Desorption of Ambient Gas Molecules and Phase Transformation of α

Article pubs.acs.org/JPCC

Desorption of Ambient Gas Molecules and Phase Transformation of α‑Fe2O3 Nanostructures during Ultrahigh Vacuum Annealing Zheng Zhang,† Jupeng Lu,‡ Tao Yun,‡ Minrui Zheng,‡ Jisheng Pan,†,‡ Chorng Haur Sow,*,‡ and Eng Soon Tok*,†,‡ †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602 ‡ Department of Physics, 2 Science Drive 3, National University of Singapore (NUS), Singapore 117542 S Supporting Information *

ABSTRACT: Desorption and readsorption of gas molecules from ambient air onto the surface of α-Fe2O3 quasi-1D nanostructures (nanoflakes and nanostrips) were studied in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XPS revealed that carbon and oxygen species were physisorbed and chemisorbed as C−C/C−H, C−O, O−CO, and O2 states on both surfaces of α-Fe2O3 quasi-1D nanostructures upon exposure in air. The physisorbed carbon species (C−C/C−H) and O2 desorbed from the surfaces when the two nanostructures were heated to 100 °C inside the vacuum chamber of XPS. Significant desorption of chemisorbed O−CO and O−C occurred above 200 °C, which resulted in a reduction of Fe2O3 into Fe3O4 for both samples between 200 and 300 °C. Complete desorption of carbon and O−C/O−CO/O2 species in O1s occurred at 400 °C, where Fe3O4 in nanoflakes (sample 1) was reduced further into FeO by excess metallic Fe from the bulk, while Fe3O4 in nanostrips (sample 2) was largely oxidized into Fe2O3 by the oxygen from the bulk of Fe2O3. Although no band bending was observed during the annealing and desorption of ambient gases, the valence band changed as the phase transformation occurred. After the annealed samples were exposed to air for two days, the same chemical states associated with C and O species were again detected on the surfaces of the two nanostructures. In addition, FeO (sample 1) was found to be oxidized into a mixture of Fe2O3 and Fe3O4 on the surface. The adsorption of gas molecules from ambient environment thus has a strong influence on the chemical and physical properties of nanostructures with large surface to volume ratio. 1D nanostructures. Examples consist of α-Fe2O3 nanoflakes by heating iron foils to 350 °C on a hot plate in air,11 α-Fe2O3 random nanobelts by oxygen plasma oxidation of Fe foil in a plasma-enhanced chemical vapor deposition (PECVD) system, and α-Fe2O3 nanostrips and nanoneedles by heating Fe foils in a tube furnace to 750 and 800 °C, respectively.20 Although there have been extensive studies focusing on the application of these nanostructures by tailoring their intrinsic properties during growth, few works have investigated the influence of external factors such as ambient gas molecules and gas pressure on the performance (i.e., field emission) of these nanostructures. Using the α-Fe2O3 nanostructures as field emitter, we recently observed an increase (36 times) in the field emission current from these nanostructures through repeatedly ramping the applied voltage from 0 to 1100 V for 40 times.20 In addition, the field emission performance was also found to become poorer after exposing to air or simulated air (O2 and N2) environment compared to vacuum or pure N2 environment. As such, we attributed the improved field emission behavior to the desorption of adsorbed oxygen or water

I. INTRODUCTION α-Fe2O3 (hematite), the most stable iron oxide state under ambient condition, has attracted significant interest due to its unique properties such as being an n-type semiconductor with small band gap (2.1 eV), high resistivity to corrosion, and low fabrication cost.1 As a result, α-Fe2O3 bulk material has been widely used as photocatalyst,2−4 gas sensor,5,6 red pigment,7 and electrodes for lithium-ion batteries applications.5,8 Quasi1D nanostructures, such as nanotubes, nanowires, nanowalls, and nanobelts, have been demonstrated recently with extraordinary electronic, optical, mechanical, and thermal properties owning to their large surface areas and possible quantum confinement effect.9 Combining the promising properties of α-Fe2O3 and the versatile applications of nanoscale materials, α-Fe2O3 quasi-1D nanostructures have been the subject of various research groups for applications in a vast array of fields including Li-ion batteries,5,10−12 water treatment,13 gas sensing,5,14 and field emission.15,16 A few approaches have been developed to fabricate α-Fe2O3 quasi-1D nanostructures. Examples include template-assisted method,5 solution-based methods (hydrothermal and solvothermal methods followed by sequential calcination),10,12−14,17 and direct thermal oxidation.18,19 We have recently used direct thermal oxidation method to synthesize various α-Fe2O3 quasi© XXXX American Chemical Society

Received: November 7, 2012 Revised: December 13, 2012

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Figure 1. SEM images of as-prepared (a) α-Fe2O3 nanoflakes grown on a hot plate at 350 °C and (a′) α-Fe2O3 nanostrips grown in a furnace at 750 °C. (b) and (b′) show the morphology from same samples taken immediately after annealing to 400 °C in the XPS vacuum chamber.

temperature of 350 °C on a hot plate, and the other is α-Fe2O3 nanostrips prepared at a high temperature of 750 °C in a tube furnace. We will show that physisorbed and chemisorbed carbon- and oxygen-related species are present on the surface and desorption occurs when the samples are heated. Clean and uncontaminated nanostructures are obtained at 400 °C. These gas molecules reabsorb on the surfaces once the nanostructures are exposed to air.

