Metal powder–pure water system for rational synthesis of metal oxide functional nanomaterials: a general, facile and green synthetic approach

Abstract

Metal oxide (MO) nanomaterials have played a pivotal role in many fields. The general, facile and green approach for rational synthesis of MO nanomaterials is highly desirable. In this work, a simple approach via an ultrasonic-assisted preoxidation and a subsequent hydrothermal oxidation (UAPO–HO) of metal powders directly in pure water without using any other chemicals has been developed as a general synthetic route to prepare MO nanomaterials. Three representative MO nanomaterials (Mn₃O₄ nanorods, ZnO nanopellets, and Fe₃O₄ nanocubes) have been successfully synthesized by this UAPO–HO approach for the first time. The properties of the newly synthesized MO nanomaterials, such like Mn₃O₄ as an electrode material for supercapacitors, ZnO as an photocatalyst for degrading organic pollutants, and Fe₃O₄ as a magnetic catalyst for disposing antibiotics, are investigated, which demonstrate attractive performance in energy storage and environmental protection. The synthetic approach developed here has the significant advantages of being chemical-utility least, product-purity high, facile, green and mild, which offers a unique clue for synthesis of MO nanomaterials.

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1. Introduction

Due to the gradual depletion of fossil fuel reserves and the increasingly serious environmental pollution, synthesis of sustainable materials and development of the correlated fabrication methods for energy conversion and environmental protection are highly desirable.^1,2 Metal oxide (MO) nanomaterials, such as manganese oxide, zinc oxide, iron oxide, and so on, have received a great deal of attention because of their promising applications as electrode materials in energy conversion and (photo)catalysts in environmental protection.^3–10 A few solution-based methods, such as chemical bath deposition, hydrothermal, solvothermal and etc., have been developed to successfully synthesize the functional MO nanomaterials.^11–20 Those methods mainly utilize the chemical reactions among a variety of chemical substances and the different solubility of solvents and solutes to obtain MO precursors.²¹ To further obtain the target MO nanomaterials, some tedious post-treatments, such as centrifugating, purifying, calcining and etc., are requisite. Meanwhile, the residuals and by-products during the synthetic process are needed suitable disposals. Obviously, the solution-based methods for synthesis of functional MO nanomaterials have the limitations of utility of large amount of chemical reagents, multi-step and tedious synthetic operations, low purity of the products and heavy environmental impacts, and etc.

As well known, pure water is non-toxic and abundant, which is an ideal substance.²² However, it has little activity under common conditions and is seldom used as a reactant. Our recent investigations indicate that under hydrothermal condition pure water has high activity and can react with metals to form MO nanostructures on aluminum, magnesium, copper, and zinc substrates,^23,24 which inspires us to extend this metal–water hydrothermal route to rationally synthesize MO nanomaterials powders by using metal powders as metal sources other than a metal substrate. This metal powder–pure water hydrothermal system is hopeful to overcome the aforementioned limitations that most solution-based methods have, and significantly decrease the utilities of chemical reagents, greatly simplifies the synthetic operations, and guarantees the high purity of the products and the environmental-benignity, facileness of the synthesis process of MO functional nanomaterials. However, the attempts to form MO nanomaterials via metal powder–pure water hydrothermal treatments are not successful. Fig. S1 (in the ESI†) shows the XRD patterns of some samples prepared by metal powder–pure water hydrothermal treatments, revealing an incomplete oxidation of metal powders that mainly caused by the powder agglomeration. Hence, to successfully synthesize the MO nanomaterials by this green and facile method based upon a metal powder–pure water system, the critical challenges such as the agglomeration and incomplete oxidation of raw metal particles, have to be addressed.

In virtue of the powerful dispersion ability of the intense ultrasound,^22,25–28 in the present work, we proposed a “UAPO–HO” (an ultrasonic-assisted preoxidation and a subsequent hydrothermal oxidation) strategy to circumvent the above challenges of agglomeration and incomplete oxidation in hydrothermal process, and to rationally synthesize MO nanomaterials using only metal powders and pure water. This synthetic process is accomplished by first thoroughly dispersing of the metal powders in pure water, taking advantage of the ultrasonic effects to surround every metal particle with water molecules and partially oxidize metal particle as MO nucleus, and then continuing and accomplishing oxidation through subsequent hydrothermal process. Compared with the reported sonohydrothermal synthesis that the special synthetic devices are indispensable,^11,29 this “UAPO–HO” approach has the significant advantages of special-devices needless, facile and mild besides the superiorities of chemical-utility least, product-purity high, post-treatments free, environmentally-benign and etc.

