Limits...
Surface reactivity enhancement on a Pd/Bi2Te3 heterostructure through robust topological surface states.

He QL, Lai YH, Lu Y, Law KT, Sou IK - Sci Rep (2013)

Bottom Line: We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure.The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation.A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

View Article: PubMed Central - PubMed

Affiliation: William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, SAR China.

ABSTRACT
We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure. The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation. The surface reactivity of the adsorbed tellurium on this heterostructure is also intensified possibly benefitted from the effective transfer of the bath electrons. A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

Show MeSH
XPS core-level and Auger spectra of S#2.(a) O KLL Auger peaks. Spectra (2a-1) and (2a-2) are O KLL spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2a-3) and (2a-4) are the corresponding spectra obtained from S#2B, respectively. The inset displays the structure of S#2. (b) Pd 3p peaks. Spectra (2b-1) and (2b-2) are Pd 3p spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. The inset shows the fitting results of Pd 3p3/2 and O 1s core-levels in Spectrum (2b-4). (c) Pd 3d peaks. Spectra (2c-1) and (2c-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. All the core-level BE shifts obtained from the regions W/BT and W/O BT are illustrated by dash lines.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3750537&req=5

f2: XPS core-level and Auger spectra of S#2.(a) O KLL Auger peaks. Spectra (2a-1) and (2a-2) are O KLL spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2a-3) and (2a-4) are the corresponding spectra obtained from S#2B, respectively. The inset displays the structure of S#2. (b) Pd 3p peaks. Spectra (2b-1) and (2b-2) are Pd 3p spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. The inset shows the fitting results of Pd 3p3/2 and O 1s core-levels in Spectrum (2b-4). (c) Pd 3d peaks. Spectra (2c-1) and (2c-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. All the core-level BE shifts obtained from the regions W/BT and W/O BT are illustrated by dash lines.

Mentions: A sample [S#2, inset of Fig. 2(a) shows its structure] fabricated with its structure identical to those of S#1 was studied using X-ray photoelectron spectroscopy (XPS) to further reveal the role of the underlying BT thin film on the surface reactivity of Pd layer. XPS could provide more quantitative and direct analysis on the oxidation of Pd and is also able to study the electron transfer by probing the chemical shift of the electronic structure of the elements involved. One piece of this sample (S#2A) was cut and transferred to the XPS system right after being unloaded from the molecular beam epitaxy (MBE) system, while another piece was exposed in dry air for 3 days (S#2B) before being loaded into the XPS system. The resulted O KLL Auger spectra of S#2A are displayed in Fig. 2(a), which show that no notable O KLL signal can be detected in the region W/O BT (Spectrum 2a-1) while a weak O KLL peak can be seen in the spectrum obtained from the region W/BT (Spectrum 2a-2). This difference in peak intensity is even more obvious in the resulted O KLL peaks of S#2B (Spectra 2a-3 and 2a-4). Standard quantitative composition analysis reveals that the atomic concentration of O in percentage in the region W/BT (35.1%) is higher than that of W/O BT (20.4%), while the corresponding O/Pd ratios are 0.209 and 0.108. As shown in Spectra (2a-3) and (2a-4) in Fig. 2(a), each of the resulted O KLL spectra from the regions W/O BT and W/BT can be deconvoluted into two components. The peaks at 509.5 eV on Spectrum (2a-3) and 509.7 eV on Spectrum (2a-4) mainly originate from the standard O Auger KLL transition13, which involves contributions from O 1s and 2p atomic orbitals. Titkov et al.14 studied the oxidation of pure Pd samples with different amount of adsorbed oxygen and under various heating temperatures using the XPS technique. Their results indicate that the kinetic energy of PdO-related O Auger peak decreases with the increase of the degree of oxidation of Pd, ranging from 514.7 to 513.1 eV for a sample mildly heated under a moderate concentration of O2 to a sample heated at an elevated temperature under a high concentration of O2 respectively. Thus, the peaks at 514.9 eV on Spectrum (2a-3) and 513.4 eV on Spectrum (2a-4) are believed to be PdO-related O Auger peaks that correspond to when Pd is weakly and strongly oxidized respectively. The above observations show that Pd underwent a stronger oxidation in the region W/BT than in the region W/O BT. This also can be concluded from the resulted Pd 3p and 3d core-level spectra shown in Fig. 2(b) and (c). Spectra (2b-1) and (2b-2) show the Pd 3p core-level spectra from the regions W/O BT and W/BT of S#2A while Spectra (2c-1) and (2c-2) show the Pd 3d core-level spectra from the corresponding regions. The corresponding spectra of S#2B are shown in Spectra (2b-3), (2b-4), (2c-3) and (2c-4), respectively. As illustrated in Fig. 2(b) and (c), the binding energies (BEs) of Pd 3p1/2, 3d3/2 and 3d5/2 core-levels in the spectra of S#2A obtained from the region W/BT shift positively by 0.10, 0.15 and 0.15 eV respectively in reference to those obtained from the region W/O BT. These shifts are even larger (0.20, 0.22 and 0.22 eV, respectively) in S#2B. Since the core-level BE increases with the oxidation state of metal atoms when a metal is oxidized, these shifts further support that the oxidation of Pd is enhanced by the underlying BT thin film. Referring back to the Pd 3p3/2 core-level, its BE is in the neighborhood of that of O 1s [see Fig. 2(b)], and if the oxidation of Pd is not significant, it will be difficult to distinguish the contributions from Pd 3p3/2 and O 1s, just like what can be seen in Spectra (2b-1), (2b-2) and (2b-3). However, in Spectrum (2b-4) that is obtained from the region W/BT of S#2B, the O 1s peak appears to be well distinguished from the Pd 3p3/2 peak, which is attributed to a much stronger oxidation of Pd comparing to that of the region W/O BT. The corresponding fitting for the O 1s core-level in Spectrum (2b-4) as shown in the inset of Fig. 2(b) indicates that its BE peak locates at 530.2 eV, which is consistent with the BE range of O 1s (530.1–530.3 eV) reported for a PdO sample14. The signature of the O 1s core-level described above acts as another strong evidence that the underlying BT thin film could significantly enhance the oxidation of Pd.


