Journal of Materials Science & Technology  2019 , 35 (8): 1601-1606 https://doi.org/10.1016/j.jmst.2019.03.020

Orginal Article

Preparation of nanostructured Cu/Zr metal mixed oxides via self-sustained oxidation of a CuZr binary amorphous alloy

Xingzhou Li, Jili Wu, Ye Pan*

School of Materials Science and Engineering, Jiangsu Key Laboratory for Advance Metallic Materials, Southeast University, Nanjing 211189, China

Corresponding authors:   *Corresponding author.E-mail address: panye@seu.edu.cn (Y. Pan).

Received: 2018-10-16

Revised:  2018-12-31

Accepted:  2019-02-15

Online:  2019-08-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

We propose and demonstrate for the first time an approach to synthesize nanostructured photocatalysts through combustion of metal alloys in amorphous state. This approach takes advantage of metastable state and composition homogeneity of amorphous alloys and produce photocatalysts with uniformly dispersed oxides at nanoscale by self-propagating reactions. Using CuZr amorphous ribbons as an example, we demonstrate a photocatalyst containing copper oxides and ZrO2 in the form of 10 nm-thick nanosheet with 2 nm nanopores. The new catalyst substantially outperforms those previously reported copper oxide catalysts in experiments of photocatalytic degradation of methylene blue and direct methanol fuel cells. This study opens up an avenue to fabricate nanostructured functional oxides in an environment friendly approach not only for photocatalyst but also for oxide based nanoelectronic and nanoionic applications.

Keywords: Amorphous alloys ; Cuprous oxide ; Degradation ; Oxidation

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Xingzhou Li, Jili Wu, Ye Pan. Preparation of nanostructured Cu/Zr metal mixed oxides via self-sustained oxidation of a CuZr binary amorphous alloy[J]. Journal of Materials Science & Technology, 2019, 35(8): 1601-1606 https://doi.org/10.1016/j.jmst.2019.03.020

1. Introduction

Metal oxides with micro/nano-structures have become an interesting topic both scientifically and technologically in recent years due to their versatile electronic properties, ranging from metallic, semiconducting, insulating to superconducting [1,1,2,3]. The synthesis of these nanostructured oxides has been developed massively as early as 1960s. Nevertheless, emerging applications have fueled an increased interest in developing innovative synthetic methods to produce nanomaterials with desired properties under greener and/or more sustainable synthetic strategies. Especially, in recent years, in view of pollution in the chemical industry, a frequently important factor in the development of sustainable nanotechnologies is the ability to produce nanomaterials in compliance with green chemistry principles [4,5].

Metal oxides are normally formed by oxidizing metals under different conditions. Historically, thermal oxidation [6,7] has been typically conducted to grow nanostructures of metal oxides, which is governed by diffusion process and intrinsically suffers from uniformity and time scale issues. There is another form of oxidation, which is faster and self-sustained, i.e., combustion. Traditionally, combustion synthesis involves exothermic and self-propagating chemical reactions between metal salts and suitable organic fuels [8,9]. It reduces consumptions of time and energy and only needs simple equipment and inexpensive reagents. And yet it provides a rather high throughput and has been successfully applied to fabricate many kinds of nanomaterials. In a sense, combustion synthesis meets the demand of green and sustainable modern industry. In view of these advantages, we propose a novel approach to extend this method, that is, combustion of metal elements (instead of metal salts) under their metastable state (solid amorphous phase), leading to nanostructured oxides characterized by uniformly dispersed oxides.

Generally, common alloys with crystalline structures undergo concentration segregation during solidification, which will cause heterogeneity of composition and structure of final materials. Interestingly, metastable metals or amorphous alloys, first fabricated by Duwez, have a monolithic phase with a homogeneous composition [10,11]. It can eliminate the effect of concentration segregation in the construction of nanostructures. In previous studies, dealloying of amorphous alloys to fabricate nanostructured materials has been reported [12,13]. Nanoporous, nanobelt and nanomesh structures utilizing Ti-based amorphous alloys were successfully constructed [14]. In comparison with crystalline materials, amorphous alloys are fundamentally different precursors for dealloying owing to their metastable state and monolithic structure with homogeneous composition and phase constitution [12,13]. Therefore, amorphous alloys are desirable reactant materials to synthesize nanostructured materials.

