Magnetic Circular Dichroism of Substoichiometric Molybdenum Oxide

Jul 8, 2019 - In such substoichiometric MoO3–x, a sharp distinction has been reported in the coloration mechanisms: a small-polaron model or localiz...
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Magnetic Circular Dichroism of Substoichiometric Molybdenum Oxide (MoO3−x) Nanoarchitectures with Polaronic Defects Taisei Kimura and Hiroshi Yao* Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan

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S Supporting Information *

ABSTRACT: Magnetic circular dichroism (MCD) is demonstrated for the first time in substoichiometric molybdenum oxide (MoO3−x) nanoarchitectures to explore the origin of their near-IR (NIR) transitions. Various nanostructures of MoO3−x are synthesized by a simple hydrothermal process using an ionic Mo(VI) or metallic Mo(0) precursor. The phase, morphology, and spectroscopic properties of the obtained nanostructures are dependent both on the reaction temperature and the molybdenum precursor. In particular, hexagonal MoO3−x nanostructures are colored blue with different degrees. To better understand the nature of the electronic states in these nanomaterials, MCD spectroscopy is conducted. A derivative-like MCD response is detected in the vis-NIR region, but it is not attributed to surface magnetoplasmonic modes because of the absence of the peak-energy shift with the increasing refractive index of the dispersion medium. Then, the bisignate MCD signal can be attributed to Faraday B-terms for small-polaronic transitions, arising from two interacting polaronic states close in energy that would give opposite signs under an applied magnetic field. We believe that MCD evaluation for polaronic nanomaterials will expand promising new applications in fields such as semiconductor-based nanophotonics and magneto-optical devices.

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lead to novel applications.6 However, despite the intensive studies on their optical properties, there is no unique model (or exists some controversy) to satisfactorily describe the origin of NIR absorption for nanostructured MoO3−x probably due to the spectroscopic and/or morphological diversity in the oxides.7,8 Meanwhile, magnetic circular dichroism (MCD) is the differential absorption of left and right circularly polarized light measured under a magnetic field and gives significant information on the electronic structures of the materials in detail, since the MCD arises from the same transitions as those seen in the normal absorption spectrum but the selection rules are different.9 Then, MCD spectroscopy has been widely applied in molecular systems as well as magnetic materials to reveal their electronic-state information.9,10 Furthermore, recent studies have shown that plasmonic noble metal (Au or Ag) nanostructures can have appreciable MCD responses in the LSPR region,11−13 exhibiting a bisignate signal that can be interpreted by a substantial increase of the Lorentz force induced by the circular movement of the free carriers or conduction electrons. Hence, MCD also serves as a powerful tool to obtain significant information on the electronic transitions or states in various plasmonic nanomaterials. In the present study, to clearly examine the blue coloration effect of MoO3−x nanostructures, that is, to explore the origin

INTRODUCTION Molybdenum oxides are important oxide semiconductor compounds with many applications in sensors, catalysis, energy-storage, photothermal therapy, and photodevices.1,2 They are found in various stoichiometries with an oxygen deficiency, ranging from full stoichiometric MoO3 to metallic MoO2, and their practical performance is known to depend on the compositions and/or structures.1−3 Upon introducing oxygen defects in the stoichiometric MoO3, Mo6+ ions are reduced to Mo5+ and finally Mo4+, so the optical appearance changes as a function of the degree of Mo6+ reduction. For example, MoO3 seems to be white in color, whereas MoO3−x (0 < x < 1) generally shows an intense blue color due to its near-IR (NIR) absorption nature. In such substoichiometric MoO3−x, a sharp distinction has been reported in the coloration mechanisms: a small-polaron model or localized surface plasmon resonance (LSPR) mechanism when the oxide materials are amorphous or crystalline, respectively.4,5 In small polarons, the carrier (electron in this case) is self-trapped at essentially one lattice ion, yielding local lattice distortion, and the corresponding transitions occur from the initial trapping site to a final equivalent one.4,5 Here, the oxygen deficiency x would then influence the electron concentration, so as x (or free carrier) increases, the property is expected to be near that of metal. Therefore, high free-carrier concentration is the main reason for producing localized surface plasmons and the carriers can oscillate resonantly with light in the finite volume of the structures.6 In this regard, the field of plasmonics in semiconducting metal oxides is rapidly growing since it can © XXXX American Chemical Society

Received: April 7, 2019 Revised: June 12, 2019 Published: July 8, 2019 A

DOI: 10.1021/acs.jpcc.9b03225 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) TEM images of the nanostructured MoO3−x samples obtained by protocol-I. (b) XRD patterns of the MoO3−x samples obtained by protocol-I. Red or green circles indicate diffraction peaks ascribed to h-MoO3 or α-MoO3, respectively.

