A heterostructured catalyst composed of poly-2,6-diaminopyridine and TiO2 microspheres used as a photoanode for efficient water splitting

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 2 1 6 e2 2 9

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A heterostructured catalyst composed of poly-2,6diaminopyridine and TiO2 microspheres used as a photoanode for efficient water splitting Yan Yu a,b, Na Zhong a, Jincan Zheng a, Shasha Tang a, Xincheng Ye a, Tao Zeng a, Weiting Yu a, Zhiqiao He a, Shuang Song a,* a b

College of Environment, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China College of Science & Technology, Ningbo University, Ningbo 315212, People’s Republic of China

highlights

graphical abstract


A novel heterostructure formed between

PDAP

and

3DTiO2

microspheres.
The photoanode achieved a high applied bias photon-to-current efficiency at low bias.
A

photocurrent

density

of

1.56 mA cm2 at 1.23 V vs. RHE under AM 1.5G.
Type II heterojunction and coexistence of Ti3þ and Ti4þ improved the PEC performance.

article info

abstract

Article history:

Photoelectrocatalytic (PEC) water splitting provides an alternative to direct solar-to-fuel pro-

Received 4 September 2019

duction. In this study, a novel heterostructure formed between a conjugated polymer [poly-2,6-

Received in revised form

diaminopyridine (PDAP)] and three-dimensional TiO2 microspheres was grown in situ on a Ti

19 October 2019

substrate (PDAP-3DTiO2MSs/Ti) and used as photoanode for water oxidation in alkaline media

Accepted 27 October 2019

under AM 1.5G illumination. The PDAP-3DTiO2MSs/Ti can produce applied bias photon-to-

Available online 28 November 2019

current efficiency of 0.85% at 0.44 V vs. Pt and a photocurrent density of 1.56 mA cm2 at 1.23 V

Keywords:

93% of its initial photocurrent being retained after 4 h of reaction. Based on physical-chemical

vs. RHE. Moreover, PDAP-3DTiO2MSs/Ti displays impressive photoelectrochemical stability with Heterostructured catalyst

characterization and photo-/electro-chemical measurements, the superior PEC water splitting

Poly-2,6-diaminopyridine

performance of PDAP-3DTiO2MSs/Ti should benefit from the coexistence of Ti3þ and Ti4þ in

TiO2 microspheres

3DTiO2MSs, the light harvest capability of PDAP and the type II heterojunction formed between

Photoanode

3DTiO2MSs and PDAP, which result in the enhanced generation and separation of photocarriers.

Water splitting

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (S. Song). https://doi.org/10.1016/j.ijhydene.2019.10.211 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 2 1 6 e2 2 9

