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44th International Conference on Environmental Systems
13-17 July 2014, Tucson, Arizona
Visible-Light-Responsive Catalysts Using Quantum Dot
Modified Ti02 for Air and Water Purification
Janelle L. Courts 1
ESC-Team QNA, Kennedy Space Center, FL, 32899
Paul E. Hintze 2
NASA, Kennedy Space Center, FL, 32899
Christain A. Clausen 3
University of Central Florida, Orlando, FL, 32816
and
Jeffrey T. Richards 4
ESC-Team QNA, Kennedy Space Center, FL, 32899
Photocatalysis, the oxidation or reduction of contaminants by light-activated catalysts,
utilizing titanium dioxide (Ti0 2 ) as the catalytic substrate has been widely studied for trace
contaminant control in both air and water applications. The interest in this process is due
primarily to its low energy consumption and capacity for catalyst regeneration. Titanium
dioxide requires ultraviolet light for activation due to its relatively large band gap energy of
3.2 eV. Traditionally, Hg-vapor fluorescent light sources are used in PCO reactors;
however, the use of mercury precludes the use of this PCO technology in a spaceflight
environment due to concerns over crew Hg exposure. The development of a visible-light-
responsive (VLR) Ti0 2 -based catalyst would eliminate the concerns over mercury
contamination. Further, VLR development would allow for the use of ambient visible solar
radiation or highly efficient LEDs, both of which would make PCO approaches more
efficient, flexible, economical, and safe. Though VLR catalyst development has been an
active area of research for the past two decades, there are few commercially available VLR
catalysts. Those VLR catalysts that are commercially available do not have adequate
catalytic activity, in the visible region, to make them competitive with those operating under
UV irradiation. This study was initiated to develop more effective VLR catalysts through a
novel method in which quantum dots (QD) consisting of narrow band gap semiconductors
(e.g., CdS, CdSe, PbS, ZnSe, etc.) are coupled to Ti0 2 via two preparation methods: 1)
photodeposition and 2) mechanical alloying using a high-speed ball mill. A library of
catalysts was developed and screened for gas and aqueous phase applications using ethanol
and 4-chlorophenol as the target contaminants, respectively. Both target compounds are
well studied in photocatalytic systems and served as model contaminants for this research.
Synthesized catalysts were compared in terms of preparation method, nature of the
quantum dots, and dosage of quantum dots.
Nomenclature
4 CP = 4-chlorophenol
CEC = controlled environment chamber
1 Scientist III, Advanced Life Support Labs, Mail Stop ESC-24, Kennedy Space Center, FL 32899.
2 Chemist, NASA NE Chemistry Branch, Mail Stop NE-L6, Kennedy Space Center, FL 32899.
3 Professor, Department of Chemistry, 4000 Central Florida Blvd., Orlando, FL 32816.
4 Scientist IV, Advanced Life Support Labs, Mail Stop ESC-24, Kennedy Space Center, FL 32899.
ICES 2014-28
eV
= electron volts
GC-FID
= gas chromatograph-flame ionization detector
LED
= light emitting diode
PCO
= photocatalytic oxidation
PDA
= photodiode array
QD
= quantum dot
rpm
= revolutions per minute
TCC
= trace contaminant control
UHPLC
= ultra high performance liquid chromatography
UV
= ultraviolet
VLR
= visible-light-repsonsive
W
= watt
XPS
= x-ray photoelectron spectroscopy
I. Introduction
T HE use of titanium dioxide as a photocatalyst has dominated the field of photocatalysis for several decades.
Applications, both realized and forecasted, encompass a broad range of processes including hydrogen
production via photocatalytic water splitting, chemical and biological purification of water and air, and synthesis of
organic compounds. The most commonly used titanium dioxide is a commercially available mixture known as
Degussa P25 and is a simple mixture of anatase (70-85%), rutile, and amorphous (minor) titania 1 and has
demonstrated high PCO activity in numerous studies 2 * " 7 . The anatase phase is known for its superior photocatalytic
activity relative to the rutile phase 8 with a band gap energy of 3.2 eV. Based on this band gap, only photons with a
wavelength of 388 nm or less (i.e. UV) have sufficient energy to activate anatase Ti0 2 9 . Traditionally, Hg-vapor
light sources are used in photocatalytic oxidation (PCO) reactors, but the presence of Hg precludes the use of these
systems in crewed spaceflight environments due to the possibility of Hg contamination (i.e., bulb breaks).
