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Electronic coupling between sulfur adsorption and oxygen vacancy in TiO2 microstructures for

room-temperature ferromagnetism

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2017 J. Phys. D: Appl. Phys. 50 365304

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As a wide band gap semiconductor (Eg = 3.2 eV), TiO2 nano-structures have attracted much attention because of their interesting applications such as gas sensing, solar-to-chemical energy conversion (water splitting), and environmental cleaning (photodecomposition of harmful materials or dirt) [1–6]. These special properties depend very much on the sur-face structure and reactant adsorption. Nanostructures such as nanotubes (NTs), nanocrystals (NCs), nanodisks, and nanow-ires have been fabricated and their optical and electronic characteristics and potential applications have been explored [7–10]. However, the development of competitive multi-func-tion materials is required.

In particular, element-doped TiO2 becomes sensitized in the visible wavelengths, and besides the photoactivity increases in the ultraviolet spectrum [11]. By transition ele-ment doping TiO2 displays low photocatalytic activity due to

the thermal instability [12], which furthermore brings about environmental concerns. So, modification of TiO2 with non-metal elements (such as, carbon, nitrogen, sulfur and iodine) has received much attention due to high efficiency of doping, low production cost and environmental friendliness [13–16]. Among these non-metallic elements, sulfur doping arouses a significant scientific interest, which demonstrates prac-tical importance for the enhancement of TiO2 photocatalytic properties [17, 18]. Sulfur-doping TiO2 also exhibits a strong antibacterial effect, and in addition the photoelectrodes are able to work in the visible spectrum region. Nevertheless, the structural aspects of sulfur integration in TiO2 are still debat-able, because the sulfur doping amount and distribution are influenced to great extent by the synthesis conditions. For example, some studies show that sulfur is on the surface of TiO2 nanoparticels in the form of S4+ and S6+ predominantly

Journal of Physics D: Applied Physics

Electronic coupling between sulfur adsorption and oxygen vacancy in TiO2 microstructures for room-temperature ferromagnetism

S Y Wu, X M Ren, J L Zhang, X L Wu and L Z Liu

Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics and Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

E-mail: lzliu@nju.edu.cn (L Z Liu) and hkxlwu@nju.edu.cn (X L Wu)

Received 10 May 2017, revised 16 July 2017Accepted for publication 20 July 2017Published 21 August 2017

AbstractThe spin electronic states coupling associated with enhanced photocatalytic activity of TiO2 microstructures annealed in Ar and O2 are designed to explore its ferromagnetism. The samples annealed in Ar show 0.023 emu g−1 magnetic moments related to adsorbed S atom concentration, which is seven times larger than that of samples annealed in O2. Finally, their saturated magnetization decreases sharply as increasing annealing temperature. Spectral analysis and density function theory calculation disclose that the ferromagnetism originates from spin electronic state coupling between adsorbed S atoms and oxygen vacancies as well as magnetic moments disappearing arises from the changes of adsorbed S atom and O vacancy distribution as annealing temperature.

Keywords: magnetism, oxygen vacancies, sulfur adsorption

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[17]. On the other hand, there are significant proofs that sulfur is integrated in the TiO2 lattice forming S–Ti–O bonds [18]. The change of electronic structure associated with micro-structure differences can lead to more desirable physical and chemical characteristics.

In many energy- and environment-related applications, improved efficiency and stability are required, and thus a better understanding of the enhancement mechanism is crucial. It has been shown that sulfur adsorption onto the nanostructure sur-face can effectively modify the reaction activity [19, 20], but the existence of oxygen vacancies (OVs) on the surface and the lack of systematic theoretical assessment make the clari-fication of the inside mechanism challenging. Therefore, it is important to clarify the relation between sulfur adsorption and OVs. Besides, some reports disclose that undoped metal oxide nanostructures exhibit room temperature ferromagnetism (FM) which is too small to be practical in spintronics [21–23]. However, when metal vacancies (VZn) are introduced in undoped ZnO crystals, ZnO is ferromagnetic at room temper-ature [24]. Ti-defected TiO2 has strong room-temperature fer-romagnetism with saturation magnetization of 0.025 emu g−1 [25]. It is generally recognized that the exchange interac-tion between localized electron spin moments resulting from cation or anion vacancies on the surface of the nanostructures accounts for the appearance of the FM [22]. According to the ‘Stoner criterion’, FM induced by the exchange of electronic wave functions at the Fermi level can be enhanced by regu-lating the vacancy distributions and adsorbed atoms [26]. If the electronic state coupling between adsorbed sulfur atoms and OVs happens, room FM can be realized, which proposes a possibility to improve surface catalytic activity by regulating the spin electronic arrangement. However, the understanding of FM induced by sulfur doping in TiO2 is quite limited so far.

