# Preparation and Catalytic Activity of M2O3/CNTs (M = Y, Nd, Sm) Nanocomposites by Solvothermal Process

Authors: Xiaojuan Zhang; Lingyun Hao
Journal: Journal of Nanomaterials
 (2018)

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## Abstract

The rare-earth oxide nanoparticles along carbon nanotubes (CNTs) (M2O3/CNTs, M = Y, Nd, Sm) were prepared by in situ solvothermal method. Products were characterized by infrared spectroscopy, X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectrometry, scanning electron microscopy, and Brunauer-Emmett-Teller method. M2O3/CNTs presented good morphology and large surface area. Furthermore, catalysis of M2O3/CNTs during the thermal decomposition of ammonium perchlorate (AP) was evaluated by differential thermal analysis (DTA). Compared with Nd2O3/CNTs and Sm2O3/CNTs, Y2O3/CNTs nanocomposites showed the best catalytic effect on the thermal decomposition of AP. With the addition of 2 wt.% Y2O3/CNTs nanocomposite, high decomposition temperature of AP decreased by 125.5°C, and the total DTA heat release increased by 2.027 kJ·g−1.


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## Body

## 1. Introduction

Rare-earth oxide (M2O3) nanoparticles have received considerable attention because of their applications in various fields, such as advanced ceramics, superconducting materials, luminescence materials, oxygen sensor, and catalysis [1–4]. In the past years, all kinds of one-dimensional and two-dimensional rare-earth nanostructures, such as rods, wires, tubes, and plates, have been synthesized by numerous groups [5, 6]. M2O3 nanoparticles exhibit good catalytic effect in the thermal decomposition of ammonium perchlorate (AP) [7], which is used as oxidizer in solid propellants [8–10]. However, M2O3 nanoparticles exhibit agglomeration, which negatively influences their catalytic properties in propellants.Owing to their unique electronic and physical properties [11, 12], carbon nanotubes (CNTs) may act as a catalyst support, on which nanoparticles may be attached to improve catalytic effect [13, 14]. M2O3/CNT nanocomposites have been prepared through numerous methods, such as chemical deposition, sol-gel, hydrothermal, atmospheric-pressure-plasma jet, and electrophoretic deposition [15–17]. For example, Wu and Chen prepared plasma-resistant Y2O3/CNT nanocomposites by an atmospheric-pressure-plasma jet method [18]. Y2O3/CNTs composites with high conductivity can be prepared in 3–5 s using atmospheric-pressure-plasma jet method, and the conventional furnace calcination would take 10–15 min at 400–500°C. And CNTs-doping has been found to be an effective method to improve the conductivity of Y2O3. Zhao et al. synthesized Nd2O3/CNT nanocomposites through chemical deposition and calcination [19]. With the surfactant served as a “bridge” between Nd(OH)3 and CNTs, the Nd2O3 nanoparticles with average particle size of 30–40 nm can be deposited on the surface of CNTs. And the obtained Nd2O3/CNTs had higher specific area and exhibited better catalysis on the thermal decomposition of AP. Mo et al. synthesized Eu2O3/CNT structures via electrophoretic deposition [20]. The electrophoretic deposition techniques produced densely packed CNTs mat-Eu2O3 film-CNTs mat heterostructure, and the capacitance-voltage measurements of the CNT mat-Eu2O3 film-CNT mat structure confirmed electrical insulation between the two CNT mats and the charge-storage capabilities of the structure.However, some of these methods lack good control of the morphology of M2O3 nanoparticles, whereas others require complex processes, expensive experimental device, or long time. Among the different methods, the solvothermal method [21] is highly advantageous because of its convenient processing, simple experimental equipment, no calcination requirement, and good dispersibility in products.However, reports on preparing M2O3/CNTs by in situ solvothermal method to catalyze the thermal decomposition of AP are limited. In this thesis, we coated M2O3 (M = Y, Nd, Sm) nanoparticles on the surface of functional CNTs and studied their catalytic properties by differential thermal analysis (DTA).


## 2. Experimental

### 2.1. Materials

Pristine CNTs (diameter: 40–60 nm, purity: 95–98%) were purchased by Shenzhen Nanotechnologies Co., Ltd. Hexadecyl trimethyl ammonium bromide (CTAB), poly(sodium 4-styrenesulfonate) (PSS), yttrium nitrate (Y(NO3)3·6H2O), neodymium nitrate (Nd(NO3)3·nH2O), samarium nitrate (Sm(NO3)3·6H2O), ethylene glycol (EG), NaAc, and polyethylene glycol were of analytical grade and obtained from Shanghai Chemical Company.


### 2.2. Synthesis of M2O3/CNTs Nanocomposites (M = Y, Nd, Sm)

The synthesis of M2O3/CNTs nanocomposites is shown in Figure 1. Pristine CNTs were purified in HNO3 at 140°C for 4 h. Afterwards, the solution was diluted with distilled water and rinsed for several times until the pH value reaches neutral. Then the purified CNTs were modified via polymer wrapping technique, where cationic CTAB (1 g) was adsorbed on CNTs to create a charged template, and anionic PSS (0.24 g) was subsequently grafted onto the surfaces of CNTs. The mixture of CNTs modified by CTAB and PSS was obtained and called functionalized CNTs (f-CNTs).Figure 1
Schematic of the formation of M2O3/CTNs nanocomposites.Around 0.03 g of f-CNTs and 0.2 g of M(NO3)3·6H2O (M = Y, Nd, Sm) were dissolved in 50 mL ethylene glycol (EG). Then, 3.6 g of NaAc and 1 g of polyethylene glycol (Mw = 4000 g·mol−1) were added with stirring for 0.5 h. The reaction system was heated to 200°C for 2.5 h in an oil bath. Finally, the M2O3/CNT (M = Y, Nd, Sm) nanocomposites were press-filtered, washed with ethanol, and dried at 40°C. The respective ratios of M2O3 : f-CNTs in experiment were labeled in Table 1.Table 1
The properties of M2O3/CNTs samples (M=Y, Nd, Sm).
Sample m M 2 O 3  (g) m f - C N T s  (g) Crystallite size(nm) Mean nanoparticle size(nm) BET surface area(m2·g−1) M2O3 : f-CNTs in experiment (wt.%) M2O3 : f-CNTs by solvothermal process (wt.%) M2O3 : f-CNTs by chemical deposition (wt.%) (a) Y2O3/CNTs 0.06 0.03 42 30 84 2.0 : 1 1.1 : 1 0.8 : 1 (b) Nd2O3/CNTs 0.08 0.03 34 45 77 2.6 : 1 0.6 : 1 0.5 : 1 (c) Sm2O3/CNTs 0.08 0.03 20 90 65 2.6 : 1 0.9 : 1 0.6 : 1For comparison, M2O3/CNTs nanocomposites were prepared by chemical deposition. f-CNTs and M(NO3)3·6H2O (M = Y, Nd, Sm) were dispersed in 50 mL of distilled water, and sodium hydroxide (NaOH) was dropped into the solution and reacted at 40°C for 1 h. Finally, the product was washed, dried, and calcined in a muffle furnace at 800°C for 2 h.