molecules from the surfaces of these nanostructures, which occur through current-induced joule heating of the nanostructures during repeated voltage ramp.20 Interestingly, besides α-Fe2O3-based nanostructures, hightemperature annealing (500 °C) of vertically aligned ZnO nanowires in UHV has also been reported to result in removal of 87% of surface carbon content from the vertically aligned ZnO nanowires. 21 Focusing on carbon nanostructures, Chernozatonskii et al. observed that a decrease in the residual gas pressure in the vacuum chamber from 10−3 to 10−5 torr can increase field emission current pronouncedly.22 These works clearly show that adsorbed gas molecules from the ambient environment in the form of oxygen-related species as well as carbon-related species can have a strong influence on the properties of these nanostructures. As such, there is a need to examine and probe the chemical states of these absorbed ambient gas molecules on these iron oxide nanostructures in more details. In this work, we simulate the current-induced heating by resistively heating α-Fe2O3 nanostructures inside an X-ray photoelectron spectroscopy (XPS) ultrahigh vacuum chamber and examine in situ the compositional change on the surfaces of these nanostructures during annealing. Two nanostructures were investigated: one is α-Fe2O3 nanoflakes prepared at a low

II. EXPERIMENTAL METHODS Iron foils (10 × 10 × 0.1 mm3) with a purity of 99.99% from Alfa Aesar were served as the substrates for the growth of αFe2O3 quasi-1D nanostructures. Prior to growth, Fe foils were polished with sandpaper (1200 grits) to remove oxide layers on the surface and were thereafter cleaned with isopropyl alcohol. To synthesize α-Fe2O3 nanoflakes (sample 1), a treated Fe foil was directly heated on a hot plate under ambient condition to 350 °C for 10 h. To fabricate α-Fe2O3 nanostrips (sample 2), polished Fe foil was heated inside a small tube furnace with a length of 30 cm at atmospheric environment to 750 °C for 4 h. Samples 1 and 2 were cut into a size of 3 × 8 mm2 and were loaded onto a Thermo Fisher heated sample block and transferred into the analysis chamber of a VG ESCALAB 220i-XL X-ray photoelectron spectroscopy to analyze the B

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surface at a temperature range between 25 and 400 °C. The temperature of the sample was monitored through a thermocouple in direct contact with the sample’s surface. The sample was typically kept at a temperature for 1 h and then cooled down to room temperature before the XPS spectra were captured. Temperature was ramped up after the XPS measurement. By doing so, thermal broadening of XPS spectra at high temperatures can be minimized. A monochromatic Al Kα (1486.6 eV) X-ray with a diameter of 700 μm is employed while the photoelectrons are collected at a normal takeoff angle (with respect to surface plane). The C1s peak from adventitious carbon at 285.0 eV is used as a reference for charge correction. The elemental composition is calculated from peak areas in the XPS spectra after subtracting a Shirleytyped background and taking into account both Scofield photoionization cross sections and the transmission function of the spectrometer. For chemical state analysis, a spectral deconvolution was performed by a curve-fitting procedure based on Lorentzians broadened by a Gaussion using the manufacturer’s software (Avantage). The error bar of binding energy is estimated to be within ±0.2 eV. The crystalline structures of the α-Fe2O3 nanostructures before and after annealing in vacuum were investigated by a Bruker general area detector diffraction system (GADDS) X-ray diffractometer (XRD) operated at a voltage of 40 kV and a current of 40 mA (Cu Kα X-ray, λ = 1.54 Å), while the morphologies of these nanostructures were studied by JEOL JSM-5600 scanning electron microscope (SEM) at an accelerating voltage of 10 kV and a tilt angle of 20°.

III. RESULTS Uniform maroon and dark-red colors were observed on the entire 1 cm × 1 cm iron foils prepared using direct thermal oxidation method on a hot plate at 350 °C (sample 1) and in a tube furnace at 750 °C (sample 2), respectively. When examined under SEM, randomly ordered nanoflakes structures were observed on sample 1 with widths of about 150−200 nm at the bases, 30 nm at the tips, and 3−5 μm of the length (Figure 1a). By contrast, more vertically aligned nanostrips ∼50 nm in thickness, 300 nm in width, and 10 μm in length were grown in sample 2 with sharp tips occasionally (Figure 1a′). The crystalline structures of these two nanostructures are studied by XRD and are shown in Figure 2. As shown in the lower black spectrum of Figure 2a, diffraction peaks attributed to body-center-cubic (BCC) iron and two types of iron oxides, namely, rhombohedra α-Fe2O3 and cubic Fe3O4, are detected in sample 1. There are several orientations in the diffraction peaks associated with α-Fe2O3 and cubic Fe3O4. From the previous high-resolution transmission electron microscope (HRTEM) observation, the nanoflakes are single crystalline α-Fe2O3 phase and free from amorphous layers.23 The different orientations of α-Fe2O3 observed in the XRD pattern are related to the various growth directions of the α-Fe2O3 nanoflakes, which can be seen in Figure 1a. The cubic Fe3O4 is a 1 μm thick, condensed layer below the nanoflakes which serves as the precursors for the growth of α-Fe2O3 nanoflakes.23 BCC Fe should come from the metallic Fe in the iron foil further below the Fe3O4 layer. The XRD pattern therefore reveals that the 0.1 mm thick iron foil does not thoroughly oxidize through oxidation on the hot plate at 350 °C. In comparison, a single rhombohedra α-Fe2O3 phase is detected in sample 2 without peaks from BCC iron (Figure 2a′), implying a complete oxidation of Fe foil into α-Fe2O3 phase. The XRD