176.5 mg of (NH4)6Mo7O24·4H2O was dissolved in a homogeneous solution mixture of water (25 mL), 0.6 M hydrochloric acid (10 mL), and PEG-400 (5 mL), and stirred for 30 min. The colorless solution was moved to a 50 mL Teflon-lined autoclave and heated at 90, 120, or 150 °C for 12 h. After cooling the reactants naturally, the blue precipitate obtained was collected by centrifugation (14 000 rpm, 5 min), followed by washing with ethanol and water three times. The nanostructured MoO3−x sample obtained at the reaction temperature of 90, 120, or 150 °C by this protocol was referred to as MP90-I, MP120-I, or MP150-I, respectively. Protocol-II. In this process, nanostructured MoO3−x samples were also synthesized in a similar manner to those of protocol-I, but metallic molybdenum powder was used as the molybdenum source.17 Briefly, 192 mg (2 mmol) of molybdenum powder was dissolved into a homogeneous solution mixture of water (25 mL), H2O2 (30%, 3 mL), and PEG-400 (5 mL) and stirred for 30 min. The orange-colored solution was then moved to a 50 mL Teflon-lined autoclave and heated at 90, 120, or 150 °C for 12 h. The precipitate was collected by centrifugation, followed by washing with ethanol and water three times. The molybdenum oxide sample obtained at 90, 120, or 150 °C via protocol-II was referred to as MP90-II, MP120-II, or MP150-II, respectively. Instrumentation. MCD spectroscopic measurements were carried out with a JASCO J-820 spectropolarimeter equipped with a permanent magnet (PM-491LB) of 1.6 T (tesla) with parallel (+1.6 T) and antiparallel (−1.6 T) fields.11,12 UV− vis−NIR absorption (or extinction) spectra were recorded with a Hitachi U-4100 spectrophotometer. Fourier-transform infrared (FT-IR) spectra were recorded with a JASCO FT/IR-550 spectrophotometer by the KBr disk pellet method. The phase structure of the products was examined by X-ray diffraction (XRD) using a Rigaku Ultima-IV X-ray diffractometer with Cu Kα radiation (λ = 1.5405 Å). Transmission electron microscopy (TEM) measurements were performed on a JEOL TEM-1011M electron microscope, operating at an

of their NIR absorption in detail, MCD spectroscopic study is carried out for the first time in substoichiometric MoO3−x nanostructures with different morphologies, which can be synthesized by a hydrothermal process. The hydrothermal synthesis utilizes reactions of some molybdenum precursors at elevated temperatures and high pressure in aqueous solution, allowing controlled phase and crystallization of the oxides. We here use two different precursors for the synthesis; ammonium molybdate(VI) and metallic molybdenum powder.14,15 In both cases, blue MoO3−x nanostructures with hexagonal (but illcrystalline) phase are typically obtained at the reaction temperature of 90 or 120 °C on a large scale. Impressively, a derivative-like MCD response is observed in the vis-NIR region (600−1200 nm), reminiscent of the magnetoplasmonic behavior of LSPR electrons; however, in ordinary absorption spectroscopic studies, the detected NIR peak positions are independent on the dispersion medium with different refractive indices, morphology, and size/shape of the oxides, inconsistent with the typical behaviors of LSPR of conduction electrons. We eventually conclude that the bisignate MCD response observed in the NIR region is due to Faraday Bterms, arising from two interacting polaronic states close in energy, which can give opposite signs.

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EXPERIMENTAL SECTION Materials. Ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O), metallic molybdenum, and molybdenum(VI) oxide powders of the highest grade were purchased from Kanto Chemical Co. Poly(ethylene glycol) 400 (PEG-400), hydrochloric acid, and hydrogen peroxide (H2O2, 30%) were received from Tokyo Chemical Industry Co. Pure water was obtained from a Yamato Auto Still WG203 ion-exchange/distillation system. Synthesis of Substoichiometric MoO3−x Nanoarchitectures: Protocol-I. In this protocol, nanostructured MoO3−x samples were synthesized by a hydrothermal process using (NH4)6Mo7O24 as the molybdenum source.16 Typically, B

DOI: 10.1021/acs.jpcc.9b03225 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) TEM images of the nanostructured MoO3−x samples prepared by protocol-II. (b) XRD patterns of the MoO3−x samples prepared by protocol-II. Red circles indicate diffraction peaks ascribed to h-MoO3, whereas ocher squares correspond to those ascribed to monoclinic MoO2.

