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On the VUV Luminescence and Degradation of UV-C Emitting Phosphors Mike Broxtermann* and Thomas Jüstel mail to: [email protected] or [email protected] Research Group Tailored Optical Materials Münster University of Applied Sciences Stegerwaldstraße 39, D-48565 Steinfurt Germany
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
3FJ
3H6
3H4
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
172
nm
YPO4:PrExc.: 160 nmEm.: 233 nm
Germicidal efficiancy (DIN 5031-10): 61%
4f15d1
3H5
0
10
20
30
40
50
60
70
80
90
100
Ref
lect
ance
/ %
BaS
O4
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
3p13p2
1p1
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
172
nm
Germicidal efficiancy (DIN 5031-10): 66%
YPO4:BiExc.: 160 nmEm.: 233 nm
3p1
MMCT0
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20
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60
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100
Ref
lect
ance
/ %
BaS
O4
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
2G7/2
2H11/2
4F9/2
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
172
nm
Germicidal efficiancy (DIN 5031-10): 57%
YPO4:NdExc.: 160 nmEm.: 233 nm
4f25d1
4IJ
0
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100
Ref
lect
ance
/ %
BaS
O4
Fig. 1 Xe-DBD Lamp operated in water.
Fig. 2 Schematic illustration of the principle of operation of two different phosphor converted Xe-excimer DBD lamp patterns.
YPO4 is a radiation stable wide band gap material (∆BG = 9.2 eV) which turns into a very efficient UVC / VUV emitter upon doping with e.g. Pr3+, Bi3+, or Nd3+
. [2],[3] Doping with Bi3+ leads to an emission band peaking at 241 nm due to a s-p transition of the [Xe]4f145d106s2 cation,
whereas a doping with Pr3+ or Nd3+ leads to a maximum emission around 235 and 192 nm, respectively. This emission is due to f-d transitions of the trivalent lanthanides.
1. Phosphor converted Xe DBD lamps
Fig. 3 VUV-excitation-, emission and reflectance spectra of the UV-C emitting phosphors YPO4:Bi and YPO4:Pr and the VUV-Emitting Phosphor YPO4:Nd, respectively
Fig. 13 Illustration of UV driven particle coating process [4]
2. UV emitting LnPO4:X phosphors for Xe DBD lamps – effective disinfection
3. Device lifetime limitations: aging effects on LnPO4 based phosphor materials within an Xe excimer discharge
4. Device lifetime improvement: Application and impact of protective inert particle coatings
Acknowledgement Literature
200 220 240 260 280 300 3200,0
0,2
0,4
0,6
0,8
1,0 rel. eff. DIN 5031-10 Em. YPO4:Bi (norm.) Em. YPO4:Pr (norm.) Em. YPO4:Nd (norm.)
relative photolytic germicidal afficacy: E.Coli
rela
tive
phot
olyt
ic g
erm
icid
al a
ffica
cy /a
.u.
Wavelength /nm
R
N O
NH
O
R
NO
HN
O
R
NO
HN
O
R
N O
NH
O
hν
200 250 300 350 400 450 500 550 600 650 700 750 800
6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
Exc.: 160 nmEm.: 233 nm
YPO4 aged
0
10
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60
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90
100
YPO4
Ref
lect
ance
/ %
BaS
O4
neat
Aged YPO4 (Xenotime)
200 250 300 350 400 450 500 550 600 650 700 750 800
6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
77K 100K 123K 150K 175K 200K 225K 250K
Energy /eV
Inte
nsity
/a.u
.
Wavelength /nm
275K 300K 325K 350K 375K 400K
Ex.: 350 nmEm.: 750 nmYPO4 aged @
425K 450K 475K 500K
0
10
20
30
40
50
60
70
80
90
100
Refle
ctan
ce /
% B
aSO
4
Aged LaPO4 (Monazite) Aged LuPO4 (Xenotime)
200 250 300 350 400 450 500 550 600 650 700 750 800
6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
/arb
. uni
ts
Wavelength /nm
Exc.: 330 nmEm.: 720 nmLuPO4 aged @
80 K 100 K 120 K 140 K 160 K 180 K 200 K 220 K 240 K 260 K 280 K 300 K
320 K 340 K 360 K 380 K 400 K 420 K 440 K 460 K 480 K 500 K
0
10
20
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40
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60
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80
90
100
Reflection @ RT
Refle
ctan
ce /
% B
aSO
4
-30 -20 -10 0 100,0
0,2
0,4
0,6
0,8
1,0
31P SS-NMR: YPO4 vs. YPO4 aged
norm
.
