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Surface structure and electrochemical properties of platinum films grown on SrTiO3(100) substrates
Surface Science 666 (2017) 14–22
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Surface structure and electrochemical properties of platinum films grown on SrTiO3 (100) substrates Masahiro Kasai∗, Hideyuki Dohi International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 819-0395, Japan
a r t i c l e Keywords: Hetero-epitaxial films Pt films SrTiO3 substrate Catalyst
i n f o
a b s t r a c t We fabricated Pt films on SrTiO3 (STO)(100) using a DC-magnetron sputtering method to investigate the preferred orientation, surface structure, and electrochemical property. A film grown at 400 °C showed the two-dimensional polycrystalline features of Pt(111). Reflection high-energy electron diffraction (RHEED) showed diffraction patterns independent of the in-plane incident angle of the film. Films grown at 600 and 700 °C exhibited a preferred orientation of Pt(100) and (110). The films exhibited the morphology of faceted islands with roughness of several tens of nm, which consisted of two kinds of domains, namely a domain with preferred orientation of (100) and one with (110). The (100) and (110) domains had 45-degree twin boundaries, which were observed as V-shaped streaks by RHEED. The (100) domain was aligned in orientation of [011]Pt//[010]STO, which suggests that the binding strength of the (110) plane at the interface was larger than that of the (100). With a further increase in the growth temperature up to 750 °C, the film primarily showed a preferred orientation of (100) with an in-plane orientation of [010]Pt//[010]STO. The film also showed an island structure; however, atomic force microscopy revealed that the top was atomically flat. © 2017 Published by Elsevier B.V.
1. Introduction Catalysts have contributed extensively to human society and industry for more than one hundred years. Catalytic reactions occur on the surfaces of catalytically active metals (ex. Pt, Pd, Ir, Fe), which are dispersed on supports (ex. Al2 O3 , ZrO2 , TiO2 ). In such reactions, the reactants are first adsorbed on the catalyst surface, and the final products are then generated through a transition state. Therefore, the relationship between the surface structure and catalytic activity has attracted much attention [1]. Advances in surface science occurred in the 1960s, and they were urged by progress in solid-state electronic devices. Various techniques that were developed to analyze silicon surfaces can also be used to study the surface of catalysts. Recently, the particle size of active metals has become increasingly smaller to achieve higher catalytic activity [2]. However, a negative particle-size effect, that is, a decrease in activity due to the nano-sizing was reported by Sun et al[3] and Bergamaski et al. [4]. The decrease in the catalytic activity was explained in terms of an increase in the fraction of edge atoms or the distance between nearest-neighboring particles. We considered that a strain in the metal due to a lattice mismatch with the support material would also be a potential cause of this effect. Changes in catalytic activity caused by lattice strain were reported by Wadayama et al [5,6]. They reported the surface structure and elec-
∗
Corresponding author. E-mail address: [email protected] (M. Kasai).
http://dx.doi.org/10.1016/j.susc.2017.08.018 Received 10 March 2017; Received in revised form 21 August 2017; Accepted 22 August 2017 Available online 24 August 2017 0039-6028/© 2017 Published by Elsevier B.V.
trochemical properties for several mono layers of Pt films, which were deposited on Pt0.75 Ni0.25 (111) single crystals using an MBE apparatus. An increase in the catalytic activity that was approximately 25 times higher than that of the single-crystal Pt(111) was exhibited. Thus, the dominant factor that determines catalytic activity is not currently understood. However, it is valuable to investigate strain-induced changes in the catalytic activity, because it can be expected that the strain can cause improvement as well as degradation in the catalytic activity. It is difficult to distinguish the contribution of strain from an alloying effect if metal substrates are used. Therefore, we considered that a “metal-onoxide” hetero-epitaxial film was suitable for studying the influence of strain, because, in general, oxide substrates do not form an alloy phase with Pt and are catalytically inert. Platinum is a well-known catalytically active metal that has a face-centered cubic (fcc) structure (Fmm) with a lattice parameter of a = 0.3923 nm. Fabrication of epitaxial Pt films on MgO [7,8], YSZ [9], and SrTiO3 [10–15] substrates has already been reported. However, these studies focused only on crystallographic interests or preparing high-quality electrodes for ferroelectric memory devices; thus, there are no reports on investigation of catalytic activity. At present nearly ten kinds of perovskite-type substrates are commercially available, which have mismatch from 7.3% tension to −6.7% compression. If we can control the strain of Pt films by using these substrates, it is expected that the strain effect on catalytic activity can be investigated. The lattice
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parameter of SrTiO3 (STO), which has a perovskite-type cubic structure (Pmm), is a = 0.3905 nm. The mismatch with Pt is small at 0.49%, which suggests that STO is an appropriate substrate for obtaining lowstrain epitaxial films. In this study, as a first step toward investigating the effect of lattice strain, we investigated the fabrication of Pt films on STO(100) using a DC-magnetron sputtering method. The preferred orientation, surface structure, and electrochemical property were examined. These results will be helpful to fabricate strain-induced films using the other kind of substrates.
orientation is about 4.2 wt%. On the other hand, the amount of (100) and (110) is comparable. Furthermore, at 750 °C, the Pt(100) amount exceeded 99 wt%. The simultaneous appearance of the Pt(110) plane with Pt(100) is closely related with the in-plane orientation of the film. We will discuss the growth mechanism in Section 3.2. 3.2. Surface structure
3.1. Preferred orientation of Pt films
3.2.1. Two-dimensional (2-D) polycrystal Fig. 3 shows the RHEED patterns of Pt films fabricated at 400 °C. The incident direction of the electron beam is indicated as [010]STO (parallel to the [010]azimuth of SrTiO3 , Fig. 3(a)) and [011]STO (parallel to the [011]azimuth of SrTiO3 , Fig. 3(b)), respectively. Streak-like patterns are observed for both the incident directions. The surface morphology investigated by AFM indicates that the surface is very flat with a roughness that is less than several unit cells as shown in Fig. 4. Thus, it is also possible that we consider that the Pt film obtained has an in-plane epitaxial relationship. However, when we carefully observe the patterns, we find that the shape of each streak line is not perfectly straight, and another interpretation of the result is shown. A two-dimensional polycrystal was presented by Ichimiya and Cohen [20]. As shown in Fig. 5(a), it is characterized by a mass of small crystals, which grew along a specified fiber axis normal to a substrate surface, randomly rotating on the axis. The surface of the small crystal has a specific crystal plane, for example Pt(111) in our present work. Usually, the reciprocal lattice of a well-ordered surface is indicated by a set of reciprocal rods. However, in the case of the two-dimensional polycrystal, the reciprocal lattice is indicated by a coaxial cylinder with multi-walls as shown in Fig. 5(b). We can consider that each wall of the cylinder corresponds to a locus of the reciprocal rod when it is rotated around the fiber axis. The intersection of the reciprocal cylinder with the Ewald sphere is indicated by curved lines. Fig. 5(c) shows a RHEED pattern, which is independent of the incident direction of the electron beam, and a simulated result for the 2-D polycrystal with Pt(111) plane is indicated in Fig. 3(a) by white lines. The authors have also confirmed such independent features of the incident direction for the diffraction pattern, by recording the pattern dynamically during a rotation of the sample. (video 1) A movie file is shown as a link.) Therefore, we concluded that the Pt film grown at 400 °C exhibited a two-dimensional polycrystalline surface. The result agrees with the discussion in the former section whereby the preferred orientation of the film is mainly dominated by the surface energy, and any contribution of interaction between Pt atoms and substrate is negligible.
