âHealingâ Effect of Graphene Oxide in Achieving Robust Dilute

Article pubs.acs.org/JPCC

“Healing” Effect of Graphene Oxide in Achieving Robust Dilute Ferromagnetism in Oxygen-Deficient Titanium Dioxide Qing Zhu,† Xijun Wang,† Jun Jiang,* and An-Wu Xu* Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physical Chemistry, Department of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: Titanium dioxide (TiO2) is an important wideband-gap semiconductor with promising application for nextgeneration spintronics. Unfortunately, the lack of inherent spin ordering enormously hinders the widening scope of TiO2, and the origination of ferromagnetic properties still needs to be comprehensively explored due to the fact that manipulating the magnetic property in semiconductor through defect engineering remains a great challenge. Here we systematically investigate the room-temperature ferromagnetism (RTFM) behavior of defective anatase TiO2−x with the exposed (001) facet grown on reduced graphene oxide (rGO). First-principles simulations were performed to examine two types of intrinsic oxygen defects in TiO2−x: vacancy on surface (VO‑Sur) and at subsurface (VO‑Sub), among which only the VO‑Sub contributes a considerable magnetism. Interestingly, simulations revealed a socalled “healing” effect for the oxygen functional groups in rGO, by removing the VO‑Sur defect of TiO2−x, which helps establish good interface and thereby ensures good coupling between rGO and VO‑Sub defects. Calculations show that the interaction of rGO with Ti3+−oxygen vacancy associates alters spin asymmetric electron distribution around VO‑Sub and consequently introduces significant spin asymmetric defect states near the Fermi level. Hence rGO triggers a significant magnetic enhancement in TiO2−x. Importantly, our findings pave a new avenue for effective design and manipulation of spin states in an undoped dilute ferromagnetic semiconductor for spintronics application.

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INTRODUCTION Spintronics utilizes the spin-related phenomena of unpaired electrons to create useful memory, sensors, and logic devices with properties that are unable to be realized by traditional charge-based devices.1−3 Dilute magnetic semiconductors (DMSs) since they hold both spin and charge degree of freedoms confined within one matrix are now envisioned as the key functional components for next-generation spin-based devices.4−6 The fabrication of room-temperature ferromagnetic semiconductors, high-efficiency spin injection, and spin manipulation has attracted considerable interest in the past few years. Achieving functional spintronic devices involves development of new materials and integration of diverse materials with atomic-level control, and extensive efforts have been made to make it applicable for spintronic technologies. Generally, transition metal doped semiconductor materials display dilute ferromagnetism. Some prototype device has been achieved in Mn-doped ZnSe, GaAs, and InAs DMSs,7−9 but the cryogenic Curie temperatures (TC, ∼100 K) required for ferromagnetic ordering have hampered its practical application. Obviously, a high TC in the DMSs is highly worthwhile when the TC is above room temperature. In recent years, ZnO- and TiO2-based DMSs have been the subject of numerous experimental and theoretical investigations.10−13 In particular, titanium dioxide (TiO2) is an important wide-band-gap functional semiconductor with promising application for nextgeneration spintronics.14 Unfortunately, the absence of intrinsic © XXXX American Chemical Society

spin ordering of common defect-free TiO2 has hampered its applications in spintronics.15 TiO2 has long been considered as an ideal candidate for achieving wide-bandgap semiconductorbased DMSs with high TC through doping with impurity element, which obtained very limited success despite numerous attempts.16 Moreover, an extraneous impurity element even might display deteriorating effect on charge separation/transfer because the elusive nature of dopant has made it difficult for practical implementation. Therefore, optimal band structure modification and high spin polarization remained as two goals in this field. Nevertheless, in-depth research about the electronic and atomic constructions is crucial to revealing the origination of the room-temperature ferromagnetism (RTFM) for the manufacturing of DMSs. Cobalt-doped TiO2 (Co2+:TiO2) has attracted intense attention since the initial report of stable ferromagnetism above 300 K.17−19 Although a variety of DMS materials were extensively studied, no unanimous conclusion for the origin of the observed RTFM can be drawn. A general belief is that metallic precipitates or clusters of Co atoms are responsible for the ferromagnetism in these materials.20,21 Up to now, the study on the DMSs has been mainly focused on the transition metal doped semiconductors with externally introduced defects Received: July 17, 2017 Revised: September 28, 2017