Figure 3. (a) XPS spectra of Mo 3d in nanostructured MoO3−x samples obtained by protocol-I. Red circles show measured spectra, and black curves are the sum of the respective components. (b) FT-IR spectra of nanostructured MoO3−x samples obtained by protocol-I. In each figure, (i), (ii), or (iii) is for MP90-I, MP120-I, or MP150-I, respectively.

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accelerating voltage of 80 kV. Elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDS) excited with an electron beam at 15 kV with a system attached to a Phenom ProX (Thermo Fisher Scientific) scanning electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted with an ESCA-3400 spectrometer (Shimadzu), using Mg Kα radiation (1253.6 eV). The shift of binding energy due to relative surface charging was corrected using the C 1s level at 284.6 eV as an internal standard.

RESULTS AND DISCUSSION

Morphology and Crystalline Phase Characterization. The morphology of the obtained MoO3−x nanosystems was characterized by TEM observations (Figure 1), and their phase or crystallinity was examined using powder XRD measurements (Figure 2). Figures 1a or 2a shows a series of TEM images of the MoO3−x samples synthesized via protocol-I or protocol-II, respectively, and Figures 1b or 2b represents their typical XRD patterns, respectively. C

DOI: 10.1021/acs.jpcc.9b03225 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) XPS spectra of Mo 3d in nanostructured MoO3−x samples obtained by protocol-II. Red circles show measured spectra, and black curves are the sum of the respective components. (b) FT-IR spectra of the MoO3−x samples obtained by protocol-II. In each figure, (i), (ii), or (iii) is for MP90-II, MP120-II, or MP150-II, respectively.

Figure 5. (a) Extinction and (b) MCD spectra of the nanostructured MoO3−x samples prepared by protocol-I. MCD spectra were obtained under an applied magnetic field of +1.6 T (blue curve) or −1.6 T (red curve). In each spectrum, (i), (ii), or (iii) is for MP90-I, MP120-I, or MP150-I, respectively. Photo of the aqueous dispersion samples is also shown.

Figure 6. (a) Extinction and (b) MCD spectra of the nanostructured MoO3−x samples prepared by protocol-II. MCD spectra were obtained under an applied magnetic field of +1.6 T (blue curve) or −1.6 T (red curve). In each spectrum, (i), (ii), or (iii) is for MP90-II, MP120-II, or MP150-II, respectively. The photo of the aqueous dispersion samples is also shown.

According to Figure 1a, different morphologies of MoO3−x nanostructures were found as a function of the hydrothermal reaction temperature in protocol-I; typically, aggregates of spherical nanoparticles (10−30 nm in diameter) for MP90-I, nanosheets for MP120-I, and aggregates of nanopolyhedron

(50−200 nm) for MP150-I. Note that small nanocubes also coexisted in sample MP120-I. The result indicates that the product morphologies strongly depend on the reaction temperature. Similarly, in protocol-II, the difference in the D

