Band Gap Module

7/31/2019 Band Gap Module

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Band Gap Tuning of Zinc Oxide Films for Solar Energy Conversion

A CASPiE Module

Created by Kyoung-Shin ChoiAssistant Professor

Department of Chemistry

Purdue University, West Lafayette, IN 47907


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Table of Contents

How to Use this Manual ..................................................................................................... 4About the Author ................................................................................................................ 5

Introduction......................................................................................................................... 61. Overview..................................................................................................................... 6

2. Semiconductors and Band Gap Energy ...................................................................... 63. Photon Absorption by Semiconductors ...................................................................... 74. Solar Energy Conversion by Semiconductors: The Use of Photon-Generated

Electron/Hole Pairs ......................................................................................................... 85. Solar Energy Spectrum and the Necessity of Band Gap Tuning ................................ 9

6. Crystal Lattice and the Unit Cell .............................................................................. 10

7. Solid Solution............................................................................................................ 128. Overview of the Activities in This Module .............................................................. 14

9. Module Calendar....................................................................................................... 15Laboratory Period 1: Preparation of ZnO Films by Spray Pyrolysis................................ 16

1. Introduction............................................................................................................... 16

2. Overview of This Research Activity......................................................................... 173. Pre-Lab Requirements .............................................................................................. 17

4. Materials Available................................................................................................... 185. Procedure .................................................................................................................. 18

6. Post-Lab Analysis of Results .................................................................................... 19

Laboratory 2: Band Gap Measurement of ZnO Films Using UV-Vis Absorption Spectraand Preparation of Zn1-xMxO Films .................................................................................. 20

1. Introduction............................................................................................................... 202. Overview of This Research Activity......................................................................... 22

3. Pre-Lab Requirements .............................................................................................. 23

4. Materials Available................................................................................................... 235. Procedure .................................................................................................................. 23

6. Post-Lab Analysis of Results .................................................................................... 24Laboratory Period 3 and 4: Band gap tuning.................................................................... 25

1. Introduction............................................................................................................... 25

2. Pre-Lab Requirements .............................................................................................. 273. Materials Available................................................................................................... 27

4. Procedure .................................................................................................................. 275. 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............................................................................................................... 292. Overview of this Research Activity.......................................................................... 303. Pre-Lab Requirements .............................................................................................. 30

4. Materials Available................................................................................................... 30

5. Procedure .................................................................................................................. 31

6. Post-Lab Analysis of Results .................................................................................... 31Submission of Samples to the Authors Lab..................................................................... 32

Procedure ...................................................................................................................... 32Glossary ............................................................................................................................ 33


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Available Metal Nitrates................................................................................................... 37

Information for the Instructor ........................................................................................... 38References......................................................................................................................... 36


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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 youwill 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 youbegin 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. Eachlaboratory period section includes an introduction of its own and an overview. These

introductions, unlike the introduction to the module, provide any background knowledgenecessary to work through that particular weeks 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 theseexercises 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 youreflect 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 theglossary 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 toin 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.


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About the Author

Dr. Kyoung-Shin Choi is an assistant professor of inorganic

chemistry at Purdue University. Her research focuses onsynthesis of semiconducting and metallic electrode materials.

The overall goal of her research group is to develop highlyefficient electrochemical and photoelectrochemical devices

including solar cells, photoelectrochemical cells, fuel cells,

and rechargeable batteries.

Recent work in the Choi research group involvesmanipulating 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 andphotocatalytic properties. From this information she plans to develop materials that can

efficiently utilize solar energy to generate electricity or to break down organicenvironmental pollutants.

Recent Publications:

Lpez, 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 Three-Dimensionally Organized Uniform Mesopores via Interfacial Surfactant Templating

Chem. Commun.2007, 3655 - 3657.


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Introduction

1. Overview

This module is designed to teach you the synthesis and characterization ofsemiconducting 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 filmswith 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 aboutthe electronic structure of a semiconductor

2, band gap energy

3, and the use of electron-

hole4

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 electricalconductivity

7at room temperature, but its electrical conductivity can be increased by the

input of energy. This can be easily understood by examining the electronic energy levelsof semiconductors.

