Photocatalytically Prepared Metal NanoclusterâOxide Semiconductor

Perspective pubs.acs.org/JPCL

Photocatalytically Prepared Metal Nanocluster−Oxide Semiconductor−Carbon Nanocomposite Electrodes for Driving Multielectron Transfer Krishnan Rajeshwar,*,† Csaba Janaky,*,‡ Wen-Yuan Lin,§ David A. Roberts,§ and Wesley Wampler§ †

Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065, United States Department of Physical Chemistry and Materials Science, University of Szeged, Szeged H6720, Hungary § Sid Richardson Carbon and Energy Company, Fort Worth, Texas 76106, United States ‡

ABSTRACT: Heterogeneous photocatalysis can be used to generate metal-nanoclusterdecorated oxide semiconductor−carbon nanocomposite matrixes for driving multielectron processes of practical import. The oxide semiconductor nanoparticles in such assemblies not only facilitate heterogeneous photocatalytic deposition of the metal nanoclusters but have several important functions that are highlighted in this Perspective. This Perspective additionally describes structure−property relationships of various mono-, bi-, and trimetallic electrocatalysts and the roles of the carbon support and the oxide semiconductor in the performance and durability of the overall architectures. Further applicability of such nanocomposites in value-added environmental remediation, such as the conversion of carbon dioxide to alcohol fuels, is discussed.

M

conductive carbon network to afford efficient carrier transport through it, and (c) has high surface area that is ideally suited to sequester and immobilize the reactant species to be electrochemically converted. The architecture as a whole thus has the virtue of supporting cascaded multielectron transfer in an efficient manner. These nanocomposite architectures are allinorganic in chemical nature, providing for intrinsic chemical and electrochemical stability, unlike their organic or biological counterparts in Nature. Even under demanding conditions as in an operating fuel cell environment, it will be shown that the oxide semiconductor imparts a stabilizing effect on the carbon component and thus on the overall stability of the nanoarchitecture. Nanocomposite Architecture, Applications, and Preparative Aspects. Most generally, the nanocomposite may be denoted as M1,M2,M3−MxOy−C, where the metal nanocluster component may consist of up to three different metals, the oxide has the general compound stoichiometry metal/oxygen of x/y, and C denotes the carbon component. The discussion below will illustrate the advantages of utilizing multiple metals in the nanocluster, and while the present discussion focuses only on metal oxides (and, more specifically, on oxide semiconductors; see below), the nanocomposite architecture is flexible enough to accommodate other types of semiconductors such as metal chalcogenides (e.g., CdX where X = S, Se, or Te).

ost electrochemical reactions of practical import such as CO2 splitting, proton reduction, and water oxidation are multielectron in nature with considerable kinetic barriers to electron transfer. They therefore require the use of carefully designed electrode surfaces to accelerate electron-transfer rates to levels that make practical sense in terms of the required energy input. Thermodynamically downhill processes such as dioxygen reduction and hydrogen (or alcohol) oxidation are also multielectron in nature, requiring the similar use of electrocatalysts. Nature has taught us valuable lessons on how to assemble such complex architectures with components that have precisely defined functionality and complementarity. Manmade structures, however, differ in two important aspects: (a) They obviously do not have the luxury of evolving over hundreds of years to achieve a targeted function because technology demands dictate drastically shortened time windows. (b) Manmade assemblies are further constrained by economic considerations, and cost always is interwoven with performance. A further distinction is that material choice and design in manmade assemblies must take chemical (or electrochemical) stability into account, whereas their counterparts in Nature have redundancy and efficient self-healing built in to counter the intrinsic instability and fragility of natural components. This Perspective focuses on one such class of nanocomposite electrochemical assemblies with metal nanoclusters (or multiple metal nanoclusters), an oxide semiconductor, and carbon as the constituents. The particular nanocomposite electrode matrix to be discussed below (a) facilitates effective dispersion of the metal nanoclusters, (b) is percolated with an electronically © 2013 American Chemical Society

Received: July 12, 2013 Accepted: September 26, 2013 Published: September 26, 2013 3468

dx.doi.org/10.1021/jz401455j | J. Phys. Chem. Lett. 2013, 4, 3468−3478

The Journal of Physical Chemistry Letters

Perspective

an aqueous medium contacting the (dissolved) metal (or multiple metal) precursor ions. The semiconductor property of the oxide semiconductor is then profitably used to generate electron−hole pairs via its photoexcitation. While the holes can be scavenged (by a reducing agent such as formate), the electrons can be used to photoreduce the metal ions. This Perspective mostly focuses on nanocomposite systems that were derived by this heterogeneous photocatalysis route. It is worth pointing out that our initial papers9−12,25 on this topic were the first to underline the synthetic utility of heterogeneous photocatalysis for deriving complex architectures based on inorganic semiconductors. (Contrastingly, heterogeneous photocatalysis has been popularly used for environmental remediation applications instead.) Scheme 1 illustrates the basis of the photocatalytic synthesis approach where the respective redox levels for the metal ion precursor species as well as an estimated electronic level for the carbon black phase are shown. The first serendipitous aspect emerging from our initial studies was the finding that metal nanocluster formation is not localized to the oxide phase in the resultant nanocomposite; rather, photodeposition of the metal occurs throughout the nanocomposite surface. Figure 1 contains a representative electron micrograph illustrating this important fact. This data trend corroborates that (a) initial photoelectron transfer across the oxide−carbon interface in the nanocomposite is very efficient and (b) these electrons then percolate throughout the carbon network via its delocalized electronic level manifold and are available for photoreducing metal ions at the numerous solid−solution interfaces in the network. These results also underline the excellent electronic contact that exists between the oxide semiconductor and the carbon components. This is clearly a prerequisite for any practical application of the nanocomposite, as further elaborated below. In situ dispersion of all of the components (as outlined above) achieves this goal. In our hands, the aqueous suspension is introduced into a custom-designed photoreactor, where photocatalytic deposition takes place under UV light illumination and constant N2 purging. Other details may be found in our original papers.9−12,25 The second, rather surprising aspect of these studies was our finding that the metal nanocluster size was significantly finer in the case of the nanocomposite relative to the situation when only the oxide semiconductor was present. In other words, the presence of the carbon component has the net effect of altering the nucleation−growth dynamics such that the metal nanoclusters do not grow in size. This trend is elaborated upon in Figure 2. It must also be emphasized that the trend seen in this comparison that chemical reduction of the metal ion precursor results in larger nanocluster size than that in the photocatalytic synthesis case arguably may not be a general one and is probably specific to our own experience.