Introduction Artificial photosynthesis is considered as a viable alternative to store solar energy as fuel [1e3]. Photoelectrocatalytic (PEC) water splitting is one of the most promising strategies for artificial photosynthesis, in which the photogenerated carriers (holes/electrons) are induced by sunlight and migrate separately to anode/cathode surface to take part in the water oxidation/reduction process. The spatially separated reaction location of this process avoids the additional process of separating H2 and O2 [4,5]. Semi-conducting photoelectrodes are the key components of a PEC cell, whose performance depends strongly on their material and structure [6e8]. TiO2 is possibly the “ideal” photoanode material due to its powerful oxidation abilities and superior charge transport characteristics, as well as chemical and optical stability [9e11]. Nonetheless, the different types of intrinsic TiO2 reported to date have photocurrent densities of 0.5 mA cm2 in the PEC water splitting reaction [12,13]. Consequently, numerous strategies have been adopted to enhance the catalytic behavior of TiO2 in photoelectroxidation of H2O in an attempt to extend the spectral response, accelerate effective carrier separation and transportation, and boost surface chemical reactions under the premise of maintaining high stability in an aqueous solution [10,14,15]. Recently, TiO2 microsheets (TiO2MSs) with a high percentage of (001) and (101) facets have become a research hotspot [16e18]. The water oxidation reaction on TiO2MSs proceeds predominantly via the surface hydroxyls of TiO2. Since TiO2 with a high percentage of (001) facets possess pronounced hydroxylation reactions in an aqueous solution, it is expected that the in situ grown TiO2MSs on Ti foil (TiO2MSs/Ti) has potential PEC water oxidation performance [19]. To maximize the active sites on a photoanode with a limited geometric area, the design of TiO2MSs/Ti with space in the third dimension (3DTiO2MSs/Ti) is highly desirable. However, stacked TiO2 particles are unfavorable for separating and directing photoinduced electrons toward the collecting electrode surface [20]. Further works are needed to improve the separation and transformation of photocarriers in 3DTiO2MSs/Ti. Hetero-structurization of TiO2 with other materials offers a feasible way to maintain the merits and simultaneously eliminate the disadvantages of 3DTiO2MSs/Ti [18,21]. Conjugated polymers have recently attracted a lot of attention for photoelectrocatalytic applications due to its unique electrical, electronic, magnetic and optical properties [22e24]. Of them, 2,6-diaminopyridine (DAP), a p-electron conjugated aromatic molecule, has been used as an efficient heteromolecular dopant for other photocatalysts in order to red-shift the absorption edge, improve electrical conductivity and suppress the recombination of photogenerated charge carriers [25e27]. Consequently, to achieve high PEC water oxidation performance, research effort toward the development of heterostructures based on poly-2,6-diaminopyridine (PDAP) and 3DTiO2MSs/Ti is required. It is expected that the electron lone pairs and conjugated p-bond systems in PDAP can function as electron donors and hole acceptors under UVevis light irradiation. The high affinity of pyridinic N atoms facilitate interfacial electron transfer via strong chemical

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bonding interactions between TiO2 and PDAP [28]. In addition, extending the p-conjugated electron system offer potential for the effective utilization of light energy [29,30]. In the present work, a unique PDAP-3DTiO2MSs/Ti heterostructured photoanode used for PEC water splitting was synthesized. This work may provide a new paradigm for the design of semiconductor backbone/light-response polymer/ water oxidation photocatalytic composite systems to simultaneously enhance the light absorption, charge separation/ transport and surface water oxidation rate.

Experimental methods Reagents and materials Ti foil (0.5 mm thick, 99.5% purity) was produced by Baoji Titanium Co., Ltd. (Shanxi, China). DAP (98%), isopropanol and HF solution (40 wt%) were supplied by Aladdin Industrial Co., Ltd. (Shanghai, China). Tetrabutylammonium hexafluorophosphate (TBAPF6, 99%), ferrocene (99%), concentrated HNO3 (98 wt%), NaOH and NaNO3 were obtained from Sigma Aldrich Co., Ltd. (St Louis, USA). Nafion-117 membranes were manufactured by Du Pont de Nemours & Co. (Wilmington, USA). N2 (99.995%) and Ar (99.999%) were obtained from Jingong Special Gas Co., Ltd. (Hangzhou, China).

Photoelectrode fabrication A simple hydrothermal method was used to synthesize the TiO2 microspheres in 3DTiO2MSs/Ti. Ti foil was cleaned in a chemical polishing solution comprised of a mixture of HF:HNO3:H2O ¼ 1:4:5 (volume ratio), followed by rinsing with distilled water. The cleaned Ti foil was then immersed in a solution comprised of HF (50 mL), isopropanol (33 mL) and H2O (61 mL) and was kept in an ultrasonic bath for 0.5 h. After being transferred into a 100 mL Teflon-lined autoclave and hydrothermally treated at 180  C for 3 h, the obtained material was washed several times with ethanol and deionized water. The electrochemical synthesis of PDAP on the Tisupported TiO2 microspheres and ITO was performed in an electrolyte containing 0.01 mol L1 2,6-diaminopyridine and 0.1 mmol L1 NaNO3 using a pulse potentiostatic method. A Ag/AgCl (saturated KCl) electrode and platinum foil were used as the reference and counter electrodes, respectively. The reaction parameters used were as follows: High potential ¼ 1.6 V, pulse width ¼ 0.2 s; low potential ¼ 0 V, pulse width ¼ 0.4 s, and pulse number ¼ 9000. Subsequently, the TiO2 electrode covered with and without PDAP film, and PDAP coated ITO were carbonized in a tube furnace under a flow of Ar gas at 450  C using a heating rate of 10  C min1 and held at this temperature for 1 h. The resultant electrodes were denoted as PDAP-3DTiO2MSs/Ti, 3DTiO2MSs/Ti and PDAP/ITO, respectively. PDAP electrodeposited onto the 3DTiO2MSs/Ti electrode free of calcination was used as a reference, which was named PDAP…3DTiO2MSs/Ti. The preparation process of the PDAP-3DTiO2MSs/Ti photoanodes is shown schematically in Fig. 1. Based on the preparation process, the size of the TiO2 nanocrystals can be tuned by varying the hydrothermal reaction time and the

