Fabrication of oriented oxide films from exfoliated yttrium hydroxide layers: Enhanced photoluminescence and unexplored behavior of energy transfer

Article abstract of DOI:10.1016/j.jallcom.2018.06.011

Layered yttrium hydroxide (LYH) nanospalates doped with different activators (Tb³⁺ and/or Eu³⁺) were hydrothermally synthesized and then successfully delaminated into ultrathin nanosheets of down to 1–3 nm thickness via intercalation

Full text of DOI:10.1016/j.jallcom.2018.06.011

Journal of Alloys and Compounds 763 (2018) 815e821
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of oriented oxide films from exfoliated yttrium hydroxide
layers: Enhanced photoluminescence and unexplored behavior of
energy transfer
,
,
b
,
,
b
,
, d
Junjie Huang a b, Tie Zhang a, Ke Ren a, Rui Zhang a **, Xiaoli Wu a *, Ji-guang Li c
a College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
b Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, University of
Technology, Guilin 541004, China
c Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang,
Liaoning 110004, China
d Advanced Materials Processing Unit, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e i n f o
a b s t r a c t
Layered yttrium hydroxide (LYH) nanospalates doped with different activators (Tb3þ and/or Eu3þ) were
hydrothermally synthesized and then successfully delaminated into ultrathin nanosheets of down to 1
e3 nm thickness via intercalation reactions and mechanical agitation. Using the LYH nanosheets, highly
[001] oriented yttrium hydroxide films were constructed through spin-coating. Based on the quasi-
topotactic transformation from yttrium hydroxide to cubic Y2O3, [111] oriented oxide films were ach-
ieved by proper annealing. For the highly oriented Y2O3:0.05Eu film, the enhanced photoluminescence
was obtained compared with the powder form due to the higher exposure of (222) facets. Through
adjusting the incorporated activation ions, films with different color were obtained, and energy transfer
from Tb3þ to Eu3þ was observed for the alternatively multilayer film, although Eu3þ and Tb3þ were
located in different layers.
Article history:
Received 28 February 2018
Received in revised form
22 May 2018
Accepted 1 June 2018
Available online 2 June 2018
Keywords:
Layered yttrium hydroxide
Nanosheets
Films
© 2018 Elsevier B.V. All rights reserved.
Luminescence
1. Introduction
LRH crystals of several microns in lateral dimension and tens to
hundreds of nanometers in thickness, which are not suitable for
Layered rare-earth hydroxide (LRH), as a relatively new type of
anionic layered compounds, has attracted much attention during
the recent years on account of their unique ability to combine the
excellent properties of the rare-earth ions with the host-guest
chemistry [1]. They may be delaminated into unilamellar or few-
layer thick nanosheets with significantly 2D morphology, which
are ideal building blocks for constructing films [2]. Another
attractive aspect is that they can incorporate selected activator ions
and generate abundant photoluminescent properties [3e9].
LRH compounds can basically be prepared by hydrothermal
crystallization [10e12] and homogeneous precipitation via reflux-
ing [13e15]. The above synthetic methodologies produce plate-like
preparing transparent luminescent films directly. On the other
hand, according to the crystal habit of LRH, it has demonstrated that
chemical precipitation at freezing temperature of 4 ꢀC can directly
produce ultrathin LRH nanosheets of 3e5 nm in thickness [16].
However, the nanosheets are assembled into aggregates like
hydrangea-flower, which are also not suitable for preparing films
directly. Therefore, hydroxide nanosheets derived from their
layered host compound are indispensable. Delaminating layered
compounds into nanosheets can be achieved by swelling the pris-
tine crystals via exchange of the interlayer anions with significantly
larger ones, followed by exfoliation in a proper medium under
mechanical agitation. The exfoliation technology has been suc-
cessful for several types of layered inorganic materials, such as
layered double hydroxides (LDHs) [17], graphite [18], metal oxides
[19], phosphates [20], and chalcogenides and h-BN [21], to name a
few.