Similarly, the choice of the carbon component in the present discussion follows the practice adopted in the polymer electrolyte fuel cell (PEFC) and Li ion battery communities, namely, of using high surface area, low-cost conductive carbons such as carbon black. The use of other types of carbon in nanocomposites such as fullerenes and graphenes in conjunction with oxide semiconductors is the subject of a recent Perspective.1 Economic considerations also drive the need for minimizing (or altogether avoiding) the extent of use of noble metals (such as Pt) in the M nanocluster component, as further elaborated below. Clearly, the nanocomposite architecture offers considerable scope for tailoring its morphology, chemical composition, and properties to fit a targeted application, and the range of possibilities perhaps is only limited by the ingenuity of the researcher(s).

Clearly, the nanocomposite architecture offers considerable scope for tailoring its morphology, chemical composition, and properties to fit a targeted application, and the range of possibilities perhaps is only limited by the ingenuity of the researcher(s). Table 1 provides a compilation of such nanocomposite systems that have been studied; this compilation, as with the others in this Perspective, is by no means exhaustive but only representative of the range examined. As seen in Table 1, the metal nanocluster component has been exclusively noble-metalderived (for obvious catalytic reasons, see below) and the oxide component spans an entire range from an electrical insulator (SiO2) to a semiconductor (TiO2, ZnO, WO3) and to an electronic conductor (MnO2). In the compilation in Table 1, early studies (even dating back to 1964) involving metal−metal oxide combinations (specifically Pt−WO3),2 where carbon was not an integral component except perhaps as an electrocatalyst support, are largely omitted. These electrode materials were mostly examined for their applicability to hydrogen oxidation. Of the metal oxides, WO3 and TiO2 clearly dominate in Table 1. The vast majority of applications targeted for these nanoarchitectures center around energy conversion, although there are also isolated instances wherein application for energy storage (as in ultracapacitors) has been considered.24 It is worth noting that in some cases, the application is value-added, as, for example, when an environmentally noxious greenhouse gas such as CO2 is converted to energy-rich alcohol products.9 Turning to preparative aspects, the least desirable route is to simply mix the separate constituents in a mechanical manner. More generally, a metal precursor (usually a complex) is used along with the metal oxide precursors, and the metal nanoclusters and the oxide are deposited in situ on the carbon matrix that is also additionally present in the reaction medium. Chemical reduction of the metal precursor and sol−gel synthesis of the oxide component have been popular routes as Table 1 illustrates, although other methods (e.g., chemical vapor deposition)15 have been used as well. Another versatile synthesis variant initially disperses both the oxide and carbon components ultrasonically (or otherwise) in

The presence of the carbon component has the net effect of altering the nucleation−growth dynamics such that the metal nanoclusters do not grow in size. On the basis of careful Scherrer analyses10 of the observed XRD peak widths, other trends can be deduced. First, the 3469


3470

TaOx ZnO

SiO2 SnO2

Nb2O5

NbO2

MnO2

TiO2

WO3

metal oxide

Pt Pt

Pt or Rh

Vulcan XC-72 glassy carbon Vulcan XC-72

Pt or Pt− Pb Pt Pt

in situ generated graphitic carbon Vulcan XC-72 Vulcan XC-72

Pt Pt

Pt

CNT array Vulcan XC-72

Vulcan XC-72

rutile powder

Pt

Pt

Vulcan XC-72

commercially obtained (Degussa-P25)

12

ORR

ethanol oxidation ORR ORR

modified polyol method (as in the sixth entry above) electrodeposition photocatalytic deposition

chemical reduction photocatalytic deposition

formic acid oxidation PEFC ORR

ORR ORR

ORR or CO stripping ORR

CO oxidation (stripping) PEFC

10 11

23 21

22

20 21

19

17 18

16

15

14

13

9,

8

6 4 7

5

4

3

ref

ORR and DMFC ORR and CO2 red. ORR

ORR PEFC PEFC

ORR and PEFC

PEFC

H2 oxidation

application

one-pot synthesis; all of the components are formed in parallel

magnetron sputtering chemical reduction with NaBH4

commercially available HiSPEC4000 catalyst

photocatalytic deposition using two different Pt precursors, namely, alcoholic platinum acetyl-acetonate or aqueous/alcoholic H2PtCl6 solution two-step chemical vapor deposition (vapor phase impregnation-decomposition)

photocatalytic deposition

Vulcan XC-72R

MWCNT or Vulcan XC-72

sol−gel route

wet chemical reaction of manganese sulfate and sodium persulfate NbO2 is magnetron sputtered onto the CNT array three different procedures used for preparing oxide nanoparticles one-pot synthesis; NbCl5/NbOEt5 is used as precursor sol−gel synthesis sol−gel synthesis from tin isopropoxide in isopropanol modified polyol method followed by thermal treatment electrodeposition precipitation

