Perovskites – and their Potential Use in Solar Energy Applications: A Current Commentary.

The following has just been published in the most recent (September 2014) issue of the journal Science Progress, of which I am an editor.The article may be downloaded for free from this link: http://stl.publisher.ingentaconnect.com/search/expand?pub=infobike%3A%2F%2Fstl%2Fsciprg%2F2014%2F00000097%2F00000003%2Fart00007&token=009719fc5a051792b68293c55505c544c5f3a3d5d68412b6e66667e2f5c752f7e687b76504c486646255c2a7b6c7a317b597c6a333f253f3568793c467d5f736a6f552b5f73bdbda2586f3d

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Introduction.

A material may be described as having a perovskite structure¹ if it has same type of crystal structure as perovskite - calcium titanium oxide (CaTiO₃) – itself does (Figure 1). Perovskite was first discovered in 1839 by Gustav Rose, in the Ural mountains in Russia, and is named after the Russian mineralogist L. A. Perovski (1792–1856), who first characterised the material. The general formula for perovskites is ABX₃, with 'A' and 'B' being two cations of significantly different sizes ('A' > 'B'), while X is an anion that binds with both cations. The perovskite structure is adopted by many oxides which have an elemental composition: ABO₃. The ideal cubic-structure has the ‘B’ cation in a 6-fold coordination, surrounded by an octahedron of anions, with the ‘A’ cation in a 12-fold cuboctahedral coordination. Cations 'A' occupy the cube corner positions (0, 0, 0), while cations 'B' occupy the body centred positions (1/2, 1/2, 1/2) with oxygen anions ‘O’ being located at face centred positions (1/2, 1/2, 0). Figure 1 shows edges for an equivalent unit cell with ‘A’ in body centre, ‘B’ at the corners, and ‘O’ in mid-edge. The requirements of relative ionic radii are quite exacting to maintain a stable cubic structure, meaning that even relatively minor degrees of buckling and distortion can result in a number of alternative versions with lower symmetry, in which the coordination numbers of either the ‘A’ cations, ‘B’ cations, or both, are reduced. Tilting of the BO₆ octahedron reduces the coordination of a too-small ‘A’ cation from 12 down to as low as 8. Conversely, when a small ‘B’ cation is brought off-centre, within its octahedral coordination, a stable bonding arrangement can be obtained. Such distortions can create an electric dipole, and it is for this reason that perovskites such as BaTiO₃, which distort in this manner, exhibit the property of ferroelectricity. The most usual non-cubic forms of perovskites are the orthorhombic and tetragonal variants. There are also some more complex perovskite structures which contain two different ‘B’-site cations, with the result that ordered and disordered variants are possible.
Figure 1 here.

Under the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSiO₃, is converted to a more dense perovskite-type polymorph, and indeed it is speculated that this particular phase of the material might be the most common mineral in the Earth.² It has a perovskite structure, with an orthorhombic distortion, and is stable at pressures from ~24 GPa to ~110 GPa. [For comparison, we may note that the pressure at the centre of the Earth is ca 300 GPa]. However, it is stable only at depths of several hundred kilometres and could not be transported back to the Earth’s surface without reforming into less dense materials. At yet greater pressures, MgSiO₃ perovskite undergoes a transformation to form post-perovskite. Although the most common perovskite compounds contain oxygen, perovskites containing fluoride anions are known, e.g. NaMgF₃. Metallic perovskite compounds also exist¹, with the general formula RT₃M, where R represents a rare-earth or other relatively large cation, T is a transition metal ion and M represents light metalloids (anions) which occupy the octahedrally coordinated ‘B’ sites, e.g. RPd₃B, RRh₃B and CeRu₃C. MgCNi₃ is a metallic perovskite compound, and is of particular interest on account of its superconducting properties. As a further category, are mixed oxide-aurides of Cs and Rb, such as Cs₃AuO, which contain large alkali metal cations in the traditional "anion" sites, bonded to O²⁻ and Au⁻ anions.

 

Properties of perovskites.

As noted, the perovskite structure is imparted with an appreciable element of structural pliancy, and the ideal cubic structure (Figure 1) can be distorted in many different ways. Thus, the octahedra may become tilted, the cations be displaced from the centres of their coordination polyhedra, and the octahedra might be distorted at the behest of electronic factors (e.g. Jahn-Teller distortions).² Accordingly, perovskite materials exhibit many unusual properties that are of theoretical interest, but which may also furnish practical applications. Such phenomena as colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin-dependent transport, high thermopower and the interleaving of structural, magnetic and transport properties are those typically observed from this family of materials. Thus, applications are found for perovskites in sensors and in catalyst electrodes for certain types of fuel cells, and they might play a future role in memory devices and spintronics devices.² Many superconducting ceramic materials (high temperature superconductors) have perovskite-like structures, generally incorporating three or more metals, copper often being one of them, and with some prevailing oxygen vacancies. In the latter regard, yttrium barium copper oxide can be made either insulating or superconducting according to its oxygen content. It is also of note that a cobalt-based perovskite material is being developed, intended to replace platinum in the catalytic converters of diesel vehicles.² In view of the limited availability of platinum, this would be a major advance. As we shall see, perovskites also offer the potential to be incorporated in efficient, and low-cost photovoltaic cells.

