Introduction

Samarium oxide (Sm2O3) is a high-k dielectric material and has extensively been studied for its potential application to alternative gate dielectric in complementary metal–oxide–semiconductors (CMOS) and resistance random access memories (RRAM) [1–4]. Wang et al. used high-k Sm2O3 thin film as a pH-sensing layer [5]. Application to catalyst has also been of interest [6–10]. Elkins et al. demonstrated that Sm2O3 could be a promising catalyst for the oxidative coupling of methane [8]. Wang et al. reported a potential application to photocatalytic dye degradation [9, 10]. Many effects have been devoted to synthesize diverse morphologies of Sm2O3. Sm(OH)3 has also been prepared since Sm2O3 could be obtained by a post-thermal dehydration [11, 12]. Facial solution-based synthetic methods include hydrothermal [13–15], precipitation [11, 16], sol–gel [18], and template [19] methods. Many different morphologies have been synthesized and reported, including nanorods [11, 15–17, 20], nanodisks [18], nanowires [21, 22], submicrospindles [23], nanoparticles [24], and nanoplates [20, 21]. Yu et al. synthesized single unit cell size (1.1 nm thick and 2.2 nm wide) Sm2O3 nanowires and nanoplates by thermal decomposition of Sm(III) acetate in the presence of oleylamine and a long-chain carboxylic acid under Ar atmosphere [21]. Panda et al. used the same chemicals but applied microwave to synthesize Sm2O3 nanorods [20]. Nejad et al. synthesized Sm2O3 nanoparticles (50–80 nm) in supercritical water condition by a hydrothermal method and reported BET surface areas of 20–32 m2/g for the synthesized nanoparticles [14]. Sm2O3 fibers were prepared by thermal calcination of samarium citrate-PVA composite nanofibers, which were prepared by an electrospinning technique [25]. Precipitation method has also widely been employed to synthesize nanorods [11, 12]. Zhang et al. reported Sm2O3 mesoporous structures by a surfactant-mediated PMMA-template assisted synthesis and a post-thermal treatment [19].

In the present study, we first prepared Sm(OH)3 and Sm2O3 nanoroll (rolled into nanotubes) sticks, which were then characterized using various experimental techniques. The morphology could be controlled by an amount of ammonia and a reaction temperature. Because new morphology generally exhibits different physicochemical properties and application performances [26] the newly found nanoroll stick morphology provides new insight for developing Sm(OH)3 and Sm2O3 nanostructures for various applications including high-k materials, catalysts, and sensors.

Experimental section

For the synthesis of Sm(OH)3 nanoroll sticks, we prepared 25 mL of ~0.04 M Sm(III) sulfate octahydrate (Sigma-Aldrich, 99.9 %) solution and added 2.0 mL of ammonia solution (~30 %) drop-wise to obtain precipitates. The solution was then transferred to a 100 mL Teflon-lined stainless autoclave and placed in an oven (140 °C) for 12 h. After that we naturally cooled the solution to room temperature, centrifuged, and washed white precipitates with deionized water and ethanol repeatedly. The powder samples were fully dried in an oven (90 °C). To obtain Sm2O3 nanoroll sticks, we calcinated the Sm(OH)3 nanoroll sticks at 450 °C for 4 h. The morphology of the samples was examined by scanning electron microscopy (SEM, Hitachi SE-4800). For the Sm2O3 nanoroll sticks, high resolution transmission electron microscopy (HRTEM) images were obtained using a Tecnai G2 F20 S-TWIN at an acceleration voltage of 200 kV. X-ray diffraction (XRD) patterns were recorded using a PANalytical X’Pert Pro MPD diffractometer with Cu Kα radiation. For the powder samples, diffuse reflectance UV–Vis–NIR absorption spectra were obtained using a Cary5000 spectrophotometer. Raman spectra were taken using a Bruker Senterra Raman spectrometer at a laser excitation wavelength of 532 nm. FT-IR spectra were obtained using a Thermo Scientific Nicolet iS10 spectrometer with ATR (attenuated total reflectance) mode.

