Biogenic N-I-codoped TiO₂ photocatalyst derived from kelp for efficient dye degradation

Abstract

The design and fabrication of efficient, cost-effective photocatalysts are required in order to ease global energy and environmental issues. In this paper, we have obtained a biogenic-TiO₂ photocatalyst through a simple one step infiltration process which can replicate well the hierarchical architecture of kelp from the macro- down to the nano-scale. In addition, the nitrogen and iodine, contained in the original plant corpus, are self-doped into the resulting samples. UV-Visible diffuse reflectance spectra of the biogenic-TiO₂ indicate that it has efficient light-harvesting capacity, especially in the region from 400 nm to 550 nm, and compared with the common N-TiO₂, its band gap absorption edge exhibits a clear red shift due to the self-doping. Moreover, the biogenic-TiO₂ possesses excellent photocatalytic properties proved by the photocatalytic activity of methylene blue degradation under solar energy irradiation. This work may provide the inspiration for the synthesis of further high performance photocatalysts based on the kelp structure and a new methodology for application of nature's plants and utilization of solar energy with biogenic materials.

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Broader context

The global energy crisis and environmental pollution have become major concerns to the public in recent years. TiO₂ is still the most commonly investigated photocatalyst which can use sunlight to destroy highly toxic molecules, remediate pollutants, produce clean energy as hydrogen and even convert solar energy to electrical power. Therefore, the development of efficient, cost-effective TiO₂ photocatalysts may be an answer to ease the current environmental and energy problems. In this paper, biogenic-TiO₂ with highly efficient light-harvesting and photocatalytic properties is fabricated by copying the elaborate architecture of kelp from the macro- and micro- down to the nanoscale and by self-doping of iodine and nitrogen, which are contained in the original plant corpus. Excellent photocatalytic properties are demonstrated due to the overall enhancement of light-harvesting by greater absorption and multiple scattering of light by the unique hierarchical architecture of kelp, as well as the narrowed band gap induced by doping. This approach demonstrates a new strategy for mimicking mother nature's elaborate creations in making materials for efficient photocatalysis and current artificial photosynthesis systems.

Introduction

If only one thousandth of the solar energy that reaches the earth could be utilized, we could still have nine times the amount of energy we need.¹ Nowadays, the global energy crisis and environmental pollution have become major threats to the lives of humans. Using solar energy more efficiently could help ease global energy and environmental issues. As photocatalysis can make use of sunlight to destroy highly toxic molecules, remediate pollutants, produce hydrogen and even electrical power,² the design and fabrication of efficient, cost-effective photocatalysts are highly demanded. Titanium dioxide is one of the most viable materials for photocatalysis because of its high oxidative power, low cost, photostability and nontoxicity.³ Unfortunately, due to its wide band gap of 3.2 eV, it can only be activated under UV irradiation which is merely a small fraction (5%) of the solar spectrum.⁴ So far, tremendous efforts have been made to enhance its photocatalytic capacity by modifying both the structures and composition. One approach is to fabricate TiO₂ with high specific surface area⁵ in the form of various nanostructures such as nanotubes, nanoplates, nanoparticles, etc. Another is to improve the composition of TiO₂ by substituting metal ions⁶ (such as Mn⁴⁺, Cr³⁺, V³⁺) at Ti sites, or by doping TiO₂ with non-metal elements (such as nitrogen,⁷sulfur,⁸carbon⁹) or noble metals.¹⁰ Although significant progress has been made, most research has only focused on one approach rather than both of them.

