Band Gap Tuning of Zinc Oxide Films for Solar Energy Conversion
A CASPiE Module
Created by Kyoung-Shin Choi
Assistant Professor
Department of Chemistry
Purdue University, West Lafayette, IN 47907
Table of Contents
How to Use this Manual ..................................................................................................... 4
About the Author ................................................................................................................ 5
Introduction......................................................................................................................... 6
1. Overview..................................................................................................................... 6
2. Semiconductors and Band Gap Energy ...................................................................... 6
3. Photon Absorption by Semiconductors ...................................................................... 7
4. Solar Energy Conversion by Semiconductors: The Use of Photon-Generated
Electron/Hole Pairs ......................................................................................................... 8
5. Solar Energy Spectrum and the Necessity of Band Gap Tuning ................................ 9
6. Crystal Lattice and the Unit Cell .............................................................................. 10
7. Solid Solution............................................................................................................ 12
8. Overview of the Activities in This Module .............................................................. 14
9. Module Calendar....................................................................................................... 15
Laboratory Period 1: Preparation of ZnO Films by Spray Pyrolysis................................ 16
1. Introduction............................................................................................................... 16
2. Overview of This Research Activity......................................................................... 17
3. Pre-Lab Requirements .............................................................................................. 17
4. Materials Available................................................................................................... 18
5. Procedure .................................................................................................................. 18
6. Post-Lab Analysis of Results .................................................................................... 19
Laboratory 2: Band Gap Measurement of ZnO Films Using UV-Vis Absorption Spectra
and Preparation of Zn1-xMxO Films .................................................................................. 20
1. Introduction............................................................................................................... 20
2. Overview of This Research Activity......................................................................... 22
3. Pre-Lab Requirements .............................................................................................. 23
4. Materials Available................................................................................................... 23
5. Procedure .................................................................................................................. 23
6. Post-Lab Analysis of Results .................................................................................... 24
Laboratory Period 3 and 4: Band gap tuning .................................................................... 25
1. Introduction............................................................................................................... 25
2. Pre-Lab Requirements .............................................................................................. 27
3. Materials Available................................................................................................... 27
4. Procedure .................................................................................................................. 27
5. Post-Lab Analysis of Results .................................................................................... 28
Laboratory Period 5 and 6: Decomposition of an Organic Dye using ZnO and Zn1-xMxO
Powders............................................................................................................................. 29
1. Introduction............................................................................................................... 29
2. Overview of this Research Activity.......................................................................... 30
3. Pre-Lab Requirements .............................................................................................. 30
4. Materials Available................................................................................................... 30
5. Procedure .................................................................................................................. 31
6. Post-Lab Analysis of Results .................................................................................... 31
Submission of Samples to the Author’s Lab..................................................................... 32
Procedure ...................................................................................................................... 32
Glossary ............................................................................................................................ 33
2
Available Metal Nitrates ................................................................................................... 37
Information for the Instructor ........................................................................................... 38
References......................................................................................................................... 36
3
How to Use this Manual
When you read the title of this module, you may realize that it is not so broad as
“chemistry” or even subdivisions such as “organic chemistry” or “analytical chemistry”.
This module focuses on a very specific area of research that is prominent in the scientific
community. If you have trouble getting an idea of what the module is about or what you
will be doing from the title, do not panic! As you work through the module, you will
come to understand the details and become a researcher in this exciting area.
As you may notice when looking through this manual, there is an introduction section
followed by several weeks of experiments. Be sure to read the introduction before you
begin attempting the experiments – it provides you with the necessary background
information to understand the big picture of what is happening in the laboratory.
After the introduction, the module is divided into sections by laboratory period. Each
laboratory period section includes an introduction of its own and an overview. These
introductions, unlike the introduction to the module, provide any background knowledge
necessary to work through that particular week’s in-lab work and analysis. The overview
is designed to help you keep the rest of the module in mind and make connections
between the different laboratory periods.
Following the overview are pre-laboratory exercises. You should complete these
exercises before attempting any of the procedures. After these exercises are a materials
list and the procedure, which provide information on what you will be doing in the
laboratory. The post-laboratory exercises following the procedures require that you
reflect on your laboratory experience, answer theoretical questions, and analyze the data
you obtained in the laboratory.
As you read through the module, you may notice that some words are bolded and have
footnotes. The definitions of the bold words can be found both in the footnotes and the
glossary in the back of the module.
It is important to keep in mind that this is a research module and not simply a set of
experiments for you to perform. Your focus should be more on how you go about
answering and developing your research question, rather than the conclusion you come to
in the end. Remember that in research, results do not always come easily or as expected;
it is the process that will help you develop into a scientist.
4
About the Author
Dr. Kyoung-Shin Choi is an assistant professor of inorganic
chemistry at Purdue University. Her research focuses on
synthesis of semiconducting and metallic electrode materials.
The overall goal of her research group is to develop highly
efficient electrochemical and photoelectrochemical devices
including solar cells, photoelectrochemical cells, fuel cells,
and rechargeable batteries.
Recent work in the Choi research group involves
manipulating the morphology of inorganic materials and investigating the effect of these
manipulations on physical and chemical properties of the materials.
Dr. Choi hopes to gain information on how the compositions of ZnO affect its optical and
photocatalytic properties. From this information she plans to develop materials that can
efficiently utilize solar energy to generate electricity or to break down organic
environmental pollutants.
Recent Publications:
• López, C. M.; Choi, K.-S. “Enhancement of electrochemical and photoelectrochemical
properties of Fibrous Zn and ZnO electrodes” Chem. Commun. 2005, 3328-3330.
• Siegfried, M. J.; Choi, K.-S. “Elucidating the Effects of Additives on Growth and
Stability of Cu2O Surfaces via Shape Transformation of Pre-Grown Crystals” J. Am.
Chem. Soc. 2006, 128, 10356-10357.
• Spray, R. L.; Choi, K.-S. “Electrochemical Synthesis of SnO2 Films Containing ThreeDimensionally Organized Uniform Mesopores via Interfacial Surfactant Templating”
Chem. Commun. 2007, 3655 - 3657.
5
Introduction
1. Overview
This module is designed to teach you the synthesis and characterization of
semiconducting films and the basics of their use for solar energy conversion. You will
learn the principles and techniques of the spray pyrolysis method to prepare ZnO films
with various compositions and investigate how the compositional changes of ZnO films
affect their optical and photocatalytic1 properties.
This introductory section describes the background needed to understand solid state
chemistry and solar energy conversion. First, there will be background information about
the electronic structure of a semiconductor2, band gap energy3, and the use of electronhole4 pairs for solar energy conversion. Next will be a discussion of the details of ZnO
crystal structures5 and the basics of composition tuning of solid state materials by the
formation of solid solutions6. After that, the introduction describe the goal and the
general overview of this module.
2. Semiconductors and Band Gap Energy
Semiconducting materials are the basis of many solid state devices, including computer
chips, diode lasers, and solar cells. A semiconductor is a material that has low electrical
conductivity7 at room temperature, but its electrical conductivity can be increased by the
input of energy. This can be easily understood by examining the electronic energy levels
of semiconductors.
A solid material is composed of an inconceivable number of atoms and contains an
infinite number of energy states8. Because these energy levels are so closely spaced, they
form bands9 instead of discrete energy states. This is the major difference between a
solid material and a single molecule that contains a finite number of atoms and possesses
discrete energy levels. In a solid material, the highest energy band that is filled with
electrons is called the valence band10. The next higher band that is empty is called the
conduction band11. The energy separation between these bands is called the band gap,
Eg.
1
Photocatalysis: The process by which the activation energy of a reaction is decreased by energy obtained
from light.
