Enhancement of Hydride-Terminated Silicon Nanocrystals for Gas-Phase Heterogeneous Carbon Dioxide Reduction
by
Annabelle Po Yin Wong
A thesis submitted in conformity with the requirements for the degree of Master of Science
Department of Chemistry University of Toronto
© Copyright by Annabelle Po Yin Wong 2017
ii
Enhancement of Hydride-Terminated Silicon Nanocrystals for
Gas-Phase Heterogeneous Carbon Dioxide Reduction
Annabelle Po Yin Wong
Master of Science
Department of Chemistry
University of Toronto
2017
Abstract
Utilization of solar energy for the conversion of carbon dioxide into fuels and chemical
feedstocks serves as a promising solution to mitigate anthropogenic greenhouse gas emissions
while providing global energy security and environmental protection. Silicon nanocrystals (ncSi)
are attractive materials for the photoreduction of CO2 due to their favorable earth-abundance,
non-toxicity, optical properties, and high surface area. Herein, through rational modification of
ncSi by introducing boron and phosphorus dopants utilizing a novel, facile sol-gel synthesis, it
was demonstrated that dopants successfully enhanced the gas-phase CO2 adsorption capacity and
solar fuel production rate. Remarkably, it was found that phosphorus-doped ncSi was the best
performer among the various ncSi samples due to the combination of the number of surface
hydrides and the addition of electronegative surface atoms. Preliminary results of the
optimization of reaction conditions in a batch photoreactor enabled an enhancement of the solar
fuel production rate.
iii
Acknowledgments
I would like to sincerely thank my supervisor, Professor Geoffrey Ozin, for his support, guidance
and encouragement throughout the course of my project at the Department of Chemistry. His
passion has shaped my appreciation for nanochemistry, while his wisdom, mentorship and “crazy
ideas” have helped me develop as a critical thinker, scientist, and innovator.
I owe particular thanks to Dr. Wei Sun and Chenxi Qian for their patience, chemical insights,
and valuable advice on the silicon nanocrystal projects. I also sincerely thank the past members
of the Silicon Team, Dr. Naoto Shirahata and Dr. Dongzhi Chen, for their in-depth discussion on
silicon. This project would also not have been possible without the invaluable discussions and
assistance from my colleagues in the Ozin group. In particular, I would like to thank Sue
Mamiche for always making sure that our lab runs smoothly, Yuchan Dong for the multi-reactor
tests and surface area measurements, Leo Diehl for the multi-reactor tests, Abdinoor Jelle for the
HRTEM and XPS analysis, Mireille Ghoussoub for the TGA, and Jia Jia and Ziqi Zhang for the
high intensity reactor tests. I would also like to acknowledge Thomas Wood, Dr. Paul O’Brien
and Amit Sandhel for their guidance on the photoreactors and answering my endless questions
about the reactor system. And to the 10 am coffee crew for helping me stay awake throughout
the day.
I would like to extend my gratitude to my collaborators in the Solar Fuels Cluster at the
University of Toronto. It was an honor to have worked with such brilliant and passionate
scientists and engineers.
Lastly, I would like to thanks my parents, my sister, and my friends for their continuous
encouragement.
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Table of Contents
Acknowledgments.......................................................................................................................... iii
Table of Contents ........................................................................................................................... iv
List of Tables ................................................................................................................................ vii
List of Schemes ............................................................................................................................ viii
List of Figures ................................................................................................................................ ix
List of Abbreviations ..................................................................................................................... xi
List of Appendices ....................................................................................................................... xiii
Chapter 1 Introduction and Background ..........................................................................................1
Introduction and background ......................................................................................................1
1.1 Solar fuels: towards a sustainable carbon neutral economy ................................................1
1.2 Hydride-terminated silicon nanocrystals for heterogeneous reduction of carbon
dioxide..................................................................................................................................2
1.3 Boron and phosphorus doped silicon nanocrystals ..............................................................3
1.3.1 Boron and phosphorus co-doped silicon nanocrystals and the surface
frustrated Lewis pairs ...............................................................................................4
1.3.2 Other attractive properties of doped silicon nanocrystals ........................................5
1.3.3 Location of dopants..................................................................................................5
1.3.4 Boron and phosphorus doped silicon nanocrystals as a candidate for
heterogeneous reduction of carbon dioxide .............................................................7
1.4 Summary of chapters ...........................................................................................................9
Chapter 2 Synthesis and Characterization of Doped and Pristine Free-Standing Hydride-
Terminated Silicon Nanocrystals ..............................................................................................10
Synthesis and characterization of doped and pristine free-standing hydride-terminated
silicon nanocrystals ...................................................................................................................10
2.1 Experimental ......................................................................................................................10
2.1.1 Preparation of boron and phosphorus doped free-standing hydride-terminated
silicon nanocrystals ................................................................................................10
v
2.1.2 Characterization .....................................................................................................12
2.2 Results and discussion .......................................................................................................13
2.2.1 Crystal phases, crystallite size, and surface area ...................................................13
2.2.2 ATR-FTIR spectroscopy ........................................................................................15
2.2.3 X-ray photoelectron spectroscopy .........................................................................17
2.2.4 Surface hydrophobicity ..........................................................................................21
2.3 Summary ............................................................................................................................22
Chapter 3 Investigation of Boron and Phosphorus Doped Silicon Nanocrystals in Gas-Phase
Heterogeneous Reduction of Carbon Dioxide ..........................................................................23
Investigation of boron and phosphorus doped silicon nanocrystals in gas-phase
heterogeneous reduction of carbon dioxide ..............................................................................23
3.1 Experimental ......................................................................................................................23
3.1.1 Sample preparation and storage .............................................................................23
3.1.2 Preparation of hydride-terminated silicon nanocrystals and molecular sieves
for water removal experiments ..............................................................................23
3.1.3 Gas-phase CO2 reduction measurements ...............................................................24
3.1.4 Data analysis ..........................................................................................................24
3.1.5 CO2 adsorption capacity ........................................................................................25
3.2 Results and discussion .......................................................................................................25
3.2.1 13CO production via reverse water-gas shift reaction ............................................25
3.2.2 CO2 adsorption capacity ........................................................................................29
3.2.3 Effect of sample storage in air ...............................................................................31
3.2.4 Utilization of only solar energy for RWGS reaction .............................................32
3.2.5 Unwanted side reaction: increase of 12CO production ...........................................34
3.2.6 Removal of water for 13CO production rate enhancement .....................................35
3.3 Summary ............................................................................................................................37
Chapter 4 Conclusion and Outlook ................................................................................................38
vi
Conclusion and outlook ............................................................................................................38
4.1 Conclusion .........................................................................................................................38
4.2 Outlook ..............................................................................................................................39
References ......................................................................................................................................42
Appendix A : Powder X-ray Diffraction Pattern ...........................................................................47
Appendix B : Irradiance Spectra ....................................................................................................50
Appendix C: Thermogravimetric Analysis ....................................................................................51
vii
List of Tables
Table 1. Estimated crystallite sizes of doped and pristine ncSi from the PXRD patterns using the
Scherrer equation and from the HRTEM images, and BET surface areas. .................................. 14
Table 2. Dispersibility of hydride-terminated doped and pristine ncSi in polar and non-polar
solvents. ........................................................................................................................................ 22
viii
List of Schemes
Scheme 1. The preparation of free-standing hydride-terminated B,P-ncSi. The blue and red
circles represent boron and phosphorus dopants, respectively. The boron dopant source, the
phosphorus dopant source, and both dopant sources were omitted for the preparation of P-ncSi,
B-ncSi and ncSi, respectively. The B,P-ncSi were drop-casted onto a borosilicate glass
microfibre filter, which were used for CO2 reduction experiments. ............................................. 11
ix
List of Figures
Figure 1. PXRD patterns of a) B-ncSi, P-ncSi, B,P-ncSi and pristine ncSi and b) a close-up
region of B,P-ncSi. A small peak at 34.4 ° is observed, indicated by the red arrow. ................... 13
Figure 2. High resolution TEM images of a) ncSi, b) B-ncSi, c) P-ncSi, d-f) B,P-ncSi. Red
circles correspond to the lattice planes of Si (111); yellow, ncBP (111); and green, ncBP (100);
scale bar, 10 nm. ........................................................................................................................... 15
Figure 3. Fourier transform infrared spectra of B-ncSi, P-ncSi, B,P-ncSi and ncSi. ................... 16
Figure 4. Si 2p X-ray photoelectron spectra of a) B-ncSi, b) P-ncSi, c) B,P-ncSi and d) ncSi. ... 18
Figure 5. X-ray photoelectron spectra of the B 1s region of a) B-ncSi and b) B,P-ncSi and P 2p
region of c) P-ncSi and d) B,P-ncSi. ............................................................................................. 19
Figure 6. X-ray photoelectron spectra of the B 1s region of a) B-ncSi embedded in SiO2 and b)
B,P-ncSi embedded in SiO2. ......................................................................................................... 20
Figure 7. Digital photographs of a) ncSi b) P-ncSi c) B-ncSi d) B,P-ncSi in EtOH. ................... 21
Figure 8. Surface area normalized 13CO production rates of B-ncSi, P-ncSi, B,P-ncSi and ncSi
for four to five consecutive runs under the irradiation of 0.20 Sun at 150 °C for 6 h with 1:1 CO2
to H2. ............................................................................................................................................. 26
Figure 9. Surface area normalized 13CO production rates of B-ncSi, P-ncSi, B,P-ncSi, and ncSi,
tested at 150 °C for 6 h with 1:1 CO2 to H2 in the dark (D) and light with 0.2 Sun (L). .............. 28
Figure 10. Surface area normalized carbon dioxide adsorption capacities of B-ncSi, P-ncSi and
ncSi at 35 °C. ................................................................................................................................ 29
Figure 11. Explanation for the increase in CO2 adsorption capacity due to the introduction of
electronegative atoms in b) B-ncSi and c) P-ncSi compared to a) ncSi. The green arrows indicate
the location of potential CO2 adsorption sites. Some bonds are omitted for simplicity. .............. 30
x
Figure 12. Surface area normalized 13CO production rates of fresh B-ncSi, P-ncSi, B,P-ncSi and
ncSi samples and samples stored in air for 7 and 14 days tested a) in the light and b) in the dark
at 150 °C. ...................................................................................................................................... 31
Figure 13. CO production rates of B-ncSi, P-ncSi, B,P-ncSi and ncSi under irradiation of 50
Suns without external heating for 4 h. .......................................................................................... 33
Figure 14. The 12CO (black) and 13CO (red) GCMS peak areas obtained from the reaction of B,P-
ncSi with CO2 and H2 in the a) light and b) dark at 150 °C over five to six consecutive runs. .... 34
Figure 15. CO production rates of ncSi only and ncSi with 3A molecular sieves for three
consecutive runs under the irradiation of 0.20 Sun at 150 °C with 1:1 ratio of 13CO2 to H2 for 3 h.
