1. Experimental
Sample Preparation
Particle Sizes
Particles of various sizes were prepared by both hand grinding and ball milling
processes. Hand grinding was performed using an agate mortar and pestle.
Alternatively, other particle sizes were obtained using a high speed ball mill.
Samples were extracted at various times to ensure different particle sizes.
Doped TiO2
Electron doped TiO2 samples were synthesized using high temperature solid-
state reactions. Stoichiometric amounts were mixed, pressed into pellets, and
sealed in evacuated quartz tubes. The sealed tubes were placed in high
temperature furnaces.
2% Ti3+ Doped TiO2
0.98 TiO2 + 0.01 Ti2O3 900 ºC TiO1.99
12 hrs
4% Ti3+ Doped TiO2
0.96 TiO2 + 0.02 Ti2O3 900 ºC TiO1.98
12 hrs
TaON
Stoichiometric amounts of TaCl5, Li3N, and Li2O were weighed out and
pelletized inside an argon chamber, sealed in an evacuated silica tube, and
heated in a high temperature furnace. In many reactions, Li ions were not fully
removed from the system, so the samples were consequently heated with iodine
in a different sealed evacuated tube (Figure 7).
TaCl5 + LiO2 + Li3N TaON + 5LiCl
LiTa(OxN1-x)3 + 0.5 I2 TaON + LiI
Analysis
Formation and purity of all the products were monitored using powder X-ray
diffraction, employing a PANalytical Xpert Pro MPD diffractometer, equipped
with a linear X’Celerator detector, with Cu Kα1 radiation. The X-ray
diffraction data were collected at room temperature in the range of 10° ≤ 2θ ≤
70° with ~0.008° steps.
Each particle size was analyzed using a consistent mass of 80:20 TiO2 and flow
rate within the Micromeritics Digisizer.
Photocatalytic Treatment of Organic Wastewater Pollutants
References
1. Mezyk, S., et al. ACS Symposium Series 1071: Aquatic Redox Chemistry, 1071, 247,
(2011).
2. Scanlon, D.O., et al., Nature Materials 12, 798–801 (2013)
Roxanne Jacobs, Minué Perez, JoAnna Milam, Stephen Mezyk, and Shahab Derakhshan
Keck Energy Materials Program
Department of Chemistry and Biochemistry California State University, Long Beach, CA 90840
Acknowledgements
This research was supported by W. M. Keck Foundation through the Keck Energy Materials
Program (KEMP) at CSULB.
Background and Significance
Typically, wastewater treatment employs an adsorptive or transformative
process1. These processes are effective however not always efficient for large
scale projects. Our focus is solid catalysts that can employ UV and visible light
energy to degrade wastewater contaminants. TiO2-assisted photocatalytic
degradation of Methylene Blue (a wastewater contaminant) was investigated
using a solar simulator. The degradation as a function of time was monitored by
samples being taken at different times throughout the exposure within the solar
simulator. The absorbance of each solution was quantified employing a UV-Vis
spectrophotometer. In addition, the role of particle size, suspension, and
electron doping, in photocatalytic performance of TiO2 was investigated. To
improve the efficiency of the process in visible range of spectrum, effort was
devoted for preparation of transition metal oxy-nitrides. A new route to
synthesize TaON was developed, without the involvement of ammonia gas,
which is typically used in synthesis of such crystalline systems. For this
purpose metathesis reaction was employed and the formation of the desired
phases in various reaction conditions were monitored by powder X-ray
diffraction technique.
Solar Experiment
The performance of each solid catalyst was examined by placing 30 mg of catalyst in 200 mL of 15.6 x 10-6 M methylene blue solution. This suspension was stirred
in a glass dish with a water bath to maintain constant temperature throughout the experiment (Figure 2). Samples were taken every 8 minutes for spectroscopy
analysis (Figure 3). The absorbance of each solution was quantified utilizing a UV-Vis spectrophotometer that monitored the degradation of methylene blue as a
function of time.
Future Work
Diffuse reflectance spectroscopy will be used to determine the wavelength of the energy absorbed, which is technique closely related to UV-Vis spectrophotometry.
This will enable us to perform further chemical modifications to tune the desired band gap for semiconducting catalysts. In addition, the surface area of the samples
will be analyzed employing BET method.
This work will continue by analysing the performance of various mixtures of doped TiO2 samples within the solar experiment to evaluate their photocatalytic
activity. The oxy-nitride synthesis method will continue to be refined with the goal of producing pure product. This new method will be continue to be applied
towards TaxNb1-xON products (0 < x < 1) in order to manipulate the conductance band and valence band at the same time.
Results and Discussion
Particle Size Dependency
Particle size analysis revealed that ball milling process resulted in an increased
average particle size, which in turn caused less suspension and reduced surface
area and photocatalytic activity of the particles (Figure 4).
Comparing the first-order decay of the wastewater contaminant using the hand
ground and ball-milled TiO2 samples, revealed that the former exhibits higher
photocatalytic activity (Figure 5).
Doped TiO2
The X-ray data for the doped TiO2 samples were compared. The peaks shift to the
left as Ti3+ content (electron doping) increases. A selected peak for undoped, 2%,
and 4% rutile samples were at 27.450°, 27.440°, and 27.428°, respectively. This is
consistent with Bragg’s Law as the d-spacing within the unit cell increases, the θ
will decrease.
TaON
Synthesis of pure TaON product was attempted, however X-ray diffraction results
continuously displayed LiTaO3 as the main product. Synthesis of pure LiTaO3 was
then performed in order to compare X-ray results to previous attempts at TaON
synthesis (Figure 6). X-ray analysis indicated that LiTa(OxN1-x)3 was likely
present. In past experiments, TaON did not show up in the X-ray results until after
the sample was washed to dissolve LiCl and reheated. After washing, LiTaO3 was
still displayed as the main product. In order to drive out the excess lithium, an
iodine de-intercalation reaction (Figure 7) was employed, yielding the highest
percentage of TaON at 38%.
Figure 6: A comparison of LiTaO3 and an attempt at TaON synthesis.
Figure 7: Iodine de-
intercalation reaction.
Figure 5: The degradation of
methylene blue follows a first-order
rate decay. The slope of the
equations correspond to the rate
constant of each catalyst.
Figure 1: Schematic representation of
photocatalytic removal of organic
pollutants.
Figure 4: Particle size distribution of the hand ground and ball-milled samples.
Figure 2: Experiment set up for
photocatalytic reactions.
Figure 3: Appearance of samples
after being exposed to light at
different time intervals