Fabrication of Iron Oxide Nanoparticles via Submerged Photosynthesis and the Morphologies under Different Light Sources

Abstract

Recently, metal oxide nanocrystallites have been synthesized through a new pathway, i.e., the submerged photosynthesis of crystallites (SPSC), and flower-like ZnO and CuO nanostructures have been successfully fabricated via this method. In this work, the SPSC process was applied for the fabrication of iron oxide and hydroxide nanoparticles. The experiments were conducted under visible light, ultraviolet light, and gamma-ray irradiation conditions and the morphologies of the obtained nanoparticles were observed and compared with that obtained without illumination. Then, the mechanism of the SPSC process for the fabrication of iron oxide nanoparticles was discussed. The results show that various kinds of morphologies of nanocrystallites were obtained on the Fe plate surface and the main morphologies are different under different conditions. For example, most FeOOH with the morphologies of nanorod and nanofiber exist by visible light irradiation; most faceted crystals of FeOOH and Fe₂O₃ with the morphologies of nanograular and nanorod exist by ultraviolet irradiation. In the SPSC process, light irradiation generates ·OH at the crystal tips and promote the crystallization in apical growth of FeOOH.

1. Introduction

Metal oxides are one of the most widely investigated inorganic materials because they are ubiquitous in nature and commonly used in technological applications. Recently, the wide range of nanoscale forms of these materials has gained much attention owing to their anticipated properties and application in different areas, such as photoelectron devices, sensors, catalysts, etc.^{1,2,3,4,5,6,7)} Among these metal oxides, studies have especially focused on iron oxide nanocrystals (NCs) in biomedical application, such as diagnostic magnetic resonance imaging (MRI),^8,9,10) thermal therapy,^11,12) and drug delivery^8,13,14) because of their superparamagnetic properties,^9,15) biocompatibility,^16,17) and non-toxicity. Additionally, because most of the iron oxides have a bandgap in visible light spectral region (~ 2 eV),^18,19,20) they are promising materials for optoelectronics and photonics applications with solar energy.¹³⁾

The shapes of iron oxide NCs have tremendous impact on their properties. Therefore, it has been a scientific and technological challenge to synthesize the iron oxide nanoparticles with customized morphologies and size.²¹⁾ Physical methods such as gas phase deposition²²⁾ and electron beam lithography²³⁾ are elaborative procedures that suffer from the inability to control the particles size in nanometer size range. The wet chemical routes to magnetic nanoparticles are simpler, more tractable and more efficient with appreciable size control, composition and sometimes even the shape of the nanoparticles.¹³⁾ In these conventional methods, the morphologies and composition of the nanoparticles depends on the type of salts used, pH of the solution, ionic strength of the media, etc., which make the process complicated.

In a previous study, Jeem et al.²⁴⁾ reported a new pathway for the synthesis of a variety of metal oxide NCs via submerged illumination in water, called the submerged photosynthesis of crystallites (SPSC). This method is completely different from typical synthetic methods for nanoparticles. In the SPSC method, the growth of metal oxide NCs is assisted by a ‘photosynthesis’ reaction, where the metal surface is irradiated with light in neutral water.²⁴⁾ Thus, the SPSC process requires only light and water and does not require the incorporation of impurity precursors. Moreover, this method is applicable at room temperature and at atmospheric pressure, producing only hydrogen gas as the by-product. These characteristics give rise to the potential application of SPSC as a green technology for metal oxide NC synthesis.

At present, flower-like NCs of zinc oxide^24,25,26) and cupric oxide²⁷⁾ have been successfully synthesized using the SPSC method. The application of SPSC method to the fabrication of other metal oxide nanoparticles has not been reported. Therefore, the possibility of applying the SPSC process on the iron oxide NCs is studied in this work. The morphologies of the products obtained under different light sources are observed and the mechanism is discussed.

