Unexpected bond activations promoted by palladium nanoparticles

Article abstract of DOI:10.1039/c3dt53649a

Thioether-phosphines, 1 and 2, were applied for the stabilisation of palladium nanoparticles (PdNPs) synthesised by a bottom-up methodology, using [Pd₂(dba)₃] as an organometallic precursor. For the phenyl containing ligand 1, small (d_mean = 1.6 nm), well-defined and dispersed nanoparticles were obtained; however, ligand 2 involving a long alkyl chain led to agglomerates. NMR and GC-MS analyses throughout the synthesis of the nanomaterials revealed partial cleavage of ligands by C-S and C-P bond activations, and XPS spectra of the isolated nanoparticles indicated the presence of both thioether-phosphines and their fragments on the metallic surface. Reactivity studies of molecular palladium systems as well as on extended palladium surfaces pointed out that cluster entities are responsible for C-heteroatom activations, triggering structure modifications of stabilisers during the synthesis of PdNPs. the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53649a

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Unexpected bond activations promoted by
palladium nanoparticles†
Cite this: Dalton Trans., 2014, 43,
9038
A. M. López-Vinasco,a I. Favier,b,c C. Pradel,b,c L. Huerta,d I. Guerrero-Ríos,a
E. Teuma,b,c M. Gómez*b,c and E. Martin*a
Thioether-phosphines, 1 and 2, were applied for the stabilisation of palladium nanoparticles (PdNPs) syn-
thesised by a bottom-up methodology, using [Pd2(dba)3] as an organometallic precursor. For the phenyl
containing ligand 1, small (dmean = 1.6 nm), well-defined and dispersed nanoparticles were obtained;
however, ligand 2 involving a long alkyl chain led to agglomerates. NMR and GC-MS analyses throughout
the synthesis of the nanomaterials revealed partial cleavage of ligands by C–S and C–P bond activations,
and XPS spectra of the isolated nanoparticles indicated the presence of both thioether-phosphines
and their fragments on the metallic surface. Reactivity studies of molecular palladium systems as well
as on extended palladium surfaces pointed out that cluster entities are responsible for C-heteroatom
activations, triggering structure modifications of stabilisers during the synthesis of PdNPs.
Received 30th December 2013,
Accepted 21st March 2014
DOI: 10.1039/c3dt53649a
www.rsc.org/dalton
thioethers have attracted attention,7 in particular for their
potential applications in synthetic chemistry.8 Additionally,
Introduction
The decomposition of organometallic precursors under appro- metallic clusters, such as those of Pt9 and MoS2,10 as well as
priate conditions represents an elegant synthetic method to Ni11 and Au12 surfaces, have proven capable of splitting C–S
form metallic nanoparticles with a well-defined composition bonds when thiophenes and thiols are involved. Moreover,
and controlled morphology.1 This bottom-up methodology copper nanoclusters containing thiolates, [Cun(SR)m], gener-
frequently uses macromolecules as stabilisers, for instance ated Cu2S nanodiscs by thermal treatment.13
polymers, dendrimers or surfactants.2 Molecular ligands are
Although C–P bond activations mediated by transition
also suitable due to their coordinative abilities as well as metals are less favoured than those corresponding to C–S clea-
for the possible tuning of electronic and steric properties of vages, metal–phosphide nanoparticles were prepared by direct
the metallic surface,3 especially relevant for catalytic pur- reaction of the corresponding metal with alkyl-phosphanes at
poses.4 However, the interaction of heteroatoms, such as nitro- high temperatures, by means of activation of C–P bonds, such
gen, oxygen, phosphorus or sulphur, with the metallic surface as for InP and FeP nanoparticles.14 C–P cleavages could be
can trigger ligand modification by bond activation processes.
also promoted by molecular clusters under thermal or photo-
In relation to C–S bond cleavages, most reported studies chemical conditions, for instance those based on Mo,15 Ru,16
concern activations promoted by molecular transition metal Os,17 Mn and Re.18 Interestingly, both C–S and C–P sequential
complexes.5 Thiophenes and related derivatives have been bond activations of furyl- and thienyl-phosphines led to the
extensively studied as model substrates for hydrodesulfuriza- formation of triruthenium clusters.19
tion processes.6 Recently, this type of bond ruptures involving
Taking into account these precedents, we wondered
whether coordinating ligands employed as stabilisers for met-
allic nanoparticles could be modified during their synthesis
starting from molecular precursors. Therefore, we became
interested in identifying the nature of ligands interacting with
palladium nanoparticles (PdNP).