Figure 2. XRD patterns of as-grown Fe2O3 nanostructures, after vacuum annealing to 400 °C and after exposure in air for 10 months for (a) α-Fe2O3 nanoflakes grown on a hot plate at 350 °C and (a′) αFe2O3 nanostrips grown in a furnace at 750 °C.

pattern is dominated by the (110) peak at 35.2°, which is associated with the preferred vertical alignment of the nanostrips as shown in Figure 1a′. The respective diffraction ring patterns on the area detector are shown in Figure S1 of the Supporting Information. The two samples were then separately loaded into the XPS chamber for vacuum annealing and in situ XPS investigation. The Fe 2p3/2 peaks of both ferrous (Fe2+) and ferric (Fe3+) compounds are broadened compared with that of metallic Fe, owning to electrostatic, spin−orbit, and crystal field interactions in Fe atoms.24 It is thus difficult to fit the Fe2p spectra in order to identify the chemical states of iron. Instead, the presence of the satellite peak and the separation between satellite peak and Fe2p3/2 are frequently relied on to differentiate different chemical states of Fe oxides, i.e., FeO, Fe3O4, and Fe2O3.25−28 For example, the Fe2p XPS spectrum from as-prepared surface of sample 1 (nanoflakes) clearly showed characteristics of αFe2O3 phase, which includes two maxima in Fe 2p3/2 peak and a satellite peak closer to Fe2p1/2 than Fe2p3/2 (Figure 3a). C

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Figure 3. XPS spectra of (a) Fe2p, (b) O1s, (c) C1s, and (d) VB of α-Fe2O3 nanoflakes prepared on a hot plate at 350 °C (top row) and (a′) Fe2p, (b′) O1s, (c′) C1s, and (d′) VB of α-Fe2O3 nanostrips grown in a furnace at 750 °C (bottom row).

Ag, and Pt) surfaces,31 but they appear between 532.7 and 534.5 eV on oxide surfaces (SnO2).32 Hence, it is likely that there are O2 molecules absorptions on the surface of these nanostructures, and the adsorbed O2 molecules overlap with the O−C/O−CO component around 533.0 eV in O1s spectrum. Three chemical states for carbon, namely, hydrocarbon-like species (C−C/C−H, 285.0 eV), epoxy/alkoxy/ ether-like species (C−O, 286.4 eV), and carboxyl-like species (O−CO, 288.7 eV), can be detected on the surface after fitting the C1s spectrum (Figure 3c). The valence band (VB) of the α-Fe2O3 is a result of p−d hybridization between O2p and Fe3d orbitals. Three distinct peaks associated with α-Fe2O3 with maxima at about 2.5, 4.7, and 7.4 eV (marked by orange arrows) can be observed in VB.25,29 Valence band maximum (VBM) is located at 1.3 eV away from the Fermi level (EF = 0 eV) (Figure 3d), clearly showing a semiconductor property of as-prepared α-Fe2O3 nanoflakes. When the nanoflakes sample was heated up, there was no change in Fe2p and VB spectra at 100 °C, indicating that nanoflakes remained as Fe2O3 phase (Figure 3a and d). At 200 °C, an additional component at 708.6 eV (marked by a green arrow) appeared at the lower B.E. side of Fe2p3/2 peak beside the two maxima belonging to Fe2O3 (Figures 3a and S2a]. Meanwhile, an extra component at 1.2 eV (marked by a green arrow) also showed up in VB. These two new components are

McIntyre et al. have used a multiplet splitting pattern consisting of five components to fit Fe2p3/2 peak based on Gupta and Sen’s calculation.29,30 In this work, we simplify to using only two components to fit Fe2p3/2 peak of Fe3+ ions by following its peak shape. These two components, however, do not share the same full-width-at-half-maximum (FWHM), with the component at higher binding energy to have a bigger FWHM value (2.5 ± 0.4 eV) than the FWHM of the component at lower binding energy (1.2 ± 0.2 eV) to account for the electrostatic, spin−orbit, and crystal field interactions in Fe atoms. The resultant fitting shows two Fe2p3/2 components at 711.2 and 710.1 eV separated by ∼1.1 eV (marked by two orange arrows) and a satellite peak at 719.2 eV (marked by an orange arrow) of 8.0 eV higher than that of Fe2p3/2 (Figure 3a). The fitting can be seen clearer from Fe2p3/2 spectra in Figure S2a of the Supporting Information, after zooming in the respective Fe2p spectra in Figure 3a. The peak position of Fe2p3/2 agrees with a range of values between 710.6 and 711.2 previously reported for Fe2O3.25−30 Three chemical states for oxygen can be identified after fitting the O1s spectrum. They are O species bonded with Fe (Fe−O, 530.1 eV), hydroxyl group bonded with Fe (Fe−OH, 531.7 eV), and O species bonded with C (O−C/O−CO, 533.0 eV) (Figure 3b).29 It is worth to note that the binding energies (B.E.) of O1s for adsorbed O2 molecules are in the range of 528.2−530.8 eV on metal (Au, D

