ARTICLE pubs.acs.org/JPCC
In Situ Raman Spectroscopy of H2 Gas Interaction with Layered MoO3 Jian Zhen Ou,*,† Jos L Campbell,‡ David Yao,† Wojtek Wlodarski,† and Kourosh Kalantar-zadeh† † ‡
School of Electrical and Computer Engineering, RMIT University, Melbourne, VIC, Australia School of Applied Chemistry, RMIT University, Melbourne, VIC, Australia
bS Supporting Information ABSTRACT: It is known that the unique layered structure of orthorhombic MoO3 (R-MoO3) facilitates the interaction with H2 gas molecules and that the surface-to-volume ratios of the crystallites play an important role in the process. MoO3 was deposited on a wide variety of transparent substrates using thermal evaporation in order to alter the surface-to-volume ratios of the crystallites. In situ Raman spectroscopy was employed to investigate the interaction between MoO3 and 1% H2 in both N2 and synthetic air environments, while incorporating Pd as a catalyst at room temperature. This study confirmed that the layered MoO3 with a high surface-tovolume ratio facilitated the H2 gas interaction. The Raman spectroscopy studies revealed that the Hþ ions mainly interacted with the doubly coordinated oxygen atoms and caused the crystal transformation from the original R-MoO3 into the mixed structure of hydrogen molybdenum bronze and substoichiometric MoO3, eventually forming oxygen vacancies and water. It was also found that the presence of O2 during the H2 gas exposure caused the recombination of a number of oxygen vacancies and reduced the available surface catalytic sites for H2.
1. INTRODUCTION Molybdenum trioxide (MoO3) has been recently attracting rapid interest due to its unique layered structure, which can be used in various types of applications, such as catalysts,1 photochromic and electrochromic devices,24 gas sensors,5,6 and batteries.7 The two most common crystal phases of MoO3 include R-MoO3 (orthorhombic structure) and metastable β-MoO3 (monoclinic structure).8,9 These two phases have very different physical and chemical properties, such as refractive indices, bandgap energies, and mechanical hardness. It has been recently demonstrated that R-MoO3 with tertiary large layers (up to several micrometers) are formed from the stacking of secondary layers with an average thickness of several tens of nanometers.10 Additionally, each of these secondary layers consists of a few double-layers (∼1.4 nm thick each) of linked distorted MoO6 octahedra. In each double-layer, MoO6 octahedra form edgesharing zigzag rows along the [001] direction and corner-sharing rows along the [100] direction. The adjacent layers along [010] are linked only by weak van der Waals forces to make the lamellar formation, while the internal interactions between atoms within the double-layers are dominated by strong ionic and covalent bondings. A detailed illustration can be seen in Figure 1. A number of techniques have been reported for the deposition of MoO3, including pulse laser deposition,11 thermal evaporation,12,13 sputtering,14 solgel,15 spray pyrolysis,16 chemical vapor deposition,17 hydrothermal,18 and electrodeposition.19 Among these techniques, the deposition of MoO3 using thermal evaporation techniques offers great advantages as they can r 2011 American Chemical Society
produce highly crystalline and stratified structures.5,6,10,12 In addition, the surface morphology of MoO3 is found to be strongly dependent on the type of substrates.13 Hydrogen (H2) gas is fast becoming the most significant player in future energy and display systems.20 The intercalation of Hþ, dissociated from the H2 gas molecules, plays a fundamental role in these systems. Therefore, it is important to fully understand the behavior of H2 gas interaction with materials incorporated in them. R-MoO3 has been reported to be a potential material for H2 sensing, displaying, and energy applications, especially in combination with catalytic metals, such as Pt or Pd.5,6,21 However, to the best of our knowledge, there are only a few reports regarding the fundamental understanding of the H2 gas reaction with layered RMoO3. Instead of investigating H2 interaction in gaseous form, Yang et al.22 conducted Raman spectroscopy studies on R-MoO3 films by using electrochemical reduction in 0.05 M H2SO4 solution to intercalate Hþ ions into the films. They concluded that the injected Hþ ions caused the transformation of the original crystal structure into hydrogen molybdenum bronze, which were bonded to both the triply and the doubly coordinated oxygen atoms. Hirata et al.23 utilized ion implantation to implant various concentrations of Hþ into MoO3. They further confirmed that the resultant chemical species, after Hþ intercalation, was hydrogen molybdenum bronze based on the Raman spectroscopy results. Received: March 5, 2011 Revised: April 21, 2011 Published: May 06, 2011 10757
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Figure 2. SEM images of MoO3 deposited on (a) quartz, (b) FTO, (c) glass, and (d) ITO substrates. Figure 1. Structural representation of a double-layer in R-MoO3.