Tested molecular sieve beads were replaced by newly regenerated beads before each run. ........ 36
xi
List of Abbreviations
ATR attenuated total reflectance
BET Brunauer-Emmett-Teller
B-ncSi boron doped silicon nanocrystals
B-ncSi-SiO2 boron doped silicon nanocrystals embedded in SiO2
B,P-ncSi boron and phosphorus co-doped silicon nanocrystals
B,P-ncSi-SiO2 boron and phosphorus co-doped silicon nanocrystals embedded in SiO2
DFT density functional theory
FLP frustrated Lewis pair
FTIR Fourier transform infrared
GC gas chromatography
GCMS gas chromatography mass spectrometry
HRTEM high resolution transmission electron microscopy
LSPR localized surface plasmon resonance
ncSi silicon nanocrystals
ncSi:H hydride-terminated silicon nanocrystals
P-ncSi phosphorus doped silicon nanocrystals
P-ncSi-SiO2 phosphorus doped silicon nanocrystals embedded in SiO2
PXRD powder X-ray diffraction
RWGS reverse water-gas shift
SiNW silicon nanowire
xii
TGA thermogravimetric analysis
XPS X-ray photoelectron spectroscopy
xiii
List of Appendices
Appendix A: Powder X-ray Diffraction Pattern……………………………………….………...59
Appendix B: Irradiance Spectra……………………………………………………….…………62
Appendix C: Thermogravimetric Analysis………………………………………………………63
1
Chapter 1 Introduction and Background
Introduction and background
1.1 Solar fuels: towards a sustainable carbon neutral economy
Anthropogenic greenhouse gas emissions, namely carbon dioxide emissions from combustion of
fossil fuels, are increasing at an alarming rate and are the highest in recorded history.1 These
emissions stem from our dependency on fossil fuel usage, leading to concerning environmental
issues such as climate change. With an increasing demand for global energy consumption and a
diminishing supply of non-renewable fossil fuels, renewable energy such as solar, wind, tidal and
geothermal have been exploited for energy generation. However, the intermittency of these
renewable sources presents a major challenge for delivering energy and electricity on demand, so
the energy generated must be stored.2 In order to mitigate anthropogenic CO2 emissions and to
supply sufficient energy for global usage, the utilization and conversion of CO2 to value-added
chemicals and fuels are key strategies to tackle both of these issues. Some common examples of
value-added chemicals and fuels are carbon monoxide, methane, other short-chain hydrocarbons,
methanol, dimethyl ether, and formic acid. However, CO2 is an extremely thermodynamically
stable molecule and requires an immense amount of energy for the bonds to dissociate and the
conversion to take place.3,4 The ultimate solution is to take advantage of the indefinitely
renewable solar energy, both light and heat generated from the sun, to catalytically convert CO2
into chemical feedstocks and fuels – known as “solar fuels”. The CO2 produced by these
renewable fuels would then be re-captured and recycled into fuels, creating a “neutral carbon
cycle”. In this way, solar energy is captured and stored in the form of chemical bonds of fuels,
which can be simply stored, released on-demand, and safely transported using existing
infrastructure.
To have a meaningful effect on climate change, renewable energy and the environment, solar
fuel production requires a global research effort. To date, the majority of the research focused on
solar-driven hydrogen production by water splitting. Hydrogen is a sustainable fuel when
produced by photocatalytic or photoelectrochemical water splitting and can be used to power, for
example, hydrogen fuel cells. However, it is not a practical choice of fuel due to safety concerns,
2
storage and transportation issues, and the lack of existing infrastructure for hydrogen.2 In order to
realize more efficient, scalable and economically viable solar fuel production methods,
development of materials for effective CO2 reduction is critical for advancement in this field to
overcome the energy crisis.
As mentioned above, CO is one of the value-added chemical feedstocks which can be produced
from CO2. One may not realize that CO is a useful chemical to produce, but in fact, CO is a
common reagent in the chemical industry. For instance, when combined with H2, this mixture
called syngas can be processed to produce liquid hydrocarbon fuels via the well-known Fischer-
Tropsch synthesis using existing technologies.5–8 The production of CO from CO2 occurs via the
reverse water-gas shift (RWGS) reaction, where CO2 is mixed with hydrogen, and carbon
monoxide and water are produced (Reaction 1).
CO2 + H2 → CO + H2O (1)
Ideally, the CO2 would be captured from the atmosphere or flue gases from industrial plants, and
the hydrogen would be produced by solar-driven water splitting to create a sustainable system.
The RWGS reaction has a high activation energy barrier, and therefore, is an energy intensive
process.5,9 Currently the reaction can be processed at temperatures above 800 °C at 1 atmosphere
with the use of solid metal catalysts in order to selectively convert CO2 to CO and to prevent the
unwanted formation of coke.5 If only this reaction could be driven by solar energy, then a
sustainable carbon-neutral carbon-cycle would be created.
1.2 Hydride-terminated silicon nanocrystals for heterogeneous reduction of carbon dioxide
Many of the catalysts being studied for CO2 reduction consists of “endangered” metal
elements4,10,11 and therefore, it is important to exploit a more sustainable option for solar fuel
production. Silicon is an attractive candidate for sustainable technology applications and for
scaling-up to realize practical solar fuel production due to its earth-abundance, low-cost and low-
toxicity. Nanostructured silicon is of particular interest in solar-to-chemical conversion
applications owing to its high surface area and narrow band gap with a near-infrared to visible
absorption spectrum.12–14 Since the first demonstration of using silicon in photoelectrolysis of
water by Candea et al. in 1976,15 silicon has been investigated for solar fuel generation.
3
However, research efforts of silicon for solar fuel production is mainly focused on producing H2
from water.12,16 Nanocrystalline silicon has also been studied as a hydrogen storage material due
to its high surface area and its ability to adsorb and desorb hydrogen.17,18
Recently, our group has demonstrated the ability of hydride-terminated silicon nanocrystals,
denoted ncSi:H, for the conversion of carbon dioxide to carbon monoxide via the RWGS
reaction in a hydrogen environment (Reaction 2). This gas-phase reaction was performed in a
batch reactor at 170 °C with a pressure of 27 psi under solar simulated irradiation, and product
gases were tested using 13C isotope tracing experiments. We have found that the surface hydrides
(Si–H) of ncSi:H are responsible for the reduction of CO2 and selectively produce CO under
these conditions.
(2)
Furthermore, we have demonstrated that this is a light-assisted reaction, which can produce CO
up to a rate of 250 μmol g-1 h-1 under 15 Suns at 150 °C. We attributed the light enhancement to
the well-known photothermal effect of ncSi. However, the CO production rate continues to
decrease over subsequent cycles. It was found that this is because the number of Si–H
diminishes, while Si–O–Si and Si–O–H groups are produced as the reaction proceeds. It was also
demonstrated that the reaction between these groups and H2 does not regenerate Si–H bonds. In
other words, the active site, Si–H, is deactivated and the reaction can only proceed
stoichiometrically. With these exciting findings, we set out to further explore the possibilities of
improving the CO2 reducing ability of ncSi, by either enhancing the reaction rate or even better,
rendering the reaction catalytic.
1.3 Boron and phosphorus doped silicon nanocrystals
Boron and phosphorus are the most commonly used dopants for modifying the electronic and
optical properties of silicon by introducing free charge carriers (holes and electrons,
respectively). They are of great importance in optoelectronic applications since the conductivity
of silicon can be tuned by carefully controlling the concentration of substitutional dopants.19–21
Some applications include but are not limited to solar cells20,22,23, light emitting diodes
(LED)14,24, bioimaging25,26, and photocatalysis.12,15,27,28
4
Motivation for doping ncSi with boron and phosphorus is the possibility of creating a surface
analogue of a molecular frustrated Lewis pair (FLP) to render the CO2 photoreduction of ncSi
catalytic, as well as the opportunity to explore other attractive properties of doped ncSi which
could enhance the reactivity of CO2 towards reduction.
1.3.1 Boron and phosphorus co-doped silicon nanocrystals and the surface frustrated Lewis pairs
The motivation of utilizing boron and phosphorus doped ncSi for CO2 reduction was inspired by
the molecular frustrated Lewis pair (FLP) developed by Stephan and coworkers. An FLP is
generated when the formation of a classical Lewis acid-base adduct is inhibited due to steric
hindrance of bulky ligands. The first FLPs developed were phosphine-borane molecules (P···B)
which are able to reversibly activate hydrogen to form the hydrogenated zwitterion
phosphonium-borate (PH+···BH-) molecules.29–31 After hydrogenation, FLPs are able to reduce
small molecules such as imines and enamines.30,31 Our motivation is to create an analogue of the
molecular FLP on the surface of ncSi with boron and phosphorus, in hopes of heterolytically
splitting H2 on P and B surface atoms which subsequently reduces CO2 to CO, rendering the
photoreduction catalytic. It is also interesting that boron phosphide nanotubes have been shown
to heterolytically dissociate hydrogen in a theoretical study.32
Recently, boron and phosphorus co-doped ncSi, from here on denoted B,P-ncSi, were developed
by Fujii and coworkers and were synthesized by co-sputtering or from hydrogen silsesquioxane
(HSQ).33 Their motivation for synthesizing B,P-ncSi was to develop ligand-free ncSi since
organic capping ligands, which are employed to prevent aggregation of nanoparticles, have
detrimental effects on charge transport. B,P-ncSi were found to be well dispersible in polar
solvents such as methanol and water and therefore could be used for large-scale solution-
processed optoelectronic fabrication and biomedical applications.33–35 Results from theoretical
studies of B,P-ncSi suggest that the polar B–P pairs are preferentially formed on the surface of
the ncSi, which induces electrostatic repulsion between individual B,P-ncSi, enabling colloidal
stability.34,36
Inspired by the above research endeavors, our curiosity was spiked to explore metal-free, earth-
abundant hydride-terminated boron and phosphorus doped ncSi for CO2 reduction, utilizing the
light and heat from the sun.