* Corresponding author. College of Materials Science and Engineering, Guilin
University of Technology, Guilin 541004, China.
** Corresponding author. College of Materials Science and Engineering, Guilin
University of Technology, Guilin 541004, China.
The crystal structure of the LRH compounds can be viewed as a
repetitive stacking of the host layer and interlayer anions along c-
E-mail addresses: rzhang@glut.edu.cn (R. Zhang), wuxiaoli@glut.edu.cn (X. Wu).
https://doi.org/10.1016/j.jallcom.2018.06.011
0925-8388/© 2018 Elsevier B.V. All rights reserved.

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J. Huang et al. / Journal of Alloys and Compounds 763 (2018) 815e821
axis (the [00l] direction), and the host layer constructed via edge
sharing of 8-fold [Ln(OH)7H2O] and 9-fold [Ln(OH)7H2O] coordi-
nation polyhedral. Due to the special structure features, the sig-
nificant 2D morphologies of the exfoliated LRHs are well suited for
the construction of highly [001] oriented film with improved
luminescence than that of LRH powder form, ascribed to that the
film maximizes the exposure more Ln activators located in the host
layer to the excitation light and thus obtain higher emission in-
tensity. Additionally, the flat surface of the highly oriented films
may reduce the scattering of the excitation light, which also result
in the higher emission intensity.
90 ꢀC to remove superfluous HNO3.
2.2. Preparation of (Y0.95Ln0.05)2(OH)5NO3·nH2O (Ln ¼ Eu, Tb) and
their exfoliated nanosheets
The first step is to synthesize (Y0.95Ln0.05)2(OH)5NO3$n-
H2O(Ln ¼ Eu, Tb) powder. For each run of the synthesis, the total
concentration of Ln3þ was kept constant at 0.02 mol/L and the Ln/
(YþLn) (Ln ¼ Eu, Tb) atomic ratio was fixed at 5 at%. In a typical
synthetic procedure, a proper amount of ammonium hydroxide
solution (25%) was added to the mixed nitrate solution until pH ~7.
After stirring for 30 min, the resultant suspension was transferred
into a Teflon lined stainless-steel autoclave and heat treated at
100 ꢀC for 12 h. After natural cooling, the hydrothermal product
was collected via centrifugation, washed with distilled water and
ethanol, and finally dried in the air at 50 ꢀC for 12 h to yield a white
powder.
Generally speaking, the photoluminescence behavior of LRH is
less than rare-earth oxides because the hydroxy in LRH structure
provide the channels of nonradiative relaxation. Y2O3 doped with
Eu3þ and Tb3þ ions are the best-known red-emitting and green-
emitting phosphors, respectively, owing to its excellent chemical
durability and stability, excellent luminescence efficiency, and high
color purity [22e24]. Due to that, Y2O3:Eu3þ and Y2O3:Tb3þ are
widely used in the field of cathode ray tubes (CRTs), field emissive
displays (FEDs), flat panel displays (FPDs), and plasma display
panels (PDPs) [25]. Among the low-dimension Y2O3:RE phosphors
(RE ¼ rare-earth element), the 0-dimensional (0D) nanoparticles
and 1-dimensional (1D) nanotubes/nanowire have been exten-
sively studied for their controllable synthesis and potentially new
optical properties [26e29]. However, the 0D and 1D oxides are not
suitable for forming untralthin luminescent film with high density
and smooth surface. 2D nanosheets are the potential ideal candi-
date for the preparation of the film with high quality. In the
solution-based processing methodology, oxide are mostly pro-
duced via controlled precursor synthesis followed by proper
annealing, and the LRH provides a 2D template for the formation of
2D oxide. Moreover, the projection in the [00l] direction for LRHs
crystal and [111] direction for the cubic oxide crystal percent close
similarities in terms of rare-earth stomic configuration, the phase
transformation from LRH to cubic oxide is a quasitopotactic one
[30e32]. Thus the [001] oriented LRH film would transform to a
[111] oriented oxide film via quasi-topotactic atomic arrangements
under proper annealing [3e7], which should result in an enhanced
exposure of the close-paked (222) facets of the oxide crystal and
thus stronger luminescence.