Ultraviolet LEDs are an emerging technology, and have proven to be a feasible excitation light source alternative.
However, currently available UV-LEDs have low lighting efficiency (13%) 6 .
The high photonic energy requirements currently needed for effective Ti0 2 PCO disallows effective use of
indoor lighting or solar radiation, which only consists of ~4-6% UV radiation at the Earth’s surface 10 , as energy
sources. Furthermore, UV-activated photocatalysis using Ti0 2 provides only a moderate reaction rate and
somewhat low quantum yield due to a low electron transfer rate and high electron-hole recombination rate 11 . Thus,
in its current state of development, Ti0 2 -assisted photocatalysis is not feasible for high throughput processes.
A solution for overcoming these limitations is to enable the Ti0 2 photocatalyst to be activated by visible light.
Developing a Ti0 2 -based catalyst that is responsive to visible region wavelengths, will allow the use of visible band
solar radiation (~45% of the solar spectrum lies in the visible region 10 ), or highly efficient blue or white LEDs in
PCO systems; thereby making PCO approaches more efficient, economical, and safe.
The development of Visible Light Responsive (VLR) PCO catalysts would facilitate the acceptance and
incorporation of PCO-based technology for numerous ISS applications including use in air trace contaminant control
(TCC) water recovery systems, low-cost hydrogen production using solar energy 12 , enhanced chemical and
microbial purification of water 13, 14 ; and potentially in the field of artificial photosynthesis. The past decade has
witnessed a shift in catalyst development towards visible light responsiveness. Efforts have included sensitization of
Ti0 2 with absorbed dye molecules 15, 16 , metal cation 17 ' 21 or anion 22 " 24 doping, narrow band gap semiconductor and
Ti0 2 coupling 25 ' 27 , to name a few. Despite the vast amount of research in this field, conclusions regarding the
relative effectiveness of any given method are difficult to draw due to the lack of consistent or standardized
experimental conditions.
Beyond the necessity of a VLR photocatalyst to possess a narrow band gap capable of utilizing visible light
photons (> 3.1 eV), it must also have suitable thermodynamic potential for the intended reactions and be resistant to
photocorrosion. As seen in Figure l 28 , the photocatalytic process begins with the absorption of a photon with energy
equal to or greater than the band gap energy of the semiconductor, which causes the formation of an electron-hole
pair. Most of the electron-hole pairs undergo recombination; however, a small percentage can move on to undergo
oxidation or reduction reactions with adsorbed molecules. Redox potential relationships between the conduction
and valence bands of the photocatalyst and target contaminant species (the species to either be oxidized or reduced)
also play a crucial role in the success of this reaction. Specifically, the redox potential of a donor species (the
molecules to be oxidized) adsorbed on the surface of the photocatalyst must be higher in energy than the valence
band position of the semiconductor in order to replenish the electron vacancies and, similarly, acceptor molecules
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(molecules to be reduced) must have a
redox potential below the conduction
band 11 . While narrow band gap
semiconductors can use visible light
photons to create election-hole pairs, this
redox potential requirement may not support
effective breakdown of contaminants. Other
considerations for possible photocatalyst
candidates include their resistance to
oxidation or reduction by water and their
overall robustness under operating
conditions (e.g., pH stability).
Several reviews on the current state of
VLR-Ti0 2 development have been
published recently 11, 29-3 1 . Kumar and
Devi 29 reviewed modified Ti0 2
photocatalysis mechanisms with respect to
interfacial charge carrier transfer dynamics;
Zaleska 31 focused on metallic- and
nonmetallic-doped Ti0 2 preparation
methods; Shon 30 discussed the effects of
various ions and modification techniques in the attempt to create a VLR-Ti0 2 catalyst; and Levine et al 11 related
various modification techniques with photocatalytic activity and photonic efficiency. Further reviews are dedicated
specifically to photocatalytic water splitting 12, 32 . All photocatalyst alteration methods have drawbacks. Dyes used
as photosensitizers are slowly photodegraded, becoming less efficient over time 33, 34 . Metal-doped catalysts are often
susceptible to thermal degradation or metal dopant molecules can serve as recombination centers rather than assist in
the photocatalytic process 20, 21 . Coupled narrow band gap semiconductors can undergo photocorrosion with the
possibility of releasing harmful side products. Drawbacks aside, many VLR catalysts have been successfully
developed and further investigation of VLR-Ti0 2 -based catalysts remains crucial to achieving high efficiency in
such systems. This paper focuses on the development of a VLR catalyst library focused on coupling narrow band
gap semiconductors with Ti0 2 via two methods: 1) photodeposition, and 2) mechanical alloying. Further, the
development of rapid screening assays for photocatalytic activity in both the aqueous and gas phases will be
discussed.
