Preparation and tunable photoluminescence of carbogenic nanoparticles confined in a microporous magnesium-aluminophosphate

Article abstract of DOI:10.1021/ic1000039

Highly photoluminescent carbogenic nanoparticles (CNPs) have been prepared in MAPO-44, a Mg-substituted aluminophosphate molecular sieve with a chabazite structure, through thermal decomposition of the occluded template or loaded organic molecules. The resulting composite phosphors can be excited by a broad range of light in the ultraviolet region, and the emission wavelength is tunable through varying the thermal treatment condition. It is demonstrated that the emission wavelength is dependent on the carbon content in the composite phosphor materials, and the higher the content, the longer the emission wavelength. Correspondingly, upon excitation at a single UV wavelength, the emission color is finely tuned from violet to orange-red for samples with various carbon contents. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectroscopy have been employed to elucidate the nature of the carbonaceous species in the phosphors. Heterogeneity or defects have been found to be prevalent in the CNPs of the composite phosphor materials, and it is these defects that form surface states responsible for the photoluminescence of the materials.

Full text of DOI:10.1021/ic1000039

Inorg. Chem. 2010, 49, 5859–5867 5859
DOI: 10.1021/ic1000039
Preparation and Tunable Photoluminescence of Carbogenic Nanoparticles
Confined in a Microporous Magnesium-Aluminophosphate
†
,‡
†,‡
‡
†
,†
Yang Xiu, Qian Gao, Guo-Dong Li, Kai-Xue Wang, and Jie-Sheng Chen*
†
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China,
‡
and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,
Jilin University, Changchun 130012, P. R. China
Received January 1, 2010
Highly photoluminescent carbogenic nanoparticles (CNPs) have been prepared in MAPO-44, a Mg-substituted
aluminophosphate molecular sieve with a chabazite structure, through thermal decomposition of the occluded
template or loaded organic molecules. The resulting composite phosphors can be excited by a broad range of light in
the ultraviolet region, and the emission wavelength is tunable through varying the thermal treatment condition. It is
demonstrated that the emission wavelength is dependent on the carbon content in the composite phosphor materials,
and the higher the content, the longer the emission wavelength. Correspondingly, upon excitation at a single UV
wavelength, the emission color is finely tuned from violet to orange-red for samples with various carbon contents. X-ray
photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectroscopy have been employed to
elucidate the nature of the carbonaceous species in the phosphors. Heterogeneity or defects have been found to be
prevalent in the CNPs of the composite phosphor materials, and it is these defects that form surface states responsible
for the photoluminescence of the materials.
3
,5
Introduction
elements. Therefore, phosphor materials which are stable,
efficient, and environmentally benign have attracted much
Phosphors are a class of luminescent materials which
find applications in a variety of fields. For example, daily
lighting, projection televisions, fluorescent tubes, industrial
tunable lasers, and X-ray intensifying screens all involve
6-12
attention recently.
Especially, with the emergence of vio-
13
let light-emitting devices and lasers, new phosphors that
can be excited in the longer-wavelength UV range (340-
400 nm) and have luminescence in the visible region may be
used to manufacture fluorescent lamps without involving
mercury. Among the new phosphors free of metal activators,
1-4
phosphors.
However, the commercially available phos-
phors are usually composed of costly or toxic metals (or metal
activators), such as germanium, cadmium, or rare-earth
6-9
the siloxane-based organic-inorganic hybrid materials
6
have been investigated extensively. For instance, Green et al.
prepared a series of highly emissive phosphors from alkoxy-
silane precursors, such as tetramethoxysilane or 3-amino-
propyltriethoxysilane (APTES), and various carboxylic acids
through a sol-gel route. In particular, the APTES-formic
acid hybrid shows a photoluminescence efficiency (quantum
yield 35 ( 1%) distinctly higher than those of the other
*
To whom correspondence should be addressed. Fax: (þ86)-21-54741297.
E-mail: chemcj@sjtu.edu.cn.
(
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Published on Web 06/08/2010
pubs.acs.org/IC

5
860 Inorganic Chemistry, Vol. 49, No. 13, 2010
Xiu et al.
we reported the preparation of highly photoluminescent
activator-free ZnO nanocrystals (quantum yield about
and the previously reported luminescent porous compounds
are usually generated either by incorporating organic dyes or
by doping with metal activators in the porous hosts.
Activator-free (or dye-free) microporous systems with tun-
able photoluminescence have not been achieved yet prior to
our work.
14
28-30
4
5%) through a surface modification approach.
Luminescent carbogenic nanoparticles (CNPs) are a new
typeof carbonaceous material whichemitin the visible region
under irradiation with UV light. In some sense, luminescent
CNPs are advantageous over the conventional phosphors
as they are chemically inert, biocompatible, and environ-
mentally benign. Various techniques have been employed
In this article, we describe the preparation and characteri-
zation of a new type of stable, wavelength-tunable, and
activator-free luminescent material, that is, host-guest phos-
phor systems with carbogenic nanoparticles confined in a
Mg-substituted microporous aluminophosphate (MgAPO-44)
solid. Upon excitation at a single wavelength, emissions with
various wavelengths are obtained from the composite phos-
phors depending on the carbon content, which can be readily
regulated through adjusting the preparation conditions.
