Selective self-redox and crystal field modulation for enhanced and tuned broadband emission in chromium-doped aluminate glasses

Article abstract of DOI:10.1039/c8tc06453a

The emerging era of big data has stimulated the development of optical materials with broad emission region to allow data transmission of higher capacity and speed. Here, broadband emission, almost covering the region of 650-1500 nm, was realized by doping transition metal ions into versatile aluminate glasses, in which abundant microstructures and certain oxidizing capacity could be guaranteed to enrich emissions. Furthermore, we aimed to improve spectroscopic properties via the simultaneous self-redox and crystal field modulation of Cr ions and matrixes, respectively. More emission ions and more efficient centers were created to enhance the emission intensity by more than one order of magnitude. In addition, proper crystal field was created to liberate a new emission band at ～708 nm. The results may allow the development of tunable fiber laser and amplifier using the materials and pave the way to precisely control the valence and crystal field to obtain efficient emission.

Full text of DOI:10.1039/c8tc06453a

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Table of Content only:
Selective self-redox and crystal field modulation have been demonstrated for enhanced and
tuned broadband emission in chromium doped versatile glass. More emission ions and more
efficient centers were created to enhance the emission intensity by one order. In addition, proper
crystal field was equipped to liberate new emission band at ~708 nm and enrich emission region.

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Journal of Materials Chemistry C
ARTICLE
Selective self-redox and crystal field modulation for enhanced and
tuned broadband emission in chromium-doped aluminate glasses
Received 00th January 20xx,
Accepted 00th January 20xx
Xingyu Li, Haoyang Luo, Yafei Wang, Xiu Wang, Jiangkun Cao, Liping Wang and Mingying Peng*
DOI: 10.1039/x0xx00000x
The emerging era of big data has stimulated the development of optical materials with broad emission region to alleviate
data transmission of higher capacity and speed. Here, broadband emission, almost covering the region of 650-1500 nm,
was realized through doping transition metal ions into versatile aluminate glasses, where abundant micro-structures and
certain oxidizing capacity could be guaranteed to enrich emissions. And we targeted to improve spectroscopic properties
via simultaneous self-redox and crystal field modulation of Cr ions and matrixes, respectively. More emission ions and
more efficient centers were created to enhance the emission intensity by more than one order. In addition, proper crystal
field was created to liberate new emission band at ~708 nm. The results may allow developing tunable fiber laser and
amplifier with the materials potentially and pave the way to precisely control over valence and crystal field to obtain
efficient emission.
www.rsc.org/
matrixes. Unfortunately, Cr activated materials, especially for Cr4+
doped glasses, usually suffer from limited Cr solubility, which
1
. Introduction
2
5-28
restricts their practical applications
. Improving emission with
Recently, the emerging era of big data proposes urgent requirements
for broadband emission materials to extend new data transmission
window to resolve data traffic eventually . Also, these optical
materials with broad emission region have been pursued in
considerable fields of bioimaging and tunable fiber lasers . Yet, the
mainly devoted rare-earth (RE) ions activated materials lack
sufficient ability to cover all important spectral regions, for instance
high efficiency under limited Cr content is of great challenge to
confront. One promising key is to transform as many Cr ions as
possible in matrixes into luminescent ions or more efficient emission
centers, which could be realized by altering Cr valence and creating
more proper crystal field, since Cr ions have been demonstrated as
1
-4
5
-8
1
6
valence-transformable and crystal field-dependent emitters .
Therefore, matrixes are in need to be explored, which could possess
the capacity to not only guarantee adequate valence state and
surroundings to enrich emission region but also prompt Cr dopant to
be efficient emitting ions.
~1200-1500 nm. Their limited and fixed emission from intrinsic f-f
forbidden transition drives it beyond the capacity to meet the needs
only by virtue of RE ions
9
-15
.