The above obtained powder is need to be exfoliated into ultra-
thin nanosheets, thus the anion exchange is the first step, the large
C
12H25SOꢁ3 (DSꢁ) was employed to exchange the NOꢁ3 . 0.05 mmol
LRH was added into an aqueous solution containing 0.25 mmol
Sodium dodecyl sulfate (SDS), following by reacting at room tem-
perature for 48 h under mechanical agitation. The exchanged
product was then dispersed into 100 mL formamide in a capped
conical beaker, and gently agitated until the solution becomes
transparent. The exfoliated LRH nanosheets were collected via
centrifugation at 25000 rpm for 1 h. The colloidal suspension
(0.5 mg/mL) was obtained by dispersing LRH nanosheets into 5 mL
ethanol, and then ultrasonic treatment for 5 min.
2.3. Preparation of oriented luminescent films
Multilayer films of the exfoliated LRH nanosheets were fabri-
cated by applying the layer-by-layer assembly procedure, that is,
colloidal suspension was dripped onto the rotating quartz glass
substrate for a certain number of times. Notably, the glass substrate
were (1) sequentially immersed in acetone, ethanol, deionized
water for ultrasound treatment for 5min, respectively, and then
placed into H2SO4/H2O2 solution (volume ratio of 3/1) at 80 ꢀC for
1 h; (2) immersed in NH4OH/H2O2/H2O solution for 10min, and
dried at 80 ꢀC; (3) immersed in CH3OH/HCl (63 wt%) solution for
20 min, and then treated in H2SO4 (98 wt%) for 20 min, finally
rinsed by deionized water.
According to the literature [33,34], the Y2O3 doped 5 at% Eu3þ
and the Y2O3 doped 5 at% Tb3þ have the optimal photo-
luminescence properties, thus in this context, efforts have been
made to fabricate oriented oxide luminescence films by employing
the exfoliated nanosheets of (Y0.95Eu0.05)2(OH)5NO3$nH2O
(LYH:0.05Eu) and (Y0.95Tb0.05)2(OH)5NO3$nH2O (LYH:0.05 Tb) as
precursor building blocks, and followed by proper annealing. Three
kinds of multilayer films (Y2O3:0.05Eu films, Y2O3:0.05 Tb films and
the mixed films formed by Y2O3:0.05Eu, and Y2O3:0.05 Tb) were
prepared via “layer-by-layer” method, and the films were charac-
terized by XRD, SEM, AFM, PL/PLE and TEM usually used in the
structure characterization of the nonlinear luminescent materials
[35e37].
2.4. Characterization techniques
Phase identification was performed via X-ray diffractometry
(XRD, X'Pert PRO, PANalytical B.V.) using nickel-filtered Cu-Ka ra-
diation operated at 40 kV/40 Ma. Fourier transform infrared spec-
troscopy (FT-IR, Model Spectrum RXI, Perkin-Elmer, Shelton,
Connecticut) was undertook by the standard KBr method.
Morphology and microstructure analysis were achieved via trans-
mission electron microscopy (TEM, Model JEM-2000FX, JEOL,
Tokyo) and field emission scanning electron microscopy (SEM,
JSM6380-LV, JEOL). Topographical images of the individual nano-
sheets were obtained via atomic force microscope (AFM, Nanosurf
easyScan 2, Switzerland). Photoluminescence properties were
measured at room temperature with FP-6500 spectrofluorometer
(JASCO, Tokyo) equipped with a 60-mm-diameter integrating
sphere (Model ISF-513, JASCO) and a 150-W Xe-lamp as the exci-
tation source.
2. Experimental
2.1. Materials
The starting rare-earth sources are Y2O3 (4 N), Tb4O7 (4 N) and
Eu2O3 (4 N), which are purchased from Huizhou Ruier Rare-Chem.