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Fig. 1 e A schematic representation of the fabrication procedure used to prepare the PDAP-3DTiO2MSs/Ti hybrid photoelectrode.

thickness of PDAP can be controlled by adjusting the pulse number. After optimizing the two variables, the optimal hydrothermal time and pulse number were determined to be 3 h and 9000, respectively, which is described in detail in the supplementary material.

Electrode characterization The crystal phases of the as-prepared electrodes were determined by X-ray diffraction (XRD, X’Pert Pro, PANalytical, Netherlands) using Cu-Ka radiation (l ¼ 0.15418 nm) at room temperature in the 2q range of 10e80 with a step size of 0.01 . The morphology and size of the catalysts were determined using field-emission scanning electron microscopy (FESEM, S4800, Hitachi, Japan) operated at 5 kV and transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin, PhilipsFEI, Netherlands) with a typical 0.2 nm point resolution at 300 kV. X-Ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system, PerkinElmer, USA) was conducted to investigate the electron-binding energy. The excitation source was Mg-Ka radiation (hv ¼ 1253.6 eV) and the X-ray gun was operated at 280 W (14 kV, 20 mA). The C 1s binding energy of carbon allotropes was taken to be 284.6 eV and used as a reference energy for calibration purposes. Electron paramagnetic resonance (EPR) spectroscopy was measured at low temperature on a Bruker A300 EPR spectrometer (Bruker, Karlsruhe, Germany) operated in the 9.44 GHz microwave frequency with a field modulator frequency of 100 kHz. UVevis diffuse reflectance spectroscopy was recorded on a UV/Vis spectrophotometer (UV/Vis DRS, UV-2550, Shimadzu, Japan) over the wavelength range of 200e800 nm. Raman spectroscopy was recorded in scanning mode on a Raman microscope (inVia, Renishaw, UK) with excitation at 532 nm. Photoluminescence spectroscopy was recorded at room temperature on a fluorescence spectrophotometer (PL spectra, FluoroMax-4P, Horiba Jobin Yvon, France) with an excitation wavelength of 325 nm. Thermogravimetric analysis was performed on a thermal analyzer (TGA, STA 409 PC, Netzsch, Germany) at a heating rate of 10  C min1 under a flow of Ar (50 mL min1). Cyclic voltammetry (CV) was recorded on a CHI 760E workstation using polymer films on ITO as the working electrode, a Pt plate as the counter electrode and Ag/Agþ electrode as the reference electrode. An acetonitrile solution containing 0.1 M TBAPF6 was used as the supporting electrolyte and the potential was calibrated against the ferrocene/ferrocenium (Fc/ Fcþ) pair [31]. Conversion of the Fc/Fcþ redox couple (EFc/Fcþ) to the normal hydrogen electrode potential (ENHE) was carried out using Eq. (1).

ENHE ¼ EFc/Fcþ þ 0.63 V

(1)

Using Ag/AgCl (saturated KCl) as the reference electrode, Pt foil as the counter electrode and 1M NaOH aqueous solution as the electrolyte, MotteSchottky measurements were conducted over a potential range of 0e1 V vs. reversible hydrogen electrode (RHE) at a frequency of 1000 Hz in the dark. Electrochemical impedance spectroscopy (EIS) at an open-circuit voltage was performed over a frequency range of 101106 Hz and an amplitude of 25 mV in the dark under AM 1.5G (100 mW cm2) illumination. Zsimpwin software was used to fit the impedance data with the equivalent circuit.