Results and discussion

Figure 1 shows the SEM images of as-prepared and post-annealed (at 450 °C) Sm(III) powder samples. The as-prepared sample showed stick morphology with widths of 300–350 nm and lengths of 4–5 μm. The edges of the sticks showed open structure, indicating a tube-like morphology. For the thermal-annealed sample, the morphology was not significantly changed, however the sticks became significantly skinnier with widths of 200–250 nm. The shrinkage is due to a thermal dehydration effect. The color of the sample was white and unchanged after thermal annealing at 450 °C.

Fig. 1
figure 1

SEM images of as-prepared (top) and 450 °C-annealed (bottom) samples. Insets show optical microscope images of the corresponding samples

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The XRD patterns of the as-prepared and 450 °C-annealed samples were obtained and are displayed in Fig. 2. The XRD patterns of the as-prepared sample matched well with those of the hexagonal phase (space group P63/m) Sm(OH)3 (JCPDS 01-083-2036) well. Three major peaks were found at 2θ = 16.0°, 28.0°, and 29.2°, assigned to the (100), (110), and (101) planes, respectively. Other crystal planes were also assigned to the corresponding XRD peaks in Fig. 2. For the formation of Sm(OH)3, a following reaction is proposed: 2Sm3+ (aq) + 3SO4 2− (aq) + 6OH− (aq) + 6NH4 + (aq) → 2Sm(OH)3 (s) + 3SO4 2− (aq) + 6NH4 + (aq) [11]. At lower concentrations of ammonia, SO4 2− ions may also be participated to form Sm(OH)(SO4). To confirm this scenario, we also synthesized with a smaller amount of ammonia (1.0 mL), and obtained a plate morphology showing a different crystal structure. This indicates that the plates consist of different Sm complexes which are under investigation. When we added 1.5 mL of ammonia we obtained mixed morphologies; SEM and XRD confirmed not shown here. Base on this, we conclude that the amount of ammonia plays a major role in determining the morphology. At a higher reaction temperature of 210 °C (not shown here), we found that the plates became thicker and the roll sticks became more like rods. For the thermal-annealed sample, the XRD patterns were drastically different from those of as-prepared sample. The patterns were in good match with those of cubic (space group Ia-3) phase Sm2O3 (JCPDS 42-1461). A major peak was found at 2θ = 28.3° and assigned to the (222) plane. Other minor peaks at 2θ = 19.9°, 32.8°, 47.0°, and 55.8° were assigned to the (211), (400), (440), and (622) planes, respectively.

Fig. 2
figure 2

Powder X-ray diffraction patterns of as-prepared (bottom) and 450 °C-annealed (top) samples. The reference XRD patterns are also shown below the corresponding XRD patterns

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Figure 3 displays the TEM and HRTEM images, and the selected area electron diffraction (SAED) patterns of Sm2O3 nanoroll sticks. As discussed above in Fig. 1, the TEM image clearly showed that a long roll sheet was folded to form a roll stick structure [22]. The SAED patterns along the [101] zone axis showed highly single crystalline nature of the nanoroll stick. The SAED pattern indicates that the crystal growth preferentially occurs along [40-4]. The HRTEM images showed clear lattice fringes. The distances between the planes were measured to be 0.32, 0.38, and 0.54 nm, which correspond the (222), (202), and (220) planes of cubic phase Sm2O3. On the basis of the results discussed above, we proposed the following growth mechanism. The nanoroll sticks may be formed by the following anisotropic growth and folding mechanism. Sm(OH)(SO4) seed with sheet morphology is initially formed, gradually transformed to Sm(OH)3 and then the sheets anisotropically grow into longer sheets. Due to different growth directions, rates, and strains upon forming from Sm(OH)(SO4) to Sm(OH)3 the sheets consequently roll up to form tubes (or roll stick-like structures at a lower temperature of 140 °C) or rods (at a high temperature of 210 °C).

Fig. 3
figure 3

TEM and HRTEM images, and SAED patterns (inset) of Sm2O3 nanoroll sticks

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The nanoroll sticks could also be synthesized for other rare earth compounds by a facial hydrothermal method by controlling the amount of ammonia and a reaction temperature. Sohn et al. have also synthesized other rare earth compounds including Dy(III) and Tb(III) and found similar morphological changes [27, 28]. Sheet morphology has commonly been obtained and converted to tube or rod morphology.

