TiO2 heterojunction for enhanced visible light photocatalysis

Applied Surface Science 434 (2018) 796–805

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In-situ fabrication of diketopyrrolopyrrole-carbazole-based conjugated polymer/TiO2 heterojunction for enhanced visible light photocatalysis夽 Long Yang a,b , Yuyan Yu a , Jianling Zhang a , Fu Chen a , Xiao Meng a , Yong Qiu b , Yi Dan a,∗ , Long Jiang a,∗ a State Key Laboratory of Polymer Materials Engineering of China (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu, 610065, China b Institute of Systems Engineering, China Academy of Engineering Physics, Mianyang, 621999, China

a r t i c l e

i n f o

Article history: Received 4 August 2017 Received in revised form 21 October 2017 Accepted 25 October 2017 Available online 27 October 2017 Keywords: Conjugated polymers Donor-acceptor Interface enforcement Photocatalytic TiO2

a b s t r a c t Aiming at developing highly efficient photocatalysts by broadening the light-harvesting region and suppressing photo-generated electron-hole recombination simultaneously, this work reports rational design and fabrication of donor-acceptor (D-A) conjugated polymer/TiO2 heterojunction catalyst with strong interfacial interactions by a facile in-situ thermal treatment. To expand the light-harvesting window, soluable conjugated copolymers with D-A architecture are prepared by Pd-mediated polycondensation of diketopyrrolopyrrole (DPP) and t-butoxycarbonyl (t-Boc) modified carbazole (Car), and used as visiblelight-harvesting antenna to couple with TiO2 nanocrystals. The DPP-Car/TiO2 composites show wide range absorption in 300–1000 nm. To improve the interfacial binding at the interface, a facile in-situ thermal treatment is carried out to cleave the pendant t-Boc groups in carbazole units and liberate the polar amino groups (–NH–) which strongly bind to the surface of TiO2 through dipole–dipole interactions, forming a heterojunction interface. This in-situ thermal treatment changes the surface elemental distribution of TiO2 , reinforces the interface bonding at the boundary of conjugated polymers/TiO2 and finally improves the photocatalytic efficiency of DPP-Car/TiO2 under visible-light irradiation. The interface changes are characterized and verified through Fourier-transform infrared spectroscopy (FT-IR), photo images, UV/Vis (solution state and powder diffuse reflection spectroscopy), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fluorescence, scanning electron microscopy(SEM) and transmission electron microscopy (TEM) techniques. This study provides a new strategy to avoid the low solubility of D-A conjugated polymers and construct highly-efficient conjugated polymer/TiO2 heterojunction by enforcing the interface contact and facilitating charge or energy transfer for the applications in photocatalysis. © 2017 Elsevier B.V. All rights reserved.

1. Introduction There has been a surge of interest recently in the development of visible-light-driven photocatalysts using conjugated polymers (CPs) as optically active materials, owing to their advantages of excellent electrical and optical activity, high absorption coefficients in the visible light region and mechanical properties [1–4]. However, the metal-free polymeric photocatalysts developed thus far have been restricted by low efficiency, mainly due to the short

夽 The authors declare no competing financial interest. ∗ Corresponding authors. E-mail addresses: [email protected] (Y. Dan), [email protected] (L. Jiang). https://doi.org/10.1016/j.apsusc.2017.10.176 0169-4332/© 2017 Elsevier B.V. All rights reserved.

exciton diffusion length (∼10 nm) and fast recombination of photoinitiated electron-hole pairs [5–7]. At present, the creation of heterojunction based on the intimate mixing of conjugated polymer (as electron donor) and n-type metal oxide semiconductor (as electron acceptor) has been recognized as one of the most promising approach to suppress the recombination of electron-hole pairs and, thus, improve the photocatalytic performance [8–12]. Various n-type metal oxide semiconductors, such as ZnO, WO3 , Cu2 O and TiO2 , have been successfully applied in the photocatalytic heterojunctions [13–23]. Particularly, TiO2 has proven thus far to be the best choices for n-type electron acceptor materials in the creation of a heterojunction with CPs due to its excellent photo-stability and relatively high reactivity [23–25]. In this field, a series of conjugated homopolymers including polyaniline, polypyr-

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

role, poly(p-phenylene) and polythiophene and their derivatives have been widely incorporated with TiO2 , resulting in heterojunctions with enhanced photocatalytic performance under both ultraviolet-light and visible-light irradiation [26–33]. Meanwhile, our very recent investigation has revealed that the photocatalytic performance of CPs/TiO2 heterojunction could be dramatically enhanced by forming covalent bonding interact between CPs and TiO2 through interfacial engineering [34]. Such a rapid progress is significant compared to any other types of current photocatalyst systems, showing promise for future applications. Besides interfacial engineering, development of new conjugated polymers as optically active layer for efficient light-harvesting and charge transport is believed to be the major breakthrough in this field [35]. In principle, an ideal CP for high photoactivity CP/TiO2 should possess the following properties [36,37]: 1) broad absorption spectra coupled with a high extinction coefficient to match with the solar radiation spectrum and to harvest as much solar energy; 2) well-matched energy levels between CP and TiO2 to guarantee sufficient driving force for charge separation while maintain suitable thermodynamic driving force for different photocatalyzed reactions, including water splitting, CO2 reduction and degradation; 3) providing “docking” sites, such as NH2 , NH–, COOH, etc, to form the desired intimate interfacial contact with TiO2 for efficient charge separation at the interface; 4) long-term stability during photocatalytic applications. According to the wealth of knowledge that exists in organic photovoltaics [38–44], donor-acceptor (D-A) conjugated polymers, or the socalled push-pull conjugated polymers, which consist of alternating electron-rich (D) and electron-deficient (A) units should be the ideal candidates to satisfy the first two criteria because their intrinsic optical and electronic properties, including light-absorption ability and energy levels, can be tuned readily by controlling the intramolecular charge transfer (ICT) from D unit to A unit. Although a few conjugated microporous polymers (CMPs) have been designed and prepared through the D-A strategy for photocatalytic applications and shown promising photocatalytic activities under visible light irradiation [45,46], coupling D-A conjugated polymers with an inorganic semiconductor to achieve higher photocatalytic activity is relatively unexplored mainly because of their insolubility and poor miscibility with polar inorganic semiconductor. Introduction of long aliphatic side chains on the rigid backbone of D-A conjugated polymer is one of the most common molecular design strategies for increasing solubility [47–49]. However, in the case of fabrication of a CP/TiO2 heterojuncton, the large aliphatic side chain will significantly inhibit the intimate contact and charge transfer between CP and TiO2 , resulting in poor photocatalytic performance. Therefore, the development of new approach to simultaneously increase the solubility of D-A conjugated polymers and the interface interaction is still a very challenging area of investigation. Our group has been pursuing conjugated polymerization design and applications on photocatalysis for a long time [34,50–54]. Considering the excellent electro-optical properties of carbazole (Car) and diketopyrrolopyrrole (DPP) based donoracceptor alternating copolymers, such as high hole carrier mobility (10−3 ∼ 10−1 cm2 V−1 s−1 ), narrow optical band gap (1.5-1.9 eV) and long-term stability under solar light radiation [55], we have developed a series of DPP-Car copolymers recently, and revealed that the solubility of DPP-Car could be significantly increased by introducing thermal-cleavable tert-butoxycarbonyl (t-Boc) side chains on NH of Car units [56]. Meanwhile, this t-Boc side chain can be quantitatively removed with thermal treatment at ∼180 ◦ C and leave the unbound NH group without lowering the electro-optical properties of DPP-Car. Moreover, the as-synthesized DPP-Car exhibits an optical band gap of 1.84 eV with LUMO level at −3.14 eV (vs. NHE) and HOMO level at −4.98 eV

