This paper is published as part of a PCCP Themed Issue on: Metal oxide nanostructures: synthesis, properties and applications

Guest Editors: Nicola Pinna, Markus Niederberger, John Martin Gregg and Jean-Francois Hochepied

Editorial

Chemistry and physics of metal oxide nanostructures Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905768d

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Thin-walled Er3+

:Y2O3 nanotubes showing up-converted fluorescence

Christoph Erk,aSofia Martin Caba,

aHolger Lange,

bStefan Werner,

b

Christian Thomsen,bMartin Steinhart,

cAndreas Berger

cand Sabine Schlecht*

a

Received 27th November 2008, Accepted 4th March 2009

First published as an Advance Article on the web 26th March 2009

DOI: 10.1039/b821304f

Up-converted red and green fluorescence has been measured in low-crystallinity Er3+:Y2O3

nanotubes upon IR excitation. A new preparation procedure for thin-walled erbium doped yttria

nanotubes using alumina templates as shape defining tools and rare earth nitrates as precursors

for a purely thermolytic production of erbium doped yttria was developed. The products with a

diameter of approximately 40–50 nm and a wall thickness of only 6–8 nm showed up-converted

fluorescence with green and red emission bands when excited at 815 nm. The fluorescence

properties are compared to the ones of bulk erbium-doped yttria obtained from the same

precursor system.

Introduction

The capability of certain trivalent rare earth ions such as Er3+

to perform infrared-to-visible up-conversion is gaining

increasing attention in the field of energy harvesting as this

effect may help to make use of an extended spectral range

of solar radiation for energy conversion.1–5 In addition,

up-conversion of IR radiation plays an important role in

lasing6,7 and in fluorescence imaging.8,9 In all up-conversion

materials, the efficiency crucially depends on the local

environment of the emitting rare earth ion and the degree of

crystallinity of the respective material.10–16 Erbium-doped

non-toxic oxides such as Er3+-doped yttria show significant

potential as up-conversion materials because of their strong

emission bands in the green (550 nm, 4S3/2 - 4I15/2) and the

red (665 nm, 4F9/2 -4I15/2) region of the visible range.14 An

additional enhancement of the luminescence can be achieved

when the material is subjected to quantum confinement.17

As the quality and the crystallinity of the rare earth doped

host oxide have a large influence on the optical properties of

the resulting materials, several approaches to nanoscale

materials such as nanorods,18 nanoparticles19,20 or nanotubes21

focused on the preparation in an autoclave to obtain an

optimum crystallinity of the products. For the preparation

of shape-defined europium-doped yttria nanotubes porous

alumina templates with a pore diameter of 70 nm in combination

with a sol–gel method were used.22 In this case, as in a number

of other hydrothermal or sol–gel routes, the formation of

the rare earth-doped yttria is conducted under alkaline

conditions.18,21 As porous alumina is known to be partially

dissolved under basic conditions, these syntheses are not ideal

for a preparation of shape defined yttria nanotubes within the

template pores. The latter is inevitable for a synthetic route to

very small thin-walled nanotubes, as soft templates cannot

provide such delicate control over the growth of relatively

small nano-objects of doped yttria. For these reasons we

developed a purely thermolytic process for the formation of

thin-walled erbium-doped yttria nanotubes within the pores

of a porous alumina template with a nominal pore diameter of

60 nm.23–27 Such nanotubes allow an investigation of the

influence of the thin and only moderately crystalline walls

on the up-conversion luminescence.

Experimental

Chemicals and materials

Y(NO3)3�6H2O and Er(NO3)3�4.7H2O were purchased from

STREM and used as received. The mesoporous anodic

alumina templates were produced at the Max-Planck-Institute

of Microstructure Physics in Halle (Saale) via an electrochemical

etching step. Details on their preparation are described

elsewhere.23–27

Preparation of Er3+:Y2O3 nanotubes and Er3+:Y2O3 bulk

powder

In a typical procedure 20 mg of Y(NO3)3�6H2O and 0.4 mg of

Er(NO3)3�4.7H2O (depending on the desired amount of

erbium) were dissolved in 2 mL of distilled water. An alumina

template (pore diameter of 60 nm) was immersed into the

aqueous precursor solution for two hours. Then, the template

was removed from the solution and dried in air for one hour.