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Figure 4. Change of Fe% and O% composition during vacuum annealing of Fe2O3 from 25 to 400 °C for (a) α-Fe2O3 nanoflakes grown on a hot plate at 350 °C and (a′) α-Fe2O3 nanostrips grown in a furnace at 750 °C.

characteristic of Fe3O4,26,27,29 suggesting the formation of αFe2O3 and Fe3O4 mixture. At 300 °C, Fe2p spectrum displayed complete features of Fe3O4 phase with Fe2p3/2 moving to lower B.E. at 710.4 eV, an increase in intensity of new component at 708.7 eV, and disappearance of the satellite peak. In the mean time, the intensity of the new component at 0.7 eV increased while the three peaks associated with α-Fe2O3 in the VB become less distinct and merged together (Figure 3d). The VBM was shifted to −0.3 eV, which could be attributed to defect/impurity states in the valence band scattering of photoelectrons.27 At 400 °C, the Fe2p spectrum exhibited the features of FeO with B.E. of Fe2p3/2 shifting further lower to 709.6 eV and the reappearance of a satellite peak at 715.6 eV (marked by gray arrows), 6.0 eV higher than that of Fe2p3/2.26,29 Meanwhile, VB also displayed the features of FeO with one strong peak at 4.0 eV and a shoulder at 1.5 eV.29 The VBM for FeO was at 0.0 eV, implying a possible increase in conductivity compared to that of Fe2O3. The Fe2p and VB spectra therefore reveal a phase transition of the nanoflakes (sample 1) from α-Fe2O3 during 25−100 °C to Fe3O4 at 200− 300 °C and finally to FeO at 400 °C. During annealing, the O−C/O−CO and adsorbed O2 component in O1s slowly decreased with an increase in temperature and eventually disappeared completely at 400 °C. On the other hand, the Fe−O and Fe−OH components persisted until 400 °C (Figure 3b). The O−CO component in C1s disappeared at 300 °C, while the C−O and C−C/C−H components were completely undetected at 400 °C (Figure

3c). The O−C/O−CO/O2 component in the O1s spectra and the three components in C1s spectra are related to the gaseous species that adsorb on the surfaces of the nanostructures. It is therefore not unexpected that they can desorb from surfaces upon being heated up in vacuum, explaining their disappearance at high temperatures. It can also be seen from Figure 3c that scaling factors (which are used to normalize the spectra to have the same height) for C1s spectra from RT to 400 °C were gradually increased, suggesting a continual decrease in C1s intensity which can occur through continuous loss of hydrocarbon from surface through desorption. Due to such desorption of adsorbate from nanoflakes surfaces, the intensity from Fe oxide increased, as evidenced by a reduction in scaling factors used for Fe2p in Figure 3a and O1s in Figure 3b. While the B.E. of Fe2p3/2 decreased from 711.2 eV at 25 °C to 709.6 eV at 400 °C, the B.E. of O1s experienced an increase from 530.1 eV at 25 °C to 530.6 eV at 400 °C. The shifts in the B.E. of O1s and Fe2p3/2 occur in the opposite directions. Therefore, the shift is attributed to chemical shift arising from a change in the chemical environment from α-Fe2O3 to FeO. This is different from the 0.2 eV shift of both Zn2p3/2 and O1s to one direction (lower B.E.) after carbon and water molecules were desorbed from the ZnO nanowires, when the nanowires were heated between 300 and 600 °C. Maffeis et al. attributed the 0.2 eV shift to upward band bending due to extra surface electrons donated by water molecules.21 E