Photoelectrochemical performance Photoelectrochemical tests were carried out using a threeelectrode configuration controlled by a potentiostat (CHI760E, CH Instrument, USA) under illumination of simulated solar light (AM 1.5G, 100 mW cm2) using a 350 W Xenon lamp. The photon flux spectrum of the AM1.5 G solar simulator is shown in Fig. 2. The as-prepared electrode, Ag/AgCl in saturated KCl and Pt foil act as the photoanode, reference electrode and counter electrode, respectively. The electrolyte was 1 M NaOH solution (pH ¼ 13.6) and reaction temperature was maintained at 25  C using a circulating water bath connected to a thermostat (THD-2015, TianHeng, China). Prior to

Fig. 2 e The photon flux spectrum of the AM1.5 G solar simulator.

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irradiation, the electrolyte was purged with pure Ar gas to degas for 30 min. The chopped photocurrent produced by the photoanode under one sun irradiation was recorded at a fixed potential of 1.23 V vs. RHE using pulsed irradiation (50 s light/dark cycles). In addition, linear sweep voltammetry (LSV) was determined at a scan rate of 50 mV s1 under continuous light irradiation. The photoelectrochemical water splitting ability was evaluated by the incident photon-to-current conversion efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE). The IPCE spectrum of each electrode was obtained at 1.23 V vs. RHE as a function of the incident photon wavelength. The monochromatic illumination was provided by focusing the light from a 350 W Xe lamp through a monochromator. The IPCE value was calculated using Eq. (2). IPCE(%) ¼ (1240  Jph)/(l  Ilight)  100%

(2)

where Jph is the photocurrent density at a specific wavelength (mA cm2), l is the incident light wavelength (nm) and Ilight is the incident light intensity (mW cm2) for each wavelength. The ABPE was obtained according to Eq. (3). ABPE(%) ¼ I(1.23 e Vbias)/Jlight  100%

(3)

where Vbias is the applied bias between the working electrode and counter electrode, I is the photocurrent density at the measured bias and Jlight is the irradiance intensity (100 mW$cm2).

Sample analysis The produced H2 and O2 gases were collected at preset intervals and was quickly injected into a gas chromatograph (GC 7890B, Agilent, USA) operating in thermal conductivity detection (TCD) mode equipped with a HP-MOLESIEVE capillary column (30 m  0.32 mm  12 mm) and Ar carrier gas.

Results and discussion Morphology and structure The morphology of the Ti-based electrodes was investigated using SEM and TEM. As shown in Fig. 3, the surface of the Ti base is covered by flower-like hierarchical spheres assembled from truncated bipyramidal structures. The diameter of the spheres was determined to be ~2 mm. The similar morphologies observed among 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti reveals a stable TiO2 topography upon electrodeposition of the polymer films. From Fig. 3(b, c), the deposited polymer film can be clearly observed on the TiO2 surface in PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti. During the preparation of the photoanode, the fact that the heat treatment at 450  C can maintain the chain structure of the polymer molecules was partially demonstrated by TGA, as shown in Fig. S1 [32,33]. In addition, no obvious change in the morphology was seen on the PDAP-3DTiO2MSs/Ti surface after the reaction (Fig. 3(d)).