MgAPO-44, a magnesium-substituted aluminophosphate
molecular sieve with a chabazite (CHA) structure (desig-
nated MAPO-CHA), has elongated cages (0.9 ꢀ 0.7 nm)
interconnected through eight-member ring windows (0.4 ꢀ
0.4 nm). The CNPs located in the pores of the MAPO-CHA
host are derived from the decomposition of the template
reagent or the loaded small organic molecules without the
need for further surface passivation, and the formation of the
CNPs does not rely on sophisticated equipment such as a
1
5-24
for the preparation of luminescent CNPs.
For instance,
15
Liu et al. reported the preparation of luminescent CNPs
from candle soot through oxidative acid treatment, and the
resulting CNPs with different emission colors were isolated
16,17
through electrophoretic separation. Sun and co-workers
developed a laser ablation technique to prepare the CNPs
from a carbon target followed by surface passivation
with diamine-terminated oligomeric PEG1500N, whereas Hu
18
et al. employed an improved one-step approach for the
synthesis of luminescent CNPs though laser irradiation of a
suspension of carbon materials in organic solvents. More
recently, luminescent carbogenic dots were obtained by Peng
20
and Travas-Sejdic via surface passivation in an aqueous
solution using carbohydrates as thestarting materials. Never-
theless, the above-mentioned approaches for CNP prepara-
tion are usually sophisticated, and the obtained CNPs always
require surface stabilization with organic (polymer) mole-
16-18
laser ablation apparatus.
In addition, the micropores of
the MAPO-CHA host lattice offer a favorable microscopic
environment for the facile regulation and stabilization of the
ultrafine CNPs.
16-20
cules to become luminescent
and dispersion in liquid to
avoid agglomeration. These requirements inevitably affect
the thermal stability of the resulting luminescent materials,
limiting their practical applications to a considerable extent.
Aluminophosphate-based molecular sieves are micropo-
rous crystalline solids with widespread applications in areas
Experimental Section
Materials. Aluminum hydroxide was purchased from Al-
drich. Absolute ethanol, acetone, glacial acetic acid, orthopho-
sphoric acid (85 wt %), hydrofluoric acid (40 wt %), and
hydrochloric acid (38 wt %) were all purchased from Beijing
25-27
such as catalysis and gas separation.
Well-defined synth-
esis conditions allow these molecular sieve materials to be
highly transparent in the UV and visible regions, and in
principle, their spatial confinement offers the advantage of
formation of nanoparticles with photoluminescent proper-
ties. However, practical applications of aluminophosphate-
based molecular sieves as host materials for accommodation
of luminescent guest species have not been very successful,
3 2 2
Chemical Factory; magnesium acetate (Mg(CH COO) 4H O)
3
and n-hexane were acquired from Tianjin No. 1 Chemical Rea-
gent Factory, whereas cyclohexylamine was acquired from
Shanghai Chemical Reagent Factory. All the reagents were of
analytical grade and used as received without further purifica-
tion. Deionized water (PURELAB Plus, PALL) was used in all
of the experiments.
Sample Preparation. A. Synthesis of MAPO-CHA. The
MAPO-CHA compound was synthesized from a hydrothermal
reaction system by following the procedure described as follows.
(
14) (a) Liu, D. P.; Li, G. D.; Su, Y.; Chen, J. S. Angew. Chem., Int. Ed.
2
006, 45, 7370. (b) Liu, D. P.; Li, G. D.; Li, J. X.; Li, X. H.; Chen, J. S. Chem.
Commun. 2007, 4131.
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P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; Luo, P. G.; Yang,
H.; Kose, M. E.; Chen, B.; Veca, L. M.; Xie, S. Y. J. Am. Chem. Soc. 2006,
Under continuous stirring, 2.345 g (30.06 mmol) of Al(OH) and
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1
2
.756 g (8.19 mmol) of Mg(CH
0 mL of water, followed by the successive addition of 6 mL
3 2 2
COO) 4H O were dispersed in
3
(
42.5 wt %, aqueous solution, 32.76 mmol) of phosphoric acid,
1
28, 7756.
17) Cao, L.; Wang, X.; Meziani, M. J.; Lu, F.; Wang, H.; Luo, P. G.; Lin,
3.62 mL (31.94 mmol) of cyclohexylamine and 1.5 mL (20 wt %,
aqueous solution, 2 mmol) of hydrofluoric acid. The cyclohexyl-
amine and the HF acid functioned as a template and a minera-
lizer, respectively. The gel mixture was stirred vigorously until
becoming homogeneous and was sealed in a 50 mL PTFE-lined
stainless steel autoclave, which was subsequently heated at
180 °C for 48 h under autogenous pressure. Afterward, the reac-
tion system was slowly cooled to room temperature (RT). The
resulting solid product was recovered by filtration, washed
thoroughly with deionized water, sonicated to remove all the
residual reagents or the loosely attached crystals, and dried at
(
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Georgakilas, V.; Giannelis, E. P. Chem. Mater. 2008, 20, 4539.