797

(Vs. NHE), which implies a broad absorption in visible light region up to ∼680 nm and a suitable energy level matching with TiO2 . Encouraged by our success in designing D-A conjugated polymers and fabricating CPs/TiO2 heterojunctions for high performance photocatalytic applications, herein, we describe our recent effort in rationally designing D-A CP/TiO2 heterojunction for photocatalytic degradation. DPP-Car/TiO2 heterojunction with enhanced interface interaction were fabricated by in-situ thermal-treatment of the mixture of t-Boc modified DPP-Car (DPP-Car-Boc) and TiO2 nanocrystals. Upon in-situ thermal-treatment of DPP-Car-Boc/TiO2 at 180 ◦ C, cleavage of t-Boc side chains will convert nonpolar aliphatic side chains to polar “docking” sites (–NH–), leading to the interface enhancement at the boundary of DPP-Car/TiO2 . The influence of the thermal-treatment on the photophysical properties and photocatalytic activities of DPP-Car/TiO2 heterojunctions was studied in detail. Additionally, the influence of molecular weight of DPP-Car and stability of the heterojunctions were also studied. To the best of our knowledge, this is the first time to fabricate CP/TiO2 photocatalytic heterojunction based on a D-A conjugated polymer, and the systematic characterization and investigation may give inspiration in the interface engineering as well as the design of high-performance photocatalysts. 2. Experimental 2.1. Materials TiO2 nanoparticles (P25, particle size 20–30 nm, BET 50 m2 /g) were purchased from Degussa without any further chemical modification. 3,6-bis(5-bromothiophen-2-yl)-2,5-di(DPP-Thio-Br) (2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione and 9-(tert-butoxycarbonyl) −9H-carbazole-3,6-diyldiboronic acid (Car-Bor) were synthesized using conditions we reported previously [56]. Methyl trioctyl ammonium chloride (Aliquat 336), Pd(PPh3 )4 and K2 CO3 were purchased from Chengdu Astatech Trading Co., Ltd and used without further purification. Methyl orange (MO) was obtained from Kelong Chemical Reagents Factory (Chengdu, China). Solvents were dried over appropriate conditions, and then distilled over CaH2 . 2.2. Instruments Fourier Transformation Infra-red Spectroscopy (FT-IR) spectra were measured on a Nicolet IS10 spectrometer (Thermo Fisher Scientific, America). Fluorescence spectra were recorded on an F4600 spectrometer (Hitachi, Japan). UV/Vis spectra of solution were measured on a UV-2300 spectrophotometer (Hitachi, Japan) and UV/Vis diffuse reflectance spectra (UV/Vis DRS) of the composites were recorded on a UV-3600 spectrophotometer (Shimadzu, Japan). X-ray diffraction (XRD) measurements were carried out with a DX-2500 spectrometer (Dandong, China). X-ray photoelectron spectroscopy AXIS Ultra DLD (Kratos Analytical) was applied to characterize the surface elements distribution. SEM images were taken on Quanta-250 scanning electron microscope (American FEI Cooperation). TEM images were obtained from Tecnai G2 F20 transmission electron microscope with maximal acceleration voltage of 200 kV. The photocurrent analysis was carried out on a CHI 760E electrochemical system (Shanghai, China) without bias under xenon lamp irradiation fitted with a cut-off filter (␭ > 420 nm). A three-electrode quartz cell was used for measurement with Pt as the counter electrode, Ag/AgCl as reference electrode, the thin film with similar thickness of TiO2 or DPP-Car/TiO2 composites on indium-tinoxide (ITO) glass as the working electrode, and 0.5 mol L−1 Na2SO4 aqueous solution as the electrolyte. Electron paramagnetic resonance (EPR) measurements were carried out

