Soft-Chemical Synthetic Nonstoichiometric Bi2O2.33 Nanoflower: A

Article pubs.acs.org/JPCC

Soft-Chemical Synthetic Nonstoichiometric Bi2O2.33 Nanoflower: A New Room-Temperature Ferromagnetic Semiconductor Hangmin Guan,*,†,‡ Xiaodong Zhang,*,† and Yi Xie† †

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China (USTC), Hefei, Anhui 230026, China ‡ Department of Chemical and Material Engineering, Hefei University, Hefei, Anhui 230062, P. R. China S Supporting Information *

ABSTRACT: Bi2O2.33 nanoflowers with pure phase were directly prepared via a solvothermal route. The magnetism behavior of the product was investigated, and the data clearly revealed the room-temperature ferromagnetism of the synthetic sample. This nonstoichiometric bismuth oxide should be the diluted magnetic semiconductors (DMSs). XPS measurements showed that the room-temperature ferromagnetism might be derived from the presence of Bi2+ in the structure. The achievement of the nonstoichiometric Bi2O2.33 DMSs might pave the way for a new understanding of the underlying ferromagnetic mechanism in DMSs materials.

1. INTRODUCTION Diluted magnetic semiconductors (DMSs) integrate the semiconducting and ferromagnetic function of electrons in the same materials to create new functionalities, which are ideal candidates for spintronic devices.1−6 Moreover, DMSs allow highly efficient spin injection and ultrafast optical switches, which are also ideal for new spin field-effect transistor devices and spin light-emitting diodes (LEDs).7−9 Therefore, pursuing new DMSs is crucial for the design of electronic devices and the study of their applications.10−12 Generally speaking, the origin of ferromagnetism (FM) in DMSs can mainly be attributed to three typical models: carrierinduced ferromagnetism (CF),13 bound magnetic polarons (BMPs),14 and charge transfer FM (CTF).15 On the basis of the Zener model, CF model is driven by the exchange interaction between carriers and localized spins. For the BMP model, ferromagnetic exchange is mediated by shallow donor electrons overlapping to create a spin-split impurity band. In comparison, CTF model is developed for the defect-related FM, which involves a spin-split defect band populated by charge transfers from a proximate charge reservoir. According to these three typical models, the defects which bring out the donor electrons for charge balance are the key factors to form the ferromagnetism in structures of DMSs.14 Up to now, the study on the DMSs has been mainly focused on the transition metal semiconductors with externally introduced defects in their structure, while little attention has been paid to the ferromagnetism in pure phase semiconductors.16−20 The fact that a lot of nonstoichiometric semiconductors have defects themselves inspired us to pursue DMSs in those materials.21−24 © 2014 American Chemical Society

Bismuth oxide as an important functional semiconductor has attracted great interest in electrochemical, optical, and photocatalytic areas due to its low cost, nontoxicity, and versatile properties.25,26 Bismuth oxide has five main crystalline phases, namely, monoclinic α-Bi2O3, tetragonal β-Bi2O3, body-centered cubic (BCC) γ-Bi2O3, face-centered cubic (FCC) δ-Bi2O3, and ω-Bi2O3.27,28 Besides these extensive five main crystalline phases of bismuth oxide, there are also two nonstoichiometric phases including Bi2O2.75 and Bi2O2.33.29 The nonstoichiometric phase Bi2O2.33 has proven to be a functional semiconductor with ultraviolet-emitting photoluminescence properties and remarkable supercapacitive performance.30,31 Valence analysis of elements has drawn our attention to the nonstoichiometric phase Bi2O2.33, which may exhibit ferromagnetism for its unique structure.29 According to the charge balance, there should be two kinds of bismuth ion (Bi3+ and Bi2+) in the Bi2O2.33 crystal, where the relative number of oxygen vacancies attains up to 23.3%. The electronic configuration of Bi2+ is [Xe] 4f145d106s26p1 with an unpaired electron. In addition, Bi atoms are very close to each other with only about 3.12 Å distance, as shown in Figure 1. The bonding in Bi2O2.33 is apparently achieved not only by the interaction between the bismuth and oxygen orbitals but also by the interaction between the orbitals of bismuth pairs.29 It makes possible the overlap between electrons of Bi2+ resulting in ferromagnetic exchange coupling between them. Therefore, the nonstoichiometric phase Bi2O2.33 may be the DMSs. Received: September 7, 2014 Revised: October 23, 2014 Published: October 27, 2014 27170

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coupled plasma atomic emission spectroscopy (ICP-AES, Atomscan Advantage).

