(2) D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. 83. Table 1 Preparation conditions leading to high formation of porous material. No.. Carrier gas. Partial pressure [Pa]. Source temperature [◦ C]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23. He He H2 He H2 He H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2. 100 100 100 100 100 100 100 100 100 100 100 200 200 200 200 400 600 600 600 1200 1200 1200 1200. 668 722 781 786 839 840 841 908 970 998 1196 743 866 1161 1222 775 907 953 1023 664 702 778 895. a. ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±. 7 5 3 6 7 10 5 4 6 3 4 9 3 4 3 3 4 4 3 9 6 3 4. Nominal MoO3 vapor pressure [Pa]a. Condensate obtained. 15 80 390 440 1500 1400 1650 6990 23,150 38,300 775,500 140 2800 485,000 1,085,000 330 6850 16,800 58,800 13 44 360 5300. Porous material Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and non-porous material Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material Porous material Porous material Porous material Porous material Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material Porous material and non-porous material Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures Porous material and small amount of crystalline structures. Estimated from Clausius–Clapeyron equation. At high temperatures the extrapolation is not reliable.. range. We report on the oxidation states, morphology, structure, thermal stability and hydration grade as well as the reducing effect of the hydrogen pressure on the porous samples. Finally, we discuss the growth kinetics of the porous material and the inﬂuence of the gas on the chemical and structural characteristics of the samples. 2. Experimental procedure. sion electron microscope (FEI TECNAI F20 G2 TEM) operating at 200 kV and under diffraction and phase contrast modes. This TEM is also equipped with energy dispersive X-ray spectroscopy for elemental chemical analysis and selected area electron diffraction (SAED) for crystallographic information. The thermal stability and water content of the samples was measured by thermogravimetric analysis (TGA, Pyris 1TGA, Perkin Elmer). These experiments were carried out by heating 1 mg of the sample, under argon, with a heating rate of 10 ◦ C min−1 from room temperature up to 800 ◦ C.. 2.1. Preparation of porous MoO3−x. 3. Results The synthesis method consisted in directly evaporating a MoO3 pellet of 0.5 g from a tungsten resistive boat under He (AGA, 99.995%, O2 < 5 ppm and H2 O < 2 ppm) or H2 (AGA, 99.995%, O2 < 5 ppm and H2 O < 4 ppm) at pressures from 100 to 1200 Pa. The procedure and the experimental setup for MoO3 evaporation have been described elsewhere [16,17]. A cold-pressed pellet of 0.5 g MoO3 was placed on the tungsten boat inside a high vaccum chamber. The chamber was evacuated until it reached a pressure of ∼10−4 Pa. A mass spectrometer was used to monitor the partial pressure of the active gases, mainly oxygen, nitrogen and water vapor. Then, the carrier gas (He or H2 ) was injected to reach a constant operating pressure (range researched: 100–1200 Pa). The pressure was measured with absolute (capacitive) pressure gauges. The tungsten boat was resistively heated up to a constant working temperature in the 650–1250 ◦ C range. This was measured using an optical pyrometer through a sapphire window. The material was collected on the interior surface of a copper semi cylinder located ∼75 mm above the tungsten boat. The collector was cooled by circulating liquid nitrogen trough it. Finally, the high vacuum chamber was left overnight to allow it to reach room temperature and was ventilated with air before opening it to remove the material deposited on the collector surface.. 3.1. Preparation conditions Each preparation condition is determined by the carrier gas, its partial pressure and the maximum temperature of the evaporation source (tungsten boat), represented by the nomenclature “condition (gas, pressure, temperature)” which will be used throughout this paper. Fig. 1 summarizes the evaporations carried out under He. 2.2. Characterization The surface chemical information of the samples was obtained from X-ray photoelectron spectroscopy (XPS, Physical Electronics system model 1257), using either Al or Mg K-␣ emission, with working pressures in the range of 10−6 to 10−7 Pa. Binding energy and oxidation states were obtained from high resolution scans (pass energy 71.55 eV and step size 0.2 eV). All spectra were analyzed using the Multipak and XPSpeak41 softwares. The energy scale was calibrated by assigning 284.8 eV to the C 1s peak corresponding to adventitious carbon. After calibration, the background from each spectrum was subtracted using a Shirley-type background. Structural information of the porous samples was obtained by X-ray diffraction using a Siemens D5000 powder diffractometer. The diffraction patterns were collected in 2–80◦ range with a scanning velocity of 0.0025◦ s−1 using a graphite monochromator and Cu K-␣ radiation (40 kV and 30 mA). The structural information was complemented by Raman spectroscopy using a confocal Raman microscope (WITEC model CRC200) equipped with an argon laser ( = 514.5 nm) and a Ne–He laser ( = 632.8 nm). The sample morphology was examined using a ﬁeld emission scanning electron microscope (FEG-SEM JSM 6330F) and a ﬁeld emission transmis-. Fig. 1. Evaporation conditions using helium (black circle) or hydrogen (star) in the pressure range 100–1200 Pa. The conditions framed with a rectangle promote the growth of porous material..