2. Experimental

2.1. SPSC Experiment

Iron plate (35 × 5 × 0.5 mm, 99.5%, Nilaco, Japan) was placed in a 4-mL cuvette with ultrapure water (Wako Pure Chemical, Japan), which was deaerated by boiling, and then irradiated under ultraviolet (UV) light (UVP, B-100AP, Analytik Jena US, λ = 365 nm) or visible light (LIGHTNINGCURE, LC8, Hamamatsu Photonics, λ: 400–600 nm) for 0–144 h in a lightproof chamber. The intensity of the UV irradiation was 10–53 mW·cm⁻². During the SPSC experiment, the pH and temperature of the water were measured using a pH/ORP meter (LAQUA, D-72, Horiba) containing a micro ToupH electrode (LAQUA, 9618S, Horiba), which is capable to measure pH change in low volume (50 μL) sample, and a long ToupH electrode (LAQUA, 9680S-10D, Horiba). The gas generated during the SPSC process was collected in a vial and analyzed by gas chromatography (GC-14B, Shimadzu).

In the gamma-ray irradiation SPSC experiment, the Fe plate was placed into a test tube with 3 mL ultrapure water and then irradiated with gamma-rays, which was performed at the ⁶⁰Co irradiation facility of the Institute of Scientific and Industrial Research (ISIR) at Osaka University. The gamma-ray dose rate was determined by the distance from the sample to the ray source. The absorbed dose was calculated by Fricke dosimetry.

2.2. Dark Condition Experiment and Superoxide Dismutase (SOD) Addition Experiment

The dark condition experiment was conducted in a lightproof chamber without illumination, in which the Fe plate was immersed in the deaerated ultrapure water for several hours. In the dark condition experiment, a heater was used to control the water temperature. SOD addition experiment was performed in gamma-ray irradiation SPSC experimental procedures. SOD from bovine erythrocytes (Sigma-Aldrich, USA) was dissolved in ultrapure water at a concentration of 1.4–3.0 g/100 mL. Then, the Fe plate was submerged in SOD solution and irradiated by gamma-ray.

2.3. Physical Characterization

X-ray diffraction (XRD) patterns of the samples were obtained using an X-ray diffractometer (Miniflex, Rigaku) equipped with a Cu Kα source operating at 40 kV and 15 mA. The surface morphologies were observed by field emission scanning electron microscopy (FE-SEM, JSM-7001FA, JEOL). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns for the crystal were obtained using a conventional transmission electron microscope (JEM-2000FX, JEOL) operated at 200 kV. The nanoparticles were characterized using an X-ray photoelectron spectroscopy (XPS, JPS-9200, JEOL) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The analyzed area of the samples was 3 mm × 3 mm (large scale). The peak positions and areas were optimized by a weighted least-squares fitting method using 70% Gaussian and 30% Lorentzian line shapes. All XPS spectra were calibrated to the C (1s) core level peak at 286.0 eV.

3. Results and Discussion

Under both dark and light irradiation conditions, NCs with various kinds of morphologies were obtained, such as nanoplate, nanorod, nanofiber, nanoball, hexagram nanorod, nanogranular, etc. Figure 1 shows the overview of the various morphologies of the nanoparticles generated under both dark and light irradiation conditions.

Fig. 1.

Overview of the various morphologies observed on the Fe plated under different conditions (Dark: dark condition; UV: UV irradiation condition; VL: visible light irradiation condition). (Online version in color.)

Although many kinds of morphologies were observed on the Fe plate after the experiment under different conditions, it is found that some kinds of morphology trends to be formed under a certain condition. Figure 2 shows the mostly observed morphologies of the NCs under light irradiation conditions. Under visible light condition, many nanofibers and nanorods were observed, as shown in Figs. 2(a)–2(c). The length of the rods and fibers increased with irradiation time. EDS results indicate that most of the nanorods and nanofibers are FeOOH, and some nanorods are Fe₂O₃. Figures 2(d)–2(f) shows the mainly morphologies observed under UV irradiation conditions. Different with the morphologies formed under visible light condition, faceted crystals such as faceted nanogranular, faceted hexagram nanorod and nanoflower were mostly observed under UV irradiation condition. The EDS results indicated that nanogranulars are Fe₂O₃, and the nanorods are FeOOH and Fe₂O₃.

Fig. 2.