### 2.3. Catalytic Activity Measurement

For investigating the catalytic effect of M2O3/CNTs nanocomposites, the mass content of M2O3/CNTs nanocomposites added in AP was 2 wt.%, and the catalytic performance of f-CNTs was also carried out in the same way as comparison.


### 2.4. Characterization

The structure was characterized by X-ray diffraction (XRD, Bruker D8). A Bruker VECTOR 22 spectrometer was used to measure infrared (IR) spectrum. The morphology and EDS were measured by scanning electron microscopy (SEM, Model S-8010) and transmission electron microscopy (TEM, Tecnai 12). A Beckman Coulter SA3100 was used to measure the Brunauer-Emmett-Teller (BET) surface areas, and DTA was characterized by a thermal analyzer (TA2100, America).


## 2.1. Materials

Pristine CNTs (diameter: 40–60 nm, purity: 95–98%) were purchased by Shenzhen Nanotechnologies Co., Ltd. Hexadecyl trimethyl ammonium bromide (CTAB), poly(sodium 4-styrenesulfonate) (PSS), yttrium nitrate (Y(NO3)3·6H2O), neodymium nitrate (Nd(NO3)3·nH2O), samarium nitrate (Sm(NO3)3·6H2O), ethylene glycol (EG), NaAc, and polyethylene glycol were of analytical grade and obtained from Shanghai Chemical Company.


## 2.2. Synthesis of M2O3/CNTs Nanocomposites (M = Y, Nd, Sm)

The synthesis of M2O3/CNTs nanocomposites is shown in Figure 1. Pristine CNTs were purified in HNO3 at 140°C for 4 h. Afterwards, the solution was diluted with distilled water and rinsed for several times until the pH value reaches neutral. Then the purified CNTs were modified via polymer wrapping technique, where cationic CTAB (1 g) was adsorbed on CNTs to create a charged template, and anionic PSS (0.24 g) was subsequently grafted onto the surfaces of CNTs. The mixture of CNTs modified by CTAB and PSS was obtained and called functionalized CNTs (f-CNTs).Figure 1
Schematic of the formation of M2O3/CTNs nanocomposites.Around 0.03 g of f-CNTs and 0.2 g of M(NO3)3·6H2O (M = Y, Nd, Sm) were dissolved in 50 mL ethylene glycol (EG). Then, 3.6 g of NaAc and 1 g of polyethylene glycol (Mw = 4000 g·mol−1) were added with stirring for 0.5 h. The reaction system was heated to 200°C for 2.5 h in an oil bath. Finally, the M2O3/CNT (M = Y, Nd, Sm) nanocomposites were press-filtered, washed with ethanol, and dried at 40°C. The respective ratios of M2O3 : f-CNTs in experiment were labeled in Table 1.Table 1
The properties of M2O3/CNTs samples (M=Y, Nd, Sm).
Sample m M 2 O 3  (g) m f - C N T s  (g) Crystallite size(nm) Mean nanoparticle size(nm) BET surface area(m2·g−1) M2O3 : f-CNTs in experiment (wt.%) M2O3 : f-CNTs by solvothermal process (wt.%) M2O3 : f-CNTs by chemical deposition (wt.%) (a) Y2O3/CNTs 0.06 0.03 42 30 84 2.0 : 1 1.1 : 1 0.8 : 1 (b) Nd2O3/CNTs 0.08 0.03 34 45 77 2.6 : 1 0.6 : 1 0.5 : 1 (c) Sm2O3/CNTs 0.08 0.03 20 90 65 2.6 : 1 0.9 : 1 0.6 : 1For comparison, M2O3/CNTs nanocomposites were prepared by chemical deposition. f-CNTs and M(NO3)3·6H2O (M = Y, Nd, Sm) were dispersed in 50 mL of distilled water, and sodium hydroxide (NaOH) was dropped into the solution and reacted at 40°C for 1 h. Finally, the product was washed, dried, and calcined in a muffle furnace at 800°C for 2 h.


## 2.3. Catalytic Activity Measurement

For investigating the catalytic effect of M2O3/CNTs nanocomposites, the mass content of M2O3/CNTs nanocomposites added in AP was 2 wt.%, and the catalytic performance of f-CNTs was also carried out in the same way as comparison.


## 2.4. Characterization

The structure was characterized by X-ray diffraction (XRD, Bruker D8). A Bruker VECTOR 22 spectrometer was used to measure infrared (IR) spectrum. The morphology and EDS were measured by scanning electron microscopy (SEM, Model S-8010) and transmission electron microscopy (TEM, Tecnai 12). A Beckman Coulter SA3100 was used to measure the Brunauer-Emmett-Teller (BET) surface areas, and DTA was characterized by a thermal analyzer (TA2100, America).


## 3. Results and Discussion

### 3.1. Materials Characterization

#### 3.1.1. IR Analysis

The IR spectra of (a) pristine CNTs and (b) f-CNTs are shown in Figure2. In Figure 2(a), stretching vibration of O-H appeared at 3480 cm−1. In Figure 2(b), characteristic absorption bands of O-H stretching vibration appeared at 3285 cm−1, and the peaks at 1714 cm−1 can be ascribed to the stretching vibration of C=O bond [22]. In addition, the peaks at 1563 and 1194 cm−1 can correspond to stretching vibration peak of COO- and C-O characteristic bands, respectively. Therefore, f-CNTs were successfully modified by HNO3.Figure 2
Fourier transform IR spectra of (a) pristine CNTs and (b) f-CNTs.


