A Study of Zinc Oxide Nanoparticles, its bioactivity and
applications
By:
Drew Williams
Devin GatherWright
Travis Watts
Dylan Terry
Abstract
Zinc oxide is an inorganic compound that occurs in the Earth’s crust as a mineral. It has
the formula ZnO and is a white powder that is insoluble in water, which is widely used
as an additive in numerous materials and products including plastics, ceramics, paints,
and ointments. It occurs naturally as the mineral zincite but because of its high industrial
demand over the years most zinc oxide is now produced synthetically. This makes zinc
oxide a very abundant mineral and thus one of its biggest advantages is that it can be
produced and used commercially at a low cost. Because of this low cost of production it
has found itself to be very useful for many industrial, medical and cosmetic applications.
More recently the availability of nanotechnology has made the use of zinc oxide
nanoparticles a more readily available option and has found its way into the commercial
production line. This study sought to look and the properties and applications of zinc
oxide nanoparticles and the possible disadvantages that come with using a particle on
the nanometer scale.
Focusing primarily on the electrical and structural properties and also the medical and
coating applications of the zinc oxide nanoparticles, the study will use qualitative and
graphical representation to assess what makes zinc oxide nanoparticles a beneficial
material that can be used in everyday life.
Table of Content
I.
Introduction
1
II.
Background
2
III.
Core Content
5
a. The Structure of Zinc Oxide Crystals and Zinc Oxide Nanoparticles 5
b. Zinc Oxide Properties
5
c. Zinc Oxide for Medical and Cosmetic Applications
6
d. Zinc Oxide as a Semiconductor
7
IV.
Conclusion
10
V.
Future Work
11
i
List of Tables and Figures
I.
Figure 1: Zinc Oxide Nanoparticles
1
ii
Introduction
Nanoparticles have one dimension that will measure 100 nanometers or less
across its body. When a material is changed to a nanoparticle often times the properties
of the material will change with it. This is because nanoparticles generally have a
greater surface area per weight ratio than most standard/larger particles. In turn this
makes nanoparticles more reactive than other materials of greater size.
Zinc oxide nanoparticles do not occur in nature and typically synthesized in order
to meet the industrial needs of the zinc oxide we use in our everyday lives. The
compound is into individual particles as small as 20 nanometers in diameter. The
transparent particles, which effectively filter out UVA and UVB light are then coated with
inert silicon and aluminum oxide layers and tend to clump together into groups that are
roughly 200-500 nanometers in diameter (Hawk, What is Nanoparticle Zinc Oxide).
Figure 1: Zinc Oxide Nanoparticles.
The use of nanotechnology is recent technology that has seen very little use when
compared to the other technologies that humans have been using. In the medical field,
nanotechnology has represented a new and enabling platform that for sees a broad
range of uses and improved technologies for biological and biomedical applications.
Researchers have begun to use nanoparticles that form clumps when they attach
themselves to a certain protein or other molecule to indicate the presence of a disease
they are searching for (Rasmussen, Zinc Oxide Nanoparticles). The use of
1
nanoparticles has also been inherited in the fight against cancer. Magnetic
nanoparticles can attach to the cancer cells in the blood stream. These nanoparticles
may allow doctors to remove the cancer cells before a new tumor begins to grow. Here
are several other uses of nanoparticles in the medical field:

Quantum Dots (crystalline nanoparticles) that identify the location of cancer cells
in the body.

Nanoparticles that deliver chemotherapy drugs directly to cancer cells.

Gold nanoparticles that allow heat from infrared lasers to be targeted on cancer
cells. ‘

A nanoparticle cream that releases nitric oxide gas to help with staph infections
(Understandingnano.com).
Although there are many uses for nanoparticles in the medical field, the most widely
used nanoparticle used for medical and cosmetic reasons is the use of zinc oxide in
sunscreen lotions and creams. This is due to zinc oxides ability to absorb ultraviolet and
infrared radiation across almost the whole electromagnetic spectrum.
