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Reductive-annealing-induced changes in Mo valence states on the surfaces of MoO3 single crystals and their high temperature transport
Current Applied Physics 19 (2019) 1379–1382
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Reductive-annealing-induced changes in Mo valence states on the surfaces of MoO3 single crystals and their high temperature transport
T
Hyeonjun Konga, Han Sol Kwona, Hyegyeong Kimb, Gwang-Soo Jeena, Jaekwang Leea, Joonhyuk Leea, Yun Seok Heoa, Jinhyung Choc, Hyoungjeen Jeena,∗ a
Department of Physics, Pusan National University, Busan, 46241, South Korea PNU Core Research Facility, Pusan National University, Busan, 46241, South Korea c Department of Physics Education, Pusan National University, Busan, 46241, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: MoO3 single crystals Mo valence state X-ray photoemission spectroscopy Real-time impedance Low temperature reduction Oxygen vacancies
Reduction of MoO3 in extreme reducing condition is a way to achieve Mo metal. However, effect of less extreme reductive-annealing, where it allows to keep the crystal structure, on physical and chemical properties of MoO3 has not been well-studied. In this work, we studied the evolution of Mo valence state during reductive annealing and its effect on high temperature transport. We found the formation of oxygen vacancies on surface of MoO3 single crystals at the low temperature, which is evidenced by increase of Mo5+ and color change. In addition, formation of Mo4+ was at the elevated temperature. For understanding the relation between bulk conductivity and Mo valence state, real-time impedance spectroscopy is employed. Use of two different gases makes it possible to distinguish impedance responses of MoO3 from those of reduced MoO3-x. Also, from time-dependent impedance measurements, we observed the evolution of transport behaviour by evolution of Mo valence state.
1. Introduction Formation of oxygen vacancies are prevalent in transition metal oxides even like the case of fractured SrTiO3 surfaces [1]. The effect of oxygen vacancies on physical properties is unavoidable due to resultant change in valence state of transition metals [2–5]. So far, oxygen vacancies in oxides have been considered as a detrimental factor for materials performance, i.e. leakages in ferroelectricity. However, recently this view is rather shifted. There is a number of works that oxygen vacancies can be functional [2,5–7]. One way is its tunability in materials’ properties such as magnetic, optical, and electronic properties due to valence state modification through oxygen intercalation and de-intercalation processes [8–10]. Many external stimuli have been used such as temperature, electric fields, and mechanical stress to create and erase oxygen vacancies in the lattices. Another scope to functionalize the materials through oxygen vacancies is centered on mobile character of oxygen ions, which can be applied for solid oxide fuel cells, lithium-air batteries, electrochromic windows [11,12]. In this regard, layered MoO3 is attractive material. It can accept foreign ions like lithium, hydrogen, and etc. via electrochemical ways as well as can be transformed to oxygen deficient Magnelli phases [13–15]. Both aspects have gotten a lot of attentions for energy
∗
applications. Especially, formation of oxygen vacancies can be exemplified as follow. MoO3 has been actively used for hole-donating layers in solar cell devices. MoO3 is used a transparent conducting layer in solar cell device due to its large band gap of ~3.2 eV. The oxygen vacancies in MoO3 affect performance of the device [16,17]. Also, it has been reported that forming oxygen vacancies in MoO3 leads to high pseudocapacitive behaviour [6]. In addition, adding oxygen vacancies increases extra electrons in the Mo d orbital. Hence, such decrease of Mo valence state is well known for increase of electronic conductivity as well as resultant color change from pale yellow to dark blue. The relation between color and valence state of Mo is well-known from oxygen vacancy formation as well as hydrogen insertion/desertion experiments [18,19]. Due to increased attentions on this material, it is not difficult to find many theoretical works on oxygen vacancy formation [17,20–22]. So far, theoretically it is commonly known that the lowest vacancy formation energy is from the oxygens involved in the van der Waals interaction among three inequivalent oxygens: oxygens with edging sharing in equatorial plane, oxygens with corner sharing in equatorial plane, and apical oxygens involved in the van der Waals interaction. In theory, the charge state of the nearby Mo is supposedly to be Mo4+ after the removal of an oxygen ion. However, a recent DFT work stated
Corresponding author. E-mail address: [email protected] (H. Jeen).