(3) 84. D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. as well as under H2 at different pressures and evaporation source temperatures. Several evaporations at pressures of 100, 600 and 1200 Pa were carried out under similar conditions of source temperature and pressure, changing only the carrier gas, in order to study the effect of the gas on the chemical and structural characteristics of the samples. The dashed line in Fig. 1 displays the maximum achievable temperature of the evaporation source when the experiments were performed under helium at different partial pressures, because if this temperature was exceeded the MoO3 pellet was expelled from the evaporation source. This behavior was not observed when hydrogen was used. In general, the amount of material obtained from each evaporation was below 0.5 g with variable yields that did not exceed 70%. The evaporations framed with a rectangle in Fig. 1 indicate the preparation conditions leading to porous material, sometimes mixed with small amounts of crystalline structures as nanoparticles, nanorods, nanoribbons and nanoplates. The formation of porous structures was promoted at low pressures, not higher than 200 Pa, pressure range that concentrated 65% of the cases with a large proportion of porous material. Other preparation conditions (not framed) lead mainly to the growth of crystalline nanostructures, which structural and morphological properties have been reported elsewhere . The preparation conditions which lead to the formation of 100% of porous material or to a mixture of porous material with small amounts of crystalline nanostructures are listed in Table 1. The formation of porous material was favored under hydrogen in samples grown at 100, 200, 600 and 1200 Pa and in a particular experiment performed at 400 Pa and 775 ◦ C (condition 16). On the contrary, these porous structures predominated under helium only at a pressure of 100 Pa. Porous samples grown under hydrogen at 100 Pa grew only at high evaporation rates, i.e., at high MoO3 vapor pressures, which require source temperatures exceeding 780 ◦ C. At low evaporation rates where the source temperature did not exceed to 720 ◦ C, nonporous samples were grown. Preparation conditions (H2 , 10 Pa, 667 ◦ C) and (H2 , 50 Pa, 715 ◦ C), carried out at the lowest operating pressures (Fig. 1), also resulted in the formation of non-porous material. At 600 Pa of hydrogen, large amounts of porous structures grew only when the source temperature exceeded 900 ◦ C, i.e., at high MoO3 vapor pressures. At higher pressures of the carrier gas (1200 Pa of hydrogen) porous material grew preferentially at low and moderate MoO3 evaporation rates, when the source temperature range was between 664 and 895 ◦ C, but not at higher source temperatures, which in turn lead to formation of crystalline structures. Only condition 1, performed under helium, and conditions 11–15 (performed at 200 Pa of hydrogen) and 19 resulted in the formation of 100% porous material, whereas other preparation conditions generated small amounts of crystalline structures mixed with high amounts of porous material.. 3.2. Characterization of the porous material 3.2.1. Chemical characterization 220.127.116.11. Purity. The analysis by EDS and XPS for all samples revealed only molybdenum and oxygen. This is similar to the ﬁndings in crystalline MoO3 nanostructures grown by the same technique . Since the spectra of porous materials do not differ from the latter, they will not be reproduced here. EDS did not reveal any elements other than molybdenum and oxygen, plus copper from the sample grid. XPS showed molybdenum, oxygen and adventitious carbon, with no tungsten contamination from the evaporation source.. Fig. 2. (a) Mo 3d spectrum and ﬁtting from a porous sample grown under condition (He, 100 Pa, 668 ◦ C). (b) O 1s peak and its ﬁtting.. 