5
1.3.2 Other attractive properties of doped silicon nanocrystals
The addition of boron and phosphorus can drastically change the properties of ncSi, which could
assist in the enhancement of CO production rates by hydrogenation of CO2. Our group has
previously demonstrated the increase of CO production rates in the RWGS reaction by
increasing the local heating of nanostructures.13,37 Furthermore, from our study on ncSi:H, we
have demonstrated that the photoreduction of CO2 can be enabled thermally (in the dark) and the
light-enhanced CO production rate was attributed to the photothermal effect. In this way, the
addition of dopants could potentially increase the CO production rates by increasing the local
temperatures of ncSi by two processes: 1) introducing defect states in the band gap of ncSi and
2) introducing localized surface plasmon resonance (LSPR).
By incorporating dopants into ncSi, sub-band gap dopant states are introduced38–40 and non-
radiative recombination of charge carriers could occur via the Shockley-Read-Hall process,
inducing local heating in the nanostructures.41 Shockley-Read-Hall recombination occurs when
excited electrons and holes recombine through these dopant trap states and the energy loss is
released as heat or light.40,42 At sufficiently high dopant concentrations, boron and phosphorus
singly-doped ncSi (B-ncSi and P-ncSi) are also known to demonstrate LSPR,43–45 which occurs
when light interacts with a nanoparticle with dimensions smaller than the incident wavelength,
resulting in a conduction electron plasmon localized at the nanoparticle.46 LSPR is an attractive
property for applications such as photothermal therapy, and hot electron generation for
photocatalysts, as it provides local heating at the nanoscale.41 In this way, if local temperatures
can be increased by non-radiative recombination and plasmonic effects induced by dopants, then
the rate of CO2 to CO conversion rate could be enhanced.
1.3.3 Location of dopants
Within the past two decades, much theoretical and experimental research have been carried out
to examine the location of boron and phosphorus dopants with respect to ncSi, whether they are
preferentially located on the surface, in the sub-surface, in the core of ncSi or in a surrounding
SiO2 matrix. Understanding the doping efficiencies and the preferential locations of dopants are
essential to control the dopant concentrations and to better understand the electronic and optical
properties of ncSi.20,36,47,48 With regards to utilization of B and P doped ncSi in the application of
CO2 reduction, it is worthwhile to examine the location of dopants to elucidate the influence of
6
the electronic and surface properties of doped ncSi on its ability to reduce CO2 into fuels. This is
especially true since new interfaces and interactions are introduced between the surface of doped
ncSi and gaseous molecules in the system.
The location and doping effects have been studied for both free-standing and silicon oxide
embedded boron and phosphorus doped ncSi. The solubility limits of boron and phosphorus in
silicon are 1% and 0.3%, respectively, which means that there should be dopant segregation on
the surface of ncSi if dopant concentrations are beyond the solubility limit.20,33
Multiple research groups have reported that B atoms preferentially reside on the surface of ncSi,
whereas P atoms are preferentially located in the core of ncSi for single- and co-doped ncSi and
silicon nanowires (SiNWs).36,47,48 Xie et al. reported that boron doped ncSi embedded in a SiO2
matrix synthesized by co-sputtering resulted in the presence of boron on both the surface and in
the core of ncSi, based on the B(0) and oxidized B peaks observed in the B 1s X-ray
photoelectron spectra.48 Fukata et al. synthesized boron and phosphorus singly- and co-doped
SiNWs by catalytic laser ablation. The change in concentrations of B and P from the surface to
the core of the SiNWs was probed by Raman spectroscopy and electron spin resonance,
respectively, using a layer-by-layer oxidation and HF etching technique. It was also found that P
tends to reside in the crystalline core of SiNWs and B near the surface of the SiNWs.49 Guerra et
al. also investigated the preferential locations of p-type (B and Al) and n-type dopants (N and P)
with respect to ncSi, by examining the lowest theoretical formation energies at various locations
of ncSi embedded in SiO2 and hydroxyl-terminated free-standing ncSi. Similar to the
experimental results discussed above, it was reported that B and Al are the most energetically
stable at the surface, whereas N and P are most stable in the core of ncSi. This is because the
diffusion barrier of B and Al atoms within the SiO2 matrix is much lower than N and P atoms,
and that B atoms are the most energetically stable when bound to 2 Si atoms and 2 oxygen atoms
(at the ncSi/SiO2 interface).36 Theoretical calculations demonstrated that in both B and P co-
doped SiNWs and ncSi, the formation of B–P bonds is energetically favorable and stabilizes B
atoms in the crystalline region of silicon near the surface of SiNW and ncSi.36,49 Fujii and
coworkers demonstrated that boron and phosphorus co-doped ncSi are able to disperse very well
in polar solvents such as methanol and water. They attributed their hydrophilicity to a negatively
charged boron-rich surface and the dispersibility in polar solvents to electrostatic repulsion of the
negatively charged co-doped ncSi.33–35
7
In this way, it is thought that the location of dopants must be independent of synthetic methods,
shape, or size of nanostructured silicon. However, Kortshagen, Pi and colleagues observed that
the opposite occurs for boron and phosphorus singly-doped ncSi synthesized by nonthermal
plasma – P atoms were experimentally found to locate near the surface of ncSi, and more B
atoms exist in the core of ncSi.47,50 They attributed this experimental result to a weaker Si–B
bond compared to Si–P on the surface of ncSi.50
The location of dopants is also helpful in determining whether the dopant element is
electronically active or not. When dopant atoms are substitutionally doped into the core of ncSi,
they are most likely electronically active. However, if dopant atoms reside on the surface of ncSi,
they may be passivated by surface oxides. Then, free charge carriers are compensated and no
longer present, rendering the ncSi electronically inactive. Also, if the B and P dopant
concentrations are equal in B,P-ncSi, then excess holes and electrons from B and P, respectively,
would be compensated.20
Pertaining to the application of reduction of CO2, it is critical to understand the distribution of
dopants with respect to the surface and volume of ncSi. For instance, it would be expected that
dopants that reside on the surface of ncSi would reduce the number of Si–H active sites, which
may decrease the CO production rates. If free charge carriers play a critical role, then having
dopants only on the surface of ncSi may have detrimental effects on the CO production rate as
free charge carriers are compensated by surface passivating atoms. On the other hand, if dopants
in the core do not enhance the CO production rate, then the synthesis method should be tailored
such that most of the dopants are on the surface of the ncSi. Understanding the role of dopants
and charge carriers could also help elucidate the mechanism of the CO2 reduction reaction. For
example, if free charge carriers are absent and the ncSi is still active, then the implication would
be that charge carriers are not directly involved in the reaction mechanism.
1.3.4 Boron and phosphorus doped silicon nanocrystals as a candidate for heterogeneous reduction of carbon dioxide
P-type silicon (p-Si) has been demonstrated to reduce CO2 in the aqueous phase.12,27,28,51,52 The
use of p-Si for CO2 photoreduction to CO can be dated back to the 1980s28,52 and research on the
modification of p-Si for CO2 reduction has been ongoing since.28 However, there are a number of
issues associated with CO2 photoreduction in the aqueous phase, including the competing
8
hydrogen evolution reaction from water, low solubility of CO2, and the scalability, energy
efficiency and economics of the process.4,12,13,28
To date, almost all boron and phosphorus doped ncSi are synthesized by chemical vapor
deposition, ion implantation, plasma synthesis, laser ablation and co-sputtering.20 The Veinot
group has developed a facile synthesis for high-purity, near monodisperse ncSi utilizing a
(HSiO1.5)n sol-gel precursor. (HSiO1.5)n can be synthesized by hydrolysis and condensation of
HSiCl3, followed by the thermal treatment of (HSiO1.5)n under a slightly reducing atmosphere.
This leads to a disproportionation reaction and nucleation and growth of silicon clusters and ncSi
are formed within a SiO2 matrix. The ncSi can then be liberated from the matrix by etching away
the SiO2 by an aqueous HF solution. Moreover, the size of ncSi can be easily tuned by the
temperature and time of the thermal treatment.53,54 A modified method is employed in the work
presented within this thesis to incorporate boron and phosphorus dopants into ncSi. The synthesis
and characterization methods are detailed in Chapter 2.
To summarize the aforementioned properties of boron and phosphorus doped ncSi, the rate of
CO production from CO2 hydrogenation would, therefore, depend on a combination of the
following:
Number of surface hydrides (the active sites)
Surface area
Electronic properties
Surface species
Photothermal properties
Presence of impurities
Adsorption capacity of CO2
Reactor test conditions
9
To the best of our knowledge, boron and phosphorus doped ncSi have not been previously
explored for gas-phase heterogeneous reduction of CO2 for solar fuel production. In this thesis,
the effect of introducing boron and phosphorus into ncSi on the light-assisted conversion of CO2
to CO via the reverse water-gas shift reaction is investigated.
1.4 Summary of chapters
This thesis is organized into 4 chapters. In Chapter 2, the experimental procedures for the
synthesis and characterization of B-ncSi, P-ncSi, B,P-ncSi and ncSi are detailed. The
determination of surface species and the location of dopants are also discussed. Chapter 3
outlines the methodologies of the gas-phase CO2 reduction experiments performed in a
photoreactor. The CO production results of B-ncSi, P-ncSi, B,P-ncSi and ncSi in the light, in the
dark, and after storage in air are compared and discussed. This chapter also presents the
preliminary results of the water removal experiment on pristine ncSi. Chapter 4 summarizes the
findings presented in this thesis and provides an outlook on future development on this work.
10
Chapter 2 Synthesis and Characterization of Doped and Pristine Free-
Standing Hydride-Terminated Silicon Nanocrystals
Synthesis and characterization of doped and pristine free-standing hydride-terminated silicon nanocrystals
2.1 Experimental
2.1.1 Preparation of boron and phosphorus doped free-standing hydride-terminated silicon nanocrystals
The synthesis of boron and phosphorus co-doped silicon nanocrystals (B,P-ncSi) is depicted in
Scheme 1. In a typical synthesis of boron and phosphorus containing cross-linked (HSiO1.5)n sol-
gel polymer, denoted B,P-(HSiO1.5)n, 15 mL of HSiCl3 (148 mmol, Sigma Aldrich) was added to
a beaker equipped with a magnetic stir bar and cooled in a dry ice/isopropanol bath (-78 °C). In a
separate vessel, 1.2 g of boric acid (19 mmol, Sigma Aldrich) and 0.8 mL of phosphoric acid (12
mmol, 85 wt%, Caledon Laboratories) were dissolved in 60 mL of distilled water. This solution
was added to the cooled HSiCl3, while stirring, in one aliquot to immediately yield a white
precipitate. The beaker was then removed from the isopropanol/dry ice bath and left at room
temperature for 30 min to ensure removal of HCl vapour and complete hydrolysis and
condensation. The white precipitate was washed by distilled water and dried by vacuum
filtration. It was then transferred to a round-bottom flask and further dried under vacuum at 100
°C overnight in an oil bath to afford B,P-(HSiO)n which is a white powder.