In addition to medical applications, zinc oxide is widely used in electrical applications
as well. Zinc oxide is a semiconductor has a relatively large direct band gap of 3.4 eV at
room temperature. With this large band gap zinc oxide has advantages such as a higher
breakdown voltage, an ability to sustain large electric fields, a low electric noise, and is
able to operate in high temperature/power operations. Zinc oxide is not new to the
semiconductor field and thus it has applications in lasers and LED applications.
Zinc oxide is a material that can be easily made synthetically and it also has a wide
variety of applications that can be used in everyday life safely. This has made it a high
demand material thus it has become a very cost effective material.
Background
Zinc oxide is an inorganic compound and rarely is formed in nature, especially in
a crystal-like form, according to znoxide.org. Rare zinc oxide that is formed in nature is
commonly red or orange in color due to a manganese impurity in the material
(znoxide.org). Zinc oxide has a wurzite hexagonal crystal structure that can be viewed
via an electron microscope (znoxide.org). Furthermore, the exact shape and size of the
2
crystal structure depends on how the crystal is formed (znoxide.org). The two main
shapes that are formed are acicular needle and plate shaped crystals, though it should
be noted that zinc oxide can be used to create a vast variety of crystal shapes by
utilizing unique deposition methods (znoxide.org).
According to the list of physical properties of zinc oxide provided by znoxide.org,
the molecular weight is 81.37 and its relative density is 5.607 respectively (znoxide.org).
Furthermore, the melting point of zinc oxide is around 1975 Co, and that’s observed
under a very high pressure (znoxide.org). The color of manufactured zinc oxide (which
is zinc oxide that does not occur in nature,) is simply white, and it should also be noted
that single crystal oxide is colorless, or transparent in nature (znoxide.org).
Furthermore, according to the list of physical properties of zinc oxide on znoxide.org,
zinc oxide is known to change lemon yellow when heated and changes back to white
when cooled (znoxide.org).
According to the list of physical properties of zinc oxide provided by znoxide.org,
zinc oxide exhibits the characteristics of an n-type semiconductor in its pure, normal
state (znoxide.org). This means that zinc oxide has a negative conductivity due to its
excess of electrons. This is caused by a large excess of zinc ions that float around the
interstitial locations in the crystal lattice (znoxide.org). When zinc oxide is doped with
other elements in order to take the place of either the zinc or the oxygen atoms in the
material, the conductivity of zinc oxide can then greatly changed over a wide range
(znoxide.org). However, p-type doping of zinc oxide has proven to be difficult due to
hydrogen atoms in the atmosphere affecting the doping process, which has recently
been rectified as you will soon see below in the electrical properties of zinc oxide
section in this report. Furthermore, according to znoxide.org, both p and n-type
materials are a must for creating pn junctions in semiconductors, thus enabling their
unique attributes and uses (znoxide.org). It should be further noted that zinc oxide
crystals possess rectification abilities and are used as rectifiers in radio receivers (at
least the very early ones) (znoxide.org).
It has previously been noted that zinc oxide crystals are transparent in visible
light, but they are also very absorbent of ultra violet light (znoxide.org). Furthermore, it
should be noted that zinc oxide’s absorption of the aforementioned ultra violet light rays
3
is much stronger than that of other pigments that can be considered white in nature
(znoxide.org). The band gap energy of zinc oxide is 3.2 ev, and this means the gap
between the valence and conducting bands in the atoms, which cause electrical current
when the electrons move between the two (znoxide.org). It should be noted that zinc
oxide is photoconductive and, according to znoxide.org, was the basis of the first
Electrofax copying machine (znoxide.org). Zinc oxide’s combination of both optical and
semiconductor properties enables doped zinc oxide’s for a slew of forthcoming cuttingedge electronic devices – for example, according to znoxide.org, solar cells need a type
of “transparent conductive coating,” and utilize zinc oxide for use in this type of coating
(znoxide.org).