Vulcan XC-72, Sid Richardson Co. carbon blacks

chemical reduction of chloroplatinic acid acid with sodium formate

Pt and Au Pt, Au and Pd Pt

Pt

Vulcan XC-72R

photocatalytic deposition chemical reduction polyol synthesis using H2PtCl6·6H2O as precursor

as above

photocatalytic deposition

Pt Pt Pt

? E-Tek Vulcan XC-72R

deposition method freeze-drying followed by thermal decomposition in vacuum or by H2 reduction (at 300 °C, 2h) chemical reduction

Pt

Pt

Vulcan XC-72

E-Tek

Pt and Ru Pt

metal(s)

Vulcan XC-72R

carbon

commercially obtained (Degussa-P25)

W dissolved in excess H2O2 (30%) and excess peroxide is decomposed with Pt black nanoscale oxide is prepared from the reaction of sodium tungstate with HCl oxide prepared as above using sodium tungstate as precursor ? hydrolysis of titanium isopropoxide in ethanol modified polyol synthesis route using Ti(OBu)4 as precursor hydrolysis of titanium isopropoxide in isopropanol

preparation method

Table 1. Representative Studies Illustrating the Range of Materials Used for Metal Nanocluster−Metal Oxide−Carbon Nanocomposites and Their Applications

The Journal of Physical Chemistry Letters Perspective



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Scheme 1. Photocatalytic Deposition of Metal Nanoclusters in the Nanocomposite Using TiO2 As an Example of the Oxide Semiconductor Componenta

a

Formate ions serve as the hole scavenger species in this scheme, and NP denotes a nanoparticle.

thermal history (e.g., surface chemical treatment or thermal graphitization), the metal nanocluster size is influenced. This presumably is because of different surface area and conductivity and the sensitivity of these properties to the carbon prehistory. Similar findings have been reported by other researchers.13 We also note the vast literature that exists on the importance of characteristics associated with the carbon support in electrocatalysis applications (for example, see the review in ref 27 and references cited therein). The relative amount and dispersion of TiO2 have an important effect (data also not shown here); therefore, the ratio of carbon/TiO2 has to be optimized. Finally, even the method of oxide preparation may have an impact on metal nanocluster size; clearly, many variables contribute toward the optimal morphology of the metal nanocluster−oxide semiconductor−carbon nanocomposite architecture, as further elaborated on in a subsequent section below. The importance of the electronic nature of the numerous interfacial contacts that exist in these nanocomposites cannot be overemphasized, and we return to this important mechanistic aspect below. The photocatalytical synthesis route is also versatile in that multiple metal nanoclusters can be incorporated into the nanostructure. As discussed in more detail below, for bimetallic catalysts, three variants of the photodeposition procedure can be envisioned, two when the metals (e.g., Pt and Au) are photodeposited in a sequential manner (Pt first followed by Au

Figure 1. Representative high-resolution transmission electron micrograph illustrating homogeneous photodeposition of the metal nanoclusters (dark spots) throughout the nanocomposite whole. Two metals (Pt and Au) were photodeposited in this example, and NP stands for nanoparticles. The carbon black was graphitized by thermal pretreatment.

average nanocluster size gradually increases with increasing metal content, perhaps not unsurprisingly (Figure 2B). Second, if our generally used carbon black is replaced by other types of carbon or if a particular carbon black is presubjected to different

Figure 2. Powder XRD patterns (A) and average nanocluster sizes of Pt (B) as a function of deposition method (photocatalytic versus chemical) and metal loading. A carbon-free photodeposition example was also included for comparison; refer to the text. (A) is reproduced with permission from J. Electrochem. Soc. (ref 10). Copyright 2008, The Electrochemical Society. 3471



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Table 2. Examples of Performance Enhancement in Selected Applications of Metal Nanocluster−Metal Oxide−Carbon Nanocomposites Relative to Cases When the Oxide Component Was Absent entry no. 1 2 3 4 5 6 7 8 9 10

comments

ref

Fuel cell performance of Pt−WO3 and Pt−TiO2 catalysts at 80 °C in humidified H2 and O2 increased with added oxide content up to a certain extent. Some of the Pt−TiOx−C composites (see the text) exhibited higher catalytic activity than Pt−C for the ORR. Pt−TiO2−C nanocomposites showed higher electrochemical activity for CO2 reduction relative to massive Pt foils of comparable geometric area. Pt−TiO2−C samples prepared by chemical vapor deposition showed a higher electrochemically active surface area and better catalytic activity for ORR compared with the Pt−C counterpart. A PEFC with an optimized Nafion composite membrane afforded a peak power density five times higher than that with a commercial Nafion 1135 membrane. The Pt−oxide−carbon materials showed higher ORR activity than the Pt−C counterpart; three different oxides, namely, TiO2, SnO2, and ZnO, were investigated. Pt−Rh−SnO2 electrocatalysts showed better selectivity than Pt−Rh, Pt, or Pt−SnO2 counterparts for ethanol oxidation. The Pt−NbO2−C electrocatalyst showed three times higher Pt mass activity for ORR than a commercial Pt−C electrocatalyst. Pt−TaOx−GC showed enhancement of ORR activity up to 12 times the activity of the unmodified Pt−GC counterpart under similar conditions. Activated carbon modified with TiO2 showed increased specific capacitance (measured at 0.65 mA/cm2) from 47.2 to 63.1 F g−1 when the composite was studied in an electrochemical capacitor environment.