Herein, we describe the synthesis of new PdNP stabilised by
thioether-phosphines. The observed reactivity of these ligands
during the PdNP formation was compared to that using both
molecular and heterogeneous palladium sources, with the aim
of understanding the palladium mediated C–S and C–P bond
activations.
aDepto. de Química Inorgánica, Facultad de Química, Universidad Nacional
Autónoma de México, Av. Universidad 3000, 04510 D.F., México.
E-mail: erikam@unam.mx; Fax: +52 55 5622 3720
bUniversité de Toulouse, UPS, LHFA, 118 route de Narbonne, 31062 Toulouse
cedex 9, France
cCNRS, LHFA UMR 5069, 31062 Toulouse cedex 9, France.
E-mail: gomez@chimie.ups-tlse.fr; Fax: +33 5 6155 8204; Tel: +33 5 6155 7738
dInstituto de Investigaciones en Materiales, Universidad Nacional Autónoma de
México, Apartado Postal 70-360, México D.F. 04510, México. Fax: +52 55 5622 4715
†Electronic supplementary information (ESI) available: Syntheses of XPS refer-
ence materials and Fig. S1–S15 and Tables S1–S2. See DOI: 10.1039/c3dt53649a
9038 | Dalton Trans., 2014, 43, 9038–9044
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more than 30% of ligand degradation was observed under the
conditions employed for the synthesis of both nanoparticles,
PdNP1 and PdNP2.
Results and discussion
Synthesis and characterization of PdNP
PdNP containing thioether-phosphines were synthesised from
With the aim of verifying the nature of the stabilisers
[Pd2(dba)3] in the presence of the corresponding ligand (1 or present on the metallic surface after the isolation of the
2) under H2 pressure (Scheme 1) following the methodology materials, ligand exchange reactions of the as-prepared PdNP
previously described.1a TEM analyses proved the significant were tested using both carbon monoxide and dodecanethiol
effect of the nature of the ligand on the material. While ligand (Scheme 2). None permitted to replace the stabilisers on the
1 led to small (1.6 0.5 nm), well-defined and dispersed nano- metallic surface even after one week of reaction. In addition,
particles (Fig. 1a), ligand 2 merely gave agglomerates (Fig. 1b). when palladium nanoparticles were treated under aerobic
The satisfactory stabilisation of PdNP1 may be related to the reflux of toluene, triphenylphosphine oxide (OvPPh3) was
π-aromatic interaction with the metallic surface besides recovered as the only product, pointing to Caryl–S bond acti-
the donor ability of the heteroatoms.3a Intriguingly, the behav- vation for both PdNP1 and PdNP2. It is important to note that
iour displayed by PdNP2 is different from what could be free ligands 1 and 2 under the same aerobic conditions exclu-
expected, given that stabilisers containing long alkyl chains sively gave the corresponding thioether-phosphine oxides,
frequently act as surfactants favouring dispersion between without any sign of cleavage bonds.
nanoparticles.20
When PdNP2 was reacted with concentrated nitric acid at
IR spectra of both PdNP1 and PdNP2 materials evidenced room temperature, OvPPh3 and 2 were mainly obtained,
the presence of aromatic ligands, THF and dibenzyliden- together with a small amount of thioether-phosphine oxide.
acetone and/or its CvC bond hydrogenated product (Fig. S1 Free ligand 2 treated with nitric acid showed low reactivity, par-
and S2†). In particular, two strong bands, present in both tially giving the thioether-phosphine oxide without the for-
ligand spectra, at 746 and 698 cm−1, were also observed for the mation of OvPPh3. These indirect analyses proved that ligand
corresponding palladium nanoparticles; these bands are 2 was present at the metallic surface. An analogous study
attributed to the out-of-plane deformation for mono- and carried out with PdNP1 only indicated the presence of
di(ortho)-substituted phenyl compounds. THF at the metallic OvPPh3 even for the reaction of free ligand 1 with nitric acid,
surface could be evidenced by strong absorptions in the range
1300–1000 cm−1. Weaker absorptions at ca. 1630 cm−1 could
be assigned to dibenzylidenacetone and its partially reduced
compound.