3.3. High resolution XPS spectra 3.3.1. Samples grown under helium Fig. 2 depicts high resolution XPS spectra from a porous sample grown under condition (He, 100 Pa, 668 ◦ C). The ﬁgure shows the Mo 3d and the O 1s photoelectron peaks with their corresponding curve ﬁt, Fig. 2(a) and (b), respectively. The best ﬁt assigned 232.7 eV to the Mo 3d5/2 peak and 235.8 eV to the Mo 3d3/2 peak, with a full width at half maximum (FWHM) of 2.0 eV and 1.9 eV, respectively. The binding energies of Mo 3d5/2 and Mo 3d3/2 are very close to the values reported for Mo6+ : 232.6 eV and 235.8 eV, respectively . Samples grown under helium maintain the chemical composition of the starting material (MoO3 pellet). Fig. 2(b) depicts the O 1s peak with its best ﬁt. The main peak in the spectrum, at 530.7 eV (FWHM ∼1.7 eV), can be attributed to the oxide, while the second peak at 532.2 eV (FWHM ∼1.9 eV) corresponds to OH groups. The ratio between OH and oxide, estimated from the peak area A, is A(532.2 eV):A(530.7 eV) = 0.15:1. 3.3.2. Samples grown under hydrogen Fig. 3 displays XPS spectra of a sample grown under condition (H2 , 100 Pa, 1196 ◦ C), the same pressure as the sample grown under helium (see Fig. 2). Fig. 3(a) shows the Mo 3d doublet which was ﬁtted with two curves assigning 233.2 eV to the Mo 3d5/2 and 236.3 eV to the Mo 3d3/2 peaks, with a FWHM of 2.5 eV and 2.7 eV, respectively. These binding energies of Mo 3d5/2 and Mo 3d3/2 are higher than the values reported for Mo6+ ions . However, these values.
(4) D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. 85. Fig. 4. SEM image showing the high porosity of a MoO3 sample obtained under condition (H2 , 100 Pa, 781 ◦ C).. presented. Examination by Raman spectroscopy, however, shows important differences and will be presented in a comparative way.. Fig. 3. (a) Mo 3d spectrum and ﬁtting of a porous sample grown under condition (H2 , 100 Pa, 1196 ◦ C). (b) O 1s peak and its best ﬁt.. are the same reported for Mo6+ in crystalline samples grown under hydrogen at 600 Pa , where the binding energies were 233.2 eV and 236.2 eV for the Mo 3d5/2 and Mo 3d3/2 peaks, respectively. No reducing effect was detected in samples grown under hydrogen at 100 Pa. Fig. 3(b) depicts the ﬁtting of the O 1s peak, which looks asymmetrical and broader than the observed for the He-grown sample. The ﬁt assigns two maxima located at 530.9 eV and 532.4 eV with a FWHM of 2.4 eV and 2.7 eV, respectively, the ﬁrst one attributed to the oxide and the second to OH groups. Their intensity ratio, estimated from the areas, is A(532.4 eV):A(530.9 eV) = 0.45:1, that is, the amount of OH groups is three times larger in the sample grown under hydrogen as compared with a sample grown under helium at the same pressure. Surface hydroxyl (OH) groups are mainly caused by the exposition of the samples to air when they are removed from the preparation chamber and storage at atmospheric pressure and room temperature and is difﬁcult to correlate with the preparation conditions. 3.4. Morphological and structural characterization Samples grown either in helium or in hydrogen look similar under SEM, TEM and XRD, and no comparative results will be. 3.4.1. Morphology Fig. 4 is the SEM image of an as-prepared MoO3 sample grown under condition (H2 , 100 Pa, 781 ◦ C), which lead principally to the formation of porous material, as shown in Table 1. The image reveals high porosity agglomerate, with pores around 25 nm and voids as large as 200 nm. The conformation and sizes of the pores are shown in the TEM images of Fig. 5. These images correspond to a MoO3 sample obtained under condition (He, 100 Pa, 668 ◦ C). In this case, the as-prepared sample was dispersed in isopropyl alcohol using an ultrasonic bath by 7 min and