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such as solvent, reaction time, and temperature all play important roles in defining the morphology and size as well as the crystalline phase.1,2,18 In Figure 1b, the XRD pattern of sample MP90-I showed rather poor crystallinity, but the resolved broad peaks could be indexed to those of the hexagonal phase of h-MoO3 (JCPDS No. 21-0569; space group P63).19 Sample MP120-I also exhibited a similar broad XRD pattern of h-MoO3, but hexagonal oxide species with very sharp diffraction peaks were also present in the sample. In contrast, the diffraction peak positions for sample MP150-I were obviously different from those of h-MoO3, and the peaks could be reasonably indexed to orthorhombic α-MoO3 (JCPDS No. 05-0508; space group Pbnm). This indicates that the phase transformation from hexagonal to orthorhombic structure occurs at an elevated reaction temperature. For products prepared by protocol-II (Figure 2b), the XRD patterns of samples MP90-II and MP120-II can also be indexed to h-MoO3 having rather ill-crystallinity. On the other hand, the XRD pattern of sample MP150-II was quite different from those of other samples but can be well-indexed to pure monoclinic molybdenum dioxide (MoO2, JCPDS No. 781069; space group P21/c).20 It is worth noting that the observed characteristic morphology of the present MoO2 nanospheres with rough surfaces is quite similar to that obtained in a different synthetic procedure, 21 so in consideration with interesting properties of MoO2 such as insolubility in strong acids and high conductivity, our synthesis protocol will also be a convenient manufacturing technology.21 Orthorhombic α-MoO3 is the most common phase and possesses a well-known layered structure of MoO3 that offers the possibility of constructing two-dimensional morphology, whereas hexagonal h-MoO3, one of the metastable phases, is composed of zigzag corner-sharing chains of MoO6 octahedra, which are also found in α-MoO3.1,2 The significant difference is the connection of individual chains through the cis-position, constructing a hexagonal crystalline structure featuring large one-dimensional tunnels.22 Interestingly, the hydrothermal synthesis conditions favor the single formation and growth of the metastable phase h-MoO3 or stable phase α-MoO3.18,23 Vibrational and Photoelectron Spectroscopy for Chemical Analysis. The oxidation state of the molybdenum atoms is associated with the amount of oxygen deficiencies, and it can be evaluated by XPS measurements as shown in Figures 3 and 4. Figures 3a or 4a presents the XPS spectra of Mo 3d core levels of the products prepared by protocol-I or those by protocol-II, respectively. In Figure 3a, a typical fourpeak shape of the Mo 3d spectrum can be found; a pair of doublet peaks at the binding energy of ∼235.2 and ∼232.0 eV, which are attributed to 3d3/2 and 3d5/2 of Mo6+ cations, and other two peaks at ∼233.8 and ∼230.8 eV that can be assigned to Mo5+ cations.16,17 The presence of Mo5+ cations leads to an increase in electron density and is thus responsible for the blue color in substoichiometric molybdenum oxides MoO3−x.1−3 In samples prepared by protocol-II at the reaction temperature of 90 or 120 °C (Figure 4a-(i) and (ii)), similar XPS spectra of the Mo 3d core levels were obtained but in sample MP150-II (Figure 4a-(iii)), the two Mo 3d peaks were more red-shifted to ∼232.9 and ∼229.7 eV, which can be due to Mo4+ cations, and other oxidation states of Mo were negligible. On the basis of XPS spectral deconvolution, the atomic ratios of Mo6+/ Mo5+ for samples MP90-I, MP120-I, and MP150-I were 0.47/1, 0.43/1, and 0.44/1, respectively. Similarly, those of samples

Figure 7. Gaussian band fits of the extinction (upper panels) and MCD spectra (lower panels) of the nanostructured MoO3−x samples prepared by protocol-I. Red curves display the reconstructed extinction and MCD spectra. Black dots indicate the experimental spectra. Deconvoluted components (1−4 and one additional offset) are also shown in green curves.

Figure 8. Gaussian band fits of the extinction (upper panels) and MCD spectra (lower panels) of the nanostructured MoO3−x samples prepared by protocol-II. Red curves display the reconstructed extinction and MCD spectra. Black dots indicate the experimental spectra. Deconvoluted components (1−4 and one additional offset) are also shown in green curves.

reaction temperature also influenced the sample morphology (Figure 2a): aggregates of nanoparticles (20−30 nm) for MP90-II, nanosheets for MP120-II, and nanospheres (40−80 nm) with rough surfaces for MP150-II. Interestingly, sample MP150-II had a quite different morphology from those of all other products prepared, suggesting the formation of molybdenum oxides with different phases. Indeed, it has often been reported that hydrothermal reaction conditions E

DOI: 10.1021/acs.jpcc.9b03225 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Summary of the Gaussian Fit Analysis Protocol-I component

MP90-I energy (cm−1)

1

9280

2

10 254

3

12 750

4

14 310

protocol-II component 1

energy (cm−1) 9560

MP120-I

intensity (a.u.)

band width (cm−1)

energy (cm−1)

0.344 (abs) −5.23 (MCD) 0.0794 (abs) 2.25 (MCD) 0.268 (abs) −7.64 (MCD) 0.304 (abs) 8.16 (MCD) MP90-II

680

9280

860

10 254

1305

12 810

1878

14 310

intensity (a.u.)

band width (cm−1)

energy (cm−1)

0.142 (abs) −0.588 (MCD)

