Use of titanium dioxide nanoparticles biosynthesized by Bacillus mycoides in quantum dot sensitized solar cells
© Órdenes-Aenishanslins et al. 2014
Received: 17 April 2014
Accepted: 15 June 2014
Published: 16 July 2014
One of the major challenges of nanotechnology during the last decade has been the development of new procedures to synthesize nanoparticles. In this context, biosynthetic methods have taken hold since they are simple, safe and eco-friendly.
In this study, we report the biosynthesis of TiO2 nanoparticles by an environmental isolate of Bacillus mycoides, a poorly described Gram-positive bacterium able to form colonies with novel morphologies. This isolate was able to produce TiO2 nanoparticles at 37°C in the presence of titanyl hydroxide. Biosynthesized nanoparticles have anatase polymorphic structure, spherical morphology, polydisperse size (40–60 nm) and an organic shell as determined by UV–vis spectroscopy, TEM, DLS and FTIR, respectively. Also, conversely to chemically produced nanoparticles, biosynthesized TiO2 do not display phototoxicity. In order to design less expensive and greener solar cells, biosynthesized nanoparticles were evaluated in Quantum Dot Sensitized Solar Cells (QDSSCs) and compared with chemically produced TiO2 nanoparticles. Solar cell parameters such as short circuit current density (I SC ) and open circuit voltage (V OC ) revealed that biosynthesized TiO2 nanoparticles can mobilize electrons in QDSSCs similarly than chemically produced TiO2.
Our results indicate that bacterial extracellular production of TiO2 nanoparticles at low temperatures represents a novel alternative for the construction of green solar cells.
The rapid advance of nanotechnology and the increasing number of applications involving nanomaterials have prompted the interest in developing simple and environmentally friendly protocols for nanoparticle synthesis.
To date, titanium nanoparticles (NPs) are one of the most required nanomaterials because of its use on different technological applications. Nanoparticulated titanium dioxide is a highly valuable material since it is used as photocatalyst degrading organic molecules in water treatment [], white pigment in paint manufacturing, additive in food and personal care products [], and composite films in biomedical sciences [], among many other applications.
Since TiO2 nanoparticles can conduct electrons as a wide-band gap semiconductor, its use as photoanode in the manufacture of Dye- and Quantum Dots- Sensitized Solar Cells (DSSCs and QDSSCs, respectively) has gained importance during the last decade [,]. This application has become more attractive during the last years due to the global need for replacing fossil fuels for energy generation. Accordingly, the research and development of non-conventional renewable energies (NCRE), particularly solar energy, which arises as a sustainable and abundant alternative to meet the high world energy demand, has been strongly stimulated [].
Current methods to produce TiO2 nanoparticles involve different chemical procedures such as sol–gel [], hydrothermal [] and solvothermal [], among others. All these methods involve high temperatures (>200°C) and in some cases elevated pressures, both conditions affecting the safety and costs of the process.
Biosynthesis of TiO 2 nanoparticles reported to date
Organism used in the biosynthesis
Crystal structure (dominant)
13 nm (TEM)
Anatase and Rutile
18 nm (XRD)
25 nm (TEM)
Anatase and Rutile
30 nm (XRD)
66-77 nm (SEM)
Leaves extract of Nyctanthes arbor-tristis
100 nm (XRD)
2. 500°C (calcined)
100–150 nm (SEM)
Aqueous extract of Jatropha curcas L. latex
Anatase and Rutile
Against pathogenic bacteria
Leaf aqueous extract of Eclipta prostrata
Annona squamosa peel extract
26 nm (XRD)
23 nm (TEM)
1. Not shown
Photocatalytic activity on aquatic biofilm
2. 500°C (to crystallize particles)
Leaves extract of Catharanthus roseus
25-110 nm (SEM)
Anatase and Rutile
65 nm (XRD)
Flower aqueous extract of Calotropis gigantea
160-220 nm (SEM)
10.52 nm (XRD)
100-500 nm (SEM)
Antibacterial and antifungal activity
8.89 nm (XRD)
65 nm (XRD)
Preparation of collagen-TiO2 wound dressing
2. 300°C (annealed)
10–80 nm (FE-SEM)
40.5 nm (XRD)
28–54 nm (SEM)
Leaves extract of Solanum trilobatum
70 nm (SEM)
No reports regarding the application of biosynthesized titanium dioxide nanoparticles in energy devices have been published to date (Table 1). The use of other biologically produced materials in sensitized solar cells has been recently reported, however these reports focused on using plant pigments or channel proteins in the photon harvest process [,].
The present work reports for the first time the use of biosynthesized titanium dioxide nanoparticles by B. mycoides, as semiconductors in the manufacture of photoanodes for QDSSCs. Along with introducing a new method for the synthesis of TiO2 nanoparticles, the present manuscript constitutes a first approach for using biosynthesized nanoparticles in solar cells and opens the interest in using other biosynthesized nanoparticles in energy devices as a way to develop greener photovoltaic technologies at low production costs.