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The insets in Fig. 3(aec) display the TEM images of 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/ Ti, all of which exhibit flower-like hierarchical structures, which is in accordance with SEM results. Moreover, the elemental mapping results (Fig. 3(e)) show that the C, N, O, F and Ti signals overlapped along the entire investigated area, implying that the good dispersion of PDAP on the TiO2 microsphere. The HRTEM image of PDAP-3DTiO2MSs/Ti provides more detailed information on the catalysts, as shown in Fig. 3(f). The crystal lattice spacing of 0.238 nm was assigned to the (001) facet of TiO2 [34], and the ultrathin polymer film was laid on top of the well-crystallized TiO2. To gain more information in regard the structural features of the polymer film, fast Fourier transform (FFT) analysis was recorded and provided in the insets of Fig. 3(e). The resultant FFT pattern shows both the spotted pattern observed for the crystalline phase of TiO2 and the diffused ring pattern of the amorphous phase of PDAP [35]. Fig. 4(a) depicts the XRD patterns of the pristine 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti, PDAP-3DTiO2MSs/Ti and PDAP/ ITO electrodes. In addition to the diffraction peak observed for the titanium substrate (JCPDS 87-1526), the other sharp peaks can be assigned to the anatase (JCPDS 21-1272) along with trace rutile (JCPDS 21-1276) phases of TiO2 [36]. PDAP… 3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti show no obvious change in the diffraction peaks when compared to pure 3DTiO2MSs/Ti, suggesting that the phase structure and crystallization of TiO2 was not affected by the introduction of PDAP [13]. Moreover, a broad diffraction peak at ~25 was observed in the XRD pattern of PDAP…3DTiO2MSs/Ti, PDAP3DTiO2MSs/Ti and PDAP/ITO, which matched well with the (002) plane of the amorphous carbon-based materials [37]. It should be noted that the sharp diffraction peaks of PDAP/ITO were attributed to the ITO glass substrate. Since the intensity of (002) was unchanged before and after PDAP calcination at 450  C under an Ar atmosphere, the graphitization did not occur during the heat treatment process. Based on the above results, the successful electrodeposition of the polymer films onto 3DTiO2MSs/Ti was confirmed and PDAP remained amorphous after the thermal treatment step. XRD was also performed on the used PDAP-3DTiO2MSs/Ti photoanode. The spectrum was identical to that recorded for the pristine photoanode (Fig. 4(a)), suggesting no phase changes occurred under the PEC reaction conditions. FTIR spectroscopy was applied to characterize the surface groups of 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP3DTiO2MSs/Ti. As seen in Fig. 4(b), PDAP…3DTiO2MSs/Ti exhibits characteristic peaks at 1693 and 3503 cm1 corresponding to the C]N stretching and NeH (1 amine) bond vibrations, respectively [32,38]. These results reveal that the electropolymerization of DAP was successfully completed because the amino groups link the pyridyl groups together in the polymer. Moreover, the distinct absorption peaks observed at 1465, 1209 and 675 cm1 were attributed to the C] C, CeN and TieO stretching vibrations, respectively [31]. The broad peak around 3147 cm1 can be clearly observed in PDAP…3DTiO2MSs/Ti, which was ascribed to the vibration of NeH (2 amine) or OeH due to the residues of polymerization or absorbed water molecules [38]. In contrast to PDAP… 3DTiO2MSs/Ti, the NeH (2 amine) stretching was not

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Fig. 3 e SEM images of (a) 3DTiO2MSs/Ti, (b) PDAP…3DTiO2MSs/Ti, (c) PDAP-3DTiO2MSs/Ti and (d) used PDAP-3DTiO2MSs/Ti; the insets are the corresponding TEM images. (e) A typical high-angle annular dark-field (HAADF) image of a single microsphere scraped from PDAP-3DTiO2MSs/Ti and the corresponding EDS mapping images of Ti, O, F, C and N recorded from the single microsphere. (f) HRTEM image of PDAP-3DTiO2MSs/Ti; the inset is the corresponding selected area FFT image.

apparent in PDAP-3DTiO2MSs/Ti, suggesting that the eNH2 groups in PDAP were decomposed or removed during the thermal treatment process conducted under an oxygendeficient atmosphere. It should be noted that the analogous FTIR spectra of PDAP-3DTiO2MSs/Ti before and after the reaction confirm the stability of the surface groups on the photoanode. XPS was conducted to probe the surface composition and oxidation states in 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti. Fig. 5(a) confirmed the presence of C, N, O, Ti and F in the PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/ Ti, which is consistent with the elemental mapping results