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L. Yang et al. / Applied Surface Science 434 (2018) 796–805

on Bruker EPR EMX Plus (Bruker Beijing Science and Technology Ltd, USA) of a frequency 9.8 GHz approximately using a standard microwave power of 0.1 mW. 2.3. Synthesis and characterization 2.3.1. DPP-Car-Boc (DPP-Car-Boc-H and DPP-Car-Boc-L) t-Boc modified DPP-Car alternating copolymers were prepared by palladium mediated polycondensation (Fig. 1). Typically, DPPThio-Br (0.50 g, 0.73 mmol) was dissolved in anhydrous toluene (25 mL) and the solution was stirred under Ar atmosphere for 10 min at 30 ◦ C. Then to the mixture, Pd(PPh3 )4 (0.08 g, 10 mol%), a solution of Car-Bor (0.26 g, 0.73 mmol) in anhydrous THF (5 mL), a catalytic amount of Aliquat 336 and K2 CO3 aqueous solution (2 M, 0.7 mL) were added consecutively. After 30 h polycondensation at 80 ◦ C under Ar protection, the mixture was cooled to ambient temperature and precipitated in a mixture of methanol (100 mL) and water (20 mL). The obtained precipitate was further purified by consecutive Soxhlet extractions with methanol, hexane, chloroform and chlorobenzene. DPP-Car-Boc with low molecular weight (DPP-Car-Boc-L) and high molecular weight (DPP-CarBoc-H) were obtained from the chloroform and chlorobenzene extraction, respectively. 2.3.2. DPP-Car-Boc-L (dark purple): Mw /Mn (GPC) = 3320/2818; 1 H NMR (CDCl3 , 400 MHz) 0.899 (12H, s), 1.258–1.397 (16H, m), 1.626 (2H, s), 1.737–1.819 (9H, m), 4.077 (4H, s), 7.447–7.715 (5H, m), 8.128–8.227 (3H, m), 8.901–8.980 (2H, m). 2.3.3. DPP-Car-Boc-H (dark purple): Mw = 5500; 1 H NMR (d-chlorobenzene, 400 MHz) 1.175 (12H), 1.344–1.402 (16H), 1.900 (2H), 2.023–2.070 (9H), 4.584 (4H), 7.076–7.009 (2H), 7.216 (1H), 7.147 (2H), 7.628 (1H), 7.932–7.988 (2H), 8.108–8.122 (2H). 2.4. Preparation of DPP-Car/TiO2 heterojunctions and evaluation of photocatalysis DPP-Car/TiO2 heterojunctions were prepared by the following procedure: TiO2 nanoparticles were activated at 120 ◦ C for 4 h to remove the physically adsorbed water and organic compounds. Then the activated TiO2 (1.0 g) were sonically dispersed in chlorobenzene solvent (100 mL) to prepare the TiO2 suspension. DPP-Car-Boc-H (20 mg) was dissolved in chlorobenzene (100 mL) at 80 ◦ C for 30 min. Then the solution was added slowly to the TiO2 suspension under stirring. After the addition, the solution mixture was heated at 40 ◦ C to remove the chlorobenzene solvent at reduced pressure and dried under vacuum to obtain the composite DPP-Car-Boc-H/TiO2 . Then, DPP-Car-Boc-H/TiO2 composite was heated under N2 flow at 150 ◦ C and 180 ◦ C for 2 h respectively to remove the pendant t-Boc group (Fig. 1), and DPP-Car-H/TiO2 heterojunction with enhanced interfacial interaction was obtained. In the same procedures, composites DPP-Car-Boc-L/TiO2 and DPPCar-L/TiO2 were also prepared with DPP-Car-Boc-L (DPP-Car-Boc with low molecular weight) and TiO2 . The photocatalytic performance of the composites and TiO2 (P25) was evaluated by the degradation of MO. 100 mg of photocatalyst and MO aqueous solution (200 mL, 10 mg/L) were added to a 500-mL beaker which was cooled by circulating water. The visible light source (500W tungsten–halogen lamp, OSRAM, Germany) was positioned over the beaker to maintain an irradiance of 19 mW/cm2 while a 420 nm cut-off filter was applied. Before irradiation, the suspension was stirred in dark for 60 min to obtain adsorptiondesorption equilibrium of the system. A portion of the samples was then withdrawn at regular time, filtered for separation of any

suspended solid and measured by UV/Vis spectroscopy immediately. The change in the concentration of MO was monitored by measuring the absorbance at ␭max (467 nm). To test the structure stability of the different DPP-Car/TiO2 heterojunctions, FT-IR and Raman spectra of DPP-Car/TiO2 were collected after three recycle tests. 3. Results and discussion Fig. S1 and Fig. S2 present the functional groups changes of the composites DPP-Car-Boc-H/TiO2 and DPP-Car-Boc-L/TiO2 with high and low molecular weight, respectively, before and after thermal treating. As seen from Fig. S1b and Fig. S2b, the 1729 cm−1 peak is ascribed to the stretching vibration of C O in the t-Boc protecting group and its disappearance indicates the successful cleavage of t-Boc groups, thus the composites of DPP-Car-H/TiO2 and DPP-Car-L/TiO2 are successfully prepared. The peaks located at 1664 cm−1 and 1668 cm−1 are assigned to the ␯C O (amide in DPP fragment) for DPP-Car-Boc-L/TiO2 and DPP-Car-Boc-H/TiO2 , respectively. After thermal treatment, it is found that the stretching vibrations of C O in the amide groups (␯C O ) in both DPP-CarBoc-L/TiO2 and DPP-Car-Boc-H/TiO2 shift to lower frequencies (red-shift), which is indicative of the hydrogen bonding along the polymer backbone (C O···H N) and between the polymer and TiO2 (C O···H O) [56]. Additionally, this reinforced interface interaction between DPP-Car and TiO2 through hydrogen bonding is further confirmed by the tremendous blue-shift of Ti-O-Ti stretching vibration from 672 cm−1 to 837 cm−1 and 841 cm−1 for composites DPP-Car-L/TiO2 and DPP-Car-H/TiO2 , respectively [57,58]. As can be seen from the photo-images (Fig. S3 and Fig. S4) of composites DPP-Car-Boc/TiO2 and DPP-Car/TiO2 , after thermal annealing and the cleavage of t-Boc pendant groups, the colors of DPP composites change noticeably, from pure blue to dark blue and olive green for DPP-Car-L/TiO2 and DPP-Car-H/TiO2 , respectively. As revealed by our previous research [56], no obvious change in electro- and optical-properties of DPP-Car was observed after thermal treatment at 180 ◦ C, though the pendant t-Boc groups were decomposed completely under that condition. Thus, the color change in this case can be rationally ascribed to enhanced interface bonding between DPP-Car and TiO2 after thermal treatment. To characterize the crystalline structures of TiO2 (P25) and DPP/TiO2 composites, powder XRD patterns are presented in Fig. S5. As shown, diffraction peak at 2␪=25.3◦ is assigned to the typical (101) crystal face in anatase phase, while the peak located at 2␪=27.4◦ belongs to the typical (110) crystal face in rutile phase. Thus the nanoparticle composites exhibit a mixed crystalline structure of anatase and rutile phases which account for 79% and 21% in ratio respectively. Additionally, the nanocrystal sizes of the composites range from 17 nm − 20 nm, according to the Sherrer formula (1), in which D is the size of nanocrystal (Å), K is the shape index (0.9), ␭ is the wavelength of X-Ray CuK␣ radiation (1.5406 Å), ␪ is the Bragg diffraction angle and ␤ is the half-peak width of nanocrystals. Unfortunately, no further useful information is obtained about the aggregation mode changes of conjugating polymers spreading over the surface of nanocrystal TiO2 , partially because the diffraction signal (located at about 25◦ ) of ␲-␲ stacking between the ␲-conjugating backbone plains is weak and covered by the diffraction peak of (101) crystalline face [56]. D=

