Fabrication and in / ex situ XPS Characterization of Rh Nanoparticles

Rh nanoparticles have been fabricated by the evaporation method using the He gas in the Rh evaporation chamber, connected with the pre-evacuation chamber of BL6N1 at Aichi Synchrotron Radiation Center (Aichi SR). The electronic and geometric properties of the Rh nanoparticles have been verified without atmosphere exposure (in situ XPS) and after atmosphere exposure (ex situ XPS) using SR-XPS and TEM. The size of Rh nanoparticles is estimated 1.8±0.5 nm in diameter and deposited on a substrate. Judging from the result of the in situ XPS analysis with photon energy of 2.0 keV and 3.5 keV, the surface of the deposited Rh nanoparticles without atmosphere exposure is the metallic state. On the other side, the outermost surface changes into Rh oxide after atmosphere exposure even in a short time. For a long time atmosphere exposure moreover, the Rh oxide increases in the depth direction, and the deep area is in the higher oxidation state. [DOI: 10.1380/ejssnt.2017.50]


INTRODUCTION
To meet the present stringent regulation for automobile emission, catalysts play an important role in purifying exhaust gases.For NO reduction, rhodium (Rh) is the essential precious metal as the active site [1][2][3].To make Rh more effective for NO reduction, Rh particle must be as small as it can be to increase the number of reaction site.As the metal particle size decreases below 10 nm, both different electronic and geometric properties can be observed in comparison with those of the bulk material [4][5][6][7].
Our first concern is the intrinsic properties of Rh nanoparticles.For this characterization of the electronic property, in situ X-ray photoelectron spectroscopy (XPS) is the suitable analytical method, excluding the influence of any adsorbed components.On the other side, catalysts for purifying the exhaust gases are used under atmosphere.Therefore, our second concern is the properties of nanoparticles in practical use condition.Ex situ XPS, that is XPS analysis after atmosphere exposure, clarifies this issue.In the process of NO reduction, NO molecules adsorb on the metallic Rh surface.Under atmosphere, Rh oxide layer is on the surface of Rh particle and the thickness of the oxide layer influences its performance [8][9][10].In the case of XPS analysis in laboratory, using Al Kα radiation (1486.6 eV), calculated inelastic mean free path of Rh is 1.9 nm [11], less than or equal to about one Rh oxide layer.In the case of XPS analysis in Synchrotron radiation (SR), increasing a photon energy enables the inelastic mean free path to increase.In this paper, XPS analysis with SR is adopted to characterize the electronic property of Rh nanoparticle in the depth direction.
The purpose of this paper is to reveal the geometric and electronic properties of Rh nanoparticles by using both transmission electron microscopy (TEM) and in/ex situ XPS with SR.

A. Fabrication of Rh nanoparticle
Rh nanoparticles were fabricated by the evaporation method in the Rh evaporation chamber.The Rh wire (4N) was evaporated and the Rh nanoparticles were grown under 50 Torr of the high purity He gas (99.9995%).The nanoparticles were transferred to the deposition chamber through the stainlesspipe and were deposited on a silicon wafer which was n-type and ⟨100⟩ orientation and a TEM grid.Si wafer has been cleaned only by ultra sonic wave with ethanol solution.

B. Analysis of Rh nanoparticle
TEM experiments were carried out using JEOL JEM-3000F, accelarating energy at 300 kV.To evaluate the size of Rh nanoparticles, another sample of the TEM grid with a short time fabrication was prepared.
Chemical states of the Rh nanoparticle were investigated by XPS at BL6N1 in Aichi Synchrotron Radiation Center.As the evaporation chamber for Rh nanoparticles was connected with the pre-evacuation chamber of the endstation, Rh nanoparticle was able to be analyzed without atmosphere exposure.A photon energy for XPS analysis was set at 2.0 keV or 3.5 keV.The spectra were calibrated by the peak position of Au 4f 7/2 (83.95 eV).The binding energies of Rh 3d 5/2 for metallic Rh, Rh 2 O 3 and RhO 2 are set at 307.2 eV, 308.2 eV and 309.5 eV, respectively [12][13][14].The deconvolution analysis of the Rh 3d 5/2 peaks was performed by CASA XPS [15] and each area of the Rh component with a different oxidation state were calculated as the existing ratio.
In/ex situ XPS conditions are 3 patterns; (a) Rh nanoparticles without atmosphere exposure (in situ XPS) (b) air exposed for 10 minutes (c) air exposed for a month.