then trapped on a microscope copper grid. Fig. 5(a) is a bright ﬁeld image showing slightly agglomerated porous material (indicated by the arrow) located on the discontinuous carbon mesh of the TEM grid. The inset in Fig. 5(a) is a SAED pattern showing a diffuse diffraction, revealing the low crystallinity of the porous structure. Fig. 5(b) is a high magniﬁcation image of the zone pointed out by the arrow in Fig. 5(a). This image reveals with better deﬁnition the porous character of the sample, constituted by overlapped MoO3 ﬂakes. The darker zones in the image are due to larger amounts of overlapped material. In general, the pore size estimated by means of TEM images of the porous samples was mesoporous, i.e., with a pore size distribution between 10 nm and 70 nm, with slightly larger pores if the material is grown under hydrogen (10–85 nm). 3.4.2. Structure The structural characterization performed by XRD, HRTEM and Raman spectroscopy revealed that the porous samples grown either in helium or hydrogen were constituted by an amorphous matrix with nanocrystalline inclusions. Fig. 6 is the XRD pattern of a MoO3 porous sample grown under condition (He, 100 Pa, 668 ◦ C). The diffractogram shows two diffuse broad peaks attributed to the (0 2 1) and (1 1 2) reﬂections of the base-centered orthorhombic phase of MoO3 , according to the PDF Card No. 35-0609. This diffractogram indicates that this sample present some very small crystalline inclusions embedded in a porous matrix. The presence of these crystalline inclusions was also detected by HRTEM examination and Raman spectroscopy, as revealed in Figs. 7 and 8, respectively. Fig. 7 shows HRTEM images of a porous sample grown under condition (H2 , 200 Pa, 743 ◦ C), leading to 100% of porous material,.
(5) 86. D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. Fig. 5. TEM images of a MoO3 sample grown under condition (He, 100 Pa, 668 ◦ C). (a) Bright ﬁeld showing porous material on the TEM mesh. The inset is the corresponding SAED pattern. (b) High magniﬁcation of the zone indicated by the arrow in (a).. Fig. 6. XRD pattern of a MoO3 porous sample grown under condition (H2 , 100 Pa, 1196 ◦ C).. as shown in Table 1. These images were recorded at the start of the TEM measurements, with scarce electron irradiation and therefore no electron beam effects. Crystallization phenomena associated with oxygen loss has been already reported for this kind of material . To avoid these effects, all micrographs shown here were recorded with low irradiation dose. The image pair of Fig. 7 reveals nanocrystalline inclusions in the amorphous porous matrix. Fig. 7(a) was recorded from the free edge of a portion of the material. Arrows 1 and 2 in Fig. 7 indicate zones that exhibit lattice fringes with better deﬁnition. The porous material consists of overlapped MoO3 ﬂakes, as shown in Fig. 7(b), with different sheets of material in different focal planes. Fig. 7(b) corresponds to material away from the edge. Arrow 3 shows well-deﬁned Moiré fringes corroborating the existence of nanocrystals in the overlapping MoO3 sheets. The Moiré patterns can be formed by interfering two sets of lines which have nearly common periodicities. There are two fundamentally different types of interference: the translational and the rotational Moiré . In this case, the Moiré pattern is generated by the parallel lines of the lattice fringes of superposed nanocrystals. Probably, these Moiré fringes have a translational and rotational component since these crystalline inclusions remain in the MoO3 sheets with random crys-. Fig. 7. HRTEM images of a MoO3 porous sample grown in hydrogen under condition (H2 , 200 Pa, 743 ◦ C). (a) Arrows 1 and 2 indicate lattice fringes. (b) Moiré fringes (arrow 1) and zones exhibiting lattice fringes (arrows 2 and 3)..