Figure 4 shows Raman and FT-IR spectra of Sm(OH)3 and Sm2O3 nanoroll sticks. For Sm(OH)3 nanoroll sticks, three major Raman peaks were observed at 302.0, 375.4, and 481.0 cm−1, which could be assigned to Ag translatory, E2g translatory, and E1g libration modes, respectively [16]. For Sm(OH)3 with a hexagonal crystal phase (P63/m), the vibrational mode representations are 4Ag + 3Bg + 2E1g + 5E2g + 2Au + 4Bu + 4E1u + 2E2u, where 4Ag, 2E1g and 5E2g are Raman active. For Sm2O3 with a cubic phase, 22 Raman active modes of 4Ag, 4Eg, and 14Fg are expected [29, 30]. The strong Raman peak at 344.7 cm−1 was assigned to the combination of Ag and Fg modes [29, 30]. For the FT-IR spectrum of Sm(OH)3, a very sharp IR peak was observed at 3608.0 cm−1 and attributed to the O–H stretching vibration [16, 31, 32]. The peak at 685.6 cm−1 was due to bending vibration of Sm–O–H [16]. For Sm2O3, a broad peak at 3400 cm−1 was due to H2O (or hydrated) adsorbed on the oxide surface. The peaks at 1412 and 1473 cm−1 were commonly observed and attributed to symmetric and asymmetric stretching of COO−, respectively [15].

Fig. 4
figure 4

Raman (left) and FT-IR (right) spectra of Sm(OH)3 and Sm2O3 nanoroll sticks

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Figure 5 displays the diffuse reflectance UV–Vis–NIR absorption spectra of Sm(OH)3 and Sm2O3 nanoroll stick powder samples. The Y-axis was converted from the reflectance data using the Kubelka–Munk method. Although the intensities were different before and after thermal annealing all the UV–Vis–NIR absorption peaks were in good match with the electronic transitions of Sm(III) [19, 33].The absorption spectra between 900 and 2000 nm correspond to the f–f transitions from the ground state of 6H5/2 to 6F1/2,3/2,5/2,7/2,9/2,11/2 and 6H15/2 energy levels as assigned on the corresponding peaks [33]. The absorption peaks found between 300 and 500 nm were assigned on the corresponding peaks. The broad absorption below 300 nm was possibly due to the O2−–Sm3+ charge-transfer [19]. Zhang et al. reported very similar UV–Vis absorption profiles between 200 and 600 nm for Sm2O3 mesoporous walls [19].

Fig. 5
figure 5

UV–Visible reflectance absorption spectra of Sm(OH)3 and Sm2O3 roll-sticks The spectral region in 900–2000 nm was 3× amplified for clear peak identification

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We examined surface activity of Sm2O3 nanoroll sticks by recording temperature-programmed hydrogen reduction profiles as shown in Fig. 6. The hydrogen consumption was gradually increased from 400 °C. The gradual increase was commonly attributed to the reduction of surface oxide [34]. Above 700 °C, two broad and strong reduction peaks appeared at 788 and 940 °C. The former (major) peak at 788 °C was plausibly due to reduction process of Sm2O3 → SmO and the latter (minor) peak to SmO → Sm.

Fig. 6
figure 6

Hydrogen temperature-programmed reduction (TPR) profile of Sm2O3 nanorods

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To show the originality of the nanoroll sticks, we summarized the previously reported results for Sm(OH)3 and Sm2O3 in Table 1. Various morphologies have previously been reported using diverse synthetic methods. However, nanoroll stick morphology has never been reported so far.

Table 1 Summary of literature reported for Sm(OH)3 and Sm2O3
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Conclusion

Hexagonal phase Sm(OH)3 nanoroll sticks were synthesized by a hydrothermal method for the first time. Highly single crystalline cubic phase Sm2O3 nanoroll sticks were obtained by post-thermal annealing at 450 °C. Growth direction of the oxide was found to be [40-4]. The crystal phases were confirmed by XRD, HRTEM, Raman, and FT-IR spectra. The UV–Vis–NIR absorption peaks of Sm(OH)3 and Sm2O3 clearly showed the electronic transitions of Sm(III). The TPR peaks at 788 and 940 °C were attributed to reductions of Sm2O3 and SmO, respectively.