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Nano 2009, 3, 2367. (b) Zhao, Q. L.; Zhang, Z. L.; Huang, B. H.; Peng, J.; Zhang,
M.; Pang, D. W. Chem. Commun. 2008, 5116.
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28) J u€ stel, T.; Wiechert, D. U.; Lau, C.; Sendor, D.; Kynast, U. Adv.
Funct. Mater. 2001, 11, 105.
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1424. (b) Chen, W.; Sammynaiken, R.; Huang, Y. J. Appl. Phys. 2000, 88, 5188.
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Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 5861
ambient temperature. The dry sample was designated as-synthe-
sized MAPO-CHA.
B. Preparation of Fully Carbonized MAPO-CHA (FC-
MAPO). The as-synthesized MAPO-CHA was placed into a
programmable tube furnace and heated from RT to 400 °C at a
-
1
temperature ramp rate of 2 °C min under an atmosphere of
nitrogen. The system was kept at 400 °C for 3 h, followed by
-
1
further heating to 550 °C at a rate of 1 °C min and a 4-h iso-
thermal hold at this temperature. The carbonization of the
template was accomplished in this process, resulting in the fully
carbonized MAPO-CHA (designated FC-MAPO) with a black
color.
C. Preparation of Luminescent MAPO-CHA (C-MAPO).
-
1
Under an atmosphere of oxygen with a flow rate of 0.5 L min
the FC-MAPO was heated in the programmable tube furnace
,
-
1
from RT to 550 at 4 °C min , followed by a 2-h isothermal hold
at 550 °C. The resulting carbon-containing sample was desig-
nated C-MAPO-A. For C-MAPO-B, -C, -D, and -E, the iso-
thermal times in the heating process were controlled to be
30 min, 15 min, 10 min, and 5 min, whereas for C-MAPO-F
the time was 5 min but the heating temperature was varied to
Figure 1. Powder X-ray diffraction patterns of as-synthesized MAPO-
CHA (a), FC-MAPO (b), C-MAPO-F (c), and FD-MAPO (d).
of 1,1-diphenyl-2-picrylhydrazyl (DPPH), whereas the spin
concentrations were determined by comparing the recorded
spectra with that of a Mn marker and DPPH, using the built-
in software of the spectrometer.
5
00 °C.
D. Preparation of Fully Detemplated MAPO-CHA (FD-
MAPO). FD-MAPO was prepared in the same manner as that
of C-MAPO-A, except that the isothermal temperature and time
were 600 °C and 12 h, respectively. Snow-white crystals were
obtained after this treatment.
Results and Discussion
General Characterization. Figure 1 shows the XRD
patterns of the as-synthesized MAPO-CHA (a), FC-
MAPO (b), C-MAPO-F (c), and FD-MAPO (d). These
samples show characteristic diffraction peaks which are
in good agreement with the standard data of MAPO-
E. Preparation of Luminescent MAPO-CHA from Loaded
Organic Molecules. In a typical preparation, the FD-MAPO
sample was submerged in acetone for 4 h, and the obtained solid
with loaded acetone was then heated to 500 °C in a rate of
31
CHA with and without template molecules. In the pat-
tern of FC-MAPO, a broad peak ranging from 12° to 36°
-
1
4
°C min in the air, followed by a 20 min isothermal hold. The
resulting solid product was designated AT-MAPO. Ethanol and
acetic acid loaded samples were prepared similarly (the calcina-
tion temperature was 550 °C instead of 500 °C in this case), and
the obtained products were designated ET-MAPO and AC-
MAPO, respectively. The n-hexane-MAPO sample was ob-
tained by the thermal treatment of n-hexane loaded FC-MAPO
(the so-called graphite peak) is observed, indicative of the
presence of a graphite-like structure of carbon. This ob-
servation suggests that the template cyclohexylamine is
decomposed by the thermal treatment in the N atmo-
2
sphere, and the resulting carbon fragments are accumu-
lated in the host lattice of the MAPO-CHA material. The
broad peak disappears for the C-MAPO-F derived from
oxidation of FC-MAPO because the amount of the carbo-
naceous matter in this sample is too low to be detected by
XRD. However, the energy dispersive spectroscopy (EDS)
analysis (Figure 2) conducted on the inner cross-section
of the crystals confirms the presence of carbon atoms,
although in a very small amount, in the partly oxidized
sample C-MAPO-F. The disappearance of the broad
carbon peak in the XRD pattern of C-MAPO-F indicates
that the oxidation process can regulate the carbon content
in the microporous host material. From Figure 1 it is also
seen that the thermal treatment does not affect the crystal-
line structure of the MAPO material, as the main diffrac-
tion peaks remain after the thermal treatment for both
C-MAPO-F and FD-MAPO (thevariation in relativepeak
intensity and position arises from the removal of the
template molecules after detemplating), and no new crys-
talline phases appear in the final product.
in a rapidly flowing atmosphere of N
was the same as that for AT-MAPO.
