Harnessing designer biotemplates for biomineralization of TiO2 with tunable photocatalytic activity

Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Harnessing designer biotemplates for biomineralization of TiO2 with tunable photocatalytic activity ⁎

Jung Kyu Kima, , Ji-ryang Janga, Muhammad Saad Salmana, Lihan Tanb, Chang-Hoon Namc, ⁎ Pil J. Yooa, Woo-Seok Choea, a

School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea Bioprocessing Technology Institute, A⁎STAR, Downstream Processing Group, 20 Biopolis Way, Centros #06-01, Singapore 138668, Singapore c School of Basic Science, Convergence College, DGIST, 333, Techno Jungang-daero, Hyeonpung-myeon, Dalseong-gun, Daegu 42988, Republic of Korea b

A R T I C LE I N FO

A B S T R A C T

Keywords: Biomineralization Designer biotemplate Photocatalyst in situ substitutional nitrogen-doping Titanium dioxide

Biomineralization is a promising material synthesis strategy for environmentally benign production of nanostructured metal oxides. An important question is whether biomineralization can be used in the biomimetic synthesis of TiO2 with tunable photocatalytic properties that are conducive to diverse solar energy conversion applications. Here, we report the biomineralization of energy-state-modified TiO2 nanoparticles, where the critical properties closely related to their photocatalytic activity can be manipulated by tailoring the nature of the designer biotemplates. For this purpose, STB1 heptapeptide was employed as a nucleation center to induce TiO2 biomineralization. Three distinctive types of biomolecules (peptide, protein, and phage) were deliberately designed to contain the STB1 nucleation core at different local densities and intermolecular distances. The degree of substitutional nitrogen-doping and the morphology are all subject to the context-dependent differential availability of STB1 in the biomineralization milieu. Phage-induced biomineralization results in TiO2 with modified energy state and wire-like network morphology, which account for significantly enhanced charge dissociation/transport performance and high photocatalytic activity. This is the first study to report that a specific peptide with biomineralizing activity exerts differential impacts on the properties of resulting biomineralization products in a context-dependent manner, and will provide a powerful new strategy for tailoring of material properties via biomineralization.

1. Introduction Biomineralization has been extensively studied as a green process for the synthesis of various metal oxide nanomaterials under mild conditions such as pH-neutral solvents, atmospheric pressure, and room temperature [1,2]. This environmentally friendly process can be facilitated by mimicking the synthesis routes of natural biomolecules or utilizing the functional groups in biomolecules such as DNA, proteins, and viruses as structure directing or stabilizing agents [2–10]. To date, this biomimicking procedure for the synthesis of metal oxide nanoparticles has gained great attention in the solar energy conversion research field. This is because the configuration, morphology, and crystallinity of metal oxide nanomaterials can be systematically tailored through various bioinspired synthesis techniques [9–11]. Recently, strategies to enhance the photocatalytic performance of TiO2, such as controlled synthesis and tailored electronic structure, have attracted great attention [12–16]. Due to type-II band alignment

⁎

of anatase/rutile mixed phase TiO2 with different band gap energies (3.03 eV for rutile and 3.20 eV for anatase), the photogenerated charge carriers can be efficiently separated. This prevents charge recombination, and thus results in superior photocatalytic activities [12]. Meanwhile, intensive efforts have also been taken to improve the energy conversion efficiency of TiO2 photocatalysts by engineering the midgap states via various conventional doping approaches, where hazardous chemicals are often used in gas-phase reactions under high temperatures and pressures [17–20]. In particular, nitrogen-doped TiO2 exhibits good efficiency in photocatalysis with broad absorption spectra up to near-infrared region [21]. Although the underpinning mechanism for enhanced photocatalytic activity of nitrogen-doped TiO2 in the visible-light region is still controversial, it is well reported that the midgap state from N-dopants induces localized holes, which suppresses the charge recombination and improves the charge dissociation [22–33]. However, the doped region in conventional TiO2 only occurs up to a few nanometers from the surface because doped-TiO2 is mostly

Corresponding author. E-mail addresses: [email protected] (J.K. Kim), [email protected] (W.-S. Choe).

https://doi.org/10.1016/j.ceramint.2018.12.134 Received 10 September 2018; Received in revised form 11 December 2018; Accepted 19 December 2018 0272-8842/ © 2018 Published by Elsevier Ltd.

Please cite this article as: Kim, J.K., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.134

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

2.3. Characterization

prepared via high-temperature/pressure gas-phase reaction [12]. Hence, obtaining a uniform distribution of N dopant atoms throughout TiO2 nanoparticles via the conventional synthesis method is still challenging. On the contrary, biomineralization enables the formation of metal oxides that are complexed with biomolecules enriched with carbon and nitrogen sources, which are capable of acting as dopants. Biomimetic synthesis of TiO2 is thus expected to be conducive to achieving well-dispersed mid-gap doping states throughout the surface and bulk of TiO2 nanoparticles at the time of biomineralization, and during the subsequent calcination processes [10,34]. In this study, we report biomineralization of TiO2 nanoparticles with outstanding photocatalytic activity. The in situ nitrogen-doping and morphology of TiO2 were tailored via biomineralization with three different biomolecules: peptide, protein, and virus (bacteriophage). These biomolecules were deliberately designed to contain the STB1 peptide moieties [35], which are capable of functioning as a nucleation core [3], in different contexts (i.e. local density and intermolecular spacing of STB1). This led us to first explore the contextual influence of biotemplates containing the active nucleation core STB1 in a rigidly confined structure on STB1-mediated TiO2 mineralization and the properties of TiO2 thus produced. The size and morphology of TiO2 nanoparticles were systematically controlled by the intermolecular distance of the biomolecules in solution and the intramolecular spacing of STB1 in the biomolecule. In addition, the formation of modified energy states in anatase/rutile mixed crystalline phase of TiO2 was facilitated due to the differential degree of substitutional nitrogendoping. Consequently, the virus-mediated biomineralization of TiO2 nanoparticles with tailored energy states and morphology facilitated significantly enhanced charge dissociation/transport performance and high photocatalytic activity.