K␭ ␤csc␪

(1)

Due to the superiority of light-harvesting capacity and molecular designing flexibility, conjugated polymers have been widely applied to modify nanoparticles TiO2 in various aspects to improve the efficiency of photo-driven devices. UV/Vis diffuse reflectance spectra (Fig. 2) are measured to evaluate the light-absorbing prop-

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

N R

S

Pd(PPh3)4

+

O

799

S

S N

N Boc

R

DPP-Car-Boc

Car-Bor

DPP-Thio-Br O

N Boc

O

R N

hermal treatment

O

S

S

CO2

Boc =

O

Fig. 1. Synthesis of the target DPP-Car copolymers.

100

5

a TiO2

80

TiO2 DPP-Car-L/TiO2 DPP-Car-H/TiO2

b 4 Absorbance(a.u.)

DPP-Car-L/TiO2 DPP-Car-H/TiO2

60

DPP-Car-Boc-L/TiO2 DPP-Car-Boc-H/TiO2

40

2

1

20

0 300

DPP-Car-Boc-L/TiO2 DPP-Car-Boc-H/TiO2

3

0 400

500

600

700

800

900

1000

300

400

500

600

700

800

900

1000

Fig. 2. (a) Diffuse reflectance spectra of TiO2 and DPP-Car/TiO2 before and after t-Boc cleavage; (b) UV/vis absorbance spectra of TiO2 and DPP-Car/TiO2 before and after t-Boc cleavage. The arrows imply the change in optical properties of DPP-Car/TiO2 after t-Boc cleavage through in situ thermal treatment at 180 ◦ C.

erty and the change of interface features before and after thermal annealing. As well known, pure TiO2 (P25) lacks effective visiblelight (>400 nm) absorption, and as shown in Fig. 2, the composites present light absorption within the range of 400–1000 nm, among which the peak of 500–700 nm is ascribed to ␲-␲* transition of the large ␲ conjugating system and the peak of 800–1000 nm is attributed to the sub-gap absorbance. Noticeably, after thermal treatment, the cleavage of thermo-labile t-Boc protecting groups and liberation of NH groups cause obvious decrease of ␲-␲* transition (Fig. 2b) and increase of sub-gap absorbance (Fig. 2a). This characterization indicates that more effective ground-state charge transfer complexes [59] form during the thermal treatment and the co-effect of excited-state DPP-Car and Ti positive ions result in the decrease of ␲-␲* transition and increase of sub-gap absorbance, which will be further proved by the XPS analysis [60]. Thus a conclusion could be drawn that in the boundary of DPP-Car conjugated polymers and nanocrystalline TiO2 particles, the liberation of polar NH groups could effectively perturb the electronic state of the interface and thus reinforce the binding energy. The intensified sub-gap absorbance which originates from charge-transfer band may bring profound effects in light harvesting, exciton dissociation, charge separation and charge migration and consequently affects the photocatalytic performance [61,62]. To investigate the emissive property and energy level alignment, the fluorescence spectra of pure TiO2 (P25) and composites DPP-Car/TiO2 were recorded in DMF (Fig. 3). From the emissive characterization, the energy levels and interface bonding effect at the boundaries of conjugated polymers and TiO2 could be gained. TiO2 nanocrystals exhibit a broad and asymmetrical emission peak

covering 300–600 nm, with an emission maximum around 379 nm (corresponding to 3.27 eV). The band originates from the recombination transition of free electrons and holes [63]. In contrast, the composites present two distinct, asymmetrical and sharp emissive peaks, one of which the maximum is located at 650 nm, the other is located at 460 nm and relatively stronger than the low-energy emissive peak (650 nm). The long-wavelength emission (650 nm) comes from the fluorescence of pristine conjugated polymers, and the short-wavelength emission peak, which is absent in the fluorescence spectra of pure conjugated polymers [56], should be from the radiation of the localized state at the boundaries of DPP-Car conjugated polymers and TiO2 nanocrystals. Consequently, effective hole-electron pairs or charge-transfer excitons could form at the interfaces of the composites [64,65]. Additionally, the emissive intensity of pristine polymer (650 nm peak) is lower than that of the localized-state boundary, which is indicative of the strong interface bonding and effective charge transfer between the conjugated polymer and TiO2 . Moreover, the intensity of 650 nm emissive peaks for the composites DPP-Car-H/TiO2 and DPP-Car-L/TiO2 are lower than that for the composites DPP-Car-Boc-H/TiO2 and DPP-CarBoc-L/TiO2 , respectively, indicating that much more excitons are formed and the excited energy or charge of conjugated polymers could be transferred to nanocrystal TiO2 more effectively after the cleavage of t-Boc groups and the liberation of polar NH groups due to the enhanced interface bonding [66]. To further verify the effective charge or energy transfer at the boundary, the fluorescence spectra of composite DPP-Car-H/TiO2 in DMF (Fig. S6) were recorded under different excitation wavelengths (280 nm, 390 nm, 410 nm and 430 nm). As can be seen from

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

1.0 0.8

a

379 460 466

TiO2 DPP-Car-Boc-L/TiO2

1.0

DPP-Car-L/TiO2

0.8

641

0.6 656

0.4

Intensity (a.u.)

