Synthesis of platinum and platinum–ruthenium-modified diamond nanoparticles

RESEARCH PAPER

Synthesis of platinum and platinum–ruthenium-modifieddiamond nanoparticles

Lyda La-Torre-Riveros • Emely Abel-Tatis •

Adrian E. Mendez-Torres • Donald A. Tryk •

Mark Prelas • Carlos R. Cabrera

Received: 18 August 2010 / Accepted: 20 December 2010 / Published online: 30 January 2011

� Springer Science+Business Media B.V. 2011

Abstract With the aim of developing dimension-

ally stable-supported catalysts for direct methanol

fuel cell application, Pt and Pt–Ru catalyst nanopar-

ticles were deposited onto undoped and boron-doped

diamond nanoparticles (BDDNPs) through a chemi-

cal reduction route using sodium borohydride as a

reducing agent. As-received commercial diamond

nanoparticles (DNPs) were purified by refluxing in

aqueous nitric acid solution. Prompt gamma neutron

activation analysis and transmission electron micros-

copy (TEM) techniques were employed to character-

ize the as-received and purified DNPs. The purified

diamond nanoparticulates, as well as the supported Pt

and Pt–Ru catalyst systems, were subjected to various

physicochemical characterizations, such as scanning

electron microscopy, energy dispersive analysis,

TEM, X-ray diffraction, inductively coupled plasma-

mass spectrometry, X-ray photoelectron spectros-

copy, and infrared spectroscopy. Physicochemical

characterization showed that the sizes of Pt and

Pt–Ru particles were only a few nanometers

(2–5 nm), and they were homogeneously dispersed

on the diamond surface (5–10 nm). The chemical

reduction method offers a simple route to prepare the

well-dispersed Pt and Pt–Ru catalyst nanoparticulates

on undoped and BDDNPs for their possible employ-

ment as an advanced electrode material in direct

methanol fuel cells.

Keywords Sodium borohydride � Chemical

reduction route � Diamond nanoparticles �Boron-doped diamond nanoparticles �Pt nanoparticles � Pt–Ru catalyst

Introduction

In the recent decades, an increase of research

activities in the area of catalyst development for fuel

cells has been seen, since the latter are considered to

be an environmentally sustainable energy system

(Viswanathan and Scibioh 2008). In spite of numer-

ous demonstration systems, the commercialization of

direct methanol fuel cells has been hampered mainly

because of two main reasons: high costs, and

durability of materials. Regarding cost reduction,

the noble metal loadings must be decreased to levels

L. La-Torre-Riveros � E. Abel-Tatis �D. A. Tryk � C. R. Cabrera (&)

Center for Advanced Nanoscale Materials, Department

of Chemistry, University of Puerto Rico, Rıo Piedras,

PO Box 70377, San Juan, PR 00936-8377, USA

e-mail: [email protected]

L. La-Torre-Riveros


A. E. Mendez-Torres � M. Prelas

Nuclear Science and Engineering Institute,

University of Missouri, Columbia, MO 65211, USA


M. Prelas


123

J Nanopart Res (2011) 13:2997–3009

DOI 10.1007/s11051-010-0196-8

below \1.0 mg cm-2 from the present 2.0–8.0

mg cm-2, depending on the specific applications.

The main approaches to the reduction of loadings

include increasing platinum utilization, developing

non-noble catalysts, and designing durable and high

performance electrodes. Dispersing catalysts on elec-

trically conducting, high surface area carbon materi-

als, is a significant step forward to achieving finer

dispersion of the metal catalyst and to achieving a

highly electrochemically active surface area (Takasu

et al. 2003). Carbon blacks are the best-known high

surface area substrates for electrocatalysts (Gloaguen

et al. 1997). Different types of carbon materials have

been tested to ascertain their suitability as catalyst

supports for fuel cell applications (Rao et al. 2005;

Scibioh et al. 2008; Antolini 2009). However, a major

problem with the carbon supports is that they lack

chemical stability when subjected to high positive

potentials, because of severe oxidation conditions

(Callstrom et al. 1990; Porcard et al. 1992). Diamond,

because of its high density and strong sp3 bonding, is

inert to oxidative attack. Though undoped diamond is

an electrical insulator, with a wide band gap (5.4 eV)

exhibiting low reactivity, and chemical and electro-

chemical inertness (Mani et al. 2002; Fischer and

Swain 2005; Fischer et al. 2004), it can be made

conductive by doping it with certain elements.

Currently, in most cases, boron is used as dopant of

diamond films, which results in a p-type semicon-

ductor. Another form in which diamond can be used

is that of nanoparticles [diamond nanoparticles

(DNPs)], which can present different electrochemical

characteristics because of its higher surface to

volume ratio and particular features (carbon–oxygen

functionalities, hydrogen terminations etc.) on its

surface, including edges and facets (Hayashi et al.