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nanostrips in sample 2 remained unchanged in morphology after annealing (Figure 1b′). This is not unexpected as the final annealing temperature (400 °C) is higher than the growth temperature of sample 1 (350 °C) but lower than the growth temperature of sample 2 (750 °C). As a result, softening and bending of some nanoflakes in sample 1 can occur. Interestingly, crystalline structures in sample 1 have changed from a mixture of BCC iron, rhombohedra Fe2O3, and cubic Fe3O4 to a mixture of BCC iron and cubic FeO after annealing, as shown by the middle red curve in Figure 2a. The structure of nanostrips in sample 2 was still dominated by rhombohedra αFe2O3(110) phase (Figure 2a′) after annealing. Thus, both XPS and XRD results suggest that while the phase and morphology of the α-Fe2O3 nanostrips (sample 2) are stable until 400 °C, the α-Fe2O3 nanoflakes (sample 1) are not stable and can be reduced to FeO after vacuum annealing at 400 °C. The samples were reloaded into the XPS chamber for investigation after two days exposure in air. The Fe2p spectrum of the nanoflakes (sample 1) exhibited combined features of Fe2O3 (a satellite peak at 718.7 eV) and Fe3O4 (a small shoulder component at 708.6 eV) (Figure 3a). The VB of nanoflakes (sample 1) also showed a weak peak at 2.8 eV and a component at 0.8 eV belonging to Fe2O3 and Fe3O4, respectively (Figure 3d). The Fe2p spectrum of the nanostrips (sample 2), however, clearly showed the characteristics of Fe2O3 with the B.E. of Fe2p3/2 having two maxima at 711.2 and 710.1 eV and a satellite peak at 719.1 eV (Figure 3a′). Three maxima representing α-Fe2O3 can be vaguely identified in the VB while the component at 0.8 eV attributed to Fe3O4 disappeared (Figure 3d′). The same three chemical states related to carbon and oxygen reappeared in the spectra of C1s and O1s for both samples. The composition of three components in C1s and O−C/O−CO/O2 component in O1s almost reach half the level of as-received surfaces for both samples (Figure 4). The results thus demonstrate readsorption of C, O-related gas molecules onto the surfaces of both nanostructures upon exposure in ambient environment. After exposure in the ambient air for 10 months, the two samples were subjected to XRD, SEM, and XPS analysis again, and similar development was observed for both samples. The morphology and structures show no changes for both samples as revealed by SEM (not shown here) and XRD (Figure 2, blue curves). XPS detects the same chemical states for Fe, O, and C as those after two days exposure in air. The nanoflakes (sample 1) were still composed of α-Fe2O3 and Fe3O4 while the nanostrips (sample 2) exhibited a single α-Fe2O3 phase as observed from both Fe2p and VB spectra (Figure 3). The composition of all three components in C1s spectrum and the O−C/O−CO/O 2 and Fe−OH components in O1s spectrum increased, while the composition of Fe and Fe−O components in O1s spectrum decreased (Figure 4). The changes in compositions suggest a continuous adsorption of gas molecules from ambient air onto the nanostructures surfaces, which enhances the adsorbate’s intensity at the expense of the underlying Fe oxides’ intensity. As a result, the composition after 10 month exposure has returned to almost the same level as before annealing.

The compositions of different components in Fe, O, and C in atomic ratio can be calculated based on the respective peak areas after spectra deconvolution in Figure 3 with consideration of Scofield photoionization cross sections and the transmission function of the spectrometer. The resulting change in the composition of Fe, three components in O, and three components in C during annealing can be seen more clearly from Figure 4a. The C−C/C−H component (black square) in C decreased significantly from 32.4% at 25 °C to 24.8% at 100 °C. It continued to decrease from 100 to 400 °C and reached 0 only at 400 °C. The O−CO component (green square) and the C−O component (red square) in C reached 0 at 300 and 400 °C, respectively. Correspondingly, the O−C/O−CO/ O2 component in O reached 0 at 400 °C. While the Fe−OH component in O remained roughly stable, the Fe−O component in both O and Fe consistently increased from 25 to 400 °C, indicating desorption of adsorbed gas molecules and the exposure of Fe oxide nanoflakes’ surfaces. The Fe2p XPS spectrum in as-prepared nanostrips (sample 2) also demonstrated characteristics of Fe2O3 with two distinct Fe2p3/2 components at 711.2 and 709.9 eV separated by ∼1.2 eV (marked by two orange arrows) and a satellite peak (719.2 eV, marked by an orange arrow) of 8.0 eV higher than that of Fe2p3/2 (Figure 3a′). The fitting of Fe2p3/2 peaks can similarly be seen clearly in Figure S2a′ of the Supporting Information. The same three chemical states were similarly present in O1s (Figure 3b′) and C1s (Figure 3c′) spectra after fitting. Three distinct peaks associated with α-Fe2O3 with maxima at about 2.4, 4.8, and 7.2 eV were similarly observed in the valence band while the VBM was at 1.3 eV (Figure 3d′). No changes in Fe2p and VB spectra were observed when the nanostrips were annealed to 100 °C in vacuum. However, the Fe2p spectrum exhibited the full characteristics of Fe3O4 at 200 °C with Fe2p3/2 moving to lower B.E. at 710.4 eV, appearance of additional component in the shoulder around 708.6 eV (marked by a green arrow), and the disappearance of the satellite peak.26,29 A featured peak of Fe3O4 at 0.8 eV also showed up in the VB, while the three peaks associated with αFe2O3 in the VB became merged and less distinguishable (Figure 3d′). The VBM was shifted lower to 0 eV. Between 300 and 400 °C, features associated with Fe2O3 such as two maxima of Fe2p3/2 at 711.1 and 710.0 eV reappeared, while the components related to Fe3O4 at 708.6 eV in Fe2p3/2 and 0.7 eV in VB weaken in intensity. During annealing, the O−CO component in C and O decreased with temperature. O−CO, C−O, and C−C/C−H components completely disappeared at 300, 400, and 400 °C, respectively. The O−C/O−CO/O2 component in O1s disappeared at 300 °C. An increase in the scaling factors for C1s spectra (Figure 3c′) and decrease in the scaling factors for Fe2p and O1s (Figure 3a′ and b′) spectra with an increase in temperature were similarly observed due to desorption of C, O-related species from α-Fe2O3 nanostrips surfaces. The development in compositions of different components in Fe, O, and C in atomic ratio during annealing are shown in Figure 4a′ based on spectra deconvolution in Figure 3a′−c′. The overall trend is similar to that of nanoflakes (sample 1) in Figure 4a as described above. When the two samples were taken out of the vacuum chamber, their morphology and crystalline structures were immediately re-examined by SEM and XRD, respectively. While the length, base width, and tip size did not change significantly, bending of some nanoflakes from the tip toward the base was observed in sample 1 after annealing (Figure 1b). However, the