Transition metal ions have been regarded as talented dopants for
generating controllable broadband emissions in view of their distinct
electronic configuration and tunability with crystal field. In particular,
chromium ions with various valence states (from 0 to +6) possess the
talent of abundant emission region for the potential electron
Starting from the view of above points, aluminate glasses were
selected as ideal matrixes regarding that the glasses, with no
traditional glass network formers, are characterized by the relatively
disorder structure, indicating enough sites to create efficient
2
9-31
emission centers
. Beyond this, there exist some superoxide ion
1
6-19
transitions
. For example, tetrahedrally coordinated tetravalent
radicals, which make it possible to adjust Cr ions to desired valence
chromium could bring about 1.1-1.6 μm emission; 1.18-1.32 and
3
2, 33
state via self-redox within glasses
.
4
+
1
.36-1.56 μm emission from Cr doped forsterite and YAG,
1
4, 20-23
Inspired by these, we successfully realized broad emission
occupying within the region of 650-1500 nm through anchoring
transition metal ions in versatile aluminate glasses. And selective
self-redox of Cr ions and providing more proper environment took
effects in improving the optical performance. That is, the emission
intensity is enhanced by a factor of more than 15 and the lifetime is
prolonged by 1.5 times successfully. Other than these, a new band at
respectively
. And trivalent chromium of octahedral
coordination within some matrixes can give birth to emission in
2
4
~0.65-1.3 μm region . Consequently, chromium ions could
theoretically endow optical materials with fully rich emission region
if abundant proper valences and microstructures were guaranteed in
The China-Germany Research Center for Photonic Materials and Devices; The
State Key Laboratory of Luminescent Materials and Devices, Guangdong
Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, School
of Materials Science and Technology, South China University of Technology,
Guangzhou 510640, P. R. China.
~
708 nm was generated, widening the spectral region, due to crystal
field evolution.
2. Experimental section
E-mail: pengmingying@scut.edu.cn.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
2.1 Sample preparation
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Cr-doped aluminate glasses, with the nominal molar dissolved gifts aluminate glasses with the certain oxidizing
View Article Online
DOI: 10.1039/C8TC06453A
compositions of 63CaO-37Al
2
O
3
-xCr
2
O
3
(x=0, 0.05%, 0.15%, capacity to modulate valence state of Cr ions. Theses enable Cr
,
0
0
.25%, 0.40%, 0.60%, 0.80%) and (63-y)CaO-37Al
2 3
O -
doped aluminate glasses to exhibit broadband emission region,
.25%Cr O -ySrO (y=0, 4, 10, 16, 22, 28, 36) were obtained in air and photoemission spectra (sample CA2.5C) are depicted in
2 3
by conventional melting and consequent quenching method, Figure 1 and compared with optical data of RE ions activated
which are denoted as CAzC (z=1000x, x=0, 0.05%, 0.15%, 0.25%, materials. When excited at 310 and 670 nm, the emission band
3
+
4+
0
.40%, 0.60%, 0.80%) and yS (y=0, 4, 10, 16, 22, 28, 36) lies at ~785 and 1175 nm, which originates from Cr and Cr
respectively. For instance, 63CaO-37Al -0. 25%Cr could be ions, respectively. The band emission region of sample CA2.5C
expressed in CA2.5C; and (63-16) CaO-37Al -0.0025Cr is apparently so broad that it almost spans from 650 to1500 nm
6SrO is represented by 16S. Raw materials, CaCO SrCO and and could not be realized by any single RE ions. Furthermore,
Al , were selected in the analytical reagent level; and 99.99% the emission region in 1200-1350 nm from Cr ions fills the
pure Cr was used as the dopant. Each 50 g batch was mixed spectral gaps of RE ions.
O
2 3
2 3
O
O
2 3
2 3
O -
1
3
3
2 3
O
2 3
O
homogeneously and put in corundum crucibles. Whereafter,
the mixtures were melted at 1550 ℃ for 0.5 h in an air
atmosphere. The melt was poured onto a stainless steel plate
and pressed with another stainless plate, by which the glasses
samples were prepared successfully. Subsequently, these
glasses samples were polished and ground for optical and
structural measurements.
