Synthesis and formation mechanism of VO₂(A) nanoplates with intrinsic peroxidase-like activity

Abstract

Monocrystalline VO₂(A) nanoplates have been synthesized via a one-pot hydrothermal process. In situ powder X-ray diffraction was used to monitor the hydrothermal synthesis and it was found that VO₂(A) nucleates and grows directly from solution after the complete hydrolysis of a 2.0 M VO(acac)₂ precursor solution, rather than involving a previously reported intermediate phase VO₂(B). A hydrating–exfoliating–splitting mechanism was established to explain the formation of the nanoplate architecture. The synthesized VO₂(A) nanoplates showed outstanding peroxidase-like activity and hence are a promising candidate for artificial peroxidase.

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1. Introduction

Vanadium oxides and their metastable structures have received considerable attention because of their diverse structures and novel physico-chemical properties. Up to now, several crystalline phases of VO_x and V–O hydrates with oxidation states from +2 to +5 have been reported, including monoclinic VO,¹ V₂O₃,² VO₂(M),³ rutile VO₂(R),⁴ tetragonal VO₂(A),⁵ monoclinic VO₂(B),⁶ VO₂(C),⁷ VO₂(D),⁸ paramontroseite VO₂(P),⁹ V₃O₇·H₂O,¹⁰ VO₂·0.5H₂O,¹¹ V₂O₄·0.25H₂O,¹² haggite V₄O₆(OH)₄,¹³ and VOOH (montroseite, orthorhombic and hollandite-type phases).^14–16 Among them, monoclinic VO₂(M) (space group P2₁/c) and tetragonal VO₂(R) (P4₂/mnm) undergo reversible phase transformation at ∼67 °C, accompanied by dramatic changes in electrical resistance and optical transmittance.¹⁷ These properties enable a wide range of applications in smart windows, gas sensors, optoelectronic switching devices, Mott field-effect transistors, for example.¹⁸ Another thermodynamically metastable polymorph VO₂(B) (C2/m) is the most common phase formed from solution synthesis. It consists of layers of distorted corner and edge-sharing VO₆ octahedra which form a tunnel structure. It has also attracted much interest in the last few years owning to its proper work potential as well as its high energy capacity when used as electrode materials in aqueous Li-ion batteries (LIBs).¹⁹ As is well known, the traditional intercalation compound VO₂(A) shows similar layered structure as VO₂(B) except for the distorted VO₆ octahedra in VO₂(B) and different stack mode between them. However, research on other metastable phases, such as layered VO₂(A), has been rarely reported.

Théobald first discovered the existence of VO₂(A) as an intermediate phase in a study of hydrothermal reaction in the V₂O₃–V₂O₅–H₂O system.²⁰ Later, the crystal structure and phase transition were thoroughly investigated by Oka.^21,22 More recently, various VO₂(A) nanostructures have been successfully fabricated. For example, Ji and coworkers selectively prepared VO₂(A) by controlling the pressure during the hydrothermal synthesis.²³ Li and coworkers reported the ultra-long VO₂(A) nanobelts synthesized hydrothermally using V₂O₅ sol as precursor and polyethylene glycol as both surfactant and reducing agent.²⁴ Xie et al. reported the large-scale fabrication of VO₂(A) nanobelts by simple hydrolysis of VO(acac)₂ and observed an intermediate phase VO₂(B).²⁵ In addition, Dai et al. prepared 1D VO₂(A) nanostructures via a one-step hydrothermal method using VOSO₄ and NH₃·H₂O as precursors; the prepared material was used as a high performance cathode in LIBs.²⁶