In this work, we have investigated the performance of H2 gas interaction with thermally evaporated layered Pd/MoO3 at room temperature with different surface-to-volume ratios via in situ Raman spectroscopy studies. Such an investigation is important as it is well known that any gas interaction behaviors greatly depend on the surface-to-volume ratio of the sensitive films.24 Additionally, Raman spectroscopy provides a simple and promising pathway to investigate gas interaction for metal oxides.22,23,25,26 The samples were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Raman spectroscopy. They were subsequently exposed to 1% H2 in N2 and synthetic air ambient, while their Raman spectra were monitored in situ. The Raman spectra studies were utilized to investigate the presence of hydrogen intercalation, formation of hydrogen molybdenum bronze, and the degree of oxygen deficiency.
2. EXPERIMENTAL DETAILS To deposit MoO3 samples, 10 mg of the MoO3 powder (China Rare Metal Material Co.) was placed at the center of a horizontal furnace. Substrates were placed at a distance from the central hot spot, where the temperature was measured to be 500 °C. The thermal deposition was carried out for 10 min using argon gas at a constant flow rate of 250 sccm. The evaporation temperature was increased slowly at the rate of 5 °C/min and cooled at the same rate after the procedure. Following the thermal evaporation process, Pd films with thicknesses of approximately 25 Å were deposited onto the samples by dc sputtering. This procedure was repeated for quartz, glass, fluoride tin oxide (FTO)-coated glass, and indium tin oxide (ITO)-coated glass substrates, which are referred to the “quartz”, “glass”, “FTO”, and “ITO” samples, respectively, in this paper. The resulting films were analyzed using the following equipment: an FEI Nova NanoSEM for scanning electron images and
EDX measurements and a Bruker D8 for XRD measurements. Raman measurements were performed using a system incorporating an Ocean Optics QE 6500 spectrometer, a 532 nm 40 mW laser as the excitation source and a notch filter utilized to prevent signals below 100 cm1. The in situ Raman spectra measurements were performed while the system was attached to a computer-controlled mass flow controller regulating the flow at 200 sccm into a customized gas testing chamber. The expressions “H2 exposure” and “air recovery” in this text are used for describing the processes of exposing the samples to “1% H2 in N2 or synthetic air for 10 min” and “synthetic air for 30 min after H2 exposure”, respectively.
3. RESULTS AND DISCUSSION 3.1. Film Characterization. The SEM images in Figure 2 show the surface morphologies of (a) quartz, (b) FTO, (c) glass, and (d) ITO samples. It is observed that the quartz sample is composed of hexagonal plates with an average width and length of 4 and 9 μm, respectively. The inset image also shows the layered nature of these hexagonal plates. The change of the substrate to FTO results in the formation of layered rectangular nanobelts rather than hexagonal plates. These rectangular nanobelts are observed to have an average width and length of approximately 5 and 40 μm, respectively. Similar to the FTO sample, the glass sample also consists of layered rectangular belts with slightly larger dimensions, lying flat on the surface of the substrate. For the ITO sample, these layered rectangular belts seem to be condensed together, forming large and flat surfaces on top of the ITO substrate. Detailed size distributions of the surface morphology for each sample can be found in the Supporting Information. The dc-sputtered Pd film is so thin (∼25 Å) that it shows no clear evidence of existence in SEM. To prove its existence, an EDX measurement is conducted, which is illustrated in Figure 3. As seen from this figure, a small peak at 2.8 keV represents the L 10758
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Figure 3. EDX pattern of Pd/MoO3.
