Rajendra Kasinath, PhD Department of Environmental

1
Nanotechnology enabled environmental
sustainability

• Rajendra Kasinath, PhD
• Department of Environmental Engineering
• Montana Tech of the University of Montana

2
The world right now..
3
Overview

• Emergence of more than a billion new consumers
  from over 20 developing countries
• Newly acquired spending capacity increasing
  global CO2 emission at an alarming rate
• For every 1 billion barrel of new oil discovered
  we are consuming 4 billion barrels
• Extraction from shale and tar sands are not
  energy efficient

4
Impetus for change

• Climate change accompanying increased green-house
  gas emissions
• The United Nations Framework Convention on
  Climate Change has called for the stabilization
  of greenhouse-gas concentrations in the
  atmosphere
• This needs to be at a level that would prevent
  dangerous anthropogenic interference with the
  climate system
• 10 TW (10 x 1012 watts) of carbon-emission-free
  power needs to be produced by the year 2050 to
  enforce this stabilization
• This is close to the power provided by all of
  todays energy sources combined

5
Impetus for change

• To meet increasing energy demand environmentally
  clean alternative energy resources need to be
  considered
• Three major options are at our disposal to tackle
  10 TW clean energy generation

6
Impetus for change

• Carbon neutral energy (fossil fuel in conjunction
  with carbon sequestration),
• Nuclear power, and
• Renewable energy

7
Impetus for change

• To produce 10 TW energy using fossil fuels
  without affecting the environment, we need to
  find secure storage for 25 billion metric tons of
  CO2 produced annually (equal to the 12,500 km3 or
  the volume of Lake Superior)
• Nuclear power would mean construction of a new 1
  GW electric nuclear fission plant everyday for
  the next 50 years (definitely somewhere on this
  planet)

8
Renewable options

• Renewable energy alternatives include
• hydroelectric resources (0.5 TW), from all tides
  ocean currents (2 TW)
• geothermal resource integrated over all available
  land area (12 TW)
• globally extractable wind power (2-4 TW), and
• solar energy striking the earth (120,000 TW)
• Solar energy stands out as the most viable choice
  to meet energy demand
• Despite this vast resource, the energy produced
  from sunlight remains less than 0.01 of the
  total energy demand

9
Nanotechnology to the rescue?

• Nanotechnology uses new paradigms and
  physiochemical laws at the nanoscale to enable
  novel, cost effective ways to enable existing
  technologies, and provide platforms to discover
  new ones

10

• Energy technologies
• Solar (photovoltaic and photosynthesis)
• Hydrogen fuel cells
• Energy storage
• Rechargeable batteries
• Super-capacitors
• Zero waste manufacturing

11
Strategies for harvesting solar energy

• Three major ways to use nanostructures for the
  design of solar energy conversion devices
• mimicking photosynthesis with donor-acceptor
  molecular assemblies and clusters
• semiconductor assisted photocatalysis to produce
  fuels such as hydrogen
• nanostructured semiconductor based solar cells

12
Donor-acceptor hybrid assemblies

• The essential roles of Chla are to
• capture solar energy
• transfer the energy to special locations, and
• bring about charge separation for the subsequent
  electron-transfer processes
• Based on the principle of photosynthesis, a
  variety of donor acceptor dyads and triads have
  been synthesized as light harvesting assemblies

13
State of the art

• Organized inorganic-organic nano-hybrids, with
  hierarchical architecture have been developed by
  assembling monolayers of organic molecules
  containing functional groups, such as
• amines, thiols, isothiocynate, and silanes, on to
  3Dprotected metal clusters (MPCs)
• functionalized with chromophores by place
  exchange reactions (e.g. porphyrin-alkanethiolate
  monolayer protected-gold nanoclusters)
• These hybrids form spherical shape clusters that
  can be employed as light harvesting antenna
• Exhibit efficient light-harvesting capability and
  suppress undesirable energy transfer quenching of
  the porphyrin singlet excited-state by the gold
  surface relative to the bulk gold