A. TEM studies of Rh nanoparticle
TEM image of the Rh nanoparticles shows in Fig. 1.Rh nanoparticles have spherical shape and were deposited with the structural property of nanoparticles.Using Fig. 2, the size of Rh nanoparticles was evaluated and the size distribution of the Rh nanoparticles is also shown in Fig. 3.The diameter of the Rh nanoparticles is in the region about 1-3.5 nm in diameter.The estimated average size and standard deviation (S.D.) are 1.8±0.5 nm.[16,17].We will confirm it by the experiment of pure O 2 exposure with Rh nanoparticles whether this Rh compound will be formed or not near future.Subsequently, the electronic properties of Rh nanoparticles in the depth direction are verified by using the photon energy at 3.5 keV. Figure 5 shows the Rh 3d of three samples (a)-(c) measured by 3.5 keV XPS and the components analysis is summarized from Table IV to Table VI.The calculated inelastic mean free path of Rh at 2.0 keV or 3.5 keV is 2.4 nm or 3.7 nm [11], respectively.The peak top position and the shape of Rh 3d spectrum of Rh nanoparticles in the case of in situ XPS for (a) is the same as with 2.0 keV.This indicates that metallic Rh exists in depth region within the mean free path of 3.7 nm for 3.5 keV.From the result of the deconvolution analysis in the case of ex situ XPS for (b) with 2 keV, metallic Rh and Rh 2 O 3 exist as 83% and 17%, respectively (Table II).Table V shows the metallic Rh component increases to 87%.Because the higher photon energy enables to detect the deeper region of Rh nanoparticles, the metallic component in the deeper region add to the spectrum.This indicates that the outermost surface of Rh nanoparticles is Rh 2 O 3 .On the other side, the peak top positions in the case of ex situ XPS for (c) with 3.5 keV shift to the higher binding energy than the Rh 3d spectrum with 2.0 keV.In the comparison between 2.0 keV and 3.5 keV spectra for (c) (see in Table III and Table VI) the oxide components of Rh 3d spectra at the higher binding energy than that of metallic states increase 75% to 81%.And Rh spectrum with the higher binding energy (310.2eV) than that of RhO 2 assigned in Table VI can be found.In general, the positive core level shift attributes to the particle size effect [6,7,13,[18][19][20][21] or the metal-support interaction [22,23] and so on.In this case, the particle size effect is excluded because this behavior cannot be seen at analysis with 2.0 keV.Moreover this result suggests that the Rh oxide increases in the depth direction, and the deep area is in the higher oxidation state.One possibility of this result would be the metal-support effect, as Rh oxidation state with the higher binding energy can exist at Rh on Al 2 O 3 after aging with high temperature in air [24,25].If a substrate has the oxide layer on its surface, Rh might has the possibility to become the higher oxidation state.We will have the further investigation and characterize this Rh state.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)

IV. CONCLUSIONS
Rh nanoparticles have been fabricated by the evaporation method.These size are estimated 1.8±0.5 nm in diameter.Judging from the result of the in situ XPS analysis with SR, the surface of the Rh nanoparticles without atmosphere exposure is the Rh metallic state.On the other side, by the ex situ XPS analysis, the outermost surface changes into Rh 2 O 3 and RhO 2 .Changing a photon energy for XPS analysis from 2.0 keV to 3.5 keV enabled to analyze the chemical state in the depth direction.

FIG. 2 :
FIG. 2: TEM image of Rh nanoparticles for the evaluation of the size of Rh nanoparticles.

FIG. 4 :
FIG. 4: Result of the deconvolution analysis of Rh 3d spectra at 2.0 keV.(a) in situ XPS, (b) ex situ XPS exposure for 10 minutes, and (c) ex situ XPS exposure for a month.

FIG. 5 :
FIG. 5: Result of the deconvolution analysis of Rh 3d spectra at 3.5 keV.(a) in situ XPS, (b) ex situ XPS exposure for 10 minutes, and (c) ex situ XPS exposure for a month.

TABLE I :
Result of the deconvolution analysis of Rh 3d 5/2 of (a) in situ XPS analysis at 2.0 keV.

TABLE II :
Result of the deconvolution analysis of Rh 3d 5/2 of (b) ex situ XPS analysis exposure for 10 minutes at 2.0 keV. the surface to Rh oxides.Figure4shows the Rh 3d 5/2 and Rh 3d 3/2 spectra of three samples (a)-(c) measured by 2.0 keV XPS and the components analysis is summarized from TableIto TableIII.The peak top of Rh nanoparticles in the case of in situ XPS of Rh 3d 5/2 for (a) is at 307.4 eV.Judging from the result of the deconvolution analysis, the surface is Rh in a metallic state.However, in the case of ex situ XPS for (b), the peak top is at 307.5 eV with a shoulder at 308.5 eV including 17% of Rh 2 O 3. These results indicate that the part of the Rh surface changes to Rh oxides by atmosphere exposure even for a short time.Moreover, in the case of ex situ XPS for (c), the peak top is at 309.5 eV.The result of the deconvolution analysis indicates that 37% of Rh 2 O 3 , 38% of RhO 2 whose 3d 5/2 peak position is 309.2 eV and 13% of Rh metal exist.This suggests that long time atmosphere exposure proceeds some Rh oxidations.Moreover, the peak with lower binding energy than that of Rh metal is also observed.One possibility of Rh state with lower binding energy might be the formation of hydrides compounds, reacting with H 2 O in the air

TABLE III :
Result of the deconvolution analysis of 3d 5/2 of (c) ex situ XPS analysis exposure for a month at 2.0 keV.

TABLE IV :
Result of the deconvolution analysis of Rh 3d 5/2 of (a) in situ XPS analysis at 3.5 keV.

TABLE V :
Result of the deconvolution analysis of Rh 3d 5/2 of (b) ex situ XPS analysis exposure for 10 minutes at 3.5 keV.

TABLE VI :
Result of the deconvolution analysis of Rh 3d 5/2 of (c) ex situ XPS analysis exposure for a month at 3.5 keV.