Zinc oxide’s chemical properties are optimal. According to znoxide.org, zinc
oxide is classified as an “amphoteric,” meaning zinc oxide reacts to both alkalis and
acids respectively (znoxide.org). Zinc oxide’s reaction with acid forms zinc sulfate,
which is a pretty common compound (znoxide.org). Inversely, when zinc oxide is mixed
with alkali, they form zincates, which are not pretty common (znoxide.org). Zinc oxide’s
and carbon monoxide’s equilibrium is at around 1,000 Co, with the lower temperatures
of zinc oxide and carbon monoxide being generally chosen (znoxide.org). Furthermore,
it should be noted that zinc oxide reacts with fatty acids like stearic by mixing and
heating zinc oxide and stearic directly and above the acid melting point (znoxide.org). It
should be further noted that zinc oxide has a low water solubility of about 0.005 g/litre
respectively (znoxide.org). Also, zinc oxide that is exposed to air is able to take in both
carbon dioxide and water vapor, thus in this act of absorption forming a material that is
known as zinc carbonate (znoxide.org). Lastly, in regards to zinc oxide’s optimal
chemical properties, zinc oxide is also able to undergo solid state reactions or
calcination at high temperatures (znoxide.org).
Some neat applications of zinc oxide include the following: rubber such as in
automobile tires; ceramics and glass; animal feed and human feed; pharmaceuticals
such as calamine lotion and sunscreen due to its antiseptic nature; paint, lubricant; fire
retardant materials; zinc salt; and phosphate coating to name a few.
4
Content
The Structure of Zinc Oxide Crystals and Zinc Oxide Nanoparticles
The properties of crystalline solids depend on the crystal structure of the
material, the manner in which atoms, ions, or molecules are spatially arranged. There is
an extremely large number of different crystal structures all having large-range atomic
order.
Zinc oxide is an inorganic compound with a formula ZnO. The structure of
zinc oxide crystals has two forms, cubic zinc blend and hexagonal wurzite. The most
common and stable of the structures is the wurzite structure. Hexagonal and zinblende
polymorphs do not have an inverse symmetry. Because of this and other properties
hexagonal ZoN results in piezoelectricity, while zincblend ZoN results in pyroelectricity.
The wurzite structure has a point group C6v , the space group is P63mc.
The lattice constants are a=3.25A and c = 5.2A which makes the ratio close to ideal for
hexagonal which is 1.633. The bonding in zinc oxide crystals is ionic. ZnO has
corresponding radii of 0.074 nm for ZN2+ and 0.140 nm for O2-. These properties are the
preferred formation for wurtzite rather than zinc blende structure as well as the strong
piezoelectricity of ZnO. Because of polar Zn-O bonds, zinc and oxygen planes are
electrically charged. To be able to maintain their neutrality, the planes reconstruct at
atomic level in most materials but not ZnO because its surfaces are already flat stable
and need no reconstruction.
Zinc Oxide Properties
The properties of crystalline solids depend on the crystal structure of the
material, the manner in which atoms, ions, or molecules are spatially arranged. There is
an extremely large number of different crystal structures all having large-range atomic
order.
Zinc oxide is an inorganic compound with a formula ZnO. The structure of
zinc oxide crystals have two forms, cubic zincblend and hexagonal wurzite. The most
common and stable of the structures is the wurzite structure. Hexagonal and zinblende
5
polymorphs do not have an inverse symmetry. Because of this and other properties
hexagonal ZoN results in piezoelectricity, while zincblend ZoN results in pyroelectricity.
The wurzite structure has a point group C6v , the space group is P63mc.