Surprisingly, GC-MS analyses of the organic phases coming
from the reaction mixtures revealed the presence of benzene in
the case of PdNP1 (Fig. S3†) and decane and decylphenyl-
thioether for PdNP2 (Fig. S4†). It is important to note that not
Scheme 2 Surface reactivity for PdNP1 and PdNP2 (Lfrag means frag-
Scheme 1 Synthesis of palladium nanoparticles PdNP1 and PdNP2,
stabilised by thioether-phosphines 1 and 2 respectively.
ments of thioether-phosphines coming from C–P and/or C–S bond
activation).
Fig. 1 TEM images of PdNPL corresponding to: PdNP1 with their size distribution histogram (a); PdNP2 (b).
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48 ppm after 1 h, assigned to the palladium(II) complex cis-[Pd-
(PS)2] (PS represents the anionic ligand Ph2P–(2-S–C6H4)).22
This compound was formed by Caryl–S (ligand 1) and Calkyl–S
(ligand 2) bond activations, via oxidative addition of the SR
moiety to Pd(0) followed by R–H reductive elimination (R =
C6H5, C10H21), giving benzene and decane for 1 and 2, respecti-
vely. After 1 h for PdNP1 and after 4 h for PdNP2, under H2
pressure, signals corresponding to Pd(0) molecular species
practically disappeared, in agreement with the formation of
nanoparticles.
GC-MS analysis of the organic phase evidenced the pres-
ence of benzene for PdNP1 and both decane and decylphenyl-
thioether for PdNP2 as observed in the synthesis of PdNP
(see above). These facts indicate that not only C–S bond
activations (Caryl–S and Calkyl–S) but also C–P bond cleavages
take place. In contrast, for PdNP1, no diphenylthioether was
observed. The formation of secondary phosphines, such as
HPPh2 and HPPh(2-SR–C6H4) (where R = Ph, decyl), or the
corresponding phosphides species could not be excluded.
Neither these phosphorus species nor their corresponding
molecular metal complexes were observed; therefore, it is
reasonable to assume their presence on the metallic surface.
XPS survey spectra for both materials PdNP1 and PdNP2
showed the presence of palladium, sulphur, carbon and
oxygen, but unfortunately phosphorus could not be detected
(Fig. S6†); the presence of phosphorus was evidenced by EDX
analyses for both nanomaterials (Fig. S7 and S8† for PdNP1
and PdNP2, respectively). Analyses of high-resolution spectra
in the binding region corresponding to Pd 3d5/2 and Pd 3d3/2
(Fig. 3, Table S1†) permitted us to propose the presence of
Pd(II) species coordinated to sulphide and thiolate species
(SPh and PS, respectively) for PdNP1 and both SC10H21 and PS
anions for PdNP2, in addition to the main Pd(0) species. This
behaviour is in agreement with that observed for PdNPs just
stabilised by [BMI][PF6] (BMI = nbutyl-methyl-imidazolium)
where Pd(0) and Pd(II) chemical states were observed on the
metallic surface by XPS studies; the major contribution corres-
ponds to Pd(0) as deduced from the binding energy region of
Fig. 2 31P NMR monitoring during the formation of PdNP2.
in the absence of palladium. The high reactivity of 1 under
oxidative conditions makes it difficult to prove the presence of
ligand 1 on the metallic surface.
As a result, a plausible PdNP degradation during the tests
to release the thioether-phosphine ligand from the metallic
surface could be presumed, but ligand degradation during the
synthesis of PdNP cannot be ruled out.
In order to identify the organic and organometallic species
involved in the formation of PdNP1 and PdNP2, the syntheses
were carried out in THF-d8 at NMR Young tube scale, allowing
31P and 1H NMR monitoring; the corresponding organic
phases were also analysed by GC-MS.
When [Pd2(dba)3] and L (1 or 2) were mixed in the absence
of H2 (Pd/L ratio = 1/0.2), two signals appeared in 31P NMR
spectra at ca. 31–34 ppm for both systems (Fig. 2 for the Pd/2
system and Fig. S5† for the Pd/1 system), which could be attrib-
uted to thioether-phosphine containing Pd(0) species
[Pd(dba)n(L)m] according to reported data for related com-
pounds.21 Under dihydrogen pressure, the intensity of signals
decreased and a new signal of low intensity appeared at
Fig. 3 High-resolution XPS spectra of the Pd 3d binding region for PdNP1 (left) and PdNP2 (right).