While most oxides are wide band gap n-type semiconductors, CuO and Cu2O, are p-type direct narrow-band-gap semiconductors with a stable chemical state [15,16]. Copper oxide nanomaterials have been studied for applications in catalysis [17,18], chemical sensors [19], field-emission devices [20], and so on. To date, various copper oxide nanostructures, for instances, nanowhiskers, nanowires and nanorods, have been grown in a wide temperature range by oxidation of various copper substrates [21]. Catalysts comprising copper oxides supported on ZrO2, which has a better thermal stability, unique surface acidity and alkalescence and redox properties, exhibit an enhanced performance compared to monolithic copper oxides [22,23]. Therefore, ZrO2-supported copper oxide is an interesting example utilized here to demonstrate our approach. Among the amorphous alloys, CuZr amorphous ribbon is a good glass former and can be readily fabricated by melt-spun method. In this study, we report using the combustion of CuZr amorphous ribbon to synthesize ZrO2-contained copper oxide nanostructured materials with enhanced catalytic activity, which can be, in principle, extended to many other material systems to synthesize multifunctional nanomaterials, including oxides, nitrides, sulfides and others.

2. Experimental

2.1. Preparation of materials

The master ingot of Cu60Zr40 was prepared by arc melting a mixture of pure Cu and Zr elements (purity ≥99.9 at.%) under a Ti-gettered Argon atmosphere. The ingot was then remelted in a quartz tube by induction melting, followed by a single roller spinning to obtain amorphous ribbons under the spinning speed of 2400 m/min.

The combustion of as-spun ribbons was performed under air atmosphere without the protection condition (Video 1# in supplementary materials). After combustion, the combusted products were ultrasonic washed with ethanol, and then the final powders were isolated via the centrifugation of the upper suspension, and finally dried in a vacuum oven at 60 °C for 24 h.

2.2. Characterization

The crystal structure of as-spun ribbons and as-prepared powders was analyzed by X-ray diffraction (XRD, D8 Bruker diffractometer with Cu Kα radiation) under operation conditions of 40 kV and 40 mA. The diffractograms were recorded in the range of 10°-80°, with a step size of 0.04° and a collecting of 0.3 s per point. The morphology of as-prepared powders was examined with a Tecnai G2 transmission electron microscope (TEM). The surface morphology and thickness were investigated using a NT-MDT Solver P47-PRO atomic force microscope (AFM). The particle size analysis was performed with a particle size analyzer (Microtrac S3500). The pore size distribution analysis was by Brunnauer-Emmett-Teller (BET) nitrogen adsorption/desorption measurement using ASAP 2100 sorption analyzer.

2.3. Photocatalytic degradation of methylene blue (MB)

The photodegradation reaction was conducted in a DW-01 photochemical reactor with a 500 W tungsten lamp (λ > 400 nm). There was a water layer between the lamp and the reaction system to remove the infrared part of the light. In a typical reaction, 80 ml of aqueous solution of methylene blue (MB, 5 mg/L), 3 ml of 30% H2O2 and 30 mg of the as-prepared powders (as catalyst) were mixed and magnetically stirred thoroughly in the dark to reach the adsorption equilibrium of the methylene blue on the catalyst before exposure to visible irradiation (10 cm distance from the solution center to the lamp). During the reaction process, 4 mL of reaction solution was withdrawn at a given time interval (2 min) and centrifuged. The centrifuged solution was then recorded in the UV-vis spectrophotometer in a scanning range of 400-800 nm at ambient temperature for an optical absorbance measurement. For the comparison, Xenon lamp was also utilized as a light source but the other conditions remain the same. The detailed experiments of photoelectric effect and electro-catalytic performance are shown in the section of Supplementary materials were prepared in a commercial infrared sintering furnace, which comprises 6 temperature zones. The length of each heating zone is 35 cm, and the width is 30 cm. The temperatures in each zone can be changed from room temperature to 1000 °C. Fig. 1 illustrates the arrangement of sample and evaporation source for our present study. When the furnace is heated and reaches the set temperatures, the substrates and the quartz boat containing Mo powders are carried to the set zones by the transmission belt from the entrance. The source materials are placed in the high temperature zone and the substrate in the low temperature one. The materials, which vapor from the source, are deposited on the substrate since it has lower temperature. In our method, MoO3 microbelts grow in air and do not involve the use of other protection gases. The advantages of using a commercial infrared sintering furnace include: (1) the substrate area can be very large; (2) easy to change deposition temperatures using the combination of different temperature zones; (3) uniformity in the temperature inside the furnace.