Transmission electron microscopy (TEM) images were obtained by using a TEM instrument (JEM ARM 200 F, JEOL). The surface areas of TiO2 were measured with a Brunauer–Emmett–Teller (BET) surface area measurement analyzer (ASAP 2000, Micromeritics Instrument Corporation). Thermal gravimetric analysis (TGA) was performed by using a thermogravimetric analysis system (Seiko Exstar 6000, Seiko Instruments Inc.) between 20 and 1000 °C, with the heating rate of 10 °C min−1. The organic content was obtained by comparing the weights (%) of the nanoparticles before (25 °C) and after (900 °C) annealing. X-ray diffraction (XRD) analysis was conducted with Cu Kα radiation by using an X-ray diffractometer (D8 Discover, Bruker) at 40 kV. X-ray photoelectron spectroscopy (XPS) measurements were recorded by an AESXPS instrument (ESCA2000, VG Microtech) equipped with an aluminum anode (Al Kα = 1486.6 eV). UV–Vis diffuse reflectance spectra were recorded by using an UV–Vis spectrophotometer (Shimadzu, UV-3600) with BaSO4 as the reference at room temperature. Low-temperature (20 K) time-correlated single photon counting (TCSPC) was performed by using a customized system with the second harmonic (SHG = 375 nm) of a tunable titanium: sapphire laser (Mira900, Coherent) with ~150 fs pulse width and 76 MHz repetition rate. Some collection optics and a monochromator (SP-2150i, Acton) setup were utilized to spectrally resolve the photoluminescence (PL) emission. For ultrafast detection, the TCSPC module (PicoHarp, PicoQuant) with MCP-PMT (R3809U-59, Hamamatsu) was used [12]. The PL was measured by using collection optics and a monochromator (SP-2150i, Acton) that was connected to a photomultiplier tube (PD174, Acton) at 20 K. 2.4. Photocatalytic activity

2. Experimental

The photocatalytic activity of biomineralized TiO2 was investigated via two different methods. To study the photodegradation of biomineralized TiO2, 10 μM rhodamine B solution was used. After a 0.05% (w/v) TiO2 solution was kept in the dark for 2 h to attain adsorption/ desorption equilibrium, it was irradiated under 1 sun AM 1.5 G. During the 1 sun illumination with a solar simulator, 1 ml of the solution was extracted at time intervals and centrifuged to obtain the supernatant. The changes in the concentration of the supernatant were measured by UV–Vis spectroscopy. To study the photocatalytic H2 production of biomineralized TiO2, a 100 ml square quartz reactor with two necks was used at ambient temperature and atmospheric pressure. Silicon rubber septa were utilized to seal the openings. A thermocouple was inserted in the reactor through one of the septa. 20 mg of sample powder was dispersed in 70 ml of a solution mixture (35 ml water and 35 ml methanol as a sacrificial agent). The mixture with the sample powder well-dispersed was bubbled with argon gas for 2 h to ensure almost complete removal of dissolved oxygen. The simulated solar radiation (i.e. 1 sun AM 1.5 G) was irradiated during the photocatalytic H2 production. During 12 h of continuous photocatalysis, the sample suspension was stirred at 600 rpm to ensure uniform light irradiation. At 30 min intervals, 1 ml of the gas was extracted and collected by using a glass injection syringe through the silicon rubber septum. The gas content was analyzed by using a gas chromatograph (Agilent technologies 7890 A GC system, USA). To examine the photoelectrochemical (PEC) water oxidation performance, linear sweep voltammetry (LSV) was carried out by using a three-electrode system with an electrochemical instrument (CHI 660, CHI Instrument Inc.) and the solar simulator (1 sun; 100 mW cm−2). To fabricate photoanode films, an ethanol-based solution mixture with 3 wt% sample powder, 5 wt% ethyl cellulose, and 15.8 wt% α-terpineol was prepared. After stirring for 16 h at 80 °C, the solution was converted into a paste. The sample paste was uniformly dispersed on precleaned FTO glass using the doctor blade method and transferred to a box furnace (Vulcan, Neytech) for calcination at a predetermined temperature for 2 h. A monochromator

2.1. Materials Constrained STB1 (CHKKPSKSC) peptide, formed via a disulfide bridge between the flanking cysteine residues, was synthesized by Cosmo Genetech (Korea). The details of the construction of recombinant plasmid (pWB1000-STB1) that can overexpress STB1 peptide-inserted LacI (LacI-S), are provided in the Supplementary Information [36]. To produce f88-STB1 (f88-S) virus, STB1 peptide genes were introduced into the recombinant pVIII region of f88 phage DNA as described elsewhere. The details are described in the Supplementary Information [37,38]. TiO2 precursor (Titanium (IV)-bisammonium-lactato-dihydroxide or TiBALDH), rhodamine B powder, ethyl cellulose, and α-terpineol were purchased from Sigma-Aldrich.

2.2. Biomineralization The peptide (STB1), protein (LacI-S), and virus (f88-S) were dissolved in 10 mM Tris buffer at pH 9.0 to yield the desired concentrations of 25 μg ml−1, 1 mg ml−1, and 1014 cfu ml−1, respectively. In a typical experiment, 500 μl of each biomolecule solution was incubated with 50 μl of 1 M TiBALDH for 1 h at room temperature. After biomineralization, 125 μl of 6 N trichloroacetic acid was added and TiO2 precipitates were collected by centrifugation (13,000 g, 5 min). The resultant precipitates were washed three times each with acetone and water. As a control experiment, a bare TiO2 sample was also synthesized by the same procedure in the absence of biotemplate. The precipitates resuspended in water were freeze-dried under − 40 °C and 20 Pa by using a freeze dryer (FDU-1200, Eyela, Japan). All the TiO2 samples mineralized by the biomolecules were calcined at 300–1000 °C for 2 h by using a digital muffle furnace (FH-05, DAIHAN Scientific Co., Ltd., Korea). 2

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

and TEM. It was observed that the number of clustered particles strongly correlated with the number of STB1 peptides present in each molecule (1 for free-STB1, 4 for LacI-S, and 150 for f88-S). For STB1induced biomineralization, at least two agglomerated TiO2 nanoparticles could be seen in the SEM and/or TEM image. For LacI-S-induced biomineralization, approximately 20 TiO2 particles were observed in individual clusters. For f88-S-induced biomineralization, TiO2 nanoparticles were observed as highly condensed wire-like clusters. The manipulation of intermolecular distances and intramolecular arrangement/spacing could control the availability of localized nucleation centers, which can lead to distinct nanostructures. More detailed discussion of intramolecular arrangement and distance of STB1 moieties are provided in the Supplementary Information [47,48]. In this study, only the molecular template effect was taken into account, with a fixed concentration of biotemplates used. The TiO2 nanoparticles biomineralized by STB1 peptide (PEP-TiO2) showed a spherical shape with an average diameter of ca. 180 nm. The TiO2 nanoparticles biomineralized by LacI-S protein (PRT-TiO2) were in the form of clusters of size 350 nm that comprised approximately 20 TiO2 nanoparticles with an average diameter of ca. 35 nm. Due to the high TiO2 affinity of the STB1 moiety in the protein (LacI-S) and its sub-ten-nanometer dimension, LacI-S resulted in spherical TiO2 nanoparticles that formed a hierarchical structure. The TiO2 nanoparticles biomineralized by f88-S virus (VIRTiO2) resulted in f88-S shape-specific wire-like network comprising spherical TiO2 nanoparticles with an average diameter of ca. 35 nm (similar diameter to PRT-TiO2). The VIR-TiO2 morphology (which was dependent on the f88-S morphology) remained the same before and after calcination at 900 °C, as observed from the TEM images in Fig. S1d and S3, respectively. For comparison, bare TiO2 (synthesized under the same experimental conditions, but in the absence of the biotemplate), with an average diameter of ca. 360 nm and irregular morphology, was also synthesized (Fig. 1a and e, and Fig. S1a and S1e). This showed that the presence of biotemplates was critical for the synthesis of TiO2 with controlled morphology (i.e. clusters and individual particles of defined sizes). Furthermore, the different morphology of biomineralized TiO2 resulted in distinct surface areas. Notably, due to the wire-like morphology of f88-S, VIR-TiO2 displayed the highest specific surface area of 78 m2 g−1, compared to ca. 2 m2 g−1 for bare TiO2, as estimated by BET method (Fig. S4). Moreover, it has been widely reported that crystallinity is an important factor in determining photocatalytic efficiency, with a larger grain size being more conducive to realizing enhanced photocatalytic activity [49–54]. In particular, anatase/rutile mixed phase TiO2 with high crystallinity is beneficial for photocatalytic activity. This is because the different band edge states between the anatase and rutile phases within a nanoparticle can efficiently dissociate and transport the photoinduced charges [12,49–53]. The crystalline phases of the biomineralized TiO2 calcined at different temperatures were characterized by XRD (Fig. 2a). The anatase/rutile mixed phase was observed for bare TiO2 and PEP-TiO2 calcined at 600 °C, and for PRT- and VIR-TiO2 calcined at 900 °C. In general, the crystalline phase transition behavior (i.e. amorphous → anatase → mixed anatase/rutile → rutile) is highly dependent on the calcination temperature, which in turn is closely affected by the amount of mineralizing biomolecules present during biomineralization [2,3,55]. It is clear from Fig. 2a that crystalline phase transition became slower in the order of bare, PEP-, PRT-, and VIRTiO2, with the increasing amount of organic content in the biomineralizing milieu. For each sample of bare, PEP-, and VIR-TiO2 with the anatase/rutile mixed phase, the anatase peaks at 2θ of 25.3° (101) and 37.8° (004) became much sharper and stronger, while the rutile peaks started to emerge at 27.4° (110), 36.1° (101), 39.2° (111), and 41.2° (210). However, in the case of PRT-TiO2 with anatase/rutile mixed phase, the anatase peaks became weaker, while the rutile peaks became much more dominant. By using the Spurr equation [56], the percentages of anatase in the anatase/rutile mixed phase samples were calculated to be ca. 69.9%, 79.1%, 24.6%, and 72.4% for bare, PEP-, PRT-,