2004; Danilenko 2004). Oxidative isothermal and

non-isothermal purification in air was found to be an

environmental friendly process in which graphite,

carbon onions, fullerene shells and graphite ribbons

are removed, between 375 and 450�C, from the

carbon soot produced in detonation processes without

loss of the diamond phase (Osswald et al. 2006).

Aqueous acidic pretreatments produce clean and

hydrogenated DNPs (Reinzler et al. 1998; La-Torre-

Riveros et al. 2005). Electrodes have been prepared

electrophoretically with DNPs, and using silicon

wafers as substrates; these electrodes have been used

to study and demonstrate the electrochemical

properties of undoped DNPs in the presence of redox

couples such as ferricyanide/ferrocyanide and hexa-

mine ruthenium(III) (La-Torre-Riveros et al. 2007).

Gold electrodes modified with a drop-coated layer of

undoped DNPs and glassy carbon electrodes modified

with DNP-mineral oil paste have shown that the

electron transfer in solution and at different pH

occurs between the undoped DNPs and the redox

couples (Holt et al. 2008, 2009; Holt 2010). This is

due to the predominant properties of diamond at the

nanoscale such as an insulating sp3 diamond core

with a surface having delocalized p bonds. Synthetic

DNP powders have been modified at their surface

through thermal and electrochemical treatments and

by deposition of metallic palladium on their surface

and used as catalyst support for the oxidation of

carbon monoxide to carbon dioxide (Bogatyreva et al.

2004). After the electrophoretic deposition process of

undoped DNPs on silicon wafers, this diamond layer

was modified by a step-and-sweep potential deposi-

tion method of platinum nanoparticle clusters. In this

experiment, the anodic current density for methanol

oxidation was higher when fewer potential cycles

were applied, showing that undoped DNPs can be

used as an electrode material (La-Torre-Riveros et al.

2010).

Polycrystalline boron-doped diamond possesses

superior morphological stability and corrosion resis-

tance, allowing for its use at elevated temperatures in

oxidizing or reducing media without the loss of

desirable properties compared to conventional sp2

carbon support materials, and is able to withstand

current densities on the order of 1 A cm-2 for days,

in both acidic and alkaline conditions, without

structural degradation (Fischer and Swain 2005;

Chen et al. 1997). Regarding the formation of

diamond-supported catalysts, first, Salazar-Banda

et al. (2006) and other research groups demonstrated

that a few metals such as Pt, Pb, Hg, Pt–RuO2, Pt–

RhO2, Pt–Sn, and Pt–Ru–Sn can be electrochemically

deposited on the surfaces of conductive diamond thin

films and on boron-doped nanoporous honey-comb

diamond films (Salazar-Banda et al. 2006; Suffredini

et al. 2006; Spataru et al. 2008; Sine et al. 2006,

2007; Honda et al. 2001; Bennett et al. 2005).

Deposition techniques have varied from pulsed

galvanostatic deposition (Bennett et al. 2005), cyclic

voltammetry (Honda et al. 2001), potentiostatic

conditions (Wang et al. 2000) to sol–gel methods

2998 J Nanopart Res (2011) 13:2997–3009

123

using several different pre- and post-treatments of the

diamond surface (Montilla et al. 2003). The electro-

chemical behavior of the undoped and boron-doped

diamond film surfaces modified by the metals men-

tioned above were studied by cyclic voltammetry,

measurements of activity for methanol oxidation and

impedance spectroscopy; these materials show prom-

ise for applications in methanol oxidation. The sizes

of the platinum particles on the electrically conduct-

ing microcrystalline and nanocrystalline diamond

thin-film electrodes ranged from 10 to 500 nm, as

observed by transmission electron microscopy (TEM)

and scanning electron microscopy (SEM) micro-

scopic techniques. Electrodeposition was found to

lead to a much more uniform dispersion of metal

nanoparticulates, and narrower (5–15 nm) size dis-

tribution, compared with chemical deposition

(20–24 nm), even though particle clusters formed

(Salazar-Banda et al. 2005). It is well known that the

preparation method of the catalysts influences their

physicochemical properties and catalytic activities.

Over the last decade, numerous preparation methods

of Pt and Pt/Ru/C have been developed, such as the

impregnation (Jeon et al. 2007; Arico et al. 1995),

microemulsion (Zhang and Chan 2003; Liu et al.

2002) and colloidal routes (Watanabe et al. 1987;

Paulus et al. 2000; Arico et al. 2003). However, some

of these methods are suitable for preparing low

loading Pt and Pt/Ru/C electrocatalyst but cannot

provide satisfactory control of the particle size and

distribution (Jeon et al. 2007; Arico et al. 1995). In

traditional methods to improve the activity of cata-

lysts, catalytic supports such as carbon blacks are

treated with HNO3, H2O2, KOH, or NaOH solutions

(Tang et al. 2007).