The X-ray diffraction patterns of the fabricated films deposited at different substrate temperatures, Ts, are shown in Fig. 1(a), (b), and (c); they were divided into three ranges of 2𝜃. At a substrate temperature of 400 °C, only a 111 peak is observed. Many researchers have reported calculation results of surface energy for various metals and their various surfaces [16–18]. The surface energy, E, of Pt has a relationship of E(100) ≳ E(110) > E(111). (Here, E(hkl) denotes the surface energy of the (hkl) plane.) The result wherein the surface with the lowest energy is observed suggests that the surface energy is dominant in determining the preferred orientation at 400 °C, while the contribution of the interaction between substrate and Pt atoms is not large. On the other hand, a preferred orientation at the (100) and (110) is observed as the substrate temperature is increased. Changes in the peak intensity for Pt111 and Pt200 are shown in Fig. 2 as the intensity ratio of I200 /I111 . Peak intensity of XRD is determined by a structure factor, a multiplicity factor, a Lorentz factor, an absorption factor and a temperature factor for powder diffraction. On the other hand, fabricated films have three preferred orientation of (100), (110) and (111). Therefore, we roughly estimated the portion of each plane by using only the structure factor F [19]. Detailed calculation process is shown in appendix. For Ts = 600 °C, although the 111 peak is clearly shown because it has a large structure factor of F2 and the intensity is indicated by a logarithmic scale, the amount of the (111)
3.2.2. Faceted island and twin boundary An investigation by AFM revealed that the roughness of the surface increased as the substrate temperature was increased. A film grown at 700 °C exhibits an island structure as shown in Fig. 6(a). The height profile shows that the length of the islands is approximately 0.1 to 0.3 μm, and the height is approximately 30 to 80 nm. Because the height of the island is comparable to the thickness of the films examined by a stylus profiler, we considered that the bottom of the islands nearly reaches the surface of the substrate. A film grown at 750 °C also exhibited an island structure as shown in Fig. 6(b), which showed a larger area than the former film because the island growth was accelerated by higher growth temperature. The height profiles shown in Fig. 6(b) indicate that the width of the islands is approximately 0.5 μm and the height is 40 to 50 nm. The top of the surface has an atomically flat surface where the roughness is within one or two unit cells of Pt along both lines B and C. Fig. 7 shows the RHEED patterns of the Pt films for Ts = 600, 700, and 750 °C, which were observed with an incident beam along the [010]STO direction and [011]STO direction, respectively. As shown in Fig. 7(a), three kinds of diffraction patterns were observed, that is, streaks shown for the [010]STO incident direction and tilted streaks for the [011] incident direction. Both the patterns are simultaneously superposed by transmission diffraction spots through a faceted island. For the
2. Experimental methods Strontium titanate (100) substrates were annealed at 950 °C for 6 h in air to eliminate mechanical stress at the surface. Observation using atomic force microscopy (AFM, Dimension Icon, Bruker) showed terraces with a width of about 100 nm and steps of a unit-cell height. The platinum films were fabricated by an ultra-high vacuum (UHV) DCmagnetron sputtering system, equipped with a load-lock chamber and a reflection high-energy electron diffraction (RHEED) apparatus. The background pressure was kept below 5.0 × 10−7 Pa at room temperature. A sputtering source was placed at 45° to the substrates, and 0.2 Pa of Ar was introduced as the sputtering gas. An input power of 20 W was applied, and the films were fabricated at substrate temperatures of 400, 600, 700, and 750 °C. The thickness of the films was typically about 50 nm. RHEED patterns were simulated using a self-made simulator. We considered two kinematic scattering processes; one is surface diffraction and another is transmission diffraction. An acceleration voltage of 20 kV was used to determine a radius of the Ewald sphere, and a camera length of 29 cm was the measured value of our vacuum chamber. An incident angle 𝜙 and rotation angle Δ were used as the fitting parameters, and an atomic scattering factor was used to calculate the spot intensity of diffraction. The crystal structure and preferred orientation of the fabricated films were examined by X-ray diffractometer (XRD, Rigaku) equipped with a Ge(220) monochrometer. A cyclic voltammogram was investigated using an electrochemical cell with a front-contact current collector. An Ag/AgCl electrode (saturated KCl) was used as a reference, and 0.5 M H2 SO4 was used as the electrolyte. The scanning range was set from −0.2 to 0.7 V (vs. Ag/AgCl) at a scanning rate of 100 mV/s. 3. Results and discussion
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Fig. 1. X-ray diffraction patterns for Pt films at varied substrate temperatures of Ts = 400, 600, 700, and 750 °C. The results are shown divided into three ranges of 2𝜃. The diffraction intensity is indicated in a logarithmic scale.
Ts = 700 °C pattern shape, the corresponding incident direction shows the same relation as the Ts = 600 °C. However, the feature of the patterns for the incident direction is reversed in the case of a substrate temperature of 750 °C. Furthermore, the superposed spots due to transmission diffraction have almost disappeared. A pseudo-one-dimensional crystal (hereinafter referred to as "1-D crystal") was reported by Lipson and Singer [21] and Lijadi et al. [22]. Fig. 8(a) shows the schematics of the 1-D crystal. The atoms are ordered in the x-direction with an equal distance composing the 1-D lattice. On the other hand, the distance of each neighboring 1-D row is randomly aligned, and furthermore, the atoms in each row are out of phase along the y-direction. Ichimiya showed that such 1-D crystals generate (but not reciprocal rods) reciprocal planes vertical to the reciprocal lattice plane [21]. As shown in Fig. 8(b), the reciprocal planes lead to a RHEED pattern, which consists of concentric circles with the incident direction along the x-axis where atoms are aligned at equal distance. On the other hand, the incidence along the y-axis results in a diffraction pattern that consists of streaks. As described in Section 3.1, for platinum films grown at 600 and 700 °C, the (100) and (110) planes are predominantly oriented normal to the surface. A model of the film growth is schematically shown
Fig. 2. Change in the integral peak intensity of Pt(200) and (111). The vertical axis presents the ratio of I200 /I111 .
Fig. 3. RHEED patterns for Pt films grown at 400 °C. (a) The incident direction of the electron beam is along [010]STO. The white lines indicate the simulation result for the 2-D polycrystal with a Pt(111) surface. (b) The incident direction along [011]STO.
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Fig. 4. AFM image and height profile of a Pt film at Ts = 400 °C.
Fig. 5. (a) Schematics of a 2-D polycrystal. An open circle indicates an atom. (b) Reciprocal planes and cross section with the Ewald sphere. (c) A RHEED pattern.
in Fig. 9(a). The film consists of three preferably oriented domains of Pt(110) [Domain A], Pt(110) [Domain B], and Pt(100) [Domain C]. The platinum atoms are indicated by blue closed circles and the top view of each unit cell is shown by a red solid line, where the lattice parameter of Pt is indicated by a. The azimuths of the STO substrate are also shown by arrows indexed as [010]STO and [001]STO. The azimuth of Pt(110) surface is aligned in relation to [001]Pt//[010]STO for domain A, and [001]Pt// [001]STO for domain B, respectively. On the other hand, Pt(100) surface in domain C grows in relation to [010]Pt//[011]STO, which means the unit cell is rotated to the epitaxial relation by 45°. Twin boundaries of 45° degrees (D1 and D2 ), which are indicated by a dotted line, are formed between domain A and domain C or B and C. The boundary consists of a row of atoms aligned at an equal distance. However, the ordering of atomic alignment is lost along the [001]STO (D1 ) and [010]STO (D2 ), because distances from the neighboring atomic √ rows are different, which are a/ 2 and a, respectively. Thus, we can presume the twin boundaries as 1-D crystals as described in Fig. 8. The results of RHEED observation are shown in Fig. 9(b)–(e) for Ts = 700 °C. Each figure shows the result of a simulation as well as a RHEED pattern observed along the [011]STO. The result of pattern simulation for domain A is shown in Fig. 9(b). White open circles indicate transmission diffraction spots with a number for the index, where the
diameter of a circle corresponds to the relative intensity of the spot. The diffraction pattern is not symmetrical due to the incident direction. For example, a 531-spot observed for domain A does not show a corresponding symmetrical spot as shown in Fig. 9(b); however, the spot is observed as the diffraction caused by domain B as shown in Fig. 9(c). A simulation result for domain C for Pt(100) is shown in Fig. 9(d), which clearly indicates that the simulation result is consistent with the observed transmission diffraction spots. We consider that the V-shaped diffraction pattern obtained shown in Fig. 9(e) resulted from diffraction by the twin boundary. As discussed above, a 1-D crystal generates reciprocal planes in reciprocal space. The cross line of the reciprocal plane with the Ewald sphere is an arc. If we consider two planes on the 00 lattice point that are inclined by 45° toward the incident direction, a projection onto a fluorescent screen is considered to be a couple of curves with a small curvature having a V-like shape. We simulated such a RHEED pattern as shown by the white lines in Fig. 9(e). The corresponding reciprocal planes were determined to be connecting two lattice points in the primitive reciprocal plane, that is, 00 - 1̄ 1 and 00 -1̄ 1̄ . The observed V-shaped lines agree well with the white lines, which supports our model for film growth in Fig. 9(a). We also performed the same analysis for the incident direction along [010]STO as shown in Fig. 11. A model of the crystal growth is shown 17
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Fig. 6. AFM images and height profiles of Pt films. (a) Ts = 700 °C, (b) Ts = 750 °C.
