Kaifu Zhang, Wei Zhou*, Xiangcheng Zhang, Bojing Sun, Lei Wang, Kai Pan, Baojiang Jiang,  Guohui Tian, and Honggang Fu*

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the  People's Republic of China, Heilongjiang University, Harbin 150080, P. R. China.

Graphical abstract

Self-floating amphiphilic macro-mesoporous black TiO2 foams are prepared via freeze-dried  method combined with molding technology and hydrogenation, which extend photoresponse to  visible-light region and exhibit excellent solar-driven photocatalytic activity due to the 3D  amphiphilic macro-mesoporous networks facilitating mass transport and adsorption, the floating  feature and Ti3+ favoring light-harvesting and the separation of photogenerated electron-hole  pairs.

Highlights

1. Self-floating amphiphilic macro-mesoporous black TiO2 foam prepared by freeze-drying  

2. With exceptional visible light photocatalytic activity and long-term stability  

3. Super amphiphilic character favors the rapid adsorption and photocatalysis  

4. Complete mineralization of floating insoluble hexadecane  

5. Favoring solar-light-harvesting directly and recycling easily

ABSTRACT

The recycle and light-harvesting of powder photocatalysts in suspended system are  bottlenecks for practical applications in photocatalysis. Herein, we demonstrate the facile  synthesis of self-floating amphiphilic black TiO2 foams with 3D macro-mesoporous architectures  through freeze-drying method combined with cast molding technology and subsequent hightemperature surface hydrogenation. Ethylenediamine plays bifunctional roles on acid-base  equilibrium and “concrete effect” on stabilizing the 3D macro-mesoporous networks against  collapsing, which also inhibit the phase transformation from anatase-to-rutile and undesirable  grain growth during hydrogenation at 600 °C. The resultant black TiO2 foams, which can float  on the water, extend the photoresponse from UV to visible-light region and exhibit excellent  solar-driven photocatalytic activity and long-term stability for complete mineralization of  floating insoluble hexadecane and some typical pesticides. Especially for floating contaminant  hexadecane, the photocatalytic reaction apparent rate constant k is ~ 7 times higher than that of  commercial Degussa P25 under AM 1.5 irradiation. This enhancement is attributed to the 3D  macro-mesoporous networks facilitating mass transport, the super amphiphilicity benefiting  rapid adsorption, the floating feature and Ti3+ in frameworks favoring light-harvesting and  spatial separation of photogenerated electron-hole pairs. The novel self-floating photocatalyst  will have real practical applications for mineralizing floating contaminants in natural  environment

Keywords: Photocatalysis, Black TiO2, Self-floating foam, Amphiphilicity, Macro-mesoporous  architecture 

1. Introduction

Semiconductor photocatalysis has attracted increasing attention due to great potentials for  solving serious environmental issues.[1-5] In particular, porous TiO2 materials have triggered  tremendous research interests because of their nontoxicity, low-cost, environmental friendliness,  high chemical stability, tunable macro/mesostructured networks, large surface areas and pore  volumes, excellent electronic and optical properties.[6-9] Numerous efforts have been carried out  to prepare high-performance porous TiO2 materials, including improving crystallinity, tuning  bandgap, fabricating heterojunctions, etc.[10-14] However, the separation efficiency of  photogenerated charge carriers and solar light utilization are still not high enough until this date.  What’s worse, the recycle of powder photocatalysts in suspended system is extremely difficult  and becomes a bottleneck for practical application.[15-16] Light-weight floating photocatalysts,  which not only favor catalysts recycle, but also illuminate directly in aqueous solution and  increase the light-harvesting efficiently, should be good candidates. Since then, various floating  photocatalysts have been constructed on the floating supporters, such as expanded perlite, highsurface area vermiculite, fly-ash cenosphere, low density polyethylene, etc.[17-23] Although the  photocatalytic performance was indeed improved obviously, it still existed some unsolved  problems, including the effective loading, the firmness on the supporter, and blocking light  transmission.[24-25] Inspired by the formation of volcanic rocks under extreme quenching  conditions, which could float on the surface of water due to quantities of closed pores in  frameworks,[26] to design and synthesize support-free self-floating porous TiO2 is possible.