A solid material is composed of an inconceivable number of atoms and contains aninfinite number ofenergy states

8. 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 possessesdiscrete 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 theconduction band

11. 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.


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e e e

h

CB

VB e-

e e eCB

VB

e-

h+

Figure 2. An electron-hole pair generated by

photon absorption.

The filling of these bands andthe magnitude of the energy gap

determine if a material is ametal, a semiconductor, or an

insulator

12

(Figure 1). A metalhas a partially filled conductionband, so there is no energy gap

between filled and unfilledregions. A significant number of

electrons can be excited by heat

into empty energy levels andmove easily throughout the

material, allowing the material to conduct electricity. An insulator possesses aconsiderable 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 migratethroughout the material. This explains why a semiconductor's electrical conductivity

increases as the temperature rises. However, the electrical conductivity of asemiconductor 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

semiconductors electrical conductivity. When a photon13 hits a semiconductor, one oftwo 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 isequal to or greater than the band gap

energy of the semiconductor. When aphoton is absorbed, its energy is

transferred to an electron in the valence

band. This electron can then be promoted

to the conduction band, where it is free tomove around within the semiconductor.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 apoor conductor of electricity.13 Photon: A particle, or quantum, of electromagnetic energy.


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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 andenvironment 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 theopposite direction, thus creating electrical potential

14and current

15flow (Figure 3a).

The photon-generated electron-hole pairs can also be directly used for reduction oroxidation 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 photon-

generated holes (h+) are used to oxidize water to oxygen and protons

H2O + 2h+1/2 O2 + 2H

+Eq. 2

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, commonlyexpressed in Amperes (A).16 Photoelectrochemical Cell: A tool which converts light into chemical and electrical energy.

photon: Particle of electromagnetic energy with energy E proportional to the

observed frequency of light () and inversely proportional to the wavelength oflight ().

E = h = hc/ q. 1

h (Plancks constant) = 6.63 x 10-34 Joulesecond (Js)c (speed of light) = 3.00 x 10

8m/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 themovement 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.


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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 manyenvironmental problems (e.g. greenhouse gas emission). Second, its use is

environmentally benign, producing only water as a by-product. It is reported that solarenergy has sufficient capacity to fully meet the global energy needs of the next century

without potentially destructive environmental consequences.

Semiconductors can also be used as photocatalysts for environmental remediation. Inthis 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 superoxideions (O2

-). These species are powerful oxidizing agents and can disintegrate harmful

organic18

pollutants in water and convert them into CO2 and H2O (Figure 4).

5. Solar Energy Spectrum and the Necessity of Band Gap TuningOnly 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.18 Organic Molecules: Molecules consisting primarily of carbon and hydrogen atoms that may also contain

oxygen, nitrogen, phosphorous, and/or sulfur atoms.

Semiconducting

electrodes

e-

h+

(a) Photovoltaics (b) Photocatalytic H2 Production

CB

VB

h

2e-

2h+

2H+

H2

1/2O2 + 2H+

H2O

e-h+

Semiconducting

particle

Figure 3. Schematic illustration of electricity generation and hydrogen production by solarenergy conversion using semiconducting materials. CB: Conduction band, VB: Valence band.

CB

VB

h2e-

2h+O2

2OH+ 2H+2H2O

Semiconductingparticle

O2

-

Organic Pollutants CO2+ H2OO2- or OH

Figure 4. Schematic illustration of photocatalytic degradation of organic pollutants by

semiconducting particles.


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1x1021

2x1021

3x1021

4x1021

1 2 3 4 5

I

II

III

PHOTON ENERGY (eV)

NUMBER

OFPHOTONS(s-1m-2eV-1)

Figure 5. Solar energy spectrum in terms of number

of photons received per second per unit area of 1 m2

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)

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.

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 IIIin Figure 5). Therefore, the narrowband gap materials are expected to

convert solar energy moreefficiently than the wide band gap

materials. However, the narrow

band gap materials are not suitablefor long-term use in

photoelectrochemical cells becausethe photon-generated electrons and

holes can react with these materials

and corrode them. Therefore, newmaterials with both good

conversion efficiency and long termstability need to be discovered to

build more efficient

photoelectrochemical cells.