Characterization. The powder X-ray diffraction (XRD) pat-
2
. The heating procedure
terns were recorded on a Rigaku D/Max 2550 X-ray diffracto-
˚
meter using Cu KR radiation (λ = 1.5418 A) over the 2θ range
of 5-40° at room temperature. A JEOL JSM-6700F scanning
electron microscope equipped with a standard secondary elec-
tron detector was used to image the inner cross-section of the
crystals, which were dehydrated and embedded in conductive
adhesive on the coppery specimen stage. Energy dispersive spe-
ctroscopy (EDS) was conducted to confirm the presence of car-
bon in the C-MAPO phosphors using an attached X-ray energy
dispersive spectroscopy system, whereas the carbon contents
of the bulk samples were determined through elemental analysis
on a Perkin-Elmer 2400 elemental analyzer. The infrared spectra
were obtained with a Bruker IFS 66 V/S FTIR spectrometer
using KBr pellets, and the Raman spectra were recorded at
ambient temperature on a Jobin-Yvon T64000 spectrometer
with an excitation wavelength of 514.5 nm. The TEM images
were taken on a JEOL JEM-3010 transmission electron micro-
scope operating at an accelerating voltage of 300 kV. N adsorp-
2
tion/desorption measurements were performed on a Micromeri-
tics ASAP 2020 M instrument after the samples were degassed at
The as-synthesized MAPO-CHA, the FC-MAPO, and
the C-MAPO-F samples are further characterized thro-
ugh FTIR spectroscopy. In the FTIR spectrum of the as-
synthesized MAPO-CHA (Figure 3Aa), there appear
-
5
2
00 °C and 1 ꢀ 10 Torr for 10 h prior to the measurements. A
Perkin-Elmer LS 55 luminescence spectrometer was used to
obtain the excitation and emission spectra for all of the solid
products. The X-ray photoelectron spectra (XPS) were recorded
on a VG ESCALAB MK II electron spectrometer, whereas the
electron spin resonance (ESR) spectroscopy was performed on a
JEOL JES-FA200 spectrometer operating in the X-band mode.
The g values were calculated by comparison with the spectrum
-1
-1
-1
bands at 1454 cm , 1508 cm , and 1606 cm which
are associated with the methylene symmetric bending
(31) Lohse, U.; Parlitz, B.; M u€ ller, D.; Schreier, E.; Bertram, R.; Fricke,
R. Microporous. Mater. 1997, 12, 39.

5
862 Inorganic Chemistry, Vol. 49, No. 13, 2010
Xiu et al.
Figure 4. TEM images of C-MAPO-E (a) and carbogenic nanoparticles
(b) obtained by removing the host lattice of C-MAPO-E with concen-
trated hydrochloric acid. Inset is the HRTEM image of a typical CNP.
-
1
(
1213 cm ), and T-O bending vibration for the dou-
-1 32
ble-6-rings (644 cm ). In Figure 3Ab and Ac, the
characteristic framework vibrations which are also pre-
sent in Figure 3Aa are observed, but the template bands
disappear for FC-MAPO and C-MAPO-F, confirming
that the thermal treatment leads to template decomposi-
tion with the framework integrity of the MAPO material
being maintained.
Figure 2. Energy dispersive spectrum of the inner cross-section of a
C-MAPO-F crystal. The carbon signals with an essentially identical
intensity are present at sites A, B, and C, indicating that the carbogenic
species were distributed homogeneously in the crystal.
Although the attempt to obtain the Raman spectra of
the oxidized samples (the C-MAPO materials) was not
successful due to their intense photoluminescent emis-
sions, the Raman spectrum of FC-MAPO, which was
used as the precursor for the formation of the C-MAPO
composite phosphors, has been recorded (Figure 3B).
-
1
-1
Two peaks centered at 1600 cm and 1355 cm appear
in the Raman spectrum with the former being attributable
to graphite-like carbon with an sp configuration.
2
33a
-
1
The slight deviation of this 1600 cm peak from that
(
the size of the graphite-like carbon in FC-MAPO is in the
-
1
1580 cm ) for ideal crystalline graphite indicates that
3
3b,c
-1
nanometer range.
with the breathing mode of A symmetry. Because this
The 1355 cm band is associated
3
3b
1
g
vibrational mode is forbidden in ideal graphite and only
observable in an imperfect graphite structure, disordered
or defective regions are considered to be present in the
carbon matter of the FC-MAPO sample.
Figure 4a shows the TEM image of the C-MAPO-E
material collected on a carbon grid. Owing to the low
contrast, the CNPs in this sample are not observed in the
TEM micrograph. However, if the MAPO host lattice is
removed with concentrated hydrochloric acid, CNPs
can be isolated through centrifugation of the acid-
treated product. As shown in Figure 4b and Figure S1
(Supporting Information), the isolated CNPs are spheri-
cal with a uniform size (ca. 4 nm). In principle, the size of
the carbogenic nanoparticles confined in the micropores
should not exceed the cage size of the inorganic MAPO
host. Therefore, the significant difference between the size
of the isolated CNPs (4 nm) and the diameter (0.9 nm) of
the CHA micropores indicates that the isolated CNPs
are formed through agglomeration of the primary carbo-
genic particles initially occluded in the C-MAPO-E host.