B-ncSi and P-ncSi were prepared by the same method, but without the addition of H3PO4 and
H3BO3, respectively. Pristine ncSi was prepared without the addition of H3PO4 and H3BO3.
11
Scheme 1. The preparation of free-standing hydride-terminated B,P-ncSi. The blue and red
circles represent boron and phosphorus dopants, respectively. The boron dopant source, the
phosphorus dopant source, and both dopant sources were omitted for the preparation of P-ncSi,
B-ncSi and ncSi, respectively. The B,P-ncSi were drop-casted onto a borosilicate glass
microfibre filter, which were used for CO2 reduction experiments.
Boron containing (HSiO1.5)n, B-(HSiO1.5)n, was prepared in the same manner by adding 1.2 g of
boric acid dissolved in 60 mL distilled water into 15 mL HSiCl3 (148 mmol, Sigma Aldrich).
Phosphorus containing (HSiO1.5)n, P-(HSiO1.5)n, was prepared by adding of a mixture of 0.8 mL
of phosphoric acid and 60 mL distilled water into 15 mL HSiCl3. Pristine (HSiO1.5)n was
prepared by adding 60 mL distilled water into 15 mL HSiCl3, without the addition of boric acid
or phosphoric acid.
B-(HSiO1.5)n, P-(HSiO1.5)n, B,P-(HSiO1.5)n or (HSiO1.5)n was transferred to a quartz reaction boat
and heated first at 400 °C for 30 min and then at 1100 °C for 2 h (18 °C min-1) under a constant
gas flow of 95% Ar/5% H2 in a quartz tube furnace. Thermal treatment of (HSiO1.5)n in a
reducing environment causes the disproportionation reaction to occur and allows nucleation and
12
growth of ncSi. The resulting brown powder is B-ncSi, P-ncSi, B,P-ncSi or ncSi, respectively,
embedded in a SiO2 matrix.
In order to liberate doped or pristine ncSi from the SiO2 matrix, 1.2 g of the brown powder was
ground in a mortar and pestle with 20 mL of EtOH to ensure uniformity of grain size and was
transferred to a Teflon beaker equipped with a magnetic stir bar. 40 mL of HF (48%, aq.,
Caledon Laboratories) were subsequently added to the beaker. Personnel should be well trained
in handling HF. This mixture was stirred for 2 h to etch away the SiO2 matrix. For B-ncSi and
B,P-ncSi, the mixture was then centrifuged at 3800 rpm for 10 min, and the HF/EtOH solution
was decanted to isolate the ncSi. The free-standing B-ncSi or B,P-ncSi were then washed with
acetone twice and isolated by centrifugation to ensure complete removal of HF. For P-ncSi and
pristine ncSi, after stirring in the HF/EtOH mixture for 2 h, free-standing P-ncSi or ncSi were
extracted into pentane and isolated by centrifugation. The final products were dried in a vacuum
oven overnight. Different extraction methods are used due to the difference in surface
hydrophilicity of ncSi.
2.1.2 Characterization
Power X-ray diffraction (PXRD) was performed on a Bruker D2-Phaser X-ray diffractometer
using Cu Kα radiation at 30 kV. Fourier-transform infrared (FTIR) spectroscopy was performed
using a Perkin Elmer Spectrum-One FT-IR fitted with a universal attenuated total reflectance
(ATR) sampling accessory with a diamond coated zinc selenide window. X-ray photoelectron
spectroscopy (XPS) was performed using an ultrahigh vacuum chamber with base pressure of
10-9 torr. A Thermo Scientific K-Alpha XPS spectrometer, with an Al Kα X-ray source
operating at 12 kV, 6 A and X-ray wavelengths of 1486.7 eV, was used. The spectra were
obtained with analyzer pass energy of 50 eV with energy spacing of 0.1 eV. Data analysis was
performed using the Thermo Scientific Avantage software. High resolution transmission electron
microscopy (HRTEM) was performed using a Hitachi HF3300 Environmental-CFE-TEM
operated at a voltage of 300 kV. HRTEM images were analyzed with the ImageJ software.
13
2.2 Results and discussion
B-ncSi, P-ncSi, B,P-ncSi and ncSi were characterized by a variety of methods to determine the
composition, size, and surface properties, which are essential to elucidate the difference in CO2
reduction activity of the various ncSi samples.
2.2.1 Crystal phases, crystallite size, and surface area
The PXRD patterns of B-ncSi, P-ncSi, B,P-ncSi and ncSi are shown in Figure 1. The diffraction
peaks at 28.7 °, 47.7 °, and 56.5 ° correspond to the (111), (220), and (311) lattice planes of
diamond cubic silicon (JCPDS #27-1402), respectively. No other phases were observed in the
diffraction patterns of B-ncSi, P-ncSi, and pristine ncSi, but in the B,P-ncSi sample a small peak
appears at about 34.4 °, corresponding to the (111) lattice plane of cubic boron phosphide
(Figure 1b, JCPDS #11-0119). Sugimoto et al. reported that during the thermal treatment of
silicon-rich borophosphosilicate films at 1200 °C under N2, B-O and P-O bonds were reduced to
cubic BP nanocrystals (ncBP), while Si(III) becomes fully oxidized to Si(IV).55 In this way,
ncBP is most likely formed in the same manner from BPO4, which can be formed by a mixture
of boric and phosphoric acid, in (HSiO1.5)n.56 The presence of ncBP is also confirmed by
HRTEM as discussed below. ncBP being very resistant to HF left residual ncBP in the sample
after etching in HF/EtOH for 6 h.
Figure 1. PXRD patterns of a) B-ncSi, P-ncSi, B,P-ncSi and pristine ncSi and b) a close-up
region of B,P-ncSi. A small peak at 34.4 ° is observed, indicated by the red arrow.
14
The crystallite sizes of the doped and pristine ncSi are estimated by the Scherrer equation and are
shown in Table 1. It is not surprising that the size of the doped ncSi is larger than pristine ncSi,
for it is known that dopants tend to accelerate the nucleation and growth of ncSi.19,57 It is also
important to note that the peaks of the doped ncSi samples appear to be asymmetrical, which
indicates a bimodal distribution of sizes.
Table 1. Estimated crystallite sizes of doped and pristine ncSi from the PXRD patterns using the
Scherrer equation and from the HRTEM images, and BET surface areas.
Crystallite Size
from PXRD
pattern (nm)
Crystallize Size
from HRTEM
images (nm)
BET Surface Area
(m2 g-1)
B-ncSi 15 5 - 15 290
P-ncSi 30 5 - 10 257
B,P-ncSi 30 5 - 20 296
ncSi 5 5 - 8 327
The PXRD patterns were also obtained prior to the liberation of doped and pristine ncSi
embedded in SiO2 (Appendix A). Sharp peaks, indicative of large crystals, were only observed in
the doped ncSi samples. This demonstrates that boron and phosphorus dopants have an effect on
the growth of ncSi during thermal treatment.
Figure 2 shows typical high resolution TEM images of ncSi, B-ncSi, P-ncSi, and B,P-ncSi
samples. Lattice spacing of 0.31 nm (circled in red), corresponding to Si (111) planes, are
observed in the images for all doped and pristine ncSi. The measured crystallite sizes of ncSi, B-
ncSi, P-ncSi, and B,P-ncSi are shown in Table 1. The large size distribution observed are
supported by the bimodal size distribution shown in the PXRD pattern. In B,P-ncSi, besides Si
(111), lattice planes with d-spacing of 0.25 nm and 0.40 nm, corresponding to the (111) and
(100) planes of cubic BP, were also observed (Figure 2f). The BP lattice planes were not
observed in the HRTEM images of ncSi, B-ncSi or P-ncSi, confirming that the crystals are only
formed in the presence of both B and P dopants and that the lattices correspond to BP. These
results support the results from PXRD.
15
Figure 2. High resolution TEM images of a) ncSi, b) B-ncSi, c) P-ncSi, d-f) B,P-ncSi. Red
circles correspond to the lattice planes of Si (111); yellow, ncBP (111); and green, ncBP (100);
scale bar, 10 nm.
2.2.2 ATR-FTIR spectroscopy
In order to probe the surface species of ncSi, ATR-FTIR spectroscopy was used to analyze the
samples. The FTIR spectra of freshly etched samples of B-ncSi, P-ncSi, B,P-ncSi and ncSi
(Figure 3) all showed two dominant peaks at ~ 2100 cm-1 and 900 cm-1, attributed to the
characteristic stretching and bending modes of Si–Hx (x = 1, 2, 3), respectively, indicating that
silicon hydrides are the dominant surface species on all samples. A small amount of C-Hx from
residual organics (i.e., solvents) is also observed at ~ 2900 cm-1 and no Si–C at 680 cm-1 is
observed for all samples. No B–H (2500 cm-1 and 1800 cm-1) or B–O (1400 cm-1) peaks are
16
observed in the IR spectra of B-ncSi and B,P-nSi , and no P–H (2276 cm-1) or P–O–P (920 cm-1)
peaks are observed in the P-ncSi and B,P-ncSi samples.
Figure 3. Fourier transform infrared spectra of B-ncSi, P-ncSi, B,P-ncSi and ncSi.
The surfaces of all samples are slightly oxidized, as evident from the OSi–O, OSi–H, and O–H
stretching modes at ~1060, 2050, and 3300 cm-1, respectively. The relative amount of surface
oxidation was approximated by taking the sum of the transmittance of the OSi–O and O–H peaks
and dividing it by the Si–H peak for each sample. The results showed that for pristine ncSi, the
oxidized Si peaks are 22% of the Si–H peak, the lowest among the 4 samples, while they are
30%, 49%, and 56% for B-ncSi, P-ncSi and B,P-ncSi, respectively. This suggests that ncSi has
the highest amount of Si–H with respect to Si–O, followed by B-ncSi, P-ncSi, and then B,P-ncSi.
The adventitious surface oxidation stemmed from oxidation by solvents or inadvertent exposure
to air and should be minimized in order to maintain the maximum number of Si–H sites to obtain
the highest CO production rates. The amount of oxidation is also examined by other
characterization methods discussed later. Since Si–H is responsible for the reduction of CO2, this
observation is crucial for understanding the difference in CO production rates of doped and
pristine ncSi.
17
2.2.3 X-ray photoelectron spectroscopy
Doped and pristine ncSi were further characterized by X-ray photoelectron spectroscopy (XPS).