Biogenic materials, which combine natural geometry with synthetic materials chemistry, are products (metals, oxides, carbides, nitrides, composites, etc.) synthesized through employing living things as templates with their structures resembling one another and elements being utilized.¹¹Biogenic-TiO₂ may realize the synergy of both structure and element-introduced improvements of TiO₂ based on a single biotemplate, and be a major advance in photochemistry to develop novel and efficient photocatalysts. On the one hand, multidimensional, sophisticated and diverse biological structures give rise to specific functions, providing inspiration for the development of materials with promising photocatalytic, optical, and electronic properties.¹² For instance, the hierarchical structures of leaves which are designed favorably for photosynthesis endow them with an extremely high light-harvesting efficiency. On the other hand, non-metallic elements derived from various kinds of organisms endowed by nature are expected to be self-doped into the prepared samples. For example, elements such as nitrogen present in plants in different forms have been successfully self-doped into TiO₂.¹³

Here, we present a novel strategy to fabricate biogenic-TiO₂ with highly efficient light-harvesting and photocatalytic properties by copying the elaborate architecture of kelp from the macro- and micro- down to the nanoscale and by the self-doping of iodine and nitrogen which are contained in the original plant corpus. In our experiments, we use algae rather than green leaves as a template due to the fact that the solar energy conversion efficiency is nearly 5% for water plants but less than 1% for land-based organisms.¹⁴ The structure of water plants, such as kelp with most of its chromatophores existing in the epidermis, can maximize the absorption of sunlight, which is beneficial to improving the light-harvesting efficiency. In addition, its porous structure increases the light scattering which enhances the solar energy conversion. Moreover, kelp is rich in natural iodine (including I⁻, IO₃⁻ and organic iodine, etc.¹⁵) which is expected to be self-doped into the resulting samples during synthesis, just as with nitrogen.¹³ Recently, many studies¹⁶ have reported that iodine-doped TiO₂ shows outstanding visible light photocatalytic activity in the decomposition of dye and organic pollutants. UV-Visible diffuse reflectance spectra of this biogenic-TiO₂ indicates that it has a highly efficient light-harvesting capacity, especially in the region from 400 nm to 550 nm. Compared with common N-TiO₂ which is calcined under the same conditions without a template, its band gap absorption edge exhibits a clear red shift. Indeed, the superior photocatalytic activity of biogenic-TiO₂ is proved by the degradation of methylene blue under solar energy irradiation. This work will probably provide a new methodology for the utilization of nature's plants and synthesis of light-harvesting biogenic materials.

Results and discussions

Structure of kelp

Kelp (Fig. 1a) is a type of large seaweed belonging to the brown algae (class Phaeophyceae) and is classified as the order Laminariales.¹⁷Scheme 1 illustrates the structures and functions of the natural kelp as well as the artificial kelp-TiO₂ which is herein named biogenic-TiO₂. Panel A shows the hierarchical structures of kelp at scales ranging from the macro- and micro- down to the nanoscale, with Fig. 1 showing images of the corresponding typical examples. As shown in the cross-section (Scheme 1(a)), it mainly contains epidermis, exodermis, endoderm and pith, endowing kelp with hierarchical porous and interconnected structures which are beneficial for efficient light-harvesting. Meanwhile, the corresponding confocal laser-scanning microscopy image (Fig. 1d) shows a cross-section of fresh kelp. The internal structure (Scheme 1(c)) of an individual epidermis cell reveals that it contains several chromatophores which make the cells look brown-green under the optical microscope (Fig. 1b), and which are observable with red fluorescence in a confocal laser-scanning microscope (Fig. 1c and e). Each chromatophore contains stacked nanolayered (about 50 nm) membranes (Scheme 1d, e), which are visible in the TEM images shown in Fig. 1f and g. A series of pigments¹⁸ of different colors including chlorophyll a and b and fucoxanthin, arranged in the membranes of the chromatophore, are responsible for light harvesting, transmission and conversion. In addition, such stacked nanolayered structures with high surface areas, allowing efficient interactions between the light-harvesting pigments and sunlight, are favorable for efficient light-harvesting and fast charge separation. In fact, the entire structure of kelp is greatly beneficial for light harvesting. For example, the less regularly arranged and porous architectures of the exodermis, endoderm and pith enhance the effective light path length and increase multiple light scattering to heighten light absorption.¹⁹ In addition, kelp becomes enriched in iodine as it grows, up to the maximum in its maturation period. Thus, besides replicating the hierarchical structures and self-doping nitrogen¹³ to achieve high energy conversion efficiencies, iodine is also expected to be self-doped into the resulting TiO₂ samples during the synthesis process in our experiments. The main forms of iodine in kelp are I⁻, IO₃⁻ and organic iodine (Scheme 1C), and I⁻ is the main species of iodine enriched by algae according to Küpper et al.²⁰Iodine in kelp will be released rapidly after it is immersed into water. Fortunately, a number of insoluble organic iodine species can be preserved in kelp's remains.²¹ The whole process is based on structure inheritance of kelp and self-doping TiO₂ with nitrogen and iodine originally existing in the kelp.