Much effort has been put into developing synthetic strategies for the fabrication of VO₂ nanostructures, using methods such as sol–gel, templating, ion implantation, precursor pyrolysis, hydrothermal and solvothermal syntheses, magnetron sputtering and gas-phase deposition.^27–31 These methods generally produced VO₂(R) or VO₂(B), but the metastable VO₂(A) was only obtained by the hydrothermal method. Therefore, hydrothermal synthesis is considered to be the most favorable and flexible method for the synthesis of phase pure VO₂(A) as well as being scalable, simple in concept, and environmentally benign. However, hydrothermal synthesis can be very complex mechanistically, as several metastable and stable hydrates and nonhydrate oxides may form upon rapid heating to high temperature in a pressurized autoclave.³² The abundance of phases and complexity of the hydrothermal phase diagram make it difficult for the formation and stabilization of each specific phase under specified conditions. It is not surprising that VO₂(A) was often reported as the intermediate phase during transition from the metastable monoclinic VO₂(B) to the more stable tetragonal rutile VO₂(R) phase, with changes in temperature, pressure, and time. This leads to an ambiguous understanding of the transformation between VO₂ polymorphs and therefore, the formation mechanism of VO₂(A).^25,33,34 Hence, it is still a great challenge to synthesize phase pure VO₂(A).

In this paper, we have investigated the facile synthesis of VO₂(A) monocrystalline nanoplates via a one-step hydrothermal process by the hydrolysis of a high concentrated (2.0 M) VO(acac)₂ solution. The formation mechanism of the nanoplates has been characterized by both ex situ characterization and in situ powder X-ray diffraction (PXRD). In addition, the activity of the nanoplates as a novel biomimetic catalyst for the oxidation of the substrate 3,3′,5,5′-tetramethylbenzidine dihydrochloride (TMB) in the presence of H₂O₂ has been determined.

2. Experimental section

2.1 Materials

Vanadyl acetylacetonate (VO(acac)₂), sodium acetate (CH₃COONa), H₂O₂ (30 wt%), and 3,3,5,5-tetramethylbenzidine dihydrochloride (TMB) were purchased from Aladin Ltd. (Shanghai, China). All chemicals used in this study were commercially available analytical grade and used without further purification.

2.2 Synthesis

VO₂(A) nanoplates were synthesized by a facile hydrothermal method using VO(acac)₂ as the vanadium source. In a typical synthesis, 30 mmol (7.96 g) of VO(acac)₂ was dispersed in 15 mL deionized water and then the mixed suspension was transferred into a 25 mL PTFE-sealed autoclave. The autoclave was heated to 220 °C and kept at that temperature for 24 h before it was cooled to room temperature in air. The resulting precipitate was filtered, washed with distilled water and ethanol three times, and finally dried at 60 °C under vacuum overnight.

2.3 Ex situ characterization

The PXRD patterns were recorded on a Rigaku D/Max-RB X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). A small amount of products were ultrasonically dispersed in ethanol and deposited on copper grids for transmission electron microscopy (TEM, JEM-2010F, JEOL) measurements. Field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL) was employed to examine the morphology of the products. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a VG ESCALAB MK II system equipped with Al KR radiation as the X-ray source. The measurements were carried out in an ultrahigh vacuum (UHV) chamber. The carbonaceous C_1s line (284.5 eV) was used as the reference for the calibration of the binding energies.

2.4 In situ PXRD

One in situ PXRD experiment was conducted on the powder diffraction beamline at the Australian Synchrotron and two laboratory-based in situ PXRD experiments were carried out using an Inel EQUINOX 3000 instrument at CSIRO Mineral Resources Flagship. For the synchrotron-based in situ PXRD experiment, the X-ray wavelength (0.6889 Å) was calibrated using a LaB₆ standard (NIST SRM 660b). For the laboratory-based in situ PXRD, a Mo Kα radiation (λ = 0.7093 Å) was used. The same sample presentation setup was used in both synchrotron- and laboratory-based in situ experiments and was described in detail elsewhere.^35–37 The starting precursor (5.4 mg VO(acac)₂ and 10.7 μL Milli-Q water) was injected into a quartz glass capillary (1 mm in diameter, 0.1 mm in wall thickness, and 40 mm in length), which was then fitted to a custom-made stainless steel holder. The precursor-containing capillary/stainless steel holder was then attached to the goniometer head of the diffractometer and heated to the synthesis temperature 220 °C by a hot air blower beneath the capillary. External N₂ pressure (3 MPa) was applied to the capillary during the synthesis to prevent vaporization of the solvent. For the synchrotron-based in situ experiments, the heating rate was 5 °C min⁻¹, which is similar to the heating rate of a laboratory scale autoclave. For the lab-based in situ experiments, faster (30 °C min⁻¹) and slower (1 °C min⁻¹) heating rates were used to study the effect of heating rate on the syntheses mechanism. The temperature was monitored by a K-type thermocouple 3.5 mm beneath the capillary. In situ PXRD patterns were collected simultaneously during the synthesis with a time resolution of ∼2 min. The capillary was oscillated continuously during the measurements, which ensured temperature homogeneity and minimized potential effects of preferred orientation. For the lab-based in situ experiments, a surveillance camera was also used for monitoring visual appearance of the contents of the capillary reaction vessel as the synthesis reactions progressed.