(Polaronix K3100 IPCE Measurement System, McScience) with a 300 W xenon lamp was used for incident photon-to-electron conversion efficiency (IPCE) measurements. The LSV and IPCE measurements were performed with a three-electrode system consisting of a photoanode, a saturated calomel reference electrode, and a Pt wire counter electrode in 1 M KOH electrolyte (pH 13.5). To determine the exact active area, the entire surface of the photoanode sample was firmly covered with a black plate, which had a round aperture of area 19.63 mm2. In this study, the solar simulator (PEC-L01, PECCELL) with a 150 W xenon lamp for 1 sun AM 1.5 G irradiation was calibrated by using a silicon reference cell (Fraunhofer ISE, certificate no. C-ISE269). 3. Results and discussion In our previous studies, the STB1 peptide (HKKPSKS), isolated by combinatorial peptide libraries, was found to recognize TiO2 with high affinity primarily through electrostatic interaction originating from the three K residues in STB1 [35,39]. It was also confirmed that the binding of STB1 to TiO2 is significantly affected by the local structure of the peptide [39,40]. This is in good agreement with the fact that the peptide geometry is a critical factor that determines the interactions between peptides and inorganic surfaces [41–43]. Besides, the binding behavior of STB1 to TiO2 was investigated in three different contexts, including free STB1 peptide, phage particles displaying STB1, and LacISTB1 fusion protein, by using a quartz crystal microbalance with energy dissipation measurement [40]. These free, phage-displayed, and LacIhosted STB1 peptides exhibited differential binding affinity to TiO2 in context-dependent manners. Furthermore, it was demonstrated that not only the affinity of STB1 to TiO2, but also the presence of a confined structure of the peptide and/or nascent peptide-titanium complex is an important factor determining the formation of peptide-precursor nucleates at the early stage of TiO2 mineralization [3]. In the present study, three different biomolecules containing the STB1 (CHKKPSKSC) peptide moiety, which was shown to be efficient in catalyzing TiO2 biomineralization, were used as biotemplates for TiO2 biomineralization. The morphology and size of the biomineralized TiO2 strongly depend on the intermolecular distances of biomolecules and intramolecular configuration of STB1 moiety during biomineralization. The biomolecules are free STB1 peptide, STB1-fused recombinant LacI protein (LacI-S), and STB1-displayed recombinant f88 bacteriophage (f88-S). LacI-S contains 4 copies of STB1 that were inserted to the Cterminal of the homotetrameric LacI protein, whereas f88-S contains 150 copies of STB1 expressed at the N-terminal of the major viral coat protein pVIII on the surface of f88 virus particles [44,45]. The sizes of the biotemplates were estimated as follows: a diameter of ca. 1.32 nm for the globular peptide (STB1), a diameter of ca. 7.2 nm for the globular protein (LacI-S), and a length of ca. 886 nm and a diameter of 6.6 nm for the wire-like virus (f88-S), as detailed in the Supplementary Information. The illustration in Scheme 1 indicates the geometries of the designer biotemplates [45,46]. The average intermolecular distances of the biomolecules were estimated to be 40, 64, and 215 nm for STB1, LacI-S, and f88-S, respectively, as detailed in the Supplementary Information. In addition, intramolecular arrangement and spacing between STB1 moieties was constant for LacI-S and f88-S, which was ca. 2 nm for LacI-S and 10 nm for f88-S. This is because the STB1 peptide was fused at specific location of LacI protein (i.e. C-terminal of individual monomer units forming the homotetramer) and f88 virus (i.e. major coat protein pVIII with a copy number of ~150) [46]. Since STB1 peptide moieties act as nucleation centers for biomineralization, the concentration of the three distinctive designer biotemplates was adjusted accordingly to ensure that the number of STB1 moieties remained constant (25 µM) during TiO2 synthesis. The properties of the three different designer biotemplates used for TiO2 biomineralization are summarized in Table S1. The morphology of the biomineralized TiO2 nanoparticles before (Fig. S1) and after (Fig. 1) calcination at 900 °C were analyzed by SEM 3

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

Scheme 1. Schematic illustration of the TiO2 biomineralization with STB1 peptide, LacI-S protein, or f88-S virus.