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DOI: 10.1021/acs.jpcc.7b07011 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) UV−visible diffuse reflectance spectra and (b) X-ray diffraction patterns of defect-free TiO2, TiO2−x, and TiO2−x/rGO. The inset is the corresponding function of Kubelka−Munk curves versus photon energy. (c) TEM and (d) HRTEM images and corresponding SEAD pattern (inset) of an individual TiO2−x/rGO nanosheet.

defects through the formation of strong interface covalent bonds. After interfacing with graphene, the saturation moment (Ms) of the TiO2−x/rGO reached up to ∼0.025 emu/g which was around 6.6 times higher than that of the TiO2−x (0.0038 emu/g). First-principles simulations were performed to examine the geometric details, electronic structure, and magnetic properties at spin-polarized density functional theory (DFT) level. Two types of intrinsic oxygen defects were identified: vacancy on surface (VO‑Sur) and vacancy at subsurface (VO‑Sub), among which only the VO‑Sub is found to contribute a considerable magnetism. According to our understandings, this is the first report to systematically examine the spatial oxygen vacancy distribution dependence of the magnetic properties in nonstoichiometric TiO2−x. The hybridizations of graphene with TiO2−x were then examined. The results show that the VO‑Sur vacancies can be easily healed by the oxygen functional groups in rGO, resulting in intimate interface. The strong coupling between the rGO and remaining VO‑Sub vacancies then plays an important role in modulating ferromagnetism in undoped TiO2−x. This helps elucidate the origin of ferromagnetism and emphasizes the probability to modulate such a defect-induced RTFM by oxygen vacancy controlling for future spintronics application. TiO2−x/rGO hybrid was obtained through a solvothermal process, and the ethanol used here acted as both solvent and reducing agent to creat VO (see experimental section in the Supporting Information). The optical response of defect-free TiO2 compared to that of defective TiO2−x and TiO2−x/rGO hybrid is shown in Figure 1a. It can be seen that defect-free TiO2 only has a certain degree of absorbance in the ultraviolet

in their structure, while little attention has been paid to the magnetism in pure phase structures. Recently, RTFM was reported in a class of undoped semiconductors. For instance, dopant-free HfO2 containing nonmagnetic Hf4+ was proved to have RTFM.22 Some other undoped oxides like In2O3, SnO2, and TiO2 were also found to have a similar phenomenon.23−25 A growing number of both experimental and theoretical evidence continues to point out that the RTFM of undoped semiconductor is strongly relevant to VO and considered to be the origin of RTFM.26,27 It has been shown that defect engineering is a key point in adjusting the functionality of semiconductor including electronic structures, charge transportation, as well as magnetism. Lattice with vacancy defects is therefore expected to provide an ideal platform to investigate the origin of ferromagnetism in undoped DMSs. In summary, there is no unifying and conclusive outcome from the existing studies, so the origin and manipulation of the magnetic properties of undoped TiO2 are still an open question. In the present work, we designed a facile and chemical strategy to directly synthesize defective TiO2−x nanosheets (TiO2−x NSs) with exposed (001) facets on reduced graphene oxide. The RTFM in the TiO2−x is derived from the creation of F-center (oxgen vacancies with localized electrons). The introduction of VO contributes a robust ferromagnetism at 300 K compared to pristine diamagnetic TiO2. Moreover, hybridization with rGO can alter the spin electron carrier distribution in TiO2−x, which increases the ferromagnetic exchange and leads to an enhanced RTFM. As graphene has unsaturated π orbitals (pz) and lacks a band gap, both characters favor electronic interactions with TiO2−x intrinsic B


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Figure 2. (a) Fitted XPS spectrum of Ti 2p of TiO2−x/rGO. (b) O 1s XPS spectrum of graphene oxide (GO) and TiO2−x/rGO nanosheets. (c) C 1s XPS spectrum of GO and (d) TiO2−x/rGO hybrids.