Surface reactivity enhancement on a Pd/Bi2Te3 heterostructure through robust topological surface states.

He QL, Lai YH, Lu Y, Law KT, Sou IK - Sci Rep (2013)

XPS core-level and Auger spectra of S#2.(a) O KLL Auger peaks. Spectra (2a-1) and (2a-2) are O KLL spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2a-3) and (2a-4) are the corresponding spectra obtained from S#2B, respectively. The inset displays the structure of S#2. (b) Pd 3p peaks. Spectra (2b-1) and (2b-2) are Pd 3p spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. The inset shows the fitting results of Pd 3p3/2 and O 1s core-levels in Spectrum (2b-4). (c) Pd 3d peaks. Spectra (2c-1) and (2c-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. All the core-level BE shifts obtained from the regions W/BT and W/O BT are illustrated by dash lines.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750537&req=5

f2: XPS core-level and Auger spectra of S#2.(a) O KLL Auger peaks. Spectra (2a-1) and (2a-2) are O KLL spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2a-3) and (2a-4) are the corresponding spectra obtained from S#2B, respectively. The inset displays the structure of S#2. (b) Pd 3p peaks. Spectra (2b-1) and (2b-2) are Pd 3p spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. The inset shows the fitting results of Pd 3p3/2 and O 1s core-levels in Spectrum (2b-4). (c) Pd 3d peaks. Spectra (2c-1) and (2c-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#2A, while Spectra (2b-3) and (2b-4) are the corresponding spectra obtained from S#2B, respectively. All the core-level BE shifts obtained from the regions W/BT and W/O BT are illustrated by dash lines.
Mentions: A sample [S#2, inset of Fig. 2(a) shows its structure] fabricated with its structure identical to those of S#1 was studied using X-ray photoelectron spectroscopy (XPS) to further reveal the role of the underlying BT thin film on the surface reactivity of Pd layer. XPS could provide more quantitative and direct analysis on the oxidation of Pd and is also able to study the electron transfer by probing the chemical shift of the electronic structure of the elements involved. One piece of this sample (S#2A) was cut and transferred to the XPS system right after being unloaded from the molecular beam epitaxy (MBE) system, while another piece was exposed in dry air for 3 days (S#2B) before being loaded into the XPS system. The resulted O KLL Auger spectra of S#2A are displayed in Fig. 2(a), which show that no notable O KLL signal can be detected in the region W/O BT (Spectrum 2a-1) while a weak O KLL peak can be seen in the spectrum obtained from the region W/BT (Spectrum 2a-2). This difference in peak intensity is even more obvious in the resulted O KLL peaks of S#2B (Spectra 2a-3 and 2a-4). Standard quantitative composition analysis reveals that the atomic concentration of O in percentage in the region W/BT (35.1%) is higher than that of W/O BT (20.4%), while the corresponding O/Pd ratios are 0.209 and 0.108. As shown in Spectra (2a-3) and (2a-4) in Fig. 2(a), each of the resulted O KLL spectra from the regions W/O BT and W/BT can be deconvoluted into two components. The peaks at 509.5 eV on Spectrum (2a-3) and 509.7 eV on Spectrum (2a-4) mainly originate from the standard O Auger KLL transition13, which involves contributions from O 1s and 2p atomic orbitals. Titkov