Particle agglomeration is very common for ultrafine
powders and nanoparticles, but within the host lattice
Figure 3. FTIR spectra (A) of as-synthesized MAPO-CHA (a), FC-
MAPO (b), and C-MAPO-F (c) and Raman spectrum (B) of FC-MAPO.
vibrations, C-N stretching vibrations, and N-H bend-
ing vibrations of the template molecules, respectively.
All the other bands are assignable to the framework
vibrations of the chabazite structure, such as the O-P-O
asymmetric stretching vibration in PO groups (1081
(33) (a) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2001, 64, 075414. (b)
Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095. (c) Ferrari, A. C.;
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cm ), the P-O-Al asymmetric stretching vibration
(
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Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 5863
in Figure 6, all six representative samples obtained under
different conditions exhibit photoluminescence at room
temperature upon excitation at 365 nm. The recorded PL
spectra show that the emission peak of C-MAPO-A to -F
varies from 420 to 550 nm. Correspondingly, the observed
emission color changes from violet through blue, cyan,
green, and yellow to orange-red. It is obvious that the
emission wavelength is dependent on the isothermal time.
Specifically, a longer time of oxidative pyrolysis leads to a
shorter wavelength for the C-MAPO emission. In order
to reveal the reason behind this observation, elemental
analysis of the C-MAPO samples has been performed,
and the analysis result shows that the carbon contents
in C-MAPO-A, -C, and -F are 0.24, 0.39, and 1.20 wt %,
respectively (Table S1, Supporting Information). These
data demonstrate that the amount of carbon in the
C-MAPO material is inversely proportional to the iso-
thermal time. In addition, the isothermal temperature for
C-MAPO-F is 500 °C, lower than that (550 °C) for other
C-MAPO samples, and as a result, the carbon content in
this sample is distinctly higher. On the basis of the analysis
result, it is concluded that the PL wavelength of the
C-MAPO material is correlated with the carbon content
in the material. Consequently, through varying the oxi-
dative pyrolysis condition and hence the carbon content,
C-MAPO phosphors with tunable emission colors, from
violet to orange-red, can be easily obtained.
To further confirm the contribution of CNPs to the
PL property of C-MAPO phosphors, FD-MAPO, AT-
MAPO, ET-MAPO, and AC-MAPO samples have been
prepared (see the Experimental Section) for comparison.
In this preparation, the fully detemplated material FD-
MAPO, which contains no carbonaceous matter at all,
is infused with acetone (AT), ethanol (ET), and acetic
acid (AC), respectively, followed by oxidative pyrolysis in
the air. The recorded PL spectra for these samples are
shown in Figure 7. It is seen that no photoluminescence is
exhibited by the FD-MAPO material (Figure 7a), indi-
cating that the MAPO host is completely nonluminous.
In contrast, the AT-MAPO, AC-MAPO, and ET-MAPO
samples emit intense light upon excitation (Figure 7a,b).
This observation demonstrates that the photolumine-
scent property of the MAPO material can be conferred
upon through generation of carbogenic species in the
pores of the material. Figure 7b shows the PL spectra of
AT-MAPO phosphors prepared at 500 °C with various
isothermal times. It is seen that the emission wavelength
decreases with increasing the isothermal time, in a manner
similar to that for the C-MAPO phosphors prepared
from the FC-MAPO precursor. Because acetone, etha-
nol, and acetic acid cannot be the direct emissive centers,
the formation of CNPs from the carbonization of these
guest molecules in the MAPO host is considered to be
responsible for the photoluminescence. For the restric-
tion of the micropores, a proportion of the infused orga-
nic molecules cannot volatize as the temperature is ele-
vated rapidly, and they decompose into carbon fragments
and accumulate within the micropores of FD-MAPO,
resulting in the photoluminescent CNPs.
Figure 5. Emission and excitation spectra of as-synthesized MAPO-
CHA (blue line) and C-MAPO-A (red line). The emission spectra were
recorded at 365 nm excitation, whereas the excitation spectra were
acquired with a detection wavelength of 420 nm.
of the microporous MAPO, the ultrafine CNPs are well-
protected and are prevented from agglomeration. The
inset of Figure 4b illustrates a lattice spacing of 0.33 nm,
2
2
which is very close to the graphite 002 lattice spacing.
After isolation from the MAPO host, the CNPs can be
easily dispersed in ethanol, acetone, and acidic or alkaline
aqueous solutions, and these dispersion systems remain
stable for at least one month without precipitation. To
clarify whether the confined CNPs affect the microporo-
sity of the host materials, N2 adsorption/desorption
measurements for the FD-MAPO and the C-MAPO-E
samples have been performed. The surface area based
on the BET model for FD-MAPO is calculated to be
2
2
6
42 cm /g, whereas that for C-MAPO-E is 529 cm /g. The
pore size for both samples is approximately 0.5 nm
Figure S2, Supporting Information). The small differ-
(
ences in BET surface area and pore size distribution of the
MAPO materials with and without CNPs indicate that
the presence of CNPs in the C-MAPO samples does not
affect the adsorption property of the host materials
significantly.