XPS does not only determine the elemental composition and oxidation states of each element,
but can also give us some insights into the location of dopants with respect to ncSi (e.g.
substitutional lattice doping or surface doping). The Si 2p spectra of B-ncSi, P-ncSi, B,P-ncSi,
and ncSi are displayed in Figure 4. The peak at a lower binding energy of ~ 100 eV is assigned
to Si(0), a Si atom that makes up the core of ncSi, whereas the peak at the higher binding energy
103-104 eV originates from Si(IV) (oxidized surface Si atoms). The fitted peaks at 100 eV and
100.7 eV in ncSi are assigned to Si 2p3/2 and Si 2p1/2, respectively (Figure 4d). A third peak at
101 eV is fitted in the Si 2p spectra, which is assigned to a silicon sub-oxide. However, in the B-
ncSi sample (Figure 4a), it appears that the ratio of the intensity of the peak at ~ 101 eV to Si(0)
is much greater in B-ncSi compared to other samples. This is because this peak at ~ 101 eV is
attributed to Si–B, which provides evidence that the incorporation of B in the Si crystal lattice
has been achieved. Therefore, there is a greater percentage of B atoms that are incorporated on
the surface or in the core of silicon in B-ncSi than in B,P-ncSi. This is supported by the B 1s
spectrum discussed below.
The ratio of Si(IV) to Si(0) also varies in different samples, which stems from adventitious
surface oxidation of ncSi. As shown in the Si 2p spectra of B-ncSi and B,P-ncSi in Figure 4a and
4c, the ratio of Si(IV) to Si(0) is the highest, approximately half of the Si is oxidized to Si(IV).
This surface oxidation most likely stemmed from the washing step with acetone.
18
Figure 4. Si 2p X-ray photoelectron spectra of a) B-ncSi, b) P-ncSi, c) B,P-ncSi and d) ncSi.
A broad peak observed in the B 1s spectra of both B-ncSi and B,P-ncSi is due to the mixed
valence states of boron – trivalent B(0), tetravalent B(0), and B(III) – which have binding
energies of 185 eV, 188 eV, and 193 eV, respectively (Figure 5). The B 1s and P 2p spectra were
not able to be fitted quantitatively due to the low signal-to-noise ratio. Trivalent and tetravalent B
are defined as B atoms that are connected to three and four B or Si atoms, respectively, and are
substitutional B atoms with zero valence charge.44 The oxidized B species at 193 eV are
attributed to surface B atoms which are oxidized in B-ncSi and B,P-ncSi. In this way, the
presence of both B(0) and oxidized B suggests that B atoms exist in the core and on the surface
of the ncSi.
19
Figure 5. X-ray photoelectron spectra of the B 1s region of a) B-ncSi and b) B,P-ncSi and P 2p
region of c) P-ncSi and d) B,P-ncSi.
The B 1s spectra of B-ncSi and B,P-ncSi embedded in SiO2, denoted B-ncSi-SiO2 and B,P-ncSi-
SiO2, respectively, were also obtained to confirm that substitutional B is present in the ncSi after
thermal treatment (Figure 6). Two peaks at ~ 188.5 eV (B(0)) and ~ 194 eV (B(III)) are observed
in B-ncSi-SiO2 and B,P-ncSi-SiO2, which indicate that B doping in the Si lattice was achieved
after thermal treatment. B atoms related to the B(III) peak likely originate from B–O residing in
the interface between ncSi and SiO2 or within the SiO2 matrix. In B-ncSi-SiO2 (Figure 6a), the
approximate ratio of the area under the B(0) and B(III) peaks is 3:7, meaning that ~ 30% of B is
doped within the ncSi lattice, whereas 70% of the B is either doped on the surface of ncSi or
reside in the SiO2 matrix. On the other hand, the B(0) peak is very weak in B,P-ncSi-SiO2
(Figure 6b). Therefore, there may only be minimal amounts of B dopant existing in the core of
ncSi, supported by the XPS of free-standing B,P-ncSi, even though the same amount of doping
reagent, H3BO3, was used in the synthesis. This is partially due to the formation of some ncBP.
20
Figure 6. X-ray photoelectron spectra of the B 1s region of a) B-ncSi embedded in SiO2 and b)
B,P-ncSi embedded in SiO2.
In the P 2p spectra of P-ncSi and B,P-ncSi, a broad peak, which stemmed from the mixed
valence states of phosphorus, is observed. The peaks of both P-ncSi and B,P-ncSi are centred at
135 eV, which is attributed to P(V) (oxidized surface P atoms). The lower binding energy of 130
eV is assigned to P(0) (substitutional P in the core of ncSi). The spectrum of P-ncSi is cut off at
the P(0) and the broad peak stretches to a high binding energy of 140 eV, which is assigned to
some P-F bonds on the sample surface. This suggests that all or almost all of the P atoms reside
on the surface of ncSi, existing as P–O and P–F, and minimal or no P atoms reside in the core of
the ncSi. For B,P-ncSi, there appears to be a very small amount of P(0). It is not surprising that
most of the P atoms reside on the surface of ncSi since the solubility limit of P (0.3%) is lower
than B (1%).20,50
From the XPS analysis, it can be concluded that 1) B-ncSi consists of both substitutional and
oxidized surface B atoms, 2) P-ncSi consists of P–O and P–F on the surface of ncSi, and 3) B,P-
ncSi consists of a very small number of substitutional B and P atoms and some surface B–O and
P–O bonds. By examining the surface species and the location of dopants of ncSi, it will allow us
to elucidate the difference in reducing power of B-ncSi, P-ncSi, B,P-ncSi, and ncSi for the
generation of solar fuels by the hydrogenation of CO2 using the light and/or heat of the sun.
21
2.2.4 Surface hydrophobicity
The surface hydrophobicity of ncSi can be observed by their ability to disperse in various
solvents. It was observed that ncSi and P-ncSi were dispersible in pentane, a non-polar solvent,
and were not dispersible in polar solvents such as ethanol, where the opposite is observed for B-
ncSi and B,P-ncSi (Table 2). As shown in Figure 7a and 7b, ncSi and P-ncSi immediately
precipitated to the bottom of the vial when EtOH was added, which is indicative that these two
samples have hydrophobic surfaces. On the other hand, a brown dispersion is observed for B-
ncSi and B,P-ncSi in EtOH with some ncSi settled to the bottom of the vial (Figure 7c and 7d). It
has been reported that B-doped silicon and B,P-ncSi composed of a boron-rich surface are
hydrophilic25,35,58,59 and that B,P-ncSi can be well dispersed in polar solvents such as MeOH and
water.35 This observation of B-ncSi and B,P-ncSi further supports the XPS results that B atoms
exist on the surface of the B-containing ncSi. The precipitate in B-ncSi and B,P-ncSi in EtOH are
most likely large crystals or ncSi that consist of less surface B atoms. Furthermore, hydride-
terminated ncSi is known to be hydrophobic, suggesting that ncSi and P-ncSi may contain more
surface hydrides than B-ncSi and B,P-ncSi. Therefore, the observation of hydrophobicity of the
various ncSi samples is not only essential to the isolation of the ncSi during synthesis, but also
provides a clue to the surface properties of the ncSi. The ability to modify the surface
hydrophobicity of ncSi without the addition of organic capping ligands is also advantageous
since ncSi can be used for various applications which uses solvents of different polarities, and in
the context of this thesis work, to avoid carbon contamination in the hydrogenation of CO2.
Figure 7. Digital photographs of a) ncSi b) P-ncSi c) B-ncSi d) B,P-ncSi in EtOH.
22
Table 2. Dispersibility of hydride-terminated doped and pristine ncSi in polar and non-polar
solvents.
Non-polar
(e.g. pentane)
Polar
(e.g. EtOH)
ncSi
P-ncSi
B-ncSi
B,P-ncSi
2.3 Summary
For the first time, B-ncSi, P-ncSi, and B,P-ncSi have been prepared from the (HSiO1.5)n sol-gel
polymer using trichlorosilane, boric acid and phosphoric acid. It was found that there is a
variation of crystallite sizes among the ncSi samples and that the ncSi crystal lattice remained
unchanged when boron and phosphorus were incorporated. No other phases except ncSi were
observed in B-ncSi and P-ncSi. However, some ncBP were observed in B,P-ncSi. It was found
that pristine ncSi consists of the highest ratio of Si–H to Si–O and B,P-ncSi consists of the least
Si–H. From the XPS analysis, it was concluded that B-ncSi consists of some substitutional B
atoms with a majority of B–O on the surface, while P-ncSi only consists of P atoms on the
surface existing as P–O and P–F, and B,P-ncSi only contains minimal amount of substitutional
and oxidized surface B and P atoms. With most of the dopant atoms residing on the surface of
ncSi, it is not surprising that we found no infrared spectroscopy evidence of LSPR due to the
small number of free charge carriers. The surface of pristine ncSi was found to be the least
oxidized and B,P-ncSi is the most oxidized, with B-ncSi and P-ncSi falling in between. Despite
the observation that P-ncSi seems to more oxidized than B-ncSi from the FTIR analysis, P-ncSi
should consist of more Si–H because its surface is hydrophobic, while B-ncSi is hydrophilic.
Characterization from FTIR spectroscopy also confirmed that all of the samples are hydride-
terminated ncSi.
23
Chapter 3 Investigation of Boron and Phosphorus Doped Silicon
Nanocrystals in Gas-Phase Heterogeneous Reduction of Carbon Dioxide
Investigation of boron and phosphorus doped silicon nanocrystals in gas-phase heterogeneous reduction of carbon dioxide
3.1 Experimental
3.1.1 Sample preparation and storage
Silicon nanocrystal samples were prepared by drop-casting sonicated dispersions of B-ncSi, P-
ncSi, B,P-ncSi or ncSi in acetone onto borosilicate glass microfibre filters, which provide
increased surface area and mechanical stability. The samples were dried under a flow of N2 gas
and subsequently under vacuum for at least 30 min to yield approximately 2 mg of ncSi on a
glass filter. Samples were kept in an inert atmosphere in the dark prior to CO2 reduction
experiments. For the oxidation experiments, samples were stored in air in the dark.
3.1.2 Preparation of hydride-terminated silicon nanocrystals and molecular sieves for water removal experiments
For the water removal experiments, pristine hydride-terminated ncSi were prepared from SiO.