shows mutually perpendicular planes of 0.19 nm and can be firmly assigned to the typical (200) and (020) atomic planes of anatase.30−32 The corresponding selected area electron diffraction (SAED) patterns also reveal that the TiO2−x nanosheets obtained here are highly single crystalline with [001] axial orientation. The chemical state of the TiO2−x/rGO hybrids and information about these chemical bonds between TiO2−x and rGO were investigateed by X-ray photoelectron spectroscopy (XPS). The valence state of Ti in TiO2−x/rGO is shown in Figure 2a. The peaks at 458.6 and 464.4 eV should be ascribed to the Ti 2p3/2 and 2p1/2, which are characteristic binding energies (BEs) for Ti4+−O bonds in TiO2.33 The broadening of the Ti 2p3/2 peak at lower BEs indicated that there exists partial Ti3+ in the TiO2−x/rGO. The incorporation of VO is associated with the valence state transformation from formal Ti4+ in defect-free TiO2 to Ti3+ in defective TiO2−x. The Ti 2p XPS of the TiO2−x/rGO hybrid can be deconvoluted into two BEs at 458.2 and 463.7 eV, respectively, which are typical characteristic peaks of trivalent Ti3+.34 Besides, we also observed a bathochromic shift and broadening of the Eg vibrational mode in the Raman spectrum, which were dominantly derived from the oxygen deficiency and asymmetric lattice structures. This result further confirms the presence of VO in these reduced TiO2−x NSs (Figure S4, Supporting Information). In Figure 2b, the single O 1s XPS peak located at a BE of 529.7 eV should be ascribed to the Ti−O−C chemical bonds. Accroding to the C 1s XPS of GO in Figure 2c, we found two new peaks with BEs located at 286.9 and 288.8 eV. The peak at BEs of 286.9 eV belong to the C−O bond, while the BEs at 288.8 eV come from the O−CO carboxyl group.35,36 Moreover, the deconvoluted peak with BEs at 284.7 eV is contributed from C−C, CC, and C−H bonds. Apparently, the signal intensities of the O-containing groups were decreased

region due to the inherent wide band gap, as reflected by the absorption edge at ∼3.2 eV. The absorption of TiO2−x in the visible-light region gradually increased to a higher level, which clarifies the color change from white to light blue (Figure S1). The broad visible/near-infrared absorption is the result of free electrons supplied by oxygen vacancies in the TiO2−x lattice, which could be trapped to form F-centers, thus giving strong absorption from visible light until NIR.28 Besides, the sample turns white after annealing at 500 °C in air, providing solid evidence for the presence of oxygen defects. At the same time we also noticed that the powder color became black after TiO2−x hybridizing with graphene (TiO2−x/rGO). This enhanced absorption could be attributed to the presence of rGO. We can calculate the band gap through plots of (αhν)1/2 vs the incident photon energy. The calculated band gaps of these samples are identical, indicating that vacancy leads to the absorption in the visible region through formation of isolated bands.29 The XRD results are shown in Figure 1b, and all of these samples exhibited typical diffraction peaks of the TiO2 anatase phase (JCPDS card no. 84-1285; space group I41/amd). These indicate virtually no phase change of XRD patterns during the process of self-doping by oxygen vacancy, as well as the addition of GO. Nevertheless, the XRD peak positions are slightly shifted to a higher diffraction angle in TiO2−x and TiO2−x/rGO. Such shifts indicate a very small change in the lattice parameter value, demonstrating that the self-modification induced by oxygen vacancy likely distorted the TiO2 matrix to make the crystal lattice shrink. The morphology of TiO2−x/ rGO was investigated by TEM. As shown in Figure 1c, graphene nanosheets are completely covered with TiO2−x, and the average side length of TiO2−x nanosheets is calculated to be around 35 nm. The edges of graphene and the extrinsic feature of TiO2−x nanosheets can be clearly observed. Figure 1d displays the HRTEM image of an individual nanosheet, which C


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Figure 3. (a) Ti K-edge X-ray absorption near-edge spectrum of the three samples including reference materials (standard Ti foil, defect-free TiO2 and TiO2−x) and details of the (b) pre- and (c) postedge region. (e) Ti K-edge extended EXAFS oscillation function k3χ(k) and (f) the corresponding Fourier transforms (FTs) for defect-free TiO2, TiO2−x, and TiO2−x/rGO.