be due to the relatively higher nitrogen contents of the host biomolecules. In addition, TiO2 calcined at lower temperatures usually showed higher nitrogen contents compared with those calcined at higher temperatures [63]. However, VIR-TiO2 calcined at 900 °C showed a higher degree of nitrogen-doping than PEP-TiO2 calcined at 600 °C. This unusual trend could be due to the significantly higher protein contents of f88 virus, in addition to the STB1 peptides per se. The use of f88-S as a biotemplate increased the ability to retain nitrogen in the biomineralized TiO2 lattice, which also accounted for the increased crystalline phase transition temperature for VIR-TiO2, as mentioned earlier. As shown in Fig. 3b, the Ti 2p3/2 and Ti 2p1/2 peaks of VIR-TiO2 were slightly shifted to higher binding energy, compared to those of bare TiO2, which implied that the oxidation state of titanium at the surface of TiO2 was slightly shifted from + 3 to + 4 [64]. Fig. 3d shows the C 1 s XPS spectra, where the peaks centered at the binding energies of ≈ 285, ≈ 286.6, and ≈ 288.8 eV can be assigned to the C-C, C-O, and C˭O bonds, respectively [65]. No significant changes in the C 1 s XPS spectra of bare and VIR-TiO2 (Fig. 3d) imply that the carbon species from the biotemplates were not present in the TiO2 after calcination at 900 °C. Additionally, uniform distribution of nitrogen dopant in VIRTiO2 was observed in the TEM-EDX (energy-dispersive X-ray spectroscopy) elemental mapping images (Fig. S6). These results demonstrate that varying extent of in situ substitutional nitrogen-doping was achieved by a simple biomineralization process through the use of deliberately designed biotemplates with varying amounts of the nitrogen source. Furthermore, the substitutional nitrogen-doping could affect the wave functions of the molecular orbitals and the density of states, producing mid-gap energy states near to the valence band (VB) of TiO2 [34,57,66–68]. Due to the nitrogen dopant, the VB XPS spectra (Fig. 3e) exhibited a blue-shift of the VB maxima in all the biomineralized anatase/rutile mixed phase TiO2. The VB maximum was 2.88 eV (with a tail of 2.52 eV), 2.51 eV (with a tail of 1.98 eV), and 2.78 eV (with a tail of 2.21 eV) for PEP-TiO2, PRT-TiO2, and VIR-TiO2, respectively. For bare TiO2, the VB onset of 2.96 eV was observed. The long tails of the VB spectra corresponded to the nitrogen-dopant, which induced the midgap energy states. It was observed that the biomolecules with a higher concentration of organic species displayed an apparent shift in the VB maximum of nitrogen-doped TiO2 with a much longer tail. The mid-gap energy states of the nitrogen-doped TiO2 were attributed to lattice defects, which produced an intense and broad background absorbance

and VIR-TiO2, respectively (Fig. 2b). In our hypothesis, there can be a correlation between the mixed crystalline phase formation of TiO2 and the spacing between adjacent STB peptides. The designer biotemplates STB1, LacI-S, and f88-S have adjacent STB1 spacings of 40 nm (intermolecular distance between free peptides), 2 nm (intramolecular spacing between STB1 moieties in LacI-S), and 10 nm (intramolecular spacing between STB1 moieties in f88-S), respectively. Biotemplates with narrow spacings between the most adjacent STB1 moieties may decrease the percentage of anatase in an anatase/rutile mixed phase sample. In addition, the grain size of anatase/rutile mixed phase can affect its photocatalytic activity, detailed characterizations of which were performed, as has been discussed in the Supplementary Information [49–54,57]. Among all the anatase/rutile mixed phase samples, VIR-TiO2 showed superior crystallinity with a favorable anatase to rutile ratio and a large grain size. The nitrogen doped in TiO2 solely originates from the organic molecules used as the biotemplates (i.e. STB1, LacI-S, and f88-S) during the time of TiO2 biomineralization. The presence of nitrogen in TiO2 was investigated by XPS (Fig. 2b and S5). The atomic concentrations of nitrogen obtained from XPS were ca. 0.56, 0.82, and 0.84 at% for PEPTiO2, PRT-TiO2, and VIR-TiO2, respectively. In the high-resolution N 1 s XPS spectra (Fig. 3a), the peaks observed at 399.2 and 400.8 eV are ascribed to substitutional (O-Ti-N) and interstitial (Ti-O-N) nitrogendoping, respectively [58–60]. VIR-TiO2 showed the highest substitutional to interstitial ratio of nitrogen-doping in comparison with both PEP- and PRT-TiO2. The percentages of substitutional nitrogen-doping were 74%, 88%, and 96% for PEP-, PRT-, and VIR-TiO2, respectively. This implies that a high extent of substitutional nitrogen-doping in VIRTiO2 was achieved by phage-templated biomineralization. The incorporation of nitrogen in VIR-TiO2 was further confirmed from the high resolution XPS spectra of Ti 2p and O 1 s as shown in Fig. 3b and c [61,62]. These XPS results closely correspond with the TGA that revealed the weight percentages of the organics including carbon and nitrogen species as 16, 38, and 44 wt% for PEP-, PRT-, and VIR-TiO2, respectively. Since the molar concentration of STB1 in all the biotemplates used was the same (25 µM), it could be inferred that the substantially higher extent of nitrogen-doping for PRT-TiO2 and VIR-TiO2, as compared to PEP-TiO2, could be attributed to the presence of LacI protein and f88 virus per se that were used as docking scaffolds to host STB1 peptide moiety. LacI protein and f88 virus are the host biomolecules for STB1 peptides, and the higher degree of nitrogen-doping can

4

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

Fig. 1. SEM (left) and TEM (right) images of TiO2 nanoparticles synthesized under the same condition without a biomolecule (bare TiO2; a and e), with STB1 peptide (PEP-TiO2; b and f), with LacI-S protein (PRT-TiO2; c and g), and with f88-S virus (VIR-TiO2; d and h). The TiO2 nanoparticles were imaged after calcination at 900 °C.

above the wavelength of 600 nm with slightly redshifted near-bandedge absorption around 395 nm region (Fig. 4a). In order to investigate the visible-light absorption and bandgaps, the Tauc plots of TiO2

samples were plotted (Fig. 4b), which indicated that the indirect bandgap energies were ca. 3.15, 3.14, 3.06, and 3.11 eV for bare TiO2, PEP-TiO2, PRT-TiO2, and VIR-TiO2, respectively. The slight decrease in 5

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

Fig. 2. (a) XRD spectra of bare, PEP-, PRT-, and VIR-TiO2 after calcination at selected temperatures to obtain anatase, anatase/rutile mixed phase, and rutile phase. “A” indicates the peaks present in anatase phase, and “R” indicates those of rutile phase. (b) Summary of the percentage of anatase in the anatase/rutile mixed phase (black), and the nitrogen concentration of the biomineralized TiO2 with anatase/rutile mixed phase (blue).