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Pd 3d.23 Deconvolutions of the experimental signals were
based on PdS, [Pd(SPh)2]n, [Pd(SC10H21)2]n and cis-[Pd(PS)2] as
Pd(II) references, and bulk palladium as the Pd(0) pattern
(Fig. S9 and S10†). Pd(0) species containing phosphines, such
as [Pd(PPh3)4], also exhibit binding energies in the regions
corresponding to the bulk metal.24
Analyses of high-resolution spectra in the binding region
corresponding to S 2p3/2 proved the presence of the thioether-
phosphine ligands 1 and 2, corroborating also the con-
tributions of sulphide and thiolate species, according to the
previously reported data.25 For a full analysis, deconvolutions
of the experimental signals were also based on [PdCl2(L)]
(where L = 1, 2), PdS, [Pd(SPh)2]n, [Pd(SC10H21)2]n and cis-[Pd-
(PS)2] (Fig. S11 and S12, Table S2†).
To more fully understand the nature of the palladium
species responsible of the observed carbon–heteroatom bond
activations, the reactivity of molecular and extended surface
palladium systems was envisaged.
Scheme 3 Partial cleavage of thioether-phosphine ligands promoted
by Pd(0). In blue, species present on the metallic surface. Detection
techniques are shown in brackets (n.d = not detected).
Molecular and extended surface reactivity
Concerning the molecular reactivity, 31P NMR monitoring of
THF mixtures of [Pd2(dba)3] and L (Pd/L ratio = 1/0.2) in the
absence of dihydrogen displayed the formation of molecular
Pd(0) species coordinated to the thioether-phosphine ligands
(Fig. S13 and S14† for 1 and 2, respectively).
Complexes containing 1 evolved from purple to dark solu-
tions with the disappearance of initial NMR signals. After 4 h,
a low intensity signal at 52 ppm emerged, indicating the for-
mation of Pd(II) thiolate species trans-[Pd(PS)2], the kinetically-
controlled isomer;22 accordingly, benzene was detected in the
organic phase (Fig. S15†). In contrast, 31P NMR spectra for the
mixture [Pd2(dba)3] with 2 did not show any signal changes
after 24 h. These results, obtained in the absence of dihydro-
gen, indicate that only the Pd/1 system led to the formation of
heterogeneous entities. Its behaviour suggests that the C–S
bond activation is induced by metallic heterogeneous
species and, consequently, palladium leaching promoted by
phosphine-thiolates takes place (trans-[Pd(PS)2]).
XPS analyses of the as-prepared materials evidenced the
presence of thioether-phosphines and the corresponding thio-
late and sulphide fragments at the metallic surface. Addition-
ally, GC-MS analyses from the organic phases corresponding
to the synthesis of PdNP displayed the formation of the
organic compounds coming from C–S and C–P bond activation
processes, under dihydrogen pressure at room temperature.
In Scheme 3, the different C-heteroatom bond cleavages
produced during the synthesis of PdNP are summarised.
A thorough analysis during the synthesis of palladium
nanoparticles by means of 31P NMR and GC-MS techniques
suggested that metallic surfaces trigger the cleavage of
C-heteroatom bonds, in agreement with the reactivity observed
using Pd/C.
This work demonstrates the high reactivity of the metallic
species generated from decomposition of molecular precursors
during the synthesis of nanoparticles, modifying the stabiliser
structure and able to induce significant consequences for the
applications of the nanomaterials, in particular in catalysis.
In relation to surface reactivity, Pd/C was chosen as the
model material. Thioether-phosphines in the presence of Pd/C
in THF under H2 (3 bar) at room temperature led to ca. 7%
degradation of ligands. GC-MS analyses of the organic phases
evidenced the formation of diphenylthioether and decylphenyl-
thioether for 1 and 2 respectively, resulting from C–P bond
Experimental
activation (ca. 2%). The corresponding products originating General
from Caryl–S and Calkyl–S bond activations (ca. 5%), benzene
(for ligand 1) and decane (for ligand 2), were also detected.
