Enhanced reactivity of high-index surface platinum hollow nanocrystals

Abstract

The precise morphological control of the surface of inorganic nanocrystals (NCs) is critical for the understanding of the unique properties of the materials at the nanoscale and useful in a wide range of applications, such as catalysis, where the development of highly active and low-cost materials represents a landmark for the development of industrial technologies. Here we show how combining solid state chemistry and colloidal synthesis allows us to prepare exotic materials, in particular, PtAg@Pt single-crystal hollow NCs with high-index planes synthesized at room temperature by controlled corrosion of silver templates, which minimize Pt consumption and maximize surface reactivity.

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Introduction

Noble metal nanocrystals (NCs) have been extensively studied recently because of their promising wide application range owing to their intriguing optical and electronic properties, which arise from their constitutional and precise morphological variations. This is dramatic in the case of heterogeneous catalysis, where low-coordination surface atomic sites are largely responsible for the observed behavior.^1,2 Consequently, it is known that the catalytic properties of metal and alloy NCs strongly depend on atomic local features such as coordination and crystallographic index planes, which in turn are determined by the method of preparation.^3–5 A very important case is that of the expensive rare metal Pt and its widespread use in low temperature polymer electrolyte (proton exchange) membrane fuel cells.⁶ Consequently, very small NCs (<5 nm) to maximize the surface to volume (mass) ratio have been synthesized to increase the (economic) competitiveness of these materials. However, such small NCs are unstable and tend to sinter or detach from the support during operation.^7,8 This has fuelled up investigation on hollow Pt NCs, as an efficient alternative to increase the surface to mass ratio.^7,9–11 Initially, hollow Pt NCs were prepared by galvanic replacement using Co¹² and Ag templates.¹³ Due to the high immiscibility of Ag and Pt at low temperature, NCs with very rough surfaces where obtained when Pt had to grow on the top of Ag. Because of this, other groups prepared Pd templates to produce hollow Pt NCs with smoother surfaces.^14,15 Recently, ultrathin (down to 3 atomic layers) hollow Pt NCs have been prepared reaching a new limit in the optimization of the surface to mass ratio of Pt NCs.¹⁶ In addition to reducing the effective mass, the increase of surface activity per unit area is also a good strategy to increase Pt NC performance. Indeed, it is well known that for most catalytic reactions, high-index planes (low coordinated atoms), associated with atomic steps, edges and kinks have been considered responsible for the enhancement of catalytic performance.² Thus, high-index surface Pt-based NCs have been sought and prepared showing strong thermal and chemical stability and enhanced catalytic activity (up to 400%).^2,17,18

In order to produce stable high-index faceted NCs, we observed that the processes of dissolution and deposition during galvanic replacement reactions could be used to control the atomic roughness of the NCs and promote the alloying of Pt and Ag despite their immiscibility.¹¹ It is important to note that solid state diffusion processes are dramatically enhanced at the nanoscale (up to 10 orders of magnitude¹⁹). Thus, corrosion processes at the nanoscale show concomitant diffusion and migration of atoms in the solid matrix, allowing the production of designed materials by unexpected ways. This is the case of transformations of NCs' composition while maintaining their morphological integrity²⁰ or the carving of complex patterns in multimetallic hollow NCs,¹⁹ where the void is an added degree of freedom to control the morphology in a regime where the structure as much as the composition determines the activity. This plasticity is manifested in enhanced diffusion coefficients,^19,21,22 and is also reflected in high tolerance to crystal strain,²³ low tolerance to crystal defects,²⁴ decrease of the melting temperature, and the ease of defect curing in NCs.^25,26 Here we show how to prepare different hollow Pt NCs and especially PtAg@Pt single-crystals showing stable high-index planes (such as {950}, {910}, {540} and/or {4−10}) synthesized at room temperature by controlled corrosion of silver templates, which minimize Pt consumption and maximize surface reactivity.