800

b

379 460

465

TiO2 DPP-Car-Boc-H/TiO2 DPP-Car-H/TiO2

0.6 0.4

0.2

0.2

0.0 300 350 400 450 500 550 600 650 700 750 800

0.0

643 658

Fig. 3. Fluorescence spectra of (a) TiO2 and low molecular weight DPP-Car/TiO2 composites before and after the cleavage of t-Boc group; (b) TiO2 and high molecular weight DPP-Car/TiO2 composites before and after the cleavage of t-Boc group. Excitation wavelength: 410 nm for the composites and 280 nm for TiO2 ; temperature: 298 K.

Fig. 4. Schematic energy level diagram of DPP-Car-L/TiO2 composite. HOMO and LUMO represent highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively.

the spectra, the fluorescence maxima range from 370 to 470 nm, corresponding to different molecular vibration states of the composites, that is to say, under the intimate interface bonding, the energy levels of conjugated polymer and TiO2 interrelate effectively and are redistributed and thus result in a series of localized charge states facilitating the effective energy or charge transfer among the boundary [67]. The light-absorbing and emissive properties are summarized in Table 1. ELUMO-CB represents the energy level gaps between the lowest unoccupied molecular orbital (LUMO) of conjugated polymers and the conduction band of nanocrystal TiO2 , which is calculated on the basis of UV/Vis reflectance diffraction spectra and electrochemical cyclic voltammetry [56]. According to the above data and analysis, the schematic energy level diagram of DPP-Car-L/TiO2 composite (Fig. 4) is plotted to illustrate the charge transfer mechanism. 3.27 eV is the energy gap related to emission process of the free exciton electron-hole recombination from conduction band (CB) to valance band (VB) transition of TiO2 . The energy of emission from the localized states[68] at the DPP-Car-L/TiO2 interface to the VB of TiO2 is 2.66 eV, while the energy gap between the trapped electrons in the ␲ orbital of conjugated chain and the VB of the TiO2 is 1.89 eV 1.84 eV represents the HOMO-LUMO energy gap of the pristine conjugated polymer DPP-Car-L. Thus the ELUMO-CB gap could be obtained. ELUMO-CB is a key factor in characterizing and evaluating the photocatalytic performance of photo-electric

devices concerning the transformation of solar energy to electric or chemical energy. Lower ELUMO-CB would benefit the charge transfer from the LUMO orbital to the conduction band, as well as more intimate contact. From Table 1, the ELUMO-CB of composites DPP-Car-L/TiO2 and DPP-Car-H/TiO2 is 0.46 eV and 0.39 eV, lower than that of pre-treatment composites DPP-Car-Boc-L/TiO2 and DPP-Car-Boc-H/TiO2 , respectively. Noticeably, the ELUMO-CB of composites with high molecular weight, DPP-Car-Boc-H/TiO2 (0.44 eV) and DPP-Car-H/TiO2 (0.39 eV), are lower than that of composites with low molecular weight, DPP-Car-Boc-L/TiO2 (0.50 eV) and DPP-Car-L/TiO2 (0.46 eV), respectively, indicating that conjugated polymers with higher molecular weight would possess lower band gaps and sensitize TiO2 more effectively. Consequently, in the composites the relative low ELUMO-CB should facilitate the charge transfer from the excited-state of conjugated polymer to the CB of inorganic nanocrystal TiO2 , and finally fulfill the photocatalysis process taking advantage of visible light irradiation. To further testify the interface enforcement effect after thermal treatment and the cleavage of t-Boc groups, XPS measurement, which is an ideal chemical surface-analysis technique owing to high sensitivity and non-destruction characteristics, was applied to investigate the elemental change at the boundary combining the conjugated polymer and TiO2 . XPS spectra (Fig. 5a) show binding energies for Ti(2p), O(1s), N(1s), C(1s) and S(2p) of about 459.0 eV, 530.0 eV, 400.0 eV, 284.5 eV and 164.0 eV, in agreement with literature data [69]. O(1s) binding peaks locate around 530 eV and four peaks of 526.7 eV, 529.1 eV, 531.4 eV and 532.3 eV are found, which originate from different oxygen binding states. Noticeably, as shown in Fig. 5b, for composites DPP-Car-Boc-H/TiO2 and DPPCar-Boc-L/TiO2 , the peak of 526.7 eV disappears and the peaks of 531.4 eV and 532.3 eV are both intensified after thermal annealing, indicating the successful cleavage of t-Boc groups and the formation of much firmer carbonyl or acyl groups [70,71] which result in the charge re-distribution (charge transfer complexes or chemical bonds) and intensified interface binding at the boundary of conjugated polymers and inorganic nanocrystals. The peak of 529.1 eV is assigned to Ti-O-Ti signal, which is more distinctly asymmetric and intensified responding of high-energy signal after the cleavage of t-Boc groups. The 529.1 eV peak change comes from the impact of Ti3+ and should play positive role in improving the photocatalytic performance of composites. Ti4+ (2p) exhibits typical double-peak feature at the positions of 457.9 eV (Ti 2p3/2 ) and 463.7 eV (Ti 2p1/2 ) with a peak separation of 5.8 eV (Fig. 5c), which agrees well with reference values for bulk TiO2 [72,73]. Compared with the peak of 457.9 eV, the peak of 463.7 eV is obviously intensified and slightly shifted to high-energy position relatively, after thermal treatment,

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

801

Table 1 Photophysical properties of DPP-Car/TiO2 composites. Composites

␭onset (nm)/Egap (eV)

␭em (nm)/Egap (eV)

ELUMO-CB (eV)

DPP-Car-Boc-L/TiO2 DPP-Car-L/TiO2 DPP-Car-Boc-H/TiO2 DPP-Car-H/TiO2

894/1.39 864/1.44 905/1.37 897/1.38

460, 641/2.70, 1.93 466, 656/2.66, 1.89 460, 643/2.70, 1.93 465, 658/2.67, 1.88

0.50 0.46 0.44 0.39

O1s

a

Ti2p N1s

b

529.1

531.4 532.3

O(1s)

S2p

C1s

526.7

DPP-Car-L/TiO2

DPP-Car-H/TiO2

Intensity (a.u.)