A widely used method to load platinum, ruthe-

nium, and various catalysts on carbonaceous support

surfaces is chemical reduction. There are many

reagents that are used as reducing agents, such as

NaBH4 (Tang et al. 2007; Joon-Hyun et al. 2008; Xu

et al. 2007; Wang and Chen 2004), KBH4 (Jian et al.

2004), H2O2 (Xiaobo et al. 2007), hydrazine hydrate

(N2H4�H2O) (Nersisyan et al. 2003; Im et al. 2004),

and plasma chemical reduction (Koo et al. 2005).

These methods have produced good results with

amorphous carbon. It should be interesting to exam-

ine the DNPs subjected to some of the methods

mentioned and explore facile and efficient methods to

prepare Pt and Pt–Ru electrocatalysts on stable

support materials. In this study, we report on the

development of dimensionally stable, high surface

area DNPs deposited with Pt and Pt–Ru nanoparticles

of small size, in the range of 2–5 nm, through a

simple chemical reduction route using sodium boro-

hydride as the reducing agent. Before the deposition

of catalyst particulates, the undoped and boron-doped

diamond particulates were treated with a strong acid

to enhance their surface characteristics.

Experimental section

Chemical purification of DNPs

At first, as-received undoped (DNPs) from the Alit

Company, Ukraine, were subjected to chemical

purification by acid reflux treatment. A concentrated

solution of nitric acid was used at high temperature

(130�C) during a prolonged treatment (42 h) (Rein-

zler et al. 1998). Boron-doped diamond nanoparticles

(BDDNPs) were obtained by an electric field-

enhanced diffusion technique (EFED), in which 1 g

of undoped DNPs was mixed with 3 g of boron

powder and subjected to the forced diffusion process.

In this process, a temperature range of 750–950�C, a

potential difference of 150 V, a pressure of

20–40 mmHg, and a laser source were the optimized

experimental conditions employed to obtain doped

DNPs (Suarez et al. 2002). The doped sample was

also cleaned by refluxing with nitric acid diluted in

distilled nanopure water (1:1) to eliminate the boron

excess and graphitic material. The doping level was

0.68 at % of boron, which was determined by X-ray

photoelectron spectroscopy (XPS).

Chemical reduction of platinum and ruthenium

on DNPs

Chemical reduction is a method in which a strong

reducing agent is used to produce metal particles

from its molecular precursor. A DNP surface may be

decorated with platinum and ruthenium nanoparticles

produced through the reaction of an excess (twice the

stoichiometric amount) of sodium borohydride

(Aldrich, 99%) as reducing agent, and H2PtCl6�XH2O

(Aldrich, 99.995%), and RuCl3�XH2O (Alfa Aesar,

99.99%) as the platinum and ruthenium sources. The

reaction process starts with a strong interaction of the

J Nanopart Res (2011) 13:2997–3009 2999

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DNPs and the platinum salt solution (0.005 M

H2PtCl6: original solution) in an appropriate concen-

tration to produce 20 wt% of metallic platinum or the

desired amount (see Table 1) of platinum and ruthe-

nium (0.005 M RuCl3: original solution), by sonicat-

ing the mixture for a period of 5 h. During this time,

platinum ions are expected to be in close contact with

the DNP surface, providing platinum nucleation sites.

After the sonication process, the reducing agent

(NaBH4: twice the stoichiometric amount necessary

to reduce a 20 wt% of a total of 0.002 g) is directly

added. The last step is carried out under vigorous

stirring until the completion of the reaction. Finally,

the sample is filtered by washing with abundant

nanopure water to eliminate the ions produced in the

reaction. The resulting catalyst powder was dried for

15 min at 115�C in air. Various quantities of diamond

DNPs and different platinum and ruthenium precur-

sor concentrations were used in preparing the

supported catalyst particulates (see Table 1).

Characterization techniques

The undoped DNPs and BDDNPs purified by chem-

ical pretreatment were subjected to structural and

morphological characterizations using TEM,

equipped with a Gatan TG120 microscope, scanning

electron microscopy combined with an energy dis-

persive analyzer (SEM-EDX), a Gemini LEO 1550

instrument, an inductively coupled plasma-mass

spectrometer (ICP-MS) from Agilent Technologies-

7500ce, X-ray diffraction (XRD) using a Rigaku

UltimaIII X-ray diffractometer, with a Cu Ka radi-

ation line, XPS with a PHI 5600 spectrometer,

equipped with a Mg Ka monochromatic X-ray source

(350 W), Fourier transformed-infrared spectrometry

(FT-IR), using a Thermo Nicolet-Continum infrared

spectroscope, and prompt gamma neutron activation

analysis (PGNAA). The experimental conditions

employed for PGNAA were: neutron flux: 6 9 1013

n/cm2 s, exposure time: 11.5 h, element used: U-235

(highly enriched uranium: [20). This analysis was

performed at the University of Missouri, Columbia.

