Eisuke
Yamamoto
*ab,
Akiko
Suzuki
a,
Makoto
Kobayashi
a and
Minoru
Osada
*ac
aDepartment of Materials Chemistry & Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya 464-8601, Japan
bPrecursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan
cInternational Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
First published on 29th June 2022
The assembly of the surfactants has been utilized as unique templates for the controlled synthesis of metal nanosheets. However, current strategies for metal nanosheets have mainly focused on the liquid-phase surfactant assembly. Herein, we found the solid-state surfactants as designable crystals suitable for nanostructural control and proposed a novel synthetic route for molecularly thin Pt metal nanosheets using solid surfactant crystals as a precursor. The 2D surfactant crystals containing planarly arranged Pt complexes were prepared, and the subsequent UV-ozone treatment and reduction process allowed us to obtain Pt metal nanosheets. Pt metal nanosheets had a distinct morphology with various thicknesses (from 1.5 nm to 3.0 nm), characteristic of 2D surfactant crystals.
Such an issue is particularly important for designing 2D metal nanosheets.11,12 2D metal nanosheets have attracted great attention because they exhibit a drastic change in their physicochemical properties owing to 2D confinement effects.13 In particular, due to their unique 2D morphology with exceptionally large exposed surface area, 2D nanosheets can significantly enhance the surface reactions, redox processes, and charge separations. The synthetic approach using the surfactant assembly is one of the solutions for generating 2D metal nanosheets.14–17 Typically, the concentrated surfactant micelle solutions form lamellar liquid crystals possessing the 2D nanospace; the use of interlayer nanospace of the lamellar liquid crystals has been well documented as an ideal template for the confined growth of 2D metal nanosheets. However, the control of the thickness and morphology of the metal nanosheets still remains a real challenge. Due to their fragile nature, lamellar liquid crystals usually lack a strong confinement force to regulate the 2D anisotropic growth; metal atoms tend to form 3D structures. In addition, the structure of the lamellar liquid crystals was easily changed by various factors, including temperature, concentration, and precursor amount.18–20 Therefore, the synthesis of the metal nanosheets using the surfactant assembly faces difficulty in the systematic design.
The previous strategies using the surfactant assembly have only focused on the liquid-phase surfactants; nevertheless, it is known that the surfactants form unique solid phases with a crystalline state under the Krafft point.21,22 The crystalline solid phase (Lc phase) surfactants have a lamellar structure with densely arranged counter ions in the interlayer space and the distinct morphology reflecting the crystal habit. Interestingly, the structure and morphology of surfactant crystals have the potential for design depending on the component's structure and composition, unlike liquid crystals.21,23–25 Therefore, templating the solid surfactant crystals with distinct thickness and morphology will allow the precise design of metal nanosheets, offering the potential for a novel technique for the nanoarchitectonics of metal nanosheets. However, to the best of our knowledge, the surfactant assembly with the solid crystalline phase has never been used for the designed synthesis of nanostructured materials.
Herein, we report a new concept for the designed synthesis of molecularly thin metal nanosheets using solid-state surfactant crystals. As typical metal nanosheets, Pt metal nanosheets were synthesized because they are potential candidates for future nanosheets devices and catalysts.26 We synthesized the surfactant crystals with planarly arranged Pt complexes as a precursor and designed the morphology and thickness of the 2D surfactant crystals via the recrystallization process. Then, the 2D surfactant crystals were treated with UV-ozone and reduced by H2(5%)/Ar. These processes lead to the synthesis of molecularly thin Pt metal nanosheets, which is inherent from the thickness and morphology of 2D surfactant crystals (Fig. 1). We have also succeeded in the structural analysis of surfactant crystals and found that there is a clear correlation between the thickness and morphology of 2D surfactant crystals and the resulting metal nanosheets. This method is potentially useful for the precise design of molecularly thin metal nanosheets with distinct morphology and thickness.
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Fig. 2 (a) Optical image of the single crystalline surfactants with Pt complexes, and (b) the refined crystal structure.27 |
As a typical example, 2D surfactant crystals were prepared via 2000 rpm coating, and the thickness of the sheet was monitored at each step. As seen from the AFM image, the thickness of the 2D surfactant crystal was evaluated to be 49 nm, which corresponds to 24 layers of the surfactant bilayer considering the crystal structure (Fig. 3(a) and (d)). XRD and XPS measurements and Raman spectroscopy analyses were conducted to confirm the composition and crystal structure. Fig. S2(a) of the ESI† shows the XRD pattern of the 2D surfactant crystals. The XRD pattern showed clear peaks at 4.5, 9.0, and 13.5°, which is assigned to 001, 002, and 003 reflections of the surfactant crystals, respectively. The Raman spectrum showed the retention of the surfactant, as shown by broad peaks derived from the stretching of C–H groups in the alkyl chain (CH2 and CH3) at 2900 cm−1 (Fig. S2(c) of the ESI†). In addition, the XPS spectrum of the 2D surfactant crystals showed clear peaks at 75.2 eV and 72.0 eV derived from Pt2+4f5/2 and Pt2+4f7/2, respectively (Fig. 1(g)). Although the peaks derived from Br 3d (68 eV) are unclear, a peak derived from Cl 2p3/2 was clearly observed at 198 eV, indicating the retention as Pt halides (Fig. S2(e) of the ESI†). These results indicate the formation of 2D surfactant crystals with the same structure as that of the single crystal bulk.