Typical morphologies of the NCs obtained under visible light condition (a–c) and UV (d–f) condition. (Online version in color.)

Comparing with illumination conditions, the NCs generated under dark conditions are quite different, as shown in Fig. 3. Figures 3(a)–3(c) show the morphologies obtained under dark condition at room temperature. The accumulation of nanofibers was initially formed, then the nanofibers grow in both length and width direction and formed flower-like NCs when the reaction time increased. Figures 3(d)–3(f) show the morphologies when the water temperature was controlled to 40°C under dark condition. The planar nanorods were mostly existed after 24 h reaction, which indicated that the increase of water temperature promoted the growth of nanofibers to nanorods under dark condition. With reaction time increases, the growth of the planar nanorods were observed and the flower-like morphologies were also observed.

Fig. 3.

Typical morphologies of the NCs obtained under dark condition at room temperature (a–c) and at 40°C (d–f). (Online version in color.)

The XRD analysis was conducted for the specimen obtained under each condition, and only iron peak (JCPDS, 00-001-1267) was detected because of the small amount of NCs on the Fe plate substrate. Therefore, the XPS spectral analysis was performed to determine the surface chemical composition on the Fe plate. Figure 4 shows the XPS spectrum of Fe 2p_3/2 for the specimens obtained under different conditions. Three Gaussian-Lorentzian fitting peaks were used to fit the experimental data. The peaks located at 711.9–712.1 eV, 710.4–710.7 eV, and 709.1–709.4 eV are the assigned as FeOOH, Fe(III) in Fe₂O₃, and Fe(II) peak, respectively.^28,29,30,31) The peak intensity and area changes indicated that Fe(II) mostly existed in the specimens obtained under dark conditions (Figs. 4(a), 4(b)), while more FeOOH and Fe₂O₃ exist under light irradiation compared with that under dark conditions. Under UV irradiation condition, the strongest Fe(III) peak was obtained, indicating the high ratio of Fe₂O₃ on the surface of the specimen.

Fig. 4.

XPS spectra of the Fe 2p_3/2 of the Fe specimens obtained under different conditions for 24 h: (a) dark condition at room temperature, (b) dark condition at 40°C, (c) visible light condition, (d) UV condition. (Online version in color.)

TEM observation of the typical morphologies obtained by 40 h UV irradiation are shown in Fig. 5. The nanorods with hexagram morphologies were observed as shown in Fig. 5(a). The SAED pattern analysis shows that these hexagram nanorods are lepidocrocite (Fe₂O₃·H₂O), and grow in a [010] direction. Figure 5(d) shows the planar nanorods with tapered tip, and the EDS analysis of it indicates a composition of Fe₂O₃. According to the above results, the photochemical reactions that occur during SPSC were deduced and a schematic illustration of the NCs growth reaction mechanism is shown in Fig. 6.

Fig. 5.

TEM images of the NCs obtained under UV condition by 40 h irradiation. (c) SEAD pattern of the P1 position in (b). (f) EDS profile of the P2 position in (e), and the inset tables shows the atom ratio of O and Fe. (Online version in color.)

Fig. 6.

A schematic view of the mechanism of iron oxide and hydroxide crystal growth in the SPSC process. (Online version in color.)

It is well known that the corrosion reactions of Fe with water include local anode reaction and local cathode reaction from the viewpoint of electric chemistry mechanism. In neutral water, the following reactions occur.   

$$\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\mathrm{\hspace{1em}}\text{Fe}\to {\text{Fe}}^{2+}+2e^{-}$$(1)

  

$$\begin{array}{l}\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\\ \frac{1}{2}{\text{O}}_2+{\text{H}}_2\text{O}+2e^{-}\to 2{\text{OH}}^{-}\end{array}$$(2)

Then, Fe²⁺ reacts with OH⁻, and the initial rust is generated:   

$${\text{Fe}}^{2+}+2{\text{OH}}^{-}\to \text{Fe}{\left(\text{OH}\right)}_2$$(3)

Fe(OH)₂ is not stable in water, and easily reacts with the dissolved oxygen to generate FeOOH by Reaction (4) and (5). The Gibbs energy ΔG of each reaction was calculated by HSC Chemistry software (Outokumpu Research Oy, Pori, Finland).   