DPP-Car-Boc-L/TiO2

DPP-Car-Boc-H/TiO2

DPP-Car-L/TiO2

DPP-Car-H/TiO2 DPP-Car-Boc-H/TiO2

DPP-Car-Boc-L/TiO2

TiO2

P25

1000

800

457.9

458.9

200

463.7

460.9

0

524

Ti(2p)

d

DPP-Car-L/TiO2

455.2

DPP-Car-Boc-L/TiO2

DPP-Car-H/TiO2

526

528

530 532 B.E. (eV) 399.5

N(1s)

534

DPP-Car-L/TiO2

DPP-Car-Boc-L/TiO2

DPP-Car-H/TiO2

DPP-Car-Boc-H/TiO2

DPP-Car-Boc-H/TiO2

TiO2

TiO2

456

458

460 462 B.E. (eV)

284.5

e

464

288.4

466

536

401.6

Intensity (a.u.)

c

600 400 B.E. (eV)

468

392

394

f

C(1s)

S(2p)

396

398

400 402 B.E. (eV)

163.9

404

406

DPP-Car-L/TiO2

DPP-Car-Boc-L/TiO 2 DPP-Car-L/TiO2 DPP-Car-Boc-L/TiO2 DPP-Car-H/TiO2

Intensity (a.u.)

282.0

DPP-Car-H/TiO2

DPP-Car-Boc-H/TiO2

DPP-Car-Boc-H/TiO2

TiO2

280 281 282 283 284 285 286 287 288 289 290 291 292

Fig. 5. XPS charts of DPP-Car/TiO2 composites. (a) full spectra; (b) O(1s); (c) Ti(2p); (d) N(1s); (e) C(1s); (f) S(2p).

TiO2

802

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

which is indicative of enhanced bonding force at the boundary. More importantly, for the composites DPP-Car-H/TiO2 and DPPCar-L/TiO2 , the newly-arrived peaks of 460.9 eV originate from the five-valance Ti cations due to the oxygen vacancy, which is beneficial for catalytic ability [74]. The intensified peak of 458.9 eV, corresponding to Ti3+ [75], which is in agreement with the O(1s) analysis, testifies that the cleavage of t-Boc groups and the liberation of NH groups induce the increasing ratio of Ti3+ cation which is greatly favorable in improving the photocatalytic performance of TiO2 nanoparticles. Furthermore, the Ti5+ binding energy (461.1 eV) of composite DPP-Car-H/TiO2 is higher than that of composite DPP-Car-L/TiO2 (460.9 eV), indicating that larger interaction force is obtained in conjugated polymer/TiO2 composite with higher molecular weight. The intensification and shift of Ti(2p) level from lower energy to higher energy originates from the defect-induced quantum entrapment, which is associated with an elevation of the upper edges of both the Ti(2p) and the conduction band by polarization. The shortening and strengthening of bonds between undercoordinated atoms densify and entrap the core electrons, which in turn polarize the dangling bond electrons of defect atoms. Additionally, the disappearance of low-energy signal peak of 455.2 eV results from the break of unstable Ti-O bond due to the volatilization of adsorbed small molecules on the surface of TiO2 nanoparticles, in agreement with the vanishing of 526.7 eV peak in O(1s) XPS signals (Fig. 5b). N(1s) and C(1s) signals (Fig. 5d and e) appear around 399.5 eV and 284.5 eV, respectively. The C(1s) signal of 282.0 eV is assigned to the surface-adsorbed small hydrocarbon molecules and the strong peak of 284.5 eV originates from C O, C N and C Ti bonding, while the higher-energy peak of 288.4 eV should correspond to the carbonyl of ester or amide groups. N(1s) signals are found at the positions of 399.5 eV and 401.6 eV, which are ascribed to the bonding of aromatic amino cations and indicative of compact interaction between the amide groups in DPP-Car polymers and Ti ions, in agreement well with the literature report [67,75]. Given it S(2p) signals (Fig. 5f), the peak around 163.9 eV also verifies that the thiophene fragments of the conjugated chains contact closely with the inorganic nanocrystals TiO2 [76]. To elucidate the morphologies and boundary features from the view of surface and interface for the composites, TEM images (Fig. S7) were given to present the size of composites and the binding state of conjugated polymers and TiO2 nanoparticles. SEM images (Fig. S8) and EDX maps (Fig. S9) were also recorded. In SEM images, the nanoparticles agglomerate and exhibit non-dense bulk features and show no significant change before and after the thermal annealing treatment, as well as the appearances in the TEM images. Conjugated polymers spread universally on the surface of inorganic particles as can be inferred from the EDX dotted maps of N and S elements (Fig. S9) and high-resolution TEM images (Fig. S7b and S7d). After the cleavage of pendant t-Boc groups, the size and crystalline structure of the composites both stay unchanged, with an average size of 30 nm in length and width and a distinct spacing of 0.355 nm in (101) crystalline face, which are in accordance well with the literature reports [67,77] and indicative of the unchanged statement of the morphology, crystal size and structure after the thermal annealing. Additionally, the conjugated polymers are found to cover the nanoparticles with a thickness of 1.1 nm, with an intimate contact facilitating the excited-state charge transfer through the interface, which is consistent with our previous findings [34]. To evaluate the visible-light driven photocatalytic performance of the composites before and after the cleavage of the pendant tBoc groups, the dye methyl orange (MO) is selected as the model pollutant to measure the UV/Vis spectra (Fig. 6 and Fig. S10) changes in the presence of photocatalyst and visible-light irradiation. The corresponding kinetic constants (k) and correlation coefficient (r2 )