Here we present the first synthesis of MO nanomaterials (M = Mn, Zn, and Fe) using the proposed UAPO–HO method. These three target products have been chosen to demonstrate the generalized effectivity of the UAPO–HO method in consideration of the different chemical activities of the corresponding raw metal powders and the versatile applications of the MO products especially in energy conversion and environmental protection. Application performance evaluation studies confirmed that the newly synthesized MO nanomaterials, such like Mn₃O₄ as an electrode material for supercapacitors, ZnO as an photocatalyst for degrading organic pollutants, and Fe₃O₄ as a magnetic catalyst for disposing antibiotics, display attractive performance and great application potentials in energy storage and environmental protection.

2. Results and discussion

2.1 “UAPO–HO” synthetic strategy

Fig. 1 schematically illustrates the proposed UAPO–HO approach for synthesis of MO nanomaterials. In the UAPO (ultrasonic-assisted peroxidation) step, metal powders are thoroughly dispersed in pure water by intense ultrasonic waves, which make every metal particle surrounding with water molecules and meanwhile produce implosive collapses of the bubbles in the solution, resulting in generation of the localized hotspots through shock wave formation or adiabatic compression within the gas phase of the collapsing bubbles.²⁷ The heat produced from cavity implosion can activate the water molecules to form hydrogen peroxide (H₂O₂), which may further decompose into extremely active H· and OH·.^22,27,30 Then the metal particles are easily oxidized by the surrounded highly active H₂O₂ and OH· to yield the nucleation of MO (eqn (1) and (2)).

M (s) + H₂O₂ (aq) → M(OH)_n (s) → MO (s) + H₂O (aq) (1)

M (s) + 2OH· (aq) → MO (s) + H₂O (aq) (2)

Fig. 1 Schematic illustration of the proposed UAPO–HO approach for synthesis of MO nanomaterials.

While in the HO (hydrothermal oxidation) step, the diffusion and ionization of water are both stronger under high pressure and high temperature in the closed container, inducing large amount of ion products of water in the system (eqn (3)).³¹ The highly active H₃O⁺ ions easily react with the previously formed MO and the unreacted metal to yield Mⁿ⁺ ions (eqn (4) and (5)), which further combine with the surrounded OH⁻ ions to form M(OH)_n, converting continuously to MO (eqn (6)).^23,24,32

2H₂O (aq) → H₃O⁺ (aq) + OH⁻ (aq) (3)

MO (s) + 2H₃O⁺ (aq) → Mⁿ⁺ (aq) + 3H₂O (aq) (4)

M (s) + H₃O⁺ (aq) → Mⁿ⁺ (aq) + H₂O + H₂ (g) (5)

Mⁿ⁺ (aq) + nOH⁻ (aq) → M(OH)_n (s) → MO (s) + nH₂O (aq) (6)

2.2 Characterization results

The as-made MO powders (M = Mn, Zn, and Fe) with a yield of about 100% have the appearances respectively shown in Fig. 2a, c and e, which are slightly different from those of the corresponding raw metal powders (Fig. S2a, d and g in the ESI†). To investigate the crystallinity and phase composition of the as-made samples, powder X-ray diffraction (XRD) measurements were carried out. The results (Fig. 2b, d and f) show the absence of any peaks other than those for the corresponding metal oxides. The strong and sharp diffraction peaks indicate that the newly synthesized MO samples are thoroughly crystallized, suggesting the manganese oxide sample is hausmannite Mn₃O₄ (JCPDS no. 24-0734), the zinc oxide sample is composed of wurtzite ZnO (JCPDS no. 36-1451), and the iron oxide sample is consisted of spinel Fe₃O₄ (JCPDS no. 19-0629). In addition, the XRD patterns of the samples after the UAPO step (Fig. S3 in the ESI†) confirm our speculation that small amount of MO were formed during UAPO, which was crucial for further complete oxidation of the metal powders and formation of the desired MO materials.

Fig. 2 Photographs and XRD patterns of the newly synthesized MO samples: (a and b) M = Mn, (c and d) M = Zn, and (e and f) M = Fe.