1330

9560

2

10 516

3

12 576

4

15 352

0.307 (abs) −4.15 (MCD) 0.266 (abs) 2.44 (MCD)

1995

12 431

2350

15 041

intensity (a.u.)

band width (cm−1)

0.215 (abs) −2.73 (MCD) 0.0620 (abs) 1.27 (MCD) 0.181 (abs) −8.62 (MCD) 0.264 (abs) 6.56 (MCD) MP120-II

680 860 1205 1935

intensity (a.u.)

band width (cm−1)

0.186 (abs) −2.17 (MCD) 0.0023 (abs) 4.16 (MCD) 0.298 (abs) −10.8 (MCD) 0.336 (abs) 7.63 (MCD)

1530 900 1900 2721

and Mo−O−Mo, respectively, and the band observed at 560− 570 cm−1 is due to the Mo−O−Mo deformation mode.19,25,26 Note that in sample MP150-I, an additional sharp peak at 1005 cm−1 is obvious and this could have originated from the layered orthorhombic α-MoO3 phase.25 Conclusively, the IR spectroscopic study agrees well with the successful formation of molybdenum oxides passivated with PEG molecules. Optical Properties. The wide-range NIR absorption of substoichiometric MoO3−x, involving blue coloration, has been basically explained by either (i) a small-polaron model or (ii) a localized surface plasmon resonance (LSPR) mechanism.4 Small polarons represent local charge transfer between the Mo5+ and Mo6+ sites and describe that their energy, hence the absorption peak position, varies with the color-center concentration, whereas surface plasmons correspond to oscillations of conductive electrons in the volume of the entire nanostructure, and the LSPR peak position should be influenced by the size, shape (morphology), and dispersion medium.7,27 To gain better insight into the physical origin of NIR absorption (blue coloration) and magneto-optical (MO) activity in the nanostructured MoO3−x materials, we examine the optical properties on the basis of vis-NIR absorption and MCD spectroscopy, whose results are presented in Figures 5 and 6. Absorption Spectroscopy. Figures 5a or 6a shows aqueousphase UV−vis−NIR absorption (or extinction) spectra of the molybdenum oxide products obtained by protocol-I or protocol-II, respectively. The aqueous dispersion image of the products is also shown in each figure. Although no peak was found in samples MP150-I and MP150-II, other four samples prepared at the reaction temperature of 90 and 120 °C, that is, MP90-I, MP120-I, MP90-II, and MP120-II, exhibited two broad extinction peaks (or one major peak with one shoulder) in the vis-NIR region. In addition, the extinction profile associated with the major peak (= high-energy peak) was asymmetric. In protocol-I samples (MP90-I and MP120-I), the peak (or shoulder) positions were 756 and 1070 nm, and in protocol-II samples (MP90-II and MP120-II), they were 765 and 1070 nm. In consideration of the fact that nanostructured

MP90-II and MP120-II were 0.54/1 and 0.58/1, respectively. Note that these compositional analyses based on XPS data give selective information on the surface region of the specimens, so the presence of surface oxygen vacancies caused by the partial reduction of Mo6+ (to Mo5+) are comparable between the substoichiometric MoO3−x samples synthesized in the fixed protocol (except for sample MP150-II). To determine the x values (or compositions) in the substoichiometric MoO3−x products, EDS analysis was conducted. We analyzed the powdery sample placed on a Si substrate and found the existence of elements Mo, O, and C in all products. Then, by assuming that detected C atoms came solely from the PEG moieties with the atomic ratio of C/O = 2:1, we could estimate the atomic ratios of Mo/O in molybdenum oxides as 2.46 (MP90-I), 2.69 (MP120-I), and 2.74 (MP150-I) for samples obtained by protocol-I or 2.42 (MP90-II), 2.54 (MP120-II), and 2.12 (MP150-II) for samples obtained by protocol-II. Note that the atomic ratio of O/Mo = 3.0−3.1 was obtained for the EDS measurement of pure MoO3 white powder. Aside from the MoO2 compound (MP150-II), oxygen deficiencies in MoO3 nanostructures are overall decreased with an increase in the hydrothermal reaction temperature; in other words, the deficiencies depend on the morphologies of MoO3−x. As noted before, the ratio of Mo6+/ Mo5+ is almost independent of the reaction temperature in each sample series, so it is expected that the vacancies would dominantly exist around the surface region of the nanostructures MoO3−x. Next, IR spectroscopy was carried out to examine the crystal lattice vibration of molybdenum oxide samples as well as the chemical states of the product surface. Figures 3b and 4b show first a small but distinct peak at 1095 cm−1 ascribed to the stretching vibration of C−O−C, suggesting oxide surface passivation with PEG moieties.24 Interestingly, the surface passivation yield was depended on the hydrothermal reaction temperature, and the higher the temperature, the lower the passivation yield. On the other hand, the lattice vibrations of molybdenum oxides are seen at ∼975 and ∼760 cm−1 that are attributed to the stretching modes of the MoO double bond F