Figure 1: Processes occurring in a photocatalyst after
electron-hole separation, including recombination of the
electron and hole at the surface (a) and ion the bulk of the
material (b), electron participation in the reduction reactions
(c), and hole participation in oxidation reactions (d). 8
II. Methods
A. Light Source and Characterization
A custom light bank consisting of six 24-W Marine Glo T5 (60.96-cm length, 1.52-cm diameter) high output
fluorescent bulbs from Hagen (Mansfield, MA) was designed as the light source for these studies. Irradiance
profiles of the light bank at varied distances were determined in a dark room using a spectroradiometer (model
OL754C, Optronics Laboratories, Orlando, FL). The light source was placed directly above the integrating sphere
of the spectroradiometer at several heights (a 0.63 5 -cm diameter light attenuating disc was used to avoid saturation
of the detector during analysis). The height determined to have the highest irradiance was used to perform gas phase
and aqueous phase assays described later.
B. Catalyst Preparation and Commerical Catalysts
A total of 45 catalysts were prepared by two separate methods: photo -deposition and mechanical alloying for
analysis. Prepared catalysts incorporated the use of metal sulfide quantum dots, metal selenide quantum dots, and/or
pure metal with bare titanium dioxide. Degussa P25 Ti0 2 (Evonik Industries, Essen, Germany) was used as the
titanium dioxide source for all catalyst preparations and also as the traditional UV-activated photocatalyst for
comparison. A commercial catalyst claiming high VLR activity, GENS NANO™ (Green Earth Nanoscience, Inc.,
Toronto, Ontario), was also compared to prepared catalysts. The VLR catalyst in GENS NANO™ is a proprietary,
modified Ti0 2 catalyst. Activation of the commercial catalyst is said to be possible with sunlight and everyday
fluorescent lighting.
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International Conference on Environmental Systems
1. Photodeposition Method
Catalysts were prepared with metal or metal-sulfide
quantum dots, following similar procedures. In a 125-ml
Erlenmeyer flask, 0.25 g of Degussa P25 titanium dioxide
and either 100 ml of 1:20 degassed ethanokwater or 100 ml
of degassed water for quantum dot or pure metal deposition,
respectively, was added. An appropriate amount of 0.1 M
metal or sulfur stock solution was added to the flask to
achieve 0.1 to 3.0% dopant loadings by weight. The flasks
were placed on a stir plate approximately 3 cm from a 15W,
T5 UV-A fluorescent bulb and allowed to react for 30
minutes (Figure 2). The flasks received a total of
approximately 100 mW of UV radiation during the reaction.
After the reaction completed, the catalysts were centrifuged
to separate them from the liquid. The catalyst was then Figure 2; Photodeposition preparation method
washed and centrifuged in deionized water three times for quantum dot formation on Ti0 2 .
before drying at 105°C overnight.
2. Mechanical Alloying Method
Similar to the photodeposition method, catalysts were prepared with metal, metal-sulfide quantum dots, or metal-
selenide quantum dots. In a 55-mL tungsten carbide milling vial, 1 gram of Degussa P25 titanium dioxide and either
the dry material (metal or quantum dot) or 2 mL of toluene containing the material was added; two tungsten carbide
balls were placed in the vial as the milling media. An appropriate amount of metal or quantum dot material was
added to achieve 1. 0-3.0 % dopant loadings by weight. The samples underwent alloying using a high-energy Spex
SamplePrep 8000M Mixer/Mill (1060 cycles/min, 5.93 cm back-and-forth/2.54 cm side-to-side motion) for 5
minutes. Samples were placed under vacuum drying for one week.