To further identify the chemical composition of the resulting three MO samples, X-ray photoelectron spectroscopy (XPS) characterization was performed. The XPS survey spectra (Fig. S4 in the ESI†) and high-resolution results (Fig. 3) confirm that the newly synthesized MO samples are Mn₃O₄, ZnO, and Fe₃O₄. Note, the O 1s high-resolution spectra of the three oxidized samples (Fig. 3b, d and f) all have a weak peak located at BE = 531.9 eV, which are related to the chemisorbed oxygen caused by the surface hydroxyl groups that produced during the hydrothermal process.³³

Fig. 3 XPS high-resolution spectra of the newly synthesized MO samples: (a) Mn 2p and (b) O 1s for Mn₃O₄, (c) Zn 2p and (d) O 1s for ZnO, and (e) Fe 2p and (f) O 1s for Fe₃O₄.

Fig. 4a, c and e respectively show typical transmission electron microscopy (TEM) images of the as-synthesized Mn₃O₄, ZnO, and Fe₃O₄, indicating the nanorod morphology with diameter of 50–200 nm for the Mn₃O₄ sample, the nanopellet morphology with an average diameter of ∼50 nm for the ZnO sample, and the nanocube morphology with an average diameter of ∼100 nm for the Fe₃O₄ sample. High-resolution TEM (HRTEM) images (Fig. 4b, d and f) further confirm the complete crystallines of the as-made MO samples. Lattice fringes with interplanar spacings d₂₁₁ = 0.249 nm and d₁₀₃ = 0.276 nm are measured (Fig. 4b), consistent with the Mn₃O₄ crystallography. Lattice fringes with interplanar spacing d₁₀₁ = 0.246 nm (Fig. 4d) and d₁₁₁ = 0.485 nm (Fig. 4f) are consistent with the wurtzite crystal structure of ZnO and the spinel crystal structure of Fe₃O₄, respectively. As compared Fig. 4a, c and e with Fig. S2b, e and h (in the ESI†), an interesting phenomenon should be noted that the morphology and size of the MO products are not constrained by their corresponding metal raw materials, reflecting the significant advantage of the proposed hydrothermal processing in controlling micromorphology of reaction products.

Fig. 4 TEM and HRTEM images of the newly synthesized MO samples: (a and b) Mn₃O₄, (c and d) ZnO, and (e and f) Fe₃O₄.

Based upon the above characterization results, the generalized effectivity of the proposed synthesis approach, that is, an ultrasonic-assisted peroxidation and a subsequent hydrothermal oxidation (UAPO–HO) of a metal powder–pure water system, has been demonstrated to successfully synthesize MO nanomaterials, such as Mn₃O₄ nanorods, ZnO nanopellets, and Fe₃O₄ nanocubes.

2.3 Application performance

To exploit functional applications of the as-made MO nanomaterials, the performance of these materials in the fields of energy storage and environment protection was evaluated.

The electrochemical performance of the as-prepared Mn₃O₄ nanorods as an electrode material for supercapacitors is shown in Fig. 5a and b. The cyclic voltammetry (CV) curves (Fig. 5a) at different scan rates all show a rectangular and symmetrical shape, indicating that an ideal reversibility in the Faraday redox reactions and a good cycle efficiency of the charge–discharge process.^34,35 The specific capacitances of the as-prepared Mn₃O₄ material based upon the CV results are calculated to be 143, 116, 101, 81, and 66 F g⁻¹ respectively at scan rates of 2, 5, 10, 25 and 50 mV s⁻¹ (the calculating details of the specific capacitances are described in the ESI†). The cycling stability result (Fig. 5b) reveals that the capacitance retention is ∼78% of the maximum capacitance after 3000 CV cycles at 50 mV s⁻¹, suggesting a good long-term electrochemical stability.^36,37 The above electrochemical performance of the as-prepared Mn₃O₄ nanorods as a supercapacitor electrode material is remarkable as compared with those of the reported Mn₃O₄ nanomaterials,^38–41 which is further confirmed by galvanostatic charge–discharge test and electrochemical impedance spectroscopy (EIS) (Fig. S5a and b and corresponding discussion in the ESI†).

Fig. 5 Performance of the newly synthesized MO nanomaterials: (a and b) Mn₃O₄ as an electrode material for supercapacitors ((a) CV curves, (b) cycling stability measured at a scan rate of 50 mV s⁻¹), (c and d) ZnO as an photocatalyst for degrading organic pollutants ((c) photodegradation dynamics, (d) reusability), and (e and f) Fe₃O₄ as a magnetic catalyst for disposing antibiotics ((e) magnetic hysteresis loops at room temperature and the inset illustrates recycling by a magnet, (f) XRD pattern of the recycled Fe₃O₄ nanocubes).