Results and discussion

As-received undoped DNPs, despite having been

subjected to stringent purification steps by the

manufacturer, still contain a low level of amorphous

carbon impurities. Therefore, the commercial DNPs

were subjected to chemical pretreatment in acid

reflux using concentrated HNO3, to remove impuri-

ties and to introduce hydrogen onto the diamond

surface. As expected, the purification process pro-

duced significant differences compared to the com-

mercial sample; i.e., better dispersion, and higher

purity. The acid reflux purification process used to

pretreat the DNPs appears to be effective.

The hydrogen content in the DNP samples as

measured by PGNAA technique was 16,300 ppm for

the as-received and 32,500 ppm for the purified

sample subjected to acid reflux treatment. This

difference in hydrogen content is due to the acid

treatment through which hydrogen atoms become

adsorbed on the surface of the DNPs under the

purification conditions. Interesting results can be seen

in the TEM micrographs (see Fig. 1a, b), in which a

Table 1 Samples under

study: Expected and

experimental amounts

of Pt and Ru

No Sample Wt% Pt Wt% Ru

(Exp) (Exp)

1 Unpurified diamond nanoparticles – –

2 Purified diamond nanoparticles in HNO3 – –

3 Diamond under reducing agent effect – –

4 Diamond nanoparticles (Purified HNO3) 20 (18.9) –

5 Diamond nanoparticles(Purified HNO3) 20 (16.1) 20 (12.6)

6 Diamond nanoparticles (Purified HNO3) 40 (48.2) –

7 Diamond nanoparticles (Purified HNO3) 35 (40.4) 15 (14.8)

8 BDD (high B conc. cleaned in HNO3) – –

9 BDD (high B conc. cleaned in HNO3) 20 (21.4) –

10 BDD (high B conc. cleaned in HNO3) 20 (22.3) 20 (16.3)

3000 J Nanopart Res (2011) 13:2997–3009

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thin layer of amorphous carbon is present in the

unpurified diamond (sample 1 in Table 1) and is

absent in the purified DNP (sample 2 in Table 1)

surface.

The doping process was performed to make the

undoped DNPs more conductive creating holes at the

diamond surface or structure. As the results from

EELS analysis had shown in our previous study, there

seems to be some substitutions in the diamond

structure (La-Torre-Riveros et al. 2010), which are

not detectable in XPS. The small difference in

electronic conductivity between undoped DNPs and

BDDNPs will be observed indirectly in the single

methanol fuel cell experiments and its efficiency,

which will be reported in a forthcoming publication.

Figure 1c shows the TEM micrograph of metallic

platinum nanoparticles chemically deposited on the

DNP surface purified in concentrated HNO3 (sample

4 in Table 1). This high-resolution TEM image

shows clearly a few well-dispersed dark platinum

nanoparticles on the diamond nano-cluster surface.

The sizes of the platinum nanoparticles in this sample

are around 3–4 nm, and particle sizes of DNPs are

between 5 and 6 nm. Ruthenium metal was also

chemically reduced together with platinum metal on

the surface of the DNPs (sample 5 in Table 1).

Figure 1d shows a magnified image of dark bimetallic

(Pt–Ru) nanoparticles. The sizes of the bimetallic

nanoparticles are around 3–5 nm, and those of the

DNPs are around 6–7 nm. Figure 1e shows the dis-

persed platinum nanoparticles of ca. 5 nm on the

BDDNPs (sample 9 in Table 1). Chemical reduction of

platinum and ruthenium metals was also performed on

BDDNPs (sample 10 in Table 1). Figure 1f shows the

atomic planes of the bimetallic nanoparticles which

correspond to platinum and ruthenium. The sizes of

these bimetallic crystals are ca. 4–5 nm.

Identification of these metals was performed using

energy dispersive X-ray fluorescence (EDX) analysis.

These results are shown in Fig. 2a, for the Pt/undoped

diamond catalyst support system (sample 4 in

Table 1), and Fig. 2b, for the Pt–Ru/undoped dia-

mond catalyst support system (sample 5 in Table 1).