Fig. 1.   XRD patterns of as-spun CuZr ribbon (a) and as-prepared powders (b).

3. Results and discussion

The X-ray diffraction (XRD) pattern of an as-spun ribbon clearly displays its amorphous nature by showing only one hump on the curve (Fig. 1(a)). After combustion, the products show sharp diffraction peaks, indicating that the combustion has converted the amorphous ribbon into a few crystalline products (Fig. 1(b)). The crystalline diffraction peaks can be indexed to (001), (110), (-111) and (111) planes of ZrO2 (PDF No. 37-1484), (110), (111) and (200) planes of Cu2O (PDF No. 65-3822), (002) and (111) planes of CuO (PDF No. 44-0706) and (111) and (200) planes of Cu (PDF No. 65-9026). Evidently, the resultant oxides via the oxidation of CuZr binary amorphous ribbon in air atmosphere are a mixture of ZrO2, Cu2O, CuO and Cu.

X-ray photoelectron microscopy (XPS) was used to characterize the chemical states of the metal elements in the combustion products. The surface layers and contaminations from air were removed by Ar ion milling before the spectra shown in Fig. 2 were collected. It has been well established that a shake-up peak at about 940-945 eV and a binding energy at 933.0-933.8 eV (Cu2p3/2) are two major XPS characteristics of CuO, while a Cu2p3/2 peak at 932.2-933.1 eV and the absence of the shake-up peak are the characteristics of Cu+ species [24]. In Fig. 2(a), the shake-up peaks appear in the range from 938 to 945 eV and the Cu2p3/2 peaks present a broad range from 930 to 937 eV. The photoelectron signal, centered at 934.7 eV with a shoulder on the low binding energy side, can be deconvoluted into two peaks. The first one is located at 934.6 eV, corresponding to Cu2+ bonded to O, namely, the formation of CuO. The other locates at 932.5 eV, corresponding to either reduced copper oxides (Cu2O) or small clusters of Cu0, which may play a significant role in the catalytic activities as discussed later. XPS of Zr3d is shown in Fig. 2(b), where the two spin orbit peaks centered at 181.7 and 184.1 eV correspond to Zr3d5/2 and Zr3d3/2, respectively. A difference of 2.4 eV between the two peaks of the doublet indicates the existence of Zr4+ [25,26]. The atomic composition of the nanosheets was estimated by XPS to be about 11.39 at.% and 7.85 at.% for Cu and Zr, respectively. The atomic ratio of Cu/Zr is 0.59/0.41, which is close to the nominal atomic ratio of Cu60Zr40 in the ribbon.

Fig. 2.   XPS analysis of as-prepared powders (a) Cu 2p; (b) Zr 2p (The vertical lines in the figures indicate the locations of different chemical states of Cu and Zr).

The morphology of combustion products was examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Sheet-like morphology can be clearly seen from the TEM micrograph in Fig. 3, where one sheet typically consists of smaller sheets. Alternatively, pore size distribution analysis indicates that there are many nanopores in each sheet, and these $\widetilde{2}$ nm nanopores are an absolute majority (the inset in Fig. 3(a)). Fig. 3(b) and (c) indicates the detailed microstructures of sheets, and suggest that there are many nano-sized crystalline regions, which are characterized by apparent lattice fringes (Fig. 3(d) and (e)), with the diameter of 10 nm $\widetilde{2}$0 nm in the amorphous matrix that does not possess the lattice fringe (Fig. 3(f)). Selected area electron diffraction (SAED) evidence the components of the crystalline regions, being consisted of ZrO2, Cu2O, CuO and Cu (Fig. 3(g)). For this, it should be supposed that the combustion in the amorphous ribbons is rapid and the competition capture of metallic atoms with oxygen atoms causes the multiple oxides segregation in the nano-size regions. The differential scanning calorimetry (DSC) further demonstrates the existence of amorphous phase due to the obvious amorphous-to-crystalline transition with exotherm peaks (Fig. S1 in supplementary materials). AFM was utilized to determine the thickness of the sheet. Fig. 4 presents the AFM profile of those nanosheets, which reveals that the thickness is about 10 nm (Fig. 4(b)). In addition, the AFM data also shows that the sheet is about 220 nm in width or length, in agreement with particle analysis shown in (Fig. 4(c)). These results suggest that oxides with nanostructures have been formed via combustion of the as-spun amorphous alloy.