Fig. 5c and d illustrate the performance of the as-prepared ZnO nanopellets as a photocatalyst for degrading organic pollutants. The color evolution of methyl orange solution from yellow to colorless upon photocatalytic degradation that shown in the top inset of Fig. 5c indicates a thorough degradation of methyl orange during 100 min, which is further quantitatively confirmed by the ∼100% degradation efficiency calculated based upon the concentration changes of the methyl orange solution (Fig. 5c). Moreover, the photocatalytic stability of the as-prepared ZnO nanopellets was investigated by reusing the sample up to 5 times, which suggests a high photocatalytic stability of the as-prepared ZnO with maintaining the high degradation efficiencies during reusing (Fig. 5d). As compared with the reported ZnO nanomaterials,^42–48 the as-prepared ZnO show better photocatalytic property, which might be mainly due to the smaller size and higher surface area of the nanopellets.

The performance of the as-synthesized Fe₃O₄ nanocubes as a magnetic catalyst for disposing antibiotics was evaluated, resulting in a ∼92% disposing efficiency of norfloxacin by acting as a heterogeneous Fenton catalyst with H₂O₂ during 60 min disposal, which is better than those of the reported Fe₃O₄ nanomaterials.^49–52 The magnetic hysteresis loop of the as-synthesized Fe₃O₄ nanocubes at room temperature under an applied magnetic field ranging from −12 to 12 kOe shows a typical super paramagnetic behavior with the values of 96.7 emu g⁻¹ and 25.1 emu g⁻¹ for the specific saturation magnetization (M_s) and magnetic remanence (M_r), respectively (Fig. 5e). The significantly high value of M_s implies that the as-made Fe₃O₄ nanocubes have superior magnetic property to other reported magnetite powders,^50,51,53 which facilitates the magnetic separation of the Fe₃O₄ catalyst from antibiotics disposing system (the inset of Fig. 5e) and can be helpful for the Fe₃O₄ recycling and reusability. The XRD pattern of the recycled Fe₃O₄ nanocubes (Fig. 5f) shows no changes to that of the pristine sample, suggesting the high stability and excellent reusability of the as-synthesized Fe₃O₄ nanocubes.

The above results demonstrate the attractive performance of the as-made MO nanomaterials and confirm their promising applications in the fields of energy storage and environment protection. However, their applications should not be confined in these two fields, which may extend to much wider areas.

3. Experimental

3.1 Materials and chemicals

Metal powders, including manganese powders (99.9% purity; average particle diameter ca. 3 μm), zinc powders (99.9% purity; average particle diameter ca. 50 nm) and iron powders (99.9% purity; average particle diameter ca. 50 nm), were purchased from Haotian nanoscience and technology Co., Ltd. (Shanghai, PR China). All chemicals (analytical pure; Sinopharm Chemical Reagent Co., Ltd. Shanghai, PR China) were used as received.

3.2 Sample preparation

Metal powders (0.20 g) were dispersed into ultrapure water (resistivity 18.2 MΩ cm; 25 mL) in a 50 mL capacity Teflon inner lining of autoclave and ultrasonically (40 kHz, 100 W, KQ3200DE, Kunshan Ultrasonic Instruments Co. Ltd., PR China) preoxidated under a temperature for a certain time. After this ultrasonic-assisted peroxidation step, the Teflon inner lining with metal powders and ultrapure water were placed into the stainless-steel shell of autoclave and heated in an oven for a certain time to undergo hydrothermal oxidation. After that, the autoclave was left to cool to room temperature. Then the products were filtered off and dried in a vacuum oven at 60 °C for 12 h.

Due to the different chemical activities of three raw metal powders, the specific conditions in the ultrasonic-assisted peroxidation step and hydrothermal oxidation step for synthesis of three different MO nanomaterials have some differences. The optimized synthetic conditions are listed in Table 1.

Table 1 Specific conditions in the ultrasonic-assisted peroxidation step and hydrothermal oxidation step for synthesis of three different MO nanomaterials

MO nanomaterials	Ultrasonic-assisted peroxidation		Hydrothermal oxidation
	Temperature/°C	Time/min	Temperature/°C	Time/h
Mn3O4	75	30	180	24
ZnO	75	60	200	18
Fe3O4	75	80	250	18

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3.3 Sample characterizations

The crystal structure of the samples was identified by X-ray diffraction (XRD, X'Pert PRO, Holland). The morphological and structural information were examined by scanning electron microscopy (SEM, Sirion 200, Japan; *Zn powder: Zeiss Auriga, Germany) and transition electron microscopy (TEM, Zeiss LIBRA 200, Germany; **Fe₃O₄: Tecnai G2 F20 S-TWIN, USA). The chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo electron ESCALAB250, USA). The magnetic property of the Fe₃O₄ sample was examined with a vibrating sample magnetometer (VSM, Mode IBHV-525, Japan).

