The influence of mechanochemical activation on the Zn–Ce–O composition properties

Introduction

It is well known that ZnO and CeO₂ are two key semiconducting oxides, which have individually received much attention due to their applications in various fields. At the same time, it is generally accepted that the properties of the composites often cannot be considered as a sample superposition of the properties of individual components because of strong surface interactions between closely packed particles in binary oxide systems. Among these systems, ZnO–CeO₂ composite is one of the most promising materials which demonstrates the better properties than individual oxides at its uses as catalyst (Enjamuri et al., 2016; He et al., 2004; Kumar et al., 2015), photocatalyst (Li et al., 2012; Saravanan et al., 2012; Xiong et al., 2017), support of catalyst (Avgouropoulos et al., 2008; Barrios et al., 2014; Laguna et al., 2011), material for electrodes (Fan et al., 2013; Hajji et al., 2014), ultraviolet filter (De Lima et al., 2009; Li et al., 2002), etc.

The promoted effect for ZnO-doped ceria (Kaviyarasu et al., 2016; Li et al., 2002; Lin et al., 2015) or CeO₂-doped zinc oxide (De Lima et al., 2009; Fan et al., 2013; Saravanan et al., 2012) samples was shown, but many publications demonstrated an improvement of the composition properties at oxides equimolar ratio or near to this value (Barrios et al., 2014; Enjamuri et al., 2016; Hajji et al., 2014; Kumar et al., 2015; Laguna et al., 2011; Li et al., 2011; Li et al., 2012; Xiong et al., 2017, etc.).

The different methods of ZnO–CeO₂ compositions’ preparation such as precipitation (Avgouropoulos et al., 2008; Barrios et al., 2014; He et al., 2004; Kumar et al., 2015; Lin et al., 2015), sol–gel (Enjamuri et al., 2016; Laguna et al., 2011; Sharma and Pallavi, 2012), flame spray pyrolysis (Xiong et al., 2017), thermal decomposition (Saravanan et al., 2012), hydrothermal treatment (Fan et al., 2013; Ma et al., 2010), solvothermal method (Warule et al., 2012) and combustion (Li et al., 2014) were reported. These methods have some disadvantages connected with the metal salts used as raw materials (necessity of anion removal), utilization of alcohols, polymers, surfactants and thermal treatment after synthesis. Direct mechanochemical synthesis of ZnO–CeO₂ composite from initial oxides free from these defects was not studied.

At the same time, it is well known that mechanochemistry deals with the synthesis of complex composites and nanocomposites from simple raw materials (for example, Avvakumov et al., 2001; Baláž, 2008; Baláž et al., 2013; Zazhigalov, 2013; Zazhigalov et al., 2016). The formation of complex oxide composites, ferrites, perovskites, sheelites, ceramics, etc. by this method was established.

In this work, the results of mechanochemical synthesis of ZnO–CeO₂ composite from equimolar mixture of simple oxides ZnO and CeO₂ and its treatment in air are reported.

Experimental

Zinc–cerium oxide with a molar ratio of ZnO–CeO₂ = 50:50 was studied. As starting materials, commercial powder ZnO (Fluka) and CeO₂ were used which were prepared by thermal treatment of ammonia solution Ce(NH₄)₂(NO₃)₆ at 120℃. Milling was carried out in a planetary ball mill Pulverisette-6 in atmospheric air at rotation frequency of 550 r/min with the reverse after every 30 minutes of milling. For all experiments, 10 g of the powder was placed in a vial (200 cm³) without any additives. Mechanochemical activation was carried out at various treatment conditions: the processing time (2, 4 and 8 hours), ball diameters (2, 5 and 10 mm) and ball-to-powder weight ratio (BPR) (5:1, 10:1 and 15:1).

Study of structural and physicochemical properties of synthesized samples was carried out by the following methods.

Powder X-ray diffraction (XRD) pattern of the samples was obtained with Cu Kα radiation (λ = 1.54056 Å at 40 kV, 30 mA) using a diffractometer PW 1830 Philips. The sample was scanned over the required range for 2θ values (10°–90°). The crystalline size (L) for the compositions was calculated using Debye–Sherrer formula, according to equation (1)

L = 0 . 9 λ / β cos θ

(1)

The samples were studied by transmission electron microscopy (TEM) method after their previous ultrasound dispersing in ethanol to use device JEM 1230 (Jeol).