2
Semiconductor: A material that has poor electrical conductivity at room temperature but increases with
the input of energy.
3
Band Gap Energy (Eg): The energy separation between the valence and conduction bands.
4
Hole: The absence of an electron in a mass which serves as a positively charged carrier of electricity.
5
Crystal Structure: A well defined and orderly lattice structure arrangement.
6
Solid Solution: A homogeneous solid of two or more materials.
7
Conductivity: The ability of a material to transmit electrons.
8
Energy State (Level): The level excitation of an electron which corresponds to a specified amount of
energy.
9
Electron Band: A range of electron excitability which determines the localization of electrons.
10
Valence Band: An electron band containing energy states which contain the valence electrons. These
electrons are localized to particular metal atoms.
11
Conduction Band: An electron band containing energy states in which electrons are free to move
throughout a mass.
6
The filling of these bands and
the magnitude of the energy gap
determine if a material is a
metal, a semiconductor, or an
insulator12 (Figure 1). A metal
has a partially filled conduction
band, so there is no energy gap
between filled and unfilled
regions. A significant number of
electrons can be excited by heat
into empty energy levels and
move easily throughout the
material, allowing the material to conduct electricity. An insulator possesses a
considerable energy gap between the valence band and the conduction band, which
makes it difficult to excite electrons from the valence band to the conduction band. As a
result an insulator does not conduct electricity.
A semiconductor is similar to an insulator, but the band gap is much smaller. Therefore,
a small number of electrons from the valence band can be promoted to the conduction
band by an energy input (e.g. thermal energy from heat). The electrons promoted to the
conduction band are no longer strongly bound to specific atoms, and can freely migrate
throughout the material. This explains why a semiconductor's electrical conductivity
increases as the temperature rises.
However, the electrical conductivity of a
semiconductor is significantly lower than that of a metal.
3. Photon Absorption by Semiconductors
Light absorption by a semiconductor can have a similar effect in increasing the
semiconductor’s electrical conductivity. When a photon13 hits a semiconductor, one of
two things can occur: it will pass through when the photon energy is lower than the band
gap energy of the semiconductor or it will
be absorbed when the photon energy is
CB e e e
CB e ee- e
equal to or greater than the band gap
energy of the semiconductor. When a
hν
photon is absorbed, its energy is
transferred to an electron in the valence
eh+
VB
VB
band. This electron can then be promoted
to the conduction band, where it is free to Figure 2. An electron-hole pair generated by
move around within the semiconductor. photon absorption.
This transition creates a hole in the
valence band that can also move through the valence band. Thus, photon absorption by
the semiconductor can create mobile electron-hole pairs (Figure 2).
12
Insulator: A material that has a wide energy gap between valence and conducting bands, and is thus a
poor conductor of electricity.
13
Photon: A particle, or quantum, of electromagnetic energy.
7
photon: Particle of electromagnetic energy with energy E proportional to the
observed frequency of light (ν) and inversely proportional to the wavelength of
light (λ).
E = hν = hc/λ
Εq. 1
h (Planck’s constant) = 6.63 x 10-34 Joule•second (J•s)
c (speed of light) = 3.00 x 108 m/s
hole: A hole is not a physical particle in the same sense as an electron in that it
represents an absence of an electron. The presence of a hole allows for the
movement of an electron. When a neighboring electron moves to the vacancy (=
hole), it appears as if the hole moves in the opposite direction of the electron.
Therefore, a hole is considered an electric charge carrier with a positive charge,
equal in magnitude but opposite in polarity to the charge on the electron.
When e2 moves to the left to fill the vacancy, h, it looks as if h moved to the right.
4. Solar Energy Conversion by Semiconductors: The Use of Photon-Generated
Electron/Hole Pairs
Once the electron-hole pairs are created by the absorption of a photon, the electrons and
holes can be used for various useful reactions including energy production and
environment remediation. For example, solar cells enable the direct conversion of light
to electricity by sending the photon-generated electrons in one direction and holes in the
opposite direction, thus creating electrical potential14 and current15 flow (Figure 3a).
The photon-generated electron-hole pairs can also be directly used for reduction or
oxidation reactions to produce fuels. Figure 3b shows how a semiconducting material in
contact with an aqueous solution can split water into oxygen and hydrogen. The photongenerated holes (h+) are used to oxidize water to oxygen and protons
Eq. 2
H2O + 2h+ → 1/2 O2 + 2H+
while the photon-generated electrons (e-) are used to reduce protons to hydrogen.
2H+ + 2e- → H2
Eq. 3
This is the basic redox reaction of a photoelectrochemical cell16 that can produce
hydrogen by solar energy conversion. Hydrogen is widely considered to be the fuel of
14
Electrical Potential: The difference in energy per unit charge, commonly expressed in volts (V) or Joules
per Coulomb (J/C).
15
Electrical Current: The movement of positive charge opposite the flow of electrons in a mass, commonly
expressed in Amperes (A).
16
Photoelectrochemical Cell: A tool which converts light into chemical and electrical energy.
8
the future for two reasons. First, it offers the possibility of escaping from the current
reliance upon globally finite resources (e.g. oil, natural gas, coal) that also create many
environmental problems (e.g. greenhouse gas emission).
Second, its use is
environmentally benign, producing only water as a by-product. It is reported that solar
energy has sufficient capacity to fully meet the global energy needs of the next century
without potentially destructive environmental consequences.
(b) Photocatalytic H2 Production
(a) Photovoltaics
H2
2e-
2H+
Semiconducting
electrodes
e-
h+
h+
e-
Semiconducting
particle
CB
hν
VB
2h+
1/2O2 + 2H+
H2O
Figure 3. Schematic illustration of electricity generation and hydrogen production by solar
energy conversion using semiconducting materials. CB: Conduction band, VB: Valence band.
Semiconductors can also be used as photocatalysts for environmental remediation. In
this case, the photon-generated electron and hole pairs can be used for redox reactions at
the surface of semiconductors, and generate hydroxyl radicals17 (OH•) and superoxide
ions (O2-). These species are powerful oxidizing agents and can disintegrate harmful
organic18 pollutants in water and convert them into CO2 and H2O (Figure 4).
O2-
Semiconducting
particle
2e-
O2
VB
2h+
CB
hν
O2- or OH●
Organic Pollutants
CO2 + H2O
2OH● + 2H+
2H2O
Figure 4. Schematic illustration of photocatalytic degradation of organic pollutants by
semiconducting particles.
5. Solar Energy Spectrum and the Necessity of Band Gap Tuning
Only photons of energy equal to or larger than the band gap energy of a photoelectrode
may be absorbed and used for conversion. Figure 5 illustrates the solar energy spectrum,
17
Free Radicals: Molecules with a lone, unpaired electron that are very reactive with organic molecules.
Organic Molecules: Molecules consisting primarily of carbon and hydrogen atoms that may also contain
oxygen, nitrogen, phosphorous, and/or sulfur atoms.
18
9
which shows the number of photons in sunlight that possess a given energy. Note that
the majority of photons have less than 3 eV of energy in the visible and infrared regions.
-1
-2
-1
NUMBER OF PHOTONS (s m eV )
Semiconductors with a narrow band gap such as Si, GaAs, CdSe and CdTe (Eg < 3eV)
can absorb both visible light and UV light (Region II and III in Figure 5). Those with
wide band gaps such as TiO2 and ZnO (Eg ≥ 3 eV) can utilize only UV light (Region III
in Figure 5). Therefore, the narrow
band gap materials are expected to
convert
solar
energy
more
efficiently than the wide band gap
4x1021
materials. However, the narrow
band gap materials are not suitable
3x1021
for
long-term
use
in
photoelectrochemical cells because
2x1021
the photon-generated electrons and
I
holes can react with these materials
1x1021
II
and corrode them. Therefore, new
III
materials
with
both
good
1
2
3
4
5
conversion efficiency and long term
PHOTON ENERGY (eV)
stability need to be discovered to
build
more
efficient Figure 5. Solar energy spectrum in terms of number2
of photons received per second per unit area of 1 m
photoelectrochemical cells.
versus photon energy on a clear sunny day with the
sun about 60 degrees above the horizon. I: IR
region, II: Visible region, and III: UV region.
