doping of ZnO nanoparticles in regard to photocatalytic activity [8, 9]. Recently nitrogen doping is also applied to improve the photocatalytic performance

Katya Milenova, Alexander Eliyas, Vladimir Blaskov, Irina Stambolova, Sasho Vassilev, Slavcho Rakovsky, Nikoleta Kasabova

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by introducing electron levels in the band gap and utilize the visible light. Among the different approaches for activation, there are several articles devoted to the effect of post-synthesis acidic treatment of TiO2 powders on their photocatalytic performance [10, 11] but only few data are available about ZnO-N preparation [12 - 14]. Ammonium nitrate was applied by Wu [12] to fabricate N-doped ZnO, whose photocatalytic efficiency was greatly enhanced in comparison to N-doped TiO2 for degradation of formaldehyde under strong visible-light irradiation. Erdogan et al. [13] have obtained p-type ZnO:N film and detected N1s XPS peak at 402.7 eV, proving the incorporation. Li and Haneda [14] have applied zinc-ammonia complex and spray pyrolysis to obtain N-ZnO powders. In our previous paper [15], we reported the superior performance of La-doped ZnO, compared to pure undoped ZnO. The higher oxidation state of La3+, compared to Zn2+ enhances the oxidation rate/degree of discoloration.

The aim of this paper is to study the influence of a nitric acid treatment on La/ZnO samples, heated subse-quently at different temperatures, on the structural and the photocatalytic properties of ZnO powders, activated by an original patented method.

EXPERIMENTAL

Synthesis of the samplesZnO powder was activated by a procedure [16],

described in Bulgarian Patent № 28915/1980 (Classifica-tion Index C 01 G 9/02). It includes firstly its dissolution in the acidic medium, then consecutive treatment by adding simultaneously NH4OH and CO2 purging, lead-ing to precipitation as Zn(OH)CO3 (ZH); followed by thermal decomposition of ZH at 400oC. The activated ZnO powder was impregnated with definite quantities of water solutions of La(NO3)3, calculated to obtain ZnO, doped with 1.5 mass % metal (with respect to the Zn amount). The samples were finally calcined at 500oC for 2 h to obtain well crystallized wurtzite phase.

The acidically treated La/ZnO powders (1g) have been obtained by impregnation in 0.1 M HNO3 acidic solution (4 ml), followed by thermal treatment at 100oC, 350oC and 500oC.

X-ray diffraction (XRD) analysisXRD patterns were recorded using a diffractometer

(Philips PW 1050) with CuKα radiation. The observed patterns were cross-matched with those available in the JCPDS database.

Аdsorption - texture analysisThe determination of the specific surface area of the

samples was carried out by the single point Brunauer-Emmet-Teller (BET) method, involving nitrogen ad-sorption from a mixture of 30 % N2 + 70 % He, at the boiling temperature of liquid nitrogen (77.4 K), using a conventional volumetric apparatus. Before measuring the specific surface area the samples were degassed at 423 K for 30 min, until the residual pressure became lower than 1.333.10-2 Pa. The nitrogen (N2) monolayer formed was used to evaluate the specific surface area (ABET) using the BET equation.

X-ray photoelectron spectroscopy (XPS)The X-ray photoelectron spectroscopy (XPS) studies

were performed in a VG Escalab II electron spectrometer using MgKα radiation with energy of 1253.6 eV under base pressure 10-7 Pa and a total instrumental resolu-tion 1eV. The binding energies (BE) were determined, utilizing the C 1s line (from an adventitious carbon) as a reference with energy of 285.0 eV. The accuracy of the measured binding energy was 0.2 eV. The N1s, O 1s, Zn 2p, La 3d5/2, photoelectron lines were recorded and corrected by subtraction of a Shirley-type background and quantified using the peak area and Scofield’s pho-toionization cross-sections.