Fig. 3.   TEM image of the combustion product. The inset shows the pore analysis with BET. (a) a representative image for as-prepared catalysts; (b-c) the local morphologies corresponding the regions in the (a) image; (d-f) the lattice fringes as referring to the regions in (b) and (c) images; (g) a representative SAED image of as-prepared catalysts.

Fig. 4.   Height profile of nanosheets (a-b) and particle size analysis (c).

In order to demonstrate that the amorphous state of the metal alloys is critical for this new nanostructure construction approach, we conducted a control experiment to see if the crystalline CuZr ribbons with the same size can be used to synthesis the same catalyst. The annealing was carried on the high vacuum annealing furnace (The vacuum was control below 5 × 10-3 Pa) and controlled at 800 K for 6 h, which is much higher than crystallization temperature, which ensures that the as-spun amorphous ribbon was turned into crystalline ribbon. However, it was found that the as-annealed ribbon could not be combusted (Video 2# in Supplementary materials). Therefore, the metastable nature (high energy) of amorphous alloy is crucial for the success of this approach of nanostructure fabrication.

We conducted a photocatalytic degradation of methylene blue (MB) to study catalytic properties of the oxide nanosheets formed by the new approach. Fig. 5(a) gives degradation kinetic curves of degradation experiments derived by UV-vis spectra, similar to that used in reference [27]. The adsorption capability of the photocatalysts for MB indicates the apparent adsorption prior to photoactivation (Fig. S2 in Supplementary materials). As the reaction proceeds, the concentration of MB decreases. The comparison in Fig. 5(a) shows that the nanosheets prepared by our approach induce a much faster degradation velocity. To reach a 97% degradation, our catalyst needs only 24 min, while it takes over 60 min for the commercial CuO and nanoporous CuO catalyst demonstrated in reference [25]. Clearly, under the same degradation condition, the degradation efficacy of our catalyst is substantially faster than that of the CuO in reference [25]. We also carried out degradation experiments under a xenon lamp, and found accelerated degradation kinetics. Moreover, further comparisons with other copper oxide family catalysts show that a relatively slow degradation is usually observed in those previous studies (Table S1, in supplementary materials). Specifically, the concentration of catalyst (30 mg/80 mL) is significantly lower than others. However, the degradation to the point of 97%, Table S1 also clearly indicates that the 24 min in the degradation time is much lower than others (except one reported in Ref. [2] in Supplementary materials). For the dyes, we used a 5 mg/L of MB solution and this concentration is also not disadvantage in the start solution. Obviously, it strongly suggests that the photo-catalytic property of current catalyst is comparable, even superior to, that of the reported copper oxide family catalysts. For the degradation of MB, there is an intrinsic difficulty in utilizing copper oxide nanostructures as photocatalysts due to the cationic nature of the MB and p-type semiconductor of copper oxide [28]. Nevertheless, Katwal et al reported a 93% degradation of MB within 120 min [29]. Lu et al employed graphene-like copper oxide nanofilms with the aid of H2O2 to decompose and decolor MB, which takes 60 min to complete [30]. However, a rigorous comparison of catalytic activities among them is difficult because of the wide range of experimental conditions used, such as lamp power, spectral range, dye concentration, presence of H2O2 in solution (to increase the photocatalytic ability of the material). These photocatalytic results have suggested that the nanosheet oxide catalyst prepared by our approach can be used as a photocatalyst that functions under visible light irradiation. In addition, it is worth noting that the synthesis of copper oxide family catalysts, such as those listed in Table S1, often consumes a much longer preparation time than our approach. On the other hand, the present powders indicate a good reusability, after 5 cycles, there is no obvious decrease in the degradation kinetics (Fig. S3, in Supplementary materials). Evidently, the as-prepared powders for MB degradation is stable in degradation. The current oxides can also be utilized to photoelectric device (Fig. S4, in Supplementary materials) and the direct methanol fuel cells (Fig. S5, in Supplementary materials).