SiO (Sigma Aldrich, –325 mesh) was transferred to a quartz reaction boat and heated at a rate of
18 °C min-1 to 900 °C and then was held at 900 °C for 1 h under a constant gas flow of 95%
Ar/5% H2 in a quartz tube furnace. In order to liberate the ncSi from the SiO2 matrix, 0.3 g of
ncSi embedded in SiO2, a brown powder, was ground in a mortar and pestle with 10 mL of EtOH
to ensure uniformity of grain size and was transferred to a Teflon beaker equipped with a
magnetic stir bar. 20 mL of HF (48%, aq., Caledon Laboratories) were subsequently added to the
beaker and the mixture was stirred for 2 h 30 min. The ncSi were then extracted from the HF
solution by pentane. The sample preparation for CO2 reduction experiments is identical to the
procedure described in Section 3.1.1, except that the ncSi were drop-casted directly from the
pentane dispersion from the extraction step onto the glass filter. 3A molecular sieves (Sigma
Aldrich, 8-12 mesh), which are crystalline microporous aluminosilicates utilized for water
adsorption, were regenerated at 290 °C overnight to remove any adsorbed water molecules.
24
3.1.3 Gas-phase CO2 reduction measurements
Gas-phase CO2 reduction experiments were conducted in a custom-fabricated 3.8 mL stainless
steel batch reactor with a fused silica view port sealed with Viton O-ring. The reactor was
evacuated by an Alcatel dry pump after placing the sample in the reactor and then was purged
with H2 gas (99.9995%) for 20 min at a flow rate of 15 mL min-1. For the water removal
experiments, three 3A molecular sieve beads were placed inside the batch reactor on the
borosilicate glass microfibre filter containing ncSi, and were replaced with three newly
regenerated beads for each run. After filling the reactor with 15 psi H2 and 15 psi 13CO2 (99.9
atomic %, Sigma Aldrich) – a 1:1 stoichiometric ratio for the RWGS reaction – the reactor was
sealed, heated to, and held at 150 °C. The pressure of the reactor was monitored by an OMEGA
OX309 pressure transducer. The temperature of the reactor was measured by a thermocouple
placed in contact with the sample and was controlled by an OMEGA CN616 6-Zone temperature
controller. The measurements under light were irradiated under a 1000 W Hortilux Blue metal
halide bulb for a period of 3 or 6 h at an intensity of 200 W m-2 (equivalent to 0.20 Sun). For
experiments performed under high intensity light, samples were exposed to a 300 W Xe lamp
(Newport) with an intensity of ~ 50 Suns focused onto the sample. The irradiance spectra of the
lamps can be found in Appendix A. Product gases were separated by a 3’ MoleSieve 13X and a
6’ HayeSep D column and analyzed by a flame ionization detector (FID) and thermal
conductivity detector (TCD) installed in a SRI-8610 gas chromatograph. Isotope tracing
experiments were conducted with an Agilent 7890A gas chromatograph mass spectrometer with
a 60 m GS-CarbonPLOT column fed into the mass spectrometer, and product gases were
detected by a TCD.
3.1.4 Data analysis
A blank borosilicate glass microfibre filter was used as a control, placed in the reactor, and
treated under the identical conditions as the ncSi samples. The 12CO detected from the blank
glass filter was subtracted from the results of the samples. Then the ratio of 12CO to 13CO
produced was estimated by the ratio of the corresponding GCMS peak areas (mass-to-charge
ratios of 28 and 29 amu, respectively). The peaks were fitted assuming that they are Gaussian
and the areas under the curves were determined by the Peak Analyzer tool of OriginPro software.
The ratio of 12CO to 13CO was then applied to the total CO production rates, calculated from the
GC peak area, to obtain the final 13CO production rates.
25
3.1.5 CO2 adsorption capacity
The CO2 adsorption capacities of doped and pristine samples were investigated using
thermogravimetric analysis (TGA) with a TA Instruments Q500 thermogravimetric analyzer. All
of the following steps were performed at 35 °C. A desorption step was first performed by
flowing N2 at a rate of 100 mL min-1 for 90 min. Then the sample was introduced to 100% dry
CO2 at a flow rate of 100 mL min-1 for 2 h and then followed by a final desorption step by
flowing N2 at a rate of 100 mL min-1 for 90 min. The weight gain resulting from the introduction
of CO2 is used for the calculation of CO2 adsorption capacity.
3.2 Results and discussion
3.2.1 13CO production via reverse water-gas shift reaction
The ability of B-ncSi, P-ncSi, B,P-ncSi, and pristine ncSi to reduce CO2 was examined in a batch
photoreactor with a 1:1 ratio of 13CO2:H2 – the stoichiometric ratio for the RWGS reaction. It is
important to note that 13C isotope tracing experiments were performed to verify that any carbon-
containing products formed did not stem from adventitious carbon residues. The samples in the
reactor were exposed to simulated solar light using a 1000 W metal halide lamp with an intensity
of 0.20 Sun and were heated at 150 °C for 6 h for multiple cycles. Other than unreacted 13CO2,
no other 13C-containing species were detected under these conditions. The results demonstrate
that hydride-terminated B-ncSi, P-ncSi, B,P-ncSi, and pristine ncSi are all able to
heterogeneously reduce CO2 to CO via the RWGS reaction of rates of μmol g-1 h-1. Since the
crystallite size distributions are quite different among the ncSi samples, the 13CO production
rates are normalized to their BET surface areas (Table 1) to ensure that the difference in rates did
not arise from the difference in surface areas. The surface area normalized rates are presented in
Figure 8. Comparing the 13CO production rates of the first run, P-ncSi exhibited the highest rate
– achieving a rate of 214 nmol m-2 h-1 – followed by ncSi, B-ncSi and B,P-ncSi with rates of 114,
71, and 51 nmol m-2 h-1, respectively. In fact, P-ncSi is able to reach a maximum rate of 275
nmol m-2 h-1 for the first run, which corresponds to 71 μmol g-1 h-1. As shown in Figure 8, the
13CO production rate decreased over consecutive runs for all ncSi samples, which is likely
because CO2 underwent a stoichiometric reaction, and not catalytic one, similar to the results
discussed in Section 1.2. This also suggests that B,P-ncSi with surface B and P atoms do not
generate a surface FLP that is able to heterolytically split H2 and subsequently reduce CO2 under
26
these conditions to render the reaction catalytic. It can be seen that the 13CO production rate
drastically decreased in the second run for all ncSi samples as most of the surface hydrides have
been consumed after the first cycle. Furthermore, the higher the rate of the first run, the greater
the decrease in rate from run 1 to 2. Then, the rate slowly decreases from run 2 to 5, as the
number of available surface hydrides decreases.
Figure 8. Surface area normalized 13CO production rates of B-ncSi, P-ncSi, B,P-ncSi and ncSi
for four to five consecutive runs under the irradiation of 0.20 Sun at 150 °C for 6 h with 1:1 CO2
to H2.
It is interesting to note that the 13CO production rates of B-containing ncSi are lower than the
ncSi, whereas the rate of P-ncSi is greater than ncSi. The lower 13CO production rates of B-ncSi
and B,P-ncSi compared to ncSi may be explained by several reasons. Large ncBP were observed
in the B,P-ncSi samples from the PXRD pattern and HRTEM images and therefore, a control
experiment was carried out to test whether the ncBP are able to reduce CO2. In attempt to afford
ncBP by etching away all of the ncSi from B,P-ncSi-SiO2, B,P-ncSi-SiO2 was stirred in an
HF/EtOH solution for 6 h. However, after 6 h, a small amount of ncSi still remained in the
product. This sample containing mostly ncBP was tested for CO2 reduction activity, but showed
27
only a small amount of 13CO produced. The ncBP are most likely not able to reduce CO2. In this
case, the inactive ncBP would only add to the mass of the B,P-ncSi sample and subsequently
decrease the “apparent” CO production rate, which if corrected for inactive ncBP, would be even
higher.
Furthermore, after liberation of B-ncSi and B,P-ncSi from the SiO2 matrix, B-ncSi and B,P-ncSi
were extracted by acetone from the HF/EtOH solution. During the extraction procedure,
formation of bubbles was observed in acetone. When the acetone was decanted and distilled
water was added to the B-ncSi, bubbles were again observed. The formation of bubbles in
acetone was most likely formed by the oxidation of surface hydrides by water in the acetone,
producing H2 gas via the following reaction,16
Si–H + H2O → Si–OH + H2 (3)
Here, Si–H (surface silicon hydride), the reducing agent of the RWGS reaction, is oxidized to
Si–OH, which is incapable of reducing CO2.13 Therefore, the CO production rates of B-ncSi and
B,P-ncSi may be lower than the pristine ncSi and P-ncSi due to the surface oxidation during the
extraction procedure. The surface oxidation is also evident from XPS and FTIR spectra. As
shown in the Si 2p XPS (Figure 4), the peak area ratio of Si(IV) to Si(0) in B-ncSi and B,P-ncSi
is much greater than ncSi and P-ncSi. From the analysis from the FTIR spectra (Figure 3), it was
found that B,P-ncSi has the highest ratio of Si–OH to Si–H bonds among all the samples and that
B-ncSi has a higher ratio of Si–OH to Si–H than ncSi. This oxidation problem may be prevented
by using anhydrous polar solvents to wash B-ncSi and B,P-ncSi during this step.
As discussed in Section 1.3, the substitutional doping of B and P into the core of ncSi introduces
excess holes and electrons, respectively. If some P atoms are doped into the core of P-ncSi, then
the presence of excess electrons from P may be a possible contributor to its high CO production
rate. P-ncSi having an excess of electrons compared to B-ncSi could render the surface Si–H
more hydridic by accepting extra electrons from P, thereby giving P-ncSi the highest reducing
power. With regards to B,P-ncSi, any excess electrons from P would be compensated by the
holes from B. As a result, B,P-ncSi most likely does not contain any excess electrons and
therefore does not exhibit an enhanced CO production rate observed for P-ncSi.
28
Figure 9. Surface area normalized 13CO production rates of B-ncSi, P-ncSi, B,P-ncSi, and ncSi,
tested at 150 °C for 6 h with 1:1 CO2 to H2 in the dark (D) and light with 0.2 Sun (L).