2.5 Peroxidase-like activity of the as-synthesized VO₂(A) nanoplates

The peroxidase-like activity of VO₂(A) nanoplates was evaluated as follows: 30 μL VO₂(A) dispersion (0.3 mg mL⁻¹) was mixed with 3 mL CH₃COONa buffer solution (0.1 M, pH 4.0) containing 100 μM TMB and 5 mM H₂O₂. The mixture was then measured in wavelength-scan mode or time-scan mode by monitoring the absorbance change of TMB at 652 nm for 600 s on a UH4150 UV-visible spectrophotometer at 25 °C.

The peroxidase-like reaction,³⁸ catalyzed by VO₂(A) nanoplates, can be described as follows:

The kinetic data were obtained under the optimum conditions by changing the concentrations of TMB and keeping the concentrations of H₂O₂ constant, or vice versa. The apparent steady-state reaction rates of the VO₂(A) nanoplates were deduced according to their absorbance data and the molar absorption coefficient of TMB-derived oxidation products (ε = 39 000 M⁻¹ cm⁻¹). The dynamics parameters (V_max and K_m) were determined using the Michaelis–Menten equation and a Lineweaver–Burk plot:

where v, [S], K_m and V_max stands for the initial velocity, substrate concentration, the Michaelis–Menten constant and the maximal reaction velocity, respectively.^38,51

3. Results and discussion

3.1 Structure and morphology

Single crystalline tetragonal VO₂(A) phase nanoplates were synthesized on a large scale under mild hydrothermal condition by the hydrolysis of a 2.0 M VO(acac)₂ solution at 220 °C for 24 h. The phase identity of the product was confirmed by PXRD (Fig. 1), where the reflections match well with those of metastable tetragonal VO₂(A) (space group: P4/ncc, ICDD database no. 70-2716). The absence of any other phases suggests the synthesis of pure VO₂(A) phase. The chemical state and the compositions of the VO₂(A) nanoplates were also confirmed by XPS, as shown in Fig. S1.†

Fig. 1 Ex situ powder X-ray diffraction pattern collected for the VO₂(A) nanoplates prepared at 220 °C for 24 h.

The morphology of the VO₂(A) nanoplates can be observed in the TEM and SEM images shown in Fig. 2. Fig. 2a displays a representative SEM image of the high yield VO₂(A) materials. The high-magnification SEM image (Fig. 2b) distinctly shows that the VO₂(A) nanostructures are composed of uniformly plate-like nanoparticles with average width measuring ∼30–50 nm, length of 100–200 nm and thickness of 20–30 nm, respectively. The TEM image shown in Fig. 2c also confirms the plate-like morphology which is typically 20–40 nm wide and 100–200 nm long, in agreement with SEM observation. The high-resolution TEM (HRTEM) image (Fig. 2d) demonstrates the well-resolved lattice fringes with an interlayer spacing of about 0.60 nm, corresponding to the (110) lattice plane of VO₂(A). Selected-area electron diffraction (SAED) pattern (inset of Fig. 2c) further validates the single crystalline nature of a single nanoplate.

Fig. 2 Typical FESEM (a and b) and TEM (c) images of the VO₂ (A) nanoplates. The inset SAED pattern is from the red circle area. (d) HRTEM image of individual VO₂ nanoplates showing the lattice fringes.