Fig. 7. RHEED patterns for incident directions of [010]STO and [011]STO. The films were grown at (a) Ts = 600 °C, (b) Ts= 700 °C, and (c) Ts= 750 °C.
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The RHEED study result is consistent with the result where a preferred orientation of the film grown at 600 °C is mainly (100) and (110) as discussed in 3.1. Diffraction by a 45-degree twin boundary between (100) and (110) was also observed. The azimuth of [010] for (100) plane was aligned parallel to the [011] of STO substrate as well as 700 °C. In contrast, for Ts = 750 °C, the RHEED patterns showed only a surface diffraction by (100) and (110), which reflects the atomically flat surface as shown in Fig. 6. Although the amount of Pt(110) preferred orientation was less than 0.3 wt%, the surface diffraction from the (110) surface was still observed. In addition, V-shaped streaks were observed, which suggests the existence of a 45-degree twin boundary. We consider that a quite small amount of Pt(110) still existed on the surface of the Pt film. The direction in-plane growth for Pt(100) was assigned as [010]Pt(100)//[010]STO, which shows that the main portion of the film was epitaxially grown. Here, we will discuss the growth behavior investigated with respect to a change in bond strength at the interface when the substrate temperature is increased. Platinum is a metal with the property of metallic binding, and SrTiO3 as a substrate material is an oxide with ionic bonding. It is thus necessary for the two materials, which have different types of crystal binding, to form chemical bonds whereby the Pt atoms should be oxidized by oxygen at the surface of the substrate. Asthagiri et al. studied Pt adsorption on SrTiO3 surfaces by DFT calculation [23,24]. They concluded that Pt atoms prefer to adsorb on top of oxygen atoms. We considered that the primary factor that decides the growth behavior is the Pt oxidation process. At 400 °C, an oxidation process scarcely occurs; therefore, the interfacial interaction at the STO surface is negligibly weak. It is appropriate to consider that the preferred orientation is almost dominated by surface energy, which resulted in the Pt(111) orientation. As the substrate temperature is increased, the binding strength at the interface becomes larger by accelerating the formation of Pt-O bonds, because the STO surface oxygen becomes unstable. However, the interfacial binding strength is insufficient to achieve a complete epitaxial relation for Ts = 600 and 700 °C. Therefore, the preferably oriented Pt(110), which shows a surface energy close to that of Pt(100), simultaneously grows. It seems that the preferable co-growth of the Pt(110) and (100) planes, which form a 45-degree twin boundary, lowers the total energy than the sole growth of Pt(100) in epitaxial relation with
Fig. 8. (a) Schematic feature of a 1-D crystal and (b) RHEED patterns for each incident direction.
in Fig. 10(a), which is the same as Fig. 9 except that the incident direction is rotated by 45°. The observed RHEED patterns for transmission diffraction are indicated in Fig. 10(b), (c), and (d) as well as the simulated results. These results showed good agreement, although several spots in Fig. 10(c) were not clearly observed, which may be due to a deviation in the sample setting. A diffraction by twin boundary D2 is observed as streaks shown in Fig. 10(e). The simulated results, which were calculated as a projection of reciprocal planes parallel to the incident direction, also show good agreement. However, a RHEED pattern by boundary D1 , which would exhibit concentric circles, was not observed in this work. 3.2.3. Change in growth mode and substrate temperature The results of our structural study are summarized in Table 1. At 600 °C, a surface structure similar to 700 °C was observed. All transmission diffractions obtained were caused by islands with Pt(100) and Pt(110) surface, and no diffraction from islands with Pt(111) appeared.
Fig. 9. RHEED patterns of the Pt film for Ts = 700 °C with an incidence [011]STO. (a) A model of the surface structure. (b) Domain A of Pt (110), (c) domain B of Pt (110), (d) domain C of Pt(100), and (e) twin boundary D1 and D2 . 19
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Fig. 10. RHEED patterns for Pt film at Ts= 700 °C with an incidence [010]STO. (a) A model of surface structure. (b) Domain A of Pt(110), (c) domain B of Pt(110), (d) domain C of √ Pt(100), and (e) twin boundary D2 . White lines indicate simulated results for the reciprocal planes with a/ 2 spacing. Table 1 Growth mode and substrate temperature Ts.
∗
Ts (°C)
Preferred orientation
Surface structure
Morphology
400 600
(111) (100), (110)≫ (111)
Flat surface with microstructure Faceted islands with roughness of several tens of nm
700
(100), (110)≫ (111)
750
(100)≫ (110), (111)
2-D polycrystal (100)/(110) 45-degree twin [010]Pt//[011]STO (100)/(110) 45-degree twin [010]Pt//[011]STO (100)/(110) 45-degree twin [010]Pt//[001]STO
Faceted islands with roughness of several tens of nm Islands with atomically flat surface
“Surface structure” shows in-plane orientation of Pt(100) domain.
the STO. This result suggests that the interfacial binding strength of the (110) plane is larger than that of the (100). A further increase in the substrate temperature to 750 °C provides a sufficient binding strength at the interface between the Pt atoms and the oxygen lattice, which achieves an in-plane orientation of (100) surface in relation to [010]Pt// [010]STO. However, a small amount of surface diffraction by Pt(110) accompanying the twin boundary is still observed, although the amount of the (110) plane estimated by XRD is less than 0.3 wt%. The islands grown at 750 °C show a complex structure as shown in Fig. 6. We consider that the twin boundary of Pt(100)/(110) exists at the corner or in the fringe of the island to lower the total energy.
sonable that the cyclic voltammogram for the Pt film grown at 600 °C in Fig. 11(c), the surface of which comprises a (100) plane, (110) plane, and facets of the island, shows a behavior similar to that of a polycrystal plate. On the other hand, a CV profile of the film grown at 400 °C shown in Fig. 11(b) exhibits a different behavior. The oxidation current from −0.1 V to 0.1 V is smaller than the polycrystalline result. However, no characteristics CV peaks were observed. Previously reported CV profiles for Pt (111) show characteristics sharp peaks around 0.6 V, which is a value converted to a potential measured versus Ag/AgCl reference. The single crystal samples usually pass through high-temperature heat treatment just below the melting point of Pt, under a hydrogen atmosphere. Presently, we cannot clearly conclude the reason; however, a reason for this may be that the surface state of the film is considered to be much different than that of the single crystals. The film grown at 400 °C shows an approximately 10 times larger current density. We consider that the result reflects the difference in surface morphology. The film grown at 400 °C has a roughness of several nanometers, which causes an increase in the surface area.