Scheme 1 Schematic illustrations of A the hierarchical structures of kelp from the macro- to the nanoscale, B synthetic structures of the artificial oxide semiconductor photocatalyst, C the porphyrin ring of the chlorophyll and different forms of natural iodine and D a possible photocatalytic mechanism in the artificial semiconductor photocatalyst.

Fig. 1 Characterization of natural kelp from the macro- to the nanoscale. a Digital picture of intact kelp. b The upper epidermis visualized by digital microscope (Keyence). c The upper epidermis visualized by fluorescence microscope. d Cross-section observed by confocal laser-scanning microscopy (CLSM). e Cross-section observed under confocal laser-scanning microscope. The epidermis cells are displayed by chlorophyll fluorescence (red) excited by 488 nm light, with insets of the magnified images of the lineate region (e₁ by TEM, e₂ by CLSM). f and gTEM images of a chromatophore.

Synthetic procedure of biogenic-TiO₂

Pieces of fresh kelp were first treated with a dilute HCl solution to remove Na, Ca, P ions etc., and to substitute Mg²⁺ ions in the porphyrins of the chlorophyll molecules with H⁺.²² Afterwards, the as-treated kelp samples were dipped into TiCl₃ solution and left to stand under vacuum, with Ti³⁺ ions being introduced into the kelp by replacing the H⁺ ions in the pheophytins, and reacting with the abundant –OH functional groups on the cellulose and hemicellulose of the cells walls of the exodermis, endoderm, and pith. After calcination at 500 °C, 600 °C and 700 °C, respectively, the N–I-codoped biogenic-TiO₂ with hierarchical porous structures was obtained by removing the biotemplates.

Composition and structural characterization of biogenic-TiO₂ derived from kelp

The X-ray diffraction (XRD) patterns shown in Fig. 2 clearly verify that biogenic-TiO₂ samples calcined at 500 °C, 600 °C and 700 °C are all pure anatase. All of the recorded peaks can be assigned to the tetragonal anatase TiO₂ (I41/amd, No. 141, Joint Committee on Powder Diffraction Standards file number 71-1168), with lattice constants a = 3.797 Å, and c = 9.579 Å. It is commonly believed that the crystal phase is a crucial factor for the photocatalytic activity of TiO₂,²³ and that the anatase form is the active phase in photocatalytic reactions.²⁴ Generally speaking, the rutile phase appears at a calcination temperature of 550 °C.²⁵ According to Kröger et al.,²⁶ the biogenic process may affect the crystal phase formation of TiO₂. Different bio-processes have different influences. Here, our results suggest that using kelp as a biotemplate and the self-doping with N and I could suppress the phase transformation from anatase to rutile even at high temperatures.

Fig. 2 XRD patterns of biogenic-TiO₂ and N-TiO₂.