$$\begin{array}{l}2\text{Fe}{\left(\text{OH}\right)}_2+{\text{H}}_2\text{O}+\frac{1}{2}{\text{O}}_2\to 2\text{Fe}{\left(\text{OH}\right)}_3\\ (\mathrm{\Delta }\mathrm{G}=-190\mathrm{k}\mathrm{J}\mathrm{a}\mathrm{t}25°\mathrm{C})\end{array}$$(4)

  

$$\text{Fe}{\left(\text{OH}\right)}_3\to \text{FeOOH}+{\text{H}}_2\text{O}\hspace{1em}(\mathrm{\Delta }\mathrm{G}=-21\mathrm{k}\mathrm{J}\mathrm{a}\mathrm{t}25°\mathrm{C})$$(5)

These initial formed Fe(OH)₂ and FeOOH act as the seeds for the further growth of the NCs.

The bandgap energy of Fe(OH)₂ and FeOOH is close to 2 eV;³²⁾ therefore, the electrons can be excited from valence band to conduction band by both visible light and UV under light irradiation conditions in the SPSC process by the photosemiconductive reaction:³³⁾   

$$\text{SC}+hv\to \text{SC}\left(e^{-}+h^{+}\right)\hspace{1em}(\mathrm{S}\mathrm{C}:\mathrm{s}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r})$$(6)

In the former study on SPSC, it is found that the SPSC process is photocatalytic, accompanied by hydroxyl radical generation via water splitting.^24,25,26) The generated electrons build up at the apical portion of the initial rust of iron hydroxides Fe(OH)₂ and FeOOH, whereas the holes left at the bottom of the concave, as shown in Fig. 6. Photochemical water-splitting reactions then build up holes at the bottom, which subsequently contribute to ·OH generation.³⁴⁾   

$${\text{H}}_2\text{O}+h^{+}\to \cdot \text{OH}+{\text{H}}^{+}\mathrm{\hspace{0.17em} }\left(\text{water}\mathrm{\hspace{0.17em} }\text{splitting}\right)$$(7)

Meanwhile, electrons accumulated at the tip. The formation of hydrated electrons induces the transform of ·OH radicals to OH⁻ ions and contributes to the generation of an alkaline atmosphere at the end of initial iron hydroxides Fe(OH)₂ and FeOOH tip.³⁵⁾   

$$e^{-}\to e_{aq}^{-}\left(\text{hydrated}\mathrm{\hspace{0.17em} }\text{electron}\mathrm{\hspace{0.17em} }\text{formation}\right)$$(8)

  

$$\cdot \text{OH}+e_{aq}^{-}\to {\text{OH}}^{-}\left({\text{OH}}^{-}\mathrm{\hspace{0.17em} }\text{formation}\right)$$(9)

Thus, the local separation of OH⁻ at the apex and H⁺ at the bottom occurs on the surface of the iron hydroxide. On the other hand, free radicals and free electrons can be easily introduced into water especially with the existence of Fe²⁺/Fe³⁺. Water can break down in several ways to form reactive products³⁶⁾ and generate O₂ in water,³⁷⁾ which could enhance Reaction (2) as well as Fe²⁺ generation by Reaction (1). Therefore, Fe²⁺ reacted with OH⁻ at the apex, and FeOOH growth in the apical direction. Here, under light irradiation conditions, Reaction (3), and the following reaction can also occur with the generation of hydrogen gas.   

$$\begin{array}{l}2\text{Fe}{\left(\text{OH}\right)}_2+2{\text{H}}_2\text{O}\to 2\text{Fe}{\left(\text{OH}\right)}_3+{\text{H}}_2\\ (\mathrm{\Delta }\mathrm{G}=47\mathrm{\hspace{0.17em} }\mathrm{k}\mathrm{J}\mathrm{a}\mathrm{t}25°\mathrm{C})\end{array}$$(10)

Although ΔG of the above reaction is positive, the photo energy from light irradiation could drive the reaction to the right. Furthermore, according to the photo-Fenton reaction, it is considered that the reaction Fe²⁺+·OH→Fe³⁺+OH⁻ could occur during the SPSC process. The further study on this is still under investigation.