et al.14 studied the oxidation of pure Pd samples with different amount of adsorbed oxygen and under various heating temperatures using the XPS technique. Their results indicate that the kinetic energy of PdO-related O Auger peak decreases with the increase of the degree of oxidation of Pd, ranging from 514.7 to 513.1 eV for a sample mildly heated under a moderate concentration of O2 to a sample heated at an elevated temperature under a high concentration of O2 respectively. Thus, the peaks at 514.9 eV on Spectrum (2a-3) and 513.4 eV on Spectrum (2a-4) are believed to be PdO-related O Auger peaks that correspond to when Pd is weakly and strongly oxidized respectively. The above observations show that Pd underwent a stronger oxidation in the region W/BT than in the region W/O BT. This also can be concluded from the resulted Pd 3p and 3d core-level spectra shown in Fig. 2(b) and (c). Spectra (2b-1) and (2b-2) show the Pd 3p core-level spectra from the regions W/O BT and W/BT of S#2A while Spectra (2c-1) and (2c-2) show the Pd 3d core-level spectra from the corresponding regions. The corresponding spectra of S#2B are shown in Spectra (2b-3), (2b-4), (2c-3) and (2c-4), respectively. As illustrated in Fig. 2(b) and (c), the binding energies (BEs) of Pd 3p1/2, 3d3/2 and 3d5/2 core-levels in the spectra of S#2A obtained from the region W/BT shift positively by 0.10, 0.15 and 0.15 eV respectively in reference to those obtained from the region W/O BT. These shifts are even larger (0.20, 0.22 and 0.22 eV, respectively) in S#2B. Since the core-level BE increases with the oxidation state of metal atoms when a metal is oxidized, these shifts further support that the oxidation of Pd is enhanced by the underlying BT thin film. Referring back to the Pd 3p3/2 core-level, its BE is in the neighborhood of that of O 1s [see Fig. 2(b)], and if the oxidation of Pd is not significant, it will be difficult to distinguish the contributions from Pd 3p3/2 and O 1s, just like what can be seen in Spectra (2b-1), (2b-2) and (2b-3). However, in Spectrum (2b-4) that is obtained from the region W/BT of S#2B, the O 1s peak appears to be well distinguished from the Pd 3p3/2 peak, which is attributed to a much stronger oxidation of Pd comparing to that of the region W/O BT. The corresponding fitting for the O 1s core-level in Spectrum (2b-4) as shown in the inset of Fig. 2(b) indicates that its BE peak locates at 530.2 eV, which is consistent with the BE range of O 1s (530.1–530.3 eV) reported for a PdO sample14. The signature of the O 1s core-level described above acts as another strong evidence that the underlying BT thin film could significantly enhance the oxidation of Pd.

Bottom Line: We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure.The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation.A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

View Article: PubMed Central - PubMed

Affiliation: William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, SAR China.

ABSTRACT
We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure. The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation. The surface reactivity of the adsorbed tellurium on this heterostructure is also intensified possibly benefitted from the effective transfer of the bath electrons. A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

Show MeSH