Figure 4. XRD patterns of the MoO3 deposited on quartz (blue line), FTO (green line), glass (red line), and ITO (black line) substrates. The labeled peaks correspond to R-MoO3 (/, JCPDS 05-0508), substoichiometric MoO3 (0), SiO2 (9), FTO ()), and ITO (O).
electronic shell of Pd, while it also indicates that the K electronic shell of Pd sits in the shoulder of the C peak. Figure 4 shows the XRD patterns of the MoO3 samples. Most of the peaks for these four samples are indexed to orthorhombic MoO3 (R-MoO3, JCPDS 05-0508, cell parameters: a = 3.962 Å, b = 13.858 Å, c = 3.697 Å). The strong diffraction peaks appear at 12.8, 25.7, and 39.1°, which correspond to the (0 2 0), (0 4 0), and (0 6 0) planes, respectively. This reveals that MoO3 layers grow with strong preferential orientations of (1 0 0) and (0 0 1).27 The strong intensities of the reflection peaks of (0 k 0) with k = 2, 4, and 6 also prove the existence of the lamellar structure.5,10 Other obvious peaks at 23.4, 52.9, and 67.6° can be assigned to (1 1 0), (2 1 1), and (0 10 0) planes, respectively. It is also noted that the peaks of the glass sample, corresponding to the (1 1 0) and (0 k 0) planes, have similar intensities, which implies the imperfect growth of rectangular nanobelts (Figure 2c).28 It can be observed that a very small amount of substoichiometric MoO3 exists in all samples because small peaks appear at 11.7, 24.7, and 37.4°, which is a common characteristic for most of the metal oxides prepared by thermal evaporation methods.29,30 The Raman spectra of four different samples are presented in Figure 5. As can be seen, strong peaks occur at 284, 666, 821, and 995 cm1 for all these samples.3133 The 284 cm1 peak represents the bending mode for the double bond (ModO) vibration. The 666 cm1 peak is assigned to the triply coordinated oxygen (Mo3O) stretching mode, which results from edge-shared oxygen atoms in common to three adjacent octahedra. The 821 cm1 peak is for the doubly coordinated oxygen (Mo2O) stretching mode, which results from corner-sharing oxygen atoms common to two octahedra, and the peak at 995 cm1 is assigned to the terminal oxygen (Mo6þdO) stretching mode,
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Figure 5. Raman spectra of the MoO3 deposited on quartz (blue line), FTO (green line), glass (red line), and ITO (black line) substrates. The inset figure represents the intensity ratio between the peaks at 821 and 666 cm1(blue line), and those at 821 and 995 cm1 (red line).