#### 3.1.2. XRD Analysis

Figure3 illustrates the XRD patterns of f-CNTs and M2O3/CNT nanocomposites. In Figure 3(d), the peaks at 2θ = 22.94° correspond to the (002) plane of f-CNTs (JCPDS No. 25-0284) [18]. From Figures 3(a) and 3(c), characteristic peaks of CNTs remain. In Figure 3(a), diffraction peaks at 29.4°, 33.9°, 40.9°, 48.7°, and 57.9° correspond to the (222), (400), (322), (440), and (622) planes of Y2O3 (JCPDS No. 74-1828), respectively [18]. In Figure 3(b), reflection peaks at 26.9°, 29.7°, 30.7°, 40.5°, 47.4°, 53.4°, and 53.4° can be indexed to the (100), (002), (101), (102), (110), (103), and (112) planes of hexagonal Nd2O3 (JCPDS No. 41-1089), respectively [19]. As shown in Figure 3(c), reflection peaks appearing at 28.2°, 32.7°, 38.6°, 45.4°, and 54.3° correspond to (222), (400), (332), (521), and (541) planes of cubic Sm2O3 (JCPDS No. 15-0813), respectively. As shown in Table 1, the crystallite size of Y2O3 deposited on CNTs calculated according to the Scherrer formula is 42 nm and those of Nd2O3 and Sm2O3 are 34 nm and 20 nm. The crystallite size of Y2O3 deposited on CNTs is bigger than that of Nd2O3 and Sm2O3, which is consistent with the broad peaks in case of Nd2O3/CNTs and Sm2O3/CNTs.Figure 3
XRD patterns of (a) Y2O3/CNTs, (b) Nd2O3/CNTs, (c) Sm2O3/CNTs, and (d) f-CNTs.


#### 3.1.3. SEM Analysis

Morphology was investigated by SEM. Figure4(a) reveals f-CNTs with a diameter of 40–60 nm and good dispersibility. In Figure 4(b), f-CNTs were decorated with a dense layer of Y2O3 nanoparticles. Figure 4(c) shows that Nd2O3 nanoparticles were homogeneously distributed over f-CNTs because the modified CNTs are covered with a negatively charged surfactant, which can ensure good coverage of positively charged Nd3+ by electrostatic attraction. In addition, Sm2O3 nanoparticles were loaded on f-CNTs, indicating a strong interaction between Sm2O3 nanoparticles and CNTs by surfactant modification (Figure 4(d)). The formation of M2O3/CNTs nanocomposites can be represented as follows:(1)M3++3OH-→EGMOH32MOH3⟶M2O3+3H2OM3+ in the solution can favorably bind with f-CNTs by electrostatic attraction, and M3+ was hydrolyzed to M(OH)3. As a result, M2O3 nanoparticles were anchored onto the surface of f-CNTs. It could be included that the modification of CNTs improved the heterogeneous nucleation; the enhanced nucleation rate favored the generation of much more nuclei and the formation of smaller M2O3 nanoparticles.Figure 4
SEM images of (a) f-CNTs, (b) Y2O3/CNTs, (c) Nd2O3/CNTs, and (d) Sm2O3/CNTs.
(a) (b) (c) (d)


#### 3.1.4. EDS Analysis

Energy-dispersive X-ray spectrometry (EDS) was used to measure the composition of M2O3/CNT nanocomposites. The peaks (2.2 keV and 2.9 keV) are originated from Au/Pd coating. Figure 5(a) shows that Y2O3/CNTs includes 21.75 wt.% of Y, 25.33 wt.% of O, and 52.92 wt.% of C. In Figure 5(b), Nd2O3/CNTs contains 13.10 wt.% of Nd, 31.82 wt.% of O, and 55.08 wt.% of C. As shown in Figure 5(c), Sm2O3/CNTs contained 11.60 wt.% of Sm, 24.53 wt.% of O, and 63.87 wt.% of C.Figure 5
EDS patterns of (a) Y2O3/CNTs, (b) Nd2O3/CNTs, and (c) Sm2O3/CNTs.
(a) (b) (c)According to the results, the relative proportions of f-CNTs and M2O3 during the final products by solvothermal process and chemical deposition were listed in Table 1. As shown in Table 1, the relative proportions of M2O3 and f-CNTs by chemical deposition were smaller than that of solvothermal process, which indicated that the M2O3 nanoparticles had better coverage on f-CNTs by solvothermal process.


#### 3.1.5. TEM Analysis

The morphology of M2O3/CNT nanocomposites was further measured by TEM as shown in Figure 6. Figure 6(a) shows TEM image of f-CNTs. Before the reaction, f-CNTs present well-graphitized walls. In Figure 6(b), Y2O3 nanoparticles attached to f-CNTs reveal slight nonuniformity and no free nanoparticles. The average particle size of Y2O3 nanoparticles was 30 nm, which is considerably smaller than that of Y2O3/CNTs produced by chemical deposition (70 nm). In Figure 6(c), Nd2O3 nanoparticles supported on f-CNTs exhibited good dispersibility and sphericity. The average particle size of Nd2O3 nanoparticles was 45 nm, which was markedly smaller than that of Nd2O3/CNTs (90 nm) produced by chemical deposition. In addition, although the coverage of Sm2O3 nanoparticles onto f-CNTs is nonuniform, no free nanoparticles were found. And the average particle size of Sm2O3 nanoparticles was 90 nm, which is smaller than that of Sm2O3/CNTs (120 nm) produced by chemical deposition. The smaller particle size of M2O3/CNTs prepared by in situ solvothermal process is advantageous in catalysis.Figure 6
TEM images of (a) f-CNTs, (b) Y2O3/CNTs, (c) Nd2O3/CNTs, and (d) Sm2O3/CNTs.
(a) (b) (c) (d)


#### 3.1.6. BET Analysis

The surface area of M2O3/CNT nanocomposites was also characterized. As shown in Table 1, the BET surface area of Y2O3/CNTs prepared by solvothermal process was 84 m2·g−1, which is considerably larger than that of Y2O3/CNTs produced by chemical deposition (74 m2·g−1) and f-CNTs (33 m2·g−1). The same finding was confirmed by measuring the surface area of Nd2O3/CNTs and Sm2O3/CNTs nanocomposites. The BET surface areas of Nd2O3/CNTs and Sm2O3/CNTs were 77 and 65 m2·g−1, which were markedly larger than those of Nd2O3/CNTs (41 m2·g−1) and Sm2O3/CNTs (58 m2·g−1) produced by chemical deposition. The large BET surface area of M2O3/CNTs is advantageous in catalysis.