2
.2 Characterization
Optical properties: The Edinburgh FS920 fluorescence
spectrometer equipped with a 450 W Xe lamp and a μF900
microsecond lamp as excitation sources was utilized to obtain
static and dynamic-state photoluminescence (PL) and
photoluminescence excitation (PLE) spectra along with
emission decay spectra. The absorption spectra were collected
with Perkin Elmer Lambda 900 Ultraviolet-Visible-Near-infrared
Figure 1 Observation of broadband emission of chromium
activated aluminate glasses in 650-1500 nm: emission spectra
of Cr doped aluminate glass (sample CA2.5C) upon excitation of
(UV-Vis-NIR) spectrophotometer.
Structural properties: X-ray diffraction (XRD) patterns were
3
10 and 670 nm, respectively.
measured for finely ground samples to verify their amorphous
characters using an X-ray diffractometer D/max-IIIA with Cu K
α
2
7
radiation (λ = 1.5418 Å). Al MAS NMR spectra were performed
3
.2. Strategies to enhance emission intensity and create new
using a Bruker AVANCE III HD 400 instruments. To analyze the
emission band in Cr activated aluminate glasses
2
7
structure evolution, Al MAS NMR spectra were recorded at 9.4
T at a frequency of 104.26 MHz using a 3.0l s pulse with a 4 mm
MAS probe. Raman spectra were performed on a Raman
spectrometer (Renishaw in Via) with a 532 nm wavelength laser
as excitation source. The electron paramagnetic resonance (EPR)
spectra were recorded under ambient temperature using a
Bruker A300 EPR spectrometer operating in the X-band
frequency (9.8 GHz). And differential scanning calorimeter
Luminescence intensity has a strong dependence on the doping
concentration of activator. Concentration optimization is a
direct strategy to improve emission intensity and was firstly
adopted in our work. Glasses doped with different Cr
contents were prepared and illustrated in the inset of Figure
(a). The glasses gradually turn to blue-green from originally
colorless when doped Cr content increases. Absorption
2 3
O
2
2 3
O
spectra can be exerted to analyze the characteristic electronic
transitions of activators in materials. Herein the UV-NIR
absorption spectra are in accordance with observed results.
(
(
NETZSCH STA 449C) was utilized to measure glass transitions
Tg) and crystallization (Tc) temperatures with the heating rate
of 10 ℃/min to analyze the glass forming ability. All the data
were collected at room temperature unless otherwise
prescribed.
With introducing Cr
2 3
O , two strong absorption bands, i.e., at
6
+
~295 and 367 nm, from Cr come into existence and obvious
enhancement, which contributes to color changes directly. The
intense absorption is associated with the charge transfer
transition and manifests the oxidizing capacity of the aluminate
glasses. Besides, a band, spanning the region of 500-1200 nm,
is observed and originated from characteristic optical
3
. Results and discussion
3
.1 Observation of broadband emission of Cr-activated
aluminate glasses in 650-1500 nm
Chromium of abundant valence state embodies the natural gift
of various electron transitions, which would theoretically
liberate outstanding optical emission region if appropriate sites
guaranteed. Aluminate glasses were featured by the disordered
structure and adequate sites options for lack of traditional glass
4+
4+
absorption of Cr . The rich absorption region of Cr ions could
be roughly divided into three parts, that is, 520-840 nm, ~1000
nm, and ~1150 nm. The first absorption region is due to the
3
3
2 1
transition of A → T and the second could be attributed to the
3
3
2 2
A → T
2
network formers. Also, a considerable amount of O species
2
| J. Name., 2012, 00, 1-3
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Figure 2. Concentration optimization of chromium dopant in Cr doped aluminate glasses. (a) absorption spectra and (b) emission
spectra under 670 nm excitation of samples CAzC (z=1000x, x=0, 0, 0.05%, 0.15%, 0.25%, 0.40%, 0.60%, 0.80%); inset in Figure 2(b)
shows the dependence of emission (~1175 nm) intensity on Cr content.