Hi-Tech. Co. Ltd (Huizhou, China). Sodium dodecyl sulfate (AR),
ammonium hydroxide solution (25%) and nitric acid (63 wt%) were
purchased from Sinopharm Chemical Reagent Co. Ltd. Rare-earth
nitrate solution was prepared by dissolving the oxide with a
proper amount of nitric acid, followed by evaporation to dryness at
3. Results and discussion
Fig. 1(a) compares XRD patterns of (Y0.95Eu0.05)2(OH)5NO3$nH2O

J. Huang et al. / Journal of Alloys and Compounds 763 (2018) 815e821
817
Fig. 1. (a) XRD patterns and (b) FT-IR spectra of NOꢁ3 -LYH and DSꢁ-LYH.
(NOꢁ3 -LYH)
and
its
exchanged
compound
peaks of LRHs. The 220 diffraction, originated from the ab plane
(the hydroxide main layer) of the crystal structure, is strong and
sharp, indicating that the host layers of LYH are well crystallized.
(Y0.95Eu0.05)2(OH)5DS$nH2O (DSꢁ-LYH). The sample of NO3ꢁ-LYH
exhibits 002, 004 and 220 diffractions, which are the characteristic
Fig. 2. FE-SEM morphologies of (a) NOꢁ3 -LYH, (b) DSꢁ-LYH, (c) and (d) show their corresponding crystal structures.

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J. Huang et al. / Journal of Alloys and Compounds 763 (2018) 815e821
Fig. 3. (a) XRD pattern, (b) TEM image, (c) Tapping-mode AFM image and (d) the AFM height profiles of the exfoliated of NOꢁ3 -LYH nanosheets. Inserts show the colloidal suspension
of the nanosheets (left), and the clearly observable Tyndall effect under laser beam irradiation (right).
After anion exchange with DSꢁ, a noticeable feature is that the clear
and systematic 00l reflections (monitored as the 002 and 004 re-
flections) shifted to the low-angle side, while the 220 reflection is at
the same position. The basal spacings calculated from the centers of
the 002 peaks are 8.3 Å for NOꢁ3 -LYH and 23.7 Å for DSꢁ-LYH,
indicating that the incorporated larger DSꢁ expands the interlayer
distance. Fig. 1(b) similarly compares the FT-IR spectra of NOꢁ3 -LYH
before and after anion exchange. It can be seen that the sharp ab-
sorption at 1384 cmꢁ1 corresponds to the n3 mode of NOꢁ3 , an
intense and broad band centered at around 3450 cmꢁ1 is attributed
to the stretching vibration of the hydroxyl group from both the
completely replaced by DSꢁ.
SEM observation, as shown in Fig. 2(a), reveals the samples of
NOꢁ3 -LYH consist of uniform and thin hexagonal platelets with a
lateral size of about 200 nm and a thickness of about 30 nm. The
morphology of exchanged platelets becomes irregular and the
thickness of the platelets has been significantly expanded, which
results from the successful incorporation of DSꢁ in the interlayer
(see Fig. 2(b)). Since the hydroxide layer has a thickness of
~0.65 nm, the calculated gallery height (d002-0.65 nm) is 1.72 nm.
This value is in agreement with the length of the DSꢁ, suggesting
that the DSꢁ long carbon chains stand vertically between the host
layers, as depicted in the insert of Fig. 2.
hydroxide layers and the interlayer water,
a band close to
1645 cmꢁ1 is assigned to the bending mode of water molecules.