Results and discussion

Platinum NCs grown on silver templates tend to grow in a Volmer–Weber (island by island) mode due to their lattice mismatch and high immiscibility at low T. Therefore, normally, few nm (2–3 nm) rough Pt NC surfaces are obtained when Ag templates are used,^9,13 as those shown in Fig. 1a. Thus, to promote PtAg alloying and a more epitaxial growth, several strategies were employed allowing metal inter-diffusion to play a significant role in NC synthesis.^27,28 Thus, by working at low T, using insoluble platinum salts (where Pt²⁺ is slowly solubilized and consumed maintaining a low but constant concentration of reactive Pt in solution), and using mild reducing agents, allows to decrease Pt²⁺ avidity for the Ag 5s electrons. It is also worth noting that in the mixture, as a by-product, there is generation of highly insoluble AgCl and AgBr compounds, which here are solubilized by the presence of surfactants.¹⁹ As a consequence of these reaction conditions, the island growth mode (rough surfaces) is avoided and interdiffusion is promoted allowing the formation of NC smooth surfaces exposing high index planes and PtAg alloys (Fig. 1d).

Fig. 1 TEM images of hollow (a) PtAg nanoflowers prepared by using H₂PtCl₆ as the precursor and BDAC as the surfactant, (b) Volmer–Weber type (island growth) of rough surface nanoboxes synthesized by using PtCl₂ as the precursor and CTAB as the surfactant. (c) PtAg dealloyed nanocages prepared by adding an excess of Pt salt. (d) PtAg@Pt alloy smooth surface nanoboxes prepared by using PtCl₂ as the precursor and BDAC as the surfactant, and (e) UV-vis spectra for the obtained NCs after addition of different volumes of added precursor (1 mM). (f) Time evolution of the UV-VIS resonance position peak during the formation of the nanoboxes shown in (d).

Thus, by using silver NCs as sacrificial templates, polyvinylpyrrolidone (PVP), benzylalkyldimethylammonium chloride (BDAC) and cetyltrimethylammonium bromide (CTAB) as surfactants and complexing agents, platinum salts (K₂PtCl₄, H₂PtCl₆, PtCl₂) as precursors and oxidizing agents, and ascorbic acid as the reducing agent, distinct hollow PtAg and Pt NCs were obtained (Fig. 1). We observed that Pt dosing was critical to control the reaction towards the desired products. Initially, when using K₂PtCl₄ as a precursor, controlled corrosion was not achieved (Fig. S1c†). However, when a more oxidizing form of Pt (H₂PtCl₆) was used, hollow PtAg nanoflowers with symmetrically distributed pores were systematically obtained (Fig. 1b). By modifying the reaction conditions, using CTAB as the surfactant and PtCl₂ as the precursor, which is poorly soluble in water, hollow NCs were formed displaying the formation of Pt islands at the NC rough surface (Fig. 1a), as a witness of a Volmer–Weber type growth due to the low affinity of the substrate for the deposited material.²⁹ This mode of growth can be seen when using a number of surfactants and precursors (Fig. S2†). In those cases, as expected, no degree of PtAg alloying was observed. However, using BDAC as the surfactant and PtCl₂ as the precursor resulted in the formation of single-crystal nanoboxes with smooth surfaces exhibiting high-index atomic planes, forming a stable and homogeneous alloy between Ag and Pt, coated by a thin overlayer of pure platinum (PtAg@Pt hollow NCs) (Fig. 1d and 2). Here, it seems that the Volmer–Weber growth was suppressed allowing an atom-by-atom Pt growth to take place.

Fig. 2 (a and b) HRTEM image of hollow PtAg@Pt nanoboxes synthesized at room temperature. The roughness is composed of monoatomic steps leading to high-index surfaces. (c) The power spectrum (FFT) shows that the NCs are monocrystalline and that no extra crystals are present. (d and e) Enlarged view of the yellow (lower left) and green (higher right) areas in (b). The red dots indicate the Pt atoms. (f) Schematic representation of (e) with alternating (yellow, green, and cyan) colours indicating the resulting projected stepped high-index crystal planes, along with their plane indices.