4 8 9 15 20 21 22 18 23 24

Figure 3. Carbon corrosion tests for SIDCAT 451, SIDCAT 452, and other commercially available catalysts. (A) Chronopotentiograms recorded at a current density of 0.4 mA/cm2 with a catalyst loading of 0.4 mg/cm2 in all of the cases. (B) Fluoride ion emission rates for various fuel cell electrocatalysts obtained at 80 °C and 75% relative humidity (RH). (B) is reproduced with permission from J. Electrochem. Soc. (ref 10). Copyright 2008, The Electrochemical Society.

note instances where the high ohmic resistance of the oxide component detracted from the overall performance; see, for example, ref 5. Thus, electrochemical impedance spectroscopy revealed that the presence of WO3 caused the ohmic resistance of Pt−C to increase to 25 from 13 mohm.5 That the oxide component was the culprit was confirmed by separate conductivity measurements on WO3 pellets.5 As discussed in the next section, another key virtue of oxide incorporation is enhanced durability of the electrode when operated in harsh conditions typical of a fuel cell environment. Improved durability was observed in PEFCs without external humidification when the oxide component (specifically TiO2) was present.7 It was shown that the oxide plays an important role in increasing the uptake of water by the membrane.7 Thermal treatment and durability tests additionally revealed that the Pt nanoclusters did not agglomerate when the oxide was present in the nanocomposite.7 Immunity from electrode poisoning also has been noted in some studies in this context.8 Thus, Pt−TiOx−C electrocatalysts exhibited better methanol tolerance than the Pt−C counterpart.8 Finally, there are also indications in the literature (for example, see refs 18 and 26) that the amount of noble metal catalyst can be reduced in the added presence of the metal oxide; this is a crucial advantage considering that cost remains a prohibitive factor in the market penetration of PEFCs.

or vice versa) or the third variant when they are photodeposited simultaneously from one deposition “bath”.11 As elaborated on later, the morphology (structure) of the resultant nanoarchitectures presents features that crucially influence their performance and durability in a given application, such as for the oxygen reduction reaction (ORR). The trimetallic case also presents similar interesting synthetic implications.12 It is also worth noting that the metals making up the nanocluster phase need not be noble metals. The architecture is versatile enough to incorporate other catalysts (even nonmetallic ones!), as exemplified by the transition-metal candidate, namely, Cu, shown in Scheme 1 above. Within the context of this Perspective, perhaps the only constraint is that they can be photodeposited on the oxide semiconductor surface. The search for alternatives to noble metal candidates obviously has an economic incentive. Nanocomposite Performance. Why are these nanocomposites of particular interest and featured in this Perspective? Table 2 contains examples of performance enhancement of these newgeneration electrode materials relative to cases where the oxide component was not present. It is impressive that these nanocomposites show superior performance whether the application considered is dioxygen reduction (fuel cell), hydrogen oxidation (fuel cell), alcohol oxidation (fuel cell), CO2 reduction (environmental remediation/electrosynthesis), or charge storage (electrochemical capacitor). However, we do 3472



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Durability in the PEFC Environment. Catalyst degradation and limited durability are two problems that hamper long-term use and thus the commercial application of PEFCs. Three key issues are as follows:28,29 • Electrode corrosion (both anode and cathode): Carbon support corrosion has been identified as one of the major mechanisms of PEFC performance degradation in automotive applications. • Platinum instability: Pt leaching occurs because of its solubility at high potentials (>0.6 V versus Ag/AgCl). • Chemical attack of the membrane: The membrane, commonly made of a synthetic polymer (e.g., Nafion), is prone to chemical attack by free radicals. These free radicals are formed by the decomposition of hydrogen peroxide (H2O2) possibly generated during PEFC operation. These reactions are not independent of one another. For example, the presence of Pt (and other noble metals) increases the corrosion rate of the carbon catalyst support, but as carbon gets corroded, metal nanoparticles may leach out or agglomerate, forming larger particles.28 Incorporating an oxide semiconductor into the electrode matrix has at least two very different beneficial effects: (a) On the basis of its interaction with the metal nanoparticles (see below), both the stability and catalytic efficiency may be improved; (ii) the oxide may enhance membrane durability by decomposing H2O2. Figure 3 illustrates the substantial effect of TiO2 incorporation on both electrocatalyst durability (Figure 3A) and membrane stability (Figure 3B). The durability was assessed by comparison of chronopotentiometric polarization curves of commercially available PEFC catalyst samples and those containing TiO2 prepared with our photocatalytic method. As seen in Figure 3A, the durability of the oxide semiconductorcontaining electrocatalysts is much longer compared to their oxide-free counterparts. Fluoride emission rates (FERs) in our own studies10 were an order of magnitude smaller for the TiO2containing catalyst (note the logarithmic scale in Figure 3B), signaling that the oxide component was effective in quenching the free radicals formed in the ORR process. In another study on Pt−WO3−C, 70% reduction in the FER at the anode and 60% FER reduction at the cathode was noted compared to membrane electrode assemblies (MEAs) prepared with Pt−C alone, without the oxide component.16

Table 3. Effect of Carbon Black Pretreatment and Metal Composition on Electrocatalyst Durability carbon SR159 SR159 SR159 SR159 SR159 SR159

Φ Φ Φ Φ Φ

1800a 1800 1800 1800 1800 (sol−gel TiO2)b

metal composition

durability (s)