than defect-free TiO2 and TiO2−x. In particular, the P2 feature is more intense and shifted slightly toward lower energies, indicating a lower coordination number and intensity since the P2 peak increases with the increase of site distortion.39 On the other hand, the order of white line intensity (TiO2−x/rGO < TiO2−x < TiO2) around 4987 eV is just the opposite of the order we observed in the pre-edge region. This behavior can be ascribed to the low electron transition capability caused by the high degree of p−d hybridization.40 Increased peak intensity in the pre-edge region is a reflectance of centrosymmetry broken. These results above imply a higher distortion degree of TiO6 octahedron units in TiO2−x/rGO compared to defect-free TiO2 and TiO2−x. The Ti species in TiO2−x/rGO exist in a lower average oxide state (Tiy+, 3 < y < 4), which is consistent with the XPS measurements described above. In addition, we also use extended X-ray absorbance fine structure (EXAFS) analysis to gain more detailed crystal structure information. The Ti Kedge k3χ(k) oscillation curve and the derived Fourier transforms (FTs) of defect-free TiO2, TiO2−x, and TiO2−x/ rGO were depicted in Figure 3d and e. The FTs present two typical peaks in the range of 1−3 Å: the first shell represents the nearest-neighbor Ti−O, and the second is Ti−Ti pair. The positions of the atomic shells correlate well with an anatase crystal structure. The spectral features of these samples are similar to each other, indicating that local anatase structure is mainly preserved.41,42 However, the Ti K-edge EXAFS of the TiO2−x/rGO hybrid presented a similar oscillation with a slight amplitude reduction, corresponding to a decrease of the maxima in the FTs. The Ti K-edge EXAFS features clearly demonstrate that the Ti atoms are in a mixture of 5-fold (TiO5) and 6-fold (TiO6) coordinate environments, and the TiO6 octahedron becomes irregular or distorted after removing oxygen atoms from the lattice. The distortion of the TiO6 octahedral structure in TiO2−x/rGO was the highest among all the samples, as reflected by the decreased peak intensity in

dramatically in the C 1s spectra of TiO2/rGO (Figure 2d), suggesting the convertion of GO to rGO. The results we obtained above confirm the reduction of GO and incorporation between TiO2−x and rGO, as well as the strong interaction between the C, O, and Ti. It was also verified that the solvent thermal treatment in ethanol leads to the formation of Ti3+ in the TiO2 lattice due to the lattice oxygen removal, which is consistent with previous reports that reaction in a reductive condition enables the partial reduction of transition metal ions accompanied by the generation of oxygen vacancy (VO), namely, [Ti4+−O−Ti4+] → [Ti3+−□−Ti3+] + (1/2)O2.37 During the formation of an oxygen vacancy, excess electrons are localized onto nearby Ti ions, reducing them from Ti(IV) to Ti(III), as VO coexists with the nearest Ti atom (VO−Ti3+ vacancy associates). For charge balance, the Ti3+ ion would exist in the defective TiO2−x compound. In Kröger−Vink notation, this is × OO + 2Ti ×Ti → V O•• + 2Ti′Ti + 1/2O2 (g)

where O×O represents a neutral oxygen on an oxygen lattice site; Ti×Ti is a neutral titanium atom on a titanium site; V•• O is positively charged vacancy at an oxygen lattice site; and TiTi ′ is negatively charged titanium (i.e., Ti(III)) at a titanium lattice site. X-ray absorption near-edge structure (XANES) and extended X-ray absorbance fine structure (EXAFS) measurment were carried out to understand the changing of crystal structure in these materials. In principle, there are triple characteristic peaks in the XANES for anatase TiO2 located at around 4972 eV, and we mark them as P1, P2, and P3, respectively (shown in Figure 3a and b). This triple pre-edge feature can be assigned to the pure electric quadruple-allowed 1s → 3d transition through hybridization of the Ti 3d−4p orbital and afford sensitive message of the detailed changing of Ti site symmetry.38 The intensity of peaks in the pre-edge of TiO2−x/rGO is stronger D


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Figure 4. (a) X-band ESR spectrum of TiO2−x and (b) TiO2−x/rGO. (c) Positron annihilation lifetime spectrum of TiO2−x and TiO2−x/rGO samples. (d) Schematic illustration of the “healing” process through the O functional group of GO cluster species filling into the VO‑Sur surface oxygen vacancy defect, which lowers the total energy of the system.