One of the approaches that can betaken to accomplish this goal is to

tailor the composition and the

electronic structure (e.g. band gapenergy) 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 andinvestigate how composition changes affect both electronic structures and optical

properties. Therefore, in order to better understand this modules 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 veryorganized, crystalline structure. This structure can be broken down into repeating three


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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 patternin which each individual atom is located an equal distance from the four surrounding

atoms.

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 onehalf of their total volume. There are four atoms located off-center within the cell which

contribute their total volume to the unit cell.

1 18 6 4 8

8 2corners faces inside

+ + =

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 arearbitrary divisions and are only useful in recognizing organized patterns within a crystalmass.

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.

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 .


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7. Solid Solution

A solid solution is a homogeneous solid of two or more materials that can exist over arange 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 asPt1-xAux (0 < x 1). This will be called a solid solution if the material has threecharacteristics:

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 sugarmolecules (solute

21) are mixed with water molecules (solvent

22) 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, orinterstitially, by fitting into the space between

solvent particles (Figure 8). Substitutional solid solutions are the type of solid solutionsfeatured 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 tothe solvent.22 Solvent: The substance that solutes are dissolved into to form a solution.

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 entirelywithin the unit cell.


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Even small amounts of solute material can affect the electrical, chemical, and physical

properties of the solvent material.

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 nothave the same crystal structure, the extent of substitutional solid solution depends onmany factors including the atomic/ionic size difference between solute and solvent

species and synthesis conditions such as temperature.

As an example, lets suppose that there are two phases, and , that have differentcrystal structures and the solubility of in is 20% at a certain synthesis condition. Thismeans that any solid solution containing equal to or less than 20% of will be ahomogeneous phase and preserve the crystal structure of. This solution would have theempirical formula 1-xx (0 x 0.2), where the sum of the subscripts must always be 1,indicating 100%. If more than 20% of is mixed with , a mixture of

0.8

0.2and a pure

phase will result (Table 1).

% % formula0 100 10 90 0.90.115 85 0.850.1520 80 0.80.225 75 0.80.2 plus pure 30 70 0.80.2 plus pure 50 50 0.80.2 plus pure

Pure SubstanceSubstitutionalSolid Solution

Interstitial

Solid Solution

Figure 8. Schematic representations of substitutional and interstitial solid solutions.


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Figure 9. The wurtzite crystal structure of

ZnO. How many oxygen ions areconnected to one zinc ion? How many

zinc ions are connected to one oxygen ion?

8. Overview of the Activities in This

Module

ZnO is a semiconductor with a wide bandgap of 3.2-3.4 eV. ZnO has a crystal

structure called wurtzite, which is

composed of alternating planes oftetrahedrally coordinated O2-

and Zn2+

ionsas shown in Figure 9. Previous studies

showed that when a portion of Zn ions in theZnO lattice was replaced by other transition

metal ions, M (M can be any transition metal

available in a nitrate, see page 37) to form asolid solution, Zn1-xMxO, the solid solution

became able to absorb a portion of visiblelight. These investigations were made using

either single crystals or pellets of ZnO.

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 toproduce these solid solutions by other synthetic methods. In addition, the synthetic

method and experimental conditions can significantly vary the maximum amount oftransition 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 anduse a UV-Vis spectrophotometer

24to 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 themaximum 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 materials composition affects its electronic band structure and optical properties.

Lastly, you will use ZnO and Zn1-xMxO to photocatalytically decompose methyl orangemolecules. In doing so, you will have the opportunity to investigate how compositions

and optical properties of semiconducting materials are related to the efficiencies of solarenergy 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.24 Spectrophotometer: A device that shoots a beam of light of a specified wavelength through a chemicalsample, and measures the absorbance or transmittance of that beam by the sample.

O2-

Zn2+


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9. Module Calendar

Table 1. Module Progression

Lab 1: Synthesis of ZnO films via spray pyrolysis. Determination ofoptimum synthetic conditions for spray pyrolysis.