The platinum and ruthenium peaks in this spectrum

are similar in intensity, which agrees with the

theoretical amounts present in the solutions during

the reductive process. Characteristic peaks at 2.2 and

2.6 keV for platinum and ruthenium, respectively, are

observed (Zhang and Chan 2003). EDX analysis

(Fig. 2c, d) also confirmed the presence of platinum

and ruthenium in Pt/boron-doped diamond and

Pt–Ru/boron-doped diamond catalyst support

Fig. 1 Transmission electron micrographs of a unpurified,

b purified, c platinum nanoparticles supported on diamond

nanoparticles surface, d platinum and ruthenium nanoparticles

supported on diamond nanoparticles surface, e platinum

nanoparticles supported on boron doped diamond nanoparticles

surface, f platinum and ruthenium nanoparticles supported on

boron doped diamond nanoparticles surface, produced by

chemical reduction method

J Nanopart Res (2011) 13:2997–3009 3001

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systems. The intensities of platinum and ruthenium in

Fig. 2d are not similar as seen in the undoped

diamond nanopowder. This may indicate that lower

amount of ruthenium was reduced and deposited on

the boron-doped diamond nanoparticulates. Signals of

Cu and Al are present in all EDX analyses corre-

sponding to the grid and sample holder, respectively.

The inductively coupled plasma-mass spectromet-

ric (ICP-MS) technique was used to quantify the

catalytic metallic particulates present in the samples

of undoped DNPs and BDDNPs (see Table 1). The

results in parenthesis show that the experimental

values are near to the expected values, which

indicates that the chemical deposition is a convenient

and efficient method to obtain the desired metal

concentration on the surface of the diamond

nanoparticulates.

The XRD technique was used to analyze the

crystallographic orientations of undoped DNPs and

BDDNPs as well as supported platinum and plati-

num–ruthenium nanoparticles on the surface of

diamond nanoparticulates. Several representative

normalized XRD patterns of the samples under study

are shown in Figs. 3 and 4. The XRD patterns of

undoped (unpurified and purified) DNPs, undoped

DNPs treated with the reducing agent, and BDDNPs

show the peaks associated with diamond. These

peaks are observed at 43.9� (111), 75.5� (220), and

91.7� (311), as shown in Fig. 3a–d. In Fig. 3a and b, a

difference in intensity of the (111) peak is observed.

The fact that the (111) peak of the sample purified in

nitric acid is more prominent indicates that it is

preferentially exposed (Fig. 3b). The absence of

amorphous carbon is confirmed through the absence

of peaks between ca. 20 and 30� which are charac-

teristic of amorphous carbon material.

Figure 3c shows the XRD spectra of the undoped

DNPs subjected to the reaction with reducing agent,

exhibiting peaks corresponding to the (111) facet of

diamond, as well as the (220) and (311) peaks which

are smaller in intensity as compared with the

unpurified, purified (HNO3), and boron-doped DNPs.

Fig. 2 Energy dispersive

X-ray fluorescence analysis

of a Pt/undoped DNPs,

b Pt–Ru/undoped DNPs,

c Pt/BDDNPs, d Pt–Ru/

BDDNPs catalyst support

systems

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This comparison reveals that some amorphous mate-

rial could exist on the diamond surface.

BDDNPs cleaned with dilute nitric acid (1:1)

reflux was characterized by the XRD technique

(Fig. 3d) and compared to the unpurified, purified

diamond, and with the diamond treated with sodium

borohydride reducing agent. This BDDNP sample

shows more prominent XRD peaks, e.g., (111), (220)

and (311), than the reduced diamond sample. Nev-

ertheless, a shoulder, between 15 and 30 (2h), which

corresponds to graphitic species that originates from

the boron-doping (EFED) is observed (Suarez et al.

2002).

Figure 4 shows X-ray diffraction patterns of Pt/

undoped DNP and Pt–Ru/undoped DNP catalyst-

supported systems, as well as on BDDNP. Charac-

teristic peaks of diamond (111), (220), and (311) are

present in all samples. Furthermore, characteristic

peaks of platinum facets (111), (200), (220), (311),

(222), (400), (331), and (420) at 40.0�, 47�, 67.8�,

81.6�, 104.6�, 119�, and 123.5�, respectively (Fachini

et al. 2003), were also present indicating the presence

of crystalline facets of platinum, ruthenium–modified

platinum, and DNPs. Figure 4a and b shows the

difference in the XRD patterns of Pt/undoped DNP

and Pt–Ru/undoped DNP-supported catalyst systems.

In the Pt–Ru/undoped DNP catalyst support system

(Fig. 4b), the peak areas of (200), (222), (400), (331),

and (420) are higher, and in addition, there are slight

peak displacements and widening attributed to the

modification of the platinum crystal structure with

ruthenium atoms. The differences in Pt peak areas

indicate that the metal nanoparticles of Pt–Ru/

undoped DNP catalyst support system contain amor-

phous material, which corresponds to the incorpo-

rated ruthenium in the platinum nanoparticles

structure. X-ray diffraction patterns of the Pt/BDDNP

and Pt–Ru/BDDNP catalyst support systems are

Fig. 3 X-ray diffraction patterns of a unpurified, b purified

diamond nanoparticles in HNO3 reflux c reduced diamond

nanoparticles (NaBH4), and d boron doped diamond nano-

particles

Fig. 4 X-ray diffraction patterns of a platinum, b platinum–

ruthenium nanoparticles supported on purified (concentrated

HNO3) undoped diamond nanoparticles surface, and X-ray

diffraction patterns of c platinum, and d platinum–ruthenium

nanoparticles supported on boron doped diamond nanoparticles

cleaned in HNO3

J Nanopart Res (2011) 13:2997–3009 3003

123

shown in Fig. 4c and d. In the Pt–Ru/BDDNP

catalyst support system, all peaks are wider than

those of Pt/BDDNP catalyst support system peaks.