14
Photoinduced electron transfer

• Semiconductor nanoparticles are known to accept
  electrons from an excited sensitizer and transfer
  the electrons to another acceptor molecule bound
  to the surface
• Has led to semiconductor nanoparticle mediated
  electron transfer between donor and acceptor
  molecules
• The non-metallic property of ultra-small metallic
  particles can also be utilized to capture
  electrons from an excited sensitizer

15
Photoinduced electron transfer
16
Catalysis with Semiconductor/metal nanocomposites

• Semiconductor nanoparticles when subjected to
  band gap excitation undergo charge separation
• Small size of particles and high recombination
  rate leads to a fraction of these charges that
  can be utilized to induce redox processes at the
  interface
• Photocatalytic processes using TiO2 and other
  semiconductors have demonstrated the need to
  overcome the limitations in achieving higher
  photo-conversion efficiencies
• Particular interest is in the use of
  semiconductor nanostructures for solar hydrogen
  production by the photocatalytic splitting of
  water
• Efforts to employ semiconductor-semiconductor or
  semiconductor- metal composite nanoparticles have
  been explored to facilitate charge rectification
  in the semiconductor nanostructures
• The deposition of a noble metal on semiconductor
  nanoparticles is beneficial for maximizing the
  efficiency of photocatalytic reactions

17
Photochemical solar cells
18
Photochemical Solar Cells

• Various strategies have been developed in recent
  years to construct photochemical solar cells
  using organized assemblies of nanostructured
  architectures
• Many of these systems have the potential to
  develop into third generation solar cells
• Four promising strategies currently employed are
• donor-acceptor based molecular clusters
• dye sensitization of semiconductor
  nanostructures,
• Quantum dot solar cells, and
• carbon nanostructure based solar cells

19
Dye Sensitized Solar Cells

• The process of utilizing sub-bandgap excitations
  with dyes is referred to as photosensitization
• Conveniently employed in silver halide
  photography and other imaging science
  applications

20
Comparison to existing solar cells
21
Photocurrent Generation Mechanism

• Primary event responsible for photocurrent
  generation is the photo induced charge separation
  between excited porphyrin and nano-entities
• Electron transfer is completed in the
  sub-nanosecond time scale
• The reduced nanoparticles inject electrons into
  SnO2 nanocrystallites and the oxidized porphyrin
  undergoes electron transfer with iodide ions

22
Quantum dot photosynthesis

• Semiconductor films provide an efficient method
  to mimic photosynthesis
• Charge separation is facilitated by semiconductor
  nanoparticle and the high porosity of mesoscopic
  semiconductor films
• enables incorporation of sensitizing dyes in
  large concentrations
• Currently Nano-TiO2 films modified with a
  ruthenium complex exhibit photo-conversion
  efficiencies in the range of 11, which is
  comparable to that of amorphous silicon-based
  photovoltaic cells

23
Quantum dot photosynthesis

• When the electrode is illuminated with visible
  light, the sensitizer molecules absorb light and
  inject electrons into the semiconductor particles
  
• These electrons are then collected at the
  conducting glass surface to generate anodic
  photocurrent

24
Quantum Dot Solar Cells

• Ordered assemblies of narrow band gap
  semiconductor nanostructures are convenient
  systems by which to harvest visible light energy
  if employed as electrodes in photoelectrochemical
  cells
• The photocurrent obtained using such nanoparticle
  assemblies is often low as fast charge
  recombination limits photocurrent generation
• By employing composite semiconductors it has been
  possible to improve the efficiency of charge
  separation through charge rectification
• Chemically and electrochemically deposited CdS
  and CdSe nanocrystallites are capable of
  injecting electrons into wider gap materials such
  as TiO2, SnO2, and ZnO generating photocurrents
  under visible light irradiation

25
Advantages

• Specific advantages of using semiconductor
  quantum dots as light harvesting assemblies in
  solar cells are due to
• The size quantization property allows tuning the
  visible response and varying the band offsets
  allows modulation of charge transfer across
  different sized particles
• Quantum dots open up new ways to utilize hot
  electrons or generate multiple charge carriers
  with a single photon