All manipulations of air- and moisture-sensitive compounds
were performed under a dry nitrogen or argon atmosphere
using standard Schlenk and vacuum-line techniques. Reagents
were purchased from commercial providers and used without
further purification. The organic solvents were purified by
standard procedures and distilled under nitrogen. Thioether-
Conclusions
In summary, we could prove that the thioether-phosphine 1 phosphine ligands 1 and 2 were prepared following literature
containing a phenyl moiety on the sulphur atom is able to give methods.26,27 NMR spectra were recorded on a Varian (Unity
1
small and well-dispersed nanoparticles. In contrast, ligand 2 Inova) 300 and on a Bruker Avance 300 (300 MHz for H NMR
leads to the formation of agglomerates.
and 121 MHz for 31P NMR). TEM images were obtained using
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a transmission electron microscope JEOL JEM 1400 running at 1 h due to the intensity decrement of complex signals and the
120 kV. EDX spectra were recorded on a JEOL JEM-2010 gradual change from a dark red solution to a black suspen-
running at 120 kV coupled to an Oxford ISIS or a Thermoscien- sion. The complex cis-[Pd(PS)2] was prepared in order to assign
tific detector. FT-IR spectra were recorded on a Perkin Elmer the signal in 31P NMR at 48 ppm according to the literature
FT-IR 1605 spectrophotometer. GC-MS analyses were per- method.22
formed on: (a) a Perkin-Elmer Clarus 500 chromatograph
General procedure for the identification of Pd molecular
equipped with an FID, an MS-detector and an SGE BPX5 capil- species at a Pd : L ratio of 1 : 0.2. An NMR tube (Wilmad
lary column; and (b) a Thermo-Electron Trace GC Ultra fitted NMR tube w/J. Young valve, 5 mm) was charged with
with an FID, an MS-detector, and a DB-5MS capillary column. [Pd2(dba)3]·CHCl3 (0.0120 mmol, 12.42 mg) and the appropri-
XPS measurements were performed using a VG Microtech ate ligand (0.0024 mmol, ratio Pd : L = 1 : 0.2) and connected to
ESCA2000 Multilab UHV system with an Al Kα X-ray source a vacuum line. THF (0.6 mL) dried and degassed was trans-
(hν = 1486.6 eV) and a CLAM4 MCD analyser. XPS spectra were ferred under an argon atmosphere and the system was manu-
obtained at 55° from the normal surface in the constant pass ally stirred at room temperature. 31P NMR spectra were
energy mode (CAE), E0 = 50 and 20 eV for survey and high recorded at different time intervals. The complex trans-
resolution narrow scan. Peak positions were referenced to the [Pd(PS)2] was prepared in order to assign the signal in 31P
Shirley background Ag 3d5/2 core level at 368.20 eV, Au 4f7/2 at NMR at 52 ppm according to the literature method.22
84.00 eV and C 1s hydrocarbon groups at 285.00 eV central
Procedure for ligand exchange reactions of PdNP1 with CO
peaks. XPS spectra were fitted with the program SDP v 4.1.28 and dodecanethiol (DDT). An NMR tube (Wilmad NMR tube
The atomic relative sensitivity factor (RSF) reported by Scofield w/J. Young valve, 5 mm) was charged with PdNP1 (ca. 10 mg)
was corrected by a transmission function of the analyser.29 and the appropriate deuterated solvent (0.6 mL, THF-d8 for CO
XPS spectra of Pd 3d and S 2p core levels showed spin–orbit and CD2Cl2 for DDT) and connected to a vacuum line. The
coupling, and the doublet separation was set to 1.21 eV and corresponding ligand, CO (3 bar) and DDT (0.0786 mmol,
5.29 eV. XPS error is based on considering a detection limit 18.8 µL, 1 : DDT ratio = 1 : 3) were added under an argon
estimated to be 0.1% in mass and uncertain propagation. For atmosphere and stirred manually. 31P NMR spectra in the case
the deconvolution analysis the uncertainty was estimated at of CO presence were recorded at 1 h, 24 h, 3 and 7 days,
5% of the binding energy. Deconvolution analyses were done whereas for the experiment in the presence of DDT, 1H and 31
P
using reference materials: synthesis and characterisation of NMR spectra were recorded at 2 h, 48 h, 5 days, 7 days and
[PdCl22] and [Pd(SC10H21)2]n are described in ESI;† [PdCl21],26 3 weeks.