These results show more clearly the presence of Pt–

Ru bimetallic nanoparticles in the sample, because

the platinum peak areas for the (200), (220), (311),

(222), (400), (331), and (420) lines of this sample are

larger than those for the Pt/BDDNP sample. This is

due to the incorporation of ruthenium on the platinum

crystal structure. Another characteristic feature in this

pattern is that the shoulder at ca. 20–30� (2h) is

absent; this indicates the absence of graphitic mate-

rial in all the four samples. All these results confirm

that the metallic platinum and ruthenium catalysts

can be chemically deposited onto the diamond

surface; they present similar XRD characteristics to

those found in the literature (Choi et al. 2008). This

reduction method commonly generates alloys of

bimetallic nanoparticles; the latter can be observed

in the XRD results, which show changes of some

characteristic peaks of platinum when they are mixed

with ruthenium. Alloy formation is important because

both platinum and ruthenium surfaces have to be

exposed to improve the catalytic activity of the

system.

The size of the metallic nanoparticle significantly

influences the resulting efficiency of a catalyst

system. This is mainly due to the surface area-to-

volume ratio, which should be high. X-ray diffraction

analysis gives useful information on the particle size.

Table 2 summarizes the percentages of metallic

nanoparticles on undoped DNPs and BDDNPs as

well as their sizes, which were obtained using the

theoretical calibration values based on Vegard’s law

(Dıaz-Morales et al. 2004) and the Scherrer equation

(Patterson 1939), respectively. In general, the particle

sizes of nanocatalysts derived from XRD are in good

agreement with those observed from the TEM images

of the corresponding samples. The sizes of the

bimetallic nanoparticles, ca. 4.7 nm, of platinum

and ruthenium in the Pt–Ru/undoped DNPs catalyst

support, determined by using JADE software, were

larger than those for platinum, ca. 3.3 nm, in the Pt/

undoped DNPs catalyst support system. This obser-

vation is attributed to the incorporation of ruthenium

on specific facets [(200), (222), (400), (331), and

(420)] rather than on all platinum facets. However, on

Pt–Ru/BDDNPs, the incorporation of ruthenium

occurs on more platinum facets [(200), (220), (311),

(222), (400), (331), and (420)], which makes the

particle surface more amorphous.

All samples prepared in this study, undoped DNPs,

BDDNPs, and Pt and Pt–Ru catalyst systems sup-

ported on them, were analyzed by using XPS.

Figure 5 shows a few representative high-resolution

XPS spectra of samples such as those of the DNPs

after the purification with concentrated nitric acid

(Fig. 5a), showing the peaks corresponding to carbon

as well as some functional groups such as alcohols

(–C–OH, *286 eV), ethers (–C–O, *287 eV), and

carboxyls (–COOH, 289 eV) in C 1s region. The O 1s-

binding energy region for this purified sample shows

the –OH- group at *532 eV and –OH- at *533 eV

from the adsorbed vapor water on the diamond surface

Table 2 Amounts (in percentage) and particle sizes of platinum, ruthenium, and diamond nanoparticles from XRD analysis by

Vegard’s law and the Scherrer equation

Sample Diamond Metal

Size (nm) Size (nm) Pt (%) Ru (%)

As received 5.1 ± 0.3 – – –

Purified in HNO3 5.0 ± 0.3 – – –

Reduced with NaBH4 5.0 ± 0.2 – – –

Pt 20%/DNP 5.4 ± 0.7 3.3 ± 0.2 100 –

Pt–Ru 20%:20%/DNP 5.2 ± 0.4 4.7 ± 0.8 83 ± 1 17 ± 1

BDDNP 5.3 ± 0.4 – – –

Pt 20%/BDDNP 5.8 ± 0.8 3.5 ± 0.3 100 –

Pt–Ru 20%:20%/BDDNP 5.1 ± 0.6 3.3 ± 0.4 83 ± 2 17 ± 1

DNP diamond nanoparticles, BDD boron-doped diamond nanoparticles

3004 J Nanopart Res (2011) 13:2997–3009

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(www.nist.gov). Figure 5b shows the high-resolution

XPS of the C 1s- and O 1s-binding energy regions for

the undoped purified DNPs after its reaction with the

reducing agent. Some of the surface functionalities

on carbon are found to be alcohols (–C–OH), ethers

(–C–O), carboxyls (–COOH), and carbonates (–CO32-).