value of each sample with anatase/rutile mixed phase was 4.9 × 10−3 min−1 for bare TiO2, 15.4 × 10−3 min−1 for PEP-TiO2, 25.9 × 10−3 min−1 for PRT-TiO2, and 48.3 × 10−3 min−1 for VIR-TiO2. This outstanding photocatalytic activity of the biomineralized TiO2 arose from the efficient charge separation and faster charge transfer that were enabled by the substitutional nitrogen-doping, which introduced shallow energy states above the VB and anatase/rutile mixed phase. The optimized calcination temperature was 600 °C for bare and PEPTiO2, and 900 °C for PRT- and VIR-TiO2. VIR-TiO2 showed the best photocatalytic activity, with an approximately threefold increase in the k value compared to PEP-TiO2. This was probably due to the larger surface area and higher atomic concentration of the substitutional nitrogen-dopants in VIR-TiO2. VIR-TiO2 also showed ca. 86% enhanced k value compared to PRT-TiO2, despite their similar degree of doping (ca. 0.8 at% nitrogen). The properties of the biomineralized TiO2 nanoparticles are summarized in Table 1. It is interesting to note that the outstanding enhancement in the photocatalytic activity of VIR-TiO2 presumably resulted from the synergistic effects of the high anatase crystallinity and the appropriate fractions of anatase and rutile in the mixed phase despite its slightly higher band gap compared with PRTTiO2. Further, the cocatalyst-free photocatalytic H2 productions from VIR-TiO2 and PEP-TiO2 were compared, as shown in Fig. 5b. As a

the band gap of PRT-TiO2 is presumably not only due to the nitrogendoping but could also be due to the higher fraction of rutile phase. Moreover, the XPS and UV–Vis results imply that the nitrogen-doping from the biomolecules might have provided shallow-hole-extracting states near to the VB edge [66–68]. Hence, the nitrogen-doping states obtained from the biomineralization could improve the charge dissociation efficiency of TiO2 nanoparticles. With these differential properties, the photocatalytic activity of the biomineralized nitrogen-doped TiO2 was investigated via the photodegradation of the organic pollutant (rhodamine B) under 1 sun illumination (Fig. 5a and S7). As a reference, the commercially available Degussa P25 TiO2 with a specific surface area of ca. 50 m2 g−1 and anatase/rutile mixed phase (80% anatase: 20% rutile) was used [12,54]. The rate constants (k) of the photodegradation reactions are summarized in Table S2. The apparent k value was calculated by using the equation C/C0 = exp(-kt), where t is the irradiation time, C0 the initial concentration of rhodamine B, and C the residual concentration of rhodamine B measured at 10 min intervals. Fig. S7e-h show the kinetics of rhodamine B photodegradation in the form of a linear plot of ln (C/C0) versus t. As expected, the samples with the anatase/rutile mixed phase exhibited the highest photocatalytic activity, due to efficient charge separations regardless of the type of biomolecule used. The k

Fig. 3. (a) N 1 s XPS spectra of bare, PEP-, PRT-, and VIR-TiO2. (b) Ti 2p, (c) O 1 s and (d) C 1 s XPS spectra. (e) VB XPS spectra of bare, PEP-, PRT-, and VIR-TiO2. 6

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

Fig. 4. (a) UV–Vis absorbance spectra and (b) transformed Kubelka-Munk function versus the photon energy of bare, PEP-, PRT-, and VIR-TiO2.

performance of the biomineralized nitrogen-doped TiO2 with anatase/ rutile mixed phase was investigated by LSV by using the three-electrode system comprising a photoanode, a saturated calomel electrode (reference electrode), and a Pt wire counter electrode (Fig. 4c and S8) under on/off chopped 1 sun irradiation. The biomineralized nitrogendoped TiO2 samples showed robust PEC performances compared to

reference, the photocatalytic H2 production of commercialized P25 TiO2 was also investigated. Under continuous 1 sun illumination, the H2 production rate obtained was 0.906 mmol g−1 h−1 (average 0.856 mmol g−1 h−1) for VIR-TiO2, 0.113 mmol g−1 h−1 (average 0.101 mmol g−1 h−1) for PEP-TiO2, and 0.044 mmol g−1 h−1 (average 0.040 mmol g−1 h−1) for P25 TiO2 [66–68]. The PEC water oxidation

Fig. 5. (a) Concentration changes (C/C0) during photocatalytic decomposition of rhodamine B, where C is the residual concentration, and C0 is the initial concentration. (b) Cycling tests of photocatalytic hydrogen production for 12 h. (c) Current density versus potential (J-V) curves recorded under on/off chopped 1 sun irradiation with a three-electrode system composed of a photoanode, a saturated calomel reference electrode, and a Pt wire counter electrode. (d) PL decay profiles obtained via TCSPC characterization at 20 K. Note that IRF indicates the instrument response function. 7

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

Table 1 Summary of the properties of biomineralized TiO2 nanoparticles. Rate constant of photodegradation (× 10−3 min−1)

Sample

Morphology/diameter of the particles (D, nm)

Degree of nitrogen- doping (at%)

Degree of doping sites (% of substitutional doping)

Bare TiO2

Irregular (D = 360) Globular (D = 180) Clustered globular (D = 35) Wire-like globular (D = 35)

–

–

0.56

74

15.4

0.82

88

25.9

0.84

96

48.3

PEP-TiO2 PRT-TiO2 VIR-TiO2

4.9

bare TiO2. The photocurrent density values observed at 1.23 V (vs RHE) were 0.024, 0.068, 0.110, and 0.135 mA cm−2 for bare, PEP-, PRT-, and VIR-TiO2, respectively. The drastic enhancement in the PEC water oxidation performance of the biomineralized nitrogen-doped TiO2 was in accordance with the increased photocatalytic activity demonstrated for the photodegradation of rhodamine B. The corresponding external quantum efficiency (EQE) spectra are shown in Fig. S9. The photon-tocurrent conversion was observed dominantly at wavelengths below 400 nm due to the onset of a photocurrent between 390 and 420 nm (corresponding to the band gaps between 2.95 and 3.18 eV). Approximately twofold increases in the EQEs of PRT- and VIR-TiO2, in comparison with bare TiO2, were found in the wavelength range between 395 and 655 nm (Fig. S9b). The EQE enhancement in the visible-light region was closely correlated to the absorbance spectrum of the biomineralized nitrogen-doped TiO2 (Fig. 4). Although the underpinning mechanism of nitrogen-doping effect on the light harvesting efficiency of TiO2 is not yet completely understood, the outstanding enhancements in the EQEs of these biomineralized nitrogen-doped TiO2 were achieved in the visible-light region presumably due to the newly intercalated mid-gap energy states that resulted from the substitutional doping of nitrogen [34]. These results imply that biomineralized nitrogen-doped TiO2 nanoparticles can harvest the incident photons over the wide range of UV–Vis spectrum, which is induced by the biomimetic nitrogen-doping, even though the PEC response in the visible-light region is a relatively minor component. In order to investigate the charge separation efficiency, the PL decay profiles of bare and VIR-TiO2 were analyzed by using TCSPC measurements (Fig. 5d), which were conducted at 20 K to minimize the interferences from the surface defect states (or surface properties) and the lattice oscillations of the metal oxide nanoparticles [12,69,70]. The PL decay profiles of the TiO2 samples were fitted with a biexponential function accounting for the two relaxation pathways to obtain the PL lifetimes. The fast component (τ1) is strongly related to the direct formation of free carriers near the band edge states, whereas the slow component (τ2) primarily corresponds to indirect exciton formation with trapped charge carriers [71,72]. The PL decay parameters are summarized in Table S3. The amplitude-weighted average lifetimes (τave) for bare TiO2 and VIR-TiO2 were 1.87 and 0.61 ns, respectively, which could be attributed to the large decrease in τ1 (from 0.82 to 0.11 ns) and the moderate decrease in τ2 (from 3.01 to 1.42 ns). This indicated that the charge separation efficiency in the biomineralized TiO2 was significantly enhanced [12]. In addition, the increased weight fraction ratio (f1/f2) between the fast and the slow component (0.62/ 0.38 for VIR-TiO2 versus 0.52/0.48 for bare TiO2) indicated that direct exciton formation was more dominant than indirect formation, and thus the fast charge transfer resulted in efficient charge dissociation for VIRTiO2, as illustrated in Scheme 2 [66–68]. In addition, the steady state PL spectra of bare and VIR-TiO2 were obtained at 20 K (Fig. S10). For the bare TiO2, two peaks were observed at 430 and 550 nm, which resulted from a direct transition between the band edges and from the oxygen vacancies/defects, respectively [71–73]. For VIR-TiO2, a redshift in the transition peak from 430 to 450 nm was observed, while a