shown in Fig. 3(e). The high-resolution C 1s XPS spectrum (Fig. 5(b)) can be deconvoluted into two peaks located at 284.6 and 287.5 eV using Shirley backgrounds and Voigt (mixed Lorentzian-Gaussian) functions, which were attributed to amorphous CeC and CeN, respectively [39]. Together with the FTIR analysis, the CeN peak is ascribed to the polymer chains of PDAP. Fig. 5(c) shows that the asymmetrical F 1s peak can be deconvoluted into two peaks at 684.8 and 687.1 eV. The peak at 684.8 eV is consistent with ≡TieF species bound to the surface of TiO2. According to the literature [40], the peak at a higher binding energy than ≡TieF is assigned to substitutional F, which replaces an oxygen vacancy. In this work, the fraction

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Fig. 4 e (a) XRD patterns and (b) FTIR spectra of the various photoelectrodes studied.

Fig. 5 e XPS spectra of the photoelectrodes studied: (a) Survey scan, (b) C 1s, (c) F 1s, (d) N 1s, (e) Ti 2p and (f) O 1s.

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follow the order: PDAP-3DTiO2MSs/Ti > PDAP…3DTiO2MSs/ Ti > 3DTiO2MSs/Ti. This sequence was also in accordance with the variation in Ti3þ since the OV is usually generated by a neighboring Ti3þ defect and acts as a redox couple (Ti3þ/Ov) [34]. Raman spectroscopy was employed to further confirm the existence of ultrathin polymer films on TiO2. As shown in Fig. 6(b), the appearance of four characteristic peaks at ~147, ~394, ~515 and ~633 cm1 correspond to the Eg, B1g, A1g and Eg vibrational modes of the anatase phase, respectively, which is consistent with the XRD results [31]. The decrease in the TiO2 peaks of PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/ Ti may be attributed to the polymer film coating on their surface. A closer observation found that the strongest peak at ~147 cm1 was slightly upshifted after surface modification of 3DTiO2MSs/Ti, which corresponds to an increase in the number of oxygen vacancies (Fig. 7(b’)) [50]. Fig. 7(b’’) displays two additional bands centered around ~1347 and ~1608 cm1 for PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti, which are the characteristic C]C backbone stretching and ringstretching modes observed for polymers [51]. Moreover, the peak intensity was stronger for PDAP-3DTiO2MSs/Ti when compared to PDAP…3DTiO2MSs/Ti due to the pre-resonance effect, indicating that more ordered and longer conjugated segments were formed in the PDAP-3DTiO2MSs/Ti composite [31,52]. In addition, almost no change in the Raman spectrum recorded for PDAP-3DTiO2MSs/Ti was observed before and after the reaction, which is consistent with the SEM, XRD and FTIR results.