CO2 reduction experiments were also performed in the dark to elucidate the effect of light on the
ncSi samples. In the dark at 150 °C, the first run of P-ncSi also exhibited the highest rate at 92
nmol m-2 h-1 and is followed by B,P-ncSi, ncSi and B-ncSi at 40, 33, and 26 nmol m-2 h-1,
respectively. The corresponding surface area corrected rates of the first run in the light and dark
are plotted in Figure 9. The results demonstrate that the RWGS reaction by hydride-terminated
doped and pristine ncSi can be enabled thermally, as previously reported by our group for ncSi.13
The 13CO production rates of all samples are enhanced in the light at 150 °C compared to the
dark at 150 °C, which clearly illustrates that this is a light-assisted reaction. With only irradiation
of 0.2 Sun, the 13CO production rates of B-ncSi, P-ncSi and ncSi in the light are approximately
2.5 times greater in the light than in the dark. This difference is much smaller for B,P-ncSi
sample most likely due to the suppressed rate from the inactive ncBP. If the experiments are
performed under an irradiation from a higher intensity lamp (e.g. 1 Sun) and with temperature
controlled at 150 °C, then a greater difference in rate between light and dark may be observed
and the photothermal effects can be compared.
29
3.2.2 CO2 adsorption capacity
The ability of doped and pristine ncSi to adsorb CO2 is measured by thermogravimetric analysis
(TGA) at 35 °C. The TGA curves can be found in Appendix C. The weight gain observed in the
plot after the introduction of CO2 is used for calculating the CO2 adsorption capacity of the
sample and the surface area corrected results are presented in Figure 10. TGA experiments were
conducted at 35 °C since it was difficult to deduce whether the weight increase arose from CO2
adsorption or the reaction of ncSi with CO2 when performed at elevated temperatures. It is
therefore assumed that this trend is similar at 150 °C, the temperature at which the CO2 reduction
experiments were performed. Interestingly, no CO2 adsorption is observed for pristine ncSi and
an enhancement of CO2 adsorption capacity compared to ncSi is observed for B-ncSi (170 nmol
CO2 m-2) and P-ncSi (90 nmol CO2 m
-2).
Figure 10. Surface area normalized carbon dioxide adsorption capacities of B-ncSi, P-ncSi and
ncSi at 35 °C.
The enhancement of CO2 adsorption capability of doped ncSi may be explained by the increase
of electronegativity of boron and phosphorus compared to silicon. The Pauling
electronegativities of Si, B, P, O, and F are 1.90, 2.04, 2.19, 3.5, and 4.0 respectively.36,60,61 From
30
the density functional theory (DFT) calculations of pristine ncSi:H, we have shown that the
oxygen atom of CO2 approaches the Si bonded to one hydride and undergoes subsequent
dissociation to yield CO and surface Si–OH.13 Following this model, the CO2 adsorption
capacity would be expected to increase as the electronegativity of the dopant increases, as
depicted in Figure 11.
Figure 11. Explanation for the increase in CO2 adsorption capacity due to the introduction of
electronegative atoms in b) B-ncSi and c) P-ncSi compared to a) ncSi. The green arrows indicate
the location of potential CO2 adsorption sites. Some bonds are omitted for simplicity.
When a dopant with higher electronegativity is introduced, electron density is drawn from
surface Si atoms to the dopant atoms (B or P), rendering the Si atom more positively charged
(δ++; Figure 11b and 11c). In this way, the electrostatic attraction between the O on CO2 and Si
should increase, thereby increasing the ncSi’s CO2 adsorption capacity. Although P is more
electronegative than B, the results demonstrated that the adsorption capacity is higher for B-ncSi
than P-ncSi. This could be attributed to the B–O and Si–O bonds on the surface of B-ncSi. In this
case, electron density is drawn towards the oxygen from B or Si, resulting in a partially positive
B or Si atom, attracting nearby CO2 molecules (Figure 11b). Surface P–O and P–F bonds would
also play a role in enhancing the CO2 adsorption since O and F are more electronegative than P
(Figure 11c). Yet, B-ncSi has a greater CO2 adsorption capacity most likely because there are
more B–O and Si–O bonds on the surface than there are P–O, P–F, and Si–O bonds on the
surface of P-ncSi, as evident from the FTIR (Figure 3) and XPS spectra (Figure 5), and the
observations on the surface hydrophobicity (Section 2.2.4). It was demonstrated that the dopant
elements, both B and P, enhance the CO2 adsorption capacity of ncSi. It would be expected that
31
the CO2 adsorption capacity of B,P-ncSi is similar to B-ncSi since these two samples have
similar surface properties as discussed earlier. Despite that both B-ncSi and P-ncSi have an
enhanced CO2 adsorption capacity, only P-ncSi yielded a higher CO production rate than ncSi. In
this way, the increased CO2 reduction activity of P-ncSi is not solely due to the increase of CO2
adsorption capacity.
3.2.3 Effect of sample storage in air
It is important to examine the stability of silicon hydrides with storage in air if they are to
become technologically viable for solar fuel generation, especially since hydride-terminated ncSi
are prone to surface oxidation. Surface oxidation of ncSi would decrease the number the surface
hydrides which are responsible for CO2 reduction and it would be interesting to examine the
effects of dopants on the stability of CO production. In this way, all ncSi samples were stored in
the air and in the dark for 7 and 14 days and their CO production rates were compared (Figure
12).
Figure 12. Surface area normalized 13CO production rates of fresh B-ncSi, P-ncSi, B,P-ncSi and
ncSi samples and samples stored in air for 7 and 14 days tested a) in the light and b) in the dark
at 150 °C.
32
Even after 14 days of storage in air, P-ncSi still produces 13CO at a rate of 48 nmol m-2 h-1 or 12
μmol g-1 h-1, approximately 5.5 times greater than ncSi (8.8 nmol m-2 h-1), under light at 150 °C.
Furthermore, the CO production rate of pristine ncSi is the lowest of all samples after 14 days
both in the light and in the dark at 150 °C. Therefore, this suggests that dopants are providing
some stability to the amount of CO produced. The rates of B-ncSi and B,P-ncSi also appear to be
more stable than ncSi and P-ncSi over time of storage. This is most likely because B-ncSi and
B,P-nSi are more oxidized and have less Si–H than ncSi and P-ncSi, leading to a slower rate of
oxidation of surface hydrides. It also appears that the effect of light decreases over storage time,
which is likely because the amount of CO produced after oxidation is limited by the number of
available Si–H.
3.2.4 Utilization of only solar energy for RWGS reaction
Up to this point, doped and pristine ncSi are shown to convert CO2 to CO using solar irradiation
with external heating. However, in order to maximize the usage of solar energy for the RWGS
reaction, experiments were performed to investigate if solar energy alone could effectively
convert 13CO2 to 13CO. In this way, ncSi samples were exposed to a high intensity simulated
solar lamp with an intensity of 50 Suns for 4 h without any additional heating from an external
source. In a realistic process, such high solar intensities can be easily achieved by employing
existing solar concentrators such as parabolic trough solar collectors.62
33
Figure 13. CO production rates of B-ncSi, P-ncSi, B,P-ncSi and ncSi under irradiation of 50
Suns without external heating for 4 h.
The CO production rate of doped and pristine ncSi are plotted in Figure 13. A fresh sample of P-
ncSi again yielded the highest CO production rate, reaching an impressive rate of 1.86 μmol m-2
h-1, which corresponds to 0.48 mmol g-1 h-1. It is then followed by ncSi, B,P-ncSi, and B-ncSi
with rates of 1.17, 0.64, and 0.55 μmol m-2 h-1. This trend is similar to the results observed under
0.20 Sun at 150 °C.
34
3.2.5 Unwanted side reaction: increase of 12CO production
Figure 14. The 12CO (black) and 13CO (red) GCMS peak areas obtained from the reaction of
B,P-ncSi with CO2 and H2 in the a) light and b) dark at 150 °C over five to six consecutive runs.
As discussed in Section 3.2.1, the 13C isotope tracing results have confirmed that the 13CO
production rate decreases over consecutive runs. However, the total CO production rate in the
light appears to be stable. It was then observed that the stability of the total CO production rate is
due to an increase of 12CO production over consecutive runs (Figure 14a). Figure 14 shows the
integrated peak areas of 12CO and 13CO obtained from the GCMS over consecutive runs; the
peak area is dependent on the mass of the sample and therefore, the CO production rate of the
sample tested in the light is still higher than one tested in the dark, in spite of the larger 13CO
peak area obtained in the dark. The increase in 12CO production appears to only occur with ncSi
and BP-ncSi samples. This result further places an emphasis on the importance of 13C isotope
tracing experiments. Also, interestingly, the percentage increase of 12CO over consecutive runs
only occurs under light at 150 °C. In the dark at 150 °C, the 12CO peak area appears to be
relatively constant over 5 cycles (Figure 14b). Therefore, the 12C side reaction seems to be a
light-assisted reaction that is competing with the 13CO2 reduction reaction. In other words, as
more surface hydrides are consumed over consecutive runs, more 12C-containing species begin to
react to yield 12CO. Interestingly, it was reported by Kang et al. that hydride-terminated ncSi are
able to react with organic molecules in the presence of light.63 However, more thorough
experiments are required to further elucidate what reactions are occurring and how they are
35
occurring. If the side reaction can be identified and suppressed, then perhaps the 13CO production
rate may be further increased.
3.2.6 Removal of water for 13CO production rate enhancement
Molecular sieves are aluminosilicates commonly used for removal of water, alcohols, and some
common gasses from chemical processes and 3A molecular sieves are able to adsorb water
molecules.64 Besides CO, water is the other product yielded from the RWGS reaction (Reaction
1). As discussed previously, water can have detrimental effects to the photoreduction rates as it
can oxidize surface Si–H to Si–OH (Reaction 3), which are no longer able to reduce CO2.13
Therefore, according to Le Châtelier's principle, by reducing or eliminating adsorbed or product
water molecules, not only can the RWGS reaction be shifted to the product side, but more
importantly, the number of surface Si–H sites may be maintained by shifting Reaction 3 to the
left hand side.
With this hypothesis, three regenerated molecular sieve beads were placed inside the batch
reactor with a ncSi sample to adsorb any water present in the reactor. They were then tested
under a 1:1 ratio of 13CO2:H2 at 150 °C in the light for 3 h. The results are presented in Figure
15.
36
Figure 15. CO production rates of ncSi only and ncSi with 3A molecular sieves for three
consecutive runs under the irradiation of 0.20 Sun at 150 °C with 1:1 ratio of 13CO2 to H2 for 3 h.
Tested molecular sieve beads were replaced by newly regenerated beads before each run.
As shown in Figure 15, without the addition of molecular sieves, pristine ncSi is able to produce
CO at a rate of 122 μmol g-1 h-1 for the first run. With the addition of molecular sieves in the
batch reactor, the CO production rate is increased to an impressive rate of 149 μmol g-1 h-1. The
decrease in CO production rates over consecutive runs is also reduced when molecular sieves
were added and even after the second run, the ncSi sample is able to produce 31 μmol g-1 h-1.