characterization to give information on the nature of vacancy size, charge state, and relative concentration. The lifetime components τ1 and τ2 have been proved to be a sign of the presence of VO‑Sub and VO‑Sur in the titanium dioxide.47−50 The derived fitting parameters are shown in the inset in Figure 4c, and the relative ratio of VO‑Sur/VO‑Sub can be roughly estimated by comparing the intensity of I2/I1. The VO‑Sur/VO‑Sub for TiO2−x/rGO was determined to be 2.24, which is apparently higher than the 1.85 for TiO2−x. In our case, we have ruled out the possibility of the existence of outer VO‑Sur in TiO2−x/rGO, indicating the enrichment of subsurface defects prepared via GO hybridization. This was understandable considering the strong interlayer chemical bonds, and the rGO shell may act as a passivation layer to effectively block inner VO from further oxidation, thus contributing to minimizing the surface energy and hence to guarantee the excellent structure stability. To explore the above formation mechanism, we performed DFT calculations to evaluate the energies of systems. Three model structures were constructed in Figure 4d: a layer of GO cluster with a C−O−C dangling bond (edge saturated by hydrogen atoms), a TiO2 (001) surface containing one VO‑Sur (TiO2−x), and the TiO2−x hybridization with the graphene oxide cluster (TiO2−x/rGO). The latter was found to evolve into a “TiO2/ graphene” structure after geometric optimization, in which the VO‑Sur defect (TiO2−x) was filled by the O atom on rGO. Total energies of the separated TiO2−x and rGO were lowered by 4.95 eV after forming this “TiO2/graphene” system, agreeing with the experimental observation that their hybridization is thermodynamically preferred. Such a defect removing process can be regarded as the result of the oxygen functional group of GO “healing” the VO‑Sur defect site. Most likely, the growth of GO on the oxygen-deficient anatase TiO2−x (001) surface results in a hybrid structure TiO2−x (VO‑Sub)/rGO, in which the rGO (or even graphene) is contacting to the TiO2−x with subsurface defect. A large number of reports have shown that VO holds great influence on the solid properties of semiconductors, including the RTFM properties of DMSs.51 The magnetization measurement results for these nanostructures are displayed in Figure 5a and b. The magnetic field dependence of the magnetization

Figure 3e, suggesting a serious distorted coordination existed in TiO2−x/rGO due to more Ti-centered octahedral coordinations missing. These results described above give clear solid evidence that the structure of TiO2−x hybrids has been more disordered on hybriding with rGO. Thus, large ordered moments and long-range ferromagnetism interaction confined within a spinpolarized band gap are highly expected. Electron spin resonance (ESR) has been proved effective to investigate unpaired electrons in nanomaterials, such as VO and paramagnetic radicals. As displayed in Figure 4a and b, comparing to the signals of defect-free TiO2 (Figure S5 in the Supporting Information), the sample TiO2−x NSs exhibit a strong ESR signal, and the calculated g values are g⊥ = 1.92 and g∥ = 1.88, which have previously been attributed to paramagnetic Ti3+ centers.43,44 In addition, ESR measurements on TiO2−x NSs have also identified a long-lived superoxide (•O2−) species at g = 2.003 associated with Ti3+ sites, which confirms the presence of surface oxygen vacancy (VO‑Sur) on the anatase (001) facets. Since VO‑Sur can efficiently govern O2 adsorption by trapping atmospheric oxygen molecules on defect sites, the extra charge associated with the oxygen vacancy transported to the O2 and resulted in a superoxide •O2− species, generating a broad ESR signal at g ≈ 2.002.45,46 It is worth noting that the •O2− signal is completely quenched for the TiO2−x/rGO NSs (Figure 4b), which is significantly different from the signal observed for TiO2−x. The ESR signal of VO‑Sur quenched after GO hybridization should ascribed to the “healing” effect of oxygen-containing functional groups of GO filling VO‑Sur defect sites, indicating that VO‑Sur is unstable and susceptible to oxygen functional groups in GO during the solvothermal treatment. Furthermore, the oxygen defects in as-prepared TiO2−x/rGO have been demonstrated to be inherent under the near surface, explaining why the observed Ms is robust. This strongly implies that the enhancement of the observed ferromagnetism of TiO2−x/rGO at oxygen-deficient conditions originates from subsurface oxygen vacancies (VO‑Sub). The vacancies in these TiO2−x samples were further examined through positron annihilation spectroscopy (PAS), which is based on the detection of two γ rays arising from the annihilation of a thermalized positron. It is considered to be the most powerful E