Table 2 The kinetic constants of MO degradation for DPP-Car/TiO2 composites. Catalysts

k×103 (min−1 )

r2

DPP-Car-Boc-L/TiO2 DPP-Car-L/TiO2 DPP-Car-Boc-H/TiO2 DPP-Car-H/TiO2

1.78 3.38 1.55 5.63

0.997 0.999 0.993 0.997

for the linear fit are provided in Table 2. Meanwhile, the change of maximal absorbance is used to reflect the decolorization ratio and accordingly characterize the photocatalytic performance. To evaluate the photocatalytic stability of DPP-Car/TiO2 composites, the photocatalysts were recycled three times. Decolorization ratio of MO with DPP-Car/TiO2 composites in different recycling time is shown in Fig. 7 after adsorption equilibrium and photocatalytic degradation of 5 h, respectively. During the process of dark-adsorption, the four composites own different adsorption capacities for MO, ranging from 16% to 43% (Fig. 7b and Fig. S11). The adsorption equilibrium is obtained within 10 min, and does not fluctuate in the next 50 min. Owing to the Val der Waals forces between the conjugated polymers and MO molecules, the composites present better MO adsorption capacity than pure TiO2 nanoparticles. Upon the irradiation of visible light, the MO molecules undergo decomposition and for different composite photocatalysts, the decolorization ratios appear as the order of DPP-Car-H/TiO2 > DPP-Car-L/TiO2 > DPP-Car-Boc-L/TiO2 > DPP-Car-Boc-H/TiO2 > TiO2 (94% > 84% > 74% > 65% > 46%). After the thermal treatment of the composites, the photocatalytic performances are significantly improved for DPP-Car-H/TiO2 and DPP-Car-L/TiO2 , as compared with their precursors. And noticeably, the composite DPP-Car-H/TiO2 exhibits the best decolorization ratio and photocatalytic speed for MO, and its kinetic constants of MO degradation is about 3.6 times larger than that of DPP-Car-BocH/TiO2 under the same experimental conditions. The significant enhancement in photocatalytic activity is confirmed by EPR measurement. From the X-band EPR signal of DPP-Car/TiO2 (Fig. S12), we can observe a clear signal at g = 1.99 which is generally ascribed to an unpaired electron trapped on an oxygen vacancy [78,79]. The sharply increase in the EPR signal intensity evidences that the thermal-treated DPP-Car-H/TiO2 contains high concentration of oxygen vacancies, explaining the increased degradation rate of MO in the presence of DPP-Car-H/TiO2 . Recycling experimental was carried out to test the durability and stability of the DPP-Car/TiO2 composites. The decolorization ratio of MO undergoes slightly decrease in the second and third cycles for all the DPP/TiO2 composites compared with that in the first cycle. And obviously the photocatalytic stability of the composites DPP-Car-H/TiO2 and DPP-Car-L/TiO2 are better than that of DPP-Car-Boc-H/TiO2 and DPP-Car-Boc-L/TiO2 which can be ascribed to the enhanced interface bonding which is elucidated clearly in the above analysis. The two main factors, the loss of photocatalytic activity during the photocatalytic reaction and the residual MO molecules or intermediates products on the surface of photocatalysts in the previous cycling run, together increase the concentration of MO solution and decrease the apparent decolorization ratio of MO and finally induce the reduction of decolorization ratio for samples toward MO. To confirm the residual MO or intermediates on the surface of photocatalysts, the FT-IR and Raman spectra (Fig. S13 and S14) of photocatalysts were recorded after usage of three cycles. The obvious increase of two peaks (1223 cm−1 and 1158 cm−1 , sulfonic acid group) indicates the residual intermediates with sulfonic acid groups, not MO (Fig. S15), which are adsorbed on the surfaces of the composites after the photocatalytic process. Noticeably, the retention of 1730 cm−1 C O stretching vibration peak indicates the chemical stability

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

0.36

DPP-Car-H/TiO2

a

0.8

MO 10min 30min 60min 1h 2h 3h 4h 5h 6h 8h

b

DPP-Car-L/TiO2

0.30 DPP-Car-H/TiO2

0.6 DPP-Car-Boc-L/TiO2

0.4

DPP-Car-Boc-H/TiO2 0.2

Absorbance (a.u.)

1.0

803

0.24 0.18 0.12 0.06

0.0

0.00 0h

1h

2h

3h

4h

5h

6h

8h

250

300

350

400

450

500

550

600

Fig. 6. (a) Photocatalytic degradation of MO in the present of DPP-Car/TiO2 composites, and (b) UV/Vis spectra change of MO with different time in the presence of DPP-Car-H/TiO2 composite.

a 1.0

DPP-Car-Boc-L/TiO2

DPP-Car-H/TiO2

DPP-Car-Boc-H/TiO2

DPP-Car-L/TiO2

0.5

b

DPP-Car-Boc-L/TiO2 DPP-Car-Boc-H/TiO2

0.4

DPP-Car-H/TiO2

0.8 (C0-C)/C0

DPP-Car-L/TiO2

0.6 0.4

0.3

0.2

0.1

0.2 0.0

0.0

1

2

3

2

3

Fig. 7. Cyclic photocatalytic degradation of MO in the present of DPP-Car/TiO2 composites (a, degradation; b, adsorption).