3.2 Ex situ characterization of the samples synthesized under different conditions

In order to investigate the growth processes of VO₂(A) tetragonal crystalline nanoplates, a series of experiments were conducted to track the evolution of morphologies and phases as a function of temperature and time by FESEM and PXRD. Temperature played an important role in controlling the size, shape, and crystal structures of the nanoparticles. By varying the hydrothermal synthesis temperature, the nanoparticles exhibited distinctly different shapes and morphologies, which is consistent with the evolution of crystalline structures. The PXRD patterns collected for the samples hydrothermally treated at 140 °C match well with pure V₂O₄·2H₂O³⁹ (ICDD database no. 13-0346) (Fig. 3a). All the peaks were the characteristic structures of layered hydrates of V₂O₄·2H₂O particles.⁴⁰ This phase was found to be stable when temperature was increased to 160 °C. Their solution synthesis and thermal stability have been previously fully analyzed by our group.³⁹ Bunches of microrods (1–8 μm in length and 0.5–1 μm in diameter) consisting of tiny nanoparticle nuclei have been observed at 140 and 160 °C, as shown in the insets of Fig. 4a and b, respectively. As the temperature increased to 170 °C, the PXRD pattern shows the coexistence of V₂O₄·2H₂O and VO₂(A) (Fig. 3c). Correspondingly, some sheet-like VO₂(A) nanoparticles were produced by consuming surrounding V₂O₄·2H₂O microrods (Fig. 4c). However, when the temperature was further increased up to 180 °C, VO₂(A) started to form slowly (Fig. 3d). The intensities of the VO₂(A) phase reflections increase continuously with elevated reaction temperatures in Fig. 3. The crystalline sizes were estimated from the (110) diffraction peak at 2θ = 14.7° using the Scherrer formula. They were calculated to be 15.0 nm for 180 °C-24 h sample, 13.2 nm for 200 °C-10 h sample, 14.8 nm for 200 °C-24 h sample and 19.2 nm for 220 °C-24 h sample, respectively. At the same time, irregular nanosheets stacked randomly and exhibited a well-dispersed morphology (Fig. 4d–f). VO₂(A) ordered nanoplates formed only when the temperature was raised to 220 °C (Fig. 4g), indicating that higher reaction temperature is beneficial for increasing crystallization rate and for uniformity of morphology of the particles.

Fig. 3 PXRD patterns of the samples synthesized at different temperatures for 24 h. (a) 140 °C, (b) 160 °C, (c) 170 °C, (d) 180 °C, (e) 200 °C-10 h, (f) 200 °C, (g) 220 °C. The ICDD patterns for V₂O₄·2H₂O and VO₂(A) are also shown. Patterns have been offset in the intensity axis for clarity.

Fig. 4 SEM images of the samples synthesized at different temperatures for 24 h. (a) 140 °C, (b) 160 °C, (c) 170 °C, (d) 180 °C, (e) 200 °C-10 h, (f) 200 °C, (g) 220 °C.

To elucidate in depth the formation mechanism of the plate-like VO₂(A) architecture, time-dependent hydrothermal synthesis experiments were carried out at 220 °C. The ex situ PXRD patterns of the phase evolution of the nanocrystals as a function of time are presented in Fig. 5 and S2a.† All of the reflections match well with the ICDD pattern for pure VO₂(A) with tetragonal symmetry (no. 70-2716), and no intermediate phases or impurities were observed. The absence of intermediate phases will be further verified by in situ PXRD experiments (to be discussed later). We also observed that the phase abundance and crystallite size increase with increasing synthesis time from 1 to 24 h, as the peaks became higher and narrower with time. The crystalline sizes estimated from the (110) diffraction peak are 9.5 nm, 12.5 nm, 14.3 nm, 14.5 nm, 16.2 nm and 19.2 nm for the reactions carried out for 1, 3, 6, 10, 18 and 24 h, respectively.