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Figure 5. (a) Room-temperature M−H curves of pure TiO2, TiO2−x, and TiO2−x/rGO. (b) Temperature dependence of ZFC and FC curve for TiO2−x/rGO NSs. Inset: the magnified region of ZFC and FC between 77 and 300 K. (c) The computed net spin up/down electron density (subtracting the spin down electrons from the spin up electrons) distribution in the hybrid system of a TiO2−x (VO‑Sub)/rGO structure, in which the 4th, 13th, and 15th Ti atoms are the nearest neighbors of the VO‑Sub defect. (d) The computed difference net spin electrons on each Ti atom by subtracting the spin electrons of the TiO2−x (VO‑Sub) structure from those of the TiO2−x (VO‑Sub)/rGO hybrid. (e) and (f) The computed electronic density of states (DOS) of certain orbitals in the TiO2−x (VO‑Sub) and TiO2−x (VO‑Sub)/rGO, respectively.

intrinsic RTFM of the TiO2−x/rGO with Curie temperature (TC) far over 300 K. Moreover, the saturation Ms of the TiO2−x/rGO can afford 0.025 emu/g at 300 K, dramatically superior to many other ferromagnetic semiconductors.53−61 The field cooling (FC) along with zero field cooling (ZFC) measurements clearly show distinct differences and diverge substantially in the wide temperature range, which is a typical ferromagnetic behavior and suggests the intrinsic RTFM of the TiO2−x/rGO NSs with Curie temperature far over 300 K. Moreover, we could not observe any block temperature in the ZFC curve, which unambiguously eliminates the existence of ferromagnetic clusters in TiO2−x/rGO NSs. Generally, studies of DMSs are difficult due to the small magnetic moments involved, which can easily be caused by contaminations from extrinsic sources. Of note, we also exclude the existence of ferromagnetic impurities through ICP-AES analysis of magnetic elements such as Fe, Co, and Ni. These impurities were not

(MH) hysteresis loop of defect-free TiO2 at room temperature clearly illustrates the diamagnetic nature of pristine TiO2, which can be ascribed to the absence of intrinsic spin ordering. The expected typical ferromagnetic behaviors of TiO2−x and TiO2−x/rGO are fully confirmed by the saturation magnetization (Ms) hysteresis loops with well-defined curves. The Ms for the oxygen-deficient TiO2−x nanosheets is substantially reached to 0.0038 emu/g, accompanied by the introduction of oxygen vacancies and Ti3+ cations observed in Figure 5a. Although the Ms is still weak, the observed magnetic ordering is fundamental, indicating that the RTFM of TiO2−x does come from the oxygen vacancies. The magnetization data collected for sample TiO2−x/rGO consistently exhibited a clear ferromagnetic behavior, achieving an Ms of 0.025 emu/g with coercivity Hc of 38 Oe, showing the typical RTFM characteristics of DMSs. This is significantly superior to the reported Co2+:TiO2 nanocrystalline and undoped TiO2,52 indicating F


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near the Fermi level and then induced a significant magnetic enhancement. The saturation magnetization value of the TiO2−x/rGO nanosheets could reach 0.025 emu/g at 300 K. Meanwhile, we find a vacancy “healing” effect, which is thermodynamically preferred, utilizing the oxygen functional groups of rGO to “heal” the surface oxygen vacancies toward well-coupled interface. The copresence of inherent ferromagnetic behaviors and band gaps confined within semiconductors provided great opportunities to finely regulate the charge and spin simultaneously while ensuring high utilization coefficient of spin-related devices, such as magnetic sensors, nonvolatile memory, spin-logic devices, and quantum computing.