transition; the third absorption part results from the Fano introducing certain SrO into aluminate glasses of notorious glass
1
antiresonance of the interaction between E and vibrationally forming ability facilitates glasses preparation remarkably, which
3
25, 26, 34
broadened T
2
. In addition to these, the absorption band further pushes aluminate glasses one more step forward to the
3
+
of Cr was believed to be at around ~450 and 650 nm according optical fiber. The quenched glasses samples tended to
to abundant researches, whose information might be obscured devitrificate and burst into fragments when removing SrO
by Cr4+ ions absorption. Yet, the trace of Cr in aluminate species. And differential scanning calorimetry (DSC) curves
3+
glasses could be confirmed by emission spectra (Figure 1).
were adopted to quantitate the improved glass forming ability
such as glass thermal stability. The formula ∆T=T -T is used to
describe the glass thermal stability where T and T refer to the
onset temperature of crystallization and glass transition
temperature, respectively. The value of ∆T of certain SrO-
contained aluminate glasses turns bigger than that of original
aluminate glasses, for example, 70, 140 and 119 K respectively
corresponds to samples 0S, 10S and 22S, which indicates the
improved glass forming ability under introducing certain SrO.
x
g
Figure 2(b) exhibits the emission spectra of aluminate glasses
with different Cr contents. The line-like emission bands with the
peak position at 1175 nm and a long tail extending down to
x
g
1
500nm were observed for all samples under 670 nm excitation.
Such asymmetric emission covers the whole second
telecommunication window. When increasing doping
4
+
concentration of Cr, the emission intensity of Cr goes rising
initially and comes to a maximum at x = 0.40 mol%, as shown in
the inset in Figure 2(b); subsequently, the emission intensity
Figure 3(a) presents the emission spectra of chromium doped
decreases, which might be induced by concentration quenching aluminate glasses under introducing different strontium oxides
effect. The emission intensity is enhanced by about 5 times as contents upon the excitation of 670 nm, characteristic emission
4
+
compared to the sample of lowest content. Herein, 0.40 mol% from Cr shows up in the range of 1050-1500 nm; meanwhile,
Cr was the relatively optimal concentration for this system, it deserves mentioning that emission intensity keeps
yet for comparison, we selected 0.25 mol% Cr as monotonically increasing along with SrO content. The intensity
2 3
O
2 3
O
experimental doping concentration in the following, since it is at the peak position of sample 34S is up to 15 times more than
lower than critical content and the luminescence intensity is that of sample 0S. This is obviously more efficient to improve
proportional to the content.
luminescence efficiency than concentration optimization (see
inset in Figure 2(b)). Moreover, the tendency of enhancement
by substitution has not exhibited an extremum point. However,
it turns to be relatively hard to form amorphous glasses by
virtue of limit cooling speed when the substituted CaO quantity
by SrO exceeds 34 mol%. If glasses were successfully prepared
via other feasible methods, such as sol-gel method and laser
spin melting method, the enhancement might have not come to
a stop.
Aluminate glasses were verified to possess certain oxidizing
capacity due to the encaged superoxide ion radical O
-
2
, which
leaves it possible to manipulate doped activators to be desired
valence state3
2, 33
. And lack of traditional glass network former
indicates relatively disordered structure and potentiates easier
microstructure modulation, which provides a beneficial
accommodation for activators
properties of Cr doped aluminate glasses could be optimized by
engineering aluminate glass structure and adjusting Cr ions
2
9, 35
. Therefore, the optical
The lifetime is the key factor to evaluate the spectroscopic
valence state. After massive experiments, SrO was, thus, performance. Luminescent decay curves of samples under the
eventually considered as charming species in view of the same testing conditions (λex=670 nm, λem=1175 nm) were
fascinating evolutions of optical properties. In addition, recorded respectively and listed as Figure 3(b). It is obvious that
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Figure 3. Strategies to improve the optical performance of Cr4+ inside Cr activated aluminate glasses. (a) emission spectra of
samples yS (y=0, 4, 10, 16, 22, 28, 34) upon 670 nm excitation; (b) emission decay curves (λex=670 nm, λem=1175 nm) of samples
yS (y=0, 16, 34); (c) time-resolved emission spectra of sample 34S (λex=670 nm).