Fig. 5 PXRD patterns collected for the products of hydrothermal synthesis at 220 °C for various times: (a) 1 h; (b) 3 h; (c) 6 h; (d) 10 h; (e) 18 h; (f) 24 h. The ICDD patterns for VO₂(A) is also shown. Patterns have been offset in the intensity axis for clarity.

When the synthesis time exceeds 30 min, single phase of VO₂(A) nanosheets assembled from nanowires were obtained (as shown in Fig. S2†). After synthesis for 1 h and 3 h, 3D urchin-like microspheres having a diameter of ∼2–5 μm were formed (Fig. 6a and c). The hierarchical urchin-like superstructures are composed of radially aligned nanobelts having a thickness of ∼30 nm, width in the range of 100–200 nm and length up to 100 nm (Fig. 6b and d). More interestingly, exfoliation from the surface of the layer associated with splitting to narrow plates can be clearly observed (highlighted by red circles in Fig. 6b). It is likely that the existence of intrinsic stress and surface energy caused the rolling up of the nanosheets and subsequent exfoliation and splitting. When the reaction time was prolonged to 10 h and above, the sheets were cleaved into well aligned plate-shaped building blocks, as demonstrated in Fig. 6f–h.

Fig. 6 SEM images of samples synthesized hydrothermally at 220 °C for various reaction times. (a and b) 1 h, (c and d) 3 h, (e) 6 h, (f) 10 h, (g) 18 h, (h) 24 h.

3.3 In situ characterization of the formation of VO₂(A) phase

The poor understanding of most hydrothermal syntheses is largely due to the challenge of carrying out in situ and real time characterization, because the high temperature and high pressure autoclave condition is not accessible to most characterization techniques. By application of the in situ PXRD technique for tracking hydrothermal and solvothermal syntheses, we have the opportunity of unveiling the ‘black box’ of hydrothermal syntheses and elucidating unambiguous synthesis mechanisms, similar to which has been achieved in recent studies.^35–37,41 Fig. 7 shows the results of the synchrotron-based in situ PXRD experiments, with the accumulated time-resolved PXRD patterns viewed down the intensity axis. In this synthesis, a heating rate of 5 °C min⁻¹ was used, very close to the heating scheme of laboratory autoclaves. It is clearly shown that hydrolysis of VO(acac)₂ occurred at around 170 °C as all diffraction peaks of VO(acac)₂ disappeared. Immediately following this, diffraction peaks of this phase appear at the same time there was a sharp increase in the intensity of small angle X-ray scattering (SAXS) tail at low angle below 1.5°. The appearance of the SAXS tail was most likely due to the formation of a great number of nuclei of VO₂(A). Subsequently, the intensity of the VO₂(A) diffraction peaks increased with time while the intensity of the SAXS tail dropped. The dropping of the SAXS tail suggests the moving of scattering to lower angles which means the formation of larger crystals. This is in agreement with the increasing sharpness of the diffraction peaks. However, a complete understanding of the SAXS feature requires a separate study using a dedicated high resolution SAXS instrument which is capable of covering of much wider q range. No other phases were observed. Hence, the synthesis of VO₂(A) follows a direct crystallization mechanism, in which VO₂(A) nucleates and subsequent grows from a supersaturated solution formed by hydrolysis of VO(acac)₂.

Fig. 7 Accumulated time-resolved synchrotron-based in situ PXRD patterns (viewing down the intensity axis) collected during the synthesis of VO₂(A) nanoplates at 220 °C with a heating rate of 5 °C min⁻¹.

To study the effect of heating rate on the synthesis mechanism, another two laboratory-based in situ PXRD experiments were carried out using a heating rate of 1 °C min⁻¹ (slow heating) and 30 °C min⁻¹ (fast heating). The results are shown in Fig. S3–S5.† It was found that varying heating rate had no effect on the synthesis mechanism. VO₂(A) nucleated and grew directly from solution following the complete hydrolysis of VO(acac)₂ with no other intermediate phases observed. However, the laboratory in situ PXRD experiments provided more information from the colour change of the reaction mixture observed by the surveillance camera (Fig. S4†). For the slow heating experiment, the starting greenish blue precursor started to turn dark at 99 °C, becoming darker until turning black at 205 °C (Fig. S4†). This phenomenon suggests that VO₂(A) nucleation may commence below 170 °C with the minor amount and poor crystalline nature of the nuclei possibly explaining why they were not detected using PXRD. These nucleation events were not detected by in situ PXRD though possibly due to the minor amount and poor crystalline nature of the nuclei.