C. Rapid Aqueous Phase Assay
For aqueous phase screening of VFR activity, 4-chlorophenol was selected as the target compound since it is a
well-studied standard for photocatalytic oxidation. A catalyst loading rate of 10 mg/mF of contaminant solution was
used. All reactions were performed in a Controlled-Environment Chamber (CEC) at 30°C; samples were allowed to
adsorb the contaminant in the dark for 30 minutes followed by 30 minutes of visible light irradiation with stirring at
50 rpm (Figure 3). Analysis of the aqueous phase assay samples was completed using the Thermo Scientific Accela
UHPFC (equipped with a Varian Polaris # C-18-A column, 100 x 2.00 mm) and photodiode array (PDA) detector.
Evaluation of 4-chlorophenol removal was completed for all catalysts (degradation products were not identified in
this rapid screening procedure). Removal of 4-chlorophenol (4CP) was calculated based the ratio of change in 4 CP
concentration to initial concentration (Equation 1).
Removal %= [4CPl Ftoat ) * 100 (1)
V [4 CP] Inltia i J v '
D. Rapid Gas Phase Assay
The rapid gas phase screening methodology utilized ethanol as the target contaminant based on its well
documented PCO mechanism. An aqueous slurry of the catalyst (5 mg/mL) was prepared and deposited onto
aluminum coupons followed by evaporation to leave a thin film of catalyst. The aluminum coupons were placed in
40-mL, borosilicate vials outfitted with gas-tight septa lids for sampling (Figure 3). All gas phase reactions were
completed in a CEC controlled to 30°C. The initial ethanol contaminant concentration was 50 ppmv to allow for
sufficient detection of reaction products. Similar to the aqueous phase process, samples were allowed to dark adsorb
for 60 minutes followed by 60 minutes of visible light irradiation. Samples were analyzed via an Agilent 6890 GC-
FID equipped with an HP Plot Q column. The appearance of acetaldehyde, the main intermediate seen in the PCO
of ethanol to C0 2 , was evaluated for VLR activity. This assay, designed to be rapid, was not optimized to allow for
full mineralization of ethanol in the small, static vials.
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E. Catalyst Characterization
1 . Diffuse Reflectance Analysis
Diffuse reflectance measurements were made using a Jasco V-670 UV/Vis spectrophotometer equipped with a
60-mm diameter integrating sphere. Catalysts were contained in a powder sample holder, which pressed the sample
against a quartz window. Percent reflectance was measured between 300 and 800 nm at 2-nm intervals, referenced
against a Spectralon certified reference standard (Labsphere, North Sutton, NH).
2. X-Ray Photoelectron Spectroscopy (XPS) Analysis
XPS analysis of catalyst samples showing appreciable VLR activity in either the gas phase or aqueous phase
assays was completed on a Thermo Scientific K-Alpha system. Identical samples prepared via both methods
outlined above were also compared to determine causes of differences in VLR activity.
III. Results
A. Light Source Characterization
Figure 4 shows the Marine Glo light bank irradiance profile and intensity at varied distances. The irradiance
profile shows a broad peak ranging from -400 to 500 nm due to the phosphor coating present on the wall of the
bulb, while the sharp peaks in irradiance present at 404, 435, 546, and 578 nm are due to emission lines from the
mercury contained in the lamps. Based on the emission spectrum, both the aqueous and gas phase assays were
completed at a distance of 5 cm from the bulb to provide as much irradiance as possible to the photocatalytic
reaction centers. Based on the distribution of wavelengths, a focus on quantum dots with absorption maxima of 480
nm or lower was implemented to best alter Ti0 2 to be VLR.
Wavelength (nm)
Figure 4: Irradiance profile of Marine Glow light bank at varied distances.
B. Rapid Aqueous Phase Assay
Of the 45 catalysts prepared, five catalysts exhibited promising levels of 4-chlorophenol removal after 30
minutes of visible light exposure as seen in Figure 5 (0.4% destruction per minute or greater). Of the top-
performing catalysts, two (1% CuS and 3% Cu) were prepared via the photodeposition method while the remaining
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Aqueous Phase Assay
three (1% CdS, 1% InP/ZnS, and 1% PbS + 1% Cu)
were prepared via the mechanical alloying method.