3.4 Application performance measurements

The electrochemical performance of Mn₃O₄ as an electrode material for supercapacitor was evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge tests and electrochemical impedance spectroscopy (EIS) with an electrochemical workstation (AUTOLAB PGSTAT302N, Switzerland). All measurements were carried out at room temperature in a three-electrode configuration with the Mn₃O₄ electrode as the working electrode, a platinum foil as the counter electrode (3.0 cm × 3.0 cm), a Hg/Hg₂SO₄ electrode as the reference electrode, and 0.5 M Na₂SO₄ aqueous solution as the electrolyte. The Mn₃O₄ working electrode was prepared by mixing 80 wt% Mn₃O₄, 10 wt% acetylene black, 10 wt% polytetrafluoroethylene (PTFE) and few drops of ethanol together as a slurry, pasting the slurry onto a 1.0 cm² area of a nickel foam current collector, then pressing and finally vacuum-drying at 60 °C for 12 h. The CV analysis was performed at different scan rates in the potential window of −0.2 to 0.6 V. The galvanostatic charge–discharge tests were carried out in the potential range of −0.2 to 0.6 V at different current densities. EIS measurements were made at open-circuit potential with 10 mV AC amplitude over a frequency range of 0.01 Hz to 100 kHz.

The photocatalytic performance of ZnO as a photocatalyst for degrading organic pollutants was evaluated using methyl orange as the model pollutant.⁵⁴ The degradation experiments were performed in a single-compartment photoreactor at room temperature. The photoirradiation was carried out with a UV lamp (λ = 254 nm). The as-prepared ZnO sample (0.25 g) was ultrasonically dispersed into a 10 mg L⁻¹ methyl orange solution (100 mL), and then the mixture was stirred for 30 min in dark to establish an adsorption/desorption equilibrium. A small amount of methyl orange solution was taken from the reactor every 20 min, which was analyzed with a UV-vis spectrophotometer (PUXI TU-1810, Beijing, PR China) after centrifugation to evaluate the degradation efficiency. The analytical wavelength selected for the optical absorbance measurements was 464 nm.⁵⁵

The catalytic and magnetic performance of Fe₃O₄ as a magnetic catalyst for disposing antibiotics was evaluated using norfloxacin as the model antibiotic. The experiments were performed in a plastic reactor at 60 °C. The as-prepared Fe₃O₄ sample (0.02 g) was ultrasonically dispersed into a 25 mg L⁻¹ norfloxacin solution (100 mL, pH = 4.0, adjusting by a dilute HCl solution) and then the mixture was ultrasonically stirred for 60 min to establish an adsorption/desorption equilibrium. After that, 2 mL H₂O₂ (0.2 M) solution was added into the mixture under ultrasonication and the disposing of norfloxacin starts. After 1 h disposal, Fe₃O₄ sample was separated by using a magnet and the reaction solution was analyzed with a UV-vis spectrophotometer (PUXI TU-1810, Beijing, PR China) to evaluate the disposal efficiency. The analytical wavelength selected for the optical absorbance measurements was 260 nm.⁵⁶

4. Conclusions

In summary, a generalized pathway called “UAPO–HO” (an ultrasonic-assisted preoxidation and a subsequent hydrothermal oxidation of metal powders directly in pure water without using any other chemicals) towards synthesis of MO nanomaterials has been developed, which features least chemical-utility, products' high-purity, needless post-treatments and process simplicity, mildness and environmental-benignity. Three MO nanomaterials (Mn₃O₄ nanorods, ZnO nanopellets, and Fe₃O₄ nanocubes) have been successfully synthesized and their great application potentials in energy storage and environmental protection (such like Mn₃O₄ as an electrode material for supercapacitors, ZnO as a photocatalyst for degrading organic pollutants, and Fe₃O₄ as a magnetic catalyst for disposing antibiotics) have been demonstrated. The proposed synthetic approach in this work will hopefully stimulate further green synthesis of other functional MO nanomaterials, which will contribute to further development of the relevant fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21273293, 21373281, 21573028), the Program for New Century Excellent Talents in University (NCET-12-0587, NCET-13-0633), the Project for Distinguished Young Scholars in Chongqing (cstc2014jcyjjq100004), the Fundamental Research Funds for the Central Universities (CDJZR14228801) and the sharing fund of Chongqing University's Large-scale Equipment.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00709k

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