Photovoltaics

The cumulative global photovoltaic generating capacity had reached around 100 GW_p (gigawatts) by the end of 2012, 85% of which is derived from crystalline Si-cells, the remainder being from polycrystalline thin film cells, mostly containing cadmium telluride/cadmium sulfide³. While thin-film cells tend to be cheaper to make and offer a shorter energy payback time⁴, most of them rely upon rare elements such as tellurium (which is as rare as gold), indium, and gallium, all of which have issues over their future supply⁵, certainly if the global photovoltaic generating capacity is to be extended into the Terawatt (TW) realm³. On grounds of their relative cheapness and that a conversion efficiency of 15% has been achieved from them (i.e. as is competitive with thin-film photovoltaic technology⁴), synthetic perovskites are being explored as foundation materials for the manufacture of high-efficiency commercial photovoltaic devices (e.g. Figure 2). As a further convenience, they can be produced using the same thin-film methodology as is used to make thin film silicon solar cells.⁶ Organic-inorganic perovskite-structured semiconductors have shown promise as high-performance light-harvesting materials in solar cells, most commonly methylammonium lead triiodide (CH₃NH₃PbI₃), initially used as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures⁶. Such materials are found to possess both a high charge carrier mobility and a high charge carrier lifetime, meaning that light-generated electrons and holes can move over sufficiently long distances that an electric current may be extracted from them, as opposed to the excitation energy being merely dissipated as heat within the cell. The effective diffusion lengths are close to 100 nm for both electrons and holes in CH₃NH₃PbI₃.^7,8 Since low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole diffusion lengths (typically about 10 nanometers), this result is significant and deserves explanation. By applying femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective-electron or selective-hole extraction materials, balanced long-range electron-hole diffusion lengths of at least 100 nanometers have been confirmed to exist in solution-processed CH₃NH₃PbI₃.  It is concluded that the high photoconversion efficiencies of these systems are a result of the fact that the optical absorption length and charge-carrier diffusion lengths are comparable, so obviating the traditional constraints of solution-processed semiconductors⁸. Low-temperature solution methods (spin-coating) are employed for the deposition of the latter kind of perovskites. This approach is likely to lead to cheaply produced devices on account of the low temperature solution methods per se, and that there is no requirement for rare elements. Solution-processed films produced by other low-temperature (< 100 ^oC) methods have the disadvantage that the resulting diffusion lengths are considerably shorter.
Figure 2 here.

Stranks et al. have reported nanostructured photovoltaic cells made with CH₃NH₃PbI_3-xCl_x (essentially the triiodide, but containing a small quantity of chloride) which in one case gave a conversion efficiency of 11.4%, but this was increased to 15.4 % when vacuum evaporation was employed. A diffusion length of > 1 µm was determined⁷ for CH₃NH₃PbI_3-xCl_x, which is an order of magnitude greater than that in the pure iodide. The carrier lifetimes are also increased in the mixed halide perovskite from those in the pure iodide.⁷ The open-circuit voltage (V_OC) typically approaches 1 V in CH₃NH₃PbI₃, while for CH₃NH₃PbI_3-xCl_x , a V_OC > 1.1 V has been observed. The band gaps (E_g) of both materials are 1.55 eV, and so the V_OC-to-E_g ratios are higher than those usually measured for similar third-generation cells. A V_OC of 1.3 V has been demonstrated for perovskites with higher band-gap energies.³ However, under working conditions, the cell presently lacks sufficient durability to be used as an actual commercial device.³ New strategies are being explored to obtain an even greater V_OC, using CH₃NH₃PbBr₃ which, when employed as a film containing Cl^─ ions, can be as high as 1.5 V⁹. Vapour-deposition has been employed to make planar heterojunction perovskite solar cells containing simplified device architectures (i.e. with complex nanostructures absent), which show a 15% solar-to-electrical power conversion, as determined under simulated full sunlight.⁶

The importance of the field is indicated by the recent ACS Selects collection (http://pubs.acs.org/JACSbeta/jvi/issue27.html), and that both the highly reputable journals Nature and Science, have highlighted⁹ perovskite photovoltaics as one of the major scientific advances of the year 2013. On the basis of the rapid developments that have been witnessed in CH₃NH₃PbX₃ (X = Cl, Br, or I) perovskite photo-sensitizers as used in solid-state mesoscopic solar cells, it is anticipated that a power-production efficiency as high as 20% might be obtained, using optimized perovskite-based solid-state solar cells.¹⁰ Indeed, by means of a low-temperature (70 °C) solution processing to make TiO₂/CH₃NH₃PbI₃ based solar cells, a power conversion efficiency (PCE) of 13.7% has been obtained, along with a high open circuit potential (V_OC) of 1110 mV, which is claimed to be the highest V_OC value measured for solution-processed TiO₂/CH₃NH₃PbI₃ solar cells. A nanocrystalline TiO₂ (rutile) hole-blocking layer was deposited on a fluorine-doped tin oxide (FTO) conducting glass substrate via hydrolysis of TiCl₄ at 70 °C, to create an electron selective contact with the photoactive CH₃NH₃PbI₃ film. It was reported that this nanocrystalline rutile is superior in its performance to a planar TiO₂ (anatase) film which was prepared by high-temperature spin coating of TiCl₄, and gave a much reduced power conversion efficiency of 3.7%. This result is explained in terms of an intimate junction being formed with a large area, so providing an effective interface between the nanocrystalline rutile TiO₂ and the CH₃NH₃PbI₃ layer, with an enhanced ability to extract and mobilise electrons.¹¹ A series of solution-processed perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbX₃, has been prepared whose optical properties may be tuned according to the nature and ratio of the halides employed (X =  Cl, Br, and I), and with different cell archetectures: thin film, and mesoporous scaffold (TiO₂ and Al₂O₃). Using impedance spectroscopy, it is found that the the charge recombination rates are decreased in the light absorber film, when Cl and Br are included in the perovskite lattice. The charge recombination rates are lower, as prepared on a mesoporous Al₂O₃ electrode, than those devices prepared on mesoporous TiO₂. In all the devices measured, an efficiency was preserved to at least 80% of the initial value one month after their preparation.¹²