In previous studies,^25,42,43 VO₂(B) was regarded as the inevitable intermediate product in the initial stage of hydrolysis of VO(acac)₂ under mild hydrothermal condition. Usually, its formation was assigned to the condensation of [VO(H₂O)₅]²⁺ complex ions and subsequent formation of vertex and edge sharing VO₆ octahedra. However, this intermediate VO₂(B) phase was not observed in the in situ PXRD experiments performed here. This may be because the precursor concentration (2.0 M) used here was dozens of times higher than the precursors used in previous studies. The hydrolysis of the higher concentration precursor resulted in a very high concentration of dissolved vanadium species (e.g., [VO(H₂O)₅]²⁺) that directly triggered nucleation and subsequent growth of the thermodynamically more stable phase VO₂(A) rather than the metastable VO₂(B) phase.²⁵ This is in agreement with the selective formation of VO₂ polymorphs being greatly dependent on synthesis parameters including pressure, temperature, pH value, and precursor concentrations.²³

Based on the above results, the formation mechanism of plate-shaped VO₂(A) can be described as follows: (a) VO(acac)₂ hydrolyzes to form [VO(H₂O)₅]²⁺ complex ions and then condenses to vertex and edge sharing VO₆ octahedra; (b) direct nucleation of VO₂(A) and growth of nanosheets with a layered structure; (c) plate-like nanostructures formation due to the exfoliation and splitting of the nanosheets in order to reduce system energy. Scheme 1 summarizes the hydrating–exfoliating–splitting process during the formation of the nanoplates. This mechanism was also proposed to elucidate the formation of other nanostructures. For example, it is observed in diverse cases such as the exfoliation formation of vanadium oxide (VO₂(B),⁴⁴ V₂O₄·2H₂O,³⁹ V₂O₄·0.25H₂O,⁴⁰ V₃O₇·H₂O⁴⁵), TiO₂,⁴⁶ MnO₂,⁴⁷ and Ag nanowires.⁴⁸

Scheme 1 Schematic illustration of the proposed formation mechanism of the VO₂(A) nanoplates.

3.4 Peroxidase-like activity

To investigate the peroxidase-like activity of the as-prepared VO₂(A) nanoplates, experiments of catalytic oxidation of chromogenic substrate TMB were conducted in the presence or absence of H₂O₂. The peroxidase-like activity was evaluated based on the intensity of the absorbance peak centered at 652 nm produced by the blue oxidation product (oxTMB). In the control experiments, mixtures of TMB + VO₂ and H₂O₂ + VO₂ only showed negligible adsorption, and mixture of TMB + H₂O₂ showed no absorptions in the scan range 400–800 nm (Fig. 8a). By contrast, after adding VO₂(A) into the TMB + H₂O₂ solution, the clear solution turned to blue and meanwhile a strong absorption peak at 652 nm were observed (Fig. 8a). These observations clearly confirm the outstanding peroxidase-like catalytic activity of VO₂(A) nanoplates toward the oxidation reaction between TMB and H₂O₂. Similar to the natural enzyme (horseradish peroxidase, HRP) and most artificial nanozymes, catalytic oxidation of TMB by VO₂(A) nanoplates is highly dependent on the concentration of VO₂(A) and pH, as shown in Fig. 8b–d. Therefore, the subsequent experiments were performed at the optimal VO₂ concentration of 3.0 μg mL⁻¹ and at pH 4.0.

Fig. 8 (a) UV-vis spectra of TMB solutions. 20 μL TMB (15 mM), 10 μL of H₂O₂ (1.5 M), 3 mL CH₃COONa–CH₃COOH buffer solution (pH = 4.0), and 30 μL VO₂(A) suspension liquid (0.3 mg mL⁻¹) were used. (b) Dependence of the peroxidase-like activity on the concentrations of VO₂(A) suspension and (c and d) dependence of the peroxidase-like activity on pH.