Under our conditions, the formation of these hollow NCs took up to 50 hours. Aliquots taken at intermediate times of the reaction showed intermediate corrosion states, in which we could appreciate also the formation of Kirkendall voids that disappeared in the final product (Fig. S1†). Finally, further oxidation of these structures with Pt²⁺ led to PtAg dealloying and the formation of highly porous Pt nanocages (Fig. 1c). During this time the colour of the solution changed with a strong redshift and a dramatic broadening of the absorbance spectrum, indicative of the formation of plasmonic cavities^13,30–32 (Fig. 1e and f). Note that these observed absorption bands are different from those obtained for other reported Pt–Ag NCs, which indeed consisted of layers of metals rather than a mixture (alloy) at the atomic level.³³

Fig. 2 shows high-resolution TEM images of the obtained PtAg@Pt smooth surface hollow NCs. The outer surface roughness is composed of monoatomic steps leading to highly reactive high-index surfaces (Fig. 2d–f, and Fig. S3 and S4†) associated with a known higher catalytic activity.^2,17,34 The atomic representation extracted from the atomic-resolved image corresponds to the 2D projection of the 3D structure. Therefore, the real roughness is smoothed by this integration and the real atom de-coordination (or very low coordination) is even higher. Note that for standard high performance Pt NCs their surfaces are much smoother.^16,35 For comparison, an example of a perfect {100} solid Pt nanocube of similar dimensions showing monoatomic flat surfaces^36,37 is presented in Fig. S5.† All these studied nanoboxes are monocrystalline (Fig. 2c) and narrowly dispersed with no other by-product present in the synthesis solution (Fig. 1d).

In Fig. 3 we present a detailed high-resolution Energy-Dispersive X-ray Spectroscopy (EDS) analysis of one of the lateral facets of a representative PtAg@Pt nanobox. The wall has a thickness of around 5 nm, the first – inner – 2.5 nm surface is composed of a PtAg alloy while the external – outer – 2.5 nm surface is mainly composed of Pt, as observed in the maps and the EDS profile (Fig. 3a–c). This anisotropy together with the formation of a stable PtAg solid solution are indicative footprints of the synthetic process. It is known that Pt in the bulk form does not readily undergo solid–solid diffusion with Ag at temperatures below 900 K.¹³

Fig. 3 (a) HAADF (S)TEM view of a PtAg@Pt nanobox. Right insets show detailed EDS analysis on one of the lateral walls. (b) EDS profile. (c) Full nanobox EDS maps. (d) XRD (111) peak of PtAg@Pt nanoboxes for different volumes of the Pt precursor added (1 mM). The diffraction peaks fall between the values of pure platinum (green dashes) and silver (red dashes) indicating that it is a metal alloy. Dashed (blue, red and black) lines indicate the position of diffraction peaks for each added volume of the Pt precursor, where can be seen its displacement towards the pure platinum value as the Pt/Ag ratio increases.

However, theoretical studies, and some limited experimental data have shown that miscibility between metal elements can be significantly altered at the nanoscale, including the formation of PtAg alloys in the immiscible area,^38–40 despite their different reduction kinetics and thermodynamic immiscibility. In fact, due to the higher number of vacancies and enhanced mobility at the nanoscale,²⁴ the formation of large PtAg-alloy single crystals at RT is enabled. Of course, when forced to mix at RT, Pt and Ag should relax the consequent crystal strains in highly atomic-uneven surfaces. These PtAg alloy NCs are then passivated and stabilized with a final Pt deposition (∼2–3 nm) which seals all the open galvanic gaps, as in the case of the originally reported Au nanoboxes.⁴¹ The PtAg alloy at RT is forcedly strained and therefore serves as a substrate for the deposition of the high-index Pt facets. The formation of the PtAg alloy is confirmed by the EDS spectra and the XRD patterns (Fig. 3d and S6†).