Furthermore, a control experiment under identical test conditions was performed by placing three
molecular sieve beads and a blank glass filter in the reactor. No 13C-containing species other than
unreacted 13CO2 were detected, confirming that the molecular sieves are not responsible for
13CO2 reduction. It is also noteworthy that the amount of 12CO produced also diminished with the
addition of molecular sieves, which suggests that the oxidation of carbon contamination species
by water may be suppressed. Molecules which are adsorbed by the beads could be verified by
obtaining a thermogravimetric analysis-mass spectrum (TGA-MS) of the used beads in the
future. Also, it is hypothesized that with higher light intensities, more water would be generated
and adsorbed by the molecular sieves, and the CO production rate per Sun could be further
37
enhanced. The 13CO production rates of ncSi, B-ncSi, and P-ncSi prepared from (HSiO1.5)n may
also be improved by applying this water removal technique. If water and carbon contamination
are the cause of the side reaction discussed in Section 3.2.5, then the 12CO production rate may
also be suppressed. Other water removal methods could also be explored. For instance, testing
ncSi in a flow reactor may assist in removing water from the system.
3.3 Summary
We have successfully converted gaseous 13CO2 to 13CO via the reverse water-gas shift reaction
utilizing hydride-terminated B-ncSi, P-ncSi, B,P-ncSi, and ncSi as reducing agents in a batch
reactor. The results have shown that this is a stoichiometric light-assisted reaction and can be
enabled by solar energy alone without any auxiliary heating sources. P-ncSi, which consists of
surface P–O and P–F species, demonstrated the highest 13CO production rate among the doped
and pristine ncSi, achieving a maximum rate of 275 nmol m-2 h-1 (equivalent to 71 μmol g-1 h-1)
under the irradiation of 0.2 Sun. Pristine ncSi demonstrated the next highest 13CO production
rate, followed by B-ncSi and B,P-ncSi. It was found that B-ncSi and P-ncSi have greater CO2
adsorption capacities than ncSi. This is likely because they contain more surface polar bonds
(i.e., Si–B, Si–P, B–O, P–O and P–F), increasing the adsorption of CO2 molecules to the more
positively charged surface Si, B, or P. Although B-ncSi showed a greater CO2 adsorption
capacity, its CO production rate is lower than ncSi. This is due to the reduced number of surface
Si–H on B-ncSi compared to ncSi and P-ncSi. A similar trend was also observed when the
samples were tested in the dark at 150 °C. P-ncSi also exhibited the highest rates after storing in
the air for 7 and 14 days and under an intense irradiation of 50 Suns. The remarkably high 13CO
production rate of P-ncSi is therefore attributed to the combination of enhanced CO2 adsorption
capacity, higher number of surface Si–H sites, and the possible enhancement of the hydridic
character and reactivity of Si–H.
Further improvement in CO production rate of pristine ncSi was also investigated. By removing
adsorbed or product water from the batch reactor using molecular sieve beads and shortening the
reaction time to 3 h, an astounding CO production rate of 149 μmol g-1 h-1 could be achieved for
the first run and a more stable rate for 3 consecutive runs was obtained.
38
Chapter 4 Conclusion and Outlook
Conclusion and outlook
4.1 Conclusion
The work presented in this thesis aims to develop nanocrystalline silicon as a superior
photoreduction agent for solar fuel production. The effects of boron and phosphorus dopants in
hydride-terminated silicon nanocrystals were investigated. Hydride-terminated B-ncSi, P-ncSi,
B,P-ncSi and ncSi were successfully prepared by sol-gel synthesis. Through various
characterization techniques, it was concluded that substitutional doping in the crystal lattice or
surface doping was successful for B-ncSi, P-ncSi and B,P-ncSi, which consist of a bimodal
distribution of crystallite sizes. B-ncSi and B,P-ncSi were found to be more oxidized and consist
of less surface hydrides than P-ncSi and ncSi.
B-ncSi, P-ncSi, B,P-ncSi and ncSi were all capable of reducing 13CO2 to 13CO via the reverse
water-gas shift reaction. All of the samples were shown to undergo a stoichiometric reaction,
likely due to the oxidation of Si–H as reported in our previous study.13 In order to probe the
effect of solar simulated light on the RWGS reaction, all of the samples were subjected to tests in
the light and in the dark. It was demonstrated that CO2 reduction can be enabled thermally and is
enhanced with light. The reducing ability of doped and pristine ncSi using an intense light
without any external heating demonstrates that the reaction can be enabled using solar energy
alone. The dopants also appeared to provide some stability to the 13CO production rate over
storage time in air.
P-ncSi exhibited the highest 13CO production rate among all of the samples, achieving a
remarkable maximum rate of 276 nmol m-2 h-1 under irradiation of only 0.2 Sun within a period
of 6 h. Adventitious oxidation of ncSi has been shown to have a negative effect on the CO
production rate as it reduces the number of Si–H available for CO2 reduction. The lower CO
production rate of B-ncSi compared to ncSi and P-ncSi was attributed to its surface oxidation
and for B,P-ncSi, it was attributed to the combination of surface oxidation and the presence of
inactive ncBP. It was found that the CO2 adsorption capacity of ncSi is enhanced when boron or
phosphorus dopants were added, and it is hypothesized that the electronegativity of surface
39
species was responsible for the enhancement. However, CO2 adsorption is not the only factor
that yielded a boost in CO production of P-ncSi. The high CO production rate of P-ncSi was
ascribed to the combination of the number of Si–H and the increased CO2 adsorption. The
addition of some substitutional P may also render the surface Si–H more hydridic due to the
excess electrons present in P-ncSi. Removal of water from the photoreactor by molecular sieves
and optimization of test conditions were also shown to be important to the enhancement of CO
production rates.
4.2 Outlook
This thesis presents significant findings to enhance the CO2 reduction capability of inexpensive,
earth-abundant and non-toxic silicon as a reducing agent using the light and/or heat from the sun.
In spite of these findings, much work in the future must be carried out to realize practical fuel
production in a CO2 refinery.
With regards to synthesis and characterization, several parameters could be optimized to control
the concentration dopants of ncSi and the location of these dopants. For instance, the choice of
silane precursor can be explored to allow slower sol-gel network formation and condensation to
better incorporate dopant elements. The rate and time of heating of doped ncSi in the SiO2 matrix
may also have an effect on the incorporation of dopants. Furthermore, the effect of dopant
concentrations has not been investigated for CO2 reduction. The balance between dopant
concentrations and the amount of surface hydride is also important as Si-H are compensated with
increase of surface dopants. Modifications of the synthesis and improvement of handling
procedures could be made to further minimize surface oxidation of ncSi in order to achieve
maximum CO production rates. Other characterization methods such as electron energy loss
spectroscopy (EELS) elemental mapping and ultraviolet photoelectron spectroscopy (UPS)
would be able to confirm the location and electronic properties of dopants. The number of Si-H
responsible for the reduction of CO2 can be quantified using TGA.
In order to optimize the surface properties of ncSi for CO2 reduction, the reaction mechanism of
the doped and pristine ncSi must also be understood. Characterization by PXRD, HRTEM, XPS,
and FTIR could be performed after the CO2 reduction experiments to examine what changes
have taken place. Other characterization techniques such as in situ FTIR and in situ TGA-MS
could assist in determining the surface species formed as the reaction proceeds in the presence of
40
light compared to in the dark at various temperatures. The electronegativity effects on CO2
adsorption can be examined with the help of DFT calculations by looking at the changes in
Bader charges when dopant atoms are introduced. The introduction of other electronegative
elements could also clarify if the increase of CO2 adsorption stems from polar surface bonds.
Theoretical models could also help identify differences in the reaction mechanism when CO2 and
H2 are introduced between doped and pristine ncSi.
With regards to CO2 reduction testing, test parameters, including but not limited to the
temperature and pressure of the reactor, the type of photoreactor (batch vs flow), intensity of the
light, mass and thickness of the sample, and time of reaction, have not been optimized yet. For
instance, if the all of the possible CO being produced are afforded within 3 h, running the test for
6 h will decrease the CO production rate by half. Therefore, it is imperative to optimize these
conditions in order to boost solar fuel production rates.
Experiments could be performed to probe the photothermal effects of doped and pristine ncSi. In
the current work, irradiation of only 0.2 Sun was utilized for CO2 reduction experiments. In order
to compare the photothermal effects of doped and pristine ncSi, the samples could be tested
under the irradiation of a lamp with a higher intensity (e.g., 1 Sun) with the temperature
controlled at 150 °C. An increase of local temperatures of the doped ncSi may be induced by
non-radiative recombination of free charge carriers and the introduction of dopant trap states.
The local temperature can be probed by Raman spectroscopy.13,37
Many challenges still need to be overcome to realize large-scale solar fuel production despite
recent advances in carbon dioxide conversion and solar fuel production. One of the bottlenecks
of utilizing hydride-terminated ncSi for solar fuel production is its inability to reduce CO2 to CO
catalytically, and one solution is to incorporate a small amount of a co-catalyst to render the
RWGS reaction catalytic. This thesis provided some techniques and insights into the
enhancement of gas-phase light-assisted heterogeneous reduction of CO2 using hydride-
terminated silicon nanocrystals – a material that is inexpensive, earth-abundant, and non-toxic.
We have demonstrated that the incorporation of dopants in ncSi successfully enhanced the CO2
adsorption capacity and the ability of ncSi to convert CO2 to CO in the gas-phase. With these
remarkable findings along with silicon’s attractive properties, silicon nanocrystals, without
41
doubt, offer the potential of realizing feasible solar fuel production to drive our society towards
carbon neutrality.
42
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Appendix A: Powder X-ray Diffraction Pattern
Powder X-ray diffraction patterns of B-ncSi, P-ncSi, B,P-ncSi, and pristine ncSi heat treated at
400 °C for 30 min and then at 1100 °C for 2 h at a heating rate of 18 °C min-1 under 95% Ar/5%
H2 in a quartz tube furnace.
B-ncSi
48
P-ncSi
B,P-ncSi
49
ncSi
50
Appendix B: Irradiance Spectra
Irradiance spectra of the 1000 W Hortilux Blue metal halide lamp, 300 W xenon lamp, and the
AM1.5 solar irradiance. The light from the 300 W xenon lamp was focused to reach an intensity
of approximately 50 Suns.
51
Appendix C: Thermogravimetric Analysis
The weight gain from the introduction of CO2 is used for the calculation of CO2 adsorption
capacity (the weight percent difference between the plateaus of the red and pink curves). All of
the TGA were performed at 35 °C.
B-ncSi
52
P-ncSi
ncSi