The apparent kinetic parameters (the Michaelis constant K_m and the maximal reaction velocity V_max) were calculated based on a series of experiments carried out at 25 °C by varying one substrate concentration each time and keeping the other constant. Typical Michaelis–Menten curves were obtained as shown in Fig. S6.† The K_m and V_max parameters were obtained using Lineweaver–Burk double reciprocal plot and their comparison with other nano-inorganic peroxidase mimics is given in Table 1. Remarkably, the K_m value (0.058 mM) of VO₂(A) nanoplates with H₂O₂ as substrate is significantly lower than that of HRP (3.7 mM), and even much lower than other nanoenzyme mimics, suggesting that VO₂(A) nanoplates has a higher affinity to H₂O₂ than HRP and other peroxidase mimics. The higher performance of VO₂(A) may be attributed to the more “active site” on the surface of the VO₂(A) nanoarchitecture when compared with HRP.^38,55 In addition, K_m of VO₂(A) nanoplates with TMB as the substrate was about 50% lower than that of HRP, while being larger relative to some other nanoparticles enzyme mimics. This result indicates that a higher TMB concentration is required to achieve maximum activity of VO₂(A) nanoplates.

Table 1 Comparison of apparent kinetic parameters among natural and artificial enzymes

Catalyst	Km (mM)		Vmax (10^−8 M s^−1)		Ref.
	TMB	H2O2	TMB	H2O2
VO2(A) nanoplates	0.165	0.058	2.4	1.4	This work
HRP	0.434	3.7	10	8.7	49
Ni(OH)2 NFs	0.023	1.76	1.4	1.2	50
NiO NFs	0.018	1.77	2.4	2.1	50
VO2(B) nanobelts	0.146	1.69	131	177	51
GO-COOH	0.0237 ± 0.001	3.99 ± 0.67	3.45 ± 0.31	3.85 ± 0.22	52
TiO2@CeOx	0.30 ± 0.04	1.39 ± 0.15	12.0 ± 0.6	55 ± 5.0	53
Sisal-like Co3O4	0.01513	0.8268			38
rGO–CFs	0.046 ± 0.001	14.72 ± 2.332	1.121 ± 0.007	21.71 ± 3.445	54

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4. Conclusions

In summary, plate-like VO₂(A) nanostructures were successfully synthesized by a facile, template-free and scalable hydrothermal synthesis. The VO₂(A) nanoparticles consist of uniform nanoplates of 30–50 nm in width, 100–200 nm in length, and 20–30 nm in thickness. The formation of VO₂(A) follows a direct crystallization mechanism as no intermediate phases were observed in both ex situ and in situ PXRD-based characterization. Based on the time-dependent experimental results, a hydrating–exfoliating–splitting model was proposed to describe the formation mechanism of the nanoplate structure. The produced VO₂(A) materials showed outstanding peroxidase-like activity and may find potential applications in biotechnology.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (51172265), the Ministry of Science and Technology of China (2014AA032802), the Education Commission of Shanghai Municipal (14ZZ099), the Materials Genome Institute of Shanghai University (14DZ2261200) and the China Postdoctoral Science and Foundation (2014M561528). This research was partially undertaken on the powder diffraction beamline at the Australian Synchrotron, Victoria, Australia, through the Science and Industry Endowment Fund Special Research Program-Synchrotron Science. We thank Mr Jingchao Song for assistance with synchrotron data collection, and Dr Helen Brand and Dr Justin Kimpton for beamline setup. The CSIRO Office of the Chief Executive (OCE) Postdoctoral Fellowship is also acknowledged for financial support.

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Footnotes

† Electronic supplementary information (ESI) available: XPS spectra of the samples, PXRD patterns and HRTEM images of the products obtained at 220 °C for 30 min, images captured by the surveillance camera at in situ experiment and details about enzyme assays (double reciprocal plots). See DOI: 10.1039/c5ra11014a

‡ These authors contributed equally to this work.

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