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intensities were rather weak and scattered, we found that the zero-crossing position did not agree with the absorption peak. As is known, a derivative-shaped MCD signal generally arises from the Zeeman splitting of an orbitally degenerate excited state, which is the so-called Faraday A-term.9,10 However, in some cases, interacting Faraday B-terms, which arise from the second order effects based on the field-induced mixing of the excited states via magnetic dipole transition moments, can give spectral bands with opposite signs (pseudo-A-term), producing a bisignate response.9,10 The difference from the A-term is that the negative and positive MCD peak positions are close to the corresponding two different electronic absorption peaks, resulting from the strong mixing of two nearly degenerate excited states by the magnetic field applied.9 Hence, to more clearly evaluate the characteristic features of the vis-NIR transitions, spectral deconvolutions of both extinction and MCD data (+1.6 T) were simultaneously conducted. Here, the deconvolution analysis is constrained by the requirement that a “single set” of Gaussian components with identical shape parameters can be used for the fitting of both absorption and MCD data.12,33 Then, to obtain a numerically adequate description, deconvolution using four Gaussian bands was successful (strictly, bands 1−4 plus one additional offset component). The spectral fits of MCD and extinction spectra for samples MP90-I and MP120-I are shown in Figure 7 and those for MP90-II and MP120-II are in Figure 8. The fitting parameters (peak energy, band width, and relative intensity for each component) are listed in Table 1. Note that the emergence of component 2 was somewhat scattered, which is probably due to its low intensity, but the trends in the deconvolution result including the band widths and peak positions are quite similar among the samples analyzed. In these MoO3−x nanostructured samples, electronic transitions observed at the energy higher than ∼11 000 cm−1 (that is, major components 3 and 4) are unambiguously attributable to two different polaronic transitions that are close in energy. This can also be corroborated by the clear appearance of an extinction shoulder (or asymmetric shape of the envelope), as indicated by the arrows in Figures 7 and 8. In addition, the difference in their peak energies (that is, shift energy) is much larger than that observed for typical magnetoplasmons in noble metal nanoparticles.11 Therefore, the derivative-shaped MCD response means that the two different polaronic transitions interact with each other under the applied magnetic field (that is, two interacting B-terms), giving a Faraday pseudo-A-term signal. Recently, theoretical calculations suggest that the oxygen vacancy site or polaron configuration with the lowest formation enthalpy in substoichiometric α-MoO3−x corresponds to that at the terminaloxygen site between two adjacent Mo5+ ions, and in this site, two polarons occupy different d orbitals of the Mo5+ ions at the defect site.5 In reality, two polaronic states should lift the degeneracy under the ill-crystalline, nanostructured phases, and thus, the two small-polaronic states would reasonably interact with each other yielding a pseudo-A-term of the MCD response. Moreover, according to a study on doped-manganite polarons, a double absorption-peak structure is found in the framework of intersite (adjacent) polaron hopping and on-site Jahn−Teller-like transitions,34 suggesting the presence of two transitions that can produce a pseudo-A-term signature. On the other hand, band 1 or 2, a minor component, should also have originated from other oxygen-related defects or small polarons involving Mo5+, since protocol-I samples with a lower Mo6+/

MoO3−x morphologies were considerably different between these samples (Figures 1a and 2a), it is less likely that these peaks (including shoulders) are ascribed to the LSPR modes. Bear in mind here that, according to the XRD profiles (Figures 1b and 2b), the sample morphologies were composed of a small-crystalline phase of molybdenum oxides, where the excess electrons can be localized and the lattice distortions are presented.28 Interestingly, in our samples, the absorption peak can be seen only for the hexagonal phase of MoO3−x nanoarchitectures. According to the IR spectra, our hexagonal molybdenum oxides are partly covered by PEG, so we can deduce that PEG attachment on the surface of MoO3−x favors the hexagonal phase, and thus blue coloration might be induced by interactions between the oxide surfaces and PEG moieties. To further confirm whether the observed peaks (or shoulders) are due to LSPR or not, we tried to measure extinction spectra in various solvents such as, methanol, ethanol, acetonitrile, and N,N-dimethylformamide, since the LSPR peak position (λLSPR) can be described by the refractive index (n) of the surrounding medium as follows29 λLSPR = λ 0 1 + 2n2