Theoretical and spectroscopic studies of perovskites.
The low-frequency resonant Raman spectrum of methylammonium lead-triiodide, adsorbed on mesoporous Al₂O₃ has been obtained. On the basis of DFT calculations of appropriate related systems, the bands at 62 cm^-1 and 94 cm^–1 are assigned respectively to the bending and stretching vibrations of the Pb–I bonds, while the bands at 119 cm^-1 and 154 cm^-1 are ascribed to librations of the organic cations. There is also a broad, unstructured band spanning the range 200–400 cm^–1 which is assigned to torsional vibrations of the methylammonium cations, and serves as a marker of the orientational disorder of the material.¹³ Electronic structure calculations have been employed to interpret the fundamental properties of bulk perovskites. Hybrid perovskites are predicted to show spontaneous electric polarization, a phenomenon which might be fine-tuned according to the selection of the organic cation. It is concluded that the presence of ferroelectric domains will form internal junctions that might assist the separation and segregation of photoexcited electron and hole pairs, with an according lowering of the recombination rate. The Wannier-Mott exciton separation and effective ionization of donor and acceptor defects are both promoted as a result of high dielectric constant and low effective mass, and it is proposed that the photoferroic effect could be used to generate a higher open circuit voltage in nanostructured films too and may be responsible  for the current–voltage hysteresis that is measured in perovskite solar cells.¹⁴ A determination was made of photovoltaic conversion in high-performing perovskite-based mesostructured solar cells, with a particular focus on the part played by the mesoporous oxide/perovskite interface. Using a number of different spectroscopic methods, in particular Stark spectroscopy, the existence of oriented permanent dipoles, consistent with the hypothesis of an ordered perovskite layer, close to the oxide surface, was demonstrated. It is concluded that one of the decisive reasons for the highly efficient transport of electrons and holes in perovskite films, could be the presence of such interfacial ordering, as promoted by specific local interactions.¹⁵ The electronic structure and chemical composition of efficient photoelectron spectroscopy with hard X-rays, was used to study CH₃NH₃PbI₃ perovskite solar cell materials deposited onto mesoporous TiO₂, so being able to measure the occupied energy levels of the perovskite in addition to the underlying TiO₂, so to determine the energy level matching at the interface. A good agreement was found between the simulated density of states and the measured valence levels, and it was concluded that similar electronic structures were formed, despite two different deposition methods being used.¹⁶ DFT calculations have been employed to investigate the intrinsic defects in CH₃NH₃PbI₃ and their relation to its photovoltaic properties. Schottky defects, as are vacancies on PbI₂ and CH₃NH₃I, do not form stable trapping sites, and so they can reduce the lifetime of carriers. However, vacancies on Pb, I, and CH₃NH₃ which originate from Frenkel defects, can act as dopants, such that methylammonium lead halides (MALHs) can become unintentionally doped. That there are no intrinsic defects in MALHs is accounted for by the ionic bonding that results from organic–inorganic hybridization.¹⁷
            By means of highly sensitive photothermal deflection and photocurrent spectroscopies, the absorption spectra of CH₃NH₃PbI₃ perovskite thin films were measured at room temperature, yielding a high absorption coefficient with an unusually sharp onset. The presence of a well-ordered microstructure is suggested by the fact that below the bandgap, the absorption is exponential over more than four decades and the Urbach energy is down to 15 meV. No evidence for deep states was found at least at the detection limit of 1 cm^–1. These results accord with the well established electronic properties of perovskite thin films, and the relatively high open-circuit voltages measured for perovskite solar cells. Evidence for a change in the composition of the material, caused by the deliberate introduction of moisture, is given by the strong reduction in the absorption at photon energies below 2.4 eV.¹⁸ It has been shown that trap states at the perovskite surface give rise to charge accumulation and consequent recombination losses in working solar cells. It is found that undercoordinated iodine ions within the perovskite structure are responsible for this effect, and supramolecular halogen bond complexation can be utilised as a means to passivate these sites successfully. Thus, solar cells are fabricated with a maximum power conversion efficiency of 15.7% and a stable power output of > 15%, under a constant 0.81 V forward bias, in simulated full sunlight. It is concluded that such means of surface passivation may pave the way to constructing more efficient perovskite solar cells.¹⁹ On the basis of DFT calculations, it is shown that that the band gap in three-dimensional (3D) hybrid perovskites CH₃NH₃PbX₃ (X = Br, I) is dominated by a massive spin–orbit coupling (SOC) in the conduction-band (CB). Both direct and isotropic optical transitions, at ambient temperature, are associated with a spin–orbit split-off band that is related to the triply degenerate CB of the cubic lattice in the absence of SOC. As a result of the dominance of the SOC, the electronic states involved in the optical absorption are but weakly perturbed by local lattice distortions.²⁰ Again using DFT calculations, the observed absorption blue shift along the I → Br → Cl series was accounted for in CH₃NH₃PbX₃ and mixed halide CH₃NH₃PbI₂X perovskites (X = Cl, Br, I). It was found that CH₃NH₃PbI₃ and the mixed CH₃NH₃PbI₂Cl or CH₃NH₃PbI_3–xCl_x perovskites exhibited a similar absorption onset at 800 nm, whereas CH₃NH₃PbI₂Br absorbs light below 700 nm. A good accord was met between the calculated band structures and the experimental trend of optical absorption frequencies. The existence of two different structural types with different electronic properties was indicated for the mixed perovskites (CH₃NH₃PbI₂X), with a relative stability that depended on the nature of the group, X. For these systems, the calculated energies of formation decrease in the order I > Br > Cl, which would accord with the observed miscibility of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃, while suggesting that the degree of chlorine incorporation into CH₃NH₃Pb(I_1–xCl_x)₃ should be smaller. The calculations further indicate that that in the PbI₄X₂ octahedra, the Cl atoms preferentially occupy the apical positions while Br atoms may occupy both apical and equatorial positions, in agreement with reported lattice parameters.²¹ Transient laser spectroscopy and microwave photoconductivity measurements were made²² on TiO₂ and Al₂O₃ mesoporous films impregnated with CH₃NH₃PbI₃ perovskite and the organic hole-transporting material spiro-OMeTAD. The results show that primary charge separation occurs at both junctions, involving simultaneously TiO₂ and spiro-OMeTAD, with ultrafast electron and hole injection occurring from the photoexcited perovskite over similar timescales. It is observed that charge recombination is appreciably slower on TiO₂ films than on Al₂O₃.