Results and discussion
Environmental isolate of Bacillus mycoides
Biosynthesis of TiO2 nanoparticles by the isolated strain of B. mycoides was carried out by exposing bacterial cultures to titanyl hydroxide at 37°C, the optimal growth temperature determined for this environmental isolate (not shown). Then, the temperature of the culture was diminished (20–25°C) to stop the reaction and a white precipitate was formed, indicative of TiO2 nanoparticles synthesis. The precipitate was purified from the culture, washed and resuspended in Mili-Q ultra pure water for subsequent studies.
To date, there are two studies reporting the use of the genus Bacillus for biosynthesis of TiO2 nanoparticles, however, these reports do not use titanyl hydroxide as precursor [] and require high temperatures for the synthesis of NPs [] (Table 1). Differences in the biosynthetic process mediated by B. mycoides suggest that different biomolecules could be involved in biosynthesis and that the produced NPs could display novel properties.
Transmission electron microscopy (TEM)
Fourier transform infrared spectroscopy (FTIR) analysis
UV-visible spectroscopy and Tauc Plot analysis
In order to determine the UV-visible absorption spectrum of biosynthesized TiO2 nanoparticles, the product of biosynthesis was washed gently with ethanol and resuspended in ultrapure water to remove organic residues that might interfere with the measurement.
Antibacterial activity of biosynthesized TiO2 nanoparticles
Toxicity of TiO2 nanoparticles has become a relevant parameter since it can determine its use in different technological applications. Almost all publications related to biosynthesis of TiO2 NPs report the toxic and/or phototoxic effects of them (Table 1). Toxicity can decrease the number of technological applications in which TiO2 NPs can be used, but is the base of its use as antimicrobials and photo-reactive compounds. The use of nanoparticles in solar cells is not the exception and in addition to proper size, composition and semiconductor properties, increased biocompatibility will strongly favor their application in harvesting solar energy.
Most of the damage that TiO2 nanoparticles produce on microorganisms has been associated to phototoxicity (Table 1). When photocatalytic activity against E. coli was evaluated, a small decrease in CFU was determined for chemically synthesized TiO2 (Figure 5b). This is due to lipid peroxidation on bacterial cell membranes produced by reactive oxygen species (OH•, O2 − and H2O2) generated when titanium dioxide NPs are irradiated with UV light []. When E. coli cultures were amended with biologically synthesized TiO2 NPs and exposed to UV-B light, no effect on cell viability was determined, indicating that biosynthesized NPs do not display phototoxicity under the evaluated conditions (Figure 5b). Based on these results we can speculate that the organic coating of nanoparticles produced by B. mycoides protects bacteria from the phototoxic damage by interacting with UV-produced radicals. In this context, additional purification steps decreasing the organic matter of NPs probably increase phototoxicity of biosynthesized TiO2 NPs.
Characterization and I-V measurement of the quantum dot sensitized solar cells
Based on the favorable properties of biosynthesized TiO2 nanoparticles as wide-band gap semiconductors with low levels of toxicity, we decided to evaluate their use on solar cells.
The control solar cells produced with chemical titanium dioxide nanoparticles display the highest values of voltage and current, confirming that the flow of electrons between the CdTe-GSH QDs and chemical TiO2 NPs works properly. Moreover, the QDSSC that uses biosynthesized TiO2 nanoparticles shows decreased values of I SC and V OC , attributable to the presence of calcined organic matter on the surface of nanoparticles after sintering of the material; this calcined matter would work as insulation in electrical conduction. However, when the organic coating of TiO2 nanoparticles was removed, V OC and I SC values are significantly increased and the performance of the solar cell using biosynthesized TiO2 nanoparticles is similar to that observed in the control. The results obtained indicate that it is possible to use the TiO2 nanoparticles produced by B. mycoides in the development of greener solar cells.
TiO2 nanoparticles biosynthesized by B. mycoides exhibit low toxicity against E. coli, probably as consequence of the organic coating.
Biosynthesized nanoparticles are able to conduct electrons in QDSSC with values near those determined in a control solar cell produced with chemically synthesized TiO2 nanoparticles.
The main projection of this work is the use of these and other green nanoparticles in the sustainable manufacturing of solar cells to develop ecologically friendly and less expensive photovoltaic panels.
Synthesis of titanyl hydroxide precursor
The reaction mixture was stirred for two hours under ice-cold condition until an opalescent suspension of TiO(OH)2 was obtained. The isopropyl alcohol (propan-2-ol) was removed by successive centrifugations and washings with Mili-Q ultrapure water. Finally, titanyl hydroxide was resuspended in water. The precursor concentration was obtained by determining the dry mass of a 1 mL solution, which was lyophilized (freeze-dried) for 24 h.
Biosynthesis of TiO2 nanoparticles
A culture of 200 μL of B. mycoides grown overnight was used to inoculate 200 mL of LB medium (dilution 1:1000). This culture solution was grown for 12 h at 37°C with constant shaking (150 RPM). Then, 40 mL of a 25 mM titanyl hydroxide solution were added and the mixture incubated at 37°C for 24 h with constant shaking. After this time, the solution was incubated at room temperature for 8 h and the appearance of a white precipitate was indicative of the production of titanium dioxide nanoparticles. The precipitate was removed from the culture by centrifuging 15 min at 3820 × g. Finally, the biosynthesis product was washed and resuspended by successive centrifugations in Mili-Q ultra pure water.