Fig. 3a and b display the well retained structures of biogenic-TiO₂ derived from kelp, which are similar to the original structures (Fig. 1d). From the FESEM images, we can see clearly the replicated hierarchical porous structures from the macro- to the nanoscale: a high surface area on the macroscale, a porous framework on the microscale, and a nanolayered structure on the nanoscale. The microstructure of the cross-section framework is highly porous with layers about 250 nm wide on the walls (Fig. 3b). The TEM image shows that the crystal sizes of biogenic-TiO₂ calcined at 500 °C, all around 18 nm (Fig. 3d), are smaller on average than those of common N-TiO₂ synthesized without a template of around 30 nm (Fig. 3c). The results coincide with the crystallite sizes of the samples calculated by employing the method proposed by Hall²⁷ from the XRD pattern (Fig. 2). The interconnected nanolayered structure, which resembles the construction of membranes in the chromatophores of kelp, exists in the products as demonstrated also by TEM images (Fig. 3e). Each layer is around 30 nm thick, a little bit thinner than the original ones (∼50 nm), which can be attributed to the shrinkage resulting from calcination, and is confirmed to be the anatase crystalline phase only by the selected area electron diffraction (SAED) pattern in the inset of Fig. 3e. Such porous and nanolayered architectures with high surface areas are favorable for the absorption of light and the generation of photon-induced carriers, and thereby should enhance the photocatalytic properties.

Fig. 3 a FESEM image of a cross-section of biogenic-TiO₂ derived from kelp calcined at 500 °C; the inset is an illustration of the cross-section of such a structure. b A magnification of the hole contained in the yellow rectangle in a. cTEM image of N-TiO₂ synthesized without a template calcined under the same conditions as biogenic-TiO₂, 500 °C. dTEM image of biogenic-TiO₂ derived from kelp calcined at 500 °C. eTEM image of the layered nanostructure template from the chromatophore in biogenic-TiO₂ calcined at 500 °C; the inset shows the SAED pattern.

Fig. 4 shows the X-ray photoelectron spectroscopy (XPS) results for biogenic-TiO₂ derived from kelp calcined at 500 °C. The high-resolution scanning spectrum of N 1s displayed in Fig. 4a shows two peaks, 398.1 eV and 399.8 eV. The 399.8 eV can be attributed to the N atoms located at the interstitial sites of the TiO₂ lattice²⁸ and the low binding energy component located at 398.1 eV could be due to substitutional N in the N–Ti–O structure, according to Sathish et al.²⁹ The inset of Fig. 4b shows the contents of the main elements detected by XPS. Compared with carbonkelp (Fig. S1 of the supporting information†), elements like Na, P, Cl and Mg of the chlorophyll in the chromatophore were not detected in biogenic-TiO₂ by XPS, proving that the dilute HCl has effectively removed them. In addition, the high-resolution XPS spectrum of the I 3d region (Fig. 4b) shows doublet peaks at 629.7 eV and 618.8 eV, respectively, suggesting that iodine exists in the biogenic-TiO₂ sample. The binding energy at 629.7 eV and 618.8 eV should be ascribed to negatively charged iodine I⁻ species.³⁰ Compared with 630.5 eV and 619.2 eV in carbonkelp (Fig. S1 of the supporting information†) calcined under the same conditions as biogenic-TiO₂, the binding energy shifts by 0.8 eV and 0.4 eV, respectively. Since the ionic radius of I⁻ (0.216 nm) is much larger than that of O²⁻ (0.124 nm) or Ti⁴⁺ (0.068 nm), the substitution of Ti⁴⁺ or O²⁻ ions in the lattice by I⁻ would obviously be difficult. So it is possible that the I⁻ species is dispersed on the surface of anatase particles³¹ or at the interstitial sites of the TiO₂ lattice. The atomic content of the I dopant is relatively low. As is well known, iodine in kelp can be released rapidly after it is immersed into water, of which 60–70% is in the form of I⁻. Fortunately, there is a dynamic process between the release and absorption of iodine; 10% of I⁻ could be absorbed again into kelp by prolonging the soaking time appropriately.²¹ We will adjust the soaking time as well as the ratio of water : kelp to improve the doping ratio in our future experiments. Fig. 4c shows a Ti 2p XPS spectrum. Two peaks were observed at 464.25 eV and 458.45 eV, which were assigned to Ti 2p^1/2 and Ti 2p^3/2 respectively, agreeing well with Ti(IV) in pure anatase titanium.³² However, no trace of Ti³⁺ was observed by XPS, although it was observed by EPR spectroscopy (see Fig. 6). This may be attributed to either low amounts of Ti³⁺ which is hard to detect by the XPS technique or because the Ti³⁺ species exists in the subsurface region which is inaccessible by XPS.³³ The O 1s XPS spectrum for biogenic-TiO₂ shown in Fig. 4d can be fitted by three peaks at 532.37 eV, 531.35 eV and 529.72 eV, suggesting three independent environments for O within biogenic-TiO₂. The peak component centred at 529.72 eV corresponds to oxygen in the oxide lattice while the 532.37 eV peak is due to surface OH groups.³⁴ The component at intermediate binding energy (ca. 531.35 eV) can be assumed to be the result of Ti–N–O or N–Ti–O structures,³⁵ as commented above in the case of the N 1s region, in agreement with Spadavecchia et al.³⁶