(6) D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. Fig. 8. Raman spectra of porous samples grown at 100 Pa. (a) under helium and (b) under hydrogen.. tallographic orientations. An example is indicated by arrows 4 and 5 of Fig. 7(b) showing parallel lines of the crystal lattice with different orientations. Crystallites in the porous material were also evidenced by Raman spectroscopy. This technique revealed differences in structural features between the samples grown under He and H2 . Fig. 8 shows the room temperature-Raman spectra of the samples whose high resolution XPS spectra were previously discussed. Curve (a) corresponds to a sample obtained under (He, 100 Pa, 668 ◦ C), whereas curve (b) corresponds to one obtained under (H2 , 100 Pa, 1196 ◦ C). In both preparation conditions, 100% of the material found in the sample was porous, as shown in Table 1. Raman spectra revealed relatively sharp peaks (curves (a) and (b) of Fig. 8), different to the broad bands expected for a completely amorphous sample. This indicates some degree of translational periodicity due to the presence of medium-range order in the porous samples. This is consistent with the nanocrystalline inclusions observed in the HRTEM micrographs (Fig. 7). The Mo–O stretching and bending vibrations in MoO3 usually appear in the 1000–600 cm−1 and 600–400 cm−1 regions, respectively . The Raman spectrum of the He-grown sample, curve (a), exhibits two weak peaks labeled as 1 and 2 and located at 847 cm−1 and 775 cm−1 , respectively, which are absent in the spectrum of the H2 -grown sample. These peaks correspond to vibrational modes of a monoclinic phase of molybdenum trioxide, called ␤-MoO3 . This metastable phase of MoO3 has been obtained in microcrystals produced heating a Mo wire in mixture of gas of Ar and O2 , and by soft chemistry methods . On the contrary, the Raman spectrum of the H2 -grown sample, curve (b), only shows the spectral features of the orthorhombic phase of the molybdenum trioxide, ␣-MoO3 , which is the thermodynamically stable phase of the compound . In the high-wavenumber region, between 1000 and 600 cm−1 , the Raman spectrum displays three well-deﬁned peaks located at 992, 818 and 662 cm−1 , attributed to the stretching vibrations of Mo O, Mo–O(2) and Mo–O(3) chemical bonds, respectively . The Raman peak at 992 cm−1 is the typical vibration of the terminal bonds (Mo O) having a stable double bond and, due to its non-polar character, the Raman band appears as the most narrow band. Moreover, this mode is very sensitive to the octahedral distortion of MoO6 , hence it is a good indicator for the two-dimensional character of the MoO3 bilayer structure . Finally, the lowwavenumber region of curve (b), in the 600–400 cm−1 range, shows a weak peak at 467 cm−1 , labeled as 3, corresponding to a bending vibration of the Mo–O(3) bonds . This peak is extremely weak in. 87. Fig. 9. Thermogravimetric curves of MoO3 samples with high porous phase content. Both curves correspond to samples grown under similar preparation conditions in helium and hydrogen.. curve (a) corresponding to the vibrational modes of ␤-MoO3 (not visible in the graph with the superposed curves). 3.4.3. Thermal stability and hydration grade The thermal stability and water content of the MoO3 porous samples grown under helium and hydrogen were studied from thermogravimetric (TG) curves. Fig. 9 shows thermogravimetric curves of two samples grown under the same preparation conditions, varying only the carrier gas. The solid line corresponds to the sample grown under condition (He, 100 Pa, 786 ◦ C) and the dotted one to the sample grown under condition (H2 , 100 Pa, 781 ◦ C). Both preparation conditions lead mainly to the formation of porous material coexisting with scarce crystalline structures. The analyses were carried out by heating a 1 mg sample in argon at 10 ◦ C min−1 from room temperature up to 800 ◦ C. The solid line shows that at 155 ◦ C the mass loss reached 15%, loss attributed to the desorption of surface water from the porous sample; Camacho-López et al. and Murugan et al. reported that water desorption from hydrated crystalline MoO3 occured between 80 and 120 ◦ C [25,26]. The water is attributed to surface adsorption during storage of the samples to air. After 155 ◦ C the sample mass remained constant up to 635 ◦ C where the decomposition of the compound started and continued at increasing temperatures. At 700 ◦ C the mass loss reached 26% and thereafter the curve fell with an estimated mass loss rate of ∼1.0% ◦ C−1 , value that remained constant until the sample lost 73.6% of its mass at 750 ◦ C. The dotted line displayed a similar behavior, showing a mass loss less pronounced than the solid line. At 130 ◦ C the mass loss reached only 9%, remaining constant up to 220 ◦ C. This initial mass loss was also attributed to desorption of H2 O molecules from the porous material. The sample mass underwent a slight decrease of ∼1.7% between 220 ◦ C and 257 ◦ C. This mass loss can be attributed to the loss of lattice H2 O molecules from the compound since crystallization water is released above 200 ◦ C . The mass remained constant above 257 ◦ C and up to 618 ◦ C, where decomposition of the compound started. At 700 ◦ C, when the mass loss was of 26%, the thermogravimetric curve decayed at the same rate depicted by the solid line. The amount of water in the H2 -grown sample, estimated from the dotted line, leads to a slightly hydrated compound of the form MoO3 ·0.14H2 O. On the other hand, the solid line corresponding to the He-grown sample under similar synthesis conditions, does not account for lattice H2 O molecules..