(Adapted from Book of Photon Tools, 1999, 1-3,
Oriel-Instrument)
One of the approaches that can be
taken to accomplish this goal is to
tailor the composition and the
electronic structure (e.g. band gap
energy) of a relatively stable wide band gap material so that it can utilize visible light. In
this module, you will modify the composition of ZnO by forming solid solutions and
investigate how composition changes affect both electronic structures and optical
properties. Therefore, in order to better understand this module’s activities it is essential
to understand the basic concept of solid solutions.
eV (electron volt): a unit of energy conventionally used as a measure of particle energies.
One electron volt is equal to the amount of energy that one electron acquires by
accelerating through a potential difference of one volt. The relationship between the
electron volt and the Joule, the SI (System International) unit of energy, is as follows:
1 eV = 1.602 x 10-19 Joule
Eq. 2
6. Crystal Lattice and the Unit Cell
The particles in the solids you will be dealing with in this module maintain a very
organized, crystalline structure. This structure can be broken down into repeating three
10
dimensional patterns called unit cells19. These particular unit cells involve a total of 18
atoms, each in a tetrahedral configuration20. The term tetrahedral refers to the pattern
in which each individual atom is located an equal distance from the four surrounding
atoms.
a)
b)
Figure 6. The configuration of 18 atoms involved in the unit cell (a). Note that
each atom in the unit cell is in a tetrahedral conformation (b).
Although this unit cell involves 18 atoms, only a total of eight are considered part of the
unit cell. This is because portions of the atoms along the edges and on the corners of the
unit cell only contribute the volume found within the cube-shaped boundaries of the cell.
Each of the eight atoms on the corners of the cell only contribute one eighth of their total
volume. The six atoms that lie in the center of each face of the cell only contribute one
half of their total volume. There are four atoms located off-center within the cell which
contribute their total volume to the unit cell.
1⎞ ⎛
1⎞
⎛
⎜ 8corners × ⎟ + ⎜ 6 faces × ⎟ + 4inside = 8
8⎠ ⎝
2⎠
⎝
Eq. 4
The unit cells are stacked around each other any number of times to become a solid mass
on a scale that can be seen visibly. It is important to remember that unit cells are
arbitrary divisions and are only useful in recognizing organized patterns within a crystal
mass.
19
Unit Cell: A small , 3-dimensional, repeating arrangement of atoms that contribute to a crystalline
structure.
20
Tetrahedral Configuration: A spatial arrangement in which four bodies are equally distant and at equally
spaced angles from a central body.
11
Figure 7. Four unit cells stacked two high and two wide. The bottom left unit cell
is highlighted. Note that the atoms at the corners and the faces are not entirely
within the unit cell.
7. Solid Solution
A solid solution is a homogeneous solid of two or more materials that can exist over a
range of compositions. For example, if a portion of Pt atoms in Pt metal is replaced by
Au atoms, the composition of the resulting material can be written as
Pt1-xAux (0 < x ≤ 1). This will be called a “solid solution” if the material has three
characteristics:
• it forms a single homogeneous phase
• it maintains the basic crystal structure of Pt metal
• Au atoms are thoroughly mixed with Pt atoms (similar to the way that sugar
molecules (solute21) are mixed with water molecules (solvent22) in sugar
solution.) Pt is a solid “solvent” and Au is a solid “solute” in this case.
The solute may be incorporated into the solvent crystal structure substitutionally, by
replacing a solvent particle in the lattice, or interstitially, by fitting into the space between
solvent particles (Figure 8). Substitutional solid solutions are the type of solid solutions
featured in this research module. Solid solutions have important technological
applications because such mixtures often have superior properties to pure materials.
21
Solute: The substance that dissolves into a solution and typically is present in small quantities relative to
the solvent.
22
Solvent: The substance that solutes are dissolved into to form a solution.
12
Even small amounts of solute material can affect the electrical, chemical, and physical
properties of the solvent material.
Substitutional
Solid Solution
Pure Substance
Interstitial
Solid Solution
Figure 8. Schematic representations of substitutional and interstitial solid solutions.
Some mixtures will readily form solid solutions over a range of concentrations, while
other mixtures will not form solid solutions at all. If two materials to be mixed possess
the same crystal structures, it may be possible to form a substitutional solid solution at all
relative concentrations of the two species. If the solvent and the solute materials do not
have the same crystal structure, the extent of substitutional solid solution depends on
many factors including the atomic/ionic size difference between solute and solvent
species and synthesis conditions such as temperature.
As an example, let’s suppose that there are two phases, α and β, that have different
crystal structures and the solubility of β in α is 20% at a certain synthesis condition. This
means that any solid solution containing equal to or less than 20% of β will be a
homogeneous phase and preserve the crystal structure of α. This solution would have the
empirical formula α1-xβx (0 ≤ x ≤ 0.2), where the sum of the subscripts must always be 1,
indicating 100%. If more than 20% of β is mixed with α, a mixture of α0.8β0.2 and a pure
β phase will result (Table 1).
%β
0
10
15
20
25
30
50
%α
100
90
85
80
75
70
50
formula
α
α0.9β0.1
α0.85β0.15
α0.8β0.2
α0.8β0.2 plus pure β
α0.8β0.2 plus pure β
α0.8β0.2 plus pure β
13
8. Overview of the Activities in This
Module
ZnO is a semiconductor with a wide band
gap of 3.2-3.4 eV. ZnO has a crystal
structure called “wurtzite”, which is
composed of alternating planes of
tetrahedrally coordinated O2- and Zn2+ ions
as shown in Figure 9. Previous studies
showed that when a portion of Zn ions in the
ZnO lattice was replaced by other transition
metal ions, M (M can be any transition metal
available in a nitrate, see page 37) to form a
solid solution, Zn1-xMxO, the solid solution
became able to absorb a portion of visible
light. These investigations were made using
either single crystals or pellets of ZnO.
O2Zn2+
Figure 9. The wurtzite crystal structure of
ZnO.
How many oxygen ions are
connected to one zinc ion? How many
zinc ions are connected to one oxygen ion?
However, single crystals or pellets of ZnO
are not suitable for use in electrochemical
cells23 because growing single crystals is an
expensive and time-consuming process and because it is difficult to ensure the
homogeneity and reproducibility of the pellet samples. Therefore, it is desirable to
produce these solid solutions by other synthetic methods. In addition, the synthetic
method and experimental conditions can significantly vary the maximum amount of
transition metal ions that can be incorporated into the wurtzite structure. These factors
leave many opportunities to prepare and study Zn1-xMxO films with new optical
properties.
In this module, you will first learn a spray pyrolysis technique to prepare ZnO films and
use a UV-Vis spectrophotometer24 to characterize the films’ optical properties. Based
on this knowledge, you will design synthetic conditions to prepare ZnO-based solid
solutions with varying concentrations of an M of your choice. You will determine the
maximum amount of M that can be incorporated in the ZnO lattice.
By measuring UV-Vis spectra of the resulting Zn1-xMxO, you will be able to investigate
how a material’s composition affects its electronic band structure and optical properties.