Subsequently, the templates were calcined in air for two hours

at 500 1C. After cooling to room temperature, the template

was immersed into 4 M sodium hydroxide solution until

the alumina layer was dissolved and a small amount of a

colourless solid was released. The nanotube suspension was

centrifuged and purified by removal of the supernatant,

addition of distilled water and sonification. This procedure

was repeated four to six times until the supernatant reacted

neutral.

a Freie Univeristat Berlin, Instiute for Chemistry and Biochemistry,Fabeckstrasse 34-36, 14195 Berlin, Germany.E-mail: schlecht@chemie.fu-berlin.de; Fax: +49 (0)30 83853310;Tel: +49 (0) 83852423

b Technische Universitat Berlin, Institute for Solid State Physics,Hardenbergstrasse 36, 10623 Berlin, Germany

cMax-Planck-Institute for Microstructure Physics, Weinberg 2,06120 Halle (Saale), Germany

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 3623–3627 | 3623

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

The released and purified nanotubes could be suspended in

water or ethanol and be dropped onto any substrate.

For the preparation of the bulk powder appropriate

amounts of yttrium (III) nitrate hexahydrate and erbium (III)

nitrate hydrate were mixed using mortar and pestle and filled

into a quartz crucible. The crucible containing the slightly pink

powder was placed in an oven and calcined at 800 1C in air for

up to 80 h.

Characterization methods

TG analysis was conducted with a Netzsch TG 209 cell, DSC

measurements were performed in corundum crucibles with a

Netzsch DSC 404 C calorimeter in nitrogen atmosphere.

X-Ray powder diffraction patterns were recorded with a

STOE Stadi P transmission powder diffractometer using

CuKa radiation.

Transmission electron microscopy was performed using a

Jeol JEM 1010 and a Philipps CM12 transmission electron

microscope operated both at 100 kV (LaB6-cathode). High

resolution TEM and EDX analysis was carried out with a

Philips CM 20 FEG (200 kV, LaB6-cathode). For sample

preparation aqueous suspensions of the nanotubes were

directly dropped onto carbon coated copper grids received

from Plano.

For the investigation of the up-conversion of the absorbed

light in Er3+:Y2O3 we performed photoluminescence

measurements (PL) with infrared excitation. The samples were

deposited on a Si wafer and excited in a reflective geometry

using a mode-locked titan sapphire laser with a pulse length of

2 ps. The excitation wavelength was tuned to 815 nm. The

luminescence of the excited samples was detected with a 35 cm

McPherson double monochromator system in subtractive

mode and a peltier cooled GaAs detector.

Results and discussion

A new preparative method for the preparation of doped

thin-walled yttria nanotubes was developed as the common

syntheses either involve basic hydrothermal conditions, which

are too aggressive for narrow porous alumina templates or the

application of slurries of the yttrium and rare earth precursors

where the grain size of the dispersed components may be larger

than the pore diameter of 60 nm of our porous alumina

templates. Therefore, a non-alkaline solution-phase approach

had to be found. Before an infiltration of the pores with the

precursor solution was conducted, the thermal degradation of

the Y(NO3)3�6H2O precursor was investigated separately to

find the optimum decomposition temperature and to

assure the formation of single-phase yttria in the thermolysis.

Calorimetric (DSC) measurements (Fig. 1) showed different

endothermic events in the range of 25–300 1C and thermo-

gravimetric control experiments revealed that a weight loss of

29% occurred when the precursor was heated to a temperature

of 300 1C. This value is in good accord with the theoretical

weight loss of 28.2% when the hexahydrate will lose its

six equivalents of water. As can be seen from Fig. 1, the

decomposition of the yttrium nitrate leading to the formation

of yttria starts at B335 1C and comes to completion just

below 400 1C. Further annealing of the resulting yttria powder

to 500 1C already gave good crystallinity and after annealing

at 800 1C sharp reflections of very pure yttria were recorded

(Fig. 2). All reflections were indexed for cubic Y2O3 (space

group Ia�3, a = 10.606(1) A, ICSD-86813) and no reflections

of any impurities were found.

Refinement of the cubic lattice parameter of the micro-

crystalline yttria obtained after annealing at 800 1C gave a

lattice parameter of a = 10.601(2) A, which is in excellent

agreement with literature data.

Doping experiments with different amounts of Er(NO3)3�4.7H2O

in the range of 1–9 at% Er3+ and subsequent co-thermolysis

of the two nitrates also yielded single-phase highly crystalline

materials.

For the preparation of thin-walled Er3+-doped nanotubes

of yttria within the 60 nm pores of porous alumina, mixtures

of yttrium nitrate and erbium nitrate with varying erbium

content were prepared. The nitrates were dissolved in distilled

water and the templates were infiltrated with this precursor

solution by immersing them into it. The infiltrated templates

were dried in air and calcined at 500 1C. Then the alumina was

selectively removed by aqueous base and the nanotubes were

released. The resulting tubes were investigated by transmission

electron microscopy (TEM) and they showed diameters in

the range of 40–50 nm (Fig. 3). Their wall thickness was

determined as 6–8 nm. Electron diffraction of larger areas of

Fig. 1 DSC measurement of the thermal decomposition of

Y(NO3)3�6H2O in the temperature range of 200–450 1C.