https://doi.org/10.1016/j.cap.2019.09.001 Received 15 April 2019; Received in revised form 28 August 2019; Accepted 2 September 2019 Available online 03 September 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
Current Applied Physics 19 (2019) 1379–1382
H. Kong, et al.
that ground state of electronic configuration after a removal of oxygen is the creation of two Mo5+ sites. In the work, due to small energy barrier between two configurations, i.e. single Mo4+ site and two Mo5+ sites, two Mo5+ sites can be dominantly seen in XPS spectra at low coverage [20]. However, most studies have been focused on reduction in extreme condition to convert MoO3 to Mo4O11, MoO2, and/or Mo, which require high temperature and rely on extreme condition like at least 5% of H2 flow [13,23–26]. This condition through excessive oxygen vacancy formation creates completely new crystal structure, which is far from the crystal structure of MoO3. This may be a critical issue in fabrication of devices, since it would create stress while functioning. In this work, we used MoO3 single crystals to probe evolution of Mo valence state upon reduction in lower temperature regime up to 500 °C and mild gas condition (3% of H2) as well as to study changes in transport behaviours. ex-situ XPS of each annealed MoO3-x single crystal were done to take snapshot of its chemical state at each temperature. Note that it has not been studied on which the reduction reaction starts in terms of temperature. From impedance study with different gas environments at different temperatures and time-dependent impedance study in reducing condition at 400 °C, we could see the effect of oxygen vacancies on evolution of impedance responses including relaxation time, bulk conductivity, and etc. in real time.
that we performed x-ray diffraction (Panalytical Xpert 3, PNU-XRD) of all the samples, and we do not get any signature of other phases (See Fig. S1), which could be possible at the elevated temperature and strong reducing condition [13,24–26]. In addition, this may indicate the reaction is limited to the surfaces. To see chemistry change upon color change, x-ray photoemission spectroscopy (XPS) was adopted. First, ex-situ XPS (AXIS SUPRA, KRATOS, PNU-XPS) of the treated samples were performed. Mo 3d spectrum of each was taken at room temperature. Note that depth profiling with XPS was performed to determine how deep from the top surfaces is affected by this thermal annealing (data not shown). However, additional defect creation by the sputtering process prevents quantitative determination of the reaction depth (See Fig. S2). To understand transport nature of each snapshot taken from XPS, real-time impedance spectroscopy in controlled gas and temperature environments was performed using an impedance analyzer (Hioki 3570), which covers from 4 Hz to 5 MHz. A cleaved platelet of MoO3 was used for the entire measurements. Silver paint was applied on both side of the platelet to form capacitor structure along b-axis of MoO3. Excitation of 0.5 V was used to collect impedance data at each measurement. We used custom-made thermal reactor with mass flow controller to control temperatures and gas environments. Zview program is used to analyze bulk conductivities.