Fig. 4 X-Ray photoelectron spectra of biogenic-TiO₂ calcined at 500 °C. a N 1s XPS spectrum. b I 3d XPS spectrum, with the inset showing atomic contents of each element. cTi 2p XPS spectrum. d O 1s XPS spectrum.

Fig. 5a shows the UV-vis diffuse reflectance spectra of biogenic-TiO₂ derived from kelp and that of common N-TiO₂. Compared with N-TiO₂ nanoparticles, the absorption measurements of the biogenic-TiO₂ samples show enhanced and stronger photoabsorption in the range of wavelengths from 400 nm to 550 nm. We attribute the light-harvesting enhancements to the hierarchical porous structures of biogenic-TiO₂ from the macro- to the nanoscale: a high surface area on the macroscale, a porous framework on the microscale, and a nanolayered structure on the nanoscale. In addition, the band-gap absorption onsets, at the edge of UV and visible light, show red-shifts of more than 20 nm. This may be caused by the self-doping of N and I derived from kelp. The indirect relationship between the band gap E_g and the absorption coefficient α is also obtained from a plot of (αhν)ⁿversus the photo energy (hν) using the following equation:³⁷

(αhν)ⁿ ∝ hν − E_g

where E_g is the band-gap of allowed transitions (eV), h is Planck's constant (6.62608 × 10⁻³⁴ J s), ν is the frequency of the light (s⁻¹), and n is a number characterizing the transition process. For an indirect TiO₂ allowed transition, we take n as 1/2.³⁶ The Tauc plot, (αhν)^1/2versus hν is shown in Fig. 5b. The band gaps for biogenic-TiO₂ samples are about 2.81 eV, 2.87 eV and 2.88 eV, respectively, which are smaller than that of N-TiO₂ (2.94 eV), and all of the values are smaller than that for pure anatase (3.2 eV). It is commonly believed that an N atom doped into the TiO₂ anatase lattice can form a donor state just above the top of the valence band,³⁸ and I doping may form an acceptor state below the conduction band.³¹ Both of these processes lead to the narrowing of the band gap of TiO₂. Thus, we assume that N–I codoping can lead to narrowing of the band gap of TiO₂ and thus improve the visible photoabsorption capability. In short, biogenic-TiO₂ successfully realizes the synergy of both the enhanced visible-light harvesting introduced by the structure and the red-shift of the absorption edge induced by self-doping of nitrogen and iodine.

Fig. 5 a UV-Vis diffuse reflectance spectra of biogenic-TiO₂ and N-TiO₂. b The corresponding Tauc plot of biogenic-TiO₂ and N-TiO₂.

