Download Syntheses and Characterization of Materials for Energy Applications

Candace H. Payne
Syntheses and Characterization of Materials for Energy Applications:
Oxynitrides
Advancement to Candidacy Committee
Chair: Michael White
Advisor: Peter Khalifah
Third Member: John Parise
First Meeting Information
Date: Monday, 15th, December, 2008
Time: 12:00 pm-2:00 pm
Place: Room 515 Graduate Chemistry Building at SUNY Stony Brook
I. OVERVIEW
A view of the world at night [1] underscores
the fact that the global demand for energy is
extensive, and it is ever increasing. Simply
put, the world needs more energy. This is the
foundation for my research effort. 13 TW of
energy per year is needed to meet our
planet’s current energy needs and energy
consumption is projected to increase by the year 2050 [2]; thus, as a society, we must search for successful
ways of meeting this increasing demand. Currently, the bulk of the earth’s energy needs is supplied by the
burning of fossil fuels. However, the burning of fossil fuels produces enormous amounts of carbon dioxide
yearly and is a major cause of global warming. Therefore, there is an enormous need for renewable fuels
whose combustion is less taxing on our environment, but are as effective as fossil fuels in meeting our
energy needs. Hydrogen is a good alternative fuel because it is can be produced renewably, and the burning
of hydrogen has no adverse effects on our environment. Hydrogen can be produced by photoelectrolysis
using the sun’s energy to drive water splitting. The sun provides a 1.76 x105 TW of energy to the earth. If
an efficient, innovative, practical, cost effective transformation of solar energy into hydrogen can be
developed, the national need for energy can be accommodated [3].
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Picture reference: http://www.theglitteringeye.com/images/Worldatnight.jpg
Lewis.N. MRS Bulletin, 38 (2007) 808
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Lewis, N.S. Norcera D. Proceedings of the National Academy of Sciences, 103 (2006) 43, 15729
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II. INTRODUCTION TO PROJECTS –
A. Visible Light Solar Water Splitting
Photosynthesis is a natural energy conversion process. The sun’s energy is converted to stored
energy in the form of carbohydrates. Similarly, photoelectrolysis of water can convert energy from the sun
into stored energy in the form of hydrogen, which can be used as a fuel. A photoelectrochemical cell is
used to do this conversion.
A photoelectrochemical cell is typically made of two electrodes; for solar water splitting
applications, the anode is made of a semiconductor, and the cathode is made of a metal like platinum.
When the semiconductor is struck by light with energy greater than its band gap energy, electrons in the
valence band of the semiconductor are excited to the conduction band, leaving holes in the valence band
(Figure 1). These holes travel to the surface of the semiconductor while the electrons move through the
bulk of the semiconductor to the cathode [4]. An electric field at the solid-electrolyte interface discourages
the electrons from recombining with holes. The holes oxidize water to form oxygen at the anode (eq. 2)
while the electrons reduce H+ to form hydrogen at the cathode (eq. 3). Eq. 4 shows the overall reaction.
(1) Semiconduc tor  h  4h   4e 
(2) 4h   2H 2 O  O 2  4H 
(3) 4e   4H   2H 2
__________________________________
(4) 2H 2 O  h  O 2  2H 2
For effective solar water splitting, the potential of the valence band edge must be lower than the potential
of eq. 2 [1.23 eV vs Normal Hydrogen Electrode (NHE)], while the potential of the conduction band edge
must be higher than the potential of equation 3 (0 eV vs NHE). Semiconductors for solar water splitting
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Ohta, T. Solar-Hydrogen Energy Systems. Pergamon Press. Toronto.
3
must have band gaps of at least 1.23 eV. However, this ‘minimum’ band gap is increased because of
thermodynamic losses and overpotentials. Thermodynamic losses (~0.4 eV) occur because of a rise in the
entropy of mixing when electrons are excited to the conduction band. Overpotentials (~0.3-0.4 eV) must
also be considered because the actual potentials of eq. 2 and eq. 3 are different from their
thermodynamically determined reduction potentials [5,6, 7].