2. Experiments
3. Results and discussion
Cleaverable MoO3 single crystals were grown by modified vertical Bridgman method. The details can be found elsewhere [27]. In short, MoO3 powders were sealed in quartz ampoules in air. The ampoules were placed in vertical furnace. We intentionally set the highest temperature at the top of MoO3 powder to minimize sublimation and take advantage of convention. Thermal annealing in reducing condition was performed. We used rapid thermal annealing (RTA) apparatus (Nextron Inc.). The temperature was controlled by four halogen lamps with reflector (OSRAM GmbH) and fast-response temperature controller. The samples sat on the center of RTA and treated for 30 min at the designated temperature. Before heating, the tube was purged for 30 min with 50 sccm of forming gas (3% of H2/97% of Ar). The sample environment was maintained until the samples were cooled down to room temperature. We chose 100, 200, 300, 400, and 500 °C as annealing temperatures. The possible reaction on the MoO3 lattices with H2 can be expressed:
Fig. 1 shows clear evidence of reduction. The color of an as-grown MoO3 single crystal is light yellow. The color of the sample annealed at 100 °C shows similar to that of as-grown MoO3. However, the sample annealed at 200 °C within 30 min changes color to dark blue. Even though the forming gas is a strong reducing agent, the reaction temperature is still low. The temperature is comparable to the case of complex oxides reduced at special reducing agent [31–33]. From the sample's color, it is hard to distinguish among samples annealed at the temperature higher than 200 °C. However, since the color of MoO2 is dark blue [19,34], this color change is a strong indicator of new valence states in this reduction. As mentioned earlier, we could not observe the formation of any different crystallographic phase. This ensures we are looking at reduction at mild condition, where no phase transformation is taken place. To get better understanding on charge valence state of Mo, we performed x-ray photoemission spectroscopy (XPS) of all the crystals at room temperature (Fig. 2(a)). We focused on Mo 3d spectra to see any change in valence state. The as-grown MoO3 single crystal has 97.5% of Mo6+. Note that in its spectrum, we could not see any Mo4+ peaks and get any signature from our curve fitting results (data not shown). This means it is possible there is some portion of natural oxygen vacancies on the MoO3 single crystal. When XPS probed the annealed samples up to 300 °C, it is noticeable gradual decrease of Mo6+ from Fig. 2(b), while increase of Mo5+ (2.5% of Mo5+ for as-grown MoO3 to 7% of Mo5+ for the one treated at 300 °C) from Fig. 2(c). However, the samples treated at the elevated temperatures higher than 400 °C, it is clearly seen that the peaks corresponding to Mo4+ are shown in the spectra from Fig. 2(d). This is rather consistent with previous report [23]. It is interesting to note that the samples treated at the higher temperature show both Mo4+ and Mo5+, since the number of oxygen vacancies are increased. This qualitatively agrees with the statement that the increase of coverage increase the chance to show Mo4+ at the catalytically active temperature [20]. Comparison of O 1s XPS spectra between as-grown MoO3 and MoO3 treated at the higher temperature shows clear difference (See Fig. S3). O 1s XPS spectrum from the sample treated at 500 °C shows not only a peak related to Mo–O but also a shoulder peak related to oxygen vacancy. However, the XPS spectrum of as-grown MoO3 only shows hardly distinguishable shoulder peak related to oxygen vacancy. From the area analysis portion corresponding to oxygen vacancy in as-grown MoO3 and MoO3 treated at
MoO3 + H2 → MoO2 + H2O
(1)
The effectiveness of oxygen vacancy formation using hydrogen molecules are well-known [26,28]. Note that we did not consider hydrogenation, since the experiments were performed in non-oxygencontaining gas flow (3% of H2/97% of Ar) and use of Pt catalyst was avoided to remove any potential catalytic reaction at the interface between Pt and MoO3. In this condition, it is expected to form nothing but oxygen vacancies [29,30]. After annealing, we clearly observed that the color of samples treated higher than 200 °C were changed to dark blue, which is an indication of chemical reaction in reducing condition (See Fig. 1). Note
Fig. 1. Photograph of MoO3 single crystals annealed at different temperatures. It is clearly seen that color changes from pale yellow to dark blue. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 1380
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Fig. 3. Impedance spectra of MoO3 single crystals annealed in different gases: air and 3% forming gas. (a) Real part of impedance as a function of frequency and (b) imaginary part of impedance as a function of frequency. Clear difference in impedance spectra can be seen due to change of Mo valency.
about an order of magnitude smaller for all the temperatures. From the Bode plot for imaginary impedance data, relaxation time was also decreased to 2.2 μs at 400 °C. Due to fast relaxation nature, meaning that it is at least shorter than 0.2 μs at 500 °C in oxygen-deficient MoO3-x, we could not observed apex from the impedance spectrum up to 5 MHz. This clearly indicated oxygen vacancies, associated with increase in the number of electrons in Mo bands, actually make the sample conducting and its relaxation is much faster. After checking clear difference in temperature dependence on impedance spectra between MoO3 in air and forming gas at each different temperature, we performed time dependent impedance spectra at 400 °C to see how impedance spectra evolve by increase of oxygen vacancies from Fig. 4. As we can see from our XPS data, at 400 °C, Mo valence state in MoO3 can reach down to 4 + state within 30 min. Thus, we chose 400 °C to see the time dynamics from Fig. 4(a) through (c). For the measurements, the sample was raised in air up to 400 °C. Fig. 2. XPS results of MoO3 single crystals treated at different temperatures: (a) XPS spectra, (b) portion of Mo6+, (c) portion of Mo5+, (d) portion of Mo4+ of crystals annealed at different temperatures. Clear changes in Mo valence states can be seen.