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Fig. 3 AFM images, height profiles, and XPS spectra of the 2D sheets: (a), (d), and (g) 2D surfactant crystals, (b), (e), and (h) 2D Pt complexes, and (c), (f), and (i) Pt metal nanosheets. |
Pt metal nanosheets were synthesized via UV-ozone treatment of 2D surfactant crystals and a subsequent reduction process under H2 (5%)/Ar flow. We characterized the samples at each step for clarifying the effects of UV-ozone treatment and reduction process under H2 (5%)/Ar flow. Firstly, the 2D surfactant crystals were treated with UV-ozone to selectively remove the organic species. The other processes, such as calcination at mild conditions, lead to the collapse of the 2D structure (Fig. S3 of the ESI†). A key for maintaining the 2D morphology is the intermediate UV-ozone treatment; the direct reduction of the 2D surfactant crystals leads to the formation of discrete large-sized Pt metal nanoparticles (Fig. S4 of the ESI†). The morphological change by UV-ozone treatment on the 2D Pt complexes was observed by AFM (Fig. 3(b)). The thickness of the 2D Pt complexes was drastically decreased from 49 nm to 4.7 nm with the retention of the surface roughness and lateral size. In addition, the composition and structure of the 2D Pt complexes were evaluated by Raman spectroscopy analysis and XRD measurement. No clear peak attributed to the stretching of C–H groups (2900 cm−1) was observed in the Raman spectrum (Fig. S2(d) in the ESI†), indicating the removal of CTA+. The removal of the surfactant corresponds to the fact that the XRD pattern did not show the peaks derived from the ordered structure (Fig. S2(b) in the ESI†). Notably, the Pt species were partially oxidized via UV-ozone treatment, as detected by the XPS measurement. The peaks derived from Pt2+4f5/2, Pt2+4f7/2, Pt4+4f7/2, and Pt4+4f5/2 were observed, and the ratio of Pt2+/Pt4+ was roughly calculated to be 1.2 (Fig. 3(h)). The mixture of Pt oxide and halide should be formed after UV-ozone treatment considering the broad peaks of Cl 2p3/2. From these results, it was confirmed that the 2D nanosheets composed of the Pt complex without organic species were formed after UV-ozone treatment.
The molecularly thin Pt metal nanosheets were synthesized by reducing the 2D Pt complexes under H2 (5%)/Ar flow. For optimizing the conditions, the reductions were carried out at 120, 200, and 300 °C. The reduction condition of the Pt complex is an important factor, and discrete nanoparticles assemblies were formed at 200 and 300 °C (Fig. S5(a) and (b) of the ESI†). We succeeded in maintaining a nanosheet structure through reduction at 120 °C (Fig. 3(c)). Note that the complete reduction of the Pt species required a long-time treatment because Pt2+ species partially remained when the nanosheets were reduced for 5 h (Fig. S5(c) and (d) of the ESI†). After reduction treatment for 20 h, the asymmetrical peaks attributed to Pt04f5/2 and Pt04f7/2 were mainly obtained, indicating the formation of Pt metal nanosheets (Fig. 3(i)). The AFM image of the sample after hydrogen reduction at 120 °C for 20 h showed a significant decrease in the thickness of the nanosheet from 4.7 to 2.3 nm (Fig. 3(e)). The surface roughness of the nanosheets is almost the same as that of the substrate. In addition, the high-resolution AFM image showed that the surface of the nanosheet was very smooth, although nanoparticles were observed (Fig. S6(a) and (b) of the ESI†). The theoretical thickness calculated by the amount of Pt species in the 2D surfactant crystals with 49 nm thickness should be approximately 0.7 nm considering 21.45 g cm−3 as the density. It is known that the AFM observation of 2D materials often overestimates the thickness due to adsorbed molecules on the nanosheet surface and water adlayers on the substrate. For example, the thickness of the monolayer graphene was evaluated to be 0.7 nm in spite of 0.4 nm being the real thickness because of the water adlayers.28 However, the measured 2.3 nm thickness is too different from the theoretical value (0.7 nm) even if there were adsorbates. Therefore, there should be some voids in the nanosheet structure, which may be due to the interparticle space of polycrystalline continuous Pt metal nanosheets. Although the obtained nanosheets should be composed of polycrystalline Pt metal nanoparticles, such polycrystalline nanosheets also tend to exhibit unique properties unlike nanoparticles and bulks.29 The obtained molecularly thin Pt metal nanosheets with large lateral size and distinct rhombus morphology should be useful for application and the fundamental investigation of their properties.