3.3. Electrochemical measurement The electrochemical properties were investigated by cyclic voltammetry (CV). In this work, Pt films grown at 400 and 600 °C were examined, because the films grown at 700 and 750 °C were electrically insulated because of their island structure. The results obtained are indicated in Fig. 11. Fig. 11(a) shows the cyclic voltammogram of a polished polycrystalline Pt plate, as a reference, during 10 cycles. Here, we will discuss the difference in the shape of the CV curves with respect to the surface structure. Cyclic voltammetry using single crystals was reported by Clavilier [25–27], Kibler [28], and Marković [29]. Their studies revealed that Pt exhibits various CV profiles depending upon the crystal plane, heat treatment, electrolyte, and scan ranges. It is rea-
4. Conclusions In conclusion, we examined Pt/STO(100) hetero-epitaxial films fabricated by a DC-magnetron sputtering system to investigate the preferred orientation, surface structure, and electrochemical property. The film grown at 400 °C showed the two-dimensional polycrystalline fea20
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Surface Science 666 (2017) 14–22
Fig. 11. Cyclic voltammograms for (a) a polished polycrystalline plate, (b) a Pt film grown at 400 °C, and (c) a Pt film grown at 600 °C.
tures of Pt(111). Films grown at 600 and 700 °C exhibited the preferred orientation of Pt(100) and (110). The Pt(100) and (110) domain have a 45-degree twin boundary, which was observed as Vshaped streaks by RHEED. The (100) domain was aligned in relation to [011]Pt//[010]STO, which suggests that the interfacial binding strength of the (110) plane at the interface is larger than that of (100). With a further increase in the growth temperature up to 750 °C, the film indicated for the most part a preferred orientation of (100) with an in-plane orientation of [010] Pt//[010]STO. Change in the growth mode for substrate temperatures was interpreted with respect to binding strength of Pt-O. Appendix We approximately estimated the portion of Pt(111), (100) and (110) by calculating peak intensity of X-ray diffraction. Cullity showed relative intensity I of powder diffraction as shown in Eq. A.1 [30]. ) ( 1 + cos2 2𝜃 −2𝑀 𝐼 = |𝐹 |2 𝑝 𝑒 (A.1) sin2 𝜃cos𝜃 Fig. A.1. Atomic scattering factor of Pt [19]. 𝜆= 1.5406 Å.
F is a structure factor, p is a multiplicity factor. The equation in parentheses indicates a Lorentz-polarization factor, where q is an incident angle, and e −2 M shows a temperature factor. The structure factor F of face-centered-cubic Pt is F= 0 (for mixed h, k, l) and 4f (for unmixed h, k, l). Here, the factor f is an atomic scattering factor given by Cromer et al. as a function of sin𝜃/𝜆 shown in Fig. A.1 [19]. We calculated relative intensities using the Eq. A.1 for Pt111, 200 and 110 peaks, omitting the temperature factor because the peaks appear in adjacent angles. The results are shown in Table. A.1. Calculated
results of Int.(Calc.) are well agreed with powder diffraction data of Int.(Powder). We applied the same method to estimate portion of Pt(111), (100) and (110). We assumed that fabricated films are composed of three crystallites which grow with the azimuth perpendicular to the surface of a substrate, and also assumed that difference of relative intensities depends on only the portion of the each orientation. We omitted multiplicity factor and Lorentz-polarization factor which describe the randomness of crystal orientation in powder, and used only structure factor F in the estimation. Calculated relative intensities of Int.(Calc.) are shown in Table A.2. If each portion of the three orientation is equal, measured relative intensities should be as same as the Int.(Calc.) X-ray measurement results are shown as Int.(Obs.) for substrate temperatures of 600 and 750°C. We can clearly see that the the 100 peak is much larger than the Int.(Clac.).
Table A.1 Relative intensities by calculation and powder diffraction data. Peak
𝜃 (deg.)
F2
p
LF∗
Int.(Calc.)
Int.(Powder)
111 200 220
19.88 23.12 33.735
66562.9 61041.5 46662.6
8 6 12
14.6 10.4 4.47
100 49.0 32.1
100 48 29.2
∗
LF: Lorentz-polarization factor. Powder results were cited from PDF 000-0008801 of International Center for Diffraction Data. 21
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Table A.2 Portion of each orientation of Pt films. Peak
111 200 220 ∗
Int. (Calc.)
100 91.7 70.1
Int.(Obs.)
[6] M. Asano, R. Kawamura, R. Sasakawa, N. Todoroki, T. Wadayama, ACS Catal. 6 (2016) 5285. [7] K. Iijima, T. Terashima, K. Yamamoto, K. Hirata, Y. Bando, Appl. Phys. Lett. 56 (1990) 527. [8] J.F.M. Cillessen, R.M. Wolf, D.M. de Leeuw, Thin Solid Films 226 (1993) 53. [9] G. Beck, C. Bachmann, R. Bretzler, R. Kmeth, Mater. Chem. Phys. 158 (2015) 107. [10] A.J. Francis, A. Salvador, J. Mater. Res. 22 (1) (2007) 89. [11] B.S. Kwak, P.N. First, A. Erbil, J. Appl. Phys. 72 (8) (1992) 3735. [12] A.D. Polli, T. Wagner, T. Gemming, M. Rühle, Surf. Sci. 448 (2000) 279. [13] X.M. Xu, J. Liu, Z. Yuan, J. Weaver, C.L. Chen, Y.R. Li, H. Gao, N. Shi, Appl. Phys. Lett. 92 (2008) 102102. [14] S.T. Christensen, J.W. Elam, B. Lee, Z. Feng, M.J. Bedzyk, M.C. Hersam, Chem. Mater. 21 (2009) 516. [15] J-J. Pyeon, J-Y. Kang, S-H. Baek, C-Y. Kang, J-S. Kim, D-S. Jeong, S-K. Kim, Chem. Mater. 27 (2015) 6779. [16] L. Vitos, A.V. Ruban, H.L. Skiriver, J. Kollár, Surf. Sci. 411 (1998) 186. [17] J-M. Zhang, F. Ma, K-W Xu, Appl. Surf. Sci. 229 (2004) 34. [18] Y-Ni Wen, J-M. Zhang, Solid State Commun. 144 (2007) 163. [19] D.T. Cromer, J.B. Mann, Acta Cryst. A24 (1968) 321. [20] A. Ichimiya, P.I. Cohen, Reflection High-Energy Electron Diffraction, Cambridge Univ. Press, 2004. [21] H. Lipson, K.E. Singer, J. Phys. C 7 (1974) 12. [22] M. Lijadi, H. Iwashige, A. Ichimiya, Surf. Sci. 357 (1996) 51. [23] A. Asthagiri, D.S. Sholl, J. Chem. Phys. 116 (2002) 9914. [24] A. Asthagiri, D.S. Sholl, Surf. Sci. 581 (2005) 66. [25] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [26] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [27] A. Rodes, M.A. Zamakhchari, K. El Achi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [28] L.A. Kibler, Preparation and Characterization of Noble Metal Single Crystal Electrode Surfaces, International Society of Electrochemistry, 2003. [29] N.M. Marković, P.N. Ross Jr., Surf. Sci. Rep. 45 (2002) 117. [30] B.D. Cullity, Elements of X-ray Diffraction, 2nd. ed., Addison-Wesley Co., 1978 Chapter 4.
Portion of orientation (wt%)
600°C
750°C
600°C
750°C
100 1468.7 479.1
100 20268.3 41.8
4.19 67.15 28.66
0.45 99.28 0.27
Relative intensities are indicated by setting the 111 intensity as “100”.
For example, in the case of 600°C quantity of the (200) is calculated as 100 × 1468.7 / 91.7 = 1601.6 in arbitrary units, which is the quantity proportional to weight of the total crystallite in the film. Thus, estimated results for portion of the orientation are shown in the Table A2. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2017.08.018. References [1] G.A. Somorjai, Y. Li, Introduction to Surface Chemistry and Catalysis, second ed., Wiley, 2010. [2] M. Arenz, K.J.J. Mayrhofer, V. Stamenkovic, B.B. Blizanac, T. Tomoyuki, P.N. Ross, N.M. Markovic, J. Am. Chem. Soc. 127 (2005) 6819. [3] Y. Sun, Y. Dai, Y. Liu, S. Chen, Phys. Chem. Chem. Phys. 14 (2012) 2278. [4] K. Bergamaski, A.L.N. Pinheiro, E. Teixeria-Neto, F.C. Nart, J. Phys. Chem. B 110 (2006) 19271. [5] T. Wadayama, N. Todoroki, Y. Yamada, T. Sugawara, K. Miyamoto, Y. Iijama, Electrochem. Commun. 12 (2010) 1112.