After anion exchange reactions, water molecules are still observ-
able with the vibration of hydroxyls (OHꢁ), while the vibration of
NOꢁ3 is no longer detected. The absorptions ranged from 1300 to
900 cmꢁ1, being characteristic of -OSO-3, appeared from the ex-
change product. Besides, the new absorptions of 2952 cmꢁ1 and
2950 cmꢁ1 are respectively derived from the symmetric and
asymmetric stretching vibrations of the -CH2- in the DSꢁ structure,
and the absorption of 2850 cmꢁ1 is derived from the stretching
vibration of -CH3. The FT-IR results suggest that NOꢁ3 has been
A transparent colloidal solution was yielded through treating
the DSꢁ-LYH with formamide, as depicted in Fig. 3(a) insert,
revealing the occurrence of delamination. The resulting colloidal
suspension was very stable, and no sediment was observed upon
long-term standing. Clear Tyndall light scattering indicated that the
presence of exfoliated LRH layers (nanosheets) dispersed in the
formamide. After centrifugating the colloidal solution, a glue-like
aggregate was recovered. The XRD pattern is shown in Fig. 3(a),
where sharp basal reflections are absent. Instead, an amorphous-
like halo is observed in the low angular region (2q
< 15ꢀ), which

J. Huang et al. / Journal of Alloys and Compounds 763 (2018) 815e821
819
Fig. 4. XRD patterns of and SEM images of the oriented NOꢁ3 - LYH film (a, c) and the corresponding oxide film (b, d), respectively. Inserts show the actual photographs and
structural transformation of films.
can be ascribed to the scattering from the random aggregate of
exfoliated nanosheets [17]. Similarly, a noticeable broad hump in
Three kinds of precursor multilayer films (Eu3þ doped, Tb3þ
doped and mixed films) were prepared by repeating the deposition
and spin-coating procedures. The film thickness grew linearly with
increasing number of cycles. It is worth mentioned that the mixed
film was fabricated by alternatively depositing the Eu-doped and
Tb-doped LRH colloidal nanosheet suspension. After annealing, the
resulting oxide films exhibited good fluorescence performance. For
the Eu3þ doped oxide film, as shown in Fig. 5(a), the excitation
spectrum exhibits a shoulder peak at about 237 nm, which could be
assigned to the charge transfer band (CTB) from the O2ꢁ: 2p to
Eu3þ: 4f electron. The center peak position of the CTB blue-shifts
from 250 to 237 nm compared to the powder form, and this may
be understood from the Eu-(O-H)-A (A ¼ NOꢁ3 ). In the powder form
of LYH: 5%Eu, interlay anion (NO3ꢁ) attract the host layer [Y(Eu)-(O-
H)-] via electrostatic attraction, which loosens H- attraction toward
the 2p electrons of O2ꢁ and thus allows an easier electron transfer
from 2p orbital of O2ꢁ to the 4f orbital of Eu3þ. However, in the film
form of LYH: 5%Eu, the exfoliated nanosheets (host layer) lacks of
the attraction from interlayer anion, and thus allows a harder
charge transfer from O2ꢁ to Eu3þ and a blue-shifting of CT band
center. While exciting Eu3þ with the peak wavelength of the CTB,
the film and powder all exhibit the transitions from the 5D0 excited
state to the 7FJ (J ¼ 1, 2, 3, 4) emission state of Eu3þ ranging from
500 to 700 nm. Due to the enhanced exposure of the (222) facets
which have higher Eu3þ occupancy, the intensity of the red emis-
sion at 613 nm is ~3.5 times that of the powder form. Fig. 5(b) de-
pict the photoluminescence properties of the Tb3þ-doped oxide
film, the excitation peak at 241 nm, 256 nm and 268 nm are
attributed to the high-spin (HS) and low-spin (LS) interconfigura-
tional Tb3þ: 4f-5d transitions. The film displayed the typical 5D4-7FJ
(J ¼ 6, 5, 4, 3) transitions, weather excited at 241 nm (HS) or 256 nm
(LS). For the mixed multilayer film, the photoluminescence spectra
the 2q
range of 20e30ꢀ is due to the scattering of liquid formamide.
TEM observation (see Fig. 3(b)) exhibits that the exfoliated nano-
sheets are rather thin. A trapping-mode AFM image in Fig. 3(c)
shows that ultrathin sheets are morphologically irregular and some
large nanosheets with lateral of 200 nm are occasionally encoun-
tered. They are estimated to be approximately 0.9e2.5 nm thick
from the AFM height profile (see Fig. 3(d)), which indicates that the
nanosheets are primarily 1e3 layers thick. All above results suggest
the delamination of LRHs is successfully completed.