Fig. 5.   Degradation kinetic curves of as-prepared powders for the degradation of methylene blue (a) and schematic mechanism of MB degradation of as-prepared powder (b).

Fig. 5(b) illustrates the cooperative effect of these nanoclusters in the nano-sheets of the as-prepared powder and elemental Cu atoms, based on the band structure of the metal-semiconductor heterojunction and plasmon-mediated charge injection [17]. ZrO2 mainly plays a structural rather than a catalytic role in the catalyst as there is no widely accepted evidences showing catalytic property in ZrO2 in MB degradation. When Cu directly contacts with copper oxides, including CuO and Cu2O, electrons will migrate from Cu to the surface of the oxide nanoparticles to reach Fermi level equilibration between the metal and the p-type semiconductors. The energy band of oxides is bent down to the space charge layer on its surface. Abundant electrons accumulate in the space charge layer, which facilitates the transfer of electrons from Cu to oxides. When the Cu/oxides are illuminated under visible light, free electrons in Cu are excited to higher energy states due to surface plasmon resonance. The photo-excited electrons are injected into the conduction band of oxides. The injected electrons react with molecular oxygen, leading to the formation of ·OH radicals. At the same time, the holes left in the Cu have a great affinity for capturing electrons from adsorbed organic molecules. On the other hand, under visible irradiation, MB can be degraded directly by a sensitization mechanism [31] on the surface of Cu as a result of electron escape, as well as indirectly through photogenerated oxidizing radicals: O2· and ·OH. Therefore, oxygen not only acts as a major acceptor of conduction band electrons, but also plays an important role in the photochemical N-deethylation process [31]. The uniformly distributed nanocusters of Cu/Cu oxides junctions significantly improves the catalytic efficency.

As shown in Fig. 6, when the combustion starts, the Zr atoms preferentially reacts with O atoms first because of a much larger negative free energy of formation of Zr-O than that of Cu-O. Since the Cu and Zr atoms are uniformly and randomly distributed in the CuZr amorphous ribbons, the primarily formed Zr oxide atom clusters tend to surround their neighboring Cu atoms, preventing the Cu atoms from segregation or being fully oxidized. This result is a highly desirable catalyst structure, i.e., uniformly distributed Cu/CuOx nanoclusters. On the other hand, the metastable state of as-spun binary amorphous CuZr alloys also provide additional energy to favor the combustion of this alloy. Specifically, during the combustion, the additional energy causes a high energy state in the alloy, and it will thermodynamically release to the equilibrium state with the aid of external environment variation, such as heat from lighting the ribbon, which triggers the combustion of ribbons. In this point, Zr atoms will be more easily to react with O atoms, forming ZrO2. In fact, the formation of ZrO2 is a exothermic reaction and thus will be beneficial to the reaction of Cu atoms. In this case, high energy state will also facilitate the Cu-O reaction. However, due to intensively rapid combustion processes, the quantity of O atoms may be enough to be provided for the reaction with Cu atoms to form the CuO. As a result, the Cu2O may be formed and some Cu atoms may be remained without possibility for reaction.

Fig. 6.   Scheme of the formation of nano-scale oxides in a nanosheet.

4. Conclusion

In summary, we have developed a novel approach to construct nanostructured functional metal oxides. The CuZr binary amorphous ribbon was used as an example for the demonstration of the approach. By combusting metal alloys in their metastable states (amorphous solid state), oxides in the form of nanosheet with about 10 nm thickness are formed. The combustion reaction starts with a very homogeneous composition in the amorphous monolithic phase and finishes within a very short time, which resolves the composition heterogeneity (segregation) issue found in other approaches and results in a very desirable nanostructure with significantly improved photocatalytic performance. This green fabrication approach requires minimal energy and avoids byproducts, such as exhaust gas and chemical pollutions. This newly developed approach has the potential to be readily extended to other material systems for synthesizing nanostructured oxides that may find interesting applications in emerging electronic, photonic and ionic devices.

Acknowledgements

Authors are grateful to Prof. J. Joshua Yang (The Department of Electrical and Computer Engineering, University of Massachusetts, Amherst) for his valuable suggestions, careful review and helpful editing of this work. Thanks for the financial support from the National Natural Science Foundation of China (No. 51671056), Jiangsu Key Laboratory for Advanced Metallic Materials (No. BM2007204).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.03.020.

The authors have declared that no competing interests exist.


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