In order to examine the paramagnetic species in the biogenic-TiO₂ samples, electron paramagnetic resonance (EPR) studies were also performed. Fig. 6 shows the EPR spectrum of biogenic-TiO₂ (calcined at 500 °C) recorded at room temperature. The symmetric signal at g = 2.004 is clearly detected. It is the most prominent electron signal and is characteristic of paramagnetic materials containing “F-centers” or oxygen vacancies.³⁹ Previous EPR studies suggest that signals with g values of less than 2.0 can be attributed to photogenerated electrons stabilized by Ti cations located at crystallization defects.⁴⁰ It is well known that these trapped electrons could reduce Ti⁴⁺ to form Ti³⁺ paramagnetic species⁴⁰ according to the following process: e⁻ + TiO₂(Ti⁴⁺) → TiO₂(Ti³⁺), or alternatively, could reduce surface-absorbed O₂ to form superoxide ions (O₂⁻).³³ The resonances at g values in the range of 2.0 to 2.08 are known to be due to photogenerated holes trapped by subsurface lattice oxygens.⁴⁰ These holes, localized on the oxygen vacancies, react with the O₂⁻ and OH⁻ to form O˙ and OH˙ radicals on the catalyst surface, both of which are reactive species and are responsible for the oxidative decomposition of organic pollutants. As shown in Fig. 6, the g values of trapped hole and electron are all observed, proving the presence of Ti³⁺ ions and the superoxide radical in the biogenic-TiO₂ sample, which could facilitate photocatalysis.

Fig. 6 EPR spectrum of biogenic-TiO₂ calcined at 500 °C.

Photocatalytic activity for methylene blue degradation

Photocatalytic activities were examined for the degradation of methylene blue under solar irradiation as shown in Fig. 7. The degradation process obeys pseudo-first-order kinetics and the degradation rate can be extracted by plotting the natural logarithm of the absorbance against irradiation time.⁴¹ From Fig. 7, the mean decomposition rates of biogenic-TiO₂ calcined at 500 °C, 600 °C and 700 °C under solar irradiation are (0.074 ± 0.003) min⁻¹, (0.072 ± 0.004) min⁻¹ and (0.077 ± 0.001) min⁻¹, respectively, about 2.18, 2.12 and 2.26 times the rate of N-TiO₂, which is (0.034 ± 0.002) min⁻¹. Compared to N-TiO₂, the biogenic-TiO₂ is endowed with superior photocatalytic properties under solar irradiation. The enhanced catalytic activities of biogenic-TiO₂ may result from the synergy of its structures and components. Hierarchical structures are favorable for enhancing the photocatalytic performance as highlighted above that the high surface area on the macroscale, the porous framework on the microscale, and the nanolayered structures on the nanoscale endow the samples with larger specific surface areas which could capture more photons and offer more absorption and reaction sites for the photocatalytic reaction. Meanwhile, the self-doping of nitrogen and iodine could enlarge the band gap absorption, and the presence of Ti³⁺ ions and the superoxide radical are both responsible for the degradation of methylene blue. Furthermore, the smaller mean diameter⁴² of biogenic-TiO₂ and higher crystallinity of the sample were helpful. It is known that well crystallized anatase may facilitate the transfer of photoelectrons from the bulk to the surface and thus inhibit their recombination with photoholes, leading to enhanced quantum efficiency.

Fig. 7 Kinetic study of the degradation of methylene blue in the presence of biogenic-TiO₂ and N-TiO₂ under solar irradiation. The error bars in the figure represent the standard deviations of three independent measurements for each data point.

Apart from its high activity, biogenic-TiO₂ also shows good stability and durability during the photocatalytic degradation of methylene blue solution. As shown in Fig. 8, after recycling 3 times, the biogenic-TiO₂ sample calcined at 500 °C can still completely degrade the methylene blue within 45 min, suggesting that biogenic-TiO₂ has a good reusability.

Fig. 8 Recycling testing of the biogenic-TiO₂ sample calcined at 500 °C for the photocatalytic degradation of methylene blue. The error bars in the figure represent the standard deviations of three independent measurements for each data point.