### 3.2. Catalytic Properties

#### 3.2.1. DTA Analysis

In Figure7(a), the transition from orthorhombic to cubic AP is in agreement with the endothermic peak at 245.2°C [23]. The thermal decomposition of AP to form volatile products corresponds to the exothermic peaks at 331°C and 452°C [24]. In Figure 7(b), the two exothermic peaks combine into one peak, and the high thermal decomposition temperature decreases substantially. With the addition of f-CNTs, the high decomposition temperature decreased by 97.4°C, and the total DTA heat release increased by 0.346 kJ·g−1. Compared with f-CNTs, M2O3/CNTs nanocomposites showed better catalytic activity (Figures 7(c) and 7(d)). Based on this result, we can deduce that f-CNTs can mainly accelerate the high decomposition of AP, because the violent decomposition of temperature was greatly reduced during this process.Figure 7
DTA curves for (a) pure AP, (b) f-CNTs + AP, (c) Y2O3/CNTs + AP, (d) Sm2O3/CNTs + AP, and (e) Nd2O3/CNTs + AP.It has been reported that many nanomaterials could catalyze the thermal decomposition of AP. For example, Fe3O4 and Co3O4 microspheres were synthesized by the hydrothermal reaction and with the addition of 2% Fe3O4 and Co3O4 microspheres could decrease the high thermal decomposition temperature of AP by 80°C and 55°C, respectively [25]. V6O13 nanobelts were controlled and synthesized by one-pot hydrothermal process and the high decomposition temperature of AP in the presence of 3% V6O13 was reduced by 67°C [26]. Zhang et al. improved the catalytic activity of α-Fe2O3 in the decomposition of AP by coating amorphous carbon on α-Fe2O3 surface, which reduced the high decomposition temperature of AP by 109°C [27].As shown in Figures7(c), 7(d), and 7(e), addition of 2.0 wt.% of Y2O3/CNTs, Sm2O3/CNTs, and Nd2O3/CNTs decreased the high thermal decomposition temperature of AP by 125.5°C, 118.4°C, and 123.8°C, respectively, and increased the total DTA heat release by 2.027, 1.243, and 1.463 kJ·g−1, respectively. Compared with Fe3O4, Co3O4 microspheres, V6O13 nanobelts, and α-Fe2O3/C, the M2O3/CNTs showed better catalytic activity in the thermal decomposition of AP.


#### 3.2.2. Catalytic Mechanism of Nanocomposites

Moreover, compared with Sm2O3/CNTs and Nd2O3/CNTs, Y2O3/CNTs showed superior catalytic effect in the thermal decomposition of AP. Thermal decomposition peak temperature and the heat release of pure AP and samples are listed in Table 2. From DTA data, the following trend of the total DTA heat release was noticed: pure AP (0.281 kJ·g−1) < f-CNTs + AP (0.627 kJ·g−1) < 2 wt.% Sm2O3/CNTs + AP (1.524 kJ·g−1) < 2 wt.% Nd2O3/CNTs + AP (1.744 kJ·g−1) < 2 wt.% Y2O3/CNTs + AP (2.308 kJ·g−1). This result was consistent with the peak area of these samples.Table 2
Temperatures peaks and heat release of AP and the mixtures.
Sample t L  (°C) t H  (°C) Δ t L  (°C) Δ t H  (°C) H  (kJ·g−1) Δ H  (kJ·g−1) (a) Pure AP 331.0 452.0   0.281 (b) Pure CNTs + AP 354.6 +23.6 −97.4 0.627 0.346 (c) 2 wt.% Y2O3/CNTs + AP 326.5 −4.5 −125.5 2.308 2.027 (d) 2 wt.% Sm2O3/CNTs + AP 333.6 +2.6 −118.4 1.524 1.243 (e) 2 wt.% Nd2O3/CNTs + AP 328.2 −2.8 −123.8 1.744 1.463 Note. tL is the low temperature decomposition peak temperature, tH is the high temperature decomposition peak temperature, and H is the total DTA heat release.With the same addition, the obtained Nd2O3/CNTs decreased the high decomposition temperature by 123.8°C, which was markedly larger than the decrement with Nd2O3/CNTs produced by chemical deposition (93.9°C) [19]. It can be concluded that M2O3/CNTs nanocomposites by in situ solvothermal method possessed smaller particle size and higher specific area, which can more effectively catalyze the thermal of decomposition of AP than M2O3/CNTs synthesized by chemical deposition.On the basis of electron transfer theory [28], the presence of partially filled 4d orbital in M3+ (M = Y3+, Nd3+, Sm3+) is advantageous for electron transfer [29]. The thermal decomposition of AP was enhanced because the positive hole in M3+ can accept electrons from the AP ion and its intermediate products. Moreover, the open pore structure of f-CNTs facilitates electron transfer and heat conduction [30] and can promote the thermal decomposition of AP. By contrast, M2O3 nanoparticles with relatively small size and good dispersibility, which enhance catalytic effect, are synthesized on f-CNTs by in situ solvothermal method.


## 3.1. Materials Characterization

### 3.1.1. IR Analysis

The IR spectra of (a) pristine CNTs and (b) f-CNTs are shown in Figure2. In Figure 2(a), stretching vibration of O-H appeared at 3480 cm−1. In Figure 2(b), characteristic absorption bands of O-H stretching vibration appeared at 3285 cm−1, and the peaks at 1714 cm−1 can be ascribed to the stretching vibration of C=O bond [22]. In addition, the peaks at 1563 and 1194 cm−1 can correspond to stretching vibration peak of COO- and C-O characteristic bands, respectively. Therefore, f-CNTs were successfully modified by HNO3.Figure 2
Fourier transform IR spectra of (a) pristine CNTs and (b) f-CNTs.