Fig. 2 X-Ray powder diffractogram of microcrystalline yttria

obtained after annealing at 800 1C and indexed for cubic Y2O3 (Ia�3).

3624 | Phys. Chem. Chem. Phys., 2009, 11, 3623–3627 This journal is �c the Owner Societies 2009

nanotubes showed the expected reflections of cubic yttria and

the reflections were indexed accordingly (Fig. 3). Bundles of

erbium doped yttria tubes were investigated by EDX analysis

and the EDX spectra revealed the presence of yttrium and

erbium in the product (Fig. 4). The amount of erbium found in

such nanotubes was in good accordance with the erbium

contents of the initial precursor solution that was infiltrated

into the template pores.

In addition to the analysis and characterization of larger

fields and bundles of nanotubes we also investigated the

products on a single tube basis. One important aspect

for the optical quality of the nanotubes is a homogeneous

distribution of the erbium dopant in the yttria host lattice.

Thus we performed line scan analysis of single nanotubes to

look for potential formation of erbium-rich regions and

phase segregated areas within the tube walls. No such

phenomena were observed and all line-scans showed the typical

concentration profile of a tubular morphology (Fig. 5). The

yttrium profile and the erbium profile are correlated and

determined by the shape of the nano-objects. No other type

of erbium ion distribution was detected in the tubes.

The second major parameter that governs the line width and

the efficiency of erbium luminescence in Er3+-doped yttria is

the crystallinity of the products and the degree of order

around the emitting rare earth ions. This is one of the most

challenging aspects of nanotube synthesis in general. Often,

thin-walled nanotubes are of poor crystallinity and annealing

at higher temperatures will destroy their tubular structure. A

closer TEM investigation of the walls of the yttria nanotubes

produced in this thermolytic approach revealed polycrystalline

walls showing mostly (222) lattice planes in HRTEM images

(Fig. 6), but also some small amorphous regions in between.

An average size of the individual nano grains of 3–4 nm was

determined by HRTEM analysis. This grain size was found for

all tubes that were investigated and no larger distribution of

grains sizes was observed. Several attempts to improve the

crystallinity of the tube walls by higher annealing temperatures

or steeper heating ramps during thermolysis of the precursors

within the pores did not succeed. The presence of the amor-

phous pore wall of the alumina template seems to decrease the

tendency of the initially formed primary grains to form larger

crystalline areas. In addition, the wall thickness of only 6–8 nm

is also disadvantageous for an ordering of more extended

areas of the tubes. The rather low tendency to develop long

range order found in these tubes cannot be attributed to the

presence of Er3+, as pure yttria tubes made from the yttrium

nitrate precursor in the same way showed the same degree of

crystallinity.

Despite the moderate crystallinity of the thermolytically

produced thin-walled nanotubes, they were investigated

Fig. 3 TEM image of bundles of Er3+-doped yttria nanotubes and

electron diffraction pattern (inset) showing (222) and (431) reflections

of cubic yttria.

Fig. 4 EDX spectrum of erbium doped yttria nanotubes with an

erbium content of 6 at% Er3+.