As is known that the wide bandgap for anatase TiO2 (~ 3.2 eV) greatly hinders the utilization  of solar light.[27] Fortunately, the recent discovery of black TiO2 materials by Chen and  coworkers via hydrogenation has caused great concern and opened up new era for tuning the  TiO2 bandstructures, which extended the photoresponse to visible light and/or near infrared region.[28] The highly localized nature of the midgap states resulted in efficient spatial  separation of photogenerated electron-hole pairs in black TiO2 based on density functional  theory, which led to high solar-driven photocatalytic performance.[29] Since then, great efforts  have been paid to synthesize various black TiO2 materials and tried to reveal the mysterious  structure.[30-37] Although the exact working mechanism of black TiO2 is still under debating,  an indisputable fact is that the solar-driven photocatalytic performance and separation efficiency  of photogenerated charge carriers are indeed improved obviously, which represents a great  breakthrough in photocatalysis.[38-41] However, the light-harvesting in aqueous solution and the  recycle issue for photocatalysts are still unsettled problems. Moreover, the photocatalyst with  super amphiphilic character would favor the adsorption and photocatalysis for various  contaminants, which is also crucial for photocatalytic reaction. Therefore, it is still a great  challenge to fabricate self-floating amphiphilic porous black TiO2 materials.Herein, we demonstrate the facile synthesis of self-floating amphiphilic black TiO2 foams with  3D macro-mesoporous architectures through freeze-drying method combined with cast molding  technology and surface hydrogenation at 600 °C. The resultant black TiO2 foams, which can  float on the water, extend the photoresponse from UV to visible light region and exhibit excellent  solar-driven photocatalytic activity and long-term stability for complete mineralization of  floating insoluble hexadecane and some typical pesticides, which is higher than that of  commercial Degussa P25 TiO2 under AM 1.5 irradiation. The novel light-weight self-floating  black TiO2 foams will have widespread practical applications in environmental fields.

2. Experimental section

2.1. Materials

Titanyl sulfate (TiOSO4, CAS: 123334-00-9), ethanediamine (C2H8N2, CAS: 107-15-3), ethanol  (C2H6O, CAS: 64-17-5) and polyacrylamide (C3xH5xNxOx, CAS: 9003-05-8) were of analytical  grade and purchased from Aladdin Reagent Corp. All chemicals were used as received without  any further purification. Deionized water was used for all experiments.

2.2. Synthesis

In a typical synthetic procedure, 2 g of TiOSO4 was dissolved in 60 mL deionized water with  stirring for 18 h at room temperature. Then the solution was turned from ivory to clear gradually,  followed by adding water/ethanediamine solution (1:1, in volume) to the sample dropwise  adjusting the pH ~ 8 with drastic agitation for 1 h to obtain uniform slurry, followed by the  addition of 0.5 g polyacrylamide, which was further stirred for 3 h. The reaction mixture in a 100  mL plastic beaker was then transferred into freezer for 6 h at -30 °C. The above samples with  liquid nitrogen frozen solid again, subsequently, placed in the freeze drier for 36 h to obtain the  white foam blocks. 0.5 g of samples was put in special stainless steel mould which was kept  under pressure of 0.6 Mpa for 60 seconds. Several as-obtained products were in the quartz tube  and horizontally placed in the furnace with temperature at 500, 600, 700 and 800 °C for 4 h and  cooled to room temperature to remove the template and improve the crystallinity. And then the  samples were calcined in hydrogen gas at 600 °C for 3 h at the rate of 100 mL min-1 to obtain the  black TiO2 foams (denoted as T500, T600, T700 and T800, respectively). Degussa P25 TiO2 was  calcined in hydrogen gas which was flowed at the rate of 100 mL min-1 at 600 °C for 3 h. Then,  black P25 was obtained as a reference (denoted as b-P25).