(7) 88. D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. In all samples the main mass loss occurred between 635 ◦ C and 800 ◦ C, and between 618 ◦ C and 800 ◦ C for the He-grown sample and the H2 -grown sample, respectively. These wide thermal ranges of decomposition reveal the presence of impurities in the compound. However, no impurities were detected by EDS and XPS. We do not have a deﬁnitive explanation for this mass loss, and temptativeley we attribute this wide thermal range of decomposition to the OH groups on the surface of the samples. 4. Effect of the hydrogen pressure on the oxidation state of the porous samples The effect of the hydrogen pressure on the oxidation state is reported here only for samples with 100% of porous material. Fig. 10 shows the XPS Mo 3d peak and ﬁtting for porous samples grown in hydrogen under (a) 200 Pa, (b) 400 Pa and (c) 600 Pa at different maximum source temperatures. Fig. 10(a) corresponds to a sample grown at a maximum source temperature of 1222 ◦ C. The best ﬁt assigns 233.3 eV to the Mo 3d5/2 and 236.4 eV to the Mo 3d3/2 peaks, with a FWHM of 2.1 and 2.2, respectively. The binding energies are close to the binding energies of Mo6+ peaks of the porous sample previously analyzed in Fig. 3(a). Fig. 10(b) shows the spectrum of a sample obtained at 400 Pa and maximum source temperature of 775 ◦ C. The ﬁt assigns 232.6 eV to the Mo 3d5/2 and 235.7 eV to the Mo 3d3/2 peaks, both with a FWHM of 2.4 eV. The observed binding energies of Mo 3d5/2 and Mo 3d3/2 peaks are slightly lower than in the sample grown at 200 Pa, but still in the range of Mo(VI). The sample grown at 600 Pa with a maximum source temperature of 1023 ◦ C was partially reduced as revealed by the Mo 3d binding energy, consistent with Mo(V), as shown in Fig. 10(c). The best ﬁt was obtained by resolving the spectrum into four overlapping peaks labeled as a, b, c, and d. The binding energies of the higher intensity Mo 3d doublet were 232.5 eV (FWHM ∼ 2.1 eV) and 235.7 eV (FWHM ∼ 2.2 eV). These values are close to the reported values for Mo(VI) . The doublet of lower intensity was ﬁtted with binding energy of 230.9 eV (FWHM ∼ 1.9 eV) and 233.8 eV (FWHM ∼ 2.3 eV) for Mo 3d5/2 and Mo 3d3/2 , respectively. These binding energy values are close to the reported values for Mo5+ [18,27]. The Mo5+ to Mo6+ ratio, estimated by the area below the ﬁtted curves, amounts to Mo5+ :Mo6+ ∼ 2:7. Hence, the results conﬁrmed the existence of suboxide of the form MoO3−x due to the presence of Mo5+ in the porous samples grown under a hydrogen pressure of 600 Pa whereas the porous samples grown under a lower pressure exhibit the same chemical composition of the starting material (MoO3 ). 5. Discussion 5.1. Growth kinetics of porous samples Fig. 1 and Table 1 show that growth of porous material is favored under low pressures of the carrier gas (100 and 200 Pa), since the gas density is not high enough to cool the MoO3 vapor. The longest mean free path under these pressures, estimated to be 0.2 mm in average (Fig. 11), combined with the high convection rate at lower pressures, as compared with the conditions at 600 and 1200 Pa, leads to faster transport away from the evaporation source causing a low supersaturation in the vicinity of the MoO3 pellet or, simply, avoiding reaching supersaturation conditions. The low coalescence rate and the fast transport of the material towards the collecting surface cooled at −196 ◦ C rapidly immobilized the clusters, leading to the formation of porous samples of amorphous matrix. There are important differences in the growth kinetics under helium and hydrogen due to the conﬁnement effect. Convection and cooling rates (the last one is the energy removal rate from the. Fig. 10. Mo 3d peaks and ﬁt for 100% porous samples grown in hydrogen under (a) 200 Pa, (b) 400 Pa and (c) 600 Pa.. atoms due to collisions with the gas) are different. High amounts of porous material grew under hydrogen at 100, 200, 600 and 1200 Pa but only at 100 Pa under helium. The better conﬁnement of the MoO3 vapor by helium at higher pressures (≥200 Pa) leads to higher supersaturation of the vapor and, therefore, to a smaller critical diameter of the embryonic particles. The low diffusion of the clusters away from the evaporation source and the high coalescence rate promotes crystalline growth, inhibiting the conditions for amorphous porous growth . On the contrary, the growth of a high amount of porous phase was possible at higher pressures.