which results from an unshared oxygen. Other peaks at 339 and 376 cm1 can be assigned to Mo3O and ModO bending modes, while the peak at 471 cm1 has the same band assignment as that of 666 cm1. In addition, the peak at 154 cm1 is assigned to the lattice mode and two weak peaks at 210 and 235 cm1 both represent the bending mode of Mo2O. The only major differences on Raman spectra of these four samples are the ratios of the intensities for peaks at 821 and 666 cm1 and those at 821 and 995 cm1. According to the inset in Figure 5, both of these two ratios increase in the sequence of quartz, FTO, glass, and ITO samples, which corresponds to their decreasing surface-to-volume ratios. This indicates the significance of forming low surface-to-volume ratio crystallites in generating strong Raman signals, in which the planar vibrations are confined within one axial direction. On the basis of the XRD results and Raman spectra, the quartz, glass, FTO, and ITO samples can be all identified as R-MoO3, despite their different surface morphologies. 3.2. H2 Gas Interaction with Layered Pd/MoO3 in a N2 Environment. Figure 6 shows the in situ Raman spectra of the (a) quartz, (b) glass, (c) FTO, and (d) ITO samples before H2 exposure, after H2 exposure in the N2 environment, and after air recovery. For the quartz sample, the most notable change after the H2 exposure is observed for the intensities of Raman peaks (assigned to Mo2O stretching mode) at 821 cm1; these peaks reduced from ∼30 000 to less than 50 counts. At the same time, Raman peaks at 154 and 471 cm1 vanished and the intensities of Raman peaks at 284, 666, and 995 cm1 reduced by a factor of 103. These results all provide evidence that the R-MoO3 phase has disappeared during H2 interaction with the crystallites. According to Figure 6a, the blue shift of the peak at 821 cm1 back to 815 cm1 and the appearance of small new HxMoO3 (x e 0.4 type I hydrogen molybdenum bronze) peaks at 204 cm1 (deformation mode), 441 cm1 (Mo3O stretching mode), and 1006 cm1(ModO stretching mode) all indicate the crystal transformation from original R-MoO3 into HxMoO3 in the presence of H2.31 In addition, according to the result presented by Eda,31 another Raman peak, which is assigned to the Mo2 O stretching mode, should appear at 750 cm1. However, it is observed that a new peak instead appears at a lower wavenumber of 718 cm1, which suggests that traces of MoO3x also appear, possibly due to the modification of the original Mo2O bond.34 Other noticeable peaks, which appear at 393, 483, 866, and 962 cm1, can be assigned to the deformation mode, Mo3O, and ModO stretching modes of MoO3x, respectively.34 It has been reported that the intercalation of a sufficient amount of 10759
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Figure 7. Detailed Raman spectra of the MoO3 films deposited on quartz (blue line), glass (red line), FTO (green line), and ITO (black line) substrates after H2 exposure in a N2 environment.
Figure 8. Detailed Raman spectra of the MoO3 films deposited on quartz (blue line), glass (red line), FTO (green line), and ITO (black line) substrates after air recovery.
Figure 6. In situ Raman spectra of the MoO3 deposited on (a) quartz, (b) glass, (c) FTO, and (d) ITO substrates before H2 exposure (blue line), after H2 exposure in a N2 environment (red line), and after air recovery (green line). The inset figures are the detailed Raman spectra after H2 exposure corresponding to different substrates.
Hþ results in the theoretical formation of OH2 groups in the HxMoO3 structure and the bond lengths between the Mo atoms and OH2 groups are significantly increased.35 The authors believe that the breakage of MoOH2 bonds in HxMoO3, due to the presence of heat from either the ambient environment or the laser beam of Raman spectroscopy, could be the reason for the appearance of MoO3x, which is similar to the case of WO3 reported by other researchers.26,36 It has been reported that there is a correlation between the shift of the Raman vibration frequency and the bond length between metal and oxygen atoms.22,37,38 By investigating the Raman shifts for the ModO, Mo2O, and Mo3O bonds upon the H2 intercalation, it is found that the Mo2O bond has the largest shift (∼100 cm1) compared with the other two, which suggests that the Hþ ions
interact preferably with sites in the sequence of doubly coordinated oxygen > terminated oxygen > triply coordinated oxygen. This finding is also in agreement with the theoretical studies conducted by Sha et al.39 and Ritter et al.40 We propose the following mechanism to explain the above findings, which are associated with the process of H2 interaction with layered Pd/MoO3 (Figure 9). The Pd film dissociates the H2 molecules into electrons and Hþ ions, which are then transferred to the nearby surface of MoO3 (Figure 9a). The absorbed Hþ ions mainly interact with the lattice corner-sharing oxygen atoms of the MoO3 first monolayer, forming theoretical OH2 groups and HxMoO3 structures (Figure 9b). The remaining Hþ ions then transfer into deeper layers of MoO3 and repeat the same interaction. H2O molecules are hence formed and released from their original positions, leaving lone oxygen vacancies and resulting in the formation of MoO3x (Figure 9c). Similar to the H2 interactive behavior of the quartz sample, the intensities of major Raman peaks of the glass, FTO, and ITO samples are dramatically reduced by a factor of 103 (Figure 6bd).3 However, for these samples (Figure 7), it is observed that the intensities of the peak at 815 cm1 are all stronger than that at 718 cm1. In addition, the peaks at 284, 666, 815, and 995 cm1 10760
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Figure 9. Illustration of the H2 gas interaction with R-MoO3.