Therefore, the nanorods and nanofibers were mostly observed on the surface of the specimens obtained by visible light irradiation.

Moreover, as the photo energy of UV light is higher than the bandgap of FeOOH, heat energy could be generated by the excess energy, and the dehydration reaction of FeOOH to Fe₂O₃ is considered by following endothermic Reaction (11).   

$$2\text{FeOOH}\to {\text{Fe}}_2{\text{O}}_3+{\text{H}}_2\text{O}$$(11)

As a result, the strong peak of Fe₂O₃ was detected by XPS spectra analysis when under UV irradiation condition.

Additionally, the gases after the experiment were collected and analyzed by gas chromatograph. The results show that hydrogen gas was generated during the SPSC process. The hydrogen generation reactions are considered as the Reaction (10) and Reaction (12):   

$$2{\text{H}}^{+}+2e^{-}\to {\text{H}}_2$$(12)

Based on the above discussion, the SPSC process of Fe oxide NCs fabrication involves photoradical reactions, which is the main source of OH⁻ for FeOOH apical growth. To confirm such photoradical reactions in the SPSC process, the additional experiment of gamma-ray irradiation was conducted. Gamma-ray irradiation can efficiently generate radiolysis products of water, such as ·OH, $e_{aq}^{-}$, H(H₂), H₂O₂, and H₃O⁺.³⁶⁾ Figure 7 shows the morphologies of the Fe specimens in the gamma-ray irradiation SPSC experiment with a dosage of 150–500 kGy in ultrapure water (Figs. 7(a)–7(c)) and in SOD solution (Figs. 7(d)–7(f)).

Fig. 7.

SEM images of the Fe plates after gamma-ray irradiation (in ultrapure water: a–c, in SOD solution: d–f).

When the Fe plate was submerged in ultrapure water, the fine acicular NCs were observed. It indicates that the radical reactions due to the gramma-ray irradiation of high linear-energy-transfer (LET) radiation, have an important impact on the morphologies of the product in the SPSC process.

It should be noted that dissolved oxygen gas also enhances the iron oxide formation during the SPSC process. The dissolved oxygen increases the concentration of H₂O₂, H₂ and superoxide anion (${\text{O}}_2^{-}$) in gamma-ray irradiated aerated water.³⁸⁾ ${\text{O}}_2^{-}$ reduces the ·OH to OH⁻ by the following reaction:   

$$\cdot \text{OH}+{\text{O}}_2^{-}\to {\text{O}}_2+{\text{OH}}^{-}$$(13)

The increase in local OH⁻ concentration may enhance the Reaction (3) and then enhance the growth of NCs during the SPSC process.

However, when the sample was submerged in SOD solution, almost no NCs were found after gamma-ray irradiation. The SOD reagent is known to capture ${\text{O}}_2^{-}$.^39,40) The suppressed growth of NCs in SOD solution suggested that ${\text{O}}_2^{-}$ plays an important role in NCs growth and revealed that crystal formation via SPSC was caused by photolysis or radiolysis in oxygenated water.

Moreover, according to the mechanism analysis of the SPSC process, the local anode reactions and local cathode reactions occur on the surface. The local cathode reactions occur mainly on the tip of the NCs and the water here is alkaline with OH⁻. While the local anode reactions occur mainly at the bottom of the surface NCs and the water here is acidic with H⁺. In the SPSC process, the pH and temperature change of the overall water was monitored during UV irradiation and a random fluctuation of pH was observed. To measure the exact locally pH change of the water, the separation of local cathode and local anode areas is necessary and the study on it will be conducted in the future work.