V vs NHE
CB
e
-
Cathode
(H+/H2)
0
Eg
1.23
Anode
(H2O/O2)
VB
h+
Fig 1. Schematic showing the positions of valence band (VB) and conduction band (CB) of a semiconductor (red) for solar water splitting; band gap (E g)
should be no less than 1.23 eV, C.B should be higher than 0 eV and VB should be lower than 1.23 eV.
5
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Murphy, A.B. Glasscock, J.A. Int. J. Hydrogen Energy 31 (2006) 1999
Weber, M. Dignam, M. J. Electrochem. Soc. 131 (1984) 197
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Although the range of energies hitting the earth is
vast, Figure 2 shows that the majority of this
energy lies between ~1.3 and ~3.0 eV at air mass
1.5*. This range of energies spans from the nearinfrared region to the ultraviolet region. Much of
this energy is in the form of visible light. For this
reason,
many
contend
that
an
effective
semiconductor should be able to split water under
Figure 2. The Spectral Distribution of sunlight under
*Air Mass 1.5 showing that the greatest part of the solar [3]
visible light, so that it uses the bulk of the sun’s
energy [7]. Semiconductors with large band gaps
can only split water under ultraviolet (UV)
radiation and they will not effectively use the sun’s energy.
To this point three facts have been established: semiconductors for solar water splitting must have
valence band edges that are lower in energy than the energy required for the oxidation of water, they should
have conduction band edges that are higher in energy than the energy required for the reduction of H+, and
they should be able to absorb visible light. In addition to these, some other characteristics are necessary for
semiconductors to split water effectively [4,7]. These include:
 Long term stability – It is necessary to find a semiconductor that does not corrode when illuminated.
Oxide semiconductors tend to be very stable because the oxidation of water is faster than the
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Van de Krol, R. Liang, Y. Schoonman, J. J. Mater. Chem. 2008, 18, 2311
*[7] At Air mass 1.5, the sun is 48 0 above the earth. The global AM 1.5 spectrum takes into account radiation incident on
the earth from all angles, and is a good quantity to consider for solar water splitting
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decomposition of the oxide. Non-oxide semiconductors, like CdS and SiC, tend to become oxidized
during photoelectrolysis.
 Overall conversion efficiency,  tot – defined as the ability of the photoelectrochemical cell to convert
solar energy into hydrogen. If an overall conversion efficiency of at least 10% [3] could be achieved, this
nation’s energy needs can be met. The overall conversion efficiency is determined by the energy of the
photons absorbed by the semiconductor (equal to or greater than the band gap energy) and the amount of
current produced when these photons hit the semiconducting anode. It also depends on the amount of
bias voltage (a voltage applied to enable an electrochemical cell to work as desired), and the redox
potential of the cell. Eq. 5 [7] is used to calculate the overall quantum efficiency for a
photoelectrochemical cell-
(5)  tot 
j (Vredox  VB )
,
Plight
where j is the photo-current density (A m-2), Plight is the intensity of incident light (W m-2), VB is the bias
voltage, and Vredox is the redox potential (1.23 eV vs NHE).
 Good charge transport – the semiconductor should allow for good transport of electrons and holes. Holes
(h+) must move to the surface of the semiconductor to be able to oxidize water (eq.2). Electrons (e-) must
move to the bulk of the semiconductor, en route to the cathode to reduce H+. If the conductivity of the
semiconductor is low, then the resistance is high and energy is lost as the holes and electrons travel.
These energy losses will lower the overall efficiency of the photoelectrochemical cell.
 Inexpensive - they should be cheap enough to allow the cost of converting solar energy into hydrogen
fuel to compete with the present cost of producing energy from fossil fuels.
 Overpotentials for the reduction/oxidation reaction must be low - holes should move at a high rate from
the semiconductor to the electrolyte so that they do not accumulate at the surface of the semiconductor.
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Such accumulation decreases the electric field at the surface of the semiconductor and leads to faster
electron-hole recombination.