Photoluminescent Properties. Figure 5 shows the exci-
tation and emission spectra for the as-synthesized
MAPO-CHA and the C-MAPO-A samples at ambient
temperature. No peaks are observed for the excitation
and emission spectra of the as-synthesized MAPO-CHA.
In contrast, the C-MAPO-A material shows strong ex-
citation and emission peaks. Upon irradiation with UV
light (365 nm), the C-MAPO-A sample exhibits a broad
emission centered at about 420 nm, and correspondingly
a violet color appears to the naked eye for the sample. The
excitation spectrum of C-MAPO-A consists of a broad
band with a maximum at 344 nm, suggesting that the
sample can be excited by the light over a broad range in
the ultraviolet region. This observation demonstrates
that, through simple thermal treatment, the nonluminous
MAPO-CHA precursor can be made highly photolumi-
nescent at ambient temperature. On the other hand, it is
noted that the fully detemplated FD-MAPO sample is not
luminous either, suggesting that the CNPs occluded in the
C-MAPO materials are responsible for the photolumi-
nescence of the phosphors.
Our experiment indicates that CNP-containing MAPO
materials with photoluminescent properties are able to
be obtained either through direct oxidative pyrolysis of
the FC-MAPO precursor or through organic loading
The luminescent property of the C-MAPO phosphors
depends on the oxidative pyrolysis condition. As shown

5
864 Inorganic Chemistry, Vol. 49, No. 13, 2010
Xiu et al.
Figure 6. PL emission spectra and photographs of C-MAPO samples under UV irradiation: (a) C-MAPO-A, 120 min, 550 °C; (b) C-MAPO-B, 30 min,
5
50 °C; (c) C-MAPO-C, 15 min, 550 °C; (d) C-MAPO-D, 10 min, 550 °C; (e) C-MAPO-E, 5 min, 550 °C; and (f) C-MAPO-F, 5 min, 500 °C. For clarity, the
PL intensities were normalized.
or other organic molecules for photoluminescence, and
the thermal stability of our materials is obvious because
they have been subjected to thermal treatment at elevated
temperatures (at least 500 °C). In contrast, the polymer-
passivated CNPs are inherently unstable toward thermal
treatment because of the decomposition of polymer mo-
lecules at elevated temperatures (nonpassivated CNPs
2
1
tend to agglomerate, losing their luminescent property).
Furthermore, we can tune the emission wavelength at a
fixed excitation wavelength simply through varying the
carbon contents in the C-MAPO materials, whereas the
previously reported CNP materials usually need to use diffe-
rent excitation wavelengths to achieve different emission
1
6,17,19,20
wavelengths.
We have also estimated the external photolumine-
scent quantum yields of the materials we obtained through
the Wrighton-Ginley-Morse method using magnesium
oxide as a reflecting standard (for details, see the Support-
3
4
ing Information). For C-MAPO-A to -F, the quantum
yields are about 12, 26, 23, 19, 12, and 7%, respectively.
These values are comparable with those for the reported
7
c,8a,b,d
siloxane-based phosphors in the solid state.
On the
other hand, it is noted that the AC-MAPO shows an ex-
ternal quantum yield of ca. 40%, distinctly higher than
those for the other samples. The high quantum yield in
combination with the high thermal stability renders our
materials promising for practical applications.
Origin of Photoluminescence for C-MAPO Materials.
Since the microporous host material MAPO-CHA is
not photoluminescent, the PL property of the composite
C-MAPO phosphors must arise from the CNPs occluded
in the host lattice. X-ray photoelectron spectroscopy
(XPS) has been employed to characterize the CNPs in
the C-MAPO samples. Figure 8 shows the C1s XPS
spectra of FC-MAPO, C-MAPO-A, C-MAPO-E, and
Figure 7. PL emission spectra (a) of AC-MAPO (red line), ET-MAPO
(blue line), and FD-MAPO (black line) and PL emission spectra (b) of
AT-MAPO prepared at 500 °C with an isothermal time of 20 min (red
line), 60 min (orange line), 150 min (green line), and 210 min (violet line).
followed by thermal treatment. Unlike the photolumi-
16-20
nescent CNP materials reported in the literature, our
materials do not need further passivation with polymers
(
34) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. J. Phys. Chem. 1974, 78,
2229.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 5865
Figure 8. C1s XPS spectra for FC-MAPO (a), C-MAPO-A (b), C-MAPO-E (c), and n-hexane-MAPO (d) with the corresponding PL spectrumin the inset.
n-hexane-MAPO. As is known, the C1s XPS spectrum of
single crystalline graphite is dominated by an intense,
asymmetric peak centered at 284.6 eV with a long tail at
noted that the full width at half-maximum (FWHM) of the
main peak in C-MAPO-A and -E is 1.69 and 1.84 eV,
distinctly larger than that for ideal graphite (0.35 eV).
4
1
3
5,36
the higher binding energy side.