### 3.1.2. XRD Analysis

Figure3 illustrates the XRD patterns of f-CNTs and M2O3/CNT nanocomposites. In Figure 3(d), the peaks at 2θ = 22.94° correspond to the (002) plane of f-CNTs (JCPDS No. 25-0284) [18]. From Figures 3(a) and 3(c), characteristic peaks of CNTs remain. In Figure 3(a), diffraction peaks at 29.4°, 33.9°, 40.9°, 48.7°, and 57.9° correspond to the (222), (400), (322), (440), and (622) planes of Y2O3 (JCPDS No. 74-1828), respectively [18]. In Figure 3(b), reflection peaks at 26.9°, 29.7°, 30.7°, 40.5°, 47.4°, 53.4°, and 53.4° can be indexed to the (100), (002), (101), (102), (110), (103), and (112) planes of hexagonal Nd2O3 (JCPDS No. 41-1089), respectively [19]. As shown in Figure 3(c), reflection peaks appearing at 28.2°, 32.7°, 38.6°, 45.4°, and 54.3° correspond to (222), (400), (332), (521), and (541) planes of cubic Sm2O3 (JCPDS No. 15-0813), respectively. As shown in Table 1, the crystallite size of Y2O3 deposited on CNTs calculated according to the Scherrer formula is 42 nm and those of Nd2O3 and Sm2O3 are 34 nm and 20 nm. The crystallite size of Y2O3 deposited on CNTs is bigger than that of Nd2O3 and Sm2O3, which is consistent with the broad peaks in case of Nd2O3/CNTs and Sm2O3/CNTs.Figure 3
XRD patterns of (a) Y2O3/CNTs, (b) Nd2O3/CNTs, (c) Sm2O3/CNTs, and (d) f-CNTs.


### 3.1.3. SEM Analysis

Morphology was investigated by SEM. Figure4(a) reveals f-CNTs with a diameter of 40–60 nm and good dispersibility. In Figure 4(b), f-CNTs were decorated with a dense layer of Y2O3 nanoparticles. Figure 4(c) shows that Nd2O3 nanoparticles were homogeneously distributed over f-CNTs because the modified CNTs are covered with a negatively charged surfactant, which can ensure good coverage of positively charged Nd3+ by electrostatic attraction. In addition, Sm2O3 nanoparticles were loaded on f-CNTs, indicating a strong interaction between Sm2O3 nanoparticles and CNTs by surfactant modification (Figure 4(d)). The formation of M2O3/CNTs nanocomposites can be represented as follows:(1)M3++3OH-→EGMOH32MOH3⟶M2O3+3H2OM3+ in the solution can favorably bind with f-CNTs by electrostatic attraction, and M3+ was hydrolyzed to M(OH)3. As a result, M2O3 nanoparticles were anchored onto the surface of f-CNTs. It could be included that the modification of CNTs improved the heterogeneous nucleation; the enhanced nucleation rate favored the generation of much more nuclei and the formation of smaller M2O3 nanoparticles.Figure 4
SEM images of (a) f-CNTs, (b) Y2O3/CNTs, (c) Nd2O3/CNTs, and (d) Sm2O3/CNTs.
(a) (b) (c) (d)


### 3.1.4. EDS Analysis

Energy-dispersive X-ray spectrometry (EDS) was used to measure the composition of M2O3/CNT nanocomposites. The peaks (2.2 keV and 2.9 keV) are originated from Au/Pd coating. Figure 5(a) shows that Y2O3/CNTs includes 21.75 wt.% of Y, 25.33 wt.% of O, and 52.92 wt.% of C. In Figure 5(b), Nd2O3/CNTs contains 13.10 wt.% of Nd, 31.82 wt.% of O, and 55.08 wt.% of C. As shown in Figure 5(c), Sm2O3/CNTs contained 11.60 wt.% of Sm, 24.53 wt.% of O, and 63.87 wt.% of C.Figure 5
EDS patterns of (a) Y2O3/CNTs, (b) Nd2O3/CNTs, and (c) Sm2O3/CNTs.
(a) (b) (c)According to the results, the relative proportions of f-CNTs and M2O3 during the final products by solvothermal process and chemical deposition were listed in Table 1. As shown in Table 1, the relative proportions of M2O3 and f-CNTs by chemical deposition were smaller than that of solvothermal process, which indicated that the M2O3 nanoparticles had better coverage on f-CNTs by solvothermal process.


### 3.1.5. TEM Analysis

The morphology of M2O3/CNT nanocomposites was further measured by TEM as shown in Figure 6. Figure 6(a) shows TEM image of f-CNTs. Before the reaction, f-CNTs present well-graphitized walls. In Figure 6(b), Y2O3 nanoparticles attached to f-CNTs reveal slight nonuniformity and no free nanoparticles. The average particle size of Y2O3 nanoparticles was 30 nm, which is considerably smaller than that of Y2O3/CNTs produced by chemical deposition (70 nm). In Figure 6(c), Nd2O3 nanoparticles supported on f-CNTs exhibited good dispersibility and sphericity. The average particle size of Nd2O3 nanoparticles was 45 nm, which was markedly smaller than that of Nd2O3/CNTs (90 nm) produced by chemical deposition. In addition, although the coverage of Sm2O3 nanoparticles onto f-CNTs is nonuniform, no free nanoparticles were found. And the average particle size of Sm2O3 nanoparticles was 90 nm, which is smaller than that of Sm2O3/CNTs (120 nm) produced by chemical deposition. The smaller particle size of M2O3/CNTs prepared by in situ solvothermal process is advantageous in catalysis.Figure 6
TEM images of (a) f-CNTs, (b) Y2O3/CNTs, (c) Nd2O3/CNTs, and (d) Sm2O3/CNTs.
(a) (b) (c) (d)


### 3.1.6. BET Analysis

The surface area of M2O3/CNT nanocomposites was also characterized. As shown in Table 1, the BET surface area of Y2O3/CNTs prepared by solvothermal process was 84 m2·g−1, which is considerably larger than that of Y2O3/CNTs produced by chemical deposition (74 m2·g−1) and f-CNTs (33 m2·g−1). The same finding was confirmed by measuring the surface area of Nd2O3/CNTs and Sm2O3/CNTs nanocomposites. The BET surface areas of Nd2O3/CNTs and Sm2O3/CNTs were 77 and 65 m2·g−1, which were markedly larger than those of Nd2O3/CNTs (41 m2·g−1) and Sm2O3/CNTs (58 m2·g−1) produced by chemical deposition. The large BET surface area of M2O3/CNTs is advantageous in catalysis.