the decay time is prolonged along with the addition of SrO. The substituting CaO with SrO, a new independent band, centered
decay curves fit well the second order exponential equation I(t) at ~708 nm, gradually develops and it becomes stronger than
=
A
1
exp(-t/τ
intensity at time t, A
lifetimes of fast and slow decays. The formula τ = (τA
(A +A ) was used to calculate the mean lifetime of samples, relatively higher crystal field . The evolution hints that two
and their average lifetimes are 124, 159 and 183 μs for samples different sites of Cr potentially coexist within higher SrO
S, 16S and 34S. The lifetime owns the same improvement content samples. Other than this, the emission intensity of 785
1
)+A
2
exp(-t/τ
2
), where I(t) is the luminescent the original broad band eventually; and the two independent
1
and A
2
are constants, τ and τ are the bands merge into an asymmetric band. The new band (~708 nm)
1
2
2
2
4
4
3+
τ
1 1
+A
τ
2 2
)
2 2
could be ascribed to the transition of T → A of Cr in
3
6
/
1
τ
1
2 2
τ
3
+
0
tendency as that of ~1175 nm emission intensity when nm goes down for ≦4 mol% SrO introduction (shown in Figure
increasing SrO content. Meanwhile, the double exponential 4(b)); subsequently, it goes rising continuously and even
characters indicate the statistical distribution of multi– overtakes that of sample 0S. These seem to be associated with
4
+
environments around Cr potentially.
the formation and ratio of new band at~708 nm (discussed
below in detail).
4
+
To justify the multi-centers of Cr in aluminate glasses, time-
resolved fluorescent spectra of sample 34S were performed
Likewise, time-resolved fluorescent spectra under 310 nm
under 670 nm excitation (Fig.3(c)). Apparently, the emission excitation were utilized to unravel the multi-luminescent
3
+
band consists of an abovementioned asymmetric line-like band centers of Cr for sample 34S in the Figure 4(c). Initially, an
and another broad band, originating from two luminescent asymmetric emission band, comprised of two bands centered
centers of Cr4+ in different environments. The peak position of at 708 and 785 nm respectively, is observed and the ~708 nm
broadband emission locates at 1225 nm, which is attributed to band dominates over the whole region. As delay is prolonged,
the 3
3
4+
2 2
T → A transition of Cr in low-field sites, and the the peak position of ~708 nm band is substituted by ~ 785 nm
1
3
4+
asymmetric band is due to the E→ A
2
transition of Cr in high- band. This is perhaps because ~ 708 nm band decays faster and
field sites . As time goes, the line-shape is evolved from broad transfers its energy to ~785 nm band. Therefore, at least two
2
7
3
+
band into a relatively narrow one, because the emission at 1225 kinds of Cr emission centers were created by introducing SrO.
nm decays more quickly than that at ~1175 nm.
From above results, two kinds of key evolutions happene to
4
+
In addition to spectral evolutions of Cr ions, the introduction aluminate glasses when substituting CaO with SrO. On one hand,
3
+
4+
of SrO results in great changes for the emission of Cr ions. As the emission intensity of Cr ions especially for Cr comes to an
shown in Figure 4(a), the red to deep-red emission spectra obvious enhancement. On another hand, the new band,
appear in typical Cr3+ emission region under 310 nm excitation. originating from Cr ions in new sites, is derived to further enrich
A broad band, centered at 785 nm, was observed for sample 0S, emission region under introducing SrO. The reasons were
4
4
3+
2 2
which comes from T → A transition of Cr . Besides, when explained below.
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3
+
Figure 4. Creation of new emission center of Cr with broadened and enhanced emission inside Cr doped aluminate glasses. (a)
normalized emission spectra of samples yS (y=0, 4, 10, 16, 22, 28, 34) under 310 nm excitation; (b) dependence of 785 nm emission
on SrO content; (c) time-resolved emission spectra of sample 34S (λex=310 nm).