500 °C is 7.3% and 14.5%, respectively. Since loss of oxygens in the MoO3 lattices can give more electrons in the lattices, it is expected that oxygen deficient MoO3-x is more conducting as the case of metallic MoO2 [16,18,19]. We performed impedance spectroscopy at different temperatures and different gas environments. Note that we used a MoO3 single crystal prepared in similar growth condition for the as-grown MoO3 used for XPS experiments. Air was used to see the temperature-dependent impedance responses of MoO3, while forming gas was used to see the temperature-dependent impedance responses of MoO3-x. Fig. 3 showed impedance spectra only at the elevated temperatures due to highly insulating nature of both samples. Since MoO3 is an insulator, it is not possible to detect impedance responses at the temperature lower than 200 °C. First we measured impedance spectra of air-annealed MoO3 single crystal to see impedance responses when negligible oxygen vacancies exist in the lattices. The real part of impedance is gotten smaller the temperature is raised. This indicates bulk conductivity is increased as it infers thermally activated hopping would be the transport mechanism for the transport. From the Bode plots of imaginary impedance, relaxation time could be deduced from the apex of the curves. It is clearly seen the relaxation time (τ) is initially 14 μs at 400 °C, but decreased to 9.6 μs at 500 °C. At the elevated temperature, relaxation is faster possibly due to much higher rate of hopping. When we compared this result with that of 3% H2 annealed MoO3-x, it is clearly seen that the bulk resistivities are a lot smaller than that of air annealed MoO3, which can be clearly seen from Bode plot of real impedance. Bulk conductivity is at least
Fig. 4. Time-dependent impedance spectra of an MoO3 single crystal at 400 °C in forming gas. (a) Nyquist plots as a function of time, (b) real part of impedance as a function of frequency, (c) imaginary part of impedance as a function of frequency. As reaction time increases, clear decrease of semi-circle, impedance values, and relaxation time can be seen due to decrease of oxygen content at the catalytically active temperature. (d) relaxation time as a function of annealing time. 1381
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After temperature stabilization, we switched gas type from air to 3% H2 forming gas, then immediately started impedance measurements. As can be seen from Nyquist plots as a function of time, it is clearly seen that progressive increase of conductivity and decrease of semi-circle in the plots as time goes from Fig. 4(b). Note that the color-codes for specific time can be clearly seen from Fig. 4(d). Note that after 30 min in the forming gas, the relaxation time measured in these isothermal measurements is similar to the results from Fig. 3. In addition, from the Bode plots of imaginary impedance as a function of frequencies from Fig. 4(c), it can be clearly seen that the peak frequencies, related to relaxation times, decrease with time evolves. Note that the relaxation time from the first measurements is the half of that of the air-annealed MoO3, indicating oxygen vacancy formation is immediately made. The relaxation time evolves from 6.7 μsec to 1.0 μsec. This nearly seven fold decrease in relaxation time indicates progressive formation of oxygen vacancies as even isothermal condition. Also, this indicates the sample changed to more conducting, which is consistent with our data.
[8]
[9]
[10]
[11] [12]
[13] [14]
4. Conclusion
[15] [16]
In summary, we studied reduction behaviour of MoO3 single crystals in mild reducing condition using ex-situ XPS and real-time impedance spectroscopy. From the both approaches, we could get valuable reduction behaviours of MoO3: no macroscopic structural change, low temperature reduction as low as 100 °C, stabilization of Mo4+ above 400 °C, clear difference in impedance behaviour between MoO3 and MoO3-x, and progressive increase of conductivity and decrease of relaxation time as the formation of oxygen vacancies. This redox character of MoO3 will be useful for developing components of energy devices.
[17] [18]
[19]
[20]
Acknowledgements
[21]
This work was supported by a 2-Year Research Grant of Pusan National University. HJK and HSK are equally contributed on this work.
