Controlling the Reaction of Nanoparticles for Hollow Metal Oxides

ly necessary to deposit precursors of the target metal oxides onto the template ... example, which is a unique metal oxide capable of storing charge a...
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Controlling the Reaction of Nanoparticles for Hollow Metal Oxides Nanostructures Yong-Gang Sun, Jun-Yu Piao, Lin-Lin Hu, De-Shan Bin, Xi-Jie Lin, Shu-Yi Duan, An-Min Cao, and Li-Jun Wan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Controlling the Reaction of Nanoparticles for Hollow Metal Oxides Nanostructures Yong-Gang Sun,†,‡ Jun-Yu Piao,†,‡ Lin-Lin Hu,† De-Shan Bin,†,‡ Xi-Jie Lin,†,‡ Shu-Yi Duan,†,‡ An-Min Cao,*,†,‡ and Li-Jun Wan*,†,‡ †

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China ‡

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

Supporting Information Placeholder ABSTRACT: Hollow nanostructures of metal oxides have found broad applications in different fields. Here, we reported a facile and versatile synthetic protocol to prepare hollow metal oxide nanospheres by modulating the chemical properties in solid nanoparticles. Our synthesis design starts with the precipitation of urea-containing metal oxalate, which is soluble in water but exists as solid nanospheres in ethanol. A controlled particle hydrolysis is achieved through the heating-induced urea decomposition, which transforms the particle composition in an outside-to-inside style: The reaction starts from the surface, and then proceeds inward to gradually form a water-insoluble shell of basic metal oxalate. Such a reaction-induced solubility difference inside nanospheres becomes highly efficient to create hollow structure through a simple water wash process. A following high temperature treatment forms hollow nanospheres of different metal oxides with structural features suited to their applications. For example, high performance anode for Li-ion intercalation pseudocapacitor was demonstrated with the hollow and mesoporous Nb2O5 nanospheres.

Hollow nanostructures of metal oxides (HNMOs) have found broad applications in a large variety of areas including catalysis, photonics, biomedicine, and energy storage.1-3 By controlling the structural features, it became possible for researchers to endow HNMOs with favorable architecture suitable for their applications.4-7 Specifically, in the field of electrochemistry, HNMOs have drawn increasing research attention due to their unique capability to deliver short mass-/charge-transport lengths, abundant reaction sites, and plentiful electrolyte reservoir and buffer space.8-11 These are favorable characters to ensure the structural and mechanical stability of the electrodes, and accordingly promise improved battery performance especially during the fast charge-discharge process. Template-based synthesis routes have witnessed tremendous success to create a large variety of hollow structures.12-17 However, they are known as time-consuming processes while only providing limited quantity and quality of the desired product.18-22 Meanwhile, to replicate the shape, a nanocasting process is usual-

ly necessary to deposit precursors of the target metal oxides onto the template, which are challenging tasks for species involving high charges like Nb5+. These cations are inherently moisture sensitive and their fast hydrolysis will easily fail the nanocasting attempt by forming independent particles.23,24 Therefore, new synthetic protocols based on efficient chemical processes are in high demand to achieve a facile design and functionalization of hollow metal oxide architecture. In this contribution, we demonstrated the possibility of constructing HNMOs directly from their solid precursor through its inner chemical control. We used the synthesis of Nb2O5 as an example, which is a unique metal oxide capable of storing charge at high rate but suffers from poor kinetics caused by its low electrical conductivity.25,26 Here, we firstly prepared solid nanospheres of urea niobium oxalate (UNO) in ethanol, which is soluble in water but will turn insoluble by hydrolyzing UNO into basic niobium oxalate (BNO) in an outside-to-inside style. A simple water wash can remove the UNO core, leaving the BNO behind to form hollow and mesoporous Nb2O5 (HM-Nb2O5) upon further calcination. Such a route for HNMOs is based on a straightforward reaction control of the chemically well-tamed solid nanoparticles, and is found to be very efficient for the production of a large variety of metal oxides. Our preliminary test showed that the prepared HM-Nb2O5 exhibited extraordinary high-rate performance for its application in Li-ion intercalation pseudocapacitor (LIIP). Scheme 1 showed the detailed procedure of our synthesis design. In light of the fast hydrolysis of Nb5+ in water, we switched to a coordination-assisted precipitation in ethanol-based solution. Experimentally, urea oxalate precipitates quickly in ethanol while the oxalate group strongly coordinates with the coexistent metal ions such as Nb5+, Ti4+, Mn2+, and Co2+.27,28 A coordination compound of UNO quickly forms in ethanol (Step 1). Urea is an essential reagent in our synthesis to play a bifunctional role. First, it acts as a constituent chemical to form the nanospheric precursor to be used as the structural scaffold. Second, its gradual decomposition upon heating initiates an intraparticle transition of UNO into BNO with favorable growth kinetics (Step 2): The reaction starts on the particle surface and then gradually proceeds inside

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(Step 3), forming a water-insoluble wall of BNO with the inner UNO remaining unchanged. The following water wash (Step 4) forms hollow BNO, which is transformed into HM-Nb2O5 (Step 5) after heating at 600 oC.