The lattice constants are a=3.25A and c = 5.2A which makes the ratio close to ideal for
hexagonal which is 1.633. the bonding in zinc oxide crystals is ionic. ZnO has a
corresponding radii of 0.074 nm for ZN2+ and 0.140 nm for O2-. These properties are the
prefered formation for wurtzite rather than zinc blende structure as well as the strong
piezoelectricity of ZnO. Because of polar Zn-O bonds, zinc and oxygen planes are
electrically charged. To be able to maintain their neurtality, the planes reconstruct at
atomic level in most materials but not ZnO because its surfaces are already flat stable
and need no reconstruction.
On another note, some of the other properties of Zinc Oxide Nanoparticles
is that it is used to absorb UV rays or scatter them. A lot of sunscreen consists of Zinc
Nanoparticles for this very reason, along with other ingredients but the main application
For this very reason, they are also used in solar cells to absorb the light
from the sun so it can be converted into useable power. When dispersed in a
semiconducting polymer, it has capabilities to convert, at 500nm, up to 40% of incident
photons.
Zinc Oxide for Medical and Cosmetic Applications
Zinc oxide nanoparticles have found itself to be very useful in our modern
society. Zinc oxides properties allow it have the ability to absorb electromagnetic waves
and can be used as a coating to absorb waves throughout the electromagnetic
spectrum on a material. It also has applications on our mobile phones as a radioactive
shield
Most of zinc oxides applications, however, lie in the medical and cosmetic fields.
Zinc oxide can be used as an antibiotic to treat a number of skin irritations. It is typically
used to treat minor burns, diaper rash, and pain relief from skin irritations. Its ability to
treat skin irritations and give pain relief has made it an important ingredient to treat
6
hemorrhoids. It is even a source of nutrients for the body as it helps the body carry out a
wide range of biochemical reactions and keeps the immune system healthy.
Probably one of the most popular applications of zinc oxide is its use as a
protective coating. Its unique ability to absorb UV rays and near infrared rays across the
broadest spectrum of ultraviolet radiation has allowed it to be approved by the FDA to
be used in sunscreen lotions and creams. This makes one of the seventeen approved
active ingredients approved for sunscreen applications. There has been some
controversy, however, about the size of the zinc particles that have been going into the
lotions and creams being used on the human body. An article by the Badger Balm
Company states that when a substance is so small that it is measured in nanometers,
the surface area to volume ratio is so great that sometimes the properties of the
substance can change. This is why the Badger Balm Company is using zinc oxide
particles that are greater than 30 nanometers. The biggest concern with nanoparticles in
cosmetics is the threat of inhalation powders. It has thus been determined that zinc
oxide is the safest, most effective sunscreen additive. The mineral will sit on top of the
skin and block almost any kind ultraviolet radiation the sun can throw at it.
Zinc Oxide as a Semiconductor
According the article “An Old Dream Fulfilled: Zinc Oxide As Semiconductor”
published on Science Daily.com, zinc oxide has significant semiconductor properties
and is classified as a n-type semiconductor, which means that current carriers in zinc
oxide possess a negative charge. However, the proper amount of doping needed to
make zinc oxide into a feasible p-type semiconductor device had yet to be discovered
by scientists until recently. According to the article, scientists at Ruhr University in
Bochum, Germany claimed via experimental evidence that hydrogen atoms in the
atmosphere interfere with zinc oxide’s doping process, which is the addition of impurity
atoms within the crystal lattice of zinc oxide, thus preventing zinc oxide from reaching its
optimum potential as a p-type semiconductor (ScienceDaily.com). To further break
down the process, the foreign or impurity atoms added during the doping process either
give away an electron (which is known as n-doping,) or gain an electron (which is
known as p-doping;) therefore, these moving electrons are responsible for enabling the
7
electric conductivity of the semiconductor material, thus allowing its conductivity
properties to be active. According to the article, impurities commonly used in
semiconductor devices, such as silicon and germanium, have proven to be ineffective
when doped with zinc oxide, especially when it comes to p-doping (current carriers
possess a positive charge,) which is partly responsible for creating semiconductor
products such as light emitting diodes (LEDs) and transistors which need both n-type
and p-type material in order to perform their unique tasks (ScienceDaily.com). However,