Technical innovations for transformative perovskite solar cells.
A low-temperature vapour-assisted solution process has been introduced²³ to make polycrystalline perovskite thin films with full surface coverage, small surface roughness, and up to microscale grain sizes. Thus, it should be possible to fabricate perovskite films and devices in a simple and highly reproducible fashion. A power conversion efficiency of 12.1% has been achieved, which is the best so far obtained from CH₃NH₃PbI₃ using a planar heterojunction configuration, and the critical kinetic and thermodynamic parameters attendant to the film growth were also investigated.²³ A method has been reported for the preparation of 6 nm-sized nanoparticles of CH₃NH₃PbBr₃ perovskites, which employs an ammonium bromide containing a chain of sufficient length (steric size) that the nanoparticles remain dispersed in a wide range of organic solvents. Since the nanoparticles are stable both in the solid state and in concentrated solutions, with no requirement for a mesoporous support, homogeneous nanoparticle thin films can be made by spin-coating onto a quartz substrate. Since both the colloidal solution and the thin film emit light over a narrow wavelength region of the visible spectrum and with a high quantum yield (ca. 20%), it is thought that the nanoparticles might find particular applications in the fabrication of optoelectronic devices.²⁴ In (CH₃NH₃)PbI₃-sensitized solar cells containing iodide-based electrolytes, (CH₃NH₃)PbI₃ is relatively stable in a nonpolar solvent, such as ethyl acetate, so long as the iodide concentration is kept low (e.g., 80 mM). According to frequency-resolved modulated photocurrent/photovoltage spectroscopy when the TiO₂ film thickness is increased from 1.8 to 8.3 μm, the transport is barely altered, but the electron-hole recombination is increased by above a factor of 10, which reduces the electron diffusion distance from 16.9 to 5.5 μm. An explanation for this is the greater degree of iodide depletion within the TiO₂ pores as the film thickness increases. Thus, for the development of (CH₃NH₃)PbI₃ or similar perovskites in potential photoelectrochemical applications, it will be necessary to find alternative, compatible redox electrolytes.²⁵ In another study, the effect of TiO₂ film thickness on charge transport and recombination in solid-state mesostructured perovskite CH₃NH₃PbI₃ (via one-step coating) solar cells, using spiro-MeOTAD as the hole conductor, has been investigated using intensity-modulated photocurrent/photovoltage spectroscopies. It is demonstrated that charge transport in perovskite cells is not dominated by electron conduction from the perovskite layer, but within the mesoporous TiO₂ network. The film-thickness was found to have little influence on the electron-hole transport and recombination processes, and yet the efficiency of perovskite cells increases as the TiO₂ film increases in thickness from 240 nm to ca 650–850 nm. This effect is a consequence of enhanced light harvesting by the thicker films, although a drop-off in the cell efficiency is found as the film thickness is further increased, which is thought to be connected with a reduced fill factor or photocurrent density²⁶. A novel metal-halide perovskite has been produced, based on the formamidinium cation (HC(NH₂)₂⁺), which displays a favourable band gap (1.47 eV) and has a broader absorption than light absorbing materials that contain the methylammonium cation (CH₃NH₃⁺), as previously documented. The high open-circuit voltage (V_oc = 0.97 V) and promising fill-factor (FF = 68.7%) yield an efficiency of 4.3%. The formation of a black trigonal (P3m1) perovskite polymorph and a yellow hexagonal nonperovskite (P63mc) polymorph is also reported, and it is concluded that to develop the cell further would necessitate the stabilization of the black trigonal (P3m1) perovskite polymorph over the yellow hexagonal nonperovskite (P63mc) polymorph.²⁷

Perovskite (CH₃NH₃)PbI₃-sensitized solid-state solar cells have been reported which contain spiro-OMeTAD, poly(3-hexylthiophene-2,5-diyl) (P3HT) and 4-(diethylamino)benzaldehyde diphenylhydrazone (DEH) as hole transport materials (HTMs), which yield a respective light-to-electricity conversion efficiency of 8.5%, 4.5%, and 1.6%, under AM 1.5G illumination with an intensity of 1000 W/m². Measurements made using photoinduced absorption spectroscopy (PIA) show that the hole transfer occurs from the (CH₃NH₃)PbI₃ to the particular HTM, following an initial excitation of (CH₃NH₃)PbI₃. The electron lifetimes (τ_e) in these devices decrease in the order spiro-OMeTAD > P3HT > DEH, so explaining the lower efficiency of the cells containing P3HT and DEH; however, the charge transport time (t_tr) is relatively insensitive to the nature of the HTM.²⁸ Copper iodide (CuI) has emerged as a potential new inorganic hole conducting material for perovskite-based thin film photovoltaics, since it has been used in a cell to yield a power conversion efficiency of 6.0%, and with excellent photocurrent stability. However ,the open-circuit voltage is much lower than is obtained from the best spiro-OMeTAD devices, as is attributed to a higher degree of recombination in CuI devices, according to results obtained from impedance spectroscopy. However, the latter technique also disclosed that the electrical conductivity in CuI is two orders of magnitude higher than in spiro-OMeTAD, meaning that significantly greater fill factors are possible²⁹.  The optical and electronic structures of three N,N-di-p-methoxyphenylamine-substituted pyrene derivatives were investigated by UV/vis spectroscopy and cyclic voltammetry, to be used as hole-transporting materials (HTMs) in mesoporous TiO₂/CH₃NH₃PbI₃/HTMs/Au solar cells. A short-circuit current density of 20.2 mA/cm², an open-circuit voltage (V_oc) of 0.886 V, and a fill factor of 69.4% were measured under an illumination of 1 sun (100 mW/cm²), which gave an overall light to electricity conversion efficiency of 12.4%. Accepting that the V_oc is slightly lower, the performance of the pyrene analogue is comparable with that of the well-studied spiro-OMeTAD, and may find future applications as an HTM in perovskite-based solar cells.³⁰

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Captions to Figures.

Figure 1. Structure of a perovskite with a chemical formula ABX₃. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti⁴⁺), and the green spheres are the A-atoms (a larger metal cation, such as Ca²⁺). Pictured is the undistorted cubic structure; the symmetry is lowered to orthorhombic, tetragonal or trigonal in many perovskites. http://upload.wikimedia.org/wikipedia/commons/5/54/Perovskite.jpg

Figure 2. Diagram showing (left) the device configuration and (right) energy levels of each layer in the device. Al: Aluminum. BCP: Bathocuproine. C₆₀: Fullerene. PEDOT:PSS: Conducting polymer. http://spie.org/Images/Graphics/Newsroom/Imported-2013/005033/005033_10_fig3.jpg

Current Commentary: Sustainable Nanotechnology.

The following will be published in the December 2014 issue of the journal Science Progress http://www.sciencereviews2000.co.uk/view/journal/science-progress, of which I am an editor - so, this is a (very early) preview!

We welcome proposals and manuscripts from potential authors on most aspects of science and technology, guided by the following description:
"The journal's objective is to provide reviews of a range of current topics, which are both in-depth in their content, and of general appeal, presenting the reader with an overview of contemporary science and technology, and its impacts on humanity." The style of the following article is intended to illustrate this.