Fig. 5 High resolution

X-ray photoelectron

spectroscopy of a C 1s and

O 1s binding energy region

for diamond nanoparticles

after purification process

with HNO3 (conc.)

b C 1s and O 1s of undoped

diamond nanoparticles after

reacting with reducing

agent, c C 1s, O 1s, and

Pt 4f of purified undoped

diamond nanoparticles in

HNO3 with chemically

reduced Pt, d C 1s, O 1s,

Pt 4f, and Ru 3p of purified

diamond nanoparticles in

HNO3 with chemically

reduced Pt and Ru, e C 1s,

O 1s and B 1s of boron

doped diamond

nanoparticles cleaned in

HNO3, f C 1s, O 1s and

Pt 4f of boron doped

diamond nanoparticles

cleaned in HNO3 with

chemically reduced Pt, and

g C 1s, O 1s, Pt 4f, and

Ru 3p of purified boron

doped diamond

nanoparticles in HNO3

with chemically reduced

Pt and Ru

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123

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The oxygen groups including alcohols (–OH) and

hydroxide (OH-) from the adsorbed vapor water are

also present. It is evident that the reducing agent

affects the diamond surface with the absence of alco-

hols and ether groups and the presence of carbonates.

Figure 5c shows the high-resolution XPS spectrum of

C 1s, O 1s, and Pt 4f for platinum chemically reduced

on undoped purified (conc. HNO3) DNPs. This spec-

trum also shows some surface functional groups, such

as ethers (–C–O), carbonates (CO32-), and carboxylic

(–COOH) acid. The oxygen high-resolution binding

energy region shows the presence of alcohols (–OH)

and hydroxides (OH-). On the other hand, the plati-

num high-resolution binding energy spectrum shows

the Pt 4f7/2 (71–72 eV) and Pt 4f5/2 (74–75 eV); which

correspond to metallic platinum and platinum oxide,

respectively (Fachini et al. 2003; www.nist.gov). In

Fig. 5d, the high-resolution XPS spectrum of Pt 4f-

and Ru 3p-binding energy region, for the chemically

reduced on the undoped purified DNPs surface, are

shown, as well as the high-resolution XPS spectra of C

1s and O 1s. Ether (–C–O), and carboxylic (–COOH)

groups are present in the C 1s-binding energy region.

As in other samples, functionalities such as alcohols

(–OH) and hydroxides are also evidenced in the oxy-

gen region. The platinum spectrum in this sample also

shows the characteristic lines of metallic platinum

(Pt 4f7/2, 71–72 eV) and platinum oxides such as PtO

and PtO2 (Pt 4f5/2, 74–75 eV). The presence of

metallic ruthenium at ca. 462.2 eV and the influence of

adsorbed water at ca. 466.5 eV are observed in the

high-resolution XPS of this metal (Ru 3p3/2 line)

(www.nist.gov).

The BDDNPs sample was cleaned in a concen-

trated nitric acid reflux system to eliminate the excess

boron present in the sample. High-resolution XPS

spectra of C 1s, O 1s, and B 1s were also obtained

(see Fig. 5e). The carbon region shows the presence

of ether (–C–O) and carboxylic groups (–COOH).

The oxygen region shows alcohol (–C–OH) and

hydroxide (OH-) groups, and, in the B 1s-binding

energy region, presents peaks for boron and B2O3 at

190.5 and 193 eV, respectively (www.nist.gov). The

amount of boron present in the purified sample was

determined by using the 1s peak areas of boron and

carbon, resulting in a calculated value of 0.68% of

boron in the doped diamond sample. However, the B

1s-binding energy peak is broad; indicating that

boron is not totally incorporated and other oxidized

species of boron are present. These boron oxides

could also be responsible for the platinum deposition

and dispersion on the diamond surface.

Platinum and ruthenium metals were chemically

reduced on the boron-doped diamond surface. High-

energy resolution analysis of these samples (Fig. 5f),

show C 1s, O 1s, and Pt 4f signals. The carbon

spectrum shows the presence of ether (–C–O) and

carboxylic (–COOH) groups. The oxygen spectrum

shows the existence of alcohol (–C–OH) and hydrox-

ide (–OH-) moieties. The platinum high-resolution

spectrum shows the Pt 4f7/2 (71–72 eV) and Pt 4f5/2

(74–75 eV) peaks which also correspond to metallic

platinum and platinum oxide, respectively. Finally,

Fig. 5g shows the C 1s-, O 1s-, Pt 4f-, and Ru 3p-

binding energy regions. Carbon spectrum shows the

presence of ether (–C–O) and carboxylic (–COOH)

groups. The oxygen spectrum shows the existence of

alcohol (–C–OH) and hydroxide (–OH-) moieties.