While non-oxide semiconductor anodes have a tendency to be unstable and themselves become
oxidized in the photoelectrochemical cell, oxide semiconductor anodes are often quite stable. Traditionally,
the majority of semiconductors used for photoelectrolysis of water are transition metal oxides. This
stability does not equate to all oxide semiconductors being adequate for visible light solar water splitting.
Two different problems exist with many transition metal oxides [8]. First, their band gaps are too large to
absorb a great portion of the sun’s energy. Transition metal oxide semiconductors have valence bands that
mainly consist of O 2p orbitals, and conduction bands that mainly consist of empty metal d orbitals. The
potential of O 2p states is normally ~3 eV. As a consequence of this, and the fact that their conduction band
edges lie at ~0 eV,
many transition metal oxides have band gaps of ~3eV (TiO2[5], SrTiO2 [9],
NaTaO3[10]). Such large band gap semiconductors are able to absorb UV rays but cannot absorb visible
light. Second, their band gaps are smaller than 3 eV and they can absorb visible light, but their conduction
band and valence band edges are not at the right positions to either oxidize water or reduce hydrogen.
Hence, they cannot split water effectively by themselves. WO3 is one example; its conduction band edge is
below 0 eV which means that it cannot reduce H+ to H2 [8].
Figure 3 [11] shows the band gap energies and indicates the valence band edges and conduction
band edges of some traditional semiconductor oxides used for solar water splitting.
8
Maeda, K. Domen, K. J. Phys. Chem. C, 111 (2007) 7851
Takahara, Y. Kondo, J, et al. JACS 124 (2002) 11256
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Kato, H. Asakura, K. Kudo, A. JACS 125 (2003) 3082
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Bak, T. Nowotny, J. Rekas, M. Sorrell, C. International Journal of Hydrogen Energy 27 (2002) 991
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Fig 3: Band gap energies, with the VB edge and CB edge for some traditional oxide semiconductors [11]
It is our goal to find oxide semiconductors that fulfill all the requirements for overall conversion
efficiency outlined earlier and for whom the above two problems do not apply. We will consider three
types of semiconductors as we attempt to engineer band gaps and band positions for these applications:
 Split band transition metal oxides – For oxides made of low spin d6 transition metals, octahedral crystal
field splitting leads to band gaps between the filled t2g states and the empty eg states [12]. This means
that the valence bands of these compounds are often higher in energy than traditional oxides. One such
split band transition metal oxide, whose valence band edge is raised in energy, is LuRhO3. It has a band
gap of 2.2 eV and it has be shown to be stable to photoelectrolysis [13].
 Mixed transition metal oxides – these compounds have a conduction band consisting of d0 orbitals, and a
valence band consisting of d10 orbitals. These d10 orbitals are often higher in energy than oxygen 2p
orbitals and take the place of O 2p states at the valence band edge. This decreases the band gap of mixed
transition metal oxides relative to traditional metal oxides.
12
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Singh, D. J. Khalifah, P et al. Chem. Mater. 18 (2006), 2696
Jarrett, H. Sleight, A. Gillson, J. J. Appl. Phys 51 (1990) 3916
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 Compounds with hybrid anion lattices – in these compounds, elements like sulfur, nitrogen or chlorine
respectively, are integrated into the oxide lattice. Integration of less electronegative elements (like
nitrogen in oxynitrides or sulfur in oxysulfides) into the oxide lattice decreases the hold of the lattice on
electrons and raises the valence band potential.
Next, is a discussion of the classes of materials that will study.
Delafossites
Delafossites (ABO2) are compounds where A is usually Cu, Ag, Au, and B is usually Ga, In, Al, Cr, Fe, Co,
Rh, Al, Ga, Y, Sc, In, and Tl [14]. The structure forms two polytypes (3R and 2H) shown in Figure 4. Both
the 3R polytype (4a) of space group R-3m and the 2H polytype (4b) of space group P63/mmc are made of
recurring layers stacked along the c axis. One layer contains A cations and the other contains edge shared
BO6 octahedra.