IV. DISCUSSION The composition of C−C/C−H was found decrease from 32.4% to 24.8% for nanoflakes 52.7% to 43.4% for nanostrips (sample temperature was increased from 25 to F

to immediately (sample 1) and 2) when the 100 °C. The

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with the electron positive element of Fe in α-Fe2O3, when they are chemisorbed on the surfaces. As a result, complete desorption of O−C and O−CO components at 200−300 °C will break a large amount of bondings between O−C/O− CO and Fe2O3, and the electron charge density will be increased around Fe atoms, reflected as the reduction of Fe2O3 to Fe3O4 as detected by XPS for both samples. However, these two samples behave differently when further annealed to 400 °C. Fe2O3 and Fe3O4 in the nanoflakes (sample 1) were further reduced to FeO at 400 °C as seen from the in situ Fe2p and VB spectra (Figure 3 a and d) and the ex situ XRD pattern (Figure 2a). At 400 °C, chemisorbed C and O species have completed desorbed (Figure 3c). We attribute the reduction at this stage to the presence of metallic Fe arising from the bulk of Fe foil. Three pathways are possible upon annealing in the absence of oxygen in the ultrahigh vacuum environment:

compositions of C−O and O−CO, however, only show significant decrease above 200 °C, and these three components in C1s spectra are completely reduced to 0 at 400 °C (Figure 4). These results indicate that there is some amount of physisorbed molecules (mainly C−C related) on the surface. They desorbed readily from surface when the temperature was ramped up from 25 to 100 °C. However, the majority of molecules (part of C−C and majority of C−O and O−CO) are chemisorbed on the surface and can only be removed above 100 °C. It is worth to note that the compositions of C−C/C−H and C−O species in C1s are significantly higher in nanostrips (sample 2) than in nanoflakes (sample 1), in both as-grown and exposed-to-air conditions (Figure 4). The difference can be attributed to different sampling depth arising from the orientation of the nanostructures. The photoelectrons coming from the sides of the vertically oriented nanostrips will have much shallower effective escape depths compared with the photoelectrons coming from the randomly oriented nanoflakes. Therefore, the photoelectrons from nanostrips are much more surface-sensitive than those from nanoflakes and reflect more signals from the top surface carbonaceous contamination, resulting in higher carbon compositions. A phase change from Fe2O3 to Fe3O4 occurred for both samples between 200 and 300 °C (Figure 3 a and a′). If we consider the change in Gibbs free energy (ΔG = ΔH − TΔS and the thermodynamic data listed in Table 1) for thermal

Δ

4Fe2O3(s) + Fe(s) → 3Fe3O4 (s) Δ

Fe3O4 (s) + Fe(s) → 4FeO(s)

CO (g) CO2 (g) O2 (g) Fe (s) FeO (s) Fe3O4 (s) Fe2O3 (s)

S (J K−1 mol−1)

−110.53 −393.51 0 0 −271.96 −1118.4 −824.2

197.67 213.74 205.138 27.28 60.75 146.4 87.4

(4)

The ΔG values at 400 °C are calculated to be −100.4, −16.1, and −37.2 kJ/mol for reactions of 2, 3, and 4, respectively. The negative ΔG values suggest that all the three reactions are favorable at 400 °C. As a result of Fe consumption to reduce Fe2O3 and Fe3O4 at 400 °C, the diffraction peaks belonging to Fe(110), Fe2O3(110), and Fe3O4(220) after annealing in the red curve were reduced to barely noise levels as compared with the peaks in the as-grown sample (black curve, Figure 2a), although Fe(200) and Fe2O3(024) are just slightly reduced in intensity by contrast. The XRD diffraction pattern on the area detector in Figure S1 of the Supporting Information clearly shows the disappearance of the Fe(110) diffraction ring around 44.7° (marked by the white arrow in Figure S1b), which suggests that the Fe(110) peak is truly consumed but not undetected due to texturing. In contrast, the Fe(200) diffraction ring at 65.1° was still clearly present. The results suggest preferential reaction of metallic Fe at (110) facet over (200) facet with Fe2O3 at (110) facet to form FeO. Therefore, Fe(110) is consumed faster than Fe(200) and becomes barely undetected after 1 h of annealing. In a separate study of initial oxidation of Fe(100), Roosendaal et al. observed that Fe(100) was oxidized into FeO (3 ML) initially between RT and 200 °C by XPS.28 Increase of O2 pressure leads to formation of Fe3O4 near the oxide/vacuum interface. Annealing of this double-oxide-layer structure [Fe3O4/FeO/Fe(100)] at 200 °C in vacuum leads to formation of a single-oxide layer containing almost Fe2+ only.28 In our work, α-Fe2O3 nanoflakes (sample 1) similarly were reduced to FeO upon annealing in vacuum at 400 °C. In sharp contrast to the nanoflakes (sample 1), Fe3O4 in the nanostrips (sample 2) was not reduced to FeO but was oxidized into Fe2O3 when annealed further to 400 °C (Figure 3 a′). This difference in results between sample 1 and 2 could be attributed to the absence of metallic Fe in sample 2 needed for reduction to occur (as initial Fe substrate was completely oxidized into Fe2O3 in the furnace during growth). Although the oxidation process in not clear at this stage, it is likely to result from the supply of oxygen from the bulk of Fe2O3 during heating in the ultrahigh vacuum. After complete desorption of

decomposition of Fe2O3 into Fe3O4 and O2 in vacuum (reaction 1), we obtain a value of +345.6 and +318.4 kJ/mol at 200 and 300 °C, respectively. The positive ΔG values indicate that decomposition reaction is not favorable at these temperatures. Therefore, this phase change is unlikely attributed to thermal decomposition of Fe2O3. Δ