3
.3 Mechanisms for enhanced luminescence intensity and new in glass, the resonance intensity of O -
goes down, and there
2
3
+
38
emission band in Cr activated aluminate glasses
exists a new resonance signal at g=1.96 due to Cr ions . We
-
infer that the decrease of O
2
resonance intensity should be
Emission characters are closely associated with the absorption
spectra, which together originate from distinct electronic
transitions. Herein absorption spectra account for
abovementioned evolution. Similarly, the featured absorption
bands from Cr6+ (280-410 nm) and Cr4+ (500-1200 nm) ions can
be resolved clearly, and absorption from Cr ions could be
picked out differently from its absorption region, overlapping
with Cr4+ ions. To further discern variations on the ratio of Cr
responsible for the formation of high-valence Cr ions. That is,
3
+
-
Cr doped could be oxidized to high-valence Cr ions by O
2
,
-
which reduces the quantity of O
2
and thus leads to decreasing
resonance intensity. And these could be explained in the
3
+
-
4+
2-
4+
-
6+
2-
3
+
formulas: 3Cr + O
2
⇌ 3Cr + 2O , 3Cr + 2O
2
⇌ 3Cr + 4O
3
+
-
6+
2-
and Cr + O
2
⇌ Cr + 2O .
EPR spectra under the same testing conditions for glasses of
ions of different valence states qualitatively, absorption spectra different SrO contents are presented in Figure 5(d). The intense
6
+
-
2
were normalized to the absorption peak of Cr (367 nm) and renounce signals for all samples occur at g=2.00 (O related)
3
+
presented in Figure 5(b). Absorption intensity in the region and g=1.96 (Cr related), and outlines of signal patterns almost
500-1200 nm) gradually keeps rising with SrO content, as stay unchanged. Yet, the intensity of O
-
(
2
signal decreases with
shown in the inset of Figure 5(b). Though the region in 500-1200 SrO content obviously and reaches its minimum under adding
3
+
4+
nm might consist of Cr and Cr ions, we could speculate that 16 mol% SrO; then the intensity starts to rise. On the other hand,
3
+
introducing SrO facilitates the formation of more low-valence Cr ions related signal (g=1.96) keeps continuously decreasing,
3
+
4+
Cr ions (Cr or Cr ) from higher valence state.
showing linear dependence on SrO content. This manifests the
constant decrease of the content of trivalent Cr ions. Regarding
4
+
-,
The formation of Cr was believed to be associated with O
and it could be detected by EPR technology . Thence, EPR was
adopted to dig out more information on Cr-related features and
structure-related characters, especially for O
EPR spectrum of glass with no Cr doping is shown in Figure 5(c).
The g value was calculated through the formula: g=hv⁄μB,
where h is the Plank’s constant, υ represents microwave
frequency, μ refers to Bohr magneton and B is magnetic field
strength. And the typical resonance at g=2.00 is observed,
2
3
+
4+
3
7
the absorption overlap between Cr and Cr ions, the
increased trend of absorption intensity in 500-1200 nm region,
3
+
-
which would actually be undermined by less Cr ions, is more
2
. As comparison,
4
+
able to reflect the increased content of Cr ions. Hence by
combining the analysis of absorption and EPR data, we could
4
+
3+
6+
deduce that Cr ions should be derived from Cr or Cr ions,
which could be modulated by introducing SrO into aluminate
glasses.
-
32, 33
which is related to superoxide ion radical O
2
.
With Cr doped
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4
+
Figure 5. Mechanisms for the enhanced emission of Cr in Cr activated aluminate glasses. (a) schematic for the redox of Cr ions
within glasses with the existence of superoxide ion radical; (b) normalized absorption spectra of samples yS (y=0, 4, 10, 16, 22, 28,
3
4); (c) the EPR spectra of aluminate glasses with Cr doped and undoped (samples CA2.5C (or 0S) and CA0C respectively); (d) EPR
spectra of samples yS (y=0, 4, 10, 16, 22, 28, 34); inset in (b): zoom at the spectral region of 510 to 840 nm.
As analyzed above, the formation of high-valence Cr is
The emission of Cr ions strongly depends on the crystal field
associated with the superoxide ion radical O
equilibrium among Cr , Cr and Cr ions depends on the glass corresponding to new sites, should be related to matrixes
2
-, and redox surrounded. Thus, appearance of the new band (~708 nm),
3
+
4+
6+
-
components closely, which determines the amounts of O
2
structure. Structure-related information is obtained below to
directly. Therefore, with SrO content increased, there should be find more clues.