Characterization of TiO2 nanoparticles
Biosynthesized TiO2 nanoparticles were characterized by UV-visible spectroscopy using a Synergy™ H1 Microplate Reader (BioTek Instrument Inc.). Absorbance spectrum between 300–700 nm (2 nm resolution) was performed and used for band gap (E bg ) determination using the Tauc relation [].
For Transmission Electron Microscopy (TEM) studies, a suspension of TiO2 nanoparticles was deposited on a copper grid and examined using a Low Voltage Transmission Electron Microscope 5 (LVEM5) (Delong Instruments) operated at 5.1 kV. The size distribution histogram was performed using ImageJ software. Dinamic Light Scattering (DLS) was performed in a Zetasizer Nano ZS (Malvern Instrument Ltd.) equipment using the protocol previously described by our group [].
For the Fourier Transform Infrared Spectroscopy (FT-IR) characterization samples were lyophilized (freeze-dried) for 24 h and the powder was mixed with KBr to form a thin pellet. FT-IR measurements were carried out using a Spectrum One FT-IR Spectrometer (Perkin Elmer Inc.) in the 400–4000 cm−1 range with a 4 cm−1 resolution.
Antibacterial activity of TiO2 nanoparticles
The antibacterial activity of biosynthesized TiO2 nanoparticles was evaluated against E. coli (BW25113). Bacterial cultures were grown in LB medium at 37°C with constant shaking (150 RPM). After 3 h incubation (OD600 = 0.3) cultures were amended with 200 μg/mL of chemically (TiO2 nanopowder from Sigma-Aldrich, ~21 nm particle size) or biologically synthesized TiO2 nanoparticles. The photocatalytic effect of nanoparticles was evaluated using the same concentrations indicated above, but irradiating the culture with UV-B light for 2 min in the presence of the nanoparticles (OD600 = 0.3).
The effect of TiO2 nanoparticles on bacterial growth was evaluated by determining the number of colony forming units (CFU) over time. Culture aliquots were taken every hour and diluted to obtain 10−1 to 10−7 serial dilutions. 5 μL of every dilution were plated on LB agar, and incubated at 37°C for 12 h. After this time, CFU were determined.
Fabrication and characterization of quantum dot sensitized solar cells
QDSSCs were produced following the protocols described by Bang et al. [], Giménez et al. [] and Pan et al. [], with some modifications. To fabricate the electrodes of QDSSCs, 10 × 10 × 2 mm size fluorine doped tin oxide coated glass (FTO glass) TEC15, with a surface resistivity of 13 [Ω/sq] and 85% transmittance was used. Conductive glasses were cleaned by successive sonication in absolute ethanol and deionized water for approximately 10 min to remove organic contaminants. The anode was prepared using a suspension of biosynthesized TiO2 nanoparticles that was deposited on the glass through spin-coating at 2000 rpm for 10 sec.
To prepare a uniform titanium dioxide film that facilitates electron transfer process in QDSSCs, it is important to remove the organic coating on the surface of biosynthesized TiO2 nanoparticles. For this reason, an additional purification step was performed. Nanoparticles were treated with 1% sodium dodecyl sulfate (SDS) and the solution was sonicated gently for a few seconds to allow disaggregation of the nanoparticles. Then, the suspension of TiO2 nanoparticles was recovered by centrifugation, washed and resuspended in Mili-Q ultra pure water. Titanium(IV) oxide nanopowder from Sigma-Aldrich and biosynthesized TiO2 nanoparticles were used to manufacture the photoanodes of QDSSCs. The electrodes (TiO2 films) underwent a sintering process at 450°C for 30 min. Sensitization of TiO2 film was performed by direct adsorption of CdTe-GSH quantum dots (QDs) []. The active area of the cells was 0.16 cm2. Moreover, the cathode or counter electrode was prepared from a solution of H2PtCl6 · 6H2O in isopropanol. 10 μL of the solution were dispensed on a FTO coated glass by spin-coating and heated 20 min at 400°C.
Then, the photoanode and the counter electrode were assembled leaving 127 μm space between them. Before sealing the cell, a drop of electrolyte was added. The electrolyte solution used was sulfide/polysulfide (S2−/S n 2−) prepared from Na2S (1.0 M), S (0.1 M) and NaOH (0.1 M) in Mili-Q ultrapure water. Characterization of solar cells was performed under constant conditions of temperature and irradiance at a one sun intensity as the light source (~100 mW · cm−2 and AM1.5).
The authors would like to acknowledge Dr. Eduardo Soto for assistance with the chemical procedures and Drs. Daniel Aguayo and Hegaly Mendoza for assistance with TEM analysis. This work was supported by FONDECYT 11110077 (JMP), FONDECYT 11110076 (DB), INACH T-19-11 (JMP, DB), Anillo ACT 1107 (JMP) and Anillo ACT 1111 (JMP, DB). A doctoral fellowship from CONICYT to JPM is also acknowledged.
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