Au 5%, Pt 15% Au 5%, Pt 15% Pd 5%, Pt 15% Pd 15%, Pt 15% Au 5%, Pd 15%, Pt 15% Au 5%, Pd 15%, Pt 15%

9100 15 000 40 000 55 000 81 700 236 000

a

The blacks were thermally graphitized prior to incorporation in the nanocomposite. In all of the cases except the sample in the last row, commercial TiO2 (Degussa P25) was also present at 10 mass %. bThe commercial oxide was replaced by oxide grown by the sol−gel technique

carbon black suspensions by the sol−gel method. These samples were even more durable than the best samples based on Degussa P25, most likely because of the intimate contact between the carbon and the metal oxide components, afforded by the in situ nature of nanocomposite preparation. Electrocatalysis of Dioxygen Reduction and Multiple Metals in the Nanocomposite. As mentioned earlier, for bimetallic electrocatalysts, three variants of the photodeposition procedure (Scheme 1) can be envisioned, two when the metals (Pt and Au in our studies) are photodeposited in a sequential manner and one when they are photodeposited simultaneously. Figure 4 contains hydrodynamic voltammograms for three

The level of durability is strongly dependent on the composition (oxide semiconductor content) as well as on the properties of the carbon black.

Figure 4. Hydrodynamic (RDE) voltammograms for four selected electrocatalyst samples supported on C−TiO2. The rotation speed was 1500 rpm; for other details see ref 11. Reproduced with permission from J. Electrochem. Soc. (ref 11). Copyright 2010, The Electrochemical Society.

The level of durability is strongly dependent on the composition (oxide semiconductor content) as well as on the properties of the carbon black. Several different carbons were studied, and it was concluded that thermal pretreatment (at 1800 °C) has a beneficial effect (Table 3). The durability of different electrocatalysts using heat-treated carbon blacks is compared in this table. As seen, successive addition of other metals besides Pt also increases the durability up to 81 700 s. Finally, even the preparation method affects the durability of the electrocatalyst assembly, perhaps not unsurprisingly. Besides using commercial TiO2 (Degussa P25), as in our initial studies,9-12,25 TiO2 was also deposited in situ in different

bimetallic species (Pt/Au, Au/Pt, and Au + Pt) and one with only Pt taken as a benchmark, all four samples with 20 mass % total metal content and 5 mass % TiO2. As seen, the Pt/Au− C−TiO2 sample (Au deposited first, then Pt) outperforms the other bimetallic and only-Pt-containing samples. This superior behavior is also confirmed by quantitative assessment of electrocatalytic performance such as the kinetic current density (jk) (Table 4). This parameter was obtained from the hydrodynamic voltammetry data using Koutecky−Levich plots, as discussed elsewhere.10,11 3473



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D). As a further example, careful analysis of the lattice fringes confirmed the seeding role of Au nanoparticles in the assembly of ternary (Pt/Pd/Au) nanoarchitectures (Figure 6D). Note that similar structures can be also revealed for the simultaneously deposited bimetallic sample (Figure 6B) but to a much smaller extent. These nanoscopic morphological differences were also confirmed by high-resolution XPS (Figure 7), where the Au 4f signals were substantially attenuated in the Pt/Au case because of Pt nanoparticle coverage on the Au “core”. For the Au/Pt samples, intense signals can be seen for both metals because both of them are dispersed at the entire C−TiO2 surface. These striking morphological differences underpin the factors that control the relative electrocatalytic activity of nanocomposites containg multiple metal nanoclusters. The enhancement in the electrocatalytic activity described above may be partially rooted in the increased electrochemically active surface area (ECSA) of the samples containing added metals other than Pt. To probe the ECSA, cyclic voltammetry (CV) analyses were performed in the potential regime spanning hydrogen adsorption (Figure 8). Note that although the total metal content is constant, striking differences are seen for the three samples. By gradually replacing Pt with Pd and Pd + Au, increasingly higher CV currents can be detected. Also note that the presence of Au at the electrode/ electrolyte interface cannot be detected in this potential window because of the absence of H adsorption on this metal. Therefore, the enhancement related to Au can only be related to the smaller particle size and the better accessibility of Pd and Pt active sites in the nanocomposite. That the ECSA is influenced by the oxide component is irrefutably seen even in early studies4 on Pt−WO3−C nanocomposites. However, the metal nanocluster size plays a crucial role here; for example, later studies5 on the same system report a 50% lowering of ECSA (as probed by CV). This decrease was attributed to an increase in Pt cluster size when the oxide component was introduced.5 Generally, increases in ECSA do not simply translate into increased electrocatalytic activity in all cases, as borne out by other studies on a different system, Pt−TiOx−C.8 Thus, some samples showed lower activity in a methanol fuel cell environment even though they had higher ECSA compared to their Pt−C counterparts. This anomalous trend was attributed by the authors8 to electronic interactions between the metal component and the substrate that are now addressed in the next section.