In addition, the morphologies of the Pt metal nanosheets synthesized using different precursors were investigated. As typical examples, Pt metal nanosheets were also synthesized using H2Pt(IV)Cl6 as a metal complex precursor (Fig. 4(a) and (b)) or CTAC as a surfactant precursor (Fig. S7(e) and (f) in the ESI†). When the metal nanosheets were synthesized using CTAC, the morphology of the nanosheets was not changed from the rhombus type (Fig. S7(g) and (h) in the ESI†). In contrast, interestingly, the use of H2Pt(IV)Cl6 leads to the morphological change of the nanosheet into a hexagon, probably reflecting the crystal habit of the surfactant crystals (Fig. 4). Although the crystal structure of the surfactant is under investigation, there should be a possibility of the control of the morphology through the choice of the precursors. Considering that the previous molecularly thin Pt metal nanosheets have undefined morphology, this method may open a platform for designing the morphology of molecularly thin Pt metal nanosheets.
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Fig. 4 (a) AFM image and (b) height profile of the Pt metal nanosheet synthesized using H2Pt(IV)Cl6 as a metal complex precursor. |
Furthermore, it is known that the thickness of metal nanosheets has a significant impact on their structures and properties, and the sub-nm level thickness control is an important issue.13 Although many methods have been proposed to synthesize Pt metal nanosheets,15,29–39 the thickness and morphology control still remain challenging issues. In order to control the thickness of the molecularly thin nanosheets, we investigated the relationship of thickness between the 2D surfactant crystals and Pt metal nanosheets. The thickness of the nanosheets was monitored at each step using 4 samples with different thicknesses (Fig. S8 in the ESI†). Table 1 shows the thickness variation for each of the four nanosheets. In all cases, the thicknesses were decreased to 10% when the 2D surfactant crystals were changed into 2D Pt complexes. Subsequently, the thickness was decreased to 7–8% when the 2D complexes were reduced to Pt metal nanosheets. Considering the relationship, the thickness of the Pt metal nanosheets could be varied quite finely via the thickness control of the 2D surfactant crystals. This method should have the potential for sub-nm thickness control of Pt metal nanosheets. In addition, this method may also have the potential for increasing the lateral size of nanosheets via the optimization of the 2D surfactant synthesis condition. Pt metal nanosheets with a large lateral size (15 μm) were also synthesized using a coincidently formed large 2D surfactant as a precursor (Fig. S9 in the ESI†). Although the synthetic condition for the 2D surfactant with a large lateral size is not optimized, such large-sized nanosheets have been preferred to investigate the fundamental property of the metal nanosheets.
No. | 2D surfactant crystal | 2D Pt complex | Pt metal nanosheet |
---|---|---|---|
1 | 48 nm | 4.5 nm | 3.0 nm |
2 | 30 nm | 3.0 nm | 2.3 nm |
3 | 29 nm | 2.8 nm | 2.2 nm |
4 | 20 nm | 2.0 nm | 1.5 nm |
We found that the calcination of Pt metal nanosheets at 800 °C suppresses the formation of the large-sized nanoparticles. The calcined nanosheets exhibited high stability against solvents, and no morphological change was observed by immersing the nanosheets into water for 1 day (Fig. 5(a), (b), and (c)). Therefore, we evaluated the structure of the Pt metal nanosheets after calcination. In the calcination process, the thickness of the nanosheets slightly changed from 2.3 nm to 2.0 nm, while the morphology was retained (Fig. S10(a) in the ESI†). The decrease in the thickness should be derived from the removal of adsorbed molecules or the shrinkage of the void volume. The surface of the nanosheets was very flat, and no particle was observed (Fig. S11(b) in the ESI†). The chemical states of the nanosheets were not changed, as shown by the XPS spectrum, which showed only peaks derived from metallic Pt species (Fig. S10(c) in the ESI†). The in-plane XRD measurement probably showed very weak peaks, which are assigned to Pt with an fcc structure (Fig. S10(d) in the ESI†). Although a complete understanding of the structure of the obtained nanosheets awaits further studies, this process may be effective for obtaining stable metal nanosheets with precisely controlled thickness.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2161128. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2nr01807a |
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