3
+
4+
6+
new redox equilibrium among Cr , Cr and Cr ions.
Considering the same Cr content for all glasses, continuous
enhancement of Cr4+ absorption intensity under introducing
SrO indicates that more Cr ions turn into tetravalence state
through redox reaction. These are in accordance with the
Emission located in the low-wavelength region (~708 nm),
was originated from relative higher field sites of Cr, as
compared to that at ~785nm. X-ray powder diffraction could
reflect the crystalline-related information and, thus, was
performed on these samples with SrO introduced. As exhibited
in the Fig6(a), obvious hump could be observed in the XRD
patterns for all samples, and there are no diffraction peaks of
crystal phase. These indicate that glasses maintained the
intrinsic amorphous state without crystals precipitated. The
shape and peak position of diffused hump are independent on
the quantity variation of SrO. Yet, there occur remarkable
changes in the intensity and width of humps. To describe this
tendency, the integrated intensity of humps (20-40°) were
calculated in Figure 6(b). The integrated intensity stays
unchanged firstly; and subsequently, beyond 10 mol%
substitution, it almost linearly increases with SrO content.
Meanwhile, we calculated the effective width (△θeff) to
quantitatively describe the variations through the
4
+
enhanced NIR emission intensity of Cr . Hence, we speculate
that oxidizing and reducing effects on Cr and Cr together
lead to the variation of Cr ratio and O amounts. Below 16 mol%
2
SrO addition, the oxidizing effect on Cr , rather than reducing
effect on Cr , dominates over the whole chain redox reaction,
amounts evidenced by EPR spectra. Yet, for
samples containing more than 16 mol% SrO, reduction effect on
Cr ions mainly takes over the chain reaction, giving rise to
superoxide ions radical contents. In addition, over the whole
routine, the balance of chain reaction mainly leans to the
middle section, which makes Cr ions quantity keep increasing.
The self-redox of Cr ions could be selective through SrO content,
and Cr ions could be transformed into the desired valence state
3
+
6+
4
+
-
3
+
6
+
-
which consumes O
2
6
+
4
+
(
the schematic is presented in Figure 5(a)).
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DOI: 10.1039/C8TC06453A
3
+
Figure 6. Mechanisms for new emission centers of Cr inside Cr activated aluminate glasses. (a) XRD patterns and (b) integrated
intensity and effective width of humps in XRD patterns for samples yS (y=0, 4, 10, 16, 22, 28, 34); (c) Tanabe-Sugano diagram
following equation:
adopted to analyze Al speciation (Figure 7(a)). The dominant
resonance with a maximum between 70 and 80 ppm, indicating
the character of tetrahedrally coordinated Al, is observed for all
∫
퐼(휃)ⅆ휃
훥휃푒푓푓
=
퐼푃
2
9, 44
samples
aluminate glasses, Al of tetrahedral coordination comes into
p
is the maximum intensity of hump being so as to serve as glass former. It deserves mentioning that
. For lack of traditional glass network former in
Where θ is the degree and I
2
7
peak. The calculated effective widths were shown in Figure 6(b). shape of the Al MAS NMR spectra remains unchanged under
Likewise, the width exhibits the opposite tendency with that of increasing SrO content. These demonstrate the reserve of
intensity variation. That is, width almost stays unchanged and microenvironment of Al species.
turns smaller gradually under ≧16 mol% SrO introduction. The
Though Al–oxygen ligand could be detected by NMR spectra,
narrowing width of humps might mirror the growth of
the concrete information around Al atom, typically for the Al-
“
microcrystallites” within glasses according to the
related bond, is ambiguous. Herein, Raman spectra were
adopted to further unearth more related information, and bond
related signals exhibit certain dependence on glasses
compositions (Figure 7(b)). For all samples, Raman spectra
consist of two broad bands in the region of low frequency 430-
3
9-41
“microcrystallites” theory and Scherrer formula
. And these,
to some extent, might lead to the strengthened crystal field,
which is in accordance with new emission band (~708 nm) of
3
+
Cr ions in relatively higher crystal field.