However, the obtained products were insoluble in these solvents; instead, to obtain a clear dispersion, we used a mixture of water and ethylene glycol (EG) at the volume ratio of 1/1. Note that the refractive index of the mixture can be estimated as 1.386, whereas that of water is 1.333.30 On this basis, a red shift of 20−30 nm for the peak position is expected in the water/EG mixture. The solvent effect is shown in the Supporting Information. Actually, no peak shift was observed upon changing the dispersion solvent, strongly indicating the absence of LSPR absorption.31 On the other hand, it is reported that polaronic transitions would interfere the solvent shift in the LSPR energy for similar substoichiometric oxide nanomaterials;32 hence these peaks that appeared in the NIR region for the present MoO3−x nanoarchitectures can be ascribed to small-polaron transitions. Magnetic Circular Dichroism. MCD spectroscopy is complementary to traditional electronic absorption spectroscopy but is able to identify in more detail the electronic structures for the materials. Figures 5b or 6b shows the aqueous-phase MCD spectra of the substoichiometric molybdenum oxide products obtained by protocol-I or by protocol-II, respectively. Aside from two samples synthesized at the reaction temperature of 150 °C, derivative-like MCD signals were obvious in the spectral region of 500−1000 nm for samples MP90-I, MP120-I, MP90-II, and MP120-II, suggesting rather strong MO effects in the small-polaron transitions. The sign of the MCD signal was completely reversed when the magnetic field was switched, confirming that signatures are not from an experimental artifact. Strikingly, the position (or energy) crossing zero is in good correspondence with that of the major absorption peak. Therefore, solely from the MCD spectral features, the distinct bisignate response may give a misunderstood interpretation that involves two circular modes of surface magnetoplasmon;11 however, our comprehensive study gave us a conclusion for the first time that multiple smallpolaron transitions contribute to the bisignate MCD responses in the nanostructured h-MoO3−x materials. Note that in the longer wavelength region (>∼1000 nm), although the MCD G

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The Journal of Physical Chemistry C Mo5+ ratio exhibited stronger absorption or MCD intensity in the low-energy region; however, their MCD behavior seems slightly different from that of components 3 and 4, that is, the absorption peak does not agree with the zero-crossing point of a bipolar MCD signal, so interactions with other different transitions may cause such a behavior and thus a more detailed study is needed for further identification.7 In any case, MCD gave more spectral information that could not easily be gained by conventional absorption spectroscopy, so the present methodology will exhibit potential usefulness for identification or utilization of semiconductor-based polaronic transitions as well as future applications for semiconductor-based nanophotonics or magneto-optical devices.

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CONCLUSIONS In summary, MCD was demonstrated for the first time in polaronic MoO3−x nanoarchitectures. Samples with various morphologies were synthesized by a simple hydrothermal process. The phase, morphology, and spectroscopic properties of the nanostructures were dependent both on the reaction temperature and the molybdenum precursor. Hexagonal-phase MoO3−x obtained were colored blue with different degrees and had absorption peaks or shoulders in the vis-NIR range. A bisignate MCD response was detected in the same energy region, but it was not attributed to surface magnetoplasmonic modes because of the absence of the peak-energy shift with the increasing refractive index of the dispersion medium. We then proposed that the bisignate MCD signal was attributed to interacting two Faraday B-terms for small-polaronic transitions, arising from two polaronic states close in energy that would give opposite signs. We believe such characteristic magnetooptical responses will give some suggestions for the development of new types of advanced magneto-optical materials in the field of nanophotonics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03225. Solvent effect on the optical absorption spectra of samples MP90-I, MP120-I, MP90-II, and MP120-II, the thermal stability of MP120-I, and additional references (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hiroshi Yao: 0000-0002-4164-0085 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by Grant-in-Aids for Scientific Research (B: 18H01808 (H.Y.)) from the Japan Society for the Promotion of Science (JSPS).

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DOI: 10.1021/acs.jpcc.9b03225 J. Phys. Chem. C XXXX, XXX, XXX−XXX