22
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Surface structure and electrochemical properties of platinum films grown on SrTiO3 (100) substrates Masahiro Kasai∗, Hideyuki Dohi International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 819-0395, Japan
a r t i c l e Keywords: Hetero-epitaxial films Pt films SrTiO3 substrate Catalyst
i n f o
a b s t r a c t We fabricated Pt films on SrTiO3 (STO)(100) using a DC-magnetron sputtering method to investigate the preferred orientation, surface structure, and electrochemical property. A film grown at 400 °C showed the two-dimensional polycrystalline features of Pt(111). Reflection high-energy electron diffraction (RHEED) showed diffraction patterns independent of the in-plane incident angle of the film. Films grown at 600 and 700 °C exhibited a preferred orientation of Pt(100) and (110). The films exhibited the morphology of faceted islands with roughness of several tens of nm, which consisted of two kinds of domains, namely a domain with preferred orientation of (100) and one with (110). The (100) and (110) domains had 45-degree twin boundaries, which were observed as V-shaped streaks by RHEED. The (100) domain was aligned in orientation of [011]Pt//[010]STO, which suggests that the binding strength of the (110) plane at the interface was larger than that of the (100). With a further increase in the growth temperature up to 750 °C, the film primarily showed a preferred orientation of (100) with an in-plane orientation of [010]Pt//[010]STO. The film also showed an island structure; however, atomic force microscopy revealed that the top was atomically flat. © 2017 Published by Elsevier B.V.
1. Introduction Catalysts have contributed extensively to human society and industry for more than one hundred years. Catalytic reactions occur on the surfaces of catalytically active metals (ex. Pt, Pd, Ir, Fe), which are dispersed on supports (ex. Al2 O3 , ZrO2 , TiO2 ). In such reactions, the reactants are first adsorbed on the catalyst surface, and the final products are then generated through a transition state. Therefore, the relationship between the surface structure and catalytic activity has attracted much attention [1]. Advances in surface science occurred in the 1960s, and they were urged by progress in solid-state electronic devices. Various techniques that were developed to analyze silicon surfaces can also be used to study the surface of catalysts. Recently, the particle size of active metals has become increasingly smaller to achieve higher catalytic activity [2]. However, a negative particle-size effect, that is, a decrease in activity due to the nano-sizing was reported by Sun et al[3] and Bergamaski et al. [4]. The decrease in the catalytic activity was explained in terms of an increase in the fraction of edge atoms or the distance between nearest-neighboring particles. We considered that a strain in the metal due to a lattice mismatch with the support material would also be a potential cause of this effect. Changes in catalytic activity caused by lattice strain were reported by Wadayama et al [5,6]. They reported the surface structure and elec-
∗
Corresponding author. E-mail address: [email protected] (M. Kasai).
http://dx.doi.org/10.1016/j.susc.2017.08.018 Received 10 March 2017; Received in revised form 21 August 2017; Accepted 22 August 2017 Available online 24 August 2017 0039-6028/© 2017 Published by Elsevier B.V.
trochemical properties for several mono layers of Pt films, which were deposited on Pt0.75 Ni0.25 (111) single crystals using an MBE apparatus. An increase in the catalytic activity that was approximately 25 times higher than that of the single-crystal Pt(111) was exhibited. Thus, the dominant factor that determines catalytic activity is not currently understood. However, it is valuable to investigate strain-induced changes in the catalytic activity, because it can be expected that the strain can cause improvement as well as degradation in the catalytic activity. It is difficult to distinguish the contribution of strain from an alloying effect if metal substrates are used. Therefore, we considered that a “metal-onoxide” hetero-epitaxial film was suitable for studying the influence of strain, because, in general, oxide substrates do not form an alloy phase with Pt and are catalytically inert. Platinum is a well-known catalytically active metal that has a face-centered cubic (fcc) structure (Fmm) with a lattice parameter of a = 0.3923 nm. Fabrication of epitaxial Pt films on MgO [7,8], YSZ [9], and SrTiO3 [10–15] substrates has already been reported. However, these studies focused only on crystallographic interests or preparing high-quality electrodes for ferroelectric memory devices; thus, there are no reports on investigation of catalytic activity. At present nearly ten kinds of perovskite-type substrates are commercially available, which have mismatch from 7.3% tension to −6.7% compression. If we can control the strain of Pt films by using these substrates, it is expected that the strain effect on catalytic activity can be investigated. The lattice
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parameter of SrTiO3 (STO), which has a perovskite-type cubic structure (Pmm), is a = 0.3905 nm. The mismatch with Pt is small at 0.49%, which suggests that STO is an appropriate substrate for obtaining lowstrain epitaxial films. In this study, as a first step toward investigating the effect of lattice strain, we investigated the fabrication of Pt films on STO(100) using a DC-magnetron sputtering method. The preferred orientation, surface structure, and electrochemical property were examined. These results will be helpful to fabricate strain-induced films using the other kind of substrates.
orientation is about 4.2 wt%. On the other hand, the amount of (100) and (110) is comparable. Furthermore, at 750 °C, the Pt(100) amount exceeded 99 wt%. The simultaneous appearance of the Pt(110) plane with Pt(100) is closely related with the in-plane orientation of the film. We will discuss the growth mechanism in Section 3.2. 3.2. Surface structure
3.1. Preferred orientation of Pt films
3.2.1. Two-dimensional (2-D) polycrystal Fig. 3 shows the RHEED patterns of Pt films fabricated at 400 °C. The incident direction of the electron beam is indicated as [010]STO (parallel to the [010]azimuth of SrTiO3 , Fig. 3(a)) and [011]STO (parallel to the [011]azimuth of SrTiO3 , Fig. 3(b)), respectively. Streak-like patterns are observed for both the incident directions. The surface morphology investigated by AFM indicates that the surface is very flat with a roughness that is less than several unit cells as shown in Fig. 4. Thus, it is also possible that we consider that the Pt film obtained has an in-plane epitaxial relationship. However, when we carefully observe the patterns, we find that the shape of each streak line is not perfectly straight, and another interpretation of the result is shown. A two-dimensional polycrystal was presented by Ichimiya and Cohen [20]. As shown in Fig. 5(a), it is characterized by a mass of small crystals, which grew along a specified fiber axis normal to a substrate surface, randomly rotating on the axis. The surface of the small crystal has a specific crystal plane, for example Pt(111) in our present work. Usually, the reciprocal lattice of a well-ordered surface is indicated by a set of reciprocal rods. However, in the case of the two-dimensional polycrystal, the reciprocal lattice is indicated by a coaxial cylinder with multi-walls as shown in Fig. 5(b). We can consider that each wall of the cylinder corresponds to a locus of the reciprocal rod when it is rotated around the fiber axis. The intersection of the reciprocal cylinder with the Ewald sphere is indicated by curved lines. Fig. 5(c) shows a RHEED pattern, which is independent of the incident direction of the electron beam, and a simulated result for the 2-D polycrystal with Pt(111) plane is indicated in Fig. 3(a) by white lines. The authors have also confirmed such independent features of the incident direction for the diffraction pattern, by recording the pattern dynamically during a rotation of the sample. (video 1) A movie file is shown as a link.) Therefore, we concluded that the Pt film grown at 400 °C exhibited a two-dimensional polycrystalline surface. The result agrees with the discussion in the former section whereby the preferred orientation of the film is mainly dominated by the surface energy, and any contribution of interaction between Pt atoms and substrate is negligible.