3R CuAlO2
(b)
(a)
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Marquardt, M. Ahsmore, N. Cann, D. Thin Solid Films 496 (2006) 146
9
2H CuAlO2
(d)
(c)
Figure 4a). Stacking of the 3R polytype of CuAlO2 as viewed along the c axis, subsequent Cu+ cation layers are not directly above each other.
4b) A unit cell of the 3R polytype viewed along the b axis showing the offset of polyhedral . 4c) Stacking of the 2H polytype of CuAlO2 as
viewed along the c axis, subsequent Cu+ cation layers are positioned directly above each other. 4d) A unit cell of the 2H polytype viewed
along the b axis showing the rotation successive layers of the polyhedral in the ab plane.
In the 3R polytype, A cations do not lie directly above each other from layer to layer but are offset from
each other in the ab plane. The 2H polytype (4c and 4d) of space group, P63/mmc contains consecutive A
cationic layers that are positioned directly above each other along the c axis ( in 4c, the subsequent layers
of atoms in the same position gives the appearance of one atom if viewed along the c direction).
These compounds are good semiconductors for solar water splitting because:
 They are known p-type conducting oxide semiconductors [15].
 They are split band transition metal oxide semiconductors. Both the valence band and the conduction
band are associated with hybrid d orbitals from the A cation. The filled hybrid orbitals in the valence
band are higher in energy than oxygen 2p orbitals. This higher potential of the valence band edge means
a narrower band gap.
 Whereas holes doped into the 2p states of oxygen show very low mobilities, holes doped into a filled d
states of these compounds are predicted to show higher mobilities [16].
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Kawazoe, H. Hosono, H. et al. Nature 389 (1997) 93
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 Delafossites are insensitive to changes in pH. When the pH of the solution is varied the band potentials
remain the same [17,18].
In recent years, some work has been done on hydrogen production by a limited number of
delafossites. CuFeO2 was shown to split water under visible light, but the quantum efficiency was quite low
(0.05–0.1 %) [19]. Additionally, Trari et al investigated hydrogen production by a series of copper
delafossites, including CuMnO2 [20], CuYO2 [21], CuLaO2 [22], and CuCrO2 [23] in an alkaline solution
of S2-, SO32- and S2O32- ions; however, oxidation of the sulfur anions considerably slowed down hydrogen
production.
Oxynitrides
(i) Synthesis of metal nitrides by nitridation in NH3
Oxynitrides contain hybrid anion lattices. The valence band edges of these compounds consists of N 2p
and O 2p orbitals, while the conduction band edge consists of metal d orbitals. The N 2p orbitals are higher
in energy than O 2p orbitals (which are the main components of the valence band edges in traditional oxide
semiconductors), leading to a smaller band gap in oxynitrides as compared to traditional metal oxides.
Moreoever, there is also the possibility of hybridization between the O 2p and the N 2p states in
oxynitrides. Whether hybrid orbitals exist or individual O 2p and N 2p orbitals exist, oxynitrides tend to
have smaller band gaps than traditional oxides; they are usually less than ~3 eV. To date, some research
work have been done on various oxynitride semiconductors for visible light solar water splitting by Domen
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Kanpal, H. Seshadri, R. Solid State Sciences 4 (2002) 1045
Korich, N. Bougelia, A. Triari, M. Int. J. Hydrogen Energy 30 (2005) 693
18
Saadi, S. bouguelia, A. Trari, M. Solar Energy 80 (2006) 272
19
Takata, T. Tanaka, A. Hara, M. Kondo, J. Domen, K. Catalysis Today 44 (1998) 17
20
Bessekhouad, Y. Trari, M. Doumerc, J. Int. J. Hydrogen Energy 28 (2003) 43
21
Trari, M. Bouguelia A. Bessekhouad, Y. Sol Energy Mater Sol Cells 90 (2006) 190
22
Saadi, S. Bouguelia, A. Derbal, A. Trari, M. J. Photochemistry and Photobiology A. 187 (2007) 97
23
Saadi, S. Bouguelia, A. Trari, M. Solar Energy 80 (2006) 272
17
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et al. These include BaTa2ON [24], TaON [25], LaTiO2N [26], and the solid solution (GaN)1-x(ZnO)x . Of
these, only the last compound has been able to split water under visible light without the use of a sacrificial
agent.