It should also be noted that the Degussa P25 catalyst
(control) had minute activity; this was likely due to
the small amount of UV radiation emitted from the
light source. The commercially-available catalyst,
GENS NANO™, showed a 15.5% removal of the
target compound after 30 minutes of exposure to
visible irradiation and was outperformed (3% and
5.7% increase in removal after 30 minutes) by two
of the catalysts prepared in house as shown in Figure
5, while the remaining three catalysts prepared in
house exhibited near equivalent activity. This is a
significant achievement and supports the need for
continued research on these top performing catalysts.
By refining these newly developed materials, it is
likely that a catalyst with much higher activity than
that seen in these initial studies can be obtained.
Figure 5: 4CP removal capacity of top-performing
catalysts in the aqueous phase assay compared to
Degussa P25 (bare Ti0 2 ) and a commercially-available
VLR catalyst (GENS NANO™). PD = photodeposition
method; MA = mechanical alloying method.
Comparison of Preparation Methods
Figure 6: Comparison of aqueous phase assay
results with repect to comparion method.
C. Rapid Gas Phase Assay
Similar to the aqueous phase assay, the top five
performing catalysts for the gas-phase photoxidation
of ethanol are shown in Figure 7. The commercially
available bare Ti0 2 , Degussa P25, was found to have
nearly 28% removal of ethanol after 30 minutes of
visible light exposure, even with a polyacrylic UV
filter in place. While the filter removed the majority
of the UV radiation, it did not completely eliminate
it; this minute amount of UV exposure is likely the
cause for bare Ti0 2 activity. Unlike aqueous phase
photocatalysis where UV (and some visible)
radiation is absorbed by water lowering the amount
reaching the catalyst for activation, there are no UV
hindrances in gas phase photocatalysis. Even the
Neither preparation method proved to be more
successful than the other in the current experiment
(Figure 6); in some cases, the mechanical alloying
method was observed to give higher VFR activity for the
same type and dosage of dopant, while in other cases the
photodeposition method prevailed. Further investigation
into the relationship between the two preparation
methods is warranted based on these results. If PCO
activity between the two methods is determined to be
equivalent for a particular catalyst, the mechanical
alloying method would be favored, as it is the easier and
faster technique. Furthermore, for catalysts prepared via
the photodeposition method with high degradation
performance, subsequent treatment of the mechanical
alloying process could increase activity further by
increasing the catalyst surface area. This was not tested
during the project, but would be of interest in any
subsequent work.
Gas Phase Assay
Figure 7: Gas phase ethanol oxidation to acetaldehyde
results for top-performing catalysts compared to Degussa
P25 and GENS NANO™ catalysts. PD = photodeposition
method; MA = mechanical alloying method.
International Conference on Environmental Systems
smallest amount of UV radiation passing through the filter will have an effect in the gas phase reaction.
The GENS NANO™ catalyst was seen to have a slight improvement of -10% over the bare Ti0 2 activity in the
gas phase. It did not however reach activity levels near any of the catalyst samples prepared in-house, compared to
results seen in the aqueous phase assay. These results show great promise for the quantum-dot modified catalysts
developed during this study; however, further work is needed to fully characterize these catalysts. Figure 7 shows
the results for the oxidation of ethanol to acetaldehyde (not a favored reaction product, as acetaldehyde is more
harmful than ethanol). Constraints based on the reactor design, primarily the static vial system, limited the
analytical techniques that could be employed thereby hindering full characterization and optimization of operating
conditions. Future testing would involve the development of a more evolved reactor design to allow for a flow-
through mode as well as optimized catalyst loading, contaminant loading, reaction time, etc. Optimally, the goal of
photocatalytic oxidation is to fully mineralize
contaminants (i.e., no intermediates/by-products); in the
results presented, this was not achieved. However, the
initial oxidation is still a worthwhile indicator of the
catalyst efficacy; evolution of the reactor and test design
will allow a more complete analysis.
Unlike the aqueous phase results, a clear difference
was seen in gas phase activity results depending on
preparation methods. Here, the photodeposition method
proved superior to mechanical alloying for all samples
(Figure 8). The discrepancy between methods is likely
due to oxidation of the metal and/or quantum dot species
during the mechanical alloying process, a conjecture
supported by XPS data (Section E), which showed
definitive differences between metal and/or quantum dot
peaks for the alternate preparation methods.
Comparison of Preparation Methods
Figure 8: Comparison of gas phase assay
with respect to preparation method.