δ31P /ppm
YPO4 YPO4 aged
YPO4 & LuPO4 structure (Xenotim, I41/amd)
-20 -10 0 10 200,0
0,2
0,4
0,6
0,8
1,0
LaPO4
LaPO4 aged
31P SS-NMR: LaPO4 vs. LaPO4 aged
norm
.
δ31P /ppm
LaPO4 structure (Monazit, P21/c)
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100
120
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160
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200
220
240
260 IUV-C YPO4:Bi IUV-C Al2O3@YPO4:Bi (4m%, UV-R, 1400°C calc.)
Inte
grat
ed U
V-C
Inte
nsity
/a.u
.
Run-time /h
0
-10
-20
-30
-40
-50
-60
-70
-80
Degradation mit YPO4:Bi Degradation Al2O3@YPO4:Bi (4m%, UV-R, 1400°C calc.)
Deg
rada
tion
/ %
Micro discharge channel
hν (>172 nm)
surface discharge
surface discharge
SiO2
SiO2
outer electrode
inner electrode
phosphor layer
phosphor layer
inner electrode
outer electrode phosphor layer
SiO2
SiO2
internal phosphor external phosphor
Xe + Xe*+ e-
Xe* + 2 Xe Xe2* + Xe Xe2* 2 Xe + hν (172nm)
250 300 350 400 450 500 550 600 650 700 750 800
5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
100 K 120 K 140 K 160 K 180 K 200 K 220 K 240 K 260 K 280 K 300 K
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
320 K 340 K 360 K 380 K 400 K 420 K 440 K 460 K 480 K 500 K
LaPO4 aged @ Exc.: 335 nmEm.: 705 nm
0
10
20
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100
Ref
lect
ance
/ %
BaS
O4
C A B
1S0
3P0
3P1
3P2
1P1
A 1S0
3P0
3P1
3P2
1P1
Exc. Em.
x
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6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
/arb
. uni
ts
Wavelength /nm
80 K 100 K 120 K 140 K 160 K 180 K 200 K 220 K 240 K 260 K 280 K 300 K
320 K 340 K 360 K 380 K 400 K 420 K 440 K 460 K 480 K 500 K
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Reflection @ RT
Ref
lect
ance
/ %
BaS
O4
1S0 SPJ
3P1 1S0
Energy level scheme for nS2 luminescence
Optical grade YPO4, LuPO4 and LaPO4 were aged analogous to the doped phosphor materials. All analysed ortho-phosphates exhibit comparable aging effects indicated by characteristic photoluminescence
Solid State NMR reveals that the bulk host material remains unaltered (within the detection limits)
The apparent photo-luminescence shows features characteristic for ns2 cations e.g. Bi3+, Sb3+, As3+
P+V P+III [Ne]3s0 [Ne]3s2
YPO4:Bi YPO4:Bi
Al2O3
172 nm 241 nm
coating
Phosphor converted Xe excimer lamps comprising YPO4:Bi (internal phosphor) exhibit a distinct and fast characteristic aging under continuous operation: • Observable greying of the actually clear white phosphor coating • Continuous decrease of UV radiation output over operation time
(after 24 h forerun) • A dimming of 30 % (L70) is already reached after ≈ 230 hours run-
time
Spectroscopy of aged YPO4 and YPO4 doped with Bi3+, Pr3+ and Gd3+, respectively reveals:
• The distinct aging effect is evident regardless the dopant cation and
is as well observed for the undoped host material YPO4 • As a detectable main symptom of aging, a novel broad emission band
appears rising from the visual beginning at around 500 - 600 nm towards the UV region
• The uprising absorption band may be accompanied by a general overall absorption (greying) and a absorption band ascribed to a reduced activator species in case of YPO4:Bi (Bi3+ Bi+/Bi2+)
• Upon PL excitation into the new host absorption bands in the UV yields deep red luminescence, regardless which or any doping. This phenomenon was further followed by the temperature dependent PL spectra of aged undoped YPO4, LaPO4, and LuPO4 (see B)
50 100 150 200 250 300 350 400 450 500 550 600 650 700
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120
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180
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240
260
IUV-C Lamp with YPO4:Bi
Inte
grat
ed U
V-C
Inte
nsity
/a.u
.