Table 4. Comparison of ORR Kinetics Parameters of the Electrocatalysts Shown in Figure 4 sample Pt−C− TiO2 Pt + Au− C−TiO2 Au/Pt− C−TiO2 Pt/Au− C−TiO2

jk (mA/cm2)

jk (A/mgPt)

E1/2 (V vs SHE)

Pt content (μg/cm2)

27

1.4

0.68

20.0

0

25

1.7

0.69

15.0

5.0

21

1.4

0.67

15.0

5.0

34

2.3

0.73

15.0

5.0

Au content (μg/cm2)

The electrocatalytic performance of the various (including mono-, bi-, and trimetallic) samples is compared in Figure 5. This comparison allows assessment of the role and the contribution of each metal component to ORR electrocatalysis. Three sets of hydrodynamic voltammograms are presented, and in each set, one metal is common to all samples, and other metals are added stepwise. As seen in Figure 5, addition of the second and third metal components always improves the performance, albeit to a varying extent. In the case of Au (Figure 5A), a dramatic increase was observed upon adding Pd or Pt; for Pd (Figure 5B), a smaller but still remarkable enhancement can be seen especially upon addition of Pt, whereas for the Pt-containing samples, a small (Figure 5C) but significant improvement was witnessed. These trends reflect the fact that ORR activity decreases in the order Pt > Pd > Au. Finally, note that the trimetallic nanocomposite electrocatalyst outperforms its bimetallic and monometallic counterparts. The order of deposition was also investigated for the trimetallic catalyst (there are many more options than those for the bimetallic samples); sequential deposition of Au, Pd, and Pt was found to be most optimal (in this order) for ORR activity.12 Morphological (HR-TEM) and spectroscopic (XPS) studies11 have uncovered important structure−property relationships for these electrocatalyst assemblies and shed further light on the mechanistic aspects of the photocatalytic deposition approach. In the case of the first sequentially deposited sample (Figure 6A, Au/Pt), when Pt was deposited first, the coexistence of individual nanoparticles can be seen, having little influence on one another. On the other hand, when gold is deposited first (Pt/Au sample), Pt deposition occurs predominantly on the initially photodeposited Au seeds, resulting in “raspberry-like” superstructures (Figure 6C and

Figure 5. Comparison of RDE data for different mono-, bi-, and trimetallic electrocatalysts in ORR. Each of the frames (A−C) shows the data starting with a different metal, Au, Pd, and Pt, respectively. Note that these data merely underline the catalytic effect exerted by each individual metal; the effect of varying the deposition sequence is not addressed here per se. 3474



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Figure 6. HR-TEM images contrasting (A) Au/Pt−C−TiO2, (B) Au+Pt−C−TiO2, and (C) Pt/Au−C−TiO2 electrocatalyst assemblies. (D) is a higher magnification image of a similar raspberry-like superstructure, observed for a trimetallic (Pt/Pd/Au) sample.

It has been shown above that the oxide component in the nanocomposite results in better (a) electrochemical activity for a given application (e.g., ORR or hydrogen oxidation), (b) durability, and (c) immunity from poisons such as CO. Now, we examine chemical and electronic factors underpinning these positive trends, bearing in mind that these mechanisms have long been under discussion within the more general context of heterogeneous catalysis, that is, even in cases where electrochemistry is not an inherent factor in the catalytic activity. Thus, strong metal support interaction (SMSI),30 atom spillover,31 and site proximity effects32 have long been discussed in heterogeneous (thermal) catalysis. Even in the early literature on Pt−WO3 catalysts, their relative immunity to CO poisoning was rationalized in terms of a bifunctional mechanism (see, for example, papers cited in ref 3), where CO and OH species are bound on neighboring Pt and oxide sites, respectively, thus promoting O transfer and oxidation of CO to CO2. The proclivity of Pt−Sn alloys to promote methanol oxidation (without CO poisoning) has similar roots in such site proximity effects. Atom spillover effects have also been rationalized in these early studies.3,4 For example, hydrogen oxidation on Pt−WO3 catalysts where the adsorbed H atoms on Pt sites are transferred to the oxide, forming tungsten bronzes.33 Even combinations of the atom spillover and the bifunctional catalysis mechanism have been invoked.3 Note that in these early studies, the carbon component does not exert a direct effect except in some instances as a support for the metal and oxide components. This is in direct contrast to the systems discussed here where the carbon is an active component in the nanocomposite electrode matrix. The SMSI mechanism is rooted in electronic interactions between the metal nanoclusters and the oxide component, and the landmark paper30 considered noble metals and transition metals in contact with TiO2, used here as a metal catalyst support. The SMSI mechanism has been discussed for Pt−

Figure 7. High-resolution XPS data in the Au and Pt 4f binding energy region for three different bimetallic electrocatalyst samples.

Figure 8. Cyclic voltammograms of three nanocomposite electrocatalysts at 20 mV/s in the potential region of H adsorption. The total metal content was 50 mass % in all cases.

Electronic and Other Interactions between the Metal Nanoclusters and Metal Oxide and Carbon Nanocomposite Components. 3475