## 3.1.1. IR Analysis

The IR spectra of (a) pristine CNTs and (b) f-CNTs are shown in Figure2. In Figure 2(a), stretching vibration of O-H appeared at 3480 cm−1. In Figure 2(b), characteristic absorption bands of O-H stretching vibration appeared at 3285 cm−1, and the peaks at 1714 cm−1 can be ascribed to the stretching vibration of C=O bond [22]. In addition, the peaks at 1563 and 1194 cm−1 can correspond to stretching vibration peak of COO- and C-O characteristic bands, respectively. Therefore, f-CNTs were successfully modified by HNO3.Figure 2
Fourier transform IR spectra of (a) pristine CNTs and (b) f-CNTs.


## 3.1.2. XRD Analysis

Figure3 illustrates the XRD patterns of f-CNTs and M2O3/CNT nanocomposites. In Figure 3(d), the peaks at 2θ = 22.94° correspond to the (002) plane of f-CNTs (JCPDS No. 25-0284) [18]. From Figures 3(a) and 3(c), characteristic peaks of CNTs remain. In Figure 3(a), diffraction peaks at 29.4°, 33.9°, 40.9°, 48.7°, and 57.9° correspond to the (222), (400), (322), (440), and (622) planes of Y2O3 (JCPDS No. 74-1828), respectively [18]. In Figure 3(b), reflection peaks at 26.9°, 29.7°, 30.7°, 40.5°, 47.4°, 53.4°, and 53.4° can be indexed to the (100), (002), (101), (102), (110), (103), and (112) planes of hexagonal Nd2O3 (JCPDS No. 41-1089), respectively [19]. As shown in Figure 3(c), reflection peaks appearing at 28.2°, 32.7°, 38.6°, 45.4°, and 54.3° correspond to (222), (400), (332), (521), and (541) planes of cubic Sm2O3 (JCPDS No. 15-0813), respectively. As shown in Table 1, the crystallite size of Y2O3 deposited on CNTs calculated according to the Scherrer formula is 42 nm and those of Nd2O3 and Sm2O3 are 34 nm and 20 nm. The crystallite size of Y2O3 deposited on CNTs is bigger than that of Nd2O3 and Sm2O3, which is consistent with the broad peaks in case of Nd2O3/CNTs and Sm2O3/CNTs.Figure 3
XRD patterns of (a) Y2O3/CNTs, (b) Nd2O3/CNTs, (c) Sm2O3/CNTs, and (d) f-CNTs.


## 3.1.3. SEM Analysis

Morphology was investigated by SEM. Figure4(a) reveals f-CNTs with a diameter of 40–60 nm and good dispersibility. In Figure 4(b), f-CNTs were decorated with a dense layer of Y2O3 nanoparticles. Figure 4(c) shows that Nd2O3 nanoparticles were homogeneously distributed over f-CNTs because the modified CNTs are covered with a negatively charged surfactant, which can ensure good coverage of positively charged Nd3+ by electrostatic attraction. In addition, Sm2O3 nanoparticles were loaded on f-CNTs, indicating a strong interaction between Sm2O3 nanoparticles and CNTs by surfactant modification (Figure 4(d)). The formation of M2O3/CNTs nanocomposites can be represented as follows:(1)M3++3OH-→EGMOH32MOH3⟶M2O3+3H2OM3+ in the solution can favorably bind with f-CNTs by electrostatic attraction, and M3+ was hydrolyzed to M(OH)3. As a result, M2O3 nanoparticles were anchored onto the surface of f-CNTs. It could be included that the modification of CNTs improved the heterogeneous nucleation; the enhanced nucleation rate favored the generation of much more nuclei and the formation of smaller M2O3 nanoparticles.Figure 4
SEM images of (a) f-CNTs, (b) Y2O3/CNTs, (c) Nd2O3/CNTs, and (d) Sm2O3/CNTs.
(a) (b) (c) (d)


## 3.1.4. EDS Analysis

Energy-dispersive X-ray spectrometry (EDS) was used to measure the composition of M2O3/CNT nanocomposites. The peaks (2.2 keV and 2.9 keV) are originated from Au/Pd coating. Figure 5(a) shows that Y2O3/CNTs includes 21.75 wt.% of Y, 25.33 wt.% of O, and 52.92 wt.% of C. In Figure 5(b), Nd2O3/CNTs contains 13.10 wt.% of Nd, 31.82 wt.% of O, and 55.08 wt.% of C. As shown in Figure 5(c), Sm2O3/CNTs contained 11.60 wt.% of Sm, 24.53 wt.% of O, and 63.87 wt.% of C.Figure 5
EDS patterns of (a) Y2O3/CNTs, (b) Nd2O3/CNTs, and (c) Sm2O3/CNTs.
(a) (b) (c)According to the results, the relative proportions of f-CNTs and M2O3 during the final products by solvothermal process and chemical deposition were listed in Table 1. As shown in Table 1, the relative proportions of M2O3 and f-CNTs by chemical deposition were smaller than that of solvothermal process, which indicated that the M2O3 nanoparticles had better coverage on f-CNTs by solvothermal process.