3. RESULTS AND DISCUSSION Figure 2 shows the XRD pattern of as-prepared Bi2O2.33. All the sharp diffraction peaks in the XRD pattern are perfectly indexed

Figure 1. Schematic illustration of the crystal structure of Bi2O2.33.29

Up to now, most of the work on bismuth oxides has focused on Bi2O3, while there have been few reports on Bi2O2.33. Bi2O2.33 generally appears as impurities in the bismuth oxides and bismuth-oxide-based materials.26,29,32 Pure nonstoichiometric Bi2O2.33 thin films were synthesized by chemical vapor deposition (CVD).33 Chen et al. synthesized Bi2O2.33 nanosheets by electrolytic corrosion of metal Bi and detected one strong UV emission at room temperature.30 However, unknown weak peaks are found in the X-ray diffractometer pattern of the above product, implying the existence of impurities. Yuan et al. synthesized orange-like Bi 2 O 2.33 microspheres via a chemistry precipitation method followed by thermal annealing.31 Therefore, it is still a challenge that the pure nonstoichiometric phase Bi2O2.33 is directly prepared by a soft chemical method. Here, we present a facile and chemical strategy to directly synthesize pure nonstoichiometric Bi2O2.33 nanoflowers by a solvothermal method. The magnetism behavior of the product is investigated. As expected, the as-obtained Bi2O2.33 indeed shows ferromagnetic properties.

Figure 2. XRD pattern of the as-prepared products.

as the Bi2O2.33 phase, in good agreement with the standard JCPDS (JCPDS 65-5490, space group I4/mmm, No. 139). No peaks of other phases are detected, which indicates that the Bi2O2.33 is the main phase in the product. Moreover, it can be seen that the peaks indexed as (002), (004), and (0,0,12) display stronger intensities compared with others. It is inferred that the pure Bi2O2.33 prefers the orientated growth along the (001) facet, which is perpendicular to the nanosheets. As illustrated by Figure 3a, TEM demonstrates the welldefined nanoflower morphology of the as-prepared Bi2O2.33. The thickness of the nanosheet forming as-synthesized nanoflowers is measured to be in the range 10−20 nm, as evidenced by the SEM image (as Supporting Information Figure S1). Figure 3c is the HRTEM image of a nanosheet, taken with the incident electron beam perpendicular to the inplane facets of the nanosheet. The two different interplanar distances of 0.272 nm are both observed, which are in good agreement with the lattice interplanar distances of (110) planes of the Bi2O2.33. The microscopic elemental analysis of the selected structure is checked by energy dispersive X-ray spectroscopy (EDS) shown in Figure 3d, which reveals the existence of Bi and O elements but no Na element. Cu element comes from the Cu net during the EDS analysis. Here, we investigate the magnetic properties of the synthetic Bi2O2.33 nanoflowers and report the observation of their roomtemperature ferromagnetism. The magnetization of the product is measured with the SQUID magnetometer. Figure 4 shows the magnetic-field dependence of magnetization (M vs H) at 300 K for as-prepared Bi2O2.33 nanoflowers after subtracting the paramagnetic background from the raw data. The roomtemperature ferromagnetic behavior is demonstrated by a hysteresis loop with a coercivity of approximately 200 Oe, as shown in the inset of Figure 4. The saturation magnetic moment Ms is evaluated to be 0.003 emu g−1, showing the

2. EXPERIMENTAL SECTION Synthesis Details. The 0.11 g NaBiO3 and 1.18 g PVP (polyvinylpyrrolidone) portions were ultrasonically dispersed in 20 mL ethanol, and then 5.0 mL of distilled water and 2.0 mL of 1 M HNO3 solution were added dropwise. After strong stirring, the above solution was sealed in a 40 mL autoclave and heated at the temperature of 100 °C for 3−5 h. The system was then allowed to cool to room temperature. The final sample was collected by centrifugation, washed with deionized water to remove any possible ionic remnants, and then dried in a vacuum at 60 °C. Characterization Methods. The sample was characterized using XRD with a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.541 78 Å). XPS measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an excitation source of Mg Kα = 1253.6 eV. The TEM images were obtained on a JEOL-2010 transmission electron microscope at an acceleration voltage of 200 kV. The field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F SEM. The magnetization was characterized by a superconducting quantum interference device (SQUID, quantum design MPMS XL-7) magnetometer in the temperature range from 4 to 300 K. The magnetic element content in the samples was determined by inductively 27171

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In order to verify this assumption, the Bi 4f X-ray photoelectron spectroscopy (XPS) spectra of the Bi2O2.33 and the commercial Bi2O3 are analyzed for comparison. Figure 5

Figure 5. XPS spectra of as prepared Bi2O2.33 nanoflowers and commercial Bi2O3 in comparison (the core-level peaks of Bi 4f).