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TiO2−C6,34 and Pt−WO3−C6 with the aid of XPS data. Shifts in the Pt 4f, W 4f (in the case of WO3), and Ti 2p (in the case of TiO2) signals and asymmetry of the Pt 4f peak on an oxide support (relative to the case when the metal was supported solely on carbon) were interpreted in terms of changes in the local electron density on the metal center.6 However, partial charge transfer alone could not account for the observed data trends, and alloy formation between Pt and Ti (or W) was invoked.6,35 Analysis of the XRD line profiles (via, for example, a Williamson−Hall plot) has also yielded further evidence for strong metal−metal oxide interactions.36 Changes in the lattice parameter from the bulk Pt case when the carbon and oxide components are additionally present were invoked to signal alloy formation between the metal and the metal in the oxide.35,36 These interactions involving the metal nanoclusters are not confined to the oxide component in the nanocomposite. Evidence for strong interaction between Pt nanoparticles and graphitic carbon is furnished by electrochemical CO stripping experiments.13 The delocalized π electron system of graphitic carbon is viewed to anchor the Pt particles in such a way that the metal valence band is significantly perturbed.13 Modification of Pt with transition metals such as Ni, Co, Fe, V, or Ti has been reported to yield higher electrocatalytic activity for ORR relative to pure Pt (for example, see papers cited in ref 23). Several hypotheses have been advanced to explain this enhancement of catalytic activity.23 A lattice contraction of the Pt−Pt bond distance is proposed to promote ORR by producing more sites for dissociative adsorption of dioxygen.23 Other studies and evidence from X-ray probes such as XANES18 suggest that the enhanced ORR activity is related to inhibition of adsorbed OH species. This inhibition has been attributed to lateral repulsion between Pt−OH and oxide surface species.18 Indeed, such mechanisms have been proposed to explain the positive role of TaOx or NbOx in enhancing the ORR electrocatalytic activity of Pt.18,23 Atom spillover and promotion of the d orbital vacancy of Pt for dioxygen adsorption by electron donation to Ta have also been invoked.23 The preceding discussion shows that the situation is far from clear-cut, and more than one mechanism could play a role in explaining the performance enhancement noted for M−MxOy− C nanocomposite electrocatalysts. Equally, this section also makes it clear that techniques beyond routine electroanalysis are needed to unravel these complications, and spectroscopic and structural analysis probes hold the key. In this regard, scanning probe microscopy and spectroscopy should prove to be invaluable, and some progress using these powerful tools has already been achieved with RuxSey-nanocluster-modified carbon and oxide support surfaces.36 Value-Added Conversion of CO2 to Alcohols and Product Selectivity in the Presence of the Carbon Component. The reactions considered above, such as hydrogen and methanol oxidation and ORR, are thermodynamically downhill, and thus, the M− MxOy−C nanocomposites function as true electrocatalysts. However, we return to entry 3 in Table 2, which states that Pt− TiO2−C nanocomposites showed higher electrochemical activity for CO2 reduction relative to that of massive Pt foils of comparable geometric area. In this preliminary work,9 these nanocomposites were used to drive a reaction in the thermodynamically uphill direction, and like the solar water splitting case, the ultimate product of the reaction is a fuel. The key here lies in lowering the kinetic barrier to CO 2

electroreduction (and minimizing the overpotential) such that the overall process makes sense from an energy conversion perspective. Within the context of this Perspective, an interesting result relates to product selectivity. Gas chromatography (GC) was employed to monitor the products of the CO2 electroreduction in the Pt−TiO2−C and Pt foil cases.9 In both cases, no evidence for the two-electron reduction product, namely, CO, was seen.9 However, interestingly enough, both methanol and isopropanol were seen for the nanocomposite case, while only methanol was detected for pure Pt (Figure 9). The fact that C−

Figure 9. GC analyses as a function of time for constant-current electrolysis of CO2-saturated solutions using (a) Pt−C−TiO2 and (b) Pt foil cathodes. Electrolysis conditions are specified in ref 9. Reproduced with permission from Electrochem. Solid-State Lett. (ref 9). Copyright 2012, The Electrochemical Society.

C bonds were formed in the former case signals the key role of the carbon component in possibly binding the intermediate products of the initial reduction step and incresing their residence time on the electrode surface. Subsequent C−C formation facilitated by site proximity effects of the sort discussed earlier is indeed an attractive mechanistic possibility. However, these remain speculative at present in the absence of more data including carefully perfomed isotope labeling experiments. Nonetheless, the perliminary data in Figure 9 exemplify that electrode architectures containing metal nanoclusters located in sites favorably juxtaposed relative to the metal oxide and carbon components offer good examples of site proximity effects. Issues and Challenges. This Perspective has attempted to introduce a new class of electrode materials based on metal 3476



Perspective

nanoclusters and oxide semiconductor nanoparticles embedded in a carbon matrix. While the results presented above and in companion Perspectives (see for example, refs 1 and 37) serve to illustrate the rich tapestry offered by these materials for both fundamental studies and possible applications, much remains to be done in terms of understanding how the three components interact within the nanocomposite architecture. What is the electronic nature of the interfaces at the metal nanocluster− oxide semiconductor, metal nanocluster−carbon, and oxide semiconductor−carbon contacts? How do the properties and chemical composition of the individual components influence the behavior and application performance of the material as a whole? What influence does alloying of the constituents in the metal nanocluster component exert on the performance of the electrode assembly as a whole? Even for a given application, such as CO2 electroreduction, why is the product distribution different for the nanocomposite relative to, say, Pt alone (cf. Figure 9)? Do intermediate product species adsorbed on adjacent sites within the nanocomposite interact in such a manner that carbon−carbon bonds are formed in the ultimate product(s)? What is the mechanistic basis for the remarkable enhancement seen in electrocatalyst durability for the nanocomposite in an ORR environment? These and many other, as yet unforeseen, questions should continue to spur further studies on this class of electrode materials.

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for Sid Richardson Carbon Co. His research interests are in materials science, primarily dealing in carbon nanomaterials.

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ACKNOWLEDGMENTS We thank our collaborators in earlier research on this topic, Dr. Norma Tacconi and Dr. Wilaiwan Chanmanee (University of Texas at Arlington), and Prof. Vijay Ramani (Illinois Institute of Technology). C.J. gratefully acknowledges funding support of the European Union under FP7-PEOPLE-2010-IOF, Grant Number 274046. Our colleagues at RenderNet Ltd. are also thanked for assistance with some of the artwork.