despite this exciting new discovery, there is still a laundry list of problems when it comes
to p-doping zinc oxide, namely the fact that hydrogen will always be present during the
doping process due to the fact that a large percentage of it makes up Earth’s
atmosphere (Science Daily.com). Furthermore, as stated previously above, hydrogen
atoms present during the process will always result in n-doping of the zinc oxide, which
as previously stated above, is the giving away of an electron and will result in an
negative charge due to the abundance of free electrons. However, according to the
article, the scientists of Ruhr University believe they can get around the n-doping hurdle
by doping the zinc oxide substrates with hydrogen, then annealing the hydrogen via a
heat treatment (ScienceDaily.com). According to the article, the Ruhr University
scientists utilized a unique technique for measurements and varying temperatures in
order to confirm that the charge carrier concentrations were corresponding with each
other. According to the article, to further explain the doping process, a relatively high
density of these change carrier corresponding concentrations is needed to ensure that
the electronic device/devices function at an optimum level. As mentioned before,
hydrogen impurities are impossible to avoid during the doping process, and therefore
result in negative doping instead of the positive doping desired due to the electrons
given away by the hydrogen atoms to the zinc oxide atoms, which fill the holes caused
by the desired p-doping, thus resulting in n-doping (ScienceDaily.com). Because of this,
the way to create the intrinsic zinc oxide desired, the doping must be conducted in a
pure, hydrogen-free environment; according to the article. Furthermore, according to the
article, it was believed by scientists and the academic world that the aforementioned
doping problems with zinc oxide were due to imperfections in the crystal lattice of zinc
oxide (Science Daily.com), until the research of the Ruhr University scientists came to
8
light, which can potentially make the production of optimal, high-performance zinc
oxide-based electronics possible once more research by the Ruhr University scientist is
conducted. According to the article, Ruhr University scientists intend to achieve pdoping by adding the appropriate foreign atoms to zinc oxide in a hydrogen-free
environment (Science Daily.com). Also, aside from zinc oxide’s aforementioned n-type
and p-type semiconductor properties, zinc oxide also possesses piezoelectric
properties. According to the Materials Science and Engineering textbook, piezoelectric
can be defined “a dielectric material in which polarization is induced by the application
of external forces” (Callister; Rethwisch). So, in short, a piezoelectric can be summed
up as a material whose polarization is created by the application of a mechanical stress,
which creates an electrical charge through the material. An example of zinc oxide’s
piezoelectric capabilities is discussed in the next paragraph.
Some electrical applications of zinc oxide include the aforementioned light-emitting
diodes (LEDS) and thin-film transistors. According to an article entitled “Zinc Oxide
Wires Boost LED Performance” published on furturity.org, engineers at Georgia Tech
University developed zinc oxide microwires that greatly enhance just how quickly and
how efficiently LEDs can convert electricity to ultraviolet light (futurity.org). Furthermore,
according to the article, these zinc oxide microwires are believed to be the “first gallium
nitride light-emitting diodes” in which the performance of the light-emitting diode has
been enhanced by the development of an electrical charge in a piezoelectric material
such as zinc oxide using the piezo-phototronic effect (futurity.org). According to the
article, this is accomplished by the application of a mechanical strain to the zinc oxide
microwires, in which the engineers at Georgia Tech University were able to create a
piezoelectric potential in the zinc oxide microwires, which enhances the speed of the
light-emitter diode’s carrier injection of electrons, thus allowing a faster charge through
the light-emitter diode (futurity.org). Also, the resulting piezoelectric potential from the
applied mechanical stress allows engineers to successfully “tune” the transport of the
charge throughout the light-emitter diode, meaning to either enhance or impede the
charge to a degree that is suitable for the application of the light-emitter diode
(furturity.org). This aforementioned enhancement is due to the fact that zinc oxide is a
piezoelectric material as mentioned previously above. Zinc oxide’s piezoelectric
9
capabilities are further explained by the article. The article states that the polarization of
ions that occurs in the crystals of zinc oxide can be mechanically compressed by an
external stress to create an electrical charge that flows through the zinc oxide.