In this respect, the change of the FM with the vacancy structure and S doping amount should be clarified in TiO2 microstructures. The sulfur-doped TiO2 (S-TiO2) are annealed at different temperatures under Ar and O2 to regulate sulfur doping amount and OV concentrations. The objective of this work is to identify and explain the origin of FM properties of the TiO2 NCs formed by hydrothermal method with both OV structure and sulfur doping in order to develop catalytic and sensing applications.

The S-TiO2 NCs were prepared by hydrothermal synthesis method [27]. The reactor with 50 ml titanyl sulfate solution with concentration of 103 g l−1 was warmed up to 96 °C in a thermostat, and then the solution was dropped into the pre-adding deionized water at a constant speed during 20 s under stirring condition. After feeding off, 20 ml boiled deionized water was added to the titanyl sulfate solution system immedi-ately. Thus the temperature was about 96 °C and remained con-stant. Keep stirring and heating in magnetic stirring water bath until the reaction was 90 min. Change the amount of washing water during filtering and washing to obtain hydrolysis product hydrated titanium dioxide (TiO2·H2O). All the TiO2·H2O par-ticles were dried at 110 °C for 2 h to obtain different TiO2·H2O samples. Then these samples were annealed at 200 °C, 600 °C, 800 °C and 1100 °C for 2 h under O2 or Ar. The samples were characterized by scanning electron microscopy (SEM),

high-resolution transmission electron microscopy (HR-TEM) (JEOL-2100), x-ray diffraction (XRD), Raman spectroscopy, Flourier transformation infrared spectroscopy (FTIR), photo-luminescence (PL) excitation spectroscopy, x-ray photoelec-tron spectroscopy (XPS), electron paramagnetic resonance (EPR) and thermogravimetric analysis (TG). The magnetic tests were conducted on a superconducting quantum interfer-ence device (SQUID). All the measurements were carried out at room temperature.

The SEM image acquired from the obtained samples annealed at 1100 °C in O2 for 2 h is shown in figure 1(a), and the cuboid morphology with 9.5 µm length and 6.2 µm width can be clearly observed. To clearly display the crystal struc-ture, the HR-TEM image acquired from this NC is depicted in figure 1(b). A lateral face of the microstructure with ~0.2 nm lattice fringe is the (1 1 0) crystalline plane. The selected-area electron diffraction (SAED) pattern in figure  1(c) can be indexed to the [0 1 0] zone axis of tetragonal rutile TiO2. To characterize the distribution of the doping S element, the energy-dispersive x-ray spectroscopy (EDS) mapping acquired from marked area in figure 1(d) was performed as shown in figures 1(e) and (g). The doping S element distrib-utes uniformly on the entire TiO2 NC, which indicates the S element has been successfully incorporated into the TiO2 NC surface.

To further investigate the structures of the samples annealed at different temperatures in O2, more spectroscopic results are displayed in figure 2. First, the XRD patterns of the TiO2 NC samples annealed in O2 at different annealing temper-atures are shown in figure  2(a). The scan rate of the XRD pattern is 0.1° s−1. The typical diffraction peaks are observed at 2θ = 25.4°, 38.4°,48.1° and 55°, which corresponds to anatase TiO2 [28]. As annealing temperature increased, the diffraction peaks become shaper and stronger indicating that the crystal quality is improved. The anatase phase has been retained even with increasing calcination temperature to 800 °C. All of the new peaks appear due to rutile TiO2 when the calcination temper ature is further increased to 1100 °C, implying the anatase TiO2 has transformed to rutile TiO2 com-pletely. Raman spectra were also utilized to analyze the struc-tures in figure 2(b). At 600 °C, the Eg and B1g mode occurs at 633 cm−1 and 135 cm−1 [29], which are related to the OV existences according to Matossi force constant model [30]. In addition, two strong modes at 209 cm−1 (S1) and 276 cm−1 (S2) are originated from doping S element, which are retained as the annealing temperature increases. It can be understood that the doping S atoms are adsorbed (or incorporated) into NC surfaces, and the doping S element cannot be completely removed by previous annealing treatments in O2. The Eg mode is blue-shifted and a series of Raman modes appears at 120–300 cm−1 with annealing temper ature increased. Doping S atoms are precipitated to TiO2 NC surface regions slowly to form more stable structure (interstitial, substitutional site or adsorption onto surface) because the NC surface has smaller formation energy. Those doping S atoms at surface can change original point group symmetry, so new Raman modes appear. The FTIR spectra are shown in figure  2(c) to disclose the bonding structures. Some adsorption peaks at 450–940 cm−1