(ii) High Pressure Synthesis of La2TiO2N2 and La2TiO3N
Some other oxynitrides which have been researched for solar water splitting applications and
some of these are shown in Table 2 [8].
Activity/µmolh-1
Photocatalyst
LaTiO2N
Ca0.25La0.75TiO2.25N0.75
CaNbO2N
TaON
CaTaO2N
SrTaO2N
BaTaO2N
LaTaO2N
Band Gap
Energy/ eV
2.0
2.0
1.9
2.5
2.4
2.1
1.9
2.0
H2
30
5.5
1.5
20
15
20
15
20
O2
41
60
46
660
0
0
0
0
Table 2: Photocatalytic activities of some oxynitrides. Pt cocatalyst used for water reduction; AgNo 3 (0.01M) used for water oxidation
Data from Domen et al. [8].
Many titanate oxides and oxynitrides are good semiconductors for this purpose. Moreover, many
perovskites are also good semiconductors for solar water splitting [27]. Perovskite related titanate
oxynitrides may be better semiconductors than these for this application.
24
Hellwig, A. Hendry, A. J. Materials Science. 29 (1994) 18
Hitoki, G. Domen, K. et al. Chem. Commun. (2002) 1698
26
Kasahara, A. Domen, K. et al. J. Phys. Chem. A, 106 (2002) 6750
27
Osterloh, F. Chem. Mater. 20 (2008) 35
25
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Cubic ABO3 perovskites are made of 3-D arrays of corner sharing BO6 octahedra. In an ideal cubic
perovskite, the A cations lie in the 12-coordinate interstices to the center of these octahedra and are at equal
distances from the oxygen anions. The structure of SrTiO3 is shown in Figure 7 below.
Figure 7. The cubic perovskite structure of SrTiO3 showing TiO6 corner-shared ocatehedra with a = 3.905 Å
The formation of perovskite-type structures is predicted by a tolerance factor first formulated by
Goldschmidt et al. [28]. The tolerance factor, t, is given by equation
(6) t 
rA  rB
2 (rB  rO )
where rA is the ionic radii of the A cation, rB is the ionic radii of the B cation, and ro is the ionic radii of the
oxygen anion. Geometeric calculations done by Goldschmidt revealed that ideally cubic perovskites the
ratio of the A-O bond length to the B-O bond length is 2 : 1, making t = 1. However, the bond length is
obtained by summing the respective A-O and B-O ionic radii, therefore, many perovskites have tolerance
factors that range from 0.75-1 [29], while the undistorted perovskites have t values ranging from 0.9-1.
28
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Goldschmidt, V. Videnskaps-Akad, S. Oslo, I. Mat. Nat 8 (1926)
Li, C. Chi Kwan Soh, K. Wu, P. J. Alloys and Compounds 372 (2004) 40
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These standard perovskites are the end members of a homologous series of structures known as
Ruddlesden-Popper phases. Figure 8 shows the n = 1 structure for Sr2TiO4, and the n = 2 structure for
Sr3Ti2O7, both of space group I4/mmm.
n = 1 Ruddlesden-Popper phase
n =2 Ruddlesden-Popper phase
Sr2TiO4
Sr2TiO4 with double c axis
(a)
Figure 8a shows the 2 x 2 x 2 supercell for the n = 1 Ruddlesden-Popper phase, Sr2TiO4 (a = 3.884 Å, c = 12.6 Å) viewed along the b axis.
Figure 8b shows the 2 x 2 x 2 supercell for the n = 2 Ruddlesden-Popper phase Sr3Ti2O7 (a = 3.899 Å, c = 20.39 Å) viewed from the b axis.