Furthermore, according to the article, due to the aforementioned electrical charge, the
ability to “tune” the charge transport (meaning the rate that the electrons flow through
the material,) at the p-n junction of the zinc oxide; meaning the boundary between the ntype and p-type portions of the semiconductor material that the electrons must cross
(futurity.org). The enabled tuning at the p-n junction enables engineers to therefore
increase the flow at which free electrons and holes left by free electrons recombine with
each other, therefore creating photons, which, according to the article, are responsible
for enriching both the light emission and injection current of the light-emitting diode
(futurity.org).
Conclusion
In conclusion zinc oxide has been a very important mineral throughout history.
Originally it was a natural product that could be found in nature, but finally became a
product that had to be synthetically made in order to fulfill demand. Once this could be
produced synthetically it made the product not so scarce and have good abundance so
that it could be used in many applications. The medical field has benefited very greatly
from the development of this product because of its uses in biological and biomedical
applications. The small particle size of zinc oxide is what has given it such great
applicability. Even though the structures have two forms the best form seems to be the
wurzite form because this form seems to be the most table and more worthy of being
used in the applications that have been listed.
The wurzite structure is also the more suitable form of zinc oxide because it is
electrically charged. This has made it suitable for the modern world because of its ability
to absorb electromagnetic waves. This affects most of us every day when we use things
like our cell phones and other things that are radioactive because zinc oxide serves as a
shield against such harmful rays. Not only does it protect from those harmful rays but it
is useful in the ointments and creams that we use daily. We even use this mineral on
10
our children. This mineral is included in the sunscreen that you put on your children so
that they can go and play in the sun without the worry of the ultraviolet rays. The
companies that use these substances in their products have to be very careful not to
make the particle to small in their products because of the discovery that once a
substance is measured in nanometers the substance has the ability to change its
properties which could cause big problems with the product and for the user.
Zinc oxide is a negatively charged particle and is classified as a semiconductor.
The atmosphere interferes with zinc oxides doping process and adds impurity with in its
crystal lattice preventing the zinc oxide from reaching its optimum potential as a p type
semiconductor. There are many problems when it comes to p doping zinc oxide.
Over all zinc oxide is a very useful substance in our everyday lives and is
involved in many of the things that everyone uses daily. Without zinc oxide much of
what we have would not exist or at least would not be of the quality that we are used to.
Zinc oxide is a very useful product that is of grat abundance and I am sure that it will be
be used in many applications in the future.
Future Work
Some of the Future works that zinc oxide nanoparticles have in store are,
in some cases, it has been looked at as an interceptor for certain kinds of tumors, and
has also be tested to destroy the tumors.
It’s also been looked at that it needs to be investigated further on its
capabilities and capacities for voltage in receiving sunlight. With today’s technologies
and advances it could one day result in having the ability to convert twice as much
sunlight as today’s solar cells to be applied to the modern power grid.
11
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Signature Page
Devin Gatherwright: researched and wrote the section on zinc oxide’s semiconductor
and electrical properties and applications. Also contributed to the background of the
report. 25%. All material came from outside sources and have been properly cited and
dated.
Drew Williams: researched and wrote the section on Zinc oxide applications in medical
and cosmetic industries. Also contributed to the introduction. 25%. All material came
from outside sources and has been properly cited and dated.
Dylan Terry: researched Zinc oxide properties. Also contributed to future works. 25%.
All material came from outside sources and has been properly cited and dated.
Travis Watts: research Zinc oxide nanoparticle structures. Also contributed to the
conclusion. 25%. All material came from outside source and has been properly cited
and dated.