6Fe2O3(s) → 4Fe3O4 (s) + O2 (g)↑

(3)

Δ

Fe2O3(s) + Fe(s) → 3FeO(s)

Table 1. Thermodynamic Data from Ref 33 at 298.15 K ΔH (kJ mol−1)

(2)

(1)

It is interesting to note that the phase change from Fe2O3 to Fe3O4 coincides with the partial desorption of the C−C/C−H component and the complete desorption of the O−CO component (shown in C1s spectra) and O−C/O−CO/O2 component (shown in O1s spectra) at 300 °C (Figure 3 b, b′ and c, c′). As explained above, C−C/C−H component physisorbs on the surface, and its desorption occurs as soon as the samples are heated to 100 °C. In this temperature regime, Fe3O4 is not detected at 100 °C for both samples. Hence, desorption of the C−C/C−H component is unlikely the cause of the phase change from Fe2O3 to Fe3O4. Instead, the phase change occurs at the temperature window that coincides with the decreases, i.e., desorption of the chemisorbed O−C and O−CO components from the surface of α-Fe2O3 nanostructures surfaces. The electron negative element of O in O−C and O−CO tends to bond G

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Figure 5. Schematic cross-sectional diagrams illustrating the behaviors when the nanoflakes (sample 1) and nanostrips (sample 2) were annealing from 25 °C to 200, 300, 400 °C and after exposure in air for 2 days.

further at 300 °C, the O in the bulk of Fe2O3 diffuses to the surface due to concentration gradient and partially oxidizes the Fe3O4 into Fe2O3 (Figure 5c′). Complete desorption takes place at 400 °C, where O from bulk of Fe2O3 nearly oxidizes the whole Fe3O4 into Fe2O3 (Figure 5d′). Upon exposure in air for two days, C, O-related species readsorb on the surface and help oxidize the outer layers of nanoflakes and the top surface from Fe3O4 into Fe2O3 (Figure 5e′). It is worth to note that adsorption of C- and O-related species readily takes place on the surfaces of either rhombohedra Fe2O3, cubic Fe3O4, or cubic FeO nanostructures in our two samples. Such adsorption behavior has been similarly observed for carbon nanotubes22 and ZnO nanowires21 and thus can be a common behavior on the surfaces of other nanostructures exposed in ambient air. The present study therefore provides evidence to support our earlier work that the removal of adsorbed gas molecules from α-Fe2O3 nanostructures by ramping the applied voltage repeatedly accounts for enhanced field emission behavior of α-Fe2O3 nanostructures.20 XRD is more a bulk analytical technique. Hence, it detects the bulk of nanoflakes in sample 1 to be cubic FeO structure after annealing in vacuum and exposure in air for two days. On the other hand, XPS is known to be a more surface-sensitive technique with a probing depth not more than 10 nm. Therefore, it reveals that the surfaces of these FeO nanoflakes have in fact been reoxidized into Fe2O3 and Fe3O4 by the O2related species in air.

O−C- and O−CO-related species from the surface, oxygen in the bulk Fe2O3 may preferentially move to the surfaces of the nanostrips due to oxygen concentration gradient and oxidize Fe3O4 into Fe2O3 at 400 °C. A schematic diagram describing the processes during annealing and after exposure in air for two days is illustrated in Figure 5 for both samples based on the studies discussed above. Sample 1 consists of randomly aligned α-Fe2O 3 nanoflakes grown out of a condensed Fe3O4 layer on top of the metallic Fe foil. The top surface of Fe3O4 layer is oxidized by the ambient air into Fe2O3 (Figure 5a). Annealing causes the physisorbed C−C/C−H species and O2 molecules to desorb from the surface initially. At 200 °C, chemisorbed O−C/O− CO species also desorb from the surface, breaking the O−Fe bonding between O−C/O−CO and Fe2O3 (Figure 5b). This is manifested as the reduction of Fe2O3 into Fe3O4 as detected by XPS. Meanwhile, a thin layer of FeO can be formed at the interface between Fe3O4 and Fe due to reduction of Fe3O4 by Fe at 200 °C.28 As the chemisorbed C, O-related species desorb further at 300 °C and Fe in the bulk of Fe foil continues diffusing to the surface due to concentration gradient, the outer layers of the nanoflakes as well as the top surface have been reduced into Fe3O4 while the FeO layer between Fe3O4 and Fe has grown thicker (Figure 5c). Complete desorption occurs at 400 °C, where the nanoflakes and the whole Fe3O4 layer have been reduced into FeO by the Fe atoms diffusing from the bulk (Figure 5d). Exposure in air for two days leads to readsorption of the C, O-related species on the surface and the reoxidation of the outer layers of nanoflakes and the top surface from FeO into Fe2O3 by the O2 in the ambient air. The layers below the nanoflakes still remain as FeO (Figure 5e). Compared with randomly aligned nanoflakes in sample 1, vertically aligned α-Fe2O3 nanostrips are grown above the bulk α-Fe2O3 in sample 2 after full oxidation of Fe foil in furnace (Figure 5a′). Desorption of chemisorbed O−C/O−CO species similarly causes reduction of outer layers of nanostrips and the top surface from α-Fe2O3 into Fe3O4 at 200 °C (Figure 5b′). While the chemisorbed C, O-related species desorb