Scheme 2. Schematic illustration of the proposed photocatalytic mechanism of metal-free biomineralized TiO2 with tailored energy states.

new peak arose at 475 nm, which accounted for the VB shift and the intercalated states attributed to the substitutional nitrogen-dopants. The intensity of the PL peak at 550 nm region was significantly reduced in the case of VIR-TiO2. This implied that the photoinduced holes were favorably extracted to the intercalated states corresponding to the PL peak at 475 nm, thereby preventing the exciton quenching at the defect states corresponding to the PL peak at 550 nm. Consequently, VIR-TiO2 with tailored morphology and energy-states resulted in significantly improved photoinduced charge separation efficiency.

4. Conclusion We have shown that the TiO2-mineralizing peptide STB1 in different contexts (i.e. free, protein docked, and virus hosted) exhibited differential biomineralizing efficacies for the biomineralization of TiO2. The resulting biomineralized TiO2 (i.e. PEP-TiO2, PRT-TiO2, and VIR-TiO2) showed distinctive physical and chemical properties such as morphology, calcination-driven crystallinity transition behavior, degree/ nature of in situ nitrogen-doping, and mid-gap energy states, all of which significantly impacted their performances as photocatalysts in the absence of a cocatalyst. This is the first attempt to prove that photocatalytic activities of biomineralized TiO2 could be critically affected by the contextual influence of a single biomineralizing nucleation core peptide present in the designer biotemplates of different architectures. In particular, VIR-TiO2 existed in a clustered network of individual nanoparticles that were aligned along the virus-shaped wires, indicating that the biomineralized TiO2 structure closely resembled the morphology of the biotemplate. Compared with PEP-TiO2 and PRT-TiO2, VIR-TIO2 exhibited the largest surface area. Furthermore, VIR-TiO2 revealed a favorable fraction of the anatase/ rutile mixed crystalline phase and increased in situ substitutional nitrogen-doping, which were largely due to the presence of abundant 8

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

organic substances that were sourced from the virus template per se. In addition, the nitrogen-doping formed new mid-gap states near the VB edge, which acted as the shallow-hole extracting states and broadened the UV–Vis absorption range. The modification of the energy states of the biomineralized VIR-TiO2 thus resulted in efficient exciton dissociation and charge separation, accounting for its superior cocatalystfree photocatalytic performance. Taken together, a novel biomineralization strategy harnessing designer biotemplates containing a biomineralizing peptide as the nucleation moiety in differential contexts was proven to be effective in controlling the properties of the resulting biomineralized products. This also provides a new insight in that careful design of the biotemplate and control of its local density is the key to tailoring the properties of mineralized products.

[18] G. Wang, X. Xiao, W. Li, Z. Lin, Z. Zhao, C. Chen, C. Wang, Y. Li, X. Huang, L. Miao, Significantly enhanced visible light photoelectrochemical activity in TiO2 nanowire arrays by nitrogen implantation, Nano Lett. 15 (2015) 4692–4698. [19] S. Bagheri, N.M. Julkapli, Synergistic effects on hydrogenated TiO2 for photodegradation of synthetic compounds pollutants, Int. J. Hydrog. Energy 41 (2016) 14652–14664. [20] S. Hoang, P.-X. Gao, Nanowire array structures for photocatalytic energy conversion and utilization: a review of design concepts, assembly and integration, and function enabling, Adv. Energy Mater. 6 (2016) 1600683. [21] X. Chen, L. Liu, Y.Y. Peter, S.S. Mao, Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011) 746–750. [22] U. Diebold, Photocatalysts: closing the gap, Nat. Chem. 3 (2011) 271–272. [23] Y.H. Hu, A highly efficient photocatalyst—hydrogenated black TiO2 for the photocatalytic splitting of water, Angew. Chem. Int. Ed. 51 (2012) 12410–12412. [24] L. Liu, Y.Y. Peter, X. Chen, S.S. Mao, D. Shen, Hydrogenation and disorder in engineered black TiO2, Phys. Rev. Lett. 111 (2013) 065505. [25] R. Asahi, T. Morikawa, H. Irie, T. Ohwaki, Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst: designs, developments, and prospects, Chem. Rev. 114 (2014) 9824–9852. [26] T. Lindgren, J. Lu, A. Hoel, C.-G. Granqvist, G.R. Torres, S.-E. Lindquist, Photoelectrochemical study of sputtered nitrogen-doped titanium dioxide thin films in aqueous electrolyte, Sol. Energy Mater. Sol. Cells 84 (2004) 145–157. [27] K.R. Reyes-Gil, E.A. Reyes-García, D. Raftery, Photoelectrochemical analysis of anion-doped TiO2 colloidal and powder thin-film electrodes, J. Electrochem. Soc. 153 (2006) A1296–A1301. [28] C. Xu, S.U.M. Khan, Photoresponse of visible light active carbon-modified -n-TiO2 thin films, Electrochem. Solid-State Lett. 10 (2007) B56–B59. [29] M.-S. Wong, W.-C. Chu, D.-S. Sun, H.-S. Huang, J.-H. Chen, P.-J. Tsai, N.-T. Lin, M.S. Yu, S.-F. Hsu, S.-L. Wang, H.-H. Chang, Visible-light-induced bactericidal activity of a nitrogen-doped titanium photocatalyst against human pathogens, Appl. Environ. Microbiol. 72 (2006) 6111–6116. [30] Q. Li, P. Wu, R. Xie, J.K. Shang, Enhanced photocatalytic disinfection of microorganisms by transition-metal-ion-modification of nitrogen-doped titanium oxide, J. Mater. Res. 25 (2010) 167–176. [31] R. Katoh, A. Furube, K.-i. Yamanaka, T. Morikawa, Charge separation and trapping in N-doped TiO2 photocatalysts: a time-resolved microwave conductivity study, J. Phys. Chem. Lett. 1 (2010) 3261–3265. [32] K.-i. Yamanaka, T. Ohwaki, T. Morikawa, Charge-carrier dynamics in Cu- or Feloaded nitrogen-doped TiO2 powder studied by femtosecond diffuse reflectance spectroscopy, J. Phys. Chem. C 117 (2013) 16448–16456. [33] K.-i. Yamanaka, T. Morikawa, Charge-carrier dynamics in nitrogen-doped TiO2 powder studied by femtosecond time-resolved diffuse reflectance spectroscopy, J. Phys. Chem. C 116 (2012) 1286–1292. [34] J. Xie, H. Xie, B.L. Su, Y.B. Cheng, X. Du, H. Zeng, M. Wang, W. Wang, H. Wang, Z. Fu, Mussel‐directed synthesis of nitrogen‐doped anatase TiO2, Angew. Chem. Int. Ed. 55 (2016) 3031–3035. [35] H. Chen, X. Su, K.-G. Neoh, W.-S. Choe, QCM-D analysis of binding mechanism of phage particles displaying a constrained heptapeptide with specific affinity to SiO2 and TiO2, Anal. Chem. 78 (2006) 4872–4879. [36] H. Chen, X. Su, K.G. Neoh, W.S. Choe, Engineering laci for self‐assembly of inorganic nanoparticles on DNA Scaffold through the understanding of laci binding to solid surfaces, Adv. Funct. Mater. 19 (2009) 1186–1192. [37] G.P. Smith, Preface, Gene 128 (1993) 1–2. [38] B. Ratnikov, P. Cieplak, J.W. Smith, Proteases and Cancer: Methods and Protocols, Humana Press, 2009. [39] H. Chen, X. Su, K.-G. Neoh, W.-S. Choe, Probing the interaction between peptides and metal oxides using point mutants of a TiO2-binding peptide, Langmuir 24 (2008) 6852–6857. [40] H. Chen, X. Su, K.-G. Neoh, W.-S. Choe, Context-dependent adsorption behavior of cyclic and linear peptides on metal oxide surfaces, Langmuir 25 (2008) 1588–1593. [41] B.R. Peelle, E.M. Krauland, K.D. Wittrup, A.M. Belcher, Probing the interface between biomolecules and inorganic materials using yeast surface display and genetic engineering, Acta Biomater. 1 (2005) 145–154. [42] C.E. Flynn, C. Mao, A. Hayhurst, J.L. Williams, G. Georgiou, B. Iverson, A.M. Belcher, Synthesis and organization of nanoscale II–VI semiconductor materials using evolved peptide specificity and viral capsid assembly, J. Mater. Chem. 13 (2003) 2414–2421. [43] W.-S. Choe, M. Sastry, C.K. Thai, H. Dai, D.T. Schwartz, F. Baneyx, Conformational control of inorganic adhesion in a designer protein engineered for cuprous oxide binding, Langmuir 23 (2007) 11347–11350. [44] N.V. Tikunova, V.V. Morozova, Phage display on the base of filamentous bacteriophages: application for recombinant antibodies selection, Acta Nat. 1 (2009) 20–28. [45] D.-Y. Jeon, K.H. Hwang, S.-J. Park, Y.-J. Kim, M.-K. Joo, S.-E. Ahn, G.-T. Kim, C.H. Nam, Controlled surface adsorption of fd filamentous phage by tuning of the pH and the functionalization of the surface, J. Appl. Phys. 109 (2011) 064701. [46] M. Lewis, G. Chang, N.C. Horton, M.A. Kercher, Crystal structure of the lactose operon repressor and its complexes with DNA and inducer, Science 271 (1996) 1247. [47] S. Kashyap, T.J. Woehl, X. Liu, S.K. Mallapragada, T. Prozorov, Nucleation of iron oxide nanoparticles mediated by Mms6 protein in situ, ACS Nano 8 (2014) 9097–9106. [48] H. Zeng, J. Xie, H. Xie, B.-L. Su, M. Wang, H. Ping, W. Wang, H. Wang, Z. Fu, Bioprocess-inspired synthesis of hierarchically porous nitrogen-doped TiO2 with high visible-light photocatalytic activity, J. Mater. Chem. A 3 (2015) 19588–19596.