2.3. Characterization

The self-floating amphiphilic macro-mesoporous black TiO2 foams were characterized by a  wide-angle X-ray diffraction over the diffraction angle range (2θ) 5−80° with a Bruker-Norius D8 advanced diffractometer, using a Cu-Kα (λ = 1.5406 Å) radiation source operated at 40 kV  and 40 mA. The X-ray photoelectron spectroscopy (XPS, Kratos, ULTRA AXIS DLD)  measurements were performed on a system with amonochromatized Al-Kα X-ray source (1486.6  eV). The transmission electron microscopy (TEM) images were conducted on a JOEL JEM  2100F operated at 200 kV. Scanning electron microscope (SEM) images were obtained from a  Hitachi S-4800 instrument working at 15 kV. Diffuse reflectance spectroscopy (DRS) was  conducted on a UV/vis spectrophotometer (Shimadzu UV-2550) in the range of 200-800 nm.  The bandgaps were estimated by extrapolating a linear part of the plots to (αhν)2 = 0. Nitrogen  adsorption-desorption isotherms at 77 K were collected on an AUTOSORB-1 (Quantachrome  Instruments) nitrogen adsorption apparatus. All samples were degassed under vacuum at 180 °C  for at least 8 h prior to the measurement. The Brunauer–Emmett–Teller (BET) equation was used  to calculate the specific surface area. Pore size distributions were obtained using the Barrett– Joyner–Halenda (BJH) method from the adsorption branch of the isotherms. The large pores  were measured through mercury intrusion porosimetry (model pore sizer 9500, Micrometritics  Co. Ltd., USA). The total organic carbon (TOC) removal was measured using the TOC analysis  equiped with analytic jena multi NIC 2100 analyzer. The electron paramagnetic resonance (EPR)  spectra were performed at room temperature with an EPR spectrometer (JES-FA 300, 9.4 GHz, 1  mW). Surface photovoltage spectroscopy (SPS) measurements were carried out with a homebuilt  apparatus equipped with a lock-in amplifier (SR830) synchronized with a light chopper (SR540).  The photoluminescence (PL) spectra were measured by a PE LS 55 spectrofluoro-photometer  with excitation wavelength of 332 nm. The •OH radicals were measured by RF-5301PC  fluorescence spectrophotometer at room temperature with the same interval.

2.4. Photocatalytic activity

The photocatalytic activity was evaluated by mineralization of different soluble and insoluble  organic pollutants such as Rhodamine B, thiobencarb, atrazine, phenol and hexadecane. The asprepared catalysts and b-P25 TiO2 powders (0.5 g) as a reference were tested in the same  conditions. The samples were added to above-mentioned solution (40 mL, 1 mg L-1) in weighing  bottle without stirring for 1 h in the dark in order to reach an adsorption-desorption equilibrium.  The suspension was irradiated by using a 300 W Xe lamp (Aulight CEL-HXF300, 100 mW  cm-2) with AM 1.5 filter as light source. All photocatalytic experiments were carried out in an  open photoreactor located at 15 cm away from the light source without stirring at constant  temperature 40 °C. To confirm the stability of the photocatalysts, we recycled the catalysts after  experiments by deionized water cleaning several times, and then drying at 60 °C for 5 h to  remove the residual reactants and reactivate the adsorption and catalytic performance. The  mineralization of pollutants before and after irradiation was tested by total organic carbon  analysis. The measurement of hydroxyl radicals (•OH) was as follows. The highly fluorescent  products hydroxy terephathalic acid (TAOH) were generated by using terephthalic acid (TA) as  fluorescence probe to react with the •OH and proportional to the formation of •OH. 0.1 g of  photocatalysts was dispersed in 40 mL of 0.5 mM TA aqueous solution, prior to expose to visible  light irradiation in vertical, the reaction suspension was stewed in the dark for 60 min to reach an  adsorption-desorption equilibrium, a xenon lamp (CHF-XM500W) equipped with an AM 1.5  filter as the excitation source to provide light emission at 200-800 nm, with an interval of 60 min,  5 mL suspension was taken out and centrifuged for the next fluorescence spectrum  measurements in 300 min by the 315 nm excitation.

2.5. Photoelectrochemical measurement

The transparent conducting glass (TCO, F-doped SnO2 layer, sheet resistance ~ 20 Ω sq-1)  substrates were washed with detergent solution and ethanol by ultrasonic 1 h in order to remove  the residue. 50 mg of as-prepared samples was added into 4 mL ethanol. By using spray  pyrolysis technology to anchor the mixture onto the TCO substrates, then transferring them to  the tubular oven calcining at 400 °C for 0.5 h in constant N2 flow. Electrochemical impedance  spectroscopy and Mott-Schottky plots were analyzed by Princeton VersaSTAT electrochemical  station in a three-electrode system, with Ag/AgCl as reference and Pt plate as counter electrode.  The samples were made into the working electrode on the TCO substrate with the area ~ 1 cm2 ,  and irradiated by 300 W xenon lamp (Autolight CEL-HXF300) with AM 1.5 filter in vertical. 1  M KOH aqueous solution (pH = 13.8) was used as an electrolyte. Electrochemical impedance  spectroscopy was measured with amplitude of 5 mV and frequencies varying from 0.05 to 10000  Hz. Mott-Schottky plots were performed using a frequency of 1 kHz in the dark.