intensify and the peak at 718 cm1 weakens in the order of FTO, glass, and ITO samples. For the glass sample, a new peak appears at a lower wavenumber of 476 cm1 compared with those of the quartz and FTO samples at 483 cm1. For the ITO sample, the Hþ intercalation effect is significantly weakened due to the formation of smaller amounts of HxMoO3 as the peak at 995 cm1 blue shifts back to 984 cm1and the 1006 cm1 peak disappears.23 This also directly affects the formation of MoO3x due to the disappearance of the peak at 483 cm1. The gradual degradation of the Hþ intercalation effect on the FTO, glass, and ITO samples can be linked to their surface-to-volume ratios, as it is obvious that the surface-to-volume ratios decrease in the order of quartz, FTO, glass, and ITO samples, according to the SEM observations presented in section 3.1. The samples with higher surface-to-volume ratios offer greater Pd catalytic surface sites for H2 interaction, hence dissociating more Hþ ions to interact with MoO3. After air recovery (Figure 8), major Raman peaks of the quartz, glass, FTO, and ITO samples slowly (30 min) recover to 45, 10, 66, and 10% of their initial patterns, respectively. For the case of the quartz and FTO samples, the appearance of Raman peaks at 154, 235, 284, 339, 376, 471, 666, 821, and 995 cm1 indicates the re-formation of R-MoO3 in the presence of O2. However, the presence of a peak at 204 cm1 and the incomplete reconstruction of the initial pattern both suggest that there is residual HxMoO3 left in the samples. In the case of the glass and ITO samples, although the peaks at 284, 339, 666, 821, and 995 cm1 intensify, it is observed that peaks at 204, 459, 483, 718, and 956 cm1 remain weak, which indicates that the presence of O2 does not fully transform their crystal structures back to R-MoO3. In addition to our previously proposed mechanism for the H2 interaction, the air recovery process can be described as follows: the incoming dry air stream encourages a portion of the water molecules to desorb from the surface. The O atoms, which are generated by the dissociation of O2 on the Pd films, transfer to the MoO3 surface and then into deeper layers to recombine with a number of the oxygen vacancies. However, the accumulation of water on the MoO3 surface may reduce the available surface catalytic sites for the dissociation of O2, which can explain the incomplete recovery of the initial crystal phase. Alternatively, the interlayer diffusion rate of O atoms in MoO3 is much slower than that of Hþ ions, which could also be a contributing factor to the slow air recovery process. 3.3. H2 Gas Interaction with Layered Pd/MoO3 in a Synthetic Air Environment. Figure 10 shows the in situ Raman spectra of the (a) quartz, (b) glass, (c) FTO, and (d) ITO samples before H2 exposure, after H2 exposure in the synthetic
Figure 10. In situ Raman spectra of MoO3 deposited on (a) quartz, (b) glass, (c) FTO, and (d) ITO substrates before H2 exposure (blue line), after H2 exposure in a synthetic air environment (red line), and after air recovery (green line). The inset figures are the detailed Raman spectra after H2 exposure corresponding to different substrates.