Acknowledgments This work was supported by National Research Foundation of Korea (NRF-2018R1D1A1B07050875). W. S. Choe thanks the financial support received from NRF (2017R1A2B2011341). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ceramint.2018.12.134. References [1] G. Zan, Q. Wu, Biomimetic and bioinspired synthesis of nanomaterials/nanostructures, Adv. Mater. 28 (2016) 2099–2147. [2] S.L. Sewell, D.W. Wright, Biomimetic synthesis of titanium dioxide utilizing the R5 peptide derived from Cylindrotheca fusiformis, Chem. Mater. 18 (2006) 3108–3113. [3] N. Choi, L. Tan, J.-R. Jang, Y.M. Um, P.J. Yoo, W.-S. Choe, The interplay of peptide sequence and local structure in TiO2 biomineralization, J. Inorg. Biochem. 115 (2012) 20–27. [4] R.R. Naik, S.J. Stringer, G. Agarwal, S.E. Jones, M.O. Stone, Biomimetic synthesis and patterning of silver nanoparticles, Nat. Mater. 1 (2002) 169–172. [5] H.R. Luckarift, M.B. Dickerson, K.H. Sandhage, J.C. Spain, Rapid, room‐temperature synthesis of antibacterial bionanocomposites of lysozyme with amorphous silica or titania, Small 2 (2006) 640–643. [6] U. Kumar, A. Shete, A.S. Harle, O. Kasyutich, W. Schwarzacher, A. Pundle, P. Poddar, Extracellular bacterial synthesis of protein-functionalized ferromagnetic Co3O4 nanocrystals and imaging of self-organization of bacterial cells under stress after exposure to metal ions, Chem. Mater. 20 (2008) 1484–1491. [7] V. Bansal, P. Poddar, A. Ahmad, M. Sastry, Room-temperature biosynthesis of ferroelectric barium titanate nanoparticles, J. Am. Chem. Soc. 128 (2006) 11958–11963. [8] G. Chen, M. Li, F. Li, S. Sun, D. Xia, Protein‐mediated synthesis of nanostructured titania with different polymorphs at room temperature, Adv. Mater. 22 (2010) 1258–1262. [9] W.J. Lee, J.M. Lee, S.T. Kochuveedu, T.H. Han, H.Y. Jeong, M. Park, J.M. Yun, J. Kwon, K. No, D.H. Kim, Biomineralized N-doped CNT/TiO2 core/shell nanowires for visible light photocatalysis, ACS Nano 6 (2011) 935–943. [10] J.K. Kim, J.-R. Jang, N. Choi, D. Hong, C.-H. Nam, P.J. Yoo, J.H. Park, W.-S. Choe, Lysozyme-mediated biomineralization of titanium-tungsten oxide hybrid nanoparticles with high photocatalytic activity, Chem. Commun. 50 (2014) 12392–12395. [11] S. Matsumura, Y. Horiguchi, T. Nishimura, H. Sakai, T. Kato, Biomineralizationinspired preparation of zinc hydroxide carbonate/polymer hybrids and their conversion into zinc oxide thin-film photocatalysts, Chem. Eur. J. 22 (2016) 7094–7101. [12] K. Zhang, L. Wang, J.K. Kim, M. Ma, G. Veerappan, C.-L. Lee, K.-j. Kong, H. Lee, J.H. Park, An order/disorder/water junction system for highly efficient co-catalystfree photocatalytic hydrogen generation, Energy Environ. Sci. 9 (2016) 499–503. [13] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455–459. [14] S.P. Albu, A. Ghicov, J.M. Macak, R. Hahn, P. Schmuki, Self-organized, freestanding TiO2 nanotube membrane for flow-through photocatalytic applications, Nano Lett. 7 (2007) 1286–1289. [15] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays, Nano Lett. 7 (2007) 69–74. [16] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett. 6 (2006) 215–218. [17] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting, Nano Lett. 6 (2006) 24–28.