## 3.1.5. TEM Analysis

The morphology of M2O3/CNT nanocomposites was further measured by TEM as shown in Figure 6. Figure 6(a) shows TEM image of f-CNTs. Before the reaction, f-CNTs present well-graphitized walls. In Figure 6(b), Y2O3 nanoparticles attached to f-CNTs reveal slight nonuniformity and no free nanoparticles. The average particle size of Y2O3 nanoparticles was 30 nm, which is considerably smaller than that of Y2O3/CNTs produced by chemical deposition (70 nm). In Figure 6(c), Nd2O3 nanoparticles supported on f-CNTs exhibited good dispersibility and sphericity. The average particle size of Nd2O3 nanoparticles was 45 nm, which was markedly smaller than that of Nd2O3/CNTs (90 nm) produced by chemical deposition. In addition, although the coverage of Sm2O3 nanoparticles onto f-CNTs is nonuniform, no free nanoparticles were found. And the average particle size of Sm2O3 nanoparticles was 90 nm, which is smaller than that of Sm2O3/CNTs (120 nm) produced by chemical deposition. The smaller particle size of M2O3/CNTs prepared by in situ solvothermal process is advantageous in catalysis.Figure 6
TEM images of (a) f-CNTs, (b) Y2O3/CNTs, (c) Nd2O3/CNTs, and (d) Sm2O3/CNTs.
(a) (b) (c) (d)


## 3.1.6. BET Analysis

The surface area of M2O3/CNT nanocomposites was also characterized. As shown in Table 1, the BET surface area of Y2O3/CNTs prepared by solvothermal process was 84 m2·g−1, which is considerably larger than that of Y2O3/CNTs produced by chemical deposition (74 m2·g−1) and f-CNTs (33 m2·g−1). The same finding was confirmed by measuring the surface area of Nd2O3/CNTs and Sm2O3/CNTs nanocomposites. The BET surface areas of Nd2O3/CNTs and Sm2O3/CNTs were 77 and 65 m2·g−1, which were markedly larger than those of Nd2O3/CNTs (41 m2·g−1) and Sm2O3/CNTs (58 m2·g−1) produced by chemical deposition. The large BET surface area of M2O3/CNTs is advantageous in catalysis.


## 3.2. Catalytic Properties

### 3.2.1. DTA Analysis

In Figure7(a), the transition from orthorhombic to cubic AP is in agreement with the endothermic peak at 245.2°C [23]. The thermal decomposition of AP to form volatile products corresponds to the exothermic peaks at 331°C and 452°C [24]. In Figure 7(b), the two exothermic peaks combine into one peak, and the high thermal decomposition temperature decreases substantially. With the addition of f-CNTs, the high decomposition temperature decreased by 97.4°C, and the total DTA heat release increased by 0.346 kJ·g−1. Compared with f-CNTs, M2O3/CNTs nanocomposites showed better catalytic activity (Figures 7(c) and 7(d)). Based on this result, we can deduce that f-CNTs can mainly accelerate the high decomposition of AP, because the violent decomposition of temperature was greatly reduced during this process.Figure 7
DTA curves for (a) pure AP, (b) f-CNTs + AP, (c) Y2O3/CNTs + AP, (d) Sm2O3/CNTs + AP, and (e) Nd2O3/CNTs + AP.It has been reported that many nanomaterials could catalyze the thermal decomposition of AP. For example, Fe3O4 and Co3O4 microspheres were synthesized by the hydrothermal reaction and with the addition of 2% Fe3O4 and Co3O4 microspheres could decrease the high thermal decomposition temperature of AP by 80°C and 55°C, respectively [25]. V6O13 nanobelts were controlled and synthesized by one-pot hydrothermal process and the high decomposition temperature of AP in the presence of 3% V6O13 was reduced by 67°C [26]. Zhang et al. improved the catalytic activity of α-Fe2O3 in the decomposition of AP by coating amorphous carbon on α-Fe2O3 surface, which reduced the high decomposition temperature of AP by 109°C [27].As shown in Figures7(c), 7(d), and 7(e), addition of 2.0 wt.% of Y2O3/CNTs, Sm2O3/CNTs, and Nd2O3/CNTs decreased the high thermal decomposition temperature of AP by 125.5°C, 118.4°C, and 123.8°C, respectively, and increased the total DTA heat release by 2.027, 1.243, and 1.463 kJ·g−1, respectively. Compared with Fe3O4, Co3O4 microspheres, V6O13 nanobelts, and α-Fe2O3/C, the M2O3/CNTs showed better catalytic activity in the thermal decomposition of AP.


### 3.2.2. Catalytic Mechanism of Nanocomposites

Moreover, compared with Sm2O3/CNTs and Nd2O3/CNTs, Y2O3/CNTs showed superior catalytic effect in the thermal decomposition of AP. Thermal decomposition peak temperature and the heat release of pure AP and samples are listed in Table 2. From DTA data, the following trend of the total DTA heat release was noticed: pure AP (0.281 kJ·g−1) < f-CNTs + AP (0.627 kJ·g−1) < 2 wt.% Sm2O3/CNTs + AP (1.524 kJ·g−1) < 2 wt.% Nd2O3/CNTs + AP (1.744 kJ·g−1) < 2 wt.% Y2O3/CNTs + AP (2.308 kJ·g−1). This result was consistent with the peak area of these samples.Table 2
Temperatures peaks and heat release of AP and the mixtures.
Sample t L  (°C) t H  (°C) Δ t L  (°C) Δ t H  (°C) H  (kJ·g−1) Δ H  (kJ·g−1) (a) Pure AP 331.0 452.0   0.281 (b) Pure CNTs + AP 354.6 +23.6 −97.4 0.627 0.346 (c) 2 wt.% Y2O3/CNTs + AP 326.5 −4.5 −125.5 2.308 2.027 (d) 2 wt.% Sm2O3/CNTs + AP 333.6 +2.6 −118.4 1.524 1.243 (e) 2 wt.% Nd2O3/CNTs + AP 328.2 −2.8 −123.8 1.744 1.463 Note. tL is the low temperature decomposition peak temperature, tH is the high temperature decomposition peak temperature, and H is the total DTA heat release.With the same addition, the obtained Nd2O3/CNTs decreased the high decomposition temperature by 123.8°C, which was markedly larger than the decrement with Nd2O3/CNTs produced by chemical deposition (93.9°C) [19]. It can be concluded that M2O3/CNTs nanocomposites by in situ solvothermal method possessed smaller particle size and higher specific area, which can more effectively catalyze the thermal of decomposition of AP than M2O3/CNTs synthesized by chemical deposition.On the basis of electron transfer theory [28], the presence of partially filled 4d orbital in M3+ (M = Y3+, Nd3+, Sm3+) is advantageous for electron transfer [29]. The thermal decomposition of AP was enhanced because the positive hole in M3+ can accept electrons from the AP ion and its intermediate products. Moreover, the open pore structure of f-CNTs facilitates electron transfer and heat conduction [30] and can promote the thermal decomposition of AP. By contrast, M2O3 nanoparticles with relatively small size and good dispersibility, which enhance catalytic effect, are synthesized on f-CNTs by in situ solvothermal method.