Lastly, you will use ZnO and Zn1-xMxO to photocatalytically decompose methyl orange
molecules. In doing so, you will have the opportunity to investigate how compositions
and optical properties of semiconducting materials are related to the efficiencies of solar
energy conversion. (Zn1-xMxO can also be written as (ZnO)1-x(MO)x with ZnO being the
solvent and MO being the solute material.)
23
Electrochemical Cells: Devices which allow the conversion between electrical and chemical energy.
Spectrophotometer: A device that shoots a beam of light of a specified wavelength through a chemical
sample, and measures the absorbance or transmittance of that beam by the sample.
24
14
9. Module Calendar
Table 1. Module Progression
Lab 1: Synthesis of ZnO films via spray pyrolysis. Determination of
optimum synthetic conditions for spray pyrolysis.
Lab 2: Measurement of UV-Vis spectra of ZnO films and band gap
determination. Synthesis of Zn1-xMxO films using spray pyrolysis.
Labs 3-4: Determination of the solubility of MO in ZnO, and study of
the effect of compositions on optical properties of Zn1-xMxO films.
Labs 5-6: Deposition of a series of Zn1-xMxO films on conducting
substrate and testing of their photocatalytic activities toward
decomposition of an organic dye molecule.
15
Laboratory Period 1: Preparation of ZnO Films by Spray Pyrolysis.
1. Introduction
Spray pyrolysis is a process in which a thin film is deposited by spraying a solution on a
heated surface, where the solutes react to form a chemical compound. The chemical
reactants are selected such that the products other than the desired compound are
volatile25 at the temperature of deposition26. Figure 10 shows a typical spraying system,
which is composed of four major parts: a unit that regulates the flow of spray solution, a
unit that controls the air pressure, a spray nozzle, and a unit that controls the temperature
of the substrate27. The significant variables in spray pyrolysis include carrier-gas flow
rate, nozzle-to-substrate distance, droplet radius, solution concentration, and substrate
temperature.
Figure 10. Spray pyrolysis system (Mooney. J. B.; Radding, S. R. Ann. Rev. Mater. Sci.
1982, 12, 81.)
For this lab, you will use a much simpler experimental set-up to produce ZnO films via
spray pyrolysis: a spray bottle and a glass slide heated on a hot plate! We will use zinc
nitrate solutions as the precursor solution. When this solution is sprayed on a hot surface,
water will evaporate and nitrate ions will be decomposed to NO2 gas leaving ZnO
deposits on the glass. The appropriate temperature to achieve this reaction is 400 °C. If a
thermocouple is available, the temperature of the hot plate can be directly measured. To
minimize the effect of the room-temperature atmosphere interfering with the
25
Volatile: The readiness at which a substance is vaporized.
Deposition: A process that settles particles out of solution.
27
Substrate: For the purpose of this module, the substrate is the material upon which deposition is made.
26
16
measurement, cover the hot plate with aluminum foil and place the tip of a thermocouple
underneath the aluminum foil.
If a thermocouple is not available, you can determine the proper settings for the hot plate
for spray pyrolysis experimentally. This can be achieved by placing a test slide on the
hot plate and spraying the solution as you gradually increase the heat setting. When you
find a setting where the spray solution evaporates as soon as it reaches the surface of the
glass slide, you are ready to make a deposition. Remember that it may take a while for
the hot plate to reach the maximum temperature allowed for each setting and for the glass
slide to achieve the same temperature. Therefore, give enough time (~20 minutes) at
each setting before you make a decision to move to the next level of heating.
Once you find the setting that results in instant evaporation of the sprayed solution on the
test slide, you are ready to make films on new glass slides. However, at this point we do
not know whether this temperature setting is high enough to decompose nitrate to
produce ZnO films. Therefore, you will make a few films at this temperature and also at
a higher temperature.
You will examine these films using a UV-Vis spectrophotometer in the next laboratory
activity to verify whether or not you have deposited ZnO. The quality of the UV-Vis
data you will obtain next time critically depends on the thickness of the film you will
prepare today. If the film is too thin, you will not observe a pronounced absorption band.
If the film is too thick, it will block the light even when there is no actual absorption,
resulting in poor quality data. (We will learn more about the UV-Vis measurement in the
next research activity.) Therefore, you will prepare films with three different thicknesses
at each temperature. The thickness of the film can be easily varied by changing the
number of times you spray the solution.
2. Overview of This Research Activity
In this research activity, you will prepare ZnO films via spray pyrolysis. The main
activities are composed of two parts. First, you will learn how to clean glass slides. ZnO
films will not adhere well to dirty slides. Also, any impurities present on the glass slides
can generate undesired side products during the spray pyrolysis process. Second, you
will perform spray pyrolysis and produce six films in total using two different
temperature settings with three different thicknesses per setting. You will examine the
appearance (e.g. thickness, evenness, smoothness) of the resulting films to think about
experimental conditions (e.g. heat setting, number of sprays) that can improve film
quality for your next research activity. You will analyze the films you prepare today
using a UV-Vis spectrophotometer in the next laboratory activity.
3. Pre-Lab Requirements
Write an introduction and outline of methods for this laboratory period. Your outline
should include a description of what you plan to do in lab, in your own words, such that
you could follow the instructions directly out of your own lab notebook. In addition to
writing the introduction and outlining the experimental steps, you will also need to
17
calculate the mass of (Zn(NO3)2·6H2O) powder needed to make 250 mL of a 0.1 M
solution to be used in creating the ZnO films.
4. Materials Available
Tweezers
Spatula
250 mL volumetric flask
Hot plate
Aluminum foil
Glass microscope slides
Spray bottle
Glass rod
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O)
Thermocouple, if available
Sharpie pen for labeling slides
Cleaning brush
5. Procedure
Cleaning Glass Slides
Clean the glass slides using soapy water and a cleaning brush. Rinse thoroughly with
deionized (DI) water. Repeat the cleaning until you can no longer see any iridescent or
oily residue on the slides. Allow slides to dry on paper towels.
Preparing Zinc Nitrate Solution
Weigh the amount of Zn(NO3)2·6H2O that you calculated you will need for the 0.1 M
Zn(NO3)2 solution. Be careful not to spill any of the material. If you do, clean the
balance area thoroughly. Recap the stock bottle of Zn(NO3)2·6H2O immediately. Place
the sample in a 250 mL volumetric flask and dissolve it in roughly 100 mL DI water. Stir
the solution with a glass rod if necessary. Gently swirling the flask will also expedite the
dissolution of solute. Add DI water to bring the volume to 250 mL.
Clean a spray bottle with soap and water and rinse it with DI water. Fill the spray bottle
with your 0.1M zinc nitrate solution.
Spray Pyrolysis
Cover the surface of a hot plate with aluminum foil and place seven glass slides at the
center of the hot plate. (One is a test slide and the others are to deposit films.)
Heat the hot plate. Start with a heat setting that uses approximately 50% of the maximum
power. It will take 15-20 minutes to reach a steady temperature. Alternatively, you can
use a thermocouple to directly measure the temperature of the hot plate. A setting that
provides 300 ºC or above should be used.
18
Cover six glass slides with aluminum foil and expose only the test glass slide. Spray zinc
nitrate solution on the glass slide from approximately 6 inches directly above the slide
and observe whether the droplets evaporate as soon as they reach the glass surface. If
not, increase the level of heat and repeat the test. Remember to give enough time for the
hot plate and glass to achieve the new temperature.
If you find a temperature where a white film forms as soon as the sprayed solution
reaches the glass slide, you are ready to make ZnO films. If the slides crack into pieces
when the spray solution hits the slide, the setting may be too high. Record the setting that
gives the best films with minimum cracking, and if possible, record information about
which hot plate you used.