4. Conclusion

Iron oxide and hydroxide NCs with various kinds of morphologies were obtained via SPSC process by using only iron and ultrapure water under light irradiation conditions, including visible light, UV, and gamma-ray. The main morphologies of the NCs obtained under each condition are different. The nanorod and nanofiber of FeOOH mostly exist by visible light irradiation; the faceted crystals of nanograular and nanorod of FeOOH and Fe₂O₃ mostly exist by UV irradiation; the fine acicular NCs mostly exist by gamma-ray irradiation. The mechanism analysis of the SPSC process shows that light irradiation generates ·OH at the crystal tips and promote the crystallization in apical growth of FeOOH. In this process, the radical reactions play an important role, which was proved by the gamma-ray irradiation experiment and SOD addition experiment.

Acknowledgements

This work was partially supported by ISIJ Research Promotion Grant (26th, 2017), and “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sport, Science and Technology (MEXT), Japan. This work was partially performed under the Research Program for CORE lab of “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “Network Joint Research Center for Materials and Devices”. The authors express their thanks to Mr. Dai. Takai, a former student at Hokkaido Unversity, for his cooperation in the experiments and data analysis, as well as the members of the Research Laboratory for Quantum Beam Science, ISIR, Osaka University, for their assistance in the gamma-ray irradiation experiments.