Keywords.

sustainable nanotechnology, silver nanoparticles, cellulose, nanocellulose, gold nanoparticles, zeolite, toxicology, C. elegans,

 
1. Can a definition for “sustainable nanotechnology” be agreed upon?

The U.S. National Nanotechnology Initiative1 defines nanotechnology as, “the manipulation of matter with at least one dimension in the range 1—100 nanometres (nm),” where the tendency is for quantum mechanical effects to become increasingly important toward the smaller end of the range. It is critical that the particular materials, and devices made from them, should possess properties that are different from the bulk (micrometric or larger) materials, as a consequence of their small size, which may include enhanced mechanical strength, chemical reactivity, electrical conductivity, magnetism and optical effects. The term sustainable has been used overly and often incorrectly, essentially to mean things that are environmentally benign, but including a degree of “greenwash” in some cases. [Greenwash is a compound word based on "whitewash", and refers to a deceptive form of promotion (spin) that portrays the products, aims or policies of an organisation in an environmentally benign (green) light]. In ecology, sustainable systems are self-sustaining, or self-regenerating (regenerative)2, as occur in nature. We may note that the phrase “sustainable agriculture” has been described2 as an oxymoron, since agriculture is by its very nature unsustainable, relying as it does on inputs of all kinds, e.g. petroleum, natural gas and water, and that it renders the soil vulnerable to erosion with the progressive and global loss of productive land3.

It is even more vexing to find a precise definition of sustainable nanotechnology, since the individual sustainability aspects of all components must be considered. ACS Sustainable Chemistry & Engineering has recently presented its second special issue concerning Sustainable Nanotechnology. Those papers featured in this issue were presented at the 2nd Sustainable Nanotechnology Organization (SNO) Conference held in November 2013. The conference was attended by over 200 delegates working in academia, industry, and government agencies. The editorial of this special issue offers the following definition4: “Sustainable nanotechnology is the research and development of nanomaterials that have economic and societal benefits with little or no negative environmental impacts. The successful application of nanotechnology is contingent upon scientific excellence that provides economic, ethical, and societal benefits.” While from the associate director of the Virginia Tech’s Center for Sustainable Nanotechnology we have5: “Sustainable nanotechnology is the development of science and technology within the 1 – 100 nanometer scale, with considerations to the long-term economic viability and a sensible use of natural resources, while minimizing negative effects to human health and the environment. Potential negative effects may be caused by engineered nanomaterials or by anthropogenic changes in the prevalence of naturally occurring nanomaterials.”

As she further stresses: “When addressing “sustainable nanotechnology”, we must address economic needs, human safety, and environmental conservation. Sustainable nanotechnology demands extra creativity and innovation in an already innovative field. How can we make materials safer to people? How can we make manufacturing less energy intensive? How can we minimize waste? These are a few good driving questions towards sustainable nanotechnology. Actually, these should be driving questions in whatever work you do, whether it is related to nanotechnology or not.” Such definitions allow us to distil the essence that the nanomaterials must have positive economic and societal benefits in their use, while effectively being “harmless”; however, issues over the manufacture of the nanomaterials themselves must also pertain, for example the likely availability of their component elements in the future, and hence how sustainable their long-term supply might prove be6. It should be noted that we are already somewhat removed from the ecological definition of sustainability, and regeneration, since the nanomaterials are required as an external and continual input to whatever systems are being “improved” by their presence. Some mitigation both of this demand, and of consequent environmental impacts, might be achieved through nano-recycling.


(2) Properties of nanoparticles.

At the nanoscale, the fraction of the total atoms in the particle that are at the surface becomes substantial, in contrast to bulk materials, and it is this very high surface area that is responsible for many of the unique properties of nanoparticles. [We may note that 1 kg of particles of 1 mm3 diameter has the same surface area as 1 mg of particles of 1 nm3 diameter]. Additionally, due to the effect of quantum confinement of the electrons, unexpected optical effects may occur: thus, nanoparticles of gold and silicon (respectively yellow and grey in their bulk forms) are reddish in colour (Fig. 1). As a further phenomenon, it was found that a sample of 2.5 nm diameter gold nanoparticles melted at ~300 °C, which is far lower than the normal 1064 °C melting point of gold7. By varying their size, shape, and chemical composition, it is possible to tune the absorption of solar radiation by nanoparticles, which in any case tend to absorb radiation more strongly than do the corresponding bulk materials, with implications for both solar PV and solar thermal applications. Nanoparticles can be created using various different methods, some of which are now outlined8.

Attrition is carried out using a mechanical device such as a ball-mill, to breakdown macro- or micro-sized materials into smaller particles, from which the nanoparticle fraction is isolated. Pyrolysis involves burning a liquid or gaseous precursor that has been forced through an orifice at high pressure, and the oxide nanoparticles are recovered from the solid product, usually by air-classification. [Air classification is a separation technique in which the material stream to be sorted is injected into a chamber which contains a column of rising air. Within the chamber, the effect of air-drag supplies an upward force on the particles which counteracts the force of gravity and lifts the material to be sorted up into the air. Since the effect of air-drag varies according to the size and shape of the particles, the latter are sorted vertically in the moving air column, and are hence separated from one another].

In order to avoid the formation of aggregates and agglomerates, ultrasonic nozzle spray pyrolysis (USP) is employed, which results in single primary particles. Thermal plasmas, which operate at temperatures in the region of 10,000 K, may be used to vapourise small micrometer-size particles from a solid, leading to the formation of nanoparticles by cooling beyond the exit point of the plasma region. RF-induction plasma torches have been used in the production of ceramic nanoparticles such as oxides, carbides, and nitrides of Ti and Si, among other materials. Nanoparticles may also be prepared using methods of radiation chemistry. In this approach, electrons, generated by radiolysis of water molecules in aqueous solutions, reduce metal cations to the corresponding metal atoms which coalesce to form nanoparticles. A surfactant is present, which surrounds the particles as they are formed and regulates their growth, and in high enough concentrations, the surfactant molecules remain in association with the nanoparticles, so preventing them from dissociating or forming clusters with other particles. The shape and size of the particles can be adjusted according to the concentrations of the materials and the dose of gamma-rays, which may be up to 10,000 Gy (1 MRad)9.

The radiolytic formation of free radicals has been studied previously using ESR and related techniques10, including in zeolite nanomaterials11. Sol-gel methods have also been found useful in the preparation of nanoparticles. The sol-gel process is used for the preparation of, typically, metal oxide materials in the fields of materials science and ceramic engineering, starting from an appropriate solution (sol) of chemicals, which acts as the precursor to an integrated network (or gel) of either discrete particles or network polymers. The sol can be deposited onto a substrate to form a film, or it may be cast into a suitable container with the desired shape to produce monolithic ceramics, glasses, fibres, membranes, and aerogels, or for the synthesis of powders (microspheres or nanospheres). The method permits the fine control of the chemical composition of a product, is inexpensive, and is carried out at low temperatures.