Expose one new glass slide and spray the solution. Spray the solution several times until
you see that a very thin white film is formed. Waiting ~30 seconds in between sprays
helps reduce slide cracking. Record how many times you sprayed. Remove this film
from the hot plate and label it. (You will need to develop a labeling system that allows
you to keep track of the conditions you used to create each slide.)
Make two more films at the same temperature with twice and three times more material
deposited on the glass.
Change the hot plate setting to a higher level, wait for the temperature to equilibrate and
then make three more films with three different thicknesses.
6. Post-Lab Analysis of Results
Report the detailed conditions (e.g. hot plate setting, temperature, number of sprays,
angle of spraying, amount of time lapsed between sprays) used to deposit each of
samples. Describe the appearance of each sample (e.g. color, smoothness, evenness).
You might find that a table is a useful way to record this information.
Discuss what experimental conditions/techniques you can vary in the future to try to
improve the evenness of the film surface.
Preparation for Next Week
Carry out the pre-lab calculations before going to lab.
19
Laboratory 2: Band Gap Measurement of ZnO Films Using UV-Vis
Absorption Spectra and Preparation of Zn1-xMxO Films
1. Introduction
UV-Vis Spectroscopy
UV-Vis spectroscopy is the measurement of the absorption of near-ultraviolet and visible
light by a sample. The absorption of light is caused by electronic transitions in the
sample. Therefore, the specific wavelengths that are absorbed and the intensity of the
absorption give us information about the electronic structure of the sample. The light
source is usually a hydrogen or deuterium lamp for UV measurements and a tungsten
lamp for visible measurements.
These light sources emit light over a broad range of wavelengths. Therefore, a very
specific wavelength can be selected for an absorption experiment by using a wavelength
separator such as a prism or grating monochromator. The wavelength separator can be
scanned over a range of wavelengths. Spectra are obtained by recording the intensity of
absorption at each wavelength over a given range (Figure 11).
Monochromator
Io
(wavelength selector)
Light source
I
Sample
Detector
Amplifier
Readout
Figure 11. Schematic diagram of a spectrophotometric experiment.
Experimental measurements are usually made in terms of transmittance (T), which is
defined as:
T = I / Io
Eq. 5
where I is the light intensity after it passes through the sample and Io is the initial light
intensity. The relation between absorbance (A) and transmittance (T) is:
A = -log T = - log (I / Io )
Eq. 6
A semiconductor can absorb light that has energy equal to or greater than the band gap
energy of the semiconductor. This absorption promotes an electron from the valence to
the conduction band, leaving behind a hole in the valence band. Each photon absorbed
thus creates one electron-hole pair.
20
Absorbance
A generic semiconductor absorption spectrum
is shown in Figure 12. This absorption
spectrum is characterized by a sharp increase
in absorption at the band gap energy. This
type of absorption behavior is due to an
electronic transition from one of the many
energy states in the valence band to one of the
many energy states in the conduction band.
The onset energy of the absorption edge28,
which represents the minimum energy
required for electronic transition from the
valence band to the conduction band, is
defined as the band gap energy, Eg.
Eg
Energy (eV)
Figure 12. General sketch of the
absorption spectrum of a semiconductor.
Question 1: Zinc oxide has a band gap energy of 3.2 eV. Using Eq. 1 and Eq. 2, calculate
the wavelength (in nm) at which the absorption edge of ZnO should appear. Does your
answer validate the use of a UV-Vis spectrophotometer to measure the band gap of ZnO?
The wavelength ranges for UV and visible light are listed in Figure 13.
Figure 13. The electromagnetic spectrum.
Preparation of ZnO-Based Solid Solutions
A solid solution is a homogeneous crystalline structure in which one or more types of
atoms or molecules are partly substituted for the original atoms and molecules without
changing the structure. In this research activity, we will prepare ZnO-based solid
solutions by replacing Zn2+ ions with other transition metal ions such as Co2+, Ni2+, Fe2+,
Mn2+, or Cu2+ while preserving the ZnO structure.
28
Absorption Edge: The point on an absorption spectrum which represents the minimum energy required
for electronic transition from the valence band to the conduction band.
21
The empirical formula of these solid solutions can be written as Zn1-xMxO (0 < x < 1)
where M is the other transition metal. For example, if 10% of the Zn2+ ions in the ZnO
structure are replaced by Co2+ ions, the formula of the resulting solid solution can be
written as Zn0.9Co0.1O. In this solid solution, Co2+ and Zn2+ ions are randomly distributed
over the sites that were originally occupied by Zn2+ ions in the ZnO structure. For each
zinc site, the probability of finding Co2+ instead of Zn2+ is 10%. This means that if you
examined 10,000 different zinc sites in this structure, you would find that approximately
1,000 sites are occupied by Co2+ ions. However, it is not possible to know in advance at
which site Co2+ ions can be found because this replacement is random and does not have
a predictable pattern.
The purpose of forming these solid solutions is to utilize visible light for the generation
of electron-hole pairs in Zn1-xMxO by reducing the band gap energy relative to that of
ZnO. If the solid solution you prepare can successfully absorb visible light, you will
notice a change of color. ZnO is white because it does not absorb any visible light. The
combination of light of all colors reflected from ZnO results in its white color.
(Remember that mixing pigment and mixing light create very different results. Mixing
pigments of all colors creates a black pigment but mixing light of all colors creates a
white light.) Therefore, if Zn1-xMxO starts to absorb a part of the visible light spectrum, it
will start to bear a color! As the value of x increases, we expect to see a more significant
color change.
The maximum value of x in Zn1-xMxO that still maintains the homogeneous ZnO
structure not only depends on the type of M (e.g. size and electronic configuration of the
metal ion) but also depends on the choice of synthetic method and detailed experimental
conditions (e.g. temperature). If the amount of M used for the synthesis of Zn1-xMxO
exceeds the maximum atomic percent that can be incorporated into the ZnO lattice, the
excess M will form its own oxide as an impurity. For example, if the spray solution
contained 50 atomic percent of cobalt when only 30 atomic percent of cobalt can be
maximally accommodated in the ZnO structure, CoO, Co2O3 or Co3O4 can be formed as
impurities in addition to Zn0.7Co0.3O.
One of the goals of this and the next few research activities is to determine the maximum
x-values of the metal you choose that can be homogeneously incorporated in the ZnO
lattice. In most of the cases, the presence of the impurities can be visually detected
because the side products possess distinctively different colors from the colors of Zn1xMxO. If you observe two or more different colors mixed in your sample, it indicates that
a portion of M is not used to form Zn1-xMxO and the ZnO lattice is already saturated with
M.
2. Overview of This Research Activity
In this research activity, you will obtain UV-Vis spectra of the films you prepared in the
previous laboratory activity and estimate their band gaps. Since the band gap energy of
zinc oxide is already known, this characterization will allow you to know whether or not
you have prepared zinc oxide films. Based on these results, you will also be able to
choose the optimum spray pyrolysis conditions (e.g. hot plate setting, thickness of
22
samples) to produce high quality ZnO films that can exhibit evident absorption edges in
their UV-Vis spectra.
Next, you will use these conditions to prepare ZnO-based solid solutions, Zn1-xMxO (M is
the metal you are investigating; 0 < x < 0.5). First, you will choose a metal to replace a
portion of Zn atoms in the ZnO lattice. Then you will prepare spray solutions that
contain both zinc nitrate and the nitrate salt of your chosen metal, M. Prepare solutions
with three different compositions (three different molar ratios between Zn and M). For
each composition, prepare films with three different thicknesses. This is to obtain a UVVis spectrum with a good signal-to-noise ratio29. Based on the homogeneity of samples
you prepare today, you will be able to choose new solution compositions to use in the
next laboratory activity. The eventual goal is to find the maximum x-value that can form
a stable Zn1-xMxO phase without forming any impurities.