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originate from stretching and bending vibration Ti-O bonds. The peaks at around 970 cm−1 and 1380 cm−1 correspond to the symmetric and asymmetric stretching vibration of S–O atomic bond [31], the decrease of whose intensity with the increased annealing temperature means the removal of the adsorbed S atoms. In addition, the characteristic absorption peak at around 1450 cm−1 is associated to the bending vibra-tion of C–H. Meanwhile, the peaks at around 1630 cm−1 and 3420 cm−1 arises from OH stretching vibration of absorbed water molecules. This analysis discloses that some S atoms are adsorbed onto TiO2 surface to form S–O bond. To investi-gate its electronic effect, the PL spectra of S-TiO2 sample were measured and are shown in figure 2(d). Compared to previous report without S-doped TiO2 [32], obvious PL peaks at 611 nm can be observed which implies that electronic structures of TiO2 are strongly affected by doping S atoms. Therefore, we can speculate that the electronic spin arrangement may be influenced at the same time. In addition, these characteriza-tions were also conducted for the samples annealed in Ar in figure S1 (stacks.iop.org/JPhysD/50/365304/mmedia), which are similar to the spectra presented in figure 2, indicating there is no obvious difference in crystal structure between them. XPS spectra were recorded in figure S2, which confirms the the existence of S and OVs. There is no the signal ascribed to Ti-S observed before annealing treatment, indicating that S is adsorbed on the surface, instead of doped in the crystal lattice further. Meanwhile, EPR spectra are shown in figure S4.The EPR signals assigned to Ti3+ appear, which is also indicative of OVs. The weight change of the prepared sample without

annealing treatment is also investigated during heating. When the temperature is above 450 °C, the weight loss is ascribed to the leaving of adsorbed sulfur above, which peaks at around 530 °C, proving that S is adsorbed on the surface further.

Subsequently, the room temperature magnetic properties of S-doped samples annealed at different temperatures under O2 and Ar are presented in figure 3. M–H curves of the as-prepared sample before annealed is also shown in figure S6, which has the largest saturation magnetization. All of the curves show obvious ferromagnetic hysteresis with nonzero residual magnetization and coercivity after subtracting the diamagnetic background, implying that the S-TiO2 exhibits room temper ature FM. The saturated magnetiza-tion of the sample annealing treatment at 200 °C under O2 (~0.0035 emu g−1) is seven times smaller than that of the same sample under Ar (~0.0226 emu g−1). Moreover, as annealing temper ature increases, the FM of these samples both becomes smaller at 600 °C, and then decreases sharply. As shown in figure S3, the concentration of OVs and doping S decreases as annealing temperature increases in O2. It is known that the doping microstructure and existence of OVs can be controlled by adopting different treatment methods and conditions. After annealing treatment under O2, some OVs were removed, meanwhile some doping S atoms became sulfur dioxide (doped S concentration becomes smaller). These results indi-cate that the saturated magnetization is positively related to the OV and doped S concentration at TiO2 surface, which is in agreement with the above discussion in figure 2. Here, it is noted that a hysteresis loop is still observed from the sample

Figure 1. (a) SEM and (b) HRTEM image of the naocube annealed in O2 at 1100 °C. (c) SAED pattern of this sample. (d) Selective area in SEM image (marked by red line) for EDS mapping. (e)–(g) The EDS mapping for Ti, O and S element, respectively.

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(1100 °C), because a few OVs and doped S atoms inevitably exist in those sample. In figure 3(b), the FM acquired from samples annealed in Ar discloses that the saturated magnet-ization is also decreased, although the OV concentration is maintained proven in figure S2(e). This is because doping S atoms began to find interstitial or substitutional sites to form more stable structures at high temperature, which is verified by the formation of Ti–S in the sample annealed at 1100 °C in Ar in figure S2(d). Meanwhile, some S atoms may be also precipitated onto TiO2 surface to form S clusters. This anal-ysis implies that both S-doped site and OV contribution are responsible for FM appearance.