The Fourier Transform Infrared Spectrometer (FT-IR) spectra were recorded on a spectrometer “Spectrum-One” (Perkin-Elmer Instruments) in atmospheric air at room temperature in absorbance mode (mass ratio of sample:KBr = 1:20) in the range of 4000–400 cm⁻¹.

Differential thermal analysis and thermogravimetry analysis (DTA-TG) were conducted on a DERIVATOGRAPH-Q apparatus of F. Paulik, J. Paulik, L. Erdey (MOM, Hungary) system in atmospheric air in the temperature range of 20–800℃ at a heating rate of 10℃/min, sample weight was equal to 200 mg.

Nitrogen sorption was used to determine the specific surface areas of the powders at the temperature of liquid nitrogen (−196℃) in a Quantachrome instrument (NOVA-2200 Gas Sorption Analyzer). The pore size distribution of the samples was calculated using the BJH method.

Photocatalytic activity of some samples was determined in water solution of dye safranin T decomposition in accordance with the protocol (Sidorchuk et al., 2012). The rate constants (K_d) were calculated according to the first-order equation for dye decomposition.

Results and discussion

The XRD pattern of initial composition of ZnO–CeO₂ = 50:50 was characterized by the presence of reflexes of wurtzite hexagonal ZnO phase (JCPD card 05-0664) with significant dominance reflex from the plane (101) (crystallite size L = 174 nm) and reflexes of a cubic fluorite structure of CeO₂ (JCPD card 34-394), centered at 2θ = 28.37°, 33.38° and 47.43° which correspond to the (111), (200) and (220), respectively (crystalline size L = 57 nm). The mechanochemical treatment of the sample (at all conditions) practically do not influence the diffractograms view (all reflexes of initial oxides are present), but the relative intensity of the basic reflexes decreases and their width increases which corresponds to the data obtained in preparation of nanocomposite by other methods (for example, Barrios et al., 2014; Enjamuri et al., 2016; Laguna et al., 2011; Li et al., 2002). This fact indicates the reduction of the initial oxides particle size (Table 1). Simultaneously, the appearance of weak gallo was observed which can be connected with a decrease of particles size or partial amorphization of the oxides. The most decrease of particles size of up to 40 nm was observed for ZnO after 2 hours of treatment (BPR = 15, d = 15 mm, Table 1), but an increase of treatment time was accompanied by slow increasing of particle dimensions of up to 53 nm after 8 hours of treatment. Analogous dependence is observed for CeO₂ particles size (Table 1). The obtained data show (Table 1) that this process of particles’ size reduction is more rapid at BPR and balls diameter increases.

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Table 1.

Influence of treatment conditions on ZnO–CeO₂ composition properties.

Treatment conditions			Size of the particles, nm		SBET, m2/g	VS·10−3, cm3/g	Kd·104, s−1
Balls diameter, mm	BPR	Time, h	ZnO	CeO2
Initial mixture			174	57	1.8	0.9	0.3
2	10	2	74	44	2.5	1.8	–
		4	51	34	2.6	2.7	1.5
		8	47	25	3.2	3.1	–
5	5	2	53	39	2.3	2.2	–
		4	46	31	2.9	2.7	1.7
		8	42	26	3.0	3.6	2.2
	10	2	48	30	2.7	3.1	–
		4	42	27	3.8	5.2	3.1
		8	43	22	5.2	8.1	3.5
	15	2	47	28	3.4	3.4	–
		4	40	20	5.1	8.0	4.3
		8	43	24	2.9	3.1	–
10	15	2	40	18	4.7	5.3	–
		4	41	21	2.8	2.0	3.3
		8	53	26	1.5	0.5	–

The change of particle sizes due to the treatment was confirmed by the TEM study. The initial composition (Figure 1(a)) consists of two types of the particles with different dimensions: (i) large crystals with size of 150–300 nm (A) and (ii) small crystals with size of 50–100 nm (B). In accordance with XRD calculation data, the first particles can be identified as ZnO and the second type of the particles as CeO₂. OPEN IN VIEWER

After mechanochemical treatment of 2 hours (Figure 1(b) to (d)), the size of the particles decreases, and at an increase of the balls diameter and BPR, this change proceeds more rapid (from 40–150 nm up to 10–40 nm). The increase of treatment time leads to the decrease of dimensions of the particles (Figure 1(b), (e) and (c), (f)), but in case of maximal power loading (maximum values of d and BPR), their partial agglomeration (which was connected with low dimension of the primary particles and high surface energy) with formation of the particles at 40–60 nm was observed (Figure 1(g) and (h)). These data correspond to the results obtained by XRD (Table 1). It is necessary to note that the formation of Zn–Ce–O nanocomposite with similar heterostructure where the small CeO₂ particles was distributed on the surface of ZnO nanoparticles (Figure 1(d), (f), (g)) at its preparation by other methods was shown earlier (He et al., 2014; Warule et al., 2012; Xiong et al., 2017).