(3) Morphology and characterisation of nanoparticles.

The terms nanotubes, nanospheres, nanoreefs, nanoboxes, nanostars and even12 nano-cabbage and (nano) sea-anemone have appeared in the literature, in reference to the apparent similarity between the various nanoparticle morphologies and the shapes of objects that are more commonly encountered by humans (Fig. 2). It is sometimes the presence of templating or directing agents, such as micellar emulsions or anodized alumina pores, that causes the various shapes to form spontaneously; alternatively, they may arise from the innate crystallographic growth patterns of the materials themselves13. As a consequence of their structural isotropy, amorphous particles tend to form spheres. Once formed, it is necessary to characterize the nanoparticles, and a range of techniques are used for this purpose, most commonly: electron microscopy (TEM, SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), x-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), ultraviolet-visible spectroscopy, Rutherford backscattering spectrometry (RBS), dual polarisation interferometry, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR). A more recently developed method has been introduced for the characterisation of nanoparticles, which is termed Tunable Resistive Pulse Sensing (TRPS) which enables the size, concentration and surface charge to be determined simultaneously for a wide variety of nanoparticles14.



(4) Recent research in sustainable nanotechnology.

That this field is one of rapid growth is emphasised by the fact that the ACS Sustainable Chemistry & Engineering journal has recently published its second special issue concerning Sustainable Nanotechnology, from which the following studies are now highlighted. To provide a “completely green and environmentally friendly” catalyst for environmental decontamination, a γ-Fe2O3-pillared montmorrilonite nanocomposite was synthesized, which was characterized using scanning electron microscopy (SEM) methods, transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). An 85% degradation of dichlorophenol (DCP) was obtained in 2.5 hours, in the presence of peroxymonosulfate, with somewhat reduced levels after 3.5 hours, with H2O2 (50%) and peracetic acid (70%) of DCP [15]. Reusable, magnetically separable, magnetite-supported copper (nanocat-Fe-CuO) 20-30 nm nanoparticles were prepared as catalysts for the synthesis of pyrazole derivatives, 4-methoxyaniline and Ullman-type condensation reactions, under mild conditions. The particles were recovered and reused six times without any loss in catalytic activity [16]. Intercellularly synthesised gold nanoparticles were characterised using surface-enhanced Raman spectroscopy (SERS).

Both intracellular and extracellular gold nanoparticles, biosynthesized by the green algae Pseudokirchneriella subcapitata were imaged sing SERS to identify surface-associated biomolecules and aid in the determination of the mechanism for the nanoparticle biosynthesis [17]. Despite the widespread use of ZnO nanoparticles (NP) in various applications, they are actually among the more toxic NPs known. Thus there is the incentive to produce safer ZnO NPs, while preserving the essential optical, electronic, and structural features of these materials. Thus, two ZnO samples of equal dimension (9.26 ± 0.11 nm) were synthesized from the same zinc acetate precursor using a forced hydrolysis process, but with different solvents, which permitted the modification of their surface structures. While the lattice parameters, optical properties, and band gap (3.44 eV) of the two ZnO NP samples were preserved, FTIR spectroscopy showed there were significant differences between them in their surface structures and surface-bound functional groups. Accordingly, the zeta potential, hydrodynamic size, and photocatalytic rate differed considerably. It was found that the ZnO NP sample with the higher zeta potential and greater catalytic activity was more cytotoxic to cancer cells by a factor of 1.5 [18]. In another study, silver and gold NPs were produced using antioxidants from extracts of natural fruits and spices blackberry, blueberry, pomegranate and turmeric). The NPs were characterized using XRD, TEM, high-resolution TEM (HR-TEM), particle size analysis, UV–vis spectroscopy, and thermogravimetric analysis [19].

Despite the fact that iron-based nanoparticles are known to be effective for the degradation of organic dyes, organochlorine compounds, and arsenic contaminants, their characterisation has been ambiguous. In a recent study, iron-based NPs were produced by reduction with green tea extract and were fully characterized by TEM, XRD, and UV–vis spectrometry. According to the XRD and TEM results, the iron formed amorphous nanosized particles, whose size depended on the reaction time. It was shown that iron(II,III) NPs prepared as green tea extract (GT–Fe nanoparticles) had negative ecotoxicological impacts on important aquatic organisms such as cyanobacterium (Synechococcus nidulans), alga (Pseudokirchneriella subcapitata), and invertebrate organisms (Daphnia magna) too20. Cacumen Platycladi (CP) extract was employed in a green bioreductive strategy to form bimetallic Au–Pd/TiO2 catalysts intended for the solvent-free oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) using an ambient pressure of O2. It was found that the uncalcined Au–Pd/TiO2 catalyst prepared at 90 °C from Au–Pd NPs (Au/Pd molar ratio: 2:1) could convert BzOH to BzH in a yield of 74.2%, and with a 95.8% selectivity, and that the catalyst preserved its activity over 7 recycling events21.

TiO2 is an efficient photocatalyst in water treatment applications, and for other purposes22. In order to adjust the band gap and electronic structure of the catalyst, a method has been presented by which an N-doped TiO2 may be obtained, which is hybridized with graphene sheets. The reaction occurs at a relatively low temperature (180 °C), in which NH3 solution is both the N source and the reaction medium. After a reaction time of 14 hours, the product contained 2.4 atom % of N. A dramatic suppression of the photoluminescence (PL) intensity was observed in the N-doped TiO2/graphene composites, as compared with undoped TiO2, demonstrating that the electron–hole pair recombination rate was dramatically reduced in the doped composite. It was further demonstrated that the N-doped TiO2/graphene photocatalyst degraded methylene blue twice as fast as a commercial Degussa P25 catalyst23. Also with water decontamination in mind, this time from Pb2+ cations, rather than organic pollutants, magnetic attapulgite/fly ash/poly(acrylic acid) (ATP/FA/PAA) ternary nanocomposite hydrogels were prepared by inverse suspension polymerization, after the inorganic materials were modified in situ. It was found that the hydrogels exhibited a high adsorption selectivity toward Pb2+ cations, which could be completely desorbed by treatment with aqueous HCl. The hydrogels are cheap to make, have a high mechanical stability and retain their magnetic properties, which suggests that they may prove to be very useful materials for the treatment of Pb2+-contaminated water24.