The X-ray diffraction patterns of the fabricated films deposited at different substrate temperatures, Ts, are shown in Fig. 1(a), (b), and (c); they were divided into three ranges of 2𝜃. At a substrate temperature of 400 °C, only a 111 peak is observed. Many researchers have reported calculation results of surface energy for various metals and their various surfaces [16–18]. The surface energy, E, of Pt has a relationship of E(100) ≳ E(110) > E(111). (Here, E(hkl) denotes the surface energy of the (hkl) plane.) The result wherein the surface with the lowest energy is observed suggests that the surface energy is dominant in determining the preferred orientation at 400 °C, while the contribution of the interaction between substrate and Pt atoms is not large. On the other hand, a preferred orientation at the (100) and (110) is observed as the substrate temperature is increased. Changes in the peak intensity for Pt111 and Pt200 are shown in Fig. 2 as the intensity ratio of I200 /I111 . Peak intensity of XRD is determined by a structure factor, a multiplicity factor, a Lorentz factor, an absorption factor and a temperature factor for powder diffraction. On the other hand, fabricated films have three preferred orientation of (100), (110) and (111). Therefore, we roughly estimated the portion of each plane by using only the structure factor F [19]. Detailed calculation process is shown in appendix. For Ts = 600 °C, although the 111 peak is clearly shown because it has a large structure factor of F2 and the intensity is indicated by a logarithmic scale, the amount of the (111)
3.2.2. Faceted island and twin boundary An investigation by AFM revealed that the roughness of the surface increased as the substrate temperature was increased. A film grown at 700 °C exhibits an island structure as shown in Fig. 6(a). The height profile shows that the length of the islands is approximately 0.1 to 0.3 μm, and the height is approximately 30 to 80 nm. Because the height of the island is comparable to the thickness of the films examined by a stylus profiler, we considered that the bottom of the islands nearly reaches the surface of the substrate. A film grown at 750 °C also exhibited an island structure as shown in Fig. 6(b), which showed a larger area than the former film because the island growth was accelerated by higher growth temperature. The height profiles shown in Fig. 6(b) indicate that the width of the islands is approximately 0.5 μm and the height is 40 to 50 nm. The top of the surface has an atomically flat surface where the roughness is within one or two unit cells of Pt along both lines B and C. Fig. 7 shows the RHEED patterns of the Pt films for Ts = 600, 700, and 750 °C, which were observed with an incident beam along the [010]STO direction and [011]STO direction, respectively. As shown in Fig. 7(a), three kinds of diffraction patterns were observed, that is, streaks shown for the [010]STO incident direction and tilted streaks for the [011] incident direction. Both the patterns are simultaneously superposed by transmission diffraction spots through a faceted island. For the
2. Experimental methods Strontium titanate (100) substrates were annealed at 950 °C for 6 h in air to eliminate mechanical stress at the surface. Observation using atomic force microscopy (AFM, Dimension Icon, Bruker) showed terraces with a width of about 100 nm and steps of a unit-cell height. The platinum films were fabricated by an ultra-high vacuum (UHV) DCmagnetron sputtering system, equipped with a load-lock chamber and a reflection high-energy electron diffraction (RHEED) apparatus. The background pressure was kept below 5.0 × 10−7 Pa at room temperature. A sputtering source was placed at 45° to the substrates, and 0.2 Pa of Ar was introduced as the sputtering gas. An input power of 20 W was applied, and the films were fabricated at substrate temperatures of 400, 600, 700, and 750 °C. The thickness of the films was typically about 50 nm. RHEED patterns were simulated using a self-made simulator. We considered two kinematic scattering processes; one is surface diffraction and another is transmission diffraction. An acceleration voltage of 20 kV was used to determine a radius of the Ewald sphere, and a camera length of 29 cm was the measured value of our vacuum chamber. An incident angle 𝜙 and rotation angle Δ were used as the fitting parameters, and an atomic scattering factor was used to calculate the spot intensity of diffraction. The crystal structure and preferred orientation of the fabricated films were examined by X-ray diffractometer (XRD, Rigaku) equipped with a Ge(220) monochrometer. A cyclic voltammogram was investigated using an electrochemical cell with a front-contact current collector. An Ag/AgCl electrode (saturated KCl) was used as a reference, and 0.5 M H2 SO4 was used as the electrolyte. The scanning range was set from −0.2 to 0.7 V (vs. Ag/AgCl) at a scanning rate of 100 mV/s. 3. Results and discussion
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Fig. 1. X-ray diffraction patterns for Pt films at varied substrate temperatures of Ts = 400, 600, 700, and 750 °C. The results are shown divided into three ranges of 2𝜃. The diffraction intensity is indicated in a logarithmic scale.
Ts = 700 °C pattern shape, the corresponding incident direction shows the same relation as the Ts = 600 °C. However, the feature of the patterns for the incident direction is reversed in the case of a substrate temperature of 750 °C. Furthermore, the superposed spots due to transmission diffraction have almost disappeared. A pseudo-one-dimensional crystal (hereinafter referred to as "1-D crystal") was reported by Lipson and Singer [21] and Lijadi et al. [22]. Fig. 8(a) shows the schematics of the 1-D crystal. The atoms are ordered in the x-direction with an equal distance composing the 1-D lattice. On the other hand, the distance of each neighboring 1-D row is randomly aligned, and furthermore, the atoms in each row are out of phase along the y-direction. Ichimiya showed that such 1-D crystals generate (but not reciprocal rods) reciprocal planes vertical to the reciprocal lattice plane [21]. As shown in Fig. 8(b), the reciprocal planes lead to a RHEED pattern, which consists of concentric circles with the incident direction along the x-axis where atoms are aligned at equal distance. On the other hand, the incidence along the y-axis results in a diffraction pattern that consists of streaks. As described in Section 3.1, for platinum films grown at 600 and 700 °C, the (100) and (110) planes are predominantly oriented normal to the surface. A model of the film growth is schematically shown
Fig. 2. Change in the integral peak intensity of Pt(200) and (111). The vertical axis presents the ratio of I200 /I111 .
Fig. 3. RHEED patterns for Pt films grown at 400 °C. (a) The incident direction of the electron beam is along [010]STO. The white lines indicate the simulation result for the 2-D polycrystal with a Pt(111) surface. (b) The incident direction along [011]STO.
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Fig. 4. AFM image and height profile of a Pt film at Ts = 400 °C.
Fig. 5. (a) Schematics of a 2-D polycrystal. An open circle indicates an atom. (b) Reciprocal planes and cross section with the Ewald sphere. (c) A RHEED pattern.
in Fig. 9(a). The film consists of three preferably oriented domains of Pt(110) [Domain A], Pt(110) [Domain B], and Pt(100) [Domain C]. The platinum atoms are indicated by blue closed circles and the top view of each unit cell is shown by a red solid line, where the lattice parameter of Pt is indicated by a. The azimuths of the STO substrate are also shown by arrows indexed as [010]STO and [001]STO. The azimuth of Pt(110) surface is aligned in relation to [001]Pt//[010]STO for domain A, and [001]Pt// [001]STO for domain B, respectively. On the other hand, Pt(100) surface in domain C grows in relation to [010]Pt//[011]STO, which means the unit cell is rotated to the epitaxial relation by 45°. Twin boundaries of 45° degrees (D1 and D2 ), which are indicated by a dotted line, are formed between domain A and domain C or B and C. The boundary consists of a row of atoms aligned at an equal distance. However, the ordering of atomic alignment is lost along the [001]STO (D1 ) and [010]STO (D2 ), because distances from the neighboring atomic √ rows are different, which are a/ 2 and a, respectively. Thus, we can presume the twin boundaries as 1-D crystals as described in Fig. 8. The results of RHEED observation are shown in Fig. 9(b)–(e) for Ts = 700 °C. Each figure shows the result of a simulation as well as a RHEED pattern observed along the [011]STO. The result of pattern simulation for domain A is shown in Fig. 9(b). White open circles indicate transmission diffraction spots with a number for the index, where the
diameter of a circle corresponds to the relative intensity of the spot. The diffraction pattern is not symmetrical due to the incident direction. For example, a 531-spot observed for domain A does not show a corresponding symmetrical spot as shown in Fig. 9(b); however, the spot is observed as the diffraction caused by domain B as shown in Fig. 9(c). A simulation result for domain C for Pt(100) is shown in Fig. 9(d), which clearly indicates that the simulation result is consistent with the observed transmission diffraction spots. We consider that the V-shaped diffraction pattern obtained shown in Fig. 9(e) resulted from diffraction by the twin boundary. As discussed above, a 1-D crystal generates reciprocal planes in reciprocal space. The cross line of the reciprocal plane with the Ewald sphere is an arc. If we consider two planes on the 00 lattice point that are inclined by 45° toward the incident direction, a projection onto a fluorescent screen is considered to be a couple of curves with a small curvature having a V-like shape. We simulated such a RHEED pattern as shown by the white lines in Fig. 9(e). The corresponding reciprocal planes were determined to be connecting two lattice points in the primitive reciprocal plane, that is, 00 - 1̄ 1 and 00 -1̄ 1̄ . The observed V-shaped lines agree well with the white lines, which supports our model for film growth in Fig. 9(a). We also performed the same analysis for the incident direction along [010]STO as shown in Fig. 11. A model of the crystal growth is shown 17
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Fig. 6. AFM images and height profiles of Pt films. (a) Ts = 700 °C, (b) Ts = 750 °C.