The main peak in the
Because the graphite peak FWHM depends on the hetero-
geneity (or defects) of the graphene layers in carbon matter
(the more heterogeneous the graphene layers, the wider
spectra (Figure8a-d) forthe carbon-containing samples is
in accordance with that of graphite in the peak position
4
2,43
(
284.6 eV), and therefore this peak is attributable to
the graphite peak), the C-MAPO samples should
contain CNPs with a considerable amount of defects such
2
36
graphite-like carbon atoms with sp hybridization. In
the spectrum of FC-MAPO, there appears a unique
shoulder at 285.9 eV, assignable to the CdN bonds
3
as sp -hybridization carbon atoms, broken bonds, and
3
7,38
43
heteroatoms. These defects are believed to play a role
formed from decomposition of the template cyclohexyl-
amine during the carbonization process. This shoulder peak
is not observed in the photoluminescent C-MAPO sam-
ples, suggesting that oxidation of the FC-MAPO removes
the CdN bonds inthecarbonmatter. Ontheother hand, in
the spectra of C-MAPO-A and -E (Figure 8b,c), there
appears a small peak at 288.5 eV besides the main peak.
Peaks at this position (288.5 eV) are considered to arise
in the photoluminescence of the C-MAPO materials.
Electron spin resonance spectroscopy (ESR) has been
performed to reveal the relationship between the defects
of CNPs and the PL property of the C-MAPO materials.
As shown in Figure 9, no ESR signal is observed for the
as-synthesized MAPO-CHA and the fully detemplated
FD-MAPO samples, whereas the carbon-containing sam-
ples all exhibit distinct ESR signals. These ESR signals at
g = 2.003, characteristic of free radicals, indicate the pre-
sence of paramagnetic centers in the C-MAPO phosphors.
The line width for AC-MAPO is 0.64 mT, and that for
C-MAPO-F is 0.58 mT. Because the MAPO molecular
sieve host lacks magnetic components, the paramagnetic
centers must be associated with the carbogenic species in
the C-MAPO materials. As demonstrated by the XPS
spectroscopy, there are many defects in the CNPs of
3
5
from carbon atoms bound to oxygen atoms, and there-
3
9,40
fore carbonyl (CdO) groups are thought to be present
in the C-MAPO-A and -E samples. Furthermore, it is
(
35) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am.
Chem. Soc. 2003, 125, 9761.
36) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.;
Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116.
37) Waltman, R. J.; Pacansky, J.; Bates, C. W. Chem. Mater. 1993, 5,
799.
38) Stankovich, S.; Piner, R. D.; Chen, X.; Wu, N.; Nguyen, S. B. T.;
Ruoff, R. S. J. Mater. Chem. 2006, 16, 155.
39) Eberhardt, W.; Sham, T. K.; Carr, R.; Krummacher, S.; Strongin,
M.; Weng, S. L.; Wesner, D. Phys. Rev. Lett. 1983, 50, 1038.
40) Su, C. Y.; Xu, Y.; Zhang, W.; Zhao, J.; Tang, X.; Tsai, C. H.; Li, L. J.
Chem. Mater. 2009, 21, 5674.
(
(
1
(
(41) Watanabe, M. O.; Itoh, S.; Mizushima, K.; Sasaki, T. Appl. Phys.
Lett. 1996, 68, 2962.
(42) Darmstadt, H.; Roy, C.; Kaliaguine, S.; Kim, T. W.; Ryoo, R. Chem.
Mater. 2003, 15, 3300.
(43) Darmstadt, H.; Roy, C. Carbon 2003, 41, 2662.
(
(

5
866 Inorganic Chemistry, Vol. 49, No. 13, 2010
Xiu et al.
Figure 9. ESR spectra of as-synthesized MAPO-CHA (a), FD-MAPO
b), AC-MAPO (c), and C-MAPO-F (d).
(
C-MAPOs. These defects are believed to be responsible for
which give rise to the
4
4,45
the presence of localized radicals,
ESR signals in the samples. Figure 10a displays the
relationship between the emission wavelength and the
electron spin concentration calculated from the corre-
sponding ESR spectra. It is seen that the emission wave-
length is proportional to the spin concentration, which
corresponds to the carbon content in the C-MAPO ma-
terial. The higher the carbon content, the larger the spin
amount, and therefore it is concluded that the emission
wavelength of the C-MAPO phosphors is dependent on
the carbon content in the materials.
Figure 10. Plots of spin concentration (a) and PL intensity (b) versus
emission wavelength for C-MAPO-A through -F.