This synthetic strategy can be applied to different Pt/Ag compositions (Fig. 3d and S6 and S7†) and sacrificial templates (Fig. 4). Thus, using Ag spheres or wires as templates, both the immiscible Volmer–Weber (rough) growth and the generation of (smooth) PtAg@Pt single crystal hollow spheres and tubes could be achieved (Fig. 4). It is important to note that Pt nanotubes, because of their shape and aspect ratio, are very appealing since they could act, for example, as both catalysts and electrodes in fuel cells, where one main drawback is the degradation of the C electrodes that support the Pt catalytic centers.⁴²

Fig. 4 TEM images of spherical (a) smooth surface PtAg NCs and (b) rough surface Volmer–Weber type of grown NCs, both prepared by using K₂PtCl₄ as the precursor and PVP as the surfactant at different molar ratios, (c and e) show SEM and (S)TEM images, respectively, of smooth surface PtAg nanotubes that can be obtained with PtCl₂ as the precursor and CTAB as the surfactant or with K₂PtCl₄ as the precursor and PVP as the surfactant, while (d) shows the TEM image of nanotubes with the rough surface Volmer–Weber type of grown NCs obtained by using PtCl₂ as the precursor and BDAC as the surfactant.

As Pt surfaces are known to be excellent catalysts, we wanted to test the surface reactivity of different types of engineered Pt NC surfaces. We compared three different types of particles with different types of Pt NCs: (i) rough surfaces produced by a Volmer–Weber growth, similar to those reported by Zhang et al.,⁴⁰ (ii) smooth ones (PtAg@Pt nanoboxes) produced by strained epitaxial growth, and (iii) treated commercial Pt black. Note that here rough surfaces display high atomic coordination and low crystallographic index planes while smooth surfaces show low atomic coordination and very high crystallographic index planes. We use processed Pt black as the control material (see methods in the ESI†). The process consists of suspending in a PVP solution the Pt black as received, leaving it for a while and then discarding the sediment to select the fraction of the smaller NPs present in Pt black. Otherwise, large crystal and aggregates will further decrease the Pt black performance when compared to the hollow NCs. The overall reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) by sodium borohydride (NaBH₄) has been chosen as a model reaction, as established in the literature,^43–45 to evaluate the surface reactivity, related to the catalytic capability of our different Pt NCs. This reaction has been proved effective to test the efficacy and selectivity of several noble metal catalysts.^28,46,47 The reduction cycles were carried out under the same conditions for all samples and then normalized to the NC concentration, total mass and total surface area (Table S1†).

The conversion rate efficiency during 10 consecutive cycles of 25 minutes reduction (with no catalyst purification between each cycle) is shown in Fig. 5 (see experimental details). Fig. 5a plots the UV-vis spectra showing the decay of the p-NP signal of normalized absorption in the 300–500 nm visible range, where it can be clearly seen how the presence of Pt NCs in the solution leads to the rapid vanishment of the p-NP absorption peak, being faster when the smooth surface (high index planes) PtAg@Pt NCs are used. In contrast, the signal coming from the p-NP is much more stable in the presence of treated Pt black. An intermediate signal decay behavior is observed when Pt NCs with rough surfaces are used. These different behaviors are very clear when plotting the maximum absorption (at 400 nm) versus time (Fig. 5b) for each individual cycle. Here the behavior associated with the different surface reactivity of the NCs is clearly seen, and translates into faster reaction rates as we engineer the NC surface. This is properly assessed when calculating the apparent kinetic rate constants (k_app), normalized to the material mass (cost) and surface area (intrinsic activity per unit area), as plotted in Fig. 5c–e. The obtained data were analyzed according to the first-order rate law. The negative natural logarithm of the absorbance at 400 nm was plotted against time and the steepest part of the curve was fitted, the slope of which was considered the apparent rate constant k_app. It is clearly shown here the superior surface reactivity (up to two orders of magnitude) and higher stability of smooth (high index) surfaces vs. the rough (low index) or controls (Pt black). Regarding k_app, it is important to note its stability and repeatability, indicating the robustness of the PtAg@Pt NCs during operation, which allows us to conclude the absence of sample poisoning under the present conditions. The observed progressive decrease in reaction yield with time is attributed to the increased presence of products in the reaction vial as the number of cycles goes by. Importantly, the k_app remains constant throughout all the cycles. Note that despite possessing a well defined Pt surface composition, the presence of Ag in the inner layer as the remnant of the original Ag template may also have some contribution to the observed catalytic behavior. According to the literature, the presence of Ag in the Pt catalyst may improve, mainly, its structural robustness after catalytic cycling.³⁸ Other effects related to modifications of the Fermi levels due to alloying and proximity effects should not be discarded and will be the subject of future studies.