It was shown that when the surfaces of membrane filters were modified using graphene-based materials, their activity toward Escherichia coli and Bacillus subtilis was improved significantly. It was concluded that part of this improved antimicrobial property was due to the formation of reactive oxygen species (ROS) by the nanomaterials, and that such surface-modification may provide enhanced filters for the treatment of water and wastewaters25. The contamination of waterways by phosphate, mostly from agricultural runoff waters, is an environmental problem since it leads to eutrophication, and the formation of algal blooms and dead zones. In this regard, it has been shown that nanoscale zero-valent iron (NZVI) particles offer promise for the recovery of phosphate from aqueous media. To determine the bioavailability of the phosphate, as sorbed onto the NZVI particles, spinach (Spinacia oleracea) and algae (Selenastrum capricornutum) grown in hydroponic solutions, were used. It was found that the concentration of algae increased by 6.7 times when the only source of phosphate was spent NZVI, in comparison with algae grown in standard all-nutrient media, which contains phosphate. An elevated iron content was measured in the roots, leaves, and stems of the spinach that had been treated with spent NZVI, respectively, as compared to the controls, which might indicate a role for iron in enhancing the overall plant growth26.

Cellulose has been converted to n-hexane using an Ir-ReOx/SiO2 (Re/Ir = 2) catalyst combined with HZSM-5 as a co-catalyst in a biphasic reaction system (n-dodecane + H2O), from which the yield of n-hexane reached 83% from ball-milled cellulose and 78% from microcrystalline cellulose. The yield remained high (71%) even when a high concentration of water was present (cellulose to water: 1:1). By recalcination of the catalyst, the yields were maintained over three cycles of the catalyst. The mechanism involves the hydrolysis of cellulose to glucose via water-soluble oligosaccharides, hydrogenation of glucose to sorbitol, and successive hydrogenolysis of sorbitol to n-hexane. The Ir-ReOx/SiO2 catalyst is active for one hydrogenation and hydrogenolysis step, while HZSM-5 enhanced both the hydrolysis of cellulose and the C–O bond hydrogenolysis activity of the Ir-ReOx/SiO2 catalyst27. 5-hydroxymethylfurfural (5-HMF) is a useful renewable biofuel and biochemical, which, it has been shown, may be produced in a “one pot” procedure by the hydrolysis of microcrystalline cellulose over a zeolite described as “Bimodal-HZ-5”, which was obtained by post-synthesis modification of H-ZSM-5 with desilication. The conversion of cellulose was 67%, with a 5-HMF yield of 46%. The catalyst was found to be reusable for four cycles, with no reduction of its activity28.

Lignin may be recovered from the Kraft process, as a component of wood-pulp. The name “Kraft” comes from the German word for “strong” in regard to the superior paper quality that was obtained from it. In a recent study, a water-soluble lignin-based polyoxyethylene ether (KL-PEG) was synthesized from kraft lignin (KL) and poly(ethylene glycol) (PEG). PEGs with various polyoxyethylene ether lengths were functionalized preferentially with epichlorohydrin using BF3-Et2O as the Lewis acid catalyst and were then grafted onto KL by blocking the phenolic hydroxyl groups. It was demonstrated that the PEG content in the KL-PEG copolymer could be controlled by varying the mass ratio of PEG to KL, and the molecular weight was controlled according to the molar ratio of epichlorohydrin to PEG. It was proposed that this novel amphiphilic KL-PEG copolymer, with its renewable lignin backbone and branched PEG groups might be used as a dispersant for a variety of materials, including agricultural suspension concentrates29.

The lithium ion battery has a high profile as an energy storage device for the potential sustainable transport of power. By means of pulse electrodeposition, dense Sn films were deposited on a Cu substrate, followed by various post-treatments at 200 °C, of which electroplating a Cu-film-coating and heat treatment in different atmospheres are representative. When the films were assembled in an Sn-based anode with a Cu coating, post-heating in argon for 12 hours, a surface was formed with Cu6Sn5/Sn as the primary phase. The latter exhibited the greatest first cycle charge/discharge capacity and largest irreversible capacity loss (IRC); in contrast, the least IRC was found for a Sn-based anode sintered in air for 48 h was surface modified by SnO. The first cycle capacity and IRC of the anode were both enhanced by defects in the Cu6Sn5/Sn phase, while the decrease in the first cycle IRC of the anode is aided by the presence of SnO30. Well-defined carbon black/polypyrrole (CB/PPy) composite hollow nanospheres have been evaluated for their potential application as electrode materials for supercapacitors. These materials were prepared via in situ chemical oxidative interfacial polymerization of pyrrole, in the presence of sulphonic acid-modified carbon black (CB-SO3H) NPs, using an inert solvent (toluene) as the soft template, in which sodium dodecyl benzenesulphonate (SDBS) was used as both the surfactant for the emulsion and the dopant for both the produced polypyrrole (PPy) and the CB-SO3H NPs. A maximum electrical conductivity of 0.045 S/cm and a specific capacitance of 29 F/g was achieved, and even after 1000 cycles, the latter parameter was reduced only by 5%, indicating an excellent cycling performance for these nanospheres31.

A series of dye-sensitized solar cells was prepared in which different 9,10-diaryl-substituted anthracene groups acted as a π-bridge with a 2,6-linkage mode. Where present, tert-butylphenyl and hexyloxyphenyl groups in the 9 and 10 positions of the anthracene unit occupied practically perpendicular orientations in respect to the conjugated ring-plane, so helping to discourage possible π–π stacking and reducing the extent of charge recombination. When an optimum arrangement of substitutents was present at the anthracene ring, a Jsc (short circuit current) of 13.42 mA cm–2, Voc (open circuit voltage) of 722 mV, and FF (fill factor) of 0.66, corresponding to an overall conversion efficiency of 6.42%, were obtained32.

A bulk hetero-junction organic polymer solar cell based on poly(3-hexylthiophene) (P3HT) and PC70BM, containing a near-infrared absorbing dye, bis[4-(2,6-di-tert-butyl)vinyl-pyrylium] squaraine (TBU-SQ), was presented, in which the dye raised the power conversion efficiency (PCE) to 4.55% from 3.47% for the binary blend alone. This improvement was attributed to the light-harvesting efficiency in the near-infrared region of the solar spectrum and the increased exciton dissociation into free charge carriers in the ternary blended film. After the film had been thermally annealed, the PCE was found to increase to 5.15%, which corresponded with a red shift and broadening of the film’s absorption profile as a result of the thermal teratement33.