V. CONCLUSION α-Fe2O3 nanoflakes and nanostrips samples are prepared through direct thermal oxidation of 0.1 mm thick Fe foil on a hot plate at 350 °C and in a tube furnace at 750 °C, respectively. Carbon and oxygen species were both physisorbed and chemisorbed as alkanes (C−C/C−H), epoxy/alkoxy/ether (C−O), carboxyl (O−CO), and O2 molecules on the surface of α-Fe2O3 quasi-1D nanostructures upon exposure in air. Part of the physisorbed C−C and O2 molecules desorbed when the two nanostructures were heated up inside the vacuum chamber H

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(17) Vayssieres, L.; Sathe, C.; Butorin, S. M.; Shuh, D. K.; Nordgren, J.; Guo, J. H. Adv. Mater. 2005, 17, 2320. (18) Fu, Y. Y.; Chen, J.; Zhang, H. Chem. Phys. Lett. 2001, 350, 491. (19) Fu, Y. Y.; Wang, R. M.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A.; Zhang, H. Chem. Phys. Lett. 2003, 379, 373. (20) Lu, J. P.; Yun, T.; Zheng, M. R.; Sow, C. H. J. Phys. Chem. C 2011, 115, 8816. (21) Maffeis, T. G. G.; Penny, M. W.; Castaing, A.; Guy, O. J.; Wilks, S. P. Surf. Sci. 2012, 606, 99. (22) Chernozatonskii, L. A.; Kosakovskaya, Z. Y.; Gulyaev, Y. V.; Sinitsyn, N. I.; Torgashov, G. V.; Zakharchenko, Y. F. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.Process., Meas., Phenom. 1996, 14, 2080. (23) Yu, T.; Zhu, Y. W.; Xu, X. J.; Yeong, K. S.; Shen, Z. X.; Chen, P.; Lim, C. T.; Thong, J. T. L.; Sow, C. H. Small 2006, 2, 80. (24) Gupta, R. P.; Sen, S. K. Phys. Rev. B 1974, 10, 71. (25) Fujii, T.; de Groot, F. M. F.; Sawatzky, G. A.; Voogt, F. C.; Hibma, T.; Okada, K. Phys. Rev. B 1999, 59, 3195. (26) Yamashita, T.; Hayes, P. Appl. Surf. Sci. 2008, 254, 2441. (27) Cai, Y. Q.; Ritter, M.; Weiss, W.; Bradshaw, A. M. Phys. Rev. B 1998, 58, 5043. (28) Roosendaal, S. J.; van Asselen, B.; Elsenaar, J. W.; Vredenberg, A. M.; Habraken, F. Surf. Sci. 1999, 442, 329. (29) McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (30) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal. 2004, 36, 1564. (31) Saliba, N.; Parker, D. H.; Koel, B. E. Surf. Sci. 1998, 410, 270. (32) Tabata, K.; Kawabe, T.; Yamaguchi, Y.; Nagasawa, Y. Catal. Surv. Asia 2003, 7, 251. (33) Kerr, J. A. CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000.

of XPS, leading to a decrease of the overall C%. Chemisorbed O−C and O−CO significantly desorbed only above 200 °C, which caused a reduction of Fe2O3 into Fe3O4 between 200 and 300 °C. The absorbed C and O species were completely desorbed at 400 °C. At this temperature, the α-Fe2O3 nanoflakes (sample 1) were reduced to FeO due to the presence of extra Fe in the foil and the tips of nanoflakes were slightly bent toward the base, while the α-Fe2O3 nanostrips (sample 2) were largely reoxidized into Fe2O3 and remained unchanged in morphology. Subjected to ambient air for two days after being taken out of the vacuum chamber, the same four chemical states related to C and O species were again detected on the surfaces of the two nanostructures. In addition, the top surface of FeO layer in sample 1 was reoxidized by the O2 species into a mixture of Fe2O3 and Fe3O4. The adsorption of C, O-related species from ambient environment can therefore be a common behavior for other nanostructures owing to their large surface to volume ratio.

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ASSOCIATED CONTENT

S Supporting Information *

XRD diffraction patterns on the area detector as well as XPS Fe2p3/2 spectra of as-grown Fe2O3 and after vacuum annealing to 400 °C for Fe2O3 prepared on a hot plate at 350 °C and in a furnace at 750 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +65-6516-2957, fax +65-6777-6126, e-mail address [email protected] (C.H.S.); tel. +65-6516-1192, fax +656777-6126, e-mail address [email protected] (E.S.T.). Notes

The authors declare no competing financial interest.

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