of DPP-Car/TiO2 composites as the less stable groups (t-Boc) stay unchanged during the photocatalytic process of three cycles. The large structural stability and intensified interface bonding should be both responsible for the excellent photocatalytic stability and performance of the composites. Combined with the above data and analysis, we assume that while DPP-Car and TiO2 were coupled by in-situ thermal treatment, a heterojunction interface would be formed through the enhanced interfacial bonding. According to the band edge position, photogenerated electron-hole pairs would transfer in the opposite direction through the heterojunction interface, that is to say, photogenerated electrons would transfer from the LUMO of DPP-Car to the CB of TiO2 , while holes would be collected in the HOMO of DPPCar, which can suppress the electron-hole recombination, leading to the improvement of photocatalytic activity, as schemed in Fig. 8 [26,80]. Photocurrent measurements (Fig. S16) were carried out to investigate the improved charge carrier separation in DPP-Car/TiO2 and the effect of thermal treatment on charge carrier separation. Fig. S16 clearly shows a sharp and reversible response of current to each light (␭ > 420 nm) switch on and switch off event in electrodes of pristine TiO2 , DPP-Car-Boc-H/TiO2 and DPP-Car-H/TiO2 . Compared with pristine TiO2 , the existence of DPP-Car conjugated polymers obviously increases the photocurrent intensity, confirming the charge transfer between DPP-Car and TiO2 , and the consequent enhanced separation efficiency. Photocurrent results also reveals that thermal-treatment of DPP-Car/TiO2 composites could further improve the separation efficiency of photo-induced

electron-hole pairs by introducing strong interactions between DPP-Car and TiO2 interface.

4. Conclusions In summary, we report the first attempt to fabricate visiblelight-driven hybrid photocatalysts based on D-A conjugated polymers. Soluble D-A conjugated polymers with broad light absorption from 400 to 1000 nm were synthesized by alternative copolymerization of DPP and t-Boc modified Car. Then, DPPCar/TiO2 photocatalytic heterojunction with light absorption in the range of 300–1000 nm was obtained by a blend method following in-situ thermal treatment to cleave nonpolar t-Boc chain and liberate the polar carbazole NH groups, which induced interface enforcement through dipolar–dipolar interaction at the boundary of conjugated polymer and TiO2 . Enhancement of the interfacial interaction by in-situ thermal treatment was verified by photo images, FT-IR, UV/Vis DRS, fluorescence and XPS analysis, and results in (1) increased adsorption capacities in the range of 800–1000 nm, (2) the generation of Ti3+ and Ti5+ ions, (3) effectively charge carriers transport between DPP-Car and TiO2 and facilitated charge carriers separation. Thus an enhanced photocatalytic performance was obtained as compared to the composite without thermal treatment. Furthermore, we revealed that molecular weight of DPP-Car plays an important role in increasing the activity of DPP-Car/TiO2 : high molecular weight DPP-Car (DPP-Car-H) exhibited better photocatalytic performance. This study suggests that in situ thermal cleavage of side chain in a D-A conjugated poly-

804

L. Yang et al. / Applied Surface Science 434 (2018) 796–805

Fig. 8. The proposed mechanism diagram of enhanced photocatalytic efficiency.

mer might be a general strategy for strengthening the interfacial interaction between conjugated polymer and inorganic semiconductor and developing polymer/inorganic semiconductor hybrid heterojunction with broad light adsorption and high efficiency. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant nos. 51173116 and 51573109) and the State Key Lab of Polymer Material Engineering Foundation (No. sklpme 2015-2-01 and 2016-3-02) for supporting this research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2017.10. 176. References [1] S. Ghosh, N.A. Kouamé, L. Ramos, S. Remita, A. Dazzi, A. Deniset-Besseau, P. Beaunier, F. Goubard, P.-H. Aubert, H. Remita, Nat. Mater. 14 (2015) 505–511. [2] S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 27 (2015) 2150–2176. [3] F. Vilela, K. Zhang, M. Antonietti, Energy Environ. Sci. 5 (2012) 7819–7832. [4] J. Wen, J. Xie, X. Chen, X. Li, Appl. Surf. Sci. 391 (2017) 72–123. [5] P.E. Shaw, A. Ruseckas, I.D. Samuel, Adv. Mater. 20 (2008) 3516–3520. [6] M.G. Schwab, M. Hamburger, X. Feng, J. Shu, H.W. Spiess, X. Wang, M. Antonietti, K. Müllen, Chem. Commun. 46 (2010) 8932–8934. [7] K. Maeda, K. Domen, J. Phys. Chem Lett. 1 (2010) 2655–2661. [8] S.J. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Energy Environ. Sci. 8 (2015) 731–759. [9] D.J. Martin, P.J.T. Reardon, S.J. Moniz, J. Tang, J. Am. Chem. Soc. 136 (2014) 12568–12571. [10] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, J. Mater. Chem. 3 (2015) 2485–2534. [11] J. Shabani Shayeh, A. Ehsani, M.R. Ganjali, P. Norouzi, B. Jaleh, Appl. Surf. Sci. 353 (2015) 594–599. [12] E. Vasilaki, M. Kaliva, N. Katsarakis, M. Vamvakaki, Appl. Surf. Sci. 399 (2017) 106–113. [13] M.D. Hernandez-Alonso, F. Fresno, S. Suarez, J.M. Coronado, Energy Environ. Sci. 2 (2009) 1231–1257. [14] T.G. Xu, L.W. Zhang, H.Y. Cheng, Y.F. Zhu, Appl. Catal. B-Environ. 101 (2011) 382–387. [15] J. Kim, C.W. Lee, W. Choi, Environ. Sci. Technol. 44 (2010) 6849–6854. [16] M.V. Dozzi, S. Marzorati, M. Longhi, M. Coduri, L. Artiglia, E. Selli, Appl. Catal. B-Environ. 186 (2016) 157–165. [17] M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J.N. Kondo, K. Domen, Chem. Commun. (1998) 357–358. [18] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem. Rev. 114 (2014) 9919–9986. [19] M. Huang, T. Wang, B. Wu, J. Lin, C. Wu, Appl. Surf. Sci. 360 (2016) 442–450. [20] Z.C. Kadirova, M. Hojamberdiev, K.-I. Katsumata, T. Isobe, N. Matsushita, A. Nakajima, K. Okada, Appl. Surf. Sci. 402 (2017) 444–455. [21] M. Rakibuddin, R. Ananthakrishnan, Appl. Surf. Sci. 362 (2016) 265–273. [22] M. Robotti, S. Dosta, C. Fernández-Rodríguez, M.J. Hernández-Rodríguez, I.G. Cano, E.P. Melián, J.M. Guilemany, Appl. Surf. Sci. 362 (2016) 274–280.

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