(8) D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. 89. Another fact leading to complex growth kinetics under hydrogen is the strong reduction of the MoO3 pellet surface even before evaporation occurs. Creation of defects on the (0 1 0) planes of a MoO3 single crystal by reaction with the H2 has already been reported . At temperatures above 280 ◦ C and pressures around 0.5 Pa the MoO3 surface is reduced forming crystallographic shear planes and three-dimensional MoO2 islands. MoO2 and MoO3 have different vapor pressures; therefore, the net molybdenum oxide evaporation rate will depend not only on the source temperature, but also on the H2 partial pressure and the elapsed time. 5.2. Effect of the hydroxyl groups We believe that the binding energy shift of 0.5 eV between the Mo 3d5/2 peak of the He-grown sample and the Mo 3d5/2 peak of the H2 -grown sample is attributed to the different amount OH groups that are formed at the surface of each porous sample. The atomic hydrogen, H+ , is strongly attached to the terminal oxygen, O2− of the distorted octahedral MoO6 that constitute the MoO3 bilayer structure . Fig. 3(a) showed that the Mo 3d5/2 binding energy of the H2 -grown sample (233.2 eV) is higher than the found in the He-grown sample (232.7 eV). This fact is due to higher formation of OH groups on the surface of the porous material grown under hydrogen. The area ratio between the curves, attributed to Mo–O and O–H bonds, is larger in the sample obtained under hydrogen, 0.45, as compared with 0.15 of the sample obtained under helium. This larger ratio in the H2 grown sample is due to an increase of intensity of the signal attributed to O–H bonds compared with the signal associated to Mo–O bonds. 5.3. Effect of the carrier gas on the structure. Fig. 11. Collision mean free path estimated from thermodynamics considerations for some conditions of Table 1 that lead to formation of high proportions of porous material: (a) mean free path as function of the partial pressure of H2 and (b) shows the mean free path estimated at 100 Pa under helium and hydrogen as function of the temperature.. under hydrogen. The collision mean free path at 600 Pa is approximately six times smaller than at 100 Pa (Fig. 11(a)), therefore, a high cooling rate and an optimal supersaturation condition would be reached, and so crystalline growth would be expected. However, formation of high amounts of porous material was observed under a hydrogen pressure of 600 Pa at high evaporation rates, when the source temperature exceeded 900 ◦ C. We believe that under these conditions the determining factor is not the collision frequency but the convection rate. The high source temperatures lead to a higher convection rate rapidly removing the clusters from the supersaturation zone. This convection reduces the collisions among the material atoms and facilitates fast transport towards the cooled surface favoring the amorphous porous formation. We would not expect porous formation in the evaporations carried out at 1200 Pa of hydrogen pressure due to the strong conﬁnement of the MoO3 vapor by the gas and the lower convection rate set up at this high pressure. However, proportionally high amounts of porous material were found in samples grown in some preparation conditions under this pressure, as shown in Table 1. At 1200 Pa the growth kinetics in vapor phase is more complex due to the strong reducing effect of hydrogen. Porous formation cannot be explained solely in terms of supersaturation, collision mean free paths and evaporation, convection and cooling rates .. The HRTEM images of the porous samples grown under helium or hydrogen exhibited that crystalline inclusions at the nano-scale were formed. The Raman spectra of the He-grown porous samples displayed some vibrational modes of ␤-MoO3 whereas the H2 -grown porous samples only exhibited the spectral features of the more stable phase ␣-MoO3 . Probably, the higher formation of OH groups and lattice H2 O molecules in the porous samples grown under H2 promote an aba (ortho) stacking sequence of MoO1/1 O2/2 O3/3 layers , which changes sometimes to aaa (mono) in the porous samples obtained under inert gas, which do not present crystallization water as shown in TG proﬁle (Fig. 9). It is known that hydrogen insertion occurs with minimal rearrangement of the MoO3 crystal . The hydrogen intercalation into MoO3 amorphous sheets would lead to reducing the electrostatic repulsions between layers of negatively charged oxide ions due to their oxygen deﬁciency by condensation in the hydrogen atmosphere and eventually to the creation of lattice H2 O molecules. 