Highly 00l oriented LRH films were self-assembled after
depositing the colloidal nanosheet suspension on a treated sub-
strate by spin-coating. During the process, the independent nano-
sheets had the tendency to reassemble into new layered materials,
thus that the obvious 002 reflection was observed in the XRD
pattern, as exhibited in Fig. 4(a). Cubic oxide films were obtained by
calcining the LRH films at 800 ꢀC for 2 h in the atmosphere of ni-
trogen (see Fig. 4 (b)). Compared to the powder form, oxide films
have stronger intensity ratio of 222 diffraction to other diffractions,
indicating the highly [111] oriented oxide films were formed. Since
the phase transformation from LRH to the cubic oxide is a quasi-
topotactic transformation, the projection in the [00l] direction for
precursor hydroxide crystal and in the [111] direction for oxide film
present close similarities in rare-earth atomic configuration, which
could account for the [111] oriented oxide film [5]. Fig. 4(c)-(d)
show SEM images of the LRH film and oxide film, respectively. For
both kinds of films, the nanosheets are in the same orientation and
the surfaces are smooth. After annealing, there is no obvious
morphological change compared with the precursor film. The
physical digital photos show both films are semitransparent (Fig. 4
inserts).

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J. Huang et al. / Journal of Alloys and Compounds 763 (2018) 815e821
Fig. 5. PL and PLE spectra of Eu-doped (a), Tb-doped (b) and (c) mixed Y2O3 films, and (d) CIE chromaticity diagram. Inserts shows multi-color emission of the oxide films under
254 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
exhibit energy transfer from Tb3þ to Eu3þ although they were
located in different layers (see Fig. 5(c)). In the PL spectrum,
monitoring the Tb3þ 546 nm emission, it produces only inter-
configurational f-d transitions of Tb3þ; however, both f-d transi-
tion band of Tb3þ and CT band of Eu3þ are obtained while moni-
toring the Eu3þ 613 nm emission. Exciting the film with CT band of
Eu-O (237 nm) or the f-d transition of Tb3þ (256 nm), the typical
obtained with hydrothermal synthesis at 100 ꢀC, and they were
successfully exfoliated into ultrathin nanosheets of 1e3 nm in
thickness via dispersing the exchanged products DSꢁ-LYH in
formamide, followed by slow stirring. Highly [001] oriented LYH
films have been constructed through spin-coating the colloidal
nanosheets on the substrates, and thus [111] oriented oxide films
were achieved after annealing. The above quasi-topotactic trans-
formation from LYH to cubic Y2O3 lead to the enhanced exposure of
(222) facets, and thus result in the PL intensity of the oxide film is
~3.5 times of that powder form. Energy transfer from Tb3þ to Eu3þ
was observed for the mixed oxide film, where Eu3þ and Tb3þ were
blended in a lattice via annealing at 800 ꢀC.
emissions of both Tb3þ and Eu3þ exist, and interestingly, the I545
/
I613 ratio (the intensity ratio of Tb3þ to Eu3þ) varies by employing
different excitation wavelength. When exciting the mixed film at
237 nm, the I545/I613 ratio is 0.88, but the value rises to 1.07 with the
excitation at 258 nm, which further confirm the energy transfer
from Tb3þ to Eu3þ. The above result is different from the previous
report, in which energy transfer between different activator ions is
restricted because the mixed multilayers of (LGdH:Eu/PSS)n and
(LGdH:Tb/PSS)m produces mixed crystallites after annealing at
800 ꢀC [7]. Herein, the mixed multilayers of LYH:Eu and LYH:Tb
yields a single Y2O3 phase blending Tb3þ and Eu3þ after the same
heat treatment. This can be ascribed to the lower phase transition
temperature for LYH than LGdH. The oxide films exhibit vivid
luminescence colors under 254 nm excitation, and the CIE chro-
maticity was labeled in Fig. 5(d).
Acknowledge
This work was financially supported by the National Natural
Science Foundation of China (No.51402059, 51502048).
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