Fig. 5 TEM image of a single Er3+-doped yttria nanotube. The

position where the line scan analysis was performed is marked as a

black bar. The yttrium and erbium profiles are shown in the inset

region.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 3623–3627 | 3625

regarding potential up-converted fluorescence. Nanorods of

Er3+ doped yttria showed up-conversion to the visible range

when excited in the infrared range at 810 nm.18 The dominant

emission bands of the trivalent erbium ion are well known to

be the 2H11/2 -4I15/2 transition (centered at 550 nm for bulk

material) and the 4S3/2 -4I15/2 transition (centered at 565 nm

for bulk material) in the bright green region of the visible

range and the 4F9/2 -4I15/2 transition (centered at 665 nm for

bulk material) leading to a red luminescence.6,14,17,28

Several nanotube samples with different concentrations of

trivalent erbium in the yttria matrix (1–9 at%) were prepared

and microcrystalline yttria with the same concentrations of

Er3+ was synthesized in parallel. Whereas the microcrystalline

samples also showed bright up-converted emission in the green

and the red region at concentrations between 5% and 9%

erbium, the nanotubes only gave satisfying up-converted

fluorescence at very low erbium contents such as 1.3 at%

Er3+. The lack of detectable visible emission for higher

concentrations can likely be attributed to concentration

quenching29 in the thin-walled tubes. The up-converted

fluorescence of the nanotubes with low erbium content was

compared to the emission bands of microcrystalline erbium

doped yttria powder (Fig. 7). Both samples exhibit the three

most intense emissions of trivalent erbium ions due to the

transitions from the 2H11/2 state, the 4S3/2 state and the 4F9/2

level to the 4I15/2 ground state. When compared to the

microcrystalline powder, the emission bands of the rare earth

doped nanotubes are significantly broadened and slightly

shifted in their positions. The 4S3/2 emission and both branches

of the red 4F9/2 emission band are red shifted by 8–11 nm. In

the case of the 4S3/2 band this leads to an emission maximum at

568 nm for the nanotubes instead of 560 nm in the bulk erbium

doped yttria. The two branches of the red 4F9/2 emission are

red-shifted from 659/681 nm for the microcrystalline material

to 668/689 nm for the nanotubes. In contrast to these two

emissions, a small but detectable blue-shift was found for the2H11/2 band. The center of this green fluorescence shifts from

550 nm for the microcrystalline powder to 545 nm for the

Er3+-doped yttria tubes.

The dominant phenomena of peak broadening and

red-shifts of the emission bands were also found by Qi

et al.16 in nanocrystalline Eu3+:Y2O3 with a particle size of

only 9 nm. As the grains in the thin walls of the Er3+:Y2O3

investigated here are even smaller, such broadening effects that

are attributed to non-homogeneous coordination spheres

around the trivalent rare earth15,16 are even more likely to be

found. In the case of Eu3+:Y2O3 it was shown, that the

coordination number of six that all trivalent ions in the

bixbyite structure type adapt is expanded to an average

coordination number of 6.8 for yttrium ions and even of

7.9 for europium ions.16 Such a pronounced distortion of the

parent structure type leads to a larger distribution of rare

earth–oxygen distances and also allows the emitting rare earth

ions to couple to a broader range of vibrational states in the

crystal structure. As each of these effects is expected to

contribute to an overall broadening of the rare earth emission

Fig. 6 TEM image of a single Er3+-doped yttria nanotube (a) with

two HRTEM areas (b and c) showing (222) lattice planes of individual

grains in the polycrystalline pore walls.Fig. 7 Up-converted room temperature luminescence spectra of thin-

walled Er3+:Y2O3 nanotubes ((a), 1.3 at% Er3+), and microcrystalline

Er3+:Y2O3 (b), both prepared thermolytically from mixtures of

erbium nitrate and yttrium nitrate.

3626 | Phys. Chem. Chem. Phys., 2009, 11, 3623–3627 This journal is �c the Owner Societies 2009

states, the changes in the structure of the fluorescence

spectrum of the Er3+:Y2O3 nanotubes can be rationalized.

The photoluminescence behaviour of rare earth metal ions in

inorganic–organic hybrid materials has been investigated by

Karmaoui et al.30,31 The structures consisted of very

thin alternating layers of various rare earth metal oxides

(B0.6 nm) and benzoate molecules (B1.2 nm) and mainly

showed rather broad reflections in their X-ray diffraction

patterns. Nevertheless, rather sharp emission bands in the

visible could be detected in e.g. gadolinium/terbium and

gadolinium/europium based materials.31 This indicates that

peak broadening can be suppressed when a high degree of

short range order is achieved, even in materials consisting of

very small crystalline domains. In contrast, if there are

relevant distortions in the first coordination sphere of the

emitting ion due to weak crystallinity a broadening of the

emission bands is observed.

Conclusions

A new thermolytic synthesis of thin-walled erbium-doped

yttria nanotubes with a diameter around 50 nm was found

and used to produce nanotubes that exhibit up-conversion

from the infrared to the visible range. With a doping level of

1.3% Er3+, a slight shift of the green and the red emission

bands was observed, most likely due to the moderate degree of

crystallinity in the tube walls and a resulting distribution of

local coordination environments of the emitting Er3+ ions.

Here, the strategy of a very low doping level to avoid rare

earth ion clustering in the relatively small volume of the thin

walls was successful and both prominent emissions were

detected. These results demonstrate that even in thin-walled

nanotubes lacking high crystallinity up-converted fluorescence

in the visible range can be obtained by Er3+ doping.

Acknowledgements

Financial support by the Fonds der Chemischen Industrie

(Kekule-Fellowship to C. E.) and the Deutsche Forschungs-

gemeinschaft (priority program 1165 ‘‘Nanowires and Nano-

tubes’’) is gratefully acknowledged. We thank Kornelia Sklarek

(MPI Halle) for the preparation of porous alumina templates.

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