of the two kinds F remained almost unchanged after the deposition of PDAP, suggesting that the activity difference of the various catalysts was independent of the form of F species present. The XPS spectrum recorded for N 1s of PDAP… 3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti are shown in Fig. 5(d). Pyridinic N (~398.3 eV) and eNH2 (~400.4 eV) are observed in PDAP…3DTiO2MSs/Ti, which are the main constituents in the polymer chain of PDAP [39,41]. In contrast, the core level of pyridinic N for PDAP-3DTiO2MSs/Ti shifts to a high binding energy, corresponding to decreased local electron density [42]. The results should induced by the strong interactions between PDAP and TiO2. PDAP and TiO2 behave as electron donors and acceptors in the PDAP-3DTiO2MSs/Ti hybrid heterostructure. Fig. 5(e) shows the Ti 2p XPS signal is composed of two doublets corresponding to Ti 2p3/2 and Ti 2p1/2 with a spineorbit coupling of ~6 eV [43]. The titanium ions in PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti exist in a mixed valence state of Ti4þ and Ti3þ [44]. According to the deconvoluted peak areas, the percentage of Ti3þ was calculated to be 13.8% and 25.6% (Table 1), respectively. The existence of Ti3þ on PDAP… 3DTiO2MSs/Ti shows that PDAP favors the stabilization of the surface Ti3þ species, preventing it from being oxidized by H2O and O2 [45,46]. Together with the N 1s high resolution spectra, the reduction of Ti4þ to a lower oxidation state after thermal treatment of PDAP…3DTiO2MSs/Ti can be attributed to the acceptance of an electron lone pair from the nitrogen atoms in PDAP [47]. XPS confirmed the presence of Ti3þ on the surface of PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti, since it is surface sensitive [48]. EPR analysis allows one to observe the sub-surface species in TiO2. In Fig. 6(a), the peak observed at g ¼ 1.998 for the samples at low temperature represents the fingerprint of Ti3þ [49]. Therefore, numerous Ti3þ defects also exist in the sub-surface of 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/ Ti and PDAP-3DTiO2MSs/Ti. As shown in Fig. 5(f), three typical peaks for O-bonding configurations such as TieOeTi, TieOeH and HeOeH can be observed in all three samples [34,39]. The TieOeTi in 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP3DTiO2MSs/Ti were observed at 529.67, 529.57 and 529.41 eV, respectively. Relative to pristine TiO2 with a lattice oxygen peak at 530.30 eV, the binding energy (BE) was shifted toward a lower value by 0.63, 0.73 and 0.89 eV for 3DTiO2MSs/Ti, PDAP… 3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti, respectively. The peak area ratios of the various O-bonding configurations to the total area were calculated and the corresponding results shown in Table 1. The percentage of TieOeH was 13.6, 18.5 and 23.6% and that of TieOeTi was 72.3, 70.7 and 68.7% for 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/ Ti, respectively. Since the surface TieOeH groups originate from the dissociation of H2O on the oxygen vacancies [34,39], it can be concluded that the OV of the as-prepared samples

Photoelectrochemical properties It is necessary to evaluate the Faraday efficiency of the photoanode in order to discriminate the effective current for water splitting from the recorded photocurrent [53,54]. The amounts of H2 and O2 produced in the PEC test and the theoretical amount of H2 and O2 calculated from the recorded current are shown in Fig. 7(a). As expected, the H2 and O2 at a rate of 32.63 and 16.02 mmol h1, respectively, resulting in a stoichiometric ratio of about 2:1. Furthermore, a Faraday efficiency of >90% was observed for the PDAP-3DTiO2MSs/Ti photoanode. The results indicate that most of the photocurrent was consumed in the water splitting reaction. As a consequence, the photocurrent was used to reflect the photoactivity of the photoanode. ABPE is used as a metric, which can be compared on an equivalent basis. The calculated ABPE is shown in Fig. 7(c) according to Fig. 7(b) and Eq. (3) [55]. The maximal ABPE was ~0.06% at 0.76 V, ~0.28% at 0.64 V and ~0.85% at 0.44 V for 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/

Table 1 e Percentages of the Ti 2p and O 1s components in the 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/ Ti photoelectrodes. Photoelectrodes

3DTiO2MSs/Ti PDAP…3DTiO2MSs/Ti PDAP-3DTiO2MSs/Ti

Ti3þ/(Ti3þþTi4þ)

TieOeTi

TieOeH

HeOeH

Area (%)

BE (eV)

Area (%)

BE (eV)

Area (%)

BE (eV)

Area (%)

e 13.8 25.6

529.67 529.57 529.41

72.3 70.7 68.7

530.68 530.44 530.40

13.6 18.5 23.6

531.99 531.78 531.69

14.1 10.8 7.7

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Fig. 6 e (a) EPR and (b) Raman spectra of the various photoelectrodes studied.

Fig. 7 e (a) H2 and O2 evolution on the PDAP-3DTiO2MSs/Ti photoelectrode at 1.23 V vs. RHE under AM 1.5G (100 mW cm¡2) irradiation. The (b) Photoelectrochemical performance, (c) ABPE and (d) IPCE of 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti photoelectrodes under AM 1.5G illumination. (e) The long-term stability test of the PDAP-3DTiO2MSs/Ti photoelectrode at 1.23 V vs. RHE under AM 1.5G irradiation.