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REFERENCES

(1) Kamat, P. V. Graphene-Based Nanoassemblies for Energy Conversion. J. Phys. Chem. Lett. 2011, 2, 242−251. (2) Kulesza, P. J.; Faulkner, L. R. Electrodeposition and Characterization of Three-Dimensional Tungsten(VI, V)-Oxide Films Containing Spherical Pt Microparticles. J. Electrochem. Soc. 1989, 136, 707− 713 ; see also references therein. (3) Chen, K. Y.; Sun, Z.; Tseung, A. C. C. Preparation and Characterization of High-Performance Pt−Ru/WO3/C Anode Catalysts for the Oxidation of Impure Hydrogen. Electrochem. Solid-State Lett. 2000, 3, 10−12. (4) Shim, J.; Lee, C.-R.; Lee, H.-K.; Lee, J.-S.; Cairns, E. J. Electrochemical Characteristics of Pt−WO3/C and Pt−TiO2/C Electrocatalysts in a Polymer Electrolyte Fuel Cell. J. Power Sources 2001, 102, 172−177. (5) Trogadas, P.; Ramani, V. Pt/C−WO3 Electrocatalysts for Degradation Mitigation in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 2008, 155, B696−B703. (6) Lewera, A.; Timperman, L.; Roguska, A.; Alonso-Vante, N. Metal−Support Interactions between Nanosized Pt and Metal Oxides (WO3 and TiO2) Studied Using X-ray Photoelectron Spectroscopy. J. Phys. Chem. C 2011, 115, 20153−20159. (7) Tian, J.; Sun, G.; Cai, M.; Mao, Q.; Xin, Q. PtTiOx/C Electrocatalysts with Improved Durability in H2/O2 PEMFCs without External Humidification. J. Electrochem. Soc. 2008, 155, B187−B193. (8) Xiong, L.; Manthiram, A. Synthesis and Characterization of Metahnol Tolerant Pt/TiOx/C Nanocomposites for Oxygen Reduction in Direct Methanol Fuel Cells. Electrochim. Acta 2004, 49, 4163− 4170. (9) de Tacconi, N. R.; Chanmanee, W.; Dennis, B. H.; MacDonnell, F. M.; Boston, D. J.; Rajeshwar, K. Electrocatalytic Reduction of Carbon Dioxide Using Pt/C−TiO2 Nanocomposite Cathode. Electrochem. Solid-State Lett. 2012, 15, B5−B8. (10) de Tacconi, N. R.; Chenthamarakshan, C. R.; Rajeshwar, K.; Lin, W.-Y.; Carlson, T. F.; Nikiel, L.; Wampler, W. A.; Sambandam, S.; Ramani, V. Photocatalytically Generated Pt/C−TiO2 Electrocatalysts with Enhanced Catalyst Dispersion for Imporved Membrane Durability in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 2008, 155, B1102−B1109. (11) de Tacconi, N. R.; Rajeshwar, K.; Chanmanee, W.; Valluri, V.; Wampler, W. A; Lin, W.-Y.; Nikiel, L. Photocatalytically Generated Bimetallic (Pt−Au/C−TiO2) Electrocatalysts for Polymer Electrolyte Fuel Cell Applications. J. Electrochem. Soc. 2010, 157, B147−B153. (12) Chanmanee, W.; de Tacconi, N. R.; Rajeshwar, K.; Lin, W.-Y.; Nikiel, L.; Wampler, W. A. Photocatalytically Generated Trimetallic (Pt−Pd−Au/C−TiO2) Nanocomposite Electrocatalyst. J. Electrochem. Soc. 2012, 159, F226−F233. (13) Ma, J.; Habrioux, A.; Guignard, N.; Alonso-Vante, N. Functionalizing Effect of Increasingly Graphitic Carbon Supports on Carbon-Supported and TiO2−Carbon Composite-Supported Pt Nanoparticles. J. Phys. Chem. C 2012, 116, 21788−21794. (14) Ruiz-Camacho, B.; Gonzalez Huerta, R. G.; Valenzuela, M. A.; Alonso-Vante, N. Preparation and Characterization of Pt/C and Pt/ TiO2 Electrocatalysts by Liquid Phase Photodeposition. Top. Catal. 2011, 54, 512−518.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.R.). *E-mail: [email protected] (C.J.). Notes

The authors declare no competing financial interest. Biographies Krishnan Rajeshwar is currently a Distinguished Professor of Chemistry and Biochemistry at the University of Texas at Arlington (UT Arlington). He began working on energy-related problems during his postdoctoral career at Colorado State University. His research spans a broad spectrum of topics in energy conversion, environmental remediation, photoelectrochemistry, and materials chemistry. For further details, see http://www.uta.edu/cos/raj/index.html. Csaba Janáky is an assistant professor at the University of Szeged, Hungary. He spent the past 2 years at UT Arlington as a Marie Curie Fellow. His scientific interests include various aspects of semiconductor (photo)electrochemistry, with special focus on conducting polymers and their hybrid assemblies (http://www2.sci.u-szeged.hu/ physchem/elchem/home.htm). Wen-Yuan Lin is a research chemist at Sid Richardson Carbon & Energy Co. He received his Ph.D. from UT Arlington and continued postdoctoral work with the same research group for 2 more years. His current efforts focus on the preparation of metal nanocluster/oxide semiconductor/carbon composite catalysts using photocatalysis and sol−gel chemistry. David A. Roberts completed his BS degree from LeTourneau University. He is an adjunct professor at Texas Christian University and an Electron Microscopist for Sid Richardson Carbon and Energy Co. His scientific interests include all forms of microanalysis with a focus on catalysis and carbon nanomaterials. Wesley Wampler received a B.S. degree in Chemistry and Biology from Tarleton State University, an M.S. degree in Organic Chemistry from UT Arlington, and a Ph.D. in Analytical Chemistry from the same institution in 1995. He is currently the Vice President of R&D 3477



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