air environment, and after air recovery. The Raman intensities of these four samples are all dramatically reduced, but not as greatly as the intensities recorded in the N2 environment (Figure 6). By closely investigating the Raman spectra after the H2 exposure, it is seen that the H2 interaction performance in the synthetic air environment is slightly different from that in N2. For the case of the quartz sample in Figure 11a, the peak at 339 cm1 blue shifts back to 320 cm1, while the original peaks at 154 and 376 cm1 are red shifted to 166 and 405 cm1, respectively. In addition, obvious peaks appear at 204, 448, 815, and 1006 cm1 and the intensity of the peak at 718 cm1 is much smaller than that at 815 cm1. These imply that the presence of O2 in the synthetic air, during the H2 interaction, causes the recombination of a number of oxygen vacancies, hence forming an equilibrium among the recombination of oxygen vacancies, the formation of OH2 groups, and the formation of oxygen vacancies. 10761
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Figure 11. Detailed Raman spectra of the MoO3 deposited on quartz (blue line), glass (red line), FTO (green line), and ITO (black line) substrates after H2 exposure in a synthetic air environment.
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4. CONCLUSION We have performed in situ Raman spectroscopy to investigate the H2 gas interaction with the layered Pd/MoO3 structures with various forms of surface morphologies both in N2 and synthetic air environments. MoO3 was synthesized using the thermal evaporation method on the quartz, glass, FTO, and ITO substrates. The SEM revealed that the surface morphologies of the crystallites were dependent on the type of substrate. Despite these differences, they were all identified as layered R-MoO3 based on the XRD and Raman measurements. During the H2 gas interaction, the Pd film dissociated the H2 gas into Hþ ions and electrons, which were then transferred into MoO3. The Hþ ions mainly interacted with the doubly coordinated oxygen and formed OH and theoretical OH2 groups, which resulted in the crystal structure transformation from original R-MoO3 into HxMoO3. Because of the breakage of the MoOH2 bond caused by the heat from either the laser or the ambient environment, part of the material appeared as MoO3x, which hence lead to the formation of water and oxygen vacancies. The latter exposure to air did not fully transform the crystal structures back to R-MoO3 at room temperature, as the surface Pd catalytic sites were blocked by the accumulated desorbed water molecules. In addition, the introduction of O2 during the H2 exposure could cause the recombination of oxygen vacancies, which resulted from the H2 interaction. This may also enhance the surface blocking effect due to the possible formation of surface water molecules caused by the dissociated Hþ ions and O atoms on the Pd sites. Interestingly, the H2 interaction intensity with MoO3 was directly linked to the surface-to-volume ratio of crystallites, with the strongest for MoO3 on quartz (highest surface-to-volume ratio) and weakest on ITO substrates (lowest surface-to-volume ratio). ’ ASSOCIATED CONTENT
bS Figure 12. Detailed Raman spectra of the MoO3 films deposited on quartz (blue line), glass (red line), FTO (green line), and ITO (black line) substrates after air recovery.
According to Figure 11a,b, it is clearly observed that major peaks of the glass, FTO, and ITO samples at 284, 666, 821, and 995 cm1 do not disappear in the spectra, which suggests that a small amount of dissociated Hþ interacts with the lattice oxygen of MoO3 to form OH2 groups. This effect becomes more obvious for the ITO sample, as no clear evidence for any new peak appearance is observed. This could be due to the blocking effect of the surface Pd catalytic sites by the accumulated water molecules, either from the desorption of the released water molecules or from the interaction between the dissociated Hþ ions and O atoms on the Pd surface. After the air recovery process, only 12, 8, 23, and 8% of the initial patterns of quartz, glass, FTO, and ITO samples are regained. These values are comparably lower than those under no O2 influence during the H2 exposure, which further confirms the blocking effect on the surface Pd catalytic sites. Similar to the description for Figure 8, Figure 12 also shows the existence of peaks at 204, 465, and 718 cm1 after the air recovery process, which indicates the presence of HxMoO3 and MoO3x, hence providing the evidence of the incomplete retransformation into R-MoO3.
Supporting Information. Detailed size distributions of the surface morphology for the samples deposited on quartz, glass, FTO, and ITO substrates. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ613 9925 3690. Fax: þ613 9925 2007. E-mail address: j.ou@ student.rmit.edu.au.
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