[22] [23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cap.2019.09.001.
[25] [26]
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Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Reductive-annealing-induced changes in Mo valence states on the surfaces of MoO3 single crystals and their high temperature transport
T
Hyeonjun Konga, Han Sol Kwona, Hyegyeong Kimb, Gwang-Soo Jeena, Jaekwang Leea, Joonhyuk Leea, Yun Seok Heoa, Jinhyung Choc, Hyoungjeen Jeena,∗ a
Department of Physics, Pusan National University, Busan, 46241, South Korea PNU Core Research Facility, Pusan National University, Busan, 46241, South Korea c Department of Physics Education, Pusan National University, Busan, 46241, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: MoO3 single crystals Mo valence state X-ray photoemission spectroscopy Real-time impedance Low temperature reduction Oxygen vacancies
Reduction of MoO3 in extreme reducing condition is a way to achieve Mo metal. However, effect of less extreme reductive-annealing, where it allows to keep the crystal structure, on physical and chemical properties of MoO3 has not been well-studied. In this work, we studied the evolution of Mo valence state during reductive annealing and its effect on high temperature transport. We found the formation of oxygen vacancies on surface of MoO3 single crystals at the low temperature, which is evidenced by increase of Mo5+ and color change. In addition, formation of Mo4+ was at the elevated temperature. For understanding the relation between bulk conductivity and Mo valence state, real-time impedance spectroscopy is employed. Use of two different gases makes it possible to distinguish impedance responses of MoO3 from those of reduced MoO3-x. Also, from time-dependent impedance measurements, we observed the evolution of transport behaviour by evolution of Mo valence state.
1. Introduction Formation of oxygen vacancies are prevalent in transition metal oxides even like the case of fractured SrTiO3 surfaces [1]. The effect of oxygen vacancies on physical properties is unavoidable due to resultant change in valence state of transition metals [2–5]. So far, oxygen vacancies in oxides have been considered as a detrimental factor for materials performance, i.e. leakages in ferroelectricity. However, recently this view is rather shifted. There is a number of works that oxygen vacancies can be functional [2,5–7]. One way is its tunability in materials’ properties such as magnetic, optical, and electronic properties due to valence state modification through oxygen intercalation and de-intercalation processes [8–10]. Many external stimuli have been used such as temperature, electric fields, and mechanical stress to create and erase oxygen vacancies in the lattices. Another scope to functionalize the materials through oxygen vacancies is centered on mobile character of oxygen ions, which can be applied for solid oxide fuel cells, lithium-air batteries, electrochromic windows [11,12]. In this regard, layered MoO3 is attractive material. It can accept foreign ions like lithium, hydrogen, and etc. via electrochemical ways as well as can be transformed to oxygen deficient Magnelli phases [13–15]. Both aspects have gotten a lot of attentions for energy
∗
applications. Especially, formation of oxygen vacancies can be exemplified as follow. MoO3 has been actively used for hole-donating layers in solar cell devices. MoO3 is used a transparent conducting layer in solar cell device due to its large band gap of ~3.2 eV. The oxygen vacancies in MoO3 affect performance of the device [16,17]. Also, it has been reported that forming oxygen vacancies in MoO3 leads to high pseudocapacitive behaviour [6]. In addition, adding oxygen vacancies increases extra electrons in the Mo d orbital. Hence, such decrease of Mo valence state is well known for increase of electronic conductivity as well as resultant color change from pale yellow to dark blue. The relation between color and valence state of Mo is well-known from oxygen vacancy formation as well as hydrogen insertion/desertion experiments [18,19]. Due to increased attentions on this material, it is not difficult to find many theoretical works on oxygen vacancy formation [17,20–22]. So far, theoretically it is commonly known that the lowest vacancy formation energy is from the oxygens involved in the van der Waals interaction among three inequivalent oxygens: oxygens with edging sharing in equatorial plane, oxygens with corner sharing in equatorial plane, and apical oxygens involved in the van der Waals interaction. In theory, the charge state of the nearby Mo is supposedly to be Mo4+ after the removal of an oxygen ion. However, a recent DFT work stated
Corresponding author. E-mail address: [email protected] (H. Jeen).