D. Catalyst Characterization
7. Diffuse Reflectance Results
Another method of studying altered Ti0 2 for visible light activity is to study the diffuse reflectance spectrum of
the material. Diffuse reflectance data allows for the calculation of a material’s band gap energy, or the energy
required to induce electron-hole separation. Beginning at high wavelengths in the visible region, the solid material
should reflect a high percentage (up to 100%) of the light coming from the source. Once the band gap energy is
reached, the material begins to absorb light, inducing election excitation, and the reflectance decreases dramatically.
This point is clearly distinguishable in diffuse reflectance data as a large change in slope in the reflectance spectrum
(Figure 9). The point at which a line of best fit is tangent to the graph is the band gap energy requirement for the
studied material.
Diffuse Reflectance Spectra (Aq. Catalysts)
120 .
0 1
300 3 SO 400 450 500 SSO
-Blank
— D^gussa P25
1%CuS (PD)
1% InP/ZnS (MA)
l%CdS (MA)
Wavelength (nm)
Diffuse Reflectance Spectra (Gas Catalysts)
120 ,
0
300 3 SO 400 450 500 SSO
Blank
Deg us sa P25
3% PbS (PD)
3%Ag(PD)
0.3% PbS (PD)
0.1% PbS (PD)
l%Cu(MA)
Wavelength (nm)
Figure 9: Diffuse reflectance spectra for top-performing catalysts.
Compared to Degussa P25, if the spectrum of the new catalyst material experiences a red-shift in the reflectance
shoulder (i.e., the spectrum is shifted to longer wavelengths), there will be a lower energy requirement for
photocatalytic activity. In all the top-performing catalysts, the spectrum exhibited this shift and explains the
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International Conference on Environmental Systems
increased activity of the catalysts with the use of visible irradiation. As seen in Figure 9, several materials possess
multiple shoulders in their reflectance spectrum. Degussa P25 appears to have two reflectance shoulders because its
formulation contains both anatase and rutile forms of Ti0 2 which possess slightly different band gaps. Other
materials studied show multiple band gaps present due to Ti0 2 and/or quantum dots or metals.
2. XPS Analysis
For those catalysts prepared using both preparation methods, x-ray photoelectron spectroscopy (XPS) was
completed to study surface differences. In most cases, there were obvious differences between oxidation states of
the metal and/or quantum dot species between the two methods. For instance, the definitive peak for lead in PbS
was intact for the sample prepared by photodeposition but was altered in the mechanically alloyed sample (it is
likely that the species was oxidized during the milling process, causing the lowered activity of the catalyst). The
process of mechanical alloying, while useful for impregnation of dopants into compounds and for increasing surface
area, has the drawback of high-energy and -temperature reactions. A compromise must be made between the
advantage of surface area improvements and possible destruction of the catalyst during preparation. Again, further
investigations with this method, both on its own and as a secondary treatment of photodeposition samples, can lead
to a better understanding of valuable VLR catalyst preparation methods.
IV. Conclusion
There has been an increasing amount of research focused on the modification of Ti0 2 to render it visible-light-
responsive. Despite the vast effort, conclusions regarding the relative effectiveness of any given method are
difficult to draw due to the lack of consistent or standardized experimental conditions. This project as allowed for
the development of successful rapid aqueous phase and gas phase assays to close this intellectual gap. Furthermore,
an initial VLR catalyst library was built based on current and novel techniques for coupling Ti0 2 with narrow-band-
gap semiconductors. Several of the catalysts produced exhibited increased VLR activity over bare Ti0 2 and over a
commercially-available VLR catalyst, GENS NANO™. This project served to gather initial information on these
top-performing catalysts including VLR activity and alterations in band gap energy and supports the need for further
investigation into these materials. The evolution of this VLR catalyst technology will allow for its incorporation
into many crucial ground-based applications as well as integration into existing ISS, and future space exploration,
systems.
Acknowledgments
This effort was supported by the Kennedy Space Center 2011 Center Innovation Fund (CIF). The authors would
like to thank the late Dr. Lanfang Levine for her dedicated support throughout the project as well as Dr. Steven
Trigwell for XPS analysis, Mr. Lawrence Koss for construction of the light banks, and Drs. Thomas Graham and
Phillip Maloney for assistance in catalyst and assay preparations.
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