3. Results and discussion

3.1. Crystal structure and morphology of self-floating amphiphilic macro-mesoporous black  TiO2 foam

In this paper, we demonstrate the facile synthesis of self-floating amphiphilic black TiO2 foams  with 3D macro-mesoporous architectures through freeze-drying method combined with cast  molding technology and surface hydrogenation at 600 °C. The illustrated formation process of  self-floating amphiphilic macro-mesoporous black TiO2 foams (SAMBTFs) is shown in Scheme  1. In this procedure, polyacrylamide is dissolved in Ti precusor and ethanediamine aqueous  solutions uniformly, which aggregates and crosslinks into primary aggregates during freezedrying process. Thereinto, polyacrylamide, as water-soluble macromoleculer polymer, plays vital  role on the formation of closed pores, which is the main reason for the self-floating character.[42]The ethanediamine molecules encircle the aggregates firmly due to the electrostatic  interaction.[43] Then, the foam structure can be formed after cast moulding technique. After  being calcined in air, the TiO2-polymer composite coverts to 3D macro-mesoporous TiO2 foam,  in which the polymer template is removed sufficiently (Fig. S1) and the crystallinity of TiO2 is  improved simultaneously. Ethylenediamine plays bifunctional roles on acid-base equilibrium and  “concrete effect” on stabilizing the 3D macro-mesoporous networks against collapsing, which  also inhibit the phase transformation from anatase-to-rutile and undesirable grain growth.[43]  Meanwhile, quantities of large closed pores are formed due to the removal of template under the  special conditions of freeze-drying process, which is critical for self-floating as volcanic rocks.  On the other hand, lots of opened pores are also generated in frameworks and on surface because  of the evaporation of water during freeze-drying conditions. The self-floating amphiphilic  macro-mesoporous black TiO2 foams are formed finally after surface hydrogenation, which  could harvest solar-light efficiently.

The crystal structure of SAMBTFs after being calcined at different temperatures and  hydrogenation at 600 °C was analyzed by X-ray diffraction and Raman techniques as shown in  Fig. 1. From T500 to T700, it clearly shows five high-intensity crystal peaks at 2θ = 25.2, 37.8,  48.1, 53.9 and 56.1°, which could be indexed as (101), (004), (200), (105), and (211) planes of  nanocrystalline anatase TiO2 (JCPDS no. 21-1272). Generally speaking, the photocatalytic  performance is highly depended on the crystal phase and crystallinity of TiO2, which are  determined by the calcination temperatures.[43] The crystallinity of anatase TiO2 is improved  obviously with increasing the calcination temperatures from Fig. 1A. The anatase crystal phase  could be maintained up to 700 °C, which is apparently higher than that of literatures.[44-45] The  average crystal size that determined from the (101) Bragg diffraction by Scherrer formula is below 20 nm (Table S1). This implies that the crystal size is not very large even after hightemperature surface hydrogenation. When the calcination temperature is above 800 °C, the phase  transformation from anatase-to-rutile appears because rutile is the most thermodynamically  stable phase of TiO2. However, without introduction of ethylenediamine, the rutile phase is  present when the calcination temperature exceeds to 600 °C. The fact suggests that  ethylenediamines indeed inhibit the phase transformation from anatase-to-rutile and undesirable  grain growth, and thus improve the crystallinity, which is favorable for photocatalysis.[43] As a  more sensitive technique for unambiguously discriminating the local order characteristics of  TiO2, Raman spectroscopy is accepted to monitor the microstructure variation.[43] As shown in  Fig. 1B, below 700 °C five high-intensity Raman vibration peaks locating at around 152, 205,  405, 520 and 645.0 cm−1 , are well ascribed to Eg, Eg, B1g, A1g (B1g) and Eg characteristic  vibration modes of anatase.[46] Compared to unhydrogenation samples and literatures, these  Raman vibration peaks are shifting, broadening and weakening, which reveal that  crystallography geometric symmetry is changed due to the structure distortion.[39] From T500 to  T700, the intensity of Raman peaks increases with the increase of calcination temperatures,  implying the improvement of anatase TiO2 crystallinity. For T800 sample, the obvious rutile  characteristic peaks are appeared, confirming the existence of phase transformation from  anatase-to-rutile, which coincide with the XRD results.

Typical IV-type hysteresis loops are observed in adsorption-desorption isotherms (Fig. S2),  implying caged mesopores based on nonlocal density functional theory with a sharp capillary  condensation step in relative pressure range of 0.6-0.96, which is representative of macromesoporous materials.[47] It should be concerned that the pore sizes are mainly distributed in  30-40 nm in BJH pore-size distribution curves, characteristic of nanostructured porous material that caused by the removal of polyacrylamide. Interestingly, the narrow pore size distribution of  large pores ~ 1.56 μm for T600 can be observed through mercury intrusion porosimetry (Fig. S3),  implying the existence of large pores in black TiO2 foams. Diffuse reflectance measurements  reveal that the bandgap is reduced after hydrogenation, because an obvious shift in the onset of  absorption from UV to visible-light region can be observed (Fig. S4A). From Fig. S4B, the  bandgaps are smaller than that of pristine anatase TiO2 (~ 3.2 eV), which are ascribed to the  efficient surface hydrogenation and maybe substantially enhance the visible-light-driven  photocatalytic activity.