3. Pre-Lab Requirements
Before coming to lab, calculate the wavelength where you expect to observe the ZnO
band gap (see introduction). You also need to plan how you will make the mixed Zn/M
nitrate solutions. Which molar ratios (between Zn and M) will you use? How will you
make the three solutions that you will use for spray pyrolysis of the Zn1-xMxO films? Do
all calculations before coming to lab. Also, answer Question 1 from the introductory
reading.
4. Materials Available
Glass slides
Tweezers
Spatula
250 mL volumetric flask
100 mL volumetric flasks
Hot plate
Aluminum foil
Spray bottle
Glass rod
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O)
The metal nitrate hydrate you are investigating (M(NO3)x·xH2O)
A UV-Vis spectrophotometer controlled by a PC
Scotch tape
5. Procedure
Measurement of UV-Vis Spectra
Attach the ZnO film outside the cuvette holder using scotch tape such that the incident
beam30 passes perpendicularly through the film. Make sure that the incident beam passes
through and evenly deposited part of your sample. Use a cleaned glass slide for
29
Signal-to-noise Ratio: A comparison of signal produced by the measurement of a sample and the
background noise produced by the device which is being used to measure.
30
Incident Beam: A ray of light that directly strikes a surface.
23
To determine the band gap:
(a) Draw the line that best fits the slope of the absorption
edge (Figure 14).
(b) Draw another line that extrapolates the background line
before the absorption edge is formed.
(c) Read the x-value of the intersection of the two lines and
convert nm to eV.
Absorbance
background correction. (See previous laboratory activity
for instructions on cleaning glass slides.)
(a)
(b)
Eg
Energy (eV)
Figure 14. How to find a band
gap energy.
Preparation of Spray Solutions to Deposit Zn1-xMxO Films
Choose a metal nitrate that will be mixed with zinc nitrate. Prepare three precursor
solutions that will result in deposition of the three molar ratios that you chose.
Spray Pyrolysis of Zn1-xMxO Films
Clean seven glass slides using last week’s procedure. (One is to test the deposition
conditions and six are to prepare films.) Heat the hot plate using the optimum setting you
identified in the previous laboratory activity to produce ZnO films. Make films with two
different thicknesses using each spray solution. Label all films.
6. Post-Lab Analysis of Results
UV-Vis Measurement
• Discuss in detail the features of the UV-Vis spectra of your films.
• How did the deposition conditions you used affect the features of the film?
• Which samples can be clearly identified as ZnO? Explain your answer.
• For each film that did not exhibit evident absorption edges, discuss possible
reasons.
Preparation of Zn1-xMxO Films
• Which Zn/M ratios did you choose to test? How did you make the spray solutions
to create the films?
• Discuss in detail the experimental conditions (e.g. hot plate setting, number of
sprays) used to make each film. Describe the colors and surface textures of each
sample.
24
Laboratory Period 3 and 4: Band gap tuning
1. Introduction
Finding the maximum atomic percent of M to form a homogeneous phase of Zn1-xMxO
In research labs at universities and in industry, many experimental scientists spend
considerable time and effort systematically modifying their experimental conditions to
achieve a specific scientific goal. These goals include (i) testing a hypothesis, (ii)
understanding the thermodynamics and/or kinetics of a reaction/system, and (iii)
improving yields and any desired properties of a final compound/product. Therefore, an
ability to efficiently design experimental conditions to complete a given task and analyze
resulting data to better design the next experiments is a critical qualification to become a
scientist.
In the next research activity, you will have an opportunity to experience this process as
you find the maximum atomic percent of M that can be incorporated into the ZnO
structure (maximum solubility of M in the ZnO structure). You will have the freedom to
choose compositions of spray solutions based on the results you obtained last week. If
the compositions you used resulted in films with a mixture of two or more colors, this
indicates that you had excess M in the spray solution that could not be incorporated in
Zn1-xMxO.
In order to identify maximum x values for Zn1-xMxO, solutions with reduced amounts of
M need to be prepared. If the composition you used resulted in films with a uniform
color, this indicates that you need to try solutions with an increased amount of M. By
repeating these processes, you will be able to identify the maximum atomic percent of M
which forms a pure ZnO-based solid solution. For example, if you have obtained the data
in Table 1, you can conclude that the transition from the homogeneous phase to the
heterogeneous phase occurred between x = 0.1 and x = 0.2. This means that the
maximum x-value in this system should be equal to or greater than 0.1 but less than 0.2.
Table 1. Possible experimental results when searching for the maximum atomic percent of M that forms a
pure ZnO-based solid solution.
Molar ratios of Zn:M in Corresponding x-value in
spray solution
Zn1-xMxO
9:1
x = 0.1
8:2
x = 0.2
7:3
x = 0.3
Film appearance
uniform color
mixture of colors
mixture of colors
In order to determine the maximum x-value accurately to the second decimal place, you
need further experiments with x-values between 0.10 and 0.20. Trying x = 0.15 can be a
reasonable starting point. If this experiment results in a homogeneous phase, x values
between 0.15 and 0.20 need to be tried for the next experiments. On the other hand, if
this experiment results in a heterogeneous phase, x values between 0.10 and 0.15 need to
be tested for the next experiments. By repeating this procedure, you will be able to find
25
the maximum atomic percent of M which forms a homogeneous ZnO-based solid
solution.
UV-Vis measurement of Zn1-xMxO
Once you identify the maximum value of x, prepare Zn1-xMxO with at least 4 different xvalues smaller than the maximum value. You may have already obtained them while you
were finding the maximum value of x. However, if the x-values are so close to one
another that their colors are indistinguishable, prepare one or two more films with new xvalues so that the resulting films can better demonstrate a gradual change of colors. Then
obtain UV-Vis spectra of these films and compare their optical properties with those of
ZnO (Figure 15a).
If incorporation of M in the ZnO lattice indeed reduces the band gap energy of ZnO, you
will observe a shift of the onset absorption energy to a lower energy (Figure 15b).
Another possible change in UV-Vis spectra caused by the presence of M in the ZnO
lattice is the appearance of absorption peaks in the visible range. The presence of these
peaks indicates that there are new electronic states created by M between the valence
band and the conduction band of ZnO (Figure 15c). Both changes make it possible for
ZnO films to absorb visible light and therefore to bear colors. Whether these changes
will be advantageous for solar energy conversion can only be determined through further
studies to determine the exact energy levels, which are out of the scope of this study. In
this module, we will focus on the understanding of the effect of compositions on the
electronic structures and optical properties of materials.
(a) ZnO
(b) Zn1-xMxO
CB
(c) Zn1-xMxO
CB
CB
Band gap
transition
New interband
states
VB
Absorbance
Absorbance
VB
Absorbance
VB
Eg
Energy (eV)
Eg
Energy (eV)
Eg
Energy (eV)
Figure 15. UV-Vis Spectra for possible modifications of the ZnO band structure. (a)
Pure, unmodified ZnO (b) Solid solution containing Zn and another M. Notice the
change in the absorption edge (c) Mixture of Zn1-xMxO in which M is in too high of
concentration to form solid solution
26
2. Pre-Lab Requirements
As last week, you need to plan how you will make the mixed Zn/M nitrate solutions.
Which molar ratios (between Zn and M) will you start with? How might you change the
solutions after analyzing the first films that you will make? Do all calculations before
coming to lab.
3. Materials Available
Glass slides
Tweezers
Spatula
250 mL volumetric flask
100 mL volumetric flasks
Hot plate
Aluminum foil
Spray bottle
Storage bottles
Glass rod
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O)
The metal nitrate hydrate you are investigating (M(NO3)x·xH2O)
A UV-Vis spectrophotometer controlled by a PC
Scotch tape
4. Procedure
Determine the maximum value of x to the second decimal place for the M of your choice
that forms a pure ZnO-based solid solution, Zn1-xMxO.