The FT-IR spectra of compositions demonstrate the broad absorption band located around 3398 cm⁻¹ and absorption band at 1645 cm⁻¹ which are characteristic for oxide compositions and correspond to stretching and deformation of O–H vibrations in absorbed water. The mechanochemical activation of the sample slightly influences on these bands intensity. The spectra for Me–O vibrations region (400–1000 cm⁻¹, Figure S1) show the presence of the Ce–O bands at 595, 617, 738, 834 and 970 cm⁻¹, and Zn–O bands at 484 and 645 cm⁻¹ which correspond to literature data for Zn–Ce–O nanocomposites (Faisal et al., 2011; Laguna et al., 2011; Sharma and Pallavi, 2012). Treatment does not influences on bands position but slightly decreases the intensity of the bands at 484, 617, 738 and 970 cm⁻¹ which can be connected with a reduction of the particles size (Table 1) and partial amorphization of the oxides (appearance of gallo according XRD data).

It was established that for all samples, nitrogen sorption isotherms belong to type IV in accordance with International Union of Pure and Applied Chemistry (IUPAC) classification. At same time, the curves of pore size distribution (Figure S2) show the essential influence of treatment on porous structure of the samples. The disappearance of micropores presented in initial oxides mixture connected with particles destruction is observed. At low energy loading (low values of d, BPR and time treatment), the appearance of two types of mesopores which was formed around the site contacts between small particles is observed. Increase of energy loading was accompanied by destruction of mesopores and the particles coalescence with macropores formation. Due to the low values of BPR, balls diameter and time treatment, the increase of specific surface area (SSA) and pore volume was established (Table 1) which can be attributed to the formation of mesopores with two dimensions (Figure S2). A decrease of these parameters of composite (S_BET, V_S) observes with an increase of treatment conditions (BRP, balls diameter and time treatment) which can be explained as secondary aggregation of the particles.

The DTA-TG data show (Figure S3) the presence of two endothermic effects in the temperature range of 105–230℃ with maximum at 118 and 212℃ which correspond to the removal of adsorbed water and the weight loss on TG curves noticed as 6%–7% and 2%–3%, respectively. Practically, the same data were reported by Barrios et al. (2014) and Ma et al. (2010). At the same time the appearance of exothermal effects without mass loss is a characteristic of the samples after mechanochemical modification (Figure S3). This exothermic peak was detected at 470–480℃ and can be attributed to crystallization process of partially amorphous phase ZnO (see XRD data).

It was established that mechanochemical treatment of ZnO–CeO₂ oxide mixture which leads to the formation of nanodispersed composite causes an increase in the photocatalytic activity of the sample in dye safranin T decomposition in water (Table 1). This fact is in agreement with the literature data which showed the high photocatalytic activity of nanocomposite Zn–Ce–O in other photocatalytic dye decomposition processes (Faisal et al., 2011; Li et al., 2011; Li et al., 2014; Saravanan et al., 2012).

Conclusions

The influence of mechanochemical treatment conditions (balls diameter, BPR, time treatment) on the properties of ZnO–CeO₂ composition with molar ratio of 50:50 was established. It was shown that the essential decrease of oxides particle size from 174 up to 40 nm for ZnO and from 57 up to 20 nm for CeO₂ (the best results were obtained at BPR = 15, balls diameter 5–10 mm and time treatment 4 and 2 hours, correspondingly) and the formation of Zn–Ce–O nanocomposite are due to the treatment. The formation of composite with bi-porous structure which contains mesopores of two sizes (at 2.1 and 30.0 nm) at small value of balls diameter and BPR (d = 2–5 mm and BPR = 5–15) was observed. An increase of specific surface area up to three times at optimal treatment conditions was established. The destruction of mesopores and partial particles aggregation with formation of the composite with macroporous structure at an increase of time treatment was observed. The formation of Zn–Ce–O nanocomposite due to mechanochemical treatment leads to an increase of ZnO–CeO₂ composition photocatalytic activity in dye safranin T decomposition in water.