9

Ceramics International xxx (xxxx) xxx–xxx

J.K. Kim et al.

[62] P. Mazierski, M. Nischk, M. Gołkowska, W. Lisowski, M. Gazda, M.J. Winiarski, T. Klimczuk, A. Zaleska-Medynska, Photocatalytic activity of nitrogen doped TiO2 nanotubes prepared by anodic oxidation: the effect of applied voltage, anodization time and amount of nitrogen dopant, Appl. Catal. B: Environ. 196 (2016) 77–88. [63] M. Sathish, B. Viswanathan, R. Viswanath, C.S. Gopinath, Synthesis, characterization, electronic structure, and photocatalytic activity of nitrogen-doped TiO2 nanocatalyst, Chem. Mater. 17 (2005) 6349–6353. [64] J.K. Kim, S.U. Chai, Y. Ji, B. Levy-Wendt, S.H. Kim, Y. Yi, T.F. Heinz, J.K. Nørskov, J.H. Park, X. Zheng, Resolving hysteresis in perovskite solar cells with rapid flameprocessed cobalt-doped TiO2, Adv. Energy Mater. 8 (2018) 1801717. [65] J.K. Kim, S.U. Chai, Y. Cho, L. Cai, S.J. Kim, S. Park, J.H. Park, X. Zheng, Ultrafast flame annealing of TiO2 paste for fabricating dye-sensitized and perovskite solar cells with enhanced efficiency, Small 13 (2017) 1702260. [66] F. Spadavecchia, G. Cappelletti, S. Ardizzone, M. Ceotto, L. Falciola, Electronic structure of pure and N-doped TiO2 nanocrystals by electrochemical experiments and first principles calculations, J. Phys. Chem. C 115 (2011) 6381–6391. [67] Z. Zhao, Q. Liu, Mechanism of higher photocatalytic activity of anatase TiO2 doped with nitrogen under visible-light irradiation from density functional theory calculation, J. Phys. D: Appl. Phys. 41 (2007) 025105. [68] J. Varley, A. Janotti, C. Van de Walle, Mechanism of visible‐light photocatalysis in nitrogen‐doped TiO2, Adv. Mater. 23 (2011) 2343–2347. [69] J.K. Kim, X. Shi, M.J. Jeong, J. Park, H.S. Han, S.H. Kim, Y. Guo, T.F. Heinz, S. Fan, C.L. Lee, J.H. Park, X. Zheng, Enhancing Mo:BiVO4 solar water splitting with patterned Au nanospheres by plasmon‐induced energy transfer, Adv. Energy Mater. 8 (2018) 1701765. [70] K. Zhang, J.K. Kim, M. Ma, S.Y. Yim, C.L. Lee, H. Shin, J.H. Park, Delocalized electron accumulation at nanorod tips: origin of efficient H2 generation, Adv. Funct. Mater. 26 (2016) 4527–4534. [71] Y. Liu, R.O. Claus, Blue light emitting nanosized TiO2 colloids, J. Am. Chem. Soc. 119 (1997) 5273–5274. [72] S. Poznyak, V. Sviridov, A. Kulak, M. Samtsov, Photoluminescence and electroluminescence at the TiO2-electrolyte interface, J. Electroanal. Chem. 340 (1992) 73–97. [73] C. Jin, B. Liu, Z. Lei, J. Sun, Structure and photoluminescence of the TiO2 films grown by atomic layer deposition using tetrakis-dimethylamino titanium and ozone, Nanoscale Res. Lett. 10 (2015) 95.

[49] A.L. Linsebigler, G. Lu, J.T. Yates Jr, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [50] D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, T. Hirai, Gold nanoparticles located at the interface of anatase/rutile TiO2 particles as active plasmonic photocatalysts for aerobic oxidation, J. Am. Chem. Soc. 134 (2012) 6309–6315. [51] Y. Zhang, J. Chen, X. Li, Preparation and photocatalytic performance of anatase/ rutile mixed-phase TiO2 nanotubes, Catal. Lett. 139 (2010) 129–133. [52] Z. Liu, X. Zhang, S. Nishimoto, M. Jin, D.A. Tryk, T. Murakami, A. Fujishima, Anatase TiO2 nanoparticles on rutile TiO2 nanorods: a heterogeneous nanostructure via layer-by-layer assembly, Langmuir 23 (2007) 10916–10919. [53] J. Yu, H. Yu, B. Cheng, M. Zhou, X. Zhao, Enhanced photocatalytic activity of TiO2 powder (P25) by hydrothermal treatment, J. Mol. Catal. A: Chem. 253 (2006) 112–118. [54] H. Liu, J.B. Joo, M. Dahl, L. Fu, Z. Zeng, Y. Yin, Crystallinity control of TiO2 hollow shells through resin-protected calcination for enhanced photocatalytic activity, Energy Environ. Sci. 8 (2015) 286–296. [55] J. Criado, C. Real, Mechanism of the inhibiting effect of phosphate on the anatase [rightward arrow] rutile transformation induced by thermal and mechanical treatment of TiO2, J. Chem. Soc., Faraday Trans. 1 (79) (1983) 2765–2771. [56] R.A. Spurr, H. Myers, Quantitative analysis of anatase-rutile mixtures with an X-ray diffractometer, Anal. Chem. 29 (1957) 760–762. [57] J. Wang, D.N. Tafen, J.P. Lewis, Z. Hong, A. Manivannan, M. Zhi, M. Li, N. Wu, Origin of photocatalytic activity of nitrogen-doped TiO2 nanobelts, J. Am. Chem. Soc. 131 (2009) 12290–12297. [58] J. Yu, X. Zhao, C.Y. Jimmy, G. Zhong, J. Han, Q. Zhao, The grain size and surface hydroxyl content of super-hydrophilic TiO2/SiO2 composite nanometer thin films, J. Mater. Sci. Lett. 20 (2001) 1745–1748. [59] V. Etacheri, M.K. Seery, S.J. Hinder, S.C. Pillai, Highly visible light active TiO2−xNx Heterojunction photocatalysts, Chem. Mater. 22 (2010) 3843–3853. [60] Y. Cao, Z. Xing, M. Hu, Z. Li, X. Wu, T. Zhao, Z. Xiu, S. Yang, W. Zhou, Mesoporous black N-TiO2−x hollow spheres as efficient visible-light-driven photocatalysts, J. Catal. 356 (2017) 246–254. [61] T. Lin, C. Yang, Z. Wang, H. Yin, X. Lu, F. Huang, J. Lin, X. Xie, M. Jiang, Effective nonmetal incorporation in black titania with enhanced solar energy utilization, Energy Environ. Sci. 7 (2014) 967–972.

10