Catalytic activity tests Reactive Black 5 (RB5) azo dye is commonly used

in the textile industry and its discharging into the water ways can cause serious environmental problems. For this reason we used this dye as a model pollutant. The photocatalytic degree of oxidative discoloring of RB5 was determined using 150 ml of aqueous dye solution with 20 ppm initial concentration. The photocatalytic activity tests have been carried out using polychromatic UV-A lamp (Sylvania BLB, 18 W), with wavelength range 315 - 400 nm (with a maximum of the irradia-tion at 365 nm). The light power density on the sample position was 0.66 mW cm-2. The process of discoloring has been monitored by an UV-Vis absorbance single beam spectrophotometer CamSpec M501 (UK), oper-ating in the wavelength range from 190 to 800 nm. All

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photocatalytic activity tests have been carried out at a constant magnetic stirring rate (400 rpm), under oxida-tive conditions (bubbling air through two frits to achieve water saturation with oxygen), at room temperature. The samples reach adsorption-desorption equilibrium in the dark within about 30 min before switching on the illumination. To test the photocatalytic activity of ZnO powders, sample aliquots of the suspension have been taken out of the reaction vessel at regular time intervals. The powder was then separated from the aliquot solution by centrifugation before the UV-Vis spectrophotometric measurement of dye concentration. After that, the ali-quot solution, together with the photocatalyst powder, were returned back into the reaction vessel to preserve constant reaction volume. The degree of discoloration is expressed as a functional dependence versus time –ln (C/Co) = f(t), where Co and C, respectively, are the initial concentration before switching on the illumination, and the residual concentration of the solution after illumi-nation for selected time interval at 599 nm (maximal absorbance specific for RB5, corresponding to the peak of the diazo bond (-N=N-).

RESULTS AND DISCUSSION

The X-ray diffraction analyses of the sample give evidence only for the formation of a wurtzite phase ZnO (PDF 36-1451). Because of its low content La is not observed (Fig. 1). One can see in Fig. 2 that the sample, treated with nitric acid, shows reduced peaks of ZnO (PDF 36-1451) and also characteristic peaks of the compounds Zn(NO3)(OH).H2O (PDF 47-0965) and

4ZnO(OH)2.Zn(NO3)2.H2O (PDF 72-0627).Further Fig. 3 illustrates the Zn2p, O1s, La 3d5/2

and N1s core level spectra of La/ZnO and La/ZnO of the acidically treated powder, after thermal treatment at 100ºC. The peaks at 1020.7 and 1043.7 eV belong to Zn 2p3/2 and Zn 2p1/2 of the La/ZnO sample and those of the acidically treated La/ZnO powder are shifted to 1021.7 and 1044.7 eV, respectively [17]. For both samples the Zn 2p3/2 peaks are sharp, demonstrating that Zn exists mainly in the form of Zn2+oxidation state [18]. The XPS spectrum of O1s peak is slightly asym-metric and was deconvoluted by Gaussian-Lorentzian function into two components - 529.7 and 531.4 eV for the non-treated La/ZnO sample and, respectively 530.5 and 532.1eV, for the acidically treated La/ZnO powders [19, 20]. The first component at lower binding energy is attributed to O2− ions in the ZnO lattice, the second, at higher energy, is ascribed to oxygen atoms in hydroxyl groups [21]. The nature of the surface hydroxyl groups is important for the physicochemical properties of the samples. The acidic-basic properties of these OH- groups

Fig. 1. X-ray diffractogram of La/ZnO sample.

Fig. 2. X-ray diffractogram of nitric acid treated La/ZnO sample (dried at 100ºC).

Sample Chemical composition/ at. %

Zn2p3/2 O1s La3d5/2 N1s

La/ZnO 46.4 50.4 3.2 -

La/ZnO a. t. 38.3 54.7 4.9 2.1

Table 1. XPS chemical composition in atomic percent of non-treated La/ZnO and acidically treated La/ZnO powder (dried at 100ºC).

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Fig. 3. XPS core level spectra of: Zn2p3/2, O1s, La 3d5/2 and N1s of La/ZnO and acidically treated La/ZnO powder (dried at 100ºC).

determine the surface character of the powders. For the non-treated La/ZnO sample, the recorded spectrum of La 3d5/2 was deconvoluted into two with binding ener-gies (834.0 and 837.6eV) [18]. The La 3d5/2 XPS signal of acidically treated powder was deconvoluted into three components, which are located at binding energy (835.1, 839.3 and 836.9 eV), Fig. 3. Nitrogen appears after the acidic treatment as N1s electronic state visible at 399.7 eV [13]. There is an ongoing debate about the surface nitrogen physisorbed species [22], but in our case nitrogen is in the form of Zn(NO3)(OH).H2O and 4ZnO(OH)2.Zn(NO3)2.H2O as evidenced by the XRD data. Table 1 presents the chemical composition of the

sample with best photocatalytic performance, before and after the acidic treatment. The Zn content is decreasing at the expense of the increase in La and N contents, which is also evidenced by the higher oxygen content.