To investigate the surface state of SAMBTFs, XPS is conducted (Fig. S5). The appearance of  two typical peaks which mainly focus on 458.5 and 464.4 eV assigned to the Ti 2p3/2 and Ti 2p1/2 for the Ti4+ species in TiO2 chemical states (Fig. S5A).[48] A broader O 1s peak with a strong  shoulder at high binding energy can be observed (Fig. S5B), which could be deconvoluted into  two peaks centered at 530.1 and 531.6 eV. The broader peak at 531.6 eV could be attributed to  Ti-OH species, implying the formation of more hydroxyl groups on TiO2 surface after  hydrogenation.[39] The presence of N 1s peak at ~ 399 eV confirms the existence of Ncontaining species (Fig. S5C), implying the efficient encircling of ethylenediamines.[43] The  valence band XPS spectra (Fig. S5D) indicate that the valence band is redshifted obviously,  which resulting in the midgap towards conduction band, further illustrates hydrogenation acting  as a key role in reducing distortion energy in improving the valence band maximum, and  conduction band minimum has no obvious change. It is beneficial to form the surface disordering,  which leads to narrowing the bandgap and absorbing visible light. Electron paramagnetic  resonance (EPR) is a technique stemming from spinning motion of the unpaired-electrons, which  can be used to detect unpaired-electrons of the atoms or molecules in qualitative and quantitative aspects, and explore its structure characteristics of the surrounding environment.[49] A strong  EPR signature of Ti3+ at g ≈ 1.98 can be observed for T600 (Fig. S6), which is the representative  for a paramagnetic Ti3+ center, implying the formation of Ti3+ in frameworks.[50-52]  Interestingly, the Ti3+ is long-term stable in air which may be ascribed to the existence of surface  nitrogen.

As revealed by scanning electron microscope (Fig. 2a, b), uniform macropores throughout the  entire networks can be observed. Transmission electron microscopy images (Fig. 2c, d) further  illustrate the macro-mesopores structure. The inset of Fig. 2c illustrates the selected area electron  diffraction pattern, which presents bright diffraction rings assigned to high crystallization and  polycrystalline structure. The appearance of the crystal lattice disorders or amorphous structure  surrounding the crystalline core is distinct to be observed in Fig. 2d, indicating the efficient  surface hydrogenation. Lattice fringe spacing is estimated to be ~ 0.35 nm, in good accordant to  the anatase TiO2 (101) facet. These facts confirm that the macro-mesopores foam-like  morphologies are essentially preserved in the calcination process. Remarkable, from the digital  photos we can clearly see the macroscopic morphology of SAMBTFs (Fig. 2e-g). It is close to a  dime for shape and size, which could float on the surface of water and favor the illumination  directly. The bifunctional ethylenediamine plays vital roles on both acid-base equilibrium and  porous networks against collapsing. Without introducing ethylenediamine, the rutile phase is  present when the calcination temperature exceeds to 600 °C. The fact suggests that  ethylenediamines indeed inhibit the phase transformation from anatase-to-rutile and undesirable  grain growth, and thus improve the crystallinity, which is favorable for photocatalysis. Taking  sodium hydroxide instead of ethylenediamine, the morphology and crystal phase are different  obviously (Fig. S7), further indicating the efficient encircling effect of ethylenediamine.

In order to investigate the amphiphilic character of SAMBTFs, the contact angle of water and  hexadecane is conducted and shown in Movie S1-S2. Obviously, the super amphiphilicity for  both water and oil can be observed because the droplet is spread thoroughly once they contact  with SAMBTFs. The super hydrophilicity is ascribed to the plenty of surface -OH and Ti3+ due  to surface hydrogenation,[53-54] which is favorable for the rapid adsorption of water soluble  contaminants and then photocatalytic degradation. While, the super lipophilicity should be  attributed to the pumped or syphonage due to the formed macro-mesoporous networks,[55]  which also favors the rapid adsorption and photodegradation of lipophilic contaminants because  photocatalysis is surface catalytic reaction. Therefore, the super amphiphilicity of the selffloating amphiphilic black TiO2 foam is vital for the rapid adsorption and the excellent  photocatalytic performance.

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