The platinum spectrum shows the Pt 4f7/2 (71–72 eV)

and Pt 4f5/2 (74–75 eV) peaks which correspond to

metallic platinum and platinum oxide, respectively.

Metallic ruthenium (*462 eV) is evidenced by the

high-resolution XPS spectrum of this metal (Ru 3p3/2

line), and the adsorbed water feature at ca. 466 eV is

also observed. It can be seen that the platinum and

Fig. 6 FT-IR spectra of a not purified diamond nanoparticles,

b purified diamond nanoparticles in concentrated HNO3,

c diamond nanoparticles put in reaction with the reducing

agent NaBH4, and d diamond nanoparticles decorated with

metallic platinum by chemical reduction using a reducing agent

(NaBH4)

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ruthenium depositions occur on the diamond surface

at the C–OH site; hence, this peak is absent in all the

samples containing platinum and platinum–ruthe-

nium catalysts. The surface organic groups of the

doped diamond are very similar to those of carbon

black. The exchange of ionic groups of surface

organic groups and metal salts may play an important

role in the formation of nanoparticles with a narrow

particle size distribution.

The FT-IR results of the unpurified and purified

DNP samples show that the only difference is the

percentage of reflectance of the m–CO feature, which

is higher in the unpurified sample (Fig. 6a, b). The

functional groups on the undoped diamond surface

are similar to those found in the literature (Kulakova

2004). The FT-IR spectrum shown in Fig. 6a for

unpurified DNPs, shows the m–OH, m–CO2, m–CO,

m–CH2, m–CH3, and m–C–O–C– characteristic IR

bands. Figure 6c for DNPs reacted with the reducing

agent NaBH4 shows, besides m–OH, m–CO, m–CH2,

m–CH3, and m–C–O–C– bands, additional bands of

m–CH at ca. 2800 and 2850 cm-1. The peak of –CH3

at ca. 1250 cm-1 is more intense than those in spectra

a and b of Fig. 6. Finally, spectrum 6d for DNPs

decorated with metallic platinum shows only m–CO,

and low intensity m–C–O–C–, m–CH2, and m–CH3

band features. This fact indicates that the nucleation

of metallic platinum started on the diamond surface

through an interaction between the metal ions and

functional groups such as alcohols (–OH) and some

of the –CH groups, and followed by the deposition.

This analysis should be verified by applying a more

sensitive technique such as far infrared spectroscopy

to determine the interaction between the platinum and

oxygen species.

Conclusions

On the basis of the materials’ characterization results,

it can be concluded that the acid reflux purification

process is efficient and increases the hydrogen

content in the undoped DNPs surface. The TEM

micrographs also corroborate the removal of amor-

phous carbon from the undoped DNPs surface, as

does the absence of amorphous carbon peaks in the

XRD spectra.

The chemical reduction method allowed us to

obtain nanoparticles in amounts near to the desired

atomic percentages in all the samples as was dem-

onstrated by the EDX and ICP results (see Table 1),

which make this method reliable for catalyst prepa-

ration with DNPs as support.

The Pt and Pt–Ru nanoparticle sizes obtained

ranged from 2 to 5 nm, as was shown in the TEM

micrographs and XRD results (see Table 2). These

results are indicative that this fast reaction is conve-

nient to obtain small nanoparticles that are desirable

to increase the surface area of a catalyst system.

The undoped DNPs and BDDNPs surfaces were

successfully used as support systems for metallic

catalyst particulates obtained by chemical reduction.

The platinum and ruthenium compounds mainly

deposit on the –OH sites, as was shown in the XPS

and FTIR spectra. This analysis will be verified by

the use of a more sensitive technique such as far

infrared spectroscopy to determine the interaction

between platinum and oxygen species.

The spectroscopic and surface characterization

showed the viability of obtaining a catalytic system

based on undoped DNPs and BDDNPs supports, and

platinum and ruthenium nanoparticles, which can be

used in applications such as direct methanol fuel cells

and will be reported in a forthcoming publication.

Acknowledgments This research was supported in part by

the NASA-URC Grant No. NNX08BA48A, NSF-EPSCoR, the

Institute for Functional Nanomaterials Grant No. OIA-

0701525, and the NSF NSEC Center for Hierarchical

Manufacturing Grant No. CHM – CMMI – 0531171. We

also acknowledge the support received from the Cornell

Center for Materials Research (CCMR-TEM) at Cornell

University. Editing of the manuscript done by Dr. D. A. Tryk

and Dr. M. A. Scibioh is gratefully acknowledged.

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Synthesis of platinum and platinum–ruthenium-modified diamond nanoparticles

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