Run-time /h
0
-10
-20
-30
-40
-50
-60
-70
-80
Degradation Lamp with YPO4:Bi
Deg
rada
tion
/ %
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
Exc.: 160 nmEm.: 233 nm
YPO4:Bi YPO4:Bi aged
0
10
20
30
40
50
60
70
80
90
100
Ref
lect
ance
/ %
BaS
O4
240 260 280 300 320 340 360 380L70 @≈230 h
Bi3+
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
Exc.: 160 nmEm.: 235 nm
YPO4:Pr YPO4:Pr aged
0
10
20
30
40
50
60
70
80
90
100
Ref
lect
ance
/ %
BaS
O4
Pr3+
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
Exc.: 160 nmEm.: 311 nm
YPO4:Gd YPO4:Gd aged
30
40
50
60
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90
100
Ref
lect
ance
/ %
BaS
O4
300 310 320
Gd3+
A) B)
Em.λmax= 241 nm Em.λmax= 235 nm Em.λmax= 192 nm
Bi+/Bi2+
host lattice
absorption of degrading host lattice
150 200 250 300 350 400 450 500 550 600 650 700 750 800
8,3 6,2 5,0 4,1 3,5 3,1 2,8 2,5 2,3 2,1 1,9 1,8 1,7 1,5
Energy /eV
Inte
nsity
(nor
m.)
Wavelength /nm
Exc.: 160 nmEm.: 233 nm
YPO4:Bi YPO4:Bi aged
0
10
20
30
40
50
60
70
80
90
100
Ref
lect
ance
/ %
BaS
O4
240 260 280 300 320 340 360 380
Particle Coating: Ideally, each and every single phosphor particle is coated with an homogeneous and surface covering coating of α-Al2O3 to prevent phosphor aging (degradation) under exposure to the Xe discharge.
We would like to express our gratitude to Prof. Dr. Rainer Pöttgen and Dr. Christopher Benndorf for conducting solid-state NMR measurements. Further thanks goes to our cooperation partners GVB GmbH, Tailorlux GmbH, the German centre for air- and space travel (DLR). Additional gratitude goes to the foundation of German economy and the German ministry of education and science for funding.
An optimal coating quality in terms of homogeneity and particle surface coverage were yielded with a photochemically driven precipitation process, recently described in literature. [4]
For further information look up literature 4 or follow
The application of an Al2O3 coating onto YPO4:Bi particles results in a reduced phosphor degradation (approximately factor ≈ 2) with respect to an uncoated material and under analogous aging conditions This positive effect appears to act upon the reduction of the activator Bi3+ as well as the degradation of the host material itself, as indicated by the comparison of spectra displayed by figure 14.
[1] U. Kogelschatz,, Plasma Chem. Plasma Process 23 (2003) 1 [2] P. Dorenbos for band gap 9,25 eV [3] T. Jüstel, P. Huppertz, W. Mayr, D.U. Wiechert, J. Luminescence 106 (2004) 225 [4] M. Broxtermann, T. Jüstel, Mater. Res. Bull. 80 (2016) 249
Fig. 4 relative photolytic disinfection efficacy for Escheria Coli according DIN 5031-10
YPO4:Bi3+/Pr3+/Nd3+ comprising lamps can be used as efficient UV-C or VUV light sources in disinfection
The germicidal efficacy typifies a useful tool to evaluate a phosphor for its disinfection suitability
Fig. 8 Solid state 31P NMR measured for aged and untreated YPO4 (left) and aged and untreated LaPO4 (right). Fig. 9 Xenotime structure Fig. 10 Monazite structure
Fig.6 Excitation, emission and reflectance spectra of aged and untreated YPO4:Pr, YPO4:Pr, YPO4
Fig. 7 Temp. dependent excitation and emission spectra measured from aged samples of YPO4, LuPO4 and LaPO4.
Fig. 11 Left: Energy level scheme for ns2 luminescence; Middle: Temp. dependent excitation and emission spectra measured from aged samples of LuPO4 supplemented by proposed electron transitions; Right: Proposed scheme for P5+ reduction due to plasma contact
Fig. 5 Left: UV-C output vs run-time for a YPO4:Bi comprising Xe lamp. Right: Excitation, emission and reflectance spectra of untreated and aged YPO4:Bi (after 700 hrs run-time)
Fig. 14 Fig. Top: UV-C output vs run-time for a YPO4:Bi and a Al2O3 coated YPO4:Bi comprising Xe lamp. Bottom: Excitation, emission and reflectance
spectra of the aged YPO4:Bi and Al2O3 coated YPO4:Bi (after 700 hrs run-time)
Fig. 13 Illustration of YPO4:Bi a particle coated with a protective but 172 nm transmitting Al2O3 layer or shell.
Reduction of P5+ by hot e- and Xe+∙ impingement in plasma contact
Dielectric barrier Xenon excimer discharge lamps are known as efficient sources of vacuum ultraviolet radiation, while the main band peaks at 172 nm.1 By the use of VUV excitable conversion materials which are applied as a coating layer onto the lamp body it is feasible to manufacture efficient UV-C, near VUV, or even UV-A/B emitting Xe excimer lamps which may perform well for the replacement of established Hg discharge lamps and respective UV radiation systems.