Carbon nanotubes (CNTs) are of interest as potential materials with which to enhance the performance of the anodes and cathodes in lithium-ion batteries, although there are some concerns that CNTs may be toxic34. A material flow analysis (MFA) with a stock dynamics and logistic model has been employed to predict the timescale for the transition from conventional Li-ion batteries in portable computers to CNT Li-ion batteries, and the according waste generation of CNTs in obsolete laptop batteries. State-specific recycling rates for electronic waste were projected, from which to estimate the quantities of CNTs in laptop batteries that will arise to be recycled, incinerated, or placed in landfill. It is concluded that as the various markets for CNT-enabled electronics begin to expand, collection and recycling facilities in the U.S., may need to inaugurate new processes or controls to reduce the potential for the emissions of and exposures to CNTs34. A survey has been made of both conventional and emerging techniques that are available for characterizing engineered nanoparticles in complex matrices35, which included microscopy (TEM, SEM, HRTEM, DLS, SNOM), chromatography (HDC, FFF), mass spectroscopy (ICP-MS, SEC-ICP/MS, MALDI, FFF-ICP-MS), sp-ICP-MS, and electrochemical techniques. The design of a portable nanoparticle analyzer based on tangential flow filtration and electrochemical detection (EC-TFF) was presented in the form of a case study, and its application for the characterization of engineered nanosilver in actual environmental samples. A 98.5% removal efficiency was obtained for Ag-NPs with varying particle sizes35.

Uncertainties are confronted by nanotechnology companies on a number of levels which concern regulation of the manufacture and use of NPs, their demand, and advances in technology. It is accordingly difficult to make predictions as to how the capacity of engineered nanomaterials or nanoenabled products is likely to expand in the future, and the influence of this on associated revenue income. However, Monte Carlo simulations may prove useful in arriving at optimal decisions regarding sustainable capacity expansion, and to aid decision makers in setting sustainable manufacturing goals by reducing any unnecessary capacity expansion and the degree of occupational exposure to NPs36.



(5) Health and environmental concerns.

In view of their high surface to volume ratio, with an associated high reactivity, NPs may present dangers both to the human organism and to the environment34. While it is known that NPs can pass through cell membranes in organisms, how they interact precisely with biological systems is relatively unknown and potentially complex: as in a recent investigation which found varying degrees of cytotoxity for ZnO NPs toward human immune cells37. It is likely that safety data that were obtained during clinical studies of prior medicines that did not contain NPs will not be entirely adequate from which to assess more recent nano-formulations, which may require their own specific safety-evaluation. Nanomaterials are widely used in various sunscreen and cosmetic formulations, but any likely risks to human health are mostly unknown at present38. Combustion processes39 in general are of concern, in that they may generate respirable NPs, and as of 2013 the Environmental Protection Agency has written the following in regard to its investigation of the safety of the following categories of NPs, in anticipation of a market based upon them that is expected to be worth $1 trillion by 2015(40):

· Carbon Nanotubes: carbon materials have a wide range of uses, ranging from composites for use in vehicles and sports equipment to integrated circuits for electronic components. The interactions between nanomaterials such as carbon nanotubes and natural organic matter strongly influence both their aggregation and deposition, which strongly affects their transport, transformation, and exposure in aquatic environments. In past research, carbon nanotubes exhibited some toxicological impacts that will be evaluated in various environmental settings in current EPA chemical safety research. EPA research will provide data, models, test methods, and best practices to discover the acute health effects of carbon nanotubes and identify methods to predict them.

· Cerium oxide: nanoscale cerium oxide is used in electronics, biomedical supplies, energy, and fuel additives. Many applications of engineered cerium oxide nanoparticles naturally disperse themselves into the environment, which increases the risk of exposure. There is ongoing exposure to new diesel emissions using fuel additives containing CeO2 nanoparticles, and the environmental and public health impacts of this new technology are unknown. EPA’s chemical safety research is assessing the environmental, ecological, and health implications of nanotechnology-enabled diesel fuel additives.

· Titanium dioxide: nano titanium dioxide is currently used in many products. Depending on the type of particle, it may be found in sunscreens, cosmetics, and paints and coatings. It is also being investigated for use in removing contaminants from drinking water.

· Nano Silver: nano silver is being incorporated into textiles and other materials to eliminate bacteria and odour from clothing, food packaging, and other items where antimicrobial properties are desirable. In collaboration with the U.S. Consumer Product Safety Commission, EPA is studying certain products to see whether they transfer nano-size silver particles in real-world scenarios. EPA is researching this topic to better understand how much nano-silver children come in contact with in their environments.

· Iron: while nano-scale iron is being investigated for many uses, including “smart fluids” for uses such as optics polishing and as a better-absorbed iron nutrient supplement, one of its more prominent current uses is to remove contamination from groundwater. This use, supported by EPA research, is being piloted at a number of sites across the U.S.40.

There is a very readable account of how nanomaterials get into the environment available41, and another which considers some of the prospects for nano-recycling42. Clearly, NPs can have an undesirable impact on the environment, since one study demonstrated that earthworms exposed to gold nanoparticles in soil produced 90% fewer offspring42. Since earthworms have been described as the “tractors” of the soil, in their contribution to the soil food web2, any die-back in the population of soil-organisms could have serious consequences for the world food supply3. Similarly, it has been shown that Ag-NPs dramatically reduce the reproduction potential of the soil nematode Caenorhabditis elegans43 (Fig. 3). It is thought that oxidative stress may play an important role in the toxicity of Ag-NPs toward C.elegans. It is reported that Ag-NPs are to be used in Nigeria to treat patients who are infected with the ebola virus, although the judiciousness of this approach is under question44.


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Captions to figures.

Fig. 1. Silicon nanopowder. http://en.wikipedia.org/wiki/Nanoparticle#mediaviewer/File:Nano_Si_640x480.jpg

Fig. 2. Nanostars of vanadium(IV) oxide. http://en.wikipedia.org/wiki/Nanoparticle#mediaviewer/File:Nanostars-it1302.jpg

Fig. 3. The soil nematode Caenorhabditis elegans http://pl.wikipedia.org/wiki/Caenorhabditis_elegans#mediaviewer/Plik:Enlarged_c_elegans.jpg