To understand the origin of the FM, we perform a den-sity functional theory (DFT) study on the rutile TiO2 film to investigate the effect of inner OVs and S doping as shown in figure  4, using the Vienna ab initio simulation package (VASP) code with projector augmented wave pseudopo-tentials [33]. Here, all the films are carved along the (1 1 0) planes. A cutoff energy of 500 eV and regular Monkhorst-Pack grid 7 × 4 × 1 k points are carried out until all forces on the free ions converge to 0.03 eV Å−1. The vacuum space is at least 18 Å, which is large enough to avoid the interaction between periodical images. Spin polarization is included throughout the calculation. Without S adsorp-tion (VS = 0.0), as shown in figure  4(a), the introduction of a certain OV (VO = 0.07) concentration cannot lead to FM. In the existence of an adsorbed S atom (VS = 0.04),

Figure 2. (a) XRD patterns (A and R represent the anatase and rutile TiO2, respectively), (b) Raman spectra and (c) FTIR spectra of TiO2 NCs annealed in O2. (d) PL spectra of TiO2 NCs annealed in O2 at 200 °C and 600 °C.

Figure 3. M–H curves acquired from TiO2 NC samples annealed in O2 (a) and Ar2 (b).

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a 1.68 µB magnetic moment at OV region (VO = 0.04) can be clearly observed. More importantly, the spin charge den-sity between spin-up and spin-down in figure 4(b) discloses that the magnetic moments do not originate from adsorbed S atoms and S–O bonds, but they are distributed in the Ti atom region nearing to OVs. This calculation confirms that electronic coupling makes the superfluous electron of an adsorbed S atom transfer to an unpaired Ti atom via S–O bonding bridge, then the spin polarization of p orbit of the Ti atom is rearranged. In addition, if the S atoms are doped into TiO2 inner (interstitial or substitutional site), the electron of S atom is localized at the doped site and no free electron is used to affect the spin arrangement of OVs (no magnetic moment occurs, so this result is not shown). As shown in figure 4(c), the increase of OV concentration causes the more spin polarization at OV positions, and a 3.75 µB magnetic moment exhibits in this case. When OV concentration is further increased in figure  4(d), the mag-netism is not enhanced significantly. This is because that free electrons originating from a certain adsorbed S atom cannot support more OVs to occur spontaneous spin polarization. According to the calculated magnetism behavior shown in figure 4, the exper imental results can be explained. Firstly, the combined action of adsorbed S atoms and OVs can lead to stronger FM, as is consistent with the experimental results

obtained from the samples annealed at 200 °C in figure 3(a). Annealing at 600 °C eliminates some OVs on the NC sur-face, so the magnetism begins to decrease. When annealing temper ature is further enhanced, the OVs further are removed and some doped S atoms also are taken away, which leads to the further reduction of the saturated magnetization. On the contrary, annealing treatment in Ar, OV concentration is retained, but the distribution of doped S atoms is changed obviously. As annealing temperature increases, the adsorbed S atoms are incorporated into the inner of TiO2 NC or form some smaller clusters which cannot result in magnetism. Adsorbed S atoms on TiO2 NC surface reduce abruptly; therefore, the magnetism also becomes smaller. The exper-imental and theoretical results indicate that the adsorbed S atoms and OVs are responsible for the FM production.

In summary, the FM acquired from S-doped TiO2 NCs pro-duced by hydrothermal synthesis method shows that as the annealing temperature increases, the saturated magnetization reduces sharply. Our spectral analysis and DFT calculation reveals that the FM originates from the electronic coupling between adsorbed S atoms and surface OVs. Our results demonstrate that suitable S-doping and oxygen vacancy can be used to initiate FM, which proposes a possibility to improve surface catalytic activity by regulating spin electronic arrangement.

Figure 4. Calculated spin density and magnetic moments for different S atom adsorptions VS and OV concentrations VO.

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Acknowledments

This work was supported by National Natural Science Foun-dation (No. 11404162), and Natural Science Foundation of Jiangsu Province (BK20171332). We also acknowledge the computational resources provide by High Performance Com-puting Centre of Nanjing University.

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