Conclusions

Biogenic-TiO₂, reproducing the structure of kelp and introducing nitrogen and iodine simultaneously in a self-doping manner, exhibits high light-harvesting and photocatalytic activity in methylene blue degradation under solar energy irradiation. Although the amount of doped iodine is not sufficient, which will be improved in future experiments, our results may represent an important first step towards the self-doping of metal oxides templated with natural plants besides N-doping. We believe that, as for TiO₂, improvements in photocatalysis, luminescence or gas-sensing properties of oxides like ZnO, In₂O₃, CeO₂, Cu₂O and so forth could also be realized by copying the special structures or self-doping of non-metallic ions derived from various kinds of organisms endowed by nature.⁴³ Besides using biogenic oxides in dye degradation, our strategy also provides inspiration for the further promise of applying them to produce hydrogen, solar cells, photoinduced sensors, photoelectrical devices and so forth.

Mother nature is amazing; some special functions or structures we have been taking decades to work out may have existed on the earth for millions of years. For instance, no artificial water-splitting devices do better than chloroplasts in nature. Dye-sensitized solar cells, consisting of iodide/triiodide redox electrolytes between two electrodes, were recently noticed to have a similar sandwich structure to kelp, in which most of the chromatophores exist in the upper and lower epidermal layers and different forms of iodine (I⁻, IO₃⁻ and organic iodine, etc.¹⁵) are in the vacuoles of the interlayer. Therefore, inspired by the epidermal layers of kelp, a biogenic-TiO₂ mesoporous film would be a novel, highly efficient photoanode structure for dye-sensitized solar cells.

Experimental

Materials

All chemical reagents were commercially available and chemically pure. Glutaraldehyde, sodium dihydrogen phosphate, disodium hydrogen phosphate, titanium trichloride and hydrochloric acid were all produced by Sinopharm Chemical Reagent Co., Ltd.

Pretreatment

Fresh kelp samples were treated with 2% glutaraldehyde/phosphate buffered saline (PBS; pH = 7.2) solution at 4 °C for 6 h for the fixation of cells and tissues. The as-treated samples were then rinsed with 0.2% PBS and pure water and kept at 0 °C for further use.

Synthesis procedure

The fixed kelp samples were treated with 5% HCl solution for 2 h. After rinsing with pure water, the as-treated samples were dipped into 5% TiCl₃ solution and left to stand under vacuum for 24 h, with the color turning to purple brown. Afterwards, the samples were rinsed with pure water and were desiccated in an aerated oven at 40 °C, 60 °C and 80 °C successively for 2 h at each temperature. Finally, the samples were calcined in air at 280 °C for 2 h and then at 500 °C, 600 °C and 700 °C for 2 h, respectively, with a ramping rate of 1 °C min⁻¹. Common N-TiO₂ was prepared from a mixture of 25% ammonia solution and 15% TiCl₃ solution with a 1 : 1 volume ratio, and calcined under the same conditions.

Characterization

Pieces of fixed original kelp and Ti-substituted samples were embedded into a tissue freezing medium (Leica Instruments GmbH) at −25 °C. Cross-sections of 15 μm thickness were prepared using a cryo-microtome (Leica CM3050-Cryostat, Leica Instruments GmbH) at −24 °C, and collected on poly-L-lysine-coated slides. Fluorescence microscopy was carried out using a Laser-Scan-Microscope (LSM 510, Carl Zeiss) including an Axioveit 200M microscope. Chlorophyll fluorescence was excited by 488 nm light from a 30 mW argon ion laser. Excitation light was separated by a FT 488/543 dichroic mirror from emission light which was passed through a LP 560 band-pass filter. UV excitation light was split first by HFT 650 then NFT 545 and 490 dichroic mirrors from emission light which was passed through a BP 435–485 band-pass filter.

The cross-sections of 15 μm samples were also collected on conductive glass slides, together with biogenic-TiO₂ derived from kelp. They were sputtered with gold and observed using a field emission scanning electron microscope (FESEM; FEI SIRION 200) operated at 5 kV.