detected since their concentrations were all below ICP-AES detection limits, and the results were consistent with XPS and EDX analysis (Figure S6 in the Supporting Information). Hence, all of the above characterizations confirm the intrinsic ferromagnetic behavior nature of the TiO2−x/rGO. In principle, the magnetic property comes from the unpaired spin up/down electrons. Theoretical simulations were then performed to examine the spin electronic structures influenced by VO‑Sub and VO‑Sur doping, as well as graphene attaching. It is found that the O vacancy defects induce weak magnetic property for TiO2−x, by accumulating extra spin up electrons, while those extra spin up electrons can be easily extracted to the graphene layer through its coupling with the surface defect VO‑Sur (no matter VO‑Sub exists or not), as illustrated in Figure S7 in the Supporting Information. Fortunately, owing to this helpful “healing” effect of rGO that removes surface VO‑Sur, we can establish a good interface between TiO2−x and graphene. The VO‑Sub sites are more thermodynamically stable than those at VO‑Sur, thus we can focus on the interplay between VO‑Sub and graphene based on the atomic models in Figure S8.62,63 Comparing to zero spin electrons in the pristine TiO2, the TiO2−x (VO‑Sub) exhibits effective spin up electrons (Figure S10 in the Supporting Information), which mostly located at the three Ti atoms (4th, 13th, 15th) around the VO‑Sub defect (Table S1 in the Supporting Information). The computed DOS diagrams identify the emergence of defect states just below the conduction band minimum (CBM). As in Figure 5c, the attaching of graphene to the TiO2−x (VO‑Sub) can further increase the spin up electrons. The differential spin electrons between the TiO2−x (VO‑Sub)/rGO hybrid and TiO2−x (VO‑Sub) displayed in Figure 5d also demonstrate the dramatic enhancements of spin up electrons and decreases of spin down electrons, in the three Ti atoms around the VO‑Sub defect (Table S2 in the Supporting Information). As a result, the computed density of states (DOS) shows spin asymmetric states for the 3d orbitals in the oxygen-deficient TiO2−x (VO‑Sub) in Figure 5e, compared to the identical spin up/down features of pure TiO2 (Figure S10 in the Supporting Information). The spin asymmetric effect of the 3d orbitals becomes even stronger in the TiO2−x (VO‑Sub)/rGO hybrid (Figure 5f). A high TC usually occurred when majority/minority spin d orbitals emerge around Fermi levels.64 Consequently, the calculated magnetic moment of TiO2−x (VO‑Sub) is ∼0.20 μB, which is increased to ∼0.99 μB by attaching graphene. The increase in ratio is about 5 times, consistent with the RTFM enhancement found in experiments (Table 1).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07011. Experimental details and additional material characterizations (PDF)

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TiO2

TiO2−x (VO‑Sub)

TiO2−x (VO‑Sub)/rGO

exp (emu/g) sim (μB)

− 0.00

0.0038 0.20

0.025 0.99

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun Jiang: 0000-0002-6116-5605 An-Wu Xu: 0000-0002-4950-0490 Author Contributions †

Q.Z. and X.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (No. 51572253, 21271165, 21633006, 21473166, 21771171), the cooperation between NSFC and Netherlands Organisation for Scientific Research (51561135011), the MOST 973 Program (No. 2014CB848900), and CAS Strategic Priority Research Program B (No. XDB01020000).

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REFERENCES

(1) Ohno, H. Making nonmagnetic semiconductors ferromagnetic. Science 1998, 281, 951−956. (2) Ando, K. Seeking room-temperature ferromagnetic semiconductors. Science 2006, 312, 1883−1885. (3) Dery, H.; Dalal, P.; Cywinski, L.; Sham, L. J. Spin-based logic in semiconductors for reconfigurable large-scale circuits. Nature 2007, 447, 573−576. (4) Viswanatha, R.; Pietryga, J.; Klimov, V.; Crooker, S. Spinpolarized Mn2+ emission from Mn-doped colloidal nanocrystals. Phys. Rev. Lett. 2011, 107, 067402. (5) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping semiconductor nanocrystals. Nature 2005, 436, 91−94. (6) Ochsenbein, S. T.; Feng, Y.; Whitaker, K. M.; Badaeva, E.; Liu, W. K.; Li, X. S.; Gamelin, D. R. Charge-controlled magnetism in colloidal doped semiconductor nanocrystals. Nat. Nanotechnol. 2009, 4, 681−687. (7) Ruster, C.; Borzenko, T.; Gould, C.; Schmidt, G.; Molenkamp, L. W.; Liu, X.; Wojtowicz, T. J.; Furdyna, J. K.; Yu, Z. G.; Flatte, M. E.

Table 1. Experimentally Measured and Computed Magnetic Moments of the Pristine TiO2, Defective TiO2−x (VO‑Sub), and Hybrid TiO2−x (VO‑Sub)/rGO Systems magnetic moment

ASSOCIATED CONTENT

S Supporting Information *

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CONCLUSIONS In this work, we found that the incorporation of graphene oxide would be a new yet convenient strategy to regulate the ferromagnetism of oxygen-deficient TiO2−x for the first time. Through altering the spin electron distribution around the subsurface oxygen vacancy (VO‑Sub) in TiO2−x, rGO created more significant spin asymmetric defect states in the PDOS G


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