https://doi.org/10.1016/j.cap.2019.09.001 Received 15 April 2019; Received in revised form 28 August 2019; Accepted 2 September 2019 Available online 03 September 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
Current Applied Physics 19 (2019) 1379–1382
H. Kong, et al.
that ground state of electronic configuration after a removal of oxygen is the creation of two Mo5+ sites. In the work, due to small energy barrier between two configurations, i.e. single Mo4+ site and two Mo5+ sites, two Mo5+ sites can be dominantly seen in XPS spectra at low coverage [20]. However, most studies have been focused on reduction in extreme condition to convert MoO3 to Mo4O11, MoO2, and/or Mo, which require high temperature and rely on extreme condition like at least 5% of H2 flow [13,23–26]. This condition through excessive oxygen vacancy formation creates completely new crystal structure, which is far from the crystal structure of MoO3. This may be a critical issue in fabrication of devices, since it would create stress while functioning. In this work, we used MoO3 single crystals to probe evolution of Mo valence state upon reduction in lower temperature regime up to 500 °C and mild gas condition (3% of H2) as well as to study changes in transport behaviours. ex-situ XPS of each annealed MoO3-x single crystal were done to take snapshot of its chemical state at each temperature. Note that it has not been studied on which the reduction reaction starts in terms of temperature. From impedance study with different gas environments at different temperatures and time-dependent impedance study in reducing condition at 400 °C, we could see the effect of oxygen vacancies on evolution of impedance responses including relaxation time, bulk conductivity, and etc. in real time.
that we performed x-ray diffraction (Panalytical Xpert 3, PNU-XRD) of all the samples, and we do not get any signature of other phases (See Fig. S1), which could be possible at the elevated temperature and strong reducing condition [13,24–26]. In addition, this may indicate the reaction is limited to the surfaces. To see chemistry change upon color change, x-ray photoemission spectroscopy (XPS) was adopted. First, ex-situ XPS (AXIS SUPRA, KRATOS, PNU-XPS) of the treated samples were performed. Mo 3d spectrum of each was taken at room temperature. Note that depth profiling with XPS was performed to determine how deep from the top surfaces is affected by this thermal annealing (data not shown). However, additional defect creation by the sputtering process prevents quantitative determination of the reaction depth (See Fig. S2). To understand transport nature of each snapshot taken from XPS, real-time impedance spectroscopy in controlled gas and temperature environments was performed using an impedance analyzer (Hioki 3570), which covers from 4 Hz to 5 MHz. A cleaved platelet of MoO3 was used for the entire measurements. Silver paint was applied on both side of the platelet to form capacitor structure along b-axis of MoO3. Excitation of 0.5 V was used to collect impedance data at each measurement. We used custom-made thermal reactor with mass flow controller to control temperatures and gas environments. Zview program is used to analyze bulk conductivities.
2. Experiments
3. Results and discussion
Cleaverable MoO3 single crystals were grown by modified vertical Bridgman method. The details can be found elsewhere [27]. In short, MoO3 powders were sealed in quartz ampoules in air. The ampoules were placed in vertical furnace. We intentionally set the highest temperature at the top of MoO3 powder to minimize sublimation and take advantage of convention. Thermal annealing in reducing condition was performed. We used rapid thermal annealing (RTA) apparatus (Nextron Inc.). The temperature was controlled by four halogen lamps with reflector (OSRAM GmbH) and fast-response temperature controller. The samples sat on the center of RTA and treated for 30 min at the designated temperature. Before heating, the tube was purged for 30 min with 50 sccm of forming gas (3% of H2/97% of Ar). The sample environment was maintained until the samples were cooled down to room temperature. We chose 100, 200, 300, 400, and 500 °C as annealing temperatures. The possible reaction on the MoO3 lattices with H2 can be expressed:
Fig. 1 shows clear evidence of reduction. The color of an as-grown MoO3 single crystal is light yellow. The color of the sample annealed at 100 °C shows similar to that of as-grown MoO3. However, the sample annealed at 200 °C within 30 min changes color to dark blue. Even though the forming gas is a strong reducing agent, the reaction temperature is still low. The temperature is comparable to the case of complex oxides reduced at special reducing agent [31–33]. From the sample's color, it is hard to distinguish among samples annealed at the temperature higher than 200 °C. However, since the color of MoO2 is dark blue [19,34], this color change is a strong indicator of new valence states in this reduction. As mentioned earlier, we could not observe the formation of any different crystallographic phase. This ensures we are looking at reduction at mild condition, where no phase transformation is taken place. To get better understanding on charge valence state of Mo, we performed x-ray photoemission spectroscopy (XPS) of all the crystals at room temperature (Fig. 2(a)). We focused on Mo 3d spectra to see any change in valence state. The as-grown MoO3 single crystal has 97.5% of Mo6+. Note that in its spectrum, we could not see any Mo4+ peaks and get any signature from our curve fitting results (data not shown). This means it is possible there is some portion of natural oxygen vacancies on the MoO3 single crystal. When XPS probed the annealed samples up to 300 °C, it is noticeable gradual decrease of Mo6+ from Fig. 2(b), while increase of Mo5+ (2.5% of Mo5+ for as-grown MoO3 to 7% of Mo5+ for the one treated at 300 °C) from Fig. 2(c). However, the samples treated at the elevated temperatures higher than 400 °C, it is clearly seen that the peaks corresponding to Mo4+ are shown in the spectra from Fig. 2(d). This is rather consistent with previous report [23]. It is interesting to note that the samples treated at the higher temperature show both Mo4+ and Mo5+, since the number of oxygen vacancies are increased. This qualitatively agrees with the statement that the increase of coverage increase the chance to show Mo4+ at the catalytically active temperature [20]. Comparison of O 1s XPS spectra between as-grown MoO3 and MoO3 treated at the higher temperature shows clear difference (See Fig. S3). O 1s XPS spectrum from the sample treated at 500 °C shows not only a peak related to Mo–O but also a shoulder peak related to oxygen vacancy. However, the XPS spectrum of as-grown MoO3 only shows hardly distinguishable shoulder peak related to oxygen vacancy. From the area analysis portion corresponding to oxygen vacancy in as-grown MoO3 and MoO3 treated at
MoO3 + H2 → MoO2 + H2O
(1)
The effectiveness of oxygen vacancy formation using hydrogen molecules are well-known [26,28]. Note that we did not consider hydrogenation, since the experiments were performed in non-oxygencontaining gas flow (3% of H2/97% of Ar) and use of Pt catalyst was avoided to remove any potential catalytic reaction at the interface between Pt and MoO3. In this condition, it is expected to form nothing but oxygen vacancies [29,30]. After annealing, we clearly observed that the color of samples treated higher than 200 °C were changed to dark blue, which is an indication of chemical reaction in reducing condition (See Fig. 1). Note
Fig. 1. Photograph of MoO3 single crystals annealed at different temperatures. It is clearly seen that color changes from pale yellow to dark blue. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 1380
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Fig. 3. Impedance spectra of MoO3 single crystals annealed in different gases: air and 3% forming gas. (a) Real part of impedance as a function of frequency and (b) imaginary part of impedance as a function of frequency. Clear difference in impedance spectra can be seen due to change of Mo valency.
about an order of magnitude smaller for all the temperatures. From the Bode plot for imaginary impedance data, relaxation time was also decreased to 2.2 μs at 400 °C. Due to fast relaxation nature, meaning that it is at least shorter than 0.2 μs at 500 °C in oxygen-deficient MoO3-x, we could not observed apex from the impedance spectrum up to 5 MHz. This clearly indicated oxygen vacancies, associated with increase in the number of electrons in Mo bands, actually make the sample conducting and its relaxation is much faster. After checking clear difference in temperature dependence on impedance spectra between MoO3 in air and forming gas at each different temperature, we performed time dependent impedance spectra at 400 °C to see how impedance spectra evolve by increase of oxygen vacancies from Fig. 4. As we can see from our XPS data, at 400 °C, Mo valence state in MoO3 can reach down to 4 + state within 30 min. Thus, we chose 400 °C to see the time dynamics from Fig. 4(a) through (c). For the measurements, the sample was raised in air up to 400 °C. Fig. 2. XPS results of MoO3 single crystals treated at different temperatures: (a) XPS spectra, (b) portion of Mo6+, (c) portion of Mo5+, (d) portion of Mo4+ of crystals annealed at different temperatures. Clear changes in Mo valence states can be seen.