Fig. 5 Conversion rate efficiency in the overall degradation of p-nitrophenol into p-aminophenol in the presence of Pt NCs in 10 consecutive 25 minutes reduction cycles. (a) Decay of the normalized absorbance in the 300–500 nm visible range, indicating the reduction of nitrophenolate ions when using the three different surface shaped PtNCs: Pt ‘Smooth’, Pt ‘Rough’ and Pt ‘Black*’ (*treated commercial Pt black) (b) decay of the normalized absorbance at 400 nm. Since the Pt ‘Rough’ and Pt ‘Smooth’ catalysed reactions involved the same amount of total nanoparticles while the Pt ‘Black’ one had 32 times more (see Table S1†), the absorbance of nitrophenolate when using Pt black was re-normalized (in terms of NCs per mL) for its comparison. The k_app values obtained for each PtNC in each reduction cycle were plotted against its respective cycle number after normalizing them as a function of (c) total number of NCs, (d) total amount of Pt surface and (e) total amount of Pt mass needed to carry out the reaction.

Pt NC's stability was further studied by analyzing the morphology and performance of the NCs before and after the reduction cycles and thermal stress. For the later, the NCs, prepared at RT, were boiled at 100 °C for 24 hours, exposed to e-beam showers (field emission 300 keV) for 30 minutes, and kept under intense artificial sun exposure for 7 consecutive days. In any case, no morphological alteration was detected (see TEM images in Fig. S9†). Additionally, after purification at the end of the 10 reduction cycles or the thermal processes, recovered NCs just performed as initially. This observed stability could be related to the pinning of the high-index planes by the underneath strained PtAg alloy and the hollow nature of these NCs (since heat is preferably generated in the bulk and dissipated at the surface). Also, it has been observed how the presence of Ag in Pt stabilizes the Pt nanostructure, what has been attributed to the shift of the d-band center of Pt in PtAg alloys.³⁸ This is important since nanobox collapse and dealloying during use would be otherwise expected.

In summary, by slowing down the reaction and controlling Pt dosing, we can obtain a set of Pt NCs with increasing reactivity per unit mass and area. This strategy enables the control of the ratio between Pt and Ag in the alloy with high morphological and structural quality, and the process provides novel insight into the advanced control of matter at the nanoscale. Finally, this process is also advantageous because it is carried out at room temperature, which results in a more cost-effective and scalable process.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors acknowledge funding from grants 2009 SGR 770, 2009 SGR 776, 2014 SGR 612, 2014 SGR 1638, XaRMAE, CSD2009-00013, MAT2006-13572-C02-02, MAT2009-14734-C02-01, MAT2010-15138, MAT2012-33330, MAT2014-51480-ERC, VALTEC09-2-0085 and VALTEC09-2-0089. J. A. acknowledges NanoAraCat funding for the use of advanced (S)TEM facilities at Instituto de Nanociencia de Aragón–Laboratorio de Microscopia Avanzada. N. G. B. acknowledges financial support through the Juan de la Cierva program (JCI-2011-09678) and European Commission for the Career Integration Grant (322153-MINE) Marie Curie Action. A patent application related to this work was filed on 14 March 2011 (European Union application number EP11158024.7).

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Footnote

† Electronic supplementary information (ESI) available: Details about the preparation, characterization and analysis of the surface of hollow nanocrystals together with the evaluation of the surface reactivity. See DOI: 10.1039/c5ta07504a

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