Lab 2: Measurement of UV-Vis spectra of ZnO films and band gapdetermination. Synthesis of Zn1-xMxO films using spray pyrolysis.

Labs 3-4: Determination of the solubility of MO in ZnO, and study ofthe effect of compositions on optical properties of Zn1-xMxO films.

Labs 5-6: Deposition of a series of Zn1-xMxO films on conductingsubstrate and testing of their photocatalytic activities towarddecomposition of an organic dye molecule.


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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 aheated surface, where the solutes react to form a chemical compound. The chemical

reactants are selected such that the products other than the desired compound arevolatile

25at the temperature ofdeposition

26. 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 temperatureof the substrate

27. The significant variables in spray pyrolysis include carrier-gas flow

rate, nozzle-to-substrate distance, droplet radius, solution concentration, and substratetemperature.

For this lab, you will use a much simpler experimental set-up to produce ZnO films viaspray 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 athermocouple 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.26 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.

Figure 10. Spray pyrolysis system (Mooney. J. B.; Radding, S. R. Ann. Rev. Mater. Sci.1982, 12, 81.)


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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 youfind a setting where the spray solution evaporates as soon as it reaches the surface of theglass 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 glassslide 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 donot 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 laboratoryactivity 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 thenext 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 slidescan generate undesired side products during the spray pyrolysis process. Second, you

will perform spray pyrolysis and produce six films in total using two differenttemperature 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 filmquality 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 thatyou could follow the instructions directly out of your own lab notebook. In addition towriting the introduction and outlining the experimental steps, you will also need to


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calculate the mass of (Zn(NO3)26H2O) powder needed to make 250 mL of a 0.1 M

solution to be used in creating the ZnO films.

4. Materials Available

Tweezers

Spatula250 mL volumetric flaskHot plate

Aluminum foilGlass microscope slides

Spray bottle

Glass rodZinc nitrate hexahydrate (Zn(NO3)26H2O)

Thermocouple, if availableSharpie pen for labeling slides

Cleaning brush

5. Procedure

Cleaning Glass SlidesClean 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 oroily residue on the slides. Allow slides to dry on paper towels.

Preparing Zinc Nitrate SolutionWeigh the amount of Zn(NO3)26H2O that you calculated you will need for the 0.1 MZn(NO

3)

2solution. Be careful not to spill any of the material. If you do, clean the

balance area thoroughly. Recap the stock bottle of Zn(NO3)26H2O immediately. Placethe 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 bottlewith your 0.1M zinc nitrate solution.

Spray PyrolysisCover the surface of a hot plate with aluminum foil and place seven glass slides at thecenter 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 canuse a thermocouple to directly measure the temperature of the hot plate. A setting thatprovides 300 C or above should be used.


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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 slideand observe whether the droplets evaporate as soon as they reach the glass surface. Ifnot, 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 solutionreaches 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 thatgives 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 sprayshelps 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 materialdeposited on the glass.

Change the hot plate setting to a higher level, wait for the temperature to equilibrate andthen 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 ofsamples. 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 WeekCarry out the pre-lab calculations before going to lab.


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Laboratory 2: Band Gap Measurement of ZnO Films Using UV-Vis

Absorption Spectra and Preparation of Zn1-xMxO Films

1. Introduction

UV-Vis SpectroscopyUV-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 theabsorption 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 tungstenlamp for visible measurements.

These light sources emit light over a broad range of wavelengths. Therefore, a veryspecific 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 ofabsorption at each wavelength over a given range (Figure 11).

Experimental measurements are usually made in terms of transmittance (T), which isdefined as:

T = I / Io Eq. 5

where I is the light intensity after it passes through the sample and Io is the initial lightintensity. 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 tothe conduction band, leaving behind a hole in the valence band. Each photon absorbed

thus creates one electron-hole pair.

Monochromator

(wavelength selector)

Light sourceSample

Detector

Amplifier Readout

Io I

Figure 11. Schematic diagram of a spectrophotometric experiment.