[Pd(SPh)2]n,30 cis/trans-[Pd(PS)2]22 and PdS were prepared as
General procedure for the thermal treatment of PdNPL and
reported in the literature. The Pd(0) component was referenced ligands with Pd/C (L = 1, 2). PdNPL (ca. 10 mg) and toluene
using the values reported in the literature for [Pd(PPh3)4],24a (10 mL) were placed in a flask. Similarly, the appropriate
elemental palladium24b,c and Pd(0)NPs.31
thioether-phosphine ligand 1 or 2 (0.0461 mmol, Pd : L ratio
General procedure for the synthesis of palladium nanoparti- 0.2), Pd/C 10% (0.2349 mmol Pd, 0.25 g) and toluene (15 mL)
cles PdNPL (L = 1, 2). [Pd2(dba)3]·CHCl3 (0.0870 mmol, 80 mg) were placed in a flask. The system was stirred at reflux temp-
and the appropriate ligand (0.0174 mmol, ratio Pd : L = 1 : 0.2) erature for 24 h. The solution was filtered over celite and the
were placed in a Fisher-Porter bottle. THF (80 mL) dried and remaining solution was concentrated at reduced pressure. The
degassed was introduced under an argon atmosphere. The residue was analysed by 31P NMR and GC-MS.
system was then pressurized with H2 (3 bar) and stirred at
Oxidative treatment of PdNPL and ligands using nitric acid.
room temperature overnight, leading to a black suspension. Distilled water (1 mL) was added to PdNPL (ca. 10 mg) or the
After replacing the residual H2 pressure by argon, a sample appropriate ligand (for 1: 10 mg, 0.027 mmol; for 2: 6 mg,
was taken for TEM and GC-MS analyses. THF was removed 0.014 mmol) was placed in a flask and cooled with an ice bath;
under reduced pressure and the remaining solid was washed nitric acid (conc. 68%) was slowly added (2 mL) and the solu-
with pentane (3 × 10 mL) and dried under reduced pressure. tion was stirred for 20 h at room temperature. The aqueous
1
The organic phase was concentrated and analysed by H NMR solution (orange for PdNP and colourless for L) was then
proving the absence of free ligand.
extracted with CH2Cl2 (1 mL). The organic phase was washed
General procedure for 31P NMR monitoring of PdNPL for- with water in order to eliminate the remaining acid and then
mation (L = 1, 2). An NMR tube (Wilmad NMR tube w/J. analysed by GC headspace in the case of ligands, GC/MS and
Young valve, 5 mm) was charged with [Pd2(dba)3]·CHCl3 31P NMR for both PdNP and ligands.
(0.0132 mmol, 13.71 mg) and the appropriate ligand
General procedure for the hydrogenation treatment of
(0.0026 mmol, ratio Pd : L = 1 : 0.2) and connected to a vacuum ligands using Pd/C. The appropriate thioether-phosphine
line. THF (0.6 mL) dried and degassed was transferred under ligand 1 or 2 (0.094 mmol, Pd : L ratio = 1 : 0.2), Pd/C 10% cata-
an argon atmosphere and stirred manually and the 31P NMR lyst (0.4698 mmol Pd, 0.5 g) and THF (20 mL) were placed in a
spectrum was recorded. The system was then pressurized with 45 mL stainless steel autoclave equipped with a magnetic
H2 (1 bar) and stirred manually at room temperature. 31P NMR stirrer. The system was then pressurized with H2 (3 bar) and
spectra were recorded every 10 minutes during the first hour stirred for 18 h at room temperature. Then, organic phases
and at 4, 6 and 24 h. Formation of PdNP was presumed after were separated and analysed by GC-MS.
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13 D. Mott, J. Yin, M. Engelhard, R. Loukrakpam, P. Chang,
G. Miller, I.-T. Bae, N. C. Das, C. Wang, J. Luo and
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Acknowledgements
This work was financially supported by the bilateral Franco-
Mexican project PCP B330/58/11, DGAPA-UNAM PAPIIT IN 14 For FeP nanoparticles, see: (a) J.-H. Chen, M.-F. Tai and
231211, CONACYT CB167443, Université Paul Sabatier and
CNRS. A. M. L.-V. thanks Conacyt for a PhD grant.
K.-M. Chi, J. Mater. Chem., 2004, 14, 296 for InP nano-
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15 I. Amor, M. E. García, M. A. Ruiz, D. Sáez, H. Hamidov and
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