In order to observe the ultrastructure of chloroplasts and the layered nanostructure after converting the bio-precursor to biogenic-TiO₂, the glutaraldehyde-fixed original kelp samples were further placed in 1% OsO₄PBS solution at 4 °C for 2 h. The specimens, dehydrated in a graded EtOH series, were embedded in a low viscosity epoxy resin and cut on a Leica UCT ultra-microtome. Sections were collected on carbon-coated copper grids and stained with uranyl acetate and lead citrate. The biogenic-TiO₂ was suspended in ethanol by ultrasonic treatment and then dropped onto copper grids. The samples were examined using a transmission electron microscope (TEM; JSM-2010, JEOL) equipped with energy dispersive X-ray spectroscopy capability (EDS, OXFORD INCA) at an applied voltage of 200 kV.

The images of the cuticle of the original leaves were obtained using a digital microscope (VHX-100, KEYENCE) and a fluorescence microscope (Lecia DM 2500).

X-Ray diffraction (XRD) measurements were performed to measure the crystal phase of the samples on a Bruker-AXS X-ray diffractometer system with Cu-Kα radiation at 40 kV and 20 mA. The spectra were recorded in the 2θ range from 20° to 90° with a scanning step of 0.02° s⁻¹.

The X-ray photoelectron spectrum (XPS) of the artificial N-doped TiO₂ photocatalyst was measured on a Thermo ESCALAB spectrometer with monochromatized Al Kα X-rays (hν = 1486.6 eV) at a pass energy of 20 eV. The C 1s peak of contaminant carbon was calibrated to 284.8 eV.

UV-Visible (UV-vis) diffuse reflectance spectra of all the samples were recorded using a Varian Cary UV-Vis-NIR spectrophotometer in the spectral range of 300–800 nm. 0.30 g of each sample was pressed between two pieces of quartz glass within the 3 × 3 cm² area to cover the aperture through which excitation light passes. A BaSiO₄ plate was used as a basis line for the spectra. All the samples were assumed to have the same quality during the measurement.

EPR measurements were carried out on a Bruker EMX-8/2.7 X-band EPR spectrometer operating in the X-band at 9.86 GHz and 2.005 mW. Portions of the solid samples (∼20 mg) were introduced into a spectroscopic quartz probe cell and the measurements were taken at room temperature.

The photocatalytic activity for methylene blue degradation was measured according to the following experiments. 0.1 g of the TiO₂ sample was dispersed in 100 mL of 10⁻⁵ mol L⁻¹methylene blue solution with pure water as the solvent. The TiO₂ suspended solution was stirred in the dark for 1 h to reach the adsorption–desorption equilibrium and then placed under a 500 W Xe lamp. The degradation was monitored by taking aliquots every 5 min. These aliquots were centrifuged and then tested under a 25Lamda UV-vis spectrometer to obtain the absorption spectra. The rate of degradation was assumed to obey pseudo-first-order kinetics and the degradation rate constant, k, was obtained according to the following equation: ln(A₀/A) = kt. A₀ is the initial absorbance, namely, the characteristic absorbance peak of the original methylene blue solution. A is the characteristic absorbance peak after a degradation time t. In the recycling experiments, the degraded solution with biogenic-TiO₂ photocatalyst suspended in it was centrifuged at 3000 r min⁻¹ for 5 min. Then the supernatant was poured out and pure water was added to the remaining precipitate for washing. The performance was repeated three times to obtain the clean biogenic-TiO₂ photocatalyst. The obtained wet biogenic-TiO₂ was dried at 25 °C for the next degradation cycle.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 50972090), the National Basic Research Program of China (No. 2006CB601200), the Dawn program of the Shanghai Education Commission (08SG15) and the Shanghai Rising-Star Program (No. 10QH1401300).

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Footnote

† Electronic supplementary information (ESI) available: XPS pattern in all elements binding energy region of kelp. See DOI: 10.1039/c0ee00363h

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