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Absorbance

Energy (eV)

Eg

Figure 12. General sketch of the

absorption spectrum of a semiconductor.

A generic semiconductor absorption spectrum

is shown in Figure 12. This absorptionspectrum is characterized by a sharp increasein absorption at the band gap energy. This

type of absorption behavior is due to an

electronic transition from one of the manyenergy states in the valence band to one of themany energy states in the conduction band.The onset energy of the absorption edge

28,

which represents the minimum energy

required for electronic transition from the

valence band to the conduction band, isdefined as the band gap energy, Eg.

Preparation of ZnO-Based Solid SolutionsA 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 withoutchanging the structure. In this research activity, we will prepare ZnO-based solidsolutions by replacing Zn

2+ions with other transition metal ions such as Co

2+, Ni

2+, Fe

2+,

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.

Figure 13. The electromagnetic spectrum.

Question 1: Zinc oxide has a band gap energy of 3.2 eV. Using Eq. 1 and Eq. 2, calculat

the wavelength (in nm) at which the absorption edge of ZnO should appear. Does you

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.


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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 ZnOstructure are replaced by Co

2+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 Co

2+

instead of Zn

2+

is 10%. This means that if youexamined 10,000 different zinc sites in this structure, you would find that approximately1,000 sites are occupied by Co

2+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 havea predictable pattern.

The purpose of forming these solid solutions is to utilize visible light for the generationof 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 willnotice 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. Mixingpigments 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, itwill 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 themetal 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, theexcess 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 bemaximally 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 ZnOlattice. 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 Zn1-

xMxO. If you observe two or more different colors mixed in your sample, it indicates thata 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 ofzinc oxide is already known, this characterization will allow you to know whether or notyou 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


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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 thatcontain both zinc nitrate and the nitrate salt of your chosen metal, M. Prepare solutionswith 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 UV-Vis spectrum with a good signal-to-noise ratio

29. 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 forma stable Zn1-xMxO phase without forming any impurities.


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/Mnitrate 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 Zn 1-xMxO films? Doall calculations before coming to lab. Also, answer Question 1 from the introductory

reading.


Glass slidesTweezers

Spatula

250 mL volumetric flask100 mL volumetric flasks

Hot plateAluminum foil

Spray bottle

Glass rodZinc nitrate hexahydrate (Zn(NO3)26H2O)

The metal nitrate hydrate you are investigating (M(NO3)xxH2O)A UV-Vis spectrophotometer controlled by a PC

Scotch tape

5. Procedure

Measurement of UV-Vis SpectraAttach 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 thebackground noise produced by the device which is being used to measure.30 Incident Beam: A ray of light that directly strikes a surface.


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(a)

(b)

Eg

Energy (eV)

Absorbance

Figure 14. How to find a band

gap energy.

background correction. (See previous laboratory activity

for instructions on cleaning glass slides.)

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 linebefore the absorption edge is formed.(c) Read the x-value of the intersection of the two lines and

convert nm to eV.

Preparation of Spray Solutions to Deposit Zn1-xMxO FilmsChoose 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 weeks procedure. (One is to test the depositionconditions 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 twodifferent thicknesses using each spray solution. Label all films.


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 possiblereasons.

Preparation of Zn1-xMxO Films Which Zn/M ratios did you choose to test? How did you make the spray solutionsto create the films?

Discuss in detail the experimental conditions (e.g. hot plate setting, number ofsprays) used to make each film. Describe the colors and surface textures of each

sample.


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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 spendconsiderable 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 analyzeresulting 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 tochoose 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, thisindicates 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. Byrepeating 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 theheterogeneous 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

spray solution

Corresponding x-value in

Zn1-xMxO

Film appearance

9:1 x = 0.1 uniform color

8:2 x = 0.2 mixture of colors

7:3 x = 0.3 mixture of colors

In order to determine the maximum x-value accurately to the second decimal place, youneed 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 tobe tested for the next experiments. By repeating this procedure, you will be able to find


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the maximum atomic percent of M which forms a homogeneous ZnO-based solid

solution.