26
Scaling into devices

• Possible application of such photocatalyst based
  hybrid cells can be visualized in outdoor
  fixtures where single stack fuel cells can be
  spread out to capture sunlight
• The rising clean energy demand will compel us in
  the near future to find hybrid devices that are
  tailored to specific applications

27
Rechargable batteries

• Most of the active research is currently focused
  on rechargeable lithium batteries (roughly 10
  billion/year)
• As compared with aqueous batteries Li-ion
  chemistry leads to an increase of 100150 on
  energy storage capability per unit weight and
  volume
• Disadvantages include low energy and power
  density, large volume change on reaction, safety
  and costs

28
Rechargable batteries

• These shortcomings are being reduced by
  nanotechnology
• Nano-materials are being researched for both the
  electrodes and the non-aqueous electrolyte
  systems
• Recent findings include
• up to six times increases in electrolyte
  conductivity by introducing nanoparticles of
  alumina, silicon or zirconia to non-aqueous
  liquid electrolytes

29
Rechargeable batteries

• Sony Corporation has commercialized a tin-based
  anode nanobattery called Nexelion
• Toshiba Corporation has announced a breakthrough
  in lithium-ion batteries that dramatically
  reduces recharge times
• the new nanobattery can recharge 80 of the
  batterys energy capacity in only 1 min,
  approximately 60 times faster than the typical
  lithium-ion batteries in use today
• nanostructuration of the cathode has also been
  explored with transition-metal dioxides,
• LiFePO4, LiMn2O4 and vanadium oxide (V2O5)
  cathodes
• Manganese oxides are especially suitable for both
  environmental and economical reasons

30
Super-capacitors

• Electrochemical capacitors (ECs), also called
  supercapacitors, are energy storage devices that
  differ from batteries
• While batteries store energy chemically,
  ultracapacitors store electricity physically, by
  separating the positive and negative charges
• These devices have attracted considerably less
  attention than batteries as energy storage
  devices
• Nanotechnology and better understanding of ionic
  behavior in small pores have rekindled interest
  in ECs

31
Super-capacitors

• The drawbacks of classical capacitors are
• high cost of premium performance electrodes due
  to miniaturization
• large requirements for long cycle life, and
• low efficiency materials that combine both high
  surface area with a low resistively are required
• Main determining factor for power density and
  maximum power output is the surface area of each
  electrode

32
Nanostructured Supercapacitors

• Use of nanostructured materials dramatically
  increases this surface area (up to 1000 m2/g
  using carbon)
• In contrast to capacitors, supercapacitors
  utilize a small volume of electrolyte
• Interacts with the surface of each electrode to
  store charge
• Offers a unique combination of high power and
  high energy performance

33
Nanostructured Supercapacitors

• Types of ECs
• pseudocapacitors,
• electrochemical double layer capacitors (EDLCs),
  and
• hybrid capacitors
• Nanostructuration of materials for
  pseudocapacitors could considerably improve their
  performance since stored charge is directly
  proportional to the electrode surface

34
Nanostructured Supercapacitors

• EDLCs are currently the most common devices
• Technologies are mainly based on blending porous
  materials (like activated carbon) with a
  conductive additive (like graphite or metals)
• A transition from activated carbon electrodes to
  carbon based nanostructures has been carried out
  order to improve the performance of these devices
  (100 kW A/kg)

35
Conclusions

• Nanotechnology or the use of nanostructures might
  pave the way for sustainable energy harvesting
  technologies
• The relevant nano-entities find applications in
  quantum dot enabled solar cells , as efficient
  charge transfer and mediation agents, and
  modified electrolytes for greater activity in
  supercapacitors

36
Conclusions

• Nanotechnology to the rescue?
• Perhaps, with new discoveries and the growing
  rise of new discovered emergent properties of
  nanomaterials
• However, as guardians of the environment we need
  to question the implications of anthropogenic
  nanomaterials exposure and their short and long
  term effects on health and planetary ecology

37
Acknowledgements

• PNWIS organization committee for the invitation
  to give this talk
• Prof. Kumar Ganesan for his guidance in
  application of nanotechnology in environmentally
  sustainable applications
• The students in our labs for being there to
  research new ideas

38
Applause..

• Any Questions?