## 3.2.1. DTA Analysis

In Figure7(a), the transition from orthorhombic to cubic AP is in agreement with the endothermic peak at 245.2°C [23]. The thermal decomposition of AP to form volatile products corresponds to the exothermic peaks at 331°C and 452°C [24]. In Figure 7(b), the two exothermic peaks combine into one peak, and the high thermal decomposition temperature decreases substantially. With the addition of f-CNTs, the high decomposition temperature decreased by 97.4°C, and the total DTA heat release increased by 0.346 kJ·g−1. Compared with f-CNTs, M2O3/CNTs nanocomposites showed better catalytic activity (Figures 7(c) and 7(d)). Based on this result, we can deduce that f-CNTs can mainly accelerate the high decomposition of AP, because the violent decomposition of temperature was greatly reduced during this process.Figure 7
DTA curves for (a) pure AP, (b) f-CNTs + AP, (c) Y2O3/CNTs + AP, (d) Sm2O3/CNTs + AP, and (e) Nd2O3/CNTs + AP.It has been reported that many nanomaterials could catalyze the thermal decomposition of AP. For example, Fe3O4 and Co3O4 microspheres were synthesized by the hydrothermal reaction and with the addition of 2% Fe3O4 and Co3O4 microspheres could decrease the high thermal decomposition temperature of AP by 80°C and 55°C, respectively [25]. V6O13 nanobelts were controlled and synthesized by one-pot hydrothermal process and the high decomposition temperature of AP in the presence of 3% V6O13 was reduced by 67°C [26]. Zhang et al. improved the catalytic activity of α-Fe2O3 in the decomposition of AP by coating amorphous carbon on α-Fe2O3 surface, which reduced the high decomposition temperature of AP by 109°C [27].As shown in Figures7(c), 7(d), and 7(e), addition of 2.0 wt.% of Y2O3/CNTs, Sm2O3/CNTs, and Nd2O3/CNTs decreased the high thermal decomposition temperature of AP by 125.5°C, 118.4°C, and 123.8°C, respectively, and increased the total DTA heat release by 2.027, 1.243, and 1.463 kJ·g−1, respectively. Compared with Fe3O4, Co3O4 microspheres, V6O13 nanobelts, and α-Fe2O3/C, the M2O3/CNTs showed better catalytic activity in the thermal decomposition of AP.


## 3.2.2. Catalytic Mechanism of Nanocomposites

Moreover, compared with Sm2O3/CNTs and Nd2O3/CNTs, Y2O3/CNTs showed superior catalytic effect in the thermal decomposition of AP. Thermal decomposition peak temperature and the heat release of pure AP and samples are listed in Table 2. From DTA data, the following trend of the total DTA heat release was noticed: pure AP (0.281 kJ·g−1) < f-CNTs + AP (0.627 kJ·g−1) < 2 wt.% Sm2O3/CNTs + AP (1.524 kJ·g−1) < 2 wt.% Nd2O3/CNTs + AP (1.744 kJ·g−1) < 2 wt.% Y2O3/CNTs + AP (2.308 kJ·g−1). This result was consistent with the peak area of these samples.Table 2
Temperatures peaks and heat release of AP and the mixtures.
Sample t L  (°C) t H  (°C) Δ t L  (°C) Δ t H  (°C) H  (kJ·g−1) Δ H  (kJ·g−1) (a) Pure AP 331.0 452.0   0.281 (b) Pure CNTs + AP 354.6 +23.6 −97.4 0.627 0.346 (c) 2 wt.% Y2O3/CNTs + AP 326.5 −4.5 −125.5 2.308 2.027 (d) 2 wt.% Sm2O3/CNTs + AP 333.6 +2.6 −118.4 1.524 1.243 (e) 2 wt.% Nd2O3/CNTs + AP 328.2 −2.8 −123.8 1.744 1.463 Note. tL is the low temperature decomposition peak temperature, tH is the high temperature decomposition peak temperature, and H is the total DTA heat release.With the same addition, the obtained Nd2O3/CNTs decreased the high decomposition temperature by 123.8°C, which was markedly larger than the decrement with Nd2O3/CNTs produced by chemical deposition (93.9°C) [19]. It can be concluded that M2O3/CNTs nanocomposites by in situ solvothermal method possessed smaller particle size and higher specific area, which can more effectively catalyze the thermal of decomposition of AP than M2O3/CNTs synthesized by chemical deposition.On the basis of electron transfer theory [28], the presence of partially filled 4d orbital in M3+ (M = Y3+, Nd3+, Sm3+) is advantageous for electron transfer [29]. The thermal decomposition of AP was enhanced because the positive hole in M3+ can accept electrons from the AP ion and its intermediate products. Moreover, the open pore structure of f-CNTs facilitates electron transfer and heat conduction [30] and can promote the thermal decomposition of AP. By contrast, M2O3 nanoparticles with relatively small size and good dispersibility, which enhance catalytic effect, are synthesized on f-CNTs by in situ solvothermal method.


## 4. Conclusions

M2O3/CNT (M = Y, Nd, Sm) nanocomposites were prepared through in situ solvothermal method, and M2O3 nanoparticles were successfully attached to f-CNTs. This method can be applied in the preparation of other metal oxide/CNTs nanocomposites. The M2O3/CNTs nanocomposites by in situ solvothermal method possessed smaller particle size and higher specific area and decreased the aggregation of M2O3 nanoparticles, which can more effectively catalyze the thermal decomposition of AP than M2O3/CNTs synthesized by chemical deposition. In addition, compared with Nd2O3/CNTs and Sm2O3/CNTs, Y2O3/CNT composites decreased the high thermal decomposition temperature of AP by 125.5°C, which exhibited the best catalytic performance.



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*Source: 3635164-2018-06-11.xml*