UV-Vis measurement of Zn1-xMxOOnce you identify the maximum value of x, prepare Zn1-xMxO with at least 4 different x-

values smaller than the maximum value. You may have already obtained them while youwere finding the maximum value of x. However, if the x-values are so close to oneanother that their colors are indistinguishable, prepare one or two more films with new x-

values so that the resulting films can better demonstrate a gradual change of colors. Thenobtain 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 valenceband 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 changeswill 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 theelectronic structures and optical properties of materials.

VBVB

CB

VB

CB

CB

Band gap

transitionNew interband

states

(a) ZnO (b) Zn1-xMxO (c) Zn1-xMxO

Energy (eV)

Absorbance

Energy (eV)

Absorbance

Energy (eV)

Absorbance

Eg Eg Eg

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 ofZn1-xMxO in which M is in too high ofconcentration to form solid solution


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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 thesolutions after analyzing the first films that you will make? Do all calculations before

coming to lab.


Glass slidesTweezers

Spatula

250 mL volumetric flask100 mL volumetric flasks

Hot plateAluminum foil

Spray bottle

Storage bottlesGlass rod

Zinc nitrate hexahydrate (Zn(NO3)26H2O)The metal nitrate hydrate you are investigating (M(NO3)xxH2O)

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. Design compositions of the spray solutions to achieve this goal efficiently.2. Make films by spray pyrolysis.3. Examine the appearance of the resulting films.4. 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-Visspectra. 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.


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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 x-values. 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 ofZn1-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?


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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 increasesthe 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 madeprimarily of carbon and hydrogen atoms, and may also contain oxygen, nitrogen, and/or

sulfur atoms. Some organic molecules (such as chlorobenzenes and phenols) can bedangerous in high levels in the water supply.

The electrons and holes created in a semiconductor when light is absorbed can be used todestroy organic molecules in water. The electrons and holes react with water to form

hydroxyl radicals, OH.

2 2

2

2 2 2

2 2 2 2

2 2

2 2

e O O

H O h OH H

O H e H O

H O O OH OH O

H O e OH OH

+ +

+

+

+ +

+ +

+ + +

+ +

The hydroxyl radicals react very quickly with organic molecules, producing carbondioxide gas as a byproduct.

In this laboratory experiment, you will test the ability of your ZnO and Zn1-xMxOpowders 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

Because methyl orange dissolves in water to make a colorful solution, the amount of dyein 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, themore methyl orange will be decomposed and the less light the solution will absorb

afterward.

Figure 16. The organic dye, methyl orange.


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You will test the powders photocatalytic activities using a combination of UV andvisible light and using only visible light to investigate whether formation of the Zn1-xMxOsolid 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 ofmaterials.

2. Overview of this Research Activity

You will use the thin films of ZnO and Zn1-xMxO that you prepared in the previous

weeks 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 atabsorbing 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-Visabsorbance 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 inabsorbance will indicate the effectiveness of your semiconductor at photocatalytically

degrading the methyl orange molecules.


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 ZnOand Zn1-xMxO films. Plan control experiments that will account for the effects of

variables such as the light from the labs overhead lights and any decrease of methyl

orange concentration in the solution due to its absorption to the surface of thesemiconductor powders.


Materials and chemicals required to prepare ZnO and Zn1-xMxO films by spray pyrolysis

Razor bladesMethyl orange dye

Stir plate and magnetic stir barUV lamp (provides UV and visible light)

Fluorescent lamp (provides visible light only)

Cuvettes

Pipettes


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5. Procedure

Prepare an aqueous solution containing 3 mg methyl orange per 10 mL solution. Planahead 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 thepowders settle to the bottom of the beaker and then transfer ~ 2 mL of the methyl orangesolution to a cuvette for UV-Vis measurements. Dont forget to zero the UV-Vis

spectrometer with water. Measure the UV-Vis absorption spectrum of the sample, returnthe 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.


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 yourdata tell you about your films ability to decompose methyl orange?


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Submission of Samples to Dr. Chois LabIf 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 tinoxide, FTO) and send them to the authors lab. We will assemble a photoelectrochemical

cell using your samples for further characterization. Deposition on a conductingsubstrate 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 youshould 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 UV-

Vis data for methyl orange solutions showing that your Zn1-xMxO sample exhibits a better

photocatalytic property than ZnO.