Fig. 7. RHEED patterns for incident directions of [010]STO and [011]STO. The films were grown at (a) Ts = 600 °C, (b) Ts= 700 °C, and (c) Ts= 750 °C.
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The RHEED study result is consistent with the result where a preferred orientation of the film grown at 600 °C is mainly (100) and (110) as discussed in 3.1. Diffraction by a 45-degree twin boundary between (100) and (110) was also observed. The azimuth of [010] for (100) plane was aligned parallel to the [011] of STO substrate as well as 700 °C. In contrast, for Ts = 750 °C, the RHEED patterns showed only a surface diffraction by (100) and (110), which reflects the atomically flat surface as shown in Fig. 6. Although the amount of Pt(110) preferred orientation was less than 0.3 wt%, the surface diffraction from the (110) surface was still observed. In addition, V-shaped streaks were observed, which suggests the existence of a 45-degree twin boundary. We consider that a quite small amount of Pt(110) still existed on the surface of the Pt film. The direction in-plane growth for Pt(100) was assigned as [010]Pt(100)//[010]STO, which shows that the main portion of the film was epitaxially grown. Here, we will discuss the growth behavior investigated with respect to a change in bond strength at the interface when the substrate temperature is increased. Platinum is a metal with the property of metallic binding, and SrTiO3 as a substrate material is an oxide with ionic bonding. It is thus necessary for the two materials, which have different types of crystal binding, to form chemical bonds whereby the Pt atoms should be oxidized by oxygen at the surface of the substrate. Asthagiri et al. studied Pt adsorption on SrTiO3 surfaces by DFT calculation [23,24]. They concluded that Pt atoms prefer to adsorb on top of oxygen atoms. We considered that the primary factor that decides the growth behavior is the Pt oxidation process. At 400 °C, an oxidation process scarcely occurs; therefore, the interfacial interaction at the STO surface is negligibly weak. It is appropriate to consider that the preferred orientation is almost dominated by surface energy, which resulted in the Pt(111) orientation. As the substrate temperature is increased, the binding strength at the interface becomes larger by accelerating the formation of Pt-O bonds, because the STO surface oxygen becomes unstable. However, the interfacial binding strength is insufficient to achieve a complete epitaxial relation for Ts = 600 and 700 °C. Therefore, the preferably oriented Pt(110), which shows a surface energy close to that of Pt(100), simultaneously grows. It seems that the preferable co-growth of the Pt(110) and (100) planes, which form a 45-degree twin boundary, lowers the total energy than the sole growth of Pt(100) in epitaxial relation with
Fig. 8. (a) Schematic feature of a 1-D crystal and (b) RHEED patterns for each incident direction.
in Fig. 10(a), which is the same as Fig. 9 except that the incident direction is rotated by 45°. The observed RHEED patterns for transmission diffraction are indicated in Fig. 10(b), (c), and (d) as well as the simulated results. These results showed good agreement, although several spots in Fig. 10(c) were not clearly observed, which may be due to a deviation in the sample setting. A diffraction by twin boundary D2 is observed as streaks shown in Fig. 10(e). The simulated results, which were calculated as a projection of reciprocal planes parallel to the incident direction, also show good agreement. However, a RHEED pattern by boundary D1 , which would exhibit concentric circles, was not observed in this work. 3.2.3. Change in growth mode and substrate temperature The results of our structural study are summarized in Table 1. At 600 °C, a surface structure similar to 700 °C was observed. All transmission diffractions obtained were caused by islands with Pt(100) and Pt(110) surface, and no diffraction from islands with Pt(111) appeared.
Fig. 9. RHEED patterns of the Pt film for Ts = 700 °C with an incidence [011]STO. (a) A model of the surface structure. (b) Domain A of Pt (110), (c) domain B of Pt (110), (d) domain C of Pt(100), and (e) twin boundary D1 and D2 . 19
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Fig. 10. RHEED patterns for Pt film at Ts= 700 °C with an incidence [010]STO. (a) A model of surface structure. (b) Domain A of Pt(110), (c) domain B of Pt(110), (d) domain C of √ Pt(100), and (e) twin boundary D2 . White lines indicate simulated results for the reciprocal planes with a/ 2 spacing. Table 1 Growth mode and substrate temperature Ts.
∗
Ts (°C)
Preferred orientation
Surface structure
Morphology
400 600
(111) (100), (110)≫ (111)
Flat surface with microstructure Faceted islands with roughness of several tens of nm
700
(100), (110)≫ (111)
750
(100)≫ (110), (111)
2-D polycrystal (100)/(110) 45-degree twin [010]Pt//[011]STO (100)/(110) 45-degree twin [010]Pt//[011]STO (100)/(110) 45-degree twin [010]Pt//[001]STO
Faceted islands with roughness of several tens of nm Islands with atomically flat surface
“Surface structure” shows in-plane orientation of Pt(100) domain.
the STO. This result suggests that the interfacial binding strength of the (110) plane is larger than that of the (100). A further increase in the substrate temperature to 750 °C provides a sufficient binding strength at the interface between the Pt atoms and the oxygen lattice, which achieves an in-plane orientation of (100) surface in relation to [010]Pt// [010]STO. However, a small amount of surface diffraction by Pt(110) accompanying the twin boundary is still observed, although the amount of the (110) plane estimated by XRD is less than 0.3 wt%. The islands grown at 750 °C show a complex structure as shown in Fig. 6. We consider that the twin boundary of Pt(100)/(110) exists at the corner or in the fringe of the island to lower the total energy.
sonable that the cyclic voltammogram for the Pt film grown at 600 °C in Fig. 11(c), the surface of which comprises a (100) plane, (110) plane, and facets of the island, shows a behavior similar to that of a polycrystal plate. On the other hand, a CV profile of the film grown at 400 °C shown in Fig. 11(b) exhibits a different behavior. The oxidation current from −0.1 V to 0.1 V is smaller than the polycrystalline result. However, no characteristics CV peaks were observed. Previously reported CV profiles for Pt (111) show characteristics sharp peaks around 0.6 V, which is a value converted to a potential measured versus Ag/AgCl reference. The single crystal samples usually pass through high-temperature heat treatment just below the melting point of Pt, under a hydrogen atmosphere. Presently, we cannot clearly conclude the reason; however, a reason for this may be that the surface state of the film is considered to be much different than that of the single crystals. The film grown at 400 °C shows an approximately 10 times larger current density. We consider that the result reflects the difference in surface morphology. The film grown at 400 °C has a roughness of several nanometers, which causes an increase in the surface area.