It is presumed that the thermal treatment of the occlu-
ded template or loaded organic molecules in the micro-
pores of the MAPO host leads to the formation of CNPs
with defects, and these defect-containing CNPs are sepa-
rated and protected by the MAPO host lattice. The
materials. That is, the higher the carbon content, the more
defects which act as luminescent centers. A higher con-
centration of luminescent centers contributes more to the
emission intensity. But if the concentration is too high,
concentration quenching of emission intensity would
dominate as observed for conventional phosphors. There-
fore, the samples (e.g., C-MAPO-F) with a distinctly high
carbon content show weaker emissions than those with
a moderate amount of carbon. The PL spectrum of FC-
MAPO shows a very weak emission (Figure S3, Support-
ing Information). Its PL intensity is so low that the
emission is not clearly observable by the naked eye under
a UV lamp. Since FC-MAPO is formed from the decom-
4
6-48
defects result in electron localization,
and a certain
amount of surface states are thus present to accommodate
these localized electrons. Upon excitation by UV light,
the CNPs with surface states are excited and the radiative
relaxation of the excited CNPs releases energy in the form
of emitted visible light, leading to photoluminescence. It
is found that the emission intensity is correlated with the
carbon content in the C-MAPO materials. With increase
in carbon content from C-MAPO-A to C-MAPO-F, the
emission intensity increases initially, followed by reaching
a maximum for the sample C-MAPO-B and a subsequent
decrease for the rest of the samples (Figure 10b). This
observation is easily rationalized on the basis of competi-
tion between concentration contribution and the quench-
ing of luminescent centers in the CNPs. As revealed by the
ESR spectroscopy, the concentration of CNP defects
which determines the amount of the surface states is
proportional to the carbon content in the C-MAPO
position of the template under a N atmosphere, it has the
2
highest carbon content among all of the C-MAPO sam-
ples. Therefore, the high carbon content is believed to
result in a high concentration of luminescent centers which
severely quench the photoluminescence.
To elucidate the effect of O-containing groups (such as
CdO and C-OH) on photoluminescence, we prepared
n-hexane-MAPO through thermal treatment of n-hex-
ane-infused FD-MAPO under an atmosphere of N . No
2
(
44) Menachem, C.; Wang, Y.; Flowers, J.; Peled, E.; Greenbaum, S. G.
J. Power Sources 1998, 76, 180.
45) Alc ꢀa ntara, R.; Ortiz, G. F.; Lavela, P.; Tirado, J. L.; Stoyanova, R.;
signal related with CdO (or C-OH) bonds except for an
intense peak of graphite-like carbon centered at 284.6 eV
with a weak shoulder at 285.2 eV assignable to carbon
(
Zhecheva, E. Chem. Mater. 2006, 18, 2293.
3
(
(
46) Enoki, T.; Kobayashi, Y. J. Mater. Chem. 2005, 15, 3999.
47) Tomita, S.; Sakurai, T.; Ohta, H.; Fujii, M.; Hayashi, S. J. Chem.
atoms with sp hybridization is observed in the C1s
XPS spectrum of the sample (Figure 8d). The n-hexane-
MAPO material free of O-containing groups also emits
visible light (inset of Figure 8d) under excitation at
Phys. 2001, 114, 7477.
48) Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. Angew.
Chem., Int. Ed. 2006, 45, 4467.
(

Article
65 nm, suggesting that the presence of O-containing
Inorganic Chemistry, Vol. 49, No. 13, 2010 5867
3
correlated with the carbon content in the phosphor materials.
At a lower carbon content, the emission intensity increases with
the content, whereas at a higher carbon content, the emission
intensity is inversely proportional to the content because of
the competition between concentration contribution and
quenching of the luminescent centers in the phosphors. In
contrast with the previously reported CNP phosphors which
are coated with organic molecules, our phosphor materials
possess high thermal stability since the materials have been
subject to elevated temperatures during the preparation pro-
cess. It is anticipated that the preparation approach we adopted
may be extended to other solid CNP phosphors with high
thermal stability and superior luminescent properties.
groups is not necessary for the photoluminescence of
the C-MAPO materials. Nevertheless, the possibility that
the CdO (or C-OH) groups help to enhance the photo-
luminescence of the CNPs is not excluded, as the AC-
MAPO sample which contains a considerable amount of
CdO (or C-OH) groups (the carbon content of AC-
MAPO is 0.36 wt % (Table S1, Supporting Information),
comparable with that of C-MAPO-C) exhibits the highest
quantum yield among all of the samples.
Conclusions
CNP-loaded magnesium-aluminophosphate solid phos-
phors with tunable emission wavelength ranging from 420 to
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20731003)
and the National Basic Research Program of China
550 nm have been prepared through thermal treatment of
the host crystals under different conditions. It is revealed that
the carbogenic species in the phosphors is responsible for the
observed photoluminescence, and the emission wavelength
depends on the carbon content in the materials. XPS and
ESR spectroscopies demonstrate that the PL property arises
from defects which render electron localization in the CNPs
possible. Upon irradiation with UV light, the surface states
with the localized electrons in the CNPs are excited, whereas
the radiative relaxation of the excited surface states releases
energy, emitting visible light. The emission intensity is also
(2007CB613303).
Supporting Information Available: TEM images of CNPs
isolated from C-MAPO-D, C-MAPO-F, and FC-MAPO; pore
size distribution of FD-MAPO and C-MAPO-E; PL emission
spectrum of FC-MAPO; results of C,H,N elemental analysis;
and the detailed procedure for calculation of the quantum
yields. This material is available free of charge via the Internet
at http://pubs.acs.org.