5.4. Effect of the hydrogen pressure on the oxidation state A reducing effect was evidenced in the porous samples when the hydrogen pressure reached 600 Pa in evaporations preformed at high source temperatures (T > 900 ◦ C). The XPS spectrum of Fig. 10(c) refers to a sample prepared under (H2 , 600 Pa, 1223 ◦ C) revealed Mo5+ mixed with Mo6+ . 600 Pa is the threshold pressure which allows a better conﬁnement the MoO3 vapor and decreases the diffusion rate of the MoO3 clusters away from the evaporation source. This lower diffusion rate increases the reaction rate between hydrogen and MoO3 clusters consisting mainly of Mo3 O9 , Mo4 O12 , Mo5 O15 . In the vapor phase process, hydrogen reduces and condenses principally the Mo3 O9 clusters that are the most stable clusters leading to the formation of porous material of the form MoO3−x . The chemical reactions of this reduction process.
(9) 90. D.E. Diaz-Droguett, V.M. Fuenzalida / Materials Chemistry and Physics 126 (2011) 82–90. were shown by Zak et al. who used MoO3−x clusters as precursors for the growth of MoS2 fullerene-like nanoparticles . 6. Conclusions Growth of porous material was promoted in evaporations performed at 100 Pa under helium or hydrogen using a wide range of source temperatures, between 650 and 1200 ◦ C. Only under hydrogen at higher pressures and under certain conditions did samples rich in the porous phase appear. The more effective conﬁnement of the MoO3 vapor by helium at higher pressures (≥200 Pa) lead to higher supersaturation, low diffusion of the clusters and high coalescence rates inhibiting the conditions for amorphous porous growth. The high formation of porous material under 600 Pa of hydrogen at high source temperatures can be explained under these conditions not by assuming that the determinating factor is the collision frequency but the convection rate, which is different from the one established under helium at the same conditions due to their differences in molar mass and thermal conductivity. Another factor that introduces important differences in the growth kinetics of the porous phase under these gases at higher pressures is the reducing effect of hydrogen on clusters of MoO3 as well as on the surface of the source material. In fact, XPS spectra detected Mo5+ in samples grown under a hydrogen pressure of 600 Pa whereas the samples grown under lower pressures exhibit the same chemical composition of the starting material (MoO3 ). Porous material grown under helium or hydrogen did not exhibit differences when examined under SEM, TEM and XRD, but exhibited differences when analyzed by thermogravimetry and Raman spectroscopy. The H2 -grown porous samples contained lattice H2 O, whereas the He-grown samples did not have it. Both kinds of samples presented a high adsorption of H2 O. The He-grown samples at 100 Pa revealed intermixed orthorhombic/monoclinic phases of MoO3 , whereas ␣-MoO3 was only found in the H2 -grown samples made at the same pressure. Probably, the major formation of OH groups and lattice H2 O molecules in these samples promote an aba (ortho) stacking sequence of MoO1/1 O2/2 O3/3 layers. Acknowledgments This work has been partially funded by the Chilean Government under contract FONDECYT 1070789 and MECESUP contract UCH0205. We acknowledge the enlightening discussions with Prof. B. Chornik and Dr. M.E. Pilleux. D. E. D-D acknowledges a CONICYT fellowship. 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