3.3. Electrochemical measurement The electrochemical properties were investigated by cyclic voltammetry (CV). In this work, Pt films grown at 400 and 600 °C were examined, because the films grown at 700 and 750 °C were electrically insulated because of their island structure. The results obtained are indicated in Fig. 11. Fig. 11(a) shows the cyclic voltammogram of a polished polycrystalline Pt plate, as a reference, during 10 cycles. Here, we will discuss the difference in the shape of the CV curves with respect to the surface structure. Cyclic voltammetry using single crystals was reported by Clavilier [25–27], Kibler [28], and Marković [29]. Their studies revealed that Pt exhibits various CV profiles depending upon the crystal plane, heat treatment, electrolyte, and scan ranges. It is rea-
4. Conclusions In conclusion, we examined Pt/STO(100) hetero-epitaxial films fabricated by a DC-magnetron sputtering system to investigate the preferred orientation, surface structure, and electrochemical property. The film grown at 400 °C showed the two-dimensional polycrystalline fea20
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Surface Science 666 (2017) 14–22
Fig. 11. Cyclic voltammograms for (a) a polished polycrystalline plate, (b) a Pt film grown at 400 °C, and (c) a Pt film grown at 600 °C.
tures of Pt(111). Films grown at 600 and 700 °C exhibited the preferred orientation of Pt(100) and (110). The Pt(100) and (110) domain have a 45-degree twin boundary, which was observed as Vshaped streaks by RHEED. The (100) domain was aligned in relation to [011]Pt//[010]STO, which suggests that the interfacial binding strength of the (110) plane at the interface is larger than that of (100). With a further increase in the growth temperature up to 750 °C, the film indicated for the most part a preferred orientation of (100) with an in-plane orientation of [010] Pt//[010]STO. Change in the growth mode for substrate temperatures was interpreted with respect to binding strength of Pt-O. Appendix We approximately estimated the portion of Pt(111), (100) and (110) by calculating peak intensity of X-ray diffraction. Cullity showed relative intensity I of powder diffraction as shown in Eq. A.1 [30]. ) ( 1 + cos2 2𝜃 −2𝑀 𝐼 = |𝐹 |2 𝑝 𝑒 (A.1) sin2 𝜃cos𝜃 Fig. A.1. Atomic scattering factor of Pt [19]. 𝜆= 1.5406 Å.
F is a structure factor, p is a multiplicity factor. The equation in parentheses indicates a Lorentz-polarization factor, where q is an incident angle, and e −2 M shows a temperature factor. The structure factor F of face-centered-cubic Pt is F= 0 (for mixed h, k, l) and 4f (for unmixed h, k, l). Here, the factor f is an atomic scattering factor given by Cromer et al. as a function of sin𝜃/𝜆 shown in Fig. A.1 [19]. We calculated relative intensities using the Eq. A.1 for Pt111, 200 and 110 peaks, omitting the temperature factor because the peaks appear in adjacent angles. The results are shown in Table. A.1. Calculated
results of Int.(Calc.) are well agreed with powder diffraction data of Int.(Powder). We applied the same method to estimate portion of Pt(111), (100) and (110). We assumed that fabricated films are composed of three crystallites which grow with the azimuth perpendicular to the surface of a substrate, and also assumed that difference of relative intensities depends on only the portion of the each orientation. We omitted multiplicity factor and Lorentz-polarization factor which describe the randomness of crystal orientation in powder, and used only structure factor F in the estimation. Calculated relative intensities of Int.(Calc.) are shown in Table A.2. If each portion of the three orientation is equal, measured relative intensities should be as same as the Int.(Calc.) X-ray measurement results are shown as Int.(Obs.) for substrate temperatures of 600 and 750°C. We can clearly see that the the 100 peak is much larger than the Int.(Clac.).
Table A.1 Relative intensities by calculation and powder diffraction data. Peak
𝜃 (deg.)
F2
p
LF∗
Int.(Calc.)
Int.(Powder)
111 200 220
19.88 23.12 33.735
66562.9 61041.5 46662.6
8 6 12
14.6 10.4 4.47
100 49.0 32.1
100 48 29.2
∗
LF: Lorentz-polarization factor. Powder results were cited from PDF 000-0008801 of International Center for Diffraction Data. 21
M. Kasai, H. Dohi
Surface Science 666 (2017) 14–22
Table A.2 Portion of each orientation of Pt films. Peak
111 200 220 ∗
Int. (Calc.)
100 91.7 70.1
Int.(Obs.)
[6] M. Asano, R. Kawamura, R. Sasakawa, N. Todoroki, T. Wadayama, ACS Catal. 6 (2016) 5285. [7] K. Iijima, T. Terashima, K. Yamamoto, K. Hirata, Y. Bando, Appl. Phys. Lett. 56 (1990) 527. [8] J.F.M. Cillessen, R.M. Wolf, D.M. de Leeuw, Thin Solid Films 226 (1993) 53. [9] G. Beck, C. Bachmann, R. Bretzler, R. Kmeth, Mater. Chem. Phys. 158 (2015) 107. [10] A.J. Francis, A. Salvador, J. Mater. Res. 22 (1) (2007) 89. [11] B.S. Kwak, P.N. First, A. Erbil, J. Appl. Phys. 72 (8) (1992) 3735. [12] A.D. Polli, T. Wagner, T. Gemming, M. Rühle, Surf. Sci. 448 (2000) 279. [13] X.M. Xu, J. Liu, Z. Yuan, J. Weaver, C.L. Chen, Y.R. Li, H. Gao, N. Shi, Appl. Phys. Lett. 92 (2008) 102102. [14] S.T. Christensen, J.W. Elam, B. Lee, Z. Feng, M.J. Bedzyk, M.C. Hersam, Chem. Mater. 21 (2009) 516. [15] J-J. Pyeon, J-Y. Kang, S-H. Baek, C-Y. Kang, J-S. Kim, D-S. Jeong, S-K. Kim, Chem. Mater. 27 (2015) 6779. [16] L. Vitos, A.V. Ruban, H.L. Skiriver, J. Kollár, Surf. Sci. 411 (1998) 186. [17] J-M. Zhang, F. Ma, K-W Xu, Appl. Surf. Sci. 229 (2004) 34. [18] Y-Ni Wen, J-M. Zhang, Solid State Commun. 144 (2007) 163. [19] D.T. Cromer, J.B. Mann, Acta Cryst. A24 (1968) 321. [20] A. Ichimiya, P.I. Cohen, Reflection High-Energy Electron Diffraction, Cambridge Univ. Press, 2004. [21] H. Lipson, K.E. Singer, J. Phys. C 7 (1974) 12. [22] M. Lijadi, H. Iwashige, A. Ichimiya, Surf. Sci. 357 (1996) 51. [23] A. Asthagiri, D.S. Sholl, J. Chem. Phys. 116 (2002) 9914. [24] A. Asthagiri, D.S. Sholl, Surf. Sci. 581 (2005) 66. [25] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [26] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [27] A. Rodes, M.A. Zamakhchari, K. El Achi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [28] L.A. Kibler, Preparation and Characterization of Noble Metal Single Crystal Electrode Surfaces, International Society of Electrochemistry, 2003. [29] N.M. Marković, P.N. Ross Jr., Surf. Sci. Rep. 45 (2002) 117. [30] B.D. Cullity, Elements of X-ray Diffraction, 2nd. ed., Addison-Wesley Co., 1978 Chapter 4.
Portion of orientation (wt%)
600°C
750°C
600°C
750°C
100 1468.7 479.1
100 20268.3 41.8
4.19 67.15 28.66
0.45 99.28 0.27
Relative intensities are indicated by setting the 111 intensity as “100”.
For example, in the case of 600°C quantity of the (200) is calculated as 100 × 1468.7 / 91.7 = 1601.6 in arbitrary units, which is the quantity proportional to weight of the total crystallite in the film. Thus, estimated results for portion of the orientation are shown in the Table A2. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2017.08.018. References [1] G.A. Somorjai, Y. Li, Introduction to Surface Chemistry and Catalysis, second ed., Wiley, 2010. [2] M. Arenz, K.J.J. Mayrhofer, V. Stamenkovic, B.B. Blizanac, T. Tomoyuki, P.N. Ross, N.M. Markovic, J. Am. Chem. Soc. 127 (2005) 6819. [3] Y. Sun, Y. Dai, Y. Liu, S. Chen, Phys. Chem. Chem. Phys. 14 (2012) 2278. [4] K. Bergamaski, A.L.N. Pinheiro, E. Teixeria-Neto, F.C. Nart, J. Phys. Chem. B 110 (2006) 19271. [5] T. Wadayama, N. Todoroki, Y. Yamada, T. Sugawara, K. Miyamoto, Y. Iijama, Electrochem. Commun. 12 (2010) 1112.
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