Fig. 4 illustrates the dependence of the photocata-lytic activities of the samples as a function of the time interval of illumination. The acidic treatment promotes substantially the adsorption capacity and the photocata-lytic activity. The temperature dependence of the photo-catalytic activity of the acidically treated La-doped ZnO samples shows best performance of the sample, treated thermally at the lowest temperature of 100oC. Fig. 5 compares the degrees of discoloration after 120 min of

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illumination depending on the temperature of thermal treatment. The degree of discoloration (95 %) is highest for La/ZnO after acidic treatment and thermal treatment at 100oC and it is almost three times higher than that of non-treated La/ZnO sample (33 %). The explanation is that the higher temperatures lead to greater losses of nitrogen [23]. The degree of ZnO crystallinity is known to become higher at higher temperatures, but this does not increase its photocatalytic activity.

The catalytic activity is dependent on the applied method of post-synthesis activation - in our case - the nitric acid treatment. The time interval of calcination at different temperatures influences the performance of the investigated photocatalysts. The rate constant k is accepted as a measure of the photocatalytic activity: ln(C/C0) = - kt (first order of reaction). The activity of the investigated samples for discoloration of Reactive Black 5 is as follows (based on rate constants k): La/

Fig. 4. Discoloration degree of the RB5 dye (concentra-tion 20 ppm), based on changes in the intensity of the absorbance peak, corresponding to azo bond (-N=N-), as a function of the time interval of illumination of La/ZnO after acidic treatment, heated at different temperatures.

Fig. 5. Effect of the heating temperature of the acidically treated La/ZnO samples on the degree of dye solution discoloration, after 120 minutes of UV-A illumination.

Sample ABET, m2g-1 Rate constants, x10-3 min-1

Degradation, %

La/ZnO a. t., 100oC 46 25.5 95 La/ZnO a. t., 350oC 44 19.4 91 La/ZnO a. t., 500oC 42 11.1 69

La/ZnO 38 3.0 33

Table 2. Specific surface area (ABET), rate constants (k) and degradation (120 min) of La/ZnO photocatalysts.

Fig. 6. Reaction course as a function of the time of illumi-nation –ln (C/C0) of La/ZnO, La/ZnO a.t., 100oC, La/ZnO a.t., 350oC and La/ZnO a.t., 500oC photocatalysts.

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ZnO a. t., 100oC (25.5 х10-3 min-1) > La/ZnO a. t., 350oC (19.4 х10-3 min-1) >La/ZnO a. t., 500oC (11.1 х10-3 min-1) >La/ZnO (3.0 х10-3 min-1). Representing the results in logarithmic scale (Fig. 6) shows that the functional de-pendence is not ideally linear. This figure gives an idea of the experimental error, when following spectropho-tometrically the course of the reaction - there are some deviations in the data points, probably due to the filtering of the suspension before measuring the absorbance. This type of error is systematic and therefore it affects in a similar way all experimental runs.

The specific surface area increase is the highest for the sample treated thermally at 100oC and this is in cor-relation with the highest photocatalytic activity (Table 2).

CONCLUSIONS Lanthanum doped ZnO powders activated in ni-

tric acid were obtained. Several factors affecting the photocatalytic activity of La/ZnO powders during the synthesis have been studied: effect of the acidic activa-tion and effect of the temperature of treatment after acidic activation. XPS date give evidence that the acidic activation results in nitrogen incorporation in the ZnO, increasing the photocatalytic activity in the oxidative dis-coloration of Reactive Black 5 model pollutant solutions. The higher temperatures of powder post-acidic thermal treatment result in lowering of the specific surface area, probably due to sintering, and losses of the incorporated nitrogen. The specific surface area increase is the high-est for the sample treated thermally at 100oC and this is in correlation with the highest photocatalytic activity.

The effect of the acidic treatment is promoting the number of hydroxyl groups on the surface in the form of Zn(NO3)(OH).H2O and 4Zn(OH)2.Zn(NO3).2H2O. In this case, in addition to the direct oxidation on the surface, we obtain a second radical chain oxidation mechanism in the water bulk phase.

AcknowledgementsThe authors thank the Bulgarian Science Fund at

the Ministry of Education and Science for the financial support by Project DFNI – T-02-16.

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