Here, the Tanabe-Sugano diagram was performed in Figure 670 cm- and relatively high frequency 670-1000 cm
(c) to qualitatively explain the phenomenon. The Y-axis respectively. The low-frequency band might result from bridge-
denotes energy, and X-axis refers to crystal field intensity. The oxygen related vibration, and high-frequency band might be
1
-1
6
3
+
Cr ions of ~785 nm emission lie in relatively low crystal field, attributed to symmetric stretching vibration of the AlO units
4
2
2
leading to low-energy transition (~785 nm). It deserves (Al in Q species, where 2 in Q denotes the number of bridging
mentioning that both sites (corresponding to ~708 and 785 nm oxygens around aluminum) . Other than these, the evolution
4
4
emission) should locate below crossover point (Dq/B≈2.1), on Raman spectra could be discerned when SrO content varies.
4
2, 43
which corresponds to broadband emission
. We could The intensity of high-frequency band, relative to the low
conclude that introducing SrO gives birth to sites of relatively frequency band, is enhanced apparently with SrO content,
higher crystal field. The sites formation of relatively higher field indicating that non-bridging oxygens become much more
provides much more proper environment for Cr ions to take dominated than before. On the one hand, Sr ions of bigger ionic
advantage of energy efficiently. Therefore, the ~708 nm radius, used to replace Ca and act as glass network modifier,
emission intensity keeps increasing in spite of the decrease of possess weaker capacity to destroy glass network relative to Ca;
3
+
Cr ions amounts. And the energy transferred to ~785 nm from but on the other hand, Ca owns much higher ionic potential to
2
-
5-
~
708 nm starts to take effects. Therefore, the emission intensity capture O of [AlO4] ionic groups and have AlO groups
4
5
-
8-
2-
at ~785 nm occurs to rise above ~4 mol% SrO content.
4 2 7
combined again (2[AlO ] →[Al O ] + O ). These effects
together make Sr ions result in much more non-bridging
oxygens than Ca ions within glass structure, which, to some
extent, means richer sites options for dopants and more easily
adjustable structure.
To acquire the reasons for crystal field variation, glass
structure is analyzed in detail. Due to the insensitivity of
conventional equipment to Ca and Sr, we aimed to probe the
2
7
environment around Al atoms, and thus Al MAS NMR was
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DOI: 10.1039/C8TC06453A
2
7
Figure 7. Structural evolutions of aluminate glasses with different SrO contents. (a) Al MAS NMR spectra and (b) Raman spectra
of samples yS (y=0, 4, 10, 16, 22, 28, 34).
From above analysis, substitution for CaO with SrO in glasses
leads to glasses structure changes but maintains the main Acknowledgements
features especially for its amorphous characters. As revealed by
We acknowledge financial supports from Guangdong Natural
2
7
Al MAS NMR, Al is coordinated by four oxygens and serves as
Science Foundation (Grant No. 2018B030308009), National
Natural Science Foundation of China (Grant No. 51672085),
Program for Innovative Research Team in University of Ministry
of Education of China (Grant No. IRT_17R38), the Joint Fund of
Ministry of Education of China (Grant No. 6141A02033225),
Local Innovative Research Team Project of “Pearl River Talent
Plan” (Grant No. 2017BT01X137) and Fundamental Research
Funds for the Central Universities.
glass network formers, which is not influenced by SrO content
significantly. In the meantime, the addition of SrO provokes
depolymerization glass network and gives rise to more
nonbridging oxygens. This enables glass to provide many
potential micro-structures for Cr ions to reside, such as
4
+
luminescent centers of tetrahedrally coordinated Cr and
3
+
octahedrally coordinated Cr . Moreover, the depolymerization
of glass network makes it relatively easy for glass to adjust its
structure, which induces two kinds of sites for Cr ions on
observation of XRD patterns (Figure 6) and spectroscopic data
Notes and references
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