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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 thelocalization 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, thusconducting electricity well.

conduction band An electron band containing energy states in which electrons are freeto 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 amass, commonly expressed in Amperes (A).

electric potential The difference in energy per unit charge, commonly expressed involts (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. 191 1.602 10eV 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.


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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 thatmay also contain oxygen, nitrogen, phosphorous, and/or sulfur atoms.

photocatalysis The process by which the activation energy of a reaction is decreasedby energy obtained from light.

photoelectrochemical cell A tool which converts light into chemical and electricalenergy.

photoelectrode A semiconducting electrode used for solar energy conversion.

photon A particle, or quantum, of electromagnetic energy.

Plancks constant (h) A constant which relates the wavelength (v) and energy () of

light, equal to 236.63 10 ( )JouleSeconds J s .

radicals Molecules with a lone, unpaired electron that are very reactive with organicmolecules.

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 sampleand 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 beamby the sample.

speed of light (c) A constant defining the speed at which electromagnetic radiation

travels in a vacuum. 83 10 /c m s= .

spray pyrolysis A process in which a thin film is deposited by spraying a solution on aheated surface, where the solutes react to form a chemical compound.


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substrate For the purpose of this module, the substrate is the material upon whichdeposition 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.


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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 SprayedThin 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, C2005, 6, 186-205.

Fox, M. A.; Dulay, M. T., Heterogeneous Photocatalysis. Chemical Review 1993, 93,

341-357.

Grtzel, 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-of-the-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 Environment2002, 291, 85-97.

Turner, J. A., Sustainable hydrogen production. Science 2004, 305, 972-974.


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Available Metal NitratesAbbr. Transition Metal Formula in Nitrate CAS Aldrich Price/500gHazards

*

Zn Zinc Zn(NO3)2 6H2O 10196-18-6 228737 38.1 Irritant

Cu Copper Cu(NO3)2 2.5 H2O 19004-19-4 223395 80.9 Causes Burns

Ni Nickel Ni(NO3)2 6H2O 13478-00-7 244074 50.7 Carcinogen

Co Cobalt Co(NO3)2 6H2O 10026-22-9 230375 112Possible Carcinogenicity,

Environment

Fe Iron Fe(NO3)3 9H2O 7782-61-8 F3002 41.1 Irritant

Mn Manganese Mn(NO3)2xH2O 15710-66-4 288640 46.1 Irritant

Cr Chromium Cr(NO3)3 9H2O 7789-02-8 239259 67.1 Irritant

V Vanadium

Ti Titanium

Sc Scandium Sc(NO3)3xH2O 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, EnvironmentPd Palladium Pd(NO3)2 2H2O 10102-05-3 76070 162.5/5g Causes Burns

Rh Rhodium RhNO3 13465-43-5 Fisher 3000/1g

Ru Ruthenium

Tc Technetium

Mo Molybdenum

Nb Niobium

Zr Zirconium ZrO(NO3)2 xH2O 14985-18-3 243493 238.5 Causes Burns

Y Yttrium Y(NO3)3 6H2O 13494-98-9 237957 216.5 Irritant

Hg Mercury Hg(NO3)2 H2O 7783-34-8 230421 166/250gIrritant, Cumulative Effects,Environment

Au Gold AuNO3 13464-77-2 VWR 245.61/1g

Pt Platinum

Ir Iridium

Os Osmium

Re Rhenium

W Tungsten

Ta Tantalum

Hf Hafnium

La Lanthanum La(NO3)3xH2O 100587-94-8 238554 160 Irritant

Ds Darmstadtium

Mt Metinerium

Hs Hassium

Bh Bohrium

Sg Seaborgium

Db Dubnium

Rf Rutherfordium

Ac Actinium

*All may cause fire when making contact with a combustible!


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Information for the InstructorIt 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 somethan 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 theyare gathering information that is new and useful.

Band Gap Module

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Transcript of Band Gap Module