500 °C is 7.3% and 14.5%, respectively. Since loss of oxygens in the MoO3 lattices can give more electrons in the lattices, it is expected that oxygen deficient MoO3-x is more conducting as the case of metallic MoO2 [16,18,19]. We performed impedance spectroscopy at different temperatures and different gas environments. Note that we used a MoO3 single crystal prepared in similar growth condition for the as-grown MoO3 used for XPS experiments. Air was used to see the temperature-dependent impedance responses of MoO3, while forming gas was used to see the temperature-dependent impedance responses of MoO3-x. Fig. 3 showed impedance spectra only at the elevated temperatures due to highly insulating nature of both samples. Since MoO3 is an insulator, it is not possible to detect impedance responses at the temperature lower than 200 °C. First we measured impedance spectra of air-annealed MoO3 single crystal to see impedance responses when negligible oxygen vacancies exist in the lattices. The real part of impedance is gotten smaller the temperature is raised. This indicates bulk conductivity is increased as it infers thermally activated hopping would be the transport mechanism for the transport. From the Bode plots of imaginary impedance, relaxation time could be deduced from the apex of the curves. It is clearly seen the relaxation time (τ) is initially 14 μs at 400 °C, but decreased to 9.6 μs at 500 °C. At the elevated temperature, relaxation is faster possibly due to much higher rate of hopping. When we compared this result with that of 3% H2 annealed MoO3-x, it is clearly seen that the bulk resistivities are a lot smaller than that of air annealed MoO3, which can be clearly seen from Bode plot of real impedance. Bulk conductivity is at least
Fig. 4. Time-dependent impedance spectra of an MoO3 single crystal at 400 °C in forming gas. (a) Nyquist plots as a function of time, (b) real part of impedance as a function of frequency, (c) imaginary part of impedance as a function of frequency. As reaction time increases, clear decrease of semi-circle, impedance values, and relaxation time can be seen due to decrease of oxygen content at the catalytically active temperature. (d) relaxation time as a function of annealing time. 1381
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After temperature stabilization, we switched gas type from air to 3% H2 forming gas, then immediately started impedance measurements. As can be seen from Nyquist plots as a function of time, it is clearly seen that progressive increase of conductivity and decrease of semi-circle in the plots as time goes from Fig. 4(b). Note that the color-codes for specific time can be clearly seen from Fig. 4(d). Note that after 30 min in the forming gas, the relaxation time measured in these isothermal measurements is similar to the results from Fig. 3. In addition, from the Bode plots of imaginary impedance as a function of frequencies from Fig. 4(c), it can be clearly seen that the peak frequencies, related to relaxation times, decrease with time evolves. Note that the relaxation time from the first measurements is the half of that of the air-annealed MoO3, indicating oxygen vacancy formation is immediately made. The relaxation time evolves from 6.7 μsec to 1.0 μsec. This nearly seven fold decrease in relaxation time indicates progressive formation of oxygen vacancies as even isothermal condition. Also, this indicates the sample changed to more conducting, which is consistent with our data.
[8]
[9]
[10]
[11] [12]
[13] [14]
4. Conclusion
[15] [16]
In summary, we studied reduction behaviour of MoO3 single crystals in mild reducing condition using ex-situ XPS and real-time impedance spectroscopy. From the both approaches, we could get valuable reduction behaviours of MoO3: no macroscopic structural change, low temperature reduction as low as 100 °C, stabilization of Mo4+ above 400 °C, clear difference in impedance behaviour between MoO3 and MoO3-x, and progressive increase of conductivity and decrease of relaxation time as the formation of oxygen vacancies. This redox character of MoO3 will be useful for developing components of energy devices.
[17] [18]
[19]
[20]
Acknowledgements
[21]
This work was supported by a 2-Year Research Grant of Pusan National University. HJK and HSK are equally contributed on this work.
[22] [23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cap.2019.09.001.
[25] [26]
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