1.
2.
3.
4.
Design compositions of the spray solutions to achieve this goal efficiently.
Make films by spray pyrolysis.
Examine the appearance of the resulting films.
Based on the results obtained from (iii), repeat (i)-(iii) until you find the
maximum x-value.
Next prepare four Zn1-xMxO films of different x-values, and measure their UV-Vis
spectra. The four films should have x values that are evenly spaced between zero and the
maximum x value that you determined. For example, if you find that any films you make
with an x value of 0.21 or above are spotty and not homogeneous, then x = 0.20
(Zn0.80M0.20O) is your maximum x value and you should make three other films with x
values of 0.05, 0.10, and 0.15.
The series of UV-Vis spectra that you obtain will show the trend in absorbance properties
of your films. To obtain the most informative and meaningful results, the x-values
should not be too close to one another.
Keep your slides and metal nitrate solutions in a safe place when finished. They will be
used in the following weeks.
27
5. Post-Lab Analysis of Results
• Record the compositions of the spray solutions you used.
o Explain why you chose these compositions.
o Describe the colors of thin films that resulted from each spray solution.
o Explain how you used this observation to narrow down the range of xvalues.
• What is the maximum x-value for your choice of M that forms a pure Zn1-xMxO?
• Describe features of UV-Vis spectra for your four different compositions of
Zn1-xMxO.
o Include print copies of the spectra in your notebook.
o What conclusion can you make from these results regarding the effect of
the incorporation of M into the ZnO structure on the electronic structure
and the optical properties of ZnO?
28
Laboratory Period 5 and 6: Decomposition of an Organic Dye using
ZnO and Zn1-xMxO Powders
1. Introduction
Up to now we have learned that tuning the composition and electronic structure of a wide
band gap semiconductor can enable efficient absorption of visible light. This increases
the number of photon-generated electron-hole pairs in the semiconductor that can be used
for many useful reactions. One such reaction is the environmental cleanup (or
remediation) of organic contaminants in water supplies. Organic molecules are made
primarily of carbon and hydrogen atoms, and may also contain oxygen, nitrogen, and/or
sulfur atoms. Some organic molecules (such as chlorobenzenes and phenols) can be
dangerous in high levels in the water supply.
The electrons and holes created in a semiconductor when light is absorbed can be used to
destroy organic molecules in water. The electrons and holes react with water to form
hydroxyl radicals, •OH.
e − + O2 → O2−
H 2O + h + →OH + H +
O2 + 2 H + + 2e − → H 2O2
H 2O2 + O2 →OH + OH − +O2
H 2O2 + e − →OH + OH −
The hydroxyl radicals react very quickly with organic molecules, producing carbon
dioxide gas as a byproduct.
In this laboratory experiment, you will test the ability of your ZnO and Zn1-xMxO
powders to decompose methyl orange, an organic dye molecule. The structure of methyl
orange in basic solutions is shown in Figure 16.
SO3
N
N
N(CH3)2
Figure 16. The organic dye, methyl orange.
Because methyl orange dissolves in water to make a colorful solution, the amount of dye
in solution can be monitored using UV-Vis spectroscopy. Therefore, the efficiency of
ZnO and Zn1-xMxO for the photocatalytic degradation of methyl orange can be easily
determined by measuring the UV-Vis spectra of the methyl orange solutions with
photocatalyst powders before and after irradiation. The more efficient the catalyst is, the
more methyl orange will be decomposed and the less light the solution will absorb
afterward.
29
You will test the powders’ photocatalytic activities using a combination of UV and
visible light and using only visible light to investigate whether formation of the Zn1-xMxO
solid solution indeed increases the utilization of visible light for the generation of
electron-hole pairs. In doing so, you will have the opportunity to observe the direct
relationship between composition, optical properties, and photocatalytic properties of
materials.
2. Overview of this Research Activity
You will use the thin films of ZnO and Zn1-xMxO that you prepared in the previous
week’s experiment. Although the maximum value of x represents the most M that can be
incorporated into the ZnO lattice, this solid composition is often not as efficient at
absorbing light as intermediate compositions are. Therefore, it is important to test both a
homogeneous ZnO film and the varying concentrations of Zn1-xMxO.
You will need to scrape the films off the glass slides using a razor blade so they can be
put in an aqueous solution with the methyl orange dye molecules. You will take UV-Vis
absorbance measurements before and after the methyl orange solutions (with the ZnO or
Zn1-xMxO powders in the solution) are exposed to UV and/or visible light. The change in
absorbance will indicate the effectiveness of your semiconductor at photocatalytically
degrading the methyl orange molecules.
3. Pre-Lab Requirements
Before coming to lab, you will need to plan your experiments. Read the general
description of the experimental procedure below. Using the materials available, how can
you determine if your Zn1-xMxO films can use more visible light than the ZnO films do?
When outlining your experiments, your team should plan to make efficient use of the
time available to you in the lab as you compare the photocatalytic abilities of your ZnO
and Zn1-xMxO films. Plan control experiments that will account for the effects of
variables such as the light from the lab’s overhead lights and any decrease of methyl
orange concentration in the solution due to its absorption to the surface of the
semiconductor powders.
4. Materials Available
Materials and chemicals required to prepare ZnO and Zn1-xMxO films by spray pyrolysis
Razor blades
Methyl orange dye
Stir plate and magnetic stir bar
UV lamp (provides UV and visible light)
Fluorescent lamp (provides visible light only)
Cuvettes
Pipettes
30
5. Procedure
Prepare an aqueous solution containing 3 mg methyl orange per 10 mL solution. Plan
ahead to know how much solution your experiments will require.
For each experiment, add 10 mg ZnO or Zn1-xMxO powder from the slides prepared in the
previous weeks to 10 mL of the methyl orange solution in a 50 mL beaker. Let the
powders settle to the bottom of the beaker and then transfer ~ 2 mL of the methyl orange
solution to a cuvette for UV-Vis measurements. Don’t forget to zero the UV-Vis
spectrometer with water. Measure the UV-Vis absorption spectrum of the sample, return
the sample to the beaker, and stir the solution for one hour under the experimental
conditions that you chose. After the hour, measure the UV-Vis absorption spectrum
again.
6. Post-Lab Analysis of Results
Using the UV-Vis spectra that you obtained, determine the peak absorption wavelength
for methyl orange. Record the absorption of your solutions at this wavelength. Calculate
the percentage change in absorption after each experimental treatment. What does your
data tell you about your films’ ability to decompose methyl orange?
31
Submission of Samples to Dr. Choi’s Lab
If the Zn1-xMxO sample(s) you prepared show better photocatalytic properties than ZnO,
you are encouraged to deposit your samples on a conducting substrate (fluorine-doped tin
oxide, FTO) and send them to the author’s lab. We will assemble a photoelectrochemical
cell using your samples for further characterization. Deposition on a conducting
substrate is necessary because the glass substrate you have been using is insulating and
thus will not conduct electrons in a photoelectrochemical device.
Procedure
Only one side of the conducting glass is conductive and this is the side on which you
should deposit films. Use a multimeter to identify the conducting side of the substrate.
The conducting side will have a resistance of 20-30 ohms.
Deposit ZnO and Zn1-xMxO films on FTO substrates using the spray conditions you
determined during the module.
Submit the resulting films and their UV-Vis spectra to your TA. Please include the UVVis data for methyl orange solutions showing that your Zn1-xMxO sample exhibits a better
photocatalytic property than ZnO.