depicts the Bi 4f peaks in the sample Bi2O2.33 and the commercial Bi2O3, respectively. In Figure 5, peaks of Bi in Bi2O3 are located at 158.8 and 164.1 eV, respectively, with a peak separation of 5.3 eV (marked 1), while Bi peaks located at 158.6 and 163.9 eV, respectively, should belong to Bi2O2.33 (marked 2). Obviously, the Bi peaks of Bi2O2.33 are asymmetric and translate to the low value in comparison to the peaks of Bi2O3. The asymmetric nature of the peak is ascribed to the various types of Bi in the Bi2O2.33 nanoflowers. The Bi 4f XPS peak of Bi2O2.33 can be fitted into two peaks by Gaussian simulation. The higher binding energy peaks at 158.8 and 164.1 eV are attributed to Bi3+ (marked 3). The lower binding energy peaks located at 158.5 and 163.8 eV are new peaks (marked 4) and should be assigned to the Bi2+ ion in the Bi2O2.33 nanoflowers.35 Generally, studies of DMSs are difficult due to the small magnetic moments involved, which can easily be caused by handling of the material with magnetic tweezers, containers, or supports, etc. To exclude the existence of magnetic impurity, an inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of magnetic elements such as Fe, Co, Ni, and Mn was carried out in our samples. The ICP-AES results show that the possible content of Fe, Co, Ni, and Mn is only 0.01%, 0.008%, 0.006%, and 0.005%, respectively. Hence, there is no magnetic impurity in our samples. In addition, there are many sodium and bismuth containing multiferroic materials.36,37 If Na from raw material NaBiO3 is still present in the final compound, this could affect the magnetic properties. We carefully check if Na is still present from the synthesis by XPS. The result shows no Na element in the final compound (as Supporting Information Figure S3) and is also consistent with the result of EDS (as Figure 3d). The magnetic signals from the holder itself were also measured (as Supporting Information Figure S4). The result shows that the intensity of magnetic signals from the holder itself is ∼10−7 emu/g, which is far weaker than that of the product (10−3− 10−4). Hence it is believed that signals of the holder contribute nothing to those of the product.

Figure 3. Bi2O2.33 nanoflowers: (a) TEM image, (b) ED pattern, (c) HRTEM image, and (d) EDS.

Figure 4. M−H curve of as-prepared Bi2O2.33 nanoflowers at 300 K.

typical room-temperature ferromagnetism characteristics of DMSs. We note that Bi2O2.33 is a typical nonstoichiometric compound including a number of oxygen defects (as Supporting Information Figure S2). For charge balance, Bi2+ ion should exist in the nonstoichiometric Bi2O2.33 compound similar to the presence of Ti2+, Ti3+ in TiO2‑x. It is known that oxygen vacancies create charge imbalance, and therefore, there is a possibility to generate Ti3+ and/or Ti2+, and unpaired 3d electron in Ti3+ or Ti2+ can lead to some magnetic moment.11,34 Hence, we suggest that the oxygen defect-induced Bi2+ with an unpaired electron might be responsible for ferromagnetism in these Bi2O2.33 nanoflowers. 27172

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(WK2060190032), and Sciences Research Program of University of Anhui Province (KJ2013B227).

Since the ICP-AES, XPS, EDS, and XRD studies confirm no existence of any impurity or secondary phases in our samples, ferromagnetism observed in the present case is not likely to be caused by the unintentional doping. According to the results of XPS spectra, Bi2+ might be responsible for ferromagnetism in the Bi2O2.33 nanoflowers. Moreover, the bonding between the bismuth pairs makes the charge transfer between Bi and Bi possible.29 Meanwhile, according to the BMPs model,14 this charge transfer favors an enhancement of curie temperature Tc to achieve the room-temperature ferromagnetism in Bi2O2.33 as expected. It should be noted that magnetic Bi2+ ions only partially bind to form the infinite chain. There are still many isolated Bi2+ and nonmagnetic Bi3+ in the Bi2O2.33 structure, as shown in Figure 1. It is worth noting that the composition of less magnetic Bi2+ ions, many isolated Bi2+ ions, and nonmagnetic Bi3+ ions is similar to that of DMSs formed by doping a nonmagnetic semiconductor with magnetic ions.1,2 Also, the results of the analysis suggest that the charge transfer only takes place in the Bi2+ sites in the infinite chain to produce the magnetic moment ordering, giving the ferromagnetism characteristic, and that many isolated Bi2+ and nonmagnetic Bi3+ only exhibit the magnetic disorder, giving the paramagnetic states, which agree well with the experimental evidence that the room-temperature ferromagnetism is in a paramagnetic background (as Supporting Information Figure S6a). Because the nonstoichiometric phase Bi2O2.33 is a functional semiconductor (as Supporting Information Figure S7),30,31 the product should be the DMSs.

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4. CONCLUSION In conclusion, pure phase Bi2O2.33 nanoflowers are successfully prepared via a one-step solvothermal route. The synthesis of such material in a pure phase is a great improvement. The magnetism behavior of this product is also investigated. As expected, Bi2O2.33 indeed shows a room-temperature ferromagnetic signal. XPS measurements showed that the oxygen defect-induced Bi2+ with an unpaired electron might be responsible for ferromagnetism in these Bi2O2.33 nanoflowers. The achievement of the nonstoichiometric Bi2O2.33 DMSs may pave the way for a new understanding of the underlying ferromagnetic mechanism in DMS materials.

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ASSOCIATED CONTENT

S Supporting Information *

SEM, XPS data, M−H, and M−T curves, electrochemical impedance spectroscopy (EIS). Structural data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone/fax: +86-551-62158439. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (11079004, 21331005, and 11321503), Chinese Academy of Science (XDB01020300), Fundamental Research Funds for the Central University 27173

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