Scheme 1. Synthesis protocol for hollow and mesoporous Nb2O5 nanospheres. Experimental details were shown in supporting information. The initial precipitation was characterized by scanning electron microscopy (SEM). As shown in Figure 1a, nanospheres with size around 450 nm formed after the reaction. The transmission electron microscopy (TEM) analysis (inset in Figure 1a) confirmed the spherical shape and showed that the particles were solid inside. Further characterizations (Figure S1) including X-Ray diffraction (XRD), Fourier-transfer infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and elemental analysis (EA) demonstrated the formation of amorphous UNO as a niobium containing compound from the coordination-assisted precipitation process(Figure S2a). The formed UNO powder is stable in ethanol but will be dissolved in water (Figure S2b).

Figure 1. (a) SEM and TEM images of the precipitate after ethanol wash. (b) TEM image of the precursor collected after reacting for 20 min at 80 oC. The sample is washed by water. (c-d) TEM (c), HRTEM (inset in figure 1c), XRD (d) result of the formed Nb2O5 product. The UNO nanospheres would experience continuous compositional change as the suspension is heated at 80 oC, which are known to induce a gradual decomposition of urea29,30 and accordingly enforced the hydrolysis of the preformed particles.31-34 After reacting for 20 minutes (t=20 min), although the particles seemed almost unchanged in shape and size after an ethanol wash, their dispersion in water forms an obvious suspension as shown in Figure S3. TEM analysis showed that they were still 450 nm nanospheres but became hollow with a 50 nm shell (Figure 1b). We found that the shell thickness gradually turned thicker (Figure S4a) as the reaction continued. Finally solid particles formed when the reaction time exceeds 2 hours (t=120 min, Figure S4b). For the prepared hollow precursor, a calcination process at 600 o C produced final metal oxide. The TEM characterization (Figure 1c) showed that the hollow feature was well preserved. The parti-

cle will shrink to around 300 nm with a shell thickness of 45 nm, while the particle wall becomes mesoporous from the high temperature treatment (Figure S5), probably due to the rearrangement after crystallization (Figure S6). XRD pattern (Figure 1d) confirmed the formation of a pure orthorhombic phase of Nb2O5 (JCPDS No. 30–0873),35,36 which was in good agreement with the high-resolution TEM (HRTEM) analysis on a randomly-selected particle (inset in Figure 1c). It is interesting that our synthesis for HM-Nb2O5 simply relies on the difference of water solubility in a solid precursor, and was able to provide a reliable tool to construct hollow structure without tedious operations. We therefore did further characterizations to probe the chemical change inside the preformed UNO nanospheres. The particles collected at different reaction stages were washed by ethanol and then characterized by X-ray photoelectron spectroscopy (XPS). Special focus has been paid to the O1s peak since its signal is sensitive to the change of those functional groups connected to oxygen inside the coordination compound. Specifically, as the reaction continued, we observed a continuous increase of the Nb-O (530.2 eV)37 at the expense of the peak from O=C-N (533.2 eV)38, showing a characteristic hydrolysis process along with the decomposition of urea in the UNO particles. Meanwhile, we managed to collect the inner component of the reacted particles (t=20 min) and compare its FT-IR spectrum to the outside counterpart. The inner part showed a similar element content and an almost identical pattern to that of the original UNO particles with characteristic signal from urea (Figure S7), which are mainly amide-related peaks located at 1641 cm-1 (–C=O amide stretching), 1562 cm-1 (–NH bending), and 1149 cm-1 (C–N stretching), respectively.38 For comparison, all these peaks almost vanished in the pattern of the outer shell, revealing a complete decomposition of the urea component. An elemental analysis (Figure S8) confirmed the disappearance of urea in the collected shell of BNO, which was measured to have a chemical formula close to NbC2O4(OH)3, a partially-hydrolyzed compound as basic niobium oxalate.