The general formula of Ruddlesden-Popper (RP) oxides is An+1BnO3n+1 where n represents the number
of planes of corner sharing BO6 octahedral layers along the c direction. Successive layers are half a unit cell
away from each other in the ab plane. The A cation, with a coordination number of 9, lies in the interlayer
region. A is usually an ion of a rare earth or alkaline earth metal, while B is usually a transition metal ion
[30]. The n =  end member (corresponds to the standard perovskite structure) and the n = 1 end member
are the most prevalent compounds in this series.
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Sharma, I. Singh, D. Bulletin of Materials Science 21 (1998) 363
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The n = 1 (A2BO4) compounds, with K2NiF4 structure type, are in space group I4/mmm. A
reformulation of the tolerance factor done by Knapp and Woodward [31] and based on data from a number
of known compounds of these phases. This new tolerance factor, tRP, is given by the following equation:
(7) t RP 
R AO
2 RBO
where RAO is the A-O bond length, and RBO is the B-O bond length and where the bond lengths are the sum
of the individual ionic radii. If tRP is less than 0.95, the n = 1 phase will tend to adopt a different structure.
High pressures can be used to access Ruddlesden-Popper compounds that are not stable at ambient
pressures. Higher pressures tend to compress anions and cations based on their charges, sizes and
coordination numbers. The amount by which they are compressed is inversely proportional to their charge
and proportional to their sizes and coordination numbers [32]. For Ruddlesden-Popper phases the A cations
tend to be larger, have a smaller charge and greater coordination geometry than the B cation; thus, on the
application of pressure the A-O and B-O bond lengths will change differently making it possible that
compounds which may not be stable at ambient pressures may be stable at high pressures. The opposite
also holds true.
LaTiO2N has a standard perovskite structure consisting of TiOxNy octahedra (x +y = 6) and belongs to
the space group P 1 . It has been previously synthesized by nitriding La2Ti2O7 at 900oC in NH3 [33, 34].
0
( 48hrs, 900 C )
La 2 Ti 2 O 7 NH
3
 LaTiO 2 N
Knapp, M. “Investigations into the Structure and Properties of Ordered Perovskites, Layered Perovskites, and Defect
Pyrochlores.” Diss. Ohio State University, Columbus. 2006.
31
32
Prewitt, C. Downs, R. Reviews in Minerology, 37 (1998) 283
Kasahara, A. Domen, K. J. Phys. Chem. A. 106 (2002) 6750
34
Kasahara, A. Domen, K. J. Phys. Chem. B. 107 (2003) 791
33
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The problem with this method is that one cannot determine how much nitrogen will be in the product
after nitridation. One alternative to ammonolysis is high pressure synthesis, which prevents evaporation of
nitrogen even at high temperatures. Although the sample sizes are small and high pressure synthesis is
difficult, under these conditions, the oxynitride can be synthesized by reaction of a precursor oxide and a
precursor nitride. The nitrogen content would be easy to determine because the amount of nitrogen in the
reactant nitride is the same as the amount in the product oxynitride. Evaporation is not an issue in high
pressure syntheses.
Additionally, LaTiO2N is a medium band gap semiconductor capable of absorbing visible light, and
of the oxynitrides shown in Table 2, it has the greatest activity for hydrogen production. Unfortunately,
LaTiO2N cannot produce O2 without the use of a sacrificial reagent. The sacrificial acceptor (oxidizing
agent) is needed to accept electrons and produce more holes in the valence band for water to be oxidized.
This indicates that that LaTiO2N has a valence band edge that is higher than 1.23 eV.
Because of this, we will attempt to make two n = 1 Ruddlesden-Popper compounds, La2TiO2N2 and
La2TiO3N with Ti oxidation states of +4 and +3 respectively. We will attempt the syntheses of these two
phases under high pressure using a diamond anvil cell (discussed later), and we will monitor the band gaps
and band positions of these compounds.
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