References

• 1)   X.  Yu,  T. J.  Marks and  A.  Facchetti: Nat. Mater., 15 (2016), 383.
• 2)   E.  Comini: Anal. Chim. Acta, 568 (2006), 28.
• 3)   C.  Wang,  L.  Yin,  L.  Zhang,  D.  Xiang and  R.  Gao: Sensors, 10 (2010), 2088.
• 4)   M. R. M.  Bin Julaihi,  P. N.  Yuh Yek,  S.  Yatsu and  S.  Watanabe: ISIJ Int., 58 (2018), 1162.
• 5)   Y.  Yamada,  C.-K.  Tsung,  W.  Huang,  Z.  Huo,  S. E.  Habas,  T.  Soejima,  C. E.  Aliaga,  G. A.  Somorjai and  P.  Yang: Nat. Chem., 3 (2011), 372.
• 6)   E.  Comini,  G.  Faglia,  G.  Sberveglieri,  Z.  Pan and  Z. L.  Wang: Appl. Phys. Lett., 81 (2002), 1869.
• 7)   Z. L.  Wang: Mater. Sci. Eng. R Rep., 64 (2009), 33.
• 8)   B.  Chertok,  B. A.  Moffat,  A. E.  David,  F.  Yu,  C.  Bergemann,  B. D.  Ross and  V. C.  Yang: Biomaterials, 29 (2008), 487.
• 9)   Y.-X. J.  Wang: Quant. Imaging Med. Surg., 1 (2011), 35.
• 10)   D.  Pouliquen,  J. J.  Le Jeune,  R.  Perdrisot,  A.  Ermias and  P.  Jallet: Magn. Reson. Imaging, 9 (1991), 275.
• 11)   S.  Laurent,  S.  Dutz,  U. O.  Häfeli and  M.  Mahmoudi: Adv. Colloid Interface Sci., 166 (2011), 8.
• 12)   B.  Samanta,  H.  Yan,  N. O.  Fischer,  J.  Shi,  D. J.  Jerry and  V. M.  Rotello: J. Mater. Chem., 18 (2008), 1204.
• 13)   A. K.  Gupta and  M.  Gupta: Biomaterials, 26 (2005), 3995.
• 14)   T. K.  Jain,  M. A.  Morales,  S. K.  Sahoo,  D. L.  Leslie-Pelecky and  V.  Labhasetwar: Mol. Pharm., 2 (2005), 194.
• 15)   R.  Weissleder,  D. D.  Stark,  B. L.  Engelstad,  B. R.  Bacon,  C. C.  Compton,  D. L.  White,  P.  Jacobs and  J.  Lewis: Am. J. Roentgenol., 152 (1989), 167.
• 16)   T. K.  Jain,  M. K.  Reddy,  M. A.  Morales,  D. L.  Leslie-Pelecky and  V.  Labhasetwar: Mol. Pharm., 5 (2008), 316.
• 17)   D.  Ling and  T.  Hyeon: Small, 9 (2013), 1450.
• 18)   L.  Vayssieres,  C.  Sathe,  S. M.  Butorin,  D. K.  Shuh,  J.  Nordgren and  J.  Guo: Adv. Mater., 17 (2005), 2320.
• 19)   K.  Sivula,  F.  Le Formal and  M.  Grätzel: ChemSusChem, 4 (2011), 432.
• 20)   M. C.  Pereira,  E. M.  Garcia,  A.  Cândido da Silva,  E.  Lorençon,  J. D.  Ardisson,  E.  Murad,  J. D.  Fabris,  T.  Matencio,  T.  de Castro Ramalho and  M. V. J.  Rocha: J. Mater. Chem., 21 (2011), 10280.
• 21)   A.  Shavel and  L. M.  Liz-Marzán: Phys. Chem. Chem. Phys., 11 (2009), 3762.
• 22)   S.  Park,  S.  Lim and  H.  Choi: Chem. Mater., 18 (2006), 5150.
• 23)   C.  Vieu,  F.  Carcenac,  A.  Pépin,  Y.  Chen,  M.  Mejias,  A.  Lebib,  L.  Manin-Ferlazzo,  L.  Couraud and  H.  Launois: Appl. Surf. Sci., 164 (2000), 111.
• 24)   M.  Jeem, M. R. bin Julaihi, J. Ishioka, S. Yatsu, K. Okamoto, T. Shibayama, T. Iwasaki, T. Kato and S. Watanabe: Sci. Rep., 5 (2015), 11429.
• 25)   M.  Jeem,  L.  Zhang,  J.  Ishioka,  T.  Shibayama,  T.  Iwasaki,  T.  Kato and  S.  Watanabe: Nano Lett., 17 (2017), 2088.
• 26)   L.  Zhang,  M.  Jeem,  K.  Okamoto and  S.  Watanabe: Sci. Rep., 8 (2018), 177.
• 27)   F.  Nishino,  M.  Jeem,  L.  Zhang,  K.  Okamoto,  S.  Okabe and  S.  Watanabe: Sci. Rep., 7 (2017), 1063.
• 28)   Y.  Guo,  E.-H.  Han and  J.  Wang: J. Mater. Sci. Technol., 31 (2015), 403.
• 29)   K.  Zhu,  C.  Jin,  Z.  Klencsár,  A.  Ganeshraja and  J.  Wang: Catalysts, 7 (2017), 138.
• 30)   O.  Lupan,  V.  Postica,  N.  Wolff,  O.  Polonskyi,  V.  Duppel,  V.  Kaidas,  E.  Lazari,  N.  Ababii,  F.  Faupel,  L.  Kienle and  R.  Adelung: Small, 13 (2017), 1062868.
• 31)   A. P.  Grosvenor,  B. A.  Kobe,  M. C.  Biesinger and  N. S.  McIntyre: Surf. Interface Anal., 36 (2004), 1564.
• 32)   E. R.  Encina,  M.  Distaso,  R. N.  Klupp Taylor and  W.  Peukert: Cryst. Growth Des., 15 (2015), 194.
• 33)   A. J.  Bard: J. Photochem., 10 (1979), 59.
• 34)   L. W.  Zhang,  H. Y.  Cheng,  R. L.  Zong and  Y. F.  Zhu: J. Phys. Chem. C, 113 (2009), 2368.
• 35)   D.  Lawless,  N.  Serpone and  D.  Meisel: J. Phys. Chem., 95 (1991), 5166.
• 36)   S.  Le Caër: Water, 3 (2011), 235.
• 37)   A.  Machulek,  F.  Quina,  F.  Gozzi,  V. O.  Silva,  L. C.  Friedrich and  J. E. F.  Moraes: Organic Pollutants Ten Years After the Stockholm Convention, Chapter 11, ed. by T. Puzyn, IntechOpen, London, (2012), 282.
• 38)   J. M.  Joseph,  B.  Seon Choi,  P.  Yakabuskie and  J.  Clara Wren: Radiat. Phys. Chem., 77 (2008), 1009.
• 39)   T.  Fukai and  M.  Ushio-Fukai: Antioxid. Redox Signal., 15 (2011), 1583.
• 40)   J. M.  McCord and  I.  Fridovich: J. Biol. Chem., 244 (1969), 6049.