Figure 2. (a) XPS analysis for the samples collected at different reaction time. (b) FT-IR spectra of the inner part and outer shell of the sample collected after 20 minute’s reaction. Our experimental observation strongly suggests a progressive inward transition from UNO to BNO. It is also reasonable to expect that the rapid decomposition of urea is essential for the UNO transition. Our control test clearly showed a close relationship between the reaction temperature and the surface transition process (Figure S9). Instead of working directly on the highly-active metal ions like Nb5+, we switched to a solid precursor whose compositional chemistry is well designed to easily achieve a desired inner compositional inhomogeneity suited to the creation of hollow structure. We found that such a synthetic protocol could be easily expanded to other metal ions to form HNMOs with either single metal ions or multi-metal ones. For example, TiO2 hollow nanospheres can be easily prepared by simply changing the reactant from NbCl5 into TiCl4 (Figure S9a). Meanwhile, this synthesis

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route is also suitable for the preparation of multi-compositional HM-metal oxides such as Ti-Nb, Nb-Mn, and Nb-Co (Figure S1115). The described synthetic protocol for HNMOs promised enormous implications for their applications in different areas including energy storage process, catalysis, and sensors. Here we demonstrated the first example of potential application by using the prepared HM-Nb2O5 as a possible anode material in LIIP. The hollow structure has been considered to be a favorable shape character to alleviate the kinetics issue related to Nb2O5 due to the advantage in high specific surface, good access for electrolytes and short diffusion paths for both ions and electrons.5 Figure 3a showed galvanostatic charge−discharge voltage profiles tested at different current densities. The HM-Nb2O5 was able to deliver an unprecedentedly-high reversible capacity of 195 mAh/g at 1C, a value close to its theoretical value of 200 mAh/g. The capacity remained at 125 mAh/g at 50 C, which showed an extraordinary rate capability of the structure-controlled Nb2O5 compared to control samples of bulk Nb2O5. (See SI for experimental details, sample was showed in Figure S16-17, denoted as B-Nb2O5.) A detailed comparison on the rate capability is shown in Figure 3b, showing the advantage of the HM-Nb2O5 sample as a high performance LIIP anode especially at high rate. The cyclic voltammetry (CV, Figure 3c) tests at different sweep rates showed broad cathodic and anodic peaks, which are characteristic pseudocapacitive features related to the two-dimensional Li+ transport within Nb2O5 crystals.35,36 A detailed analysis on the CV result (Figure 3d) revealed that the capacitance of the HM-Nb2O5 anode was mainly contributed by a capacitive mechanism rather than the diffusion-limited one, which explains well its outstanding rate capability compared to the control sample. The electrochemical impedance spectrum (EIS) tests and their fitted curves confirmed a much lower charge transfer and surface film resistance (Rct) in the HM-Nb2O5 anode (Figure 3e). It is noted that the HM-Nb2O5 electrodes are very stable in cycling. A high reversible capacity of 168 mAh/g at 1 C was achieved after 2000 cycles with a capacity retention ratio of 86%. A detailed comparison of our result to those recently published ones is listed in Figure S18. It is clear that our hollow and mesoporous sample showed advantage in high rate performance.

In conclusion, we reported an effective synthetic protocol for the creation of hollow metal oxides by modulating the chemical inhomogeneity inside solid nanospheres. Using Nb2O5 as an example, we firstly formed a precursor of urea niobium oxalate from a simple precipitation process, whose compositional chemistry is well designed for a tamed hydrolysis process: The gradual urea decomposition will result in a progressive inward structural transformation of the particle, which creates a desired inner compositional inhomogeneity suited to the creation of hollow structure. Our investigation of the structure evolution and formation mechanism of precursor particles provided a reliable tool to build a large variety of hollow metal oxides. Our preliminary results show that the prepared HM-Nb2O5 showed much improved electrochemical performance when used as an anode material in the Li-ion intercalation pseudocapacitor.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Experimental details and supporting figures (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No 51672282), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010101).

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Figure 3. Electrochemical performance of Nb2O5/Li half-cells. (a) Galvanostatic charge-discharge curves of HM-Nb2O5 electrodes and (b) rate capability test for B-Nb2O5 and HM-Nb2O5 electrodes (1 to 50 C). (c) CV curves of HM-Nb2O5 electrode at various sweep rates from 0.2 to 2.0 mV/s. (d) Capacitive contribution of B-Nb2O5 and HM-Nb2O5 electrodes with separation between total capacity (solid line) and capacitive capacity (shaded regions) at various sweep rates. (e) Nyquist plots in a frequency range of 100 kHz to 0.1 Hz and (f) cyclability tests at 1 C for B-Nb2O5 and HM-Nb2O5 electrodes.

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