32
Glossary
absorption edge The point on an absorption spectrum which represents the minimum
energy required for electronic transition from the valence band to the conduction band.
band (electron band) A range of electron excitability which determines the
localization of electrons.
band gap (Eg)
The energy separation between the valence and conduction bands.
conductor A material that has no gap between valence and conduction bands, thus
conducting electricity well.
conduction band An electron band containing energy states in which electrons are free
to move throughout a mass.
Coulomb (C)
A unit of electric charge.
crystal structure A well defined and orderly lattice structure arrangement.
deposition A process that settles particles out of solution.
electrical conductivity The ability of a material to transmit electrons.
electrical current The movement of positive charge opposite the flow of electrons in a
mass, commonly expressed in Amperes (A).
electric potential The difference in energy per unit charge, commonly expressed in
volts (V) or Joules per Coulomb (J/C).
electrochemical cells Devices which allow the conversion between electrical and
chemical energy.
electron volt (eV) A unit of energy conventionally used to describe the amount of
energy that one electron acquires by accelerating through a potential difference of one
volt. 1eV = 1.602 × 10−19 Joules .
energy state (level) The level excitation of an electron which corresponds to a
specified amount of energy.
hole The absence of an electron in a mass which serves as a positively charged carrier
of electricity.
incident beam A ray of light that directly strikes a surface.
33
insulator A material that has a wide energy gap between valence and conducting
bands, thus refusing to conduct electrical current.
lattice
The arrangement of molecules or atoms in a solid mass in relation to each other.
organic molecules Molecules consisting primarily of carbon and hydrogen atoms that
may also contain oxygen, nitrogen, phosphorous, and/or sulfur atoms.
photocatalysis The process by which the activation energy of a reaction is decreased
by energy obtained from light.
photoelectrochemical cell A tool which converts light into chemical and electrical
energy.
photoelectrode A semiconducting electrode used for solar energy conversion.
photon
A particle, or quantum, of electromagnetic energy.
Planck’s constant (h) A constant which relates the wavelength (v) and energy (λ) of
light, equal to 6.63 × 1023 JouleSeconds ( J × s ) .
radicals Molecules with a lone, unpaired electron that are very reactive with organic
molecules.
semiconductor A material that has poor electrical conductivity at room temperature but
increases with the input of energy.
signal-to-noise ratio A comparison of signal produced by the measurement of a sample
and the background noise produced by the device which is being used to measure.
solid solution
A homogeneous solid of two or more materials.
solute The substance that dissolves into a solution and typically is present in small
quantities relative to the solvent.
solvent
The substance that solutes are dissolved into to form a solution.
spectrophotometer A device that shoots a beam of light of a specified wavelength
through a chemical sample, and measures the absorbance or transmittance of that beam
by the sample.
speed of light (c) A constant defining the speed at which electromagnetic radiation
travels in a vacuum. c = 3 ×108 m / s .
spray pyrolysis A process in which a thin film is deposited by spraying a solution on a
heated surface, where the solutes react to form a chemical compound.
34
substrate For the purpose of this module, the substrate is the material upon which
deposition is made.
valence band An electron band containing energy states which contain the valence
electrons. These electrons are localized to particular metal atoms.
volatility The readiness at which a substance is vaporized.
35
References
Atkins, P. W., Physical Chemistry. 3rd ed.; WH Freeman and Company: New York, NY,
1986.
Bahadur, L.; Rao, T. N., Photoelectrochemical Studies of Cobalt-Doped ZnO Sprayed
Thin Film Semiconductor Electrodes in Acetonitrile Medium. Solar Energy
Materials 1992, 27, 347.
Chatterjee, D.; Dasgupta, S., Visible light induced photocatalytic degradation of organic
pollutants. Journal of Photochchemistry and Photobiology, C 2005, 6, 186-205.
Fox, M. A.; Dulay, M. T., Heterogeneous Photocatalysis. Chemical Review 1993, 93,
341-357.
Grätzel, M., Photoelectrochemical cells. Nature 2001, 414, 338-344.
Kotz, J. C.; Treichel, P. M.; Weaver, G. C., Chemistry and Chemical Reactivity. 6th ed.;
Thomson Learning, Inc.: Belmont, CA, 2006.
Lewis, N. S., Light work with water. Nature 2001, 414, 589-590.
Miles, R. W.; Hynes, K. M.; Forbes, I., Photovoltaic solar cells: An overview of state-ofthe-art cell development and environmental issues. Progress in Crystal Growth
and Characterization of Materials 2005, 51, 1-42.
Mooney, J. B.; Radding, S. R., Annual Review of Materials Science 1982, 12, 81.
Robertm, D.; Malato, S., Solar photocatalysis: a clean process for water detoxifiation.
Science of the Total Environment 2002, 291, 85-97.
Turner, J. A., Sustainable hydrogen production. Science 2004, 305, 972-974.
36
Available Metal Nitrates
Aldrich Price/500g Hazards*
Abbr. Transition Metal Formula in Nitrate
CAS
Zn
Zinc
Zn(NO3)2 · 6H2O
10196-18-6 228737 38.1
Cu
Copper
Cu(NO3)2 · 2.5 H2O 19004-19-4 223395 80.9
Ni
Nickel
Ni(NO3)2 · 6H2O
13478-00-7
244074 50.7
Co
Cobalt
Co(NO3)2 · 6H2O
10026-22-9
230375 112
Carcinogen
Possible Carcinogenicity,
Environment
Fe
Iron
Fe(NO3)3 · 9H2O
7782-61-8
F3002 41.1
Irritant
Mn
Manganese
Mn(NO3)2·xH2O
15710-66-4
288640 46.1
Irritant
Cr
V
Ti
Chromium
Vanadium
Titanium
Cr(NO3)3 · 9H2O
7789-02-8
239259 67.1
Irritant
Sc
Scandium
Sc(NO3)3·xH2O
107552-14-7 325902 301/5g
Cd
Cadmium
Cd(NO3)2 · 4H2O
10022-68-1
642045 88
Carcinogen, Environment
Ag
Silver
AgNO3
7761-88-8
209139 569
Causes Burns, Environment
Pd
Rh
Ru
Tc
Mo
Nb
Palladium
Rhodium
Ruthenium
Technetium
Molybdenum
Niobium
Pd(NO3)2 · 2H2O
RhNO3
10102-05-3
13465-43-5
76070
Fisher
Zr
Zirconium
ZrO(NO3)2 · xH2O
14985-18-3
243493 238.5
Y
Yttrium
Y(NO3)3 · 6H2O
13494-98-9
237957 216.5
Hg
Au
Pt
Ir
Os
Re
W
Ta
Hf
Mercury
Gold
Platinum
Iridium
Osmium
Rhenium
Tungsten
Tantalum
Hafnium
Hg(NO3)2 · H2O
AuNO3
7783-34-8
13464-77-2
La
Ds
Mt
Hs
Bh
Sg
Db
Rf
Ac
Lanthanum
Darmstadtium
Metinerium
Hassium
Bohrium
Seaborgium
Dubnium
Rutherfordium
Actinium
La(NO3)3·xH2O
100587-94-8 238554 160
*
Irritant
Causes Burns
162.5/5g Causes Burns
3000/1g
Causes Burns
Irritant
Irritant, Cumulative Effects,
230421 166/250g Environment
VWR 245.61/1g
Irritant
All may cause fire when making contact with a combustible!
37
Information for the Instructor
It is strongly suggested that the instructor implementing this module speak to the author
about which transition elements to use in this experiment. Although there are a large
variety of transition elements available in nitrates, Dr. Choi may have more data on some
than others. It is important that the students are not repeating established experiments
and choosing different elements to make available for them is one way to ensure that they
are gathering information that is new and useful.
38