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Ti, respectively. The results indicate that the PDAP-3DTiO2MSs/Ti electrode displays the best PEC performance among the three electrodes studied, since it could reach the highest ABPE at the lowest applied potential. In addition to the ABPE obtained under full solar spectrum light, information on the photoactivity at different wavelengths is important for photocatalytic applications [15,56]. IPCE was used to evaluate the spectral response of the PEC and quantum efficiency of photocurrent generation (Fig. 7(d)). All the electrodes have a spectral response in the wavelength region of 300e430 nm with a sudden IPCE change at ~400 nm, which is consistent with their UVevis spectra and estimated band gap energies. The largest IPCE of 87% was found at a wavelength of 400 nm for PDAP-3DTiO2MSs/Ti. The catalytic stability of PDAP-3DTiO2MSs/Ti in alkaline media was assessed in a continuous process with catalytic current monitoring [34]. As shown in Fig. 7(e), in the long-term

PEC process, the photocurrent of PDAP-3DTiO2MSs/Ti was almost 1.45 mA cm2. The negligible decay in the catalytic current indicates the good stability of PDAP-3DTiO2MSs/Ti.

Enhanced mechanism It is well-known that photocatalysis involve two crucial processes, semiconductor photoexcitation and the migration and separation of photogenerated charge carriers [57]. Photoexcitation driven by photon energy depends on the band gap of the semiconductor. The differences in the band gap can be observed for the different samples (Fig. 8(a)). According to Eq. (4), the band gap was calculated to be 1.49, 3.18, 3.13 and 3.02 eV for PDAP/ITO, 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti, respectively. Therefore, the absorption edge of the PDAP-3DTiO2MSs/Ti composite was slightly red-shifted when compared with 3DTiO2MSs/Ti and

Fig. 8 e (a) UVevis DRS, (b) MotteSchottky plot dark, (c) CV curves, (d) PL emission spectra obtained for the photoelectrodes studied. (e) Illumination EIS Nyquist plots measured under the open-circle potential; the inset give the equivalent circuit used to fit the impedance data and (f) Bode plots under chopped AM 1.5G illumination at 1.23 V vs. RHE of photoelectrodes.

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PDAP…3DTiO2MSs/Ti. Beyond 400 nm, a broad absorption band was observed for TiO2 induced by Ti3þ defects/OV, as confirmed by XPS and EPR spectroscopy [58]. Eg ¼ 1239$

8 l

(4)

While the band gap is an important parameter to indicate the light absorption process, the donor concentration (ND) is a more important parameter from the viewpoint of the electron mobility [59]. Therefore, MottSchottky plots were obtained to gain information on the ND of 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti, which can be calculated using Eq. (5) based on the data shown in Fig. 8(b) [60]. ND ¼

 2
1  2 d 1 C eεε0 dE

(5)

where e ¼ 1.6  1019 C, ε0 ¼ 8.86  1014 F$cm1, and ε ¼ 80 for TiO2; the ND values obtained from the linear fitting method were ~1.38  1019, ~1.63  1019 and ~3.21  1020 cm3 for 3DTiO2MSs/Ti, PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/ Ti, respectively, as reported in Table 2. Obviously, the ND value for PDAP-3DTiO2MSs/Ti was ~19.69-fold higher than that found for PDAP…3DTiO2MSs/Ti and ~23.26-fold higher than that observed for the 3DTiO2MSs/Ti electrode. Based on the previously reported work and XPS analysis [61], the higher ND value observed for PDAP-3DTiO2MSs/Ti was attributed to the presence of more oxygen vacancies in its framework, which serve as the main electron donor for TiO2. Considering these results, PDAP-3DTiO2MSs/Ti can effectively absorb incident light and provide more photogenerated carriers for water splitting under AM 1.5G illumination when compared with 3DTiO2MSs/Ti and PDAP…3DTiO2MSs/Ti. In addition, the charge separation and charge-carrier mobility play vital roles in improving the photoactivity of a photoanode. As indicated by XPS and EPR, PDAP facilitates the generation and stabilization of the mixed-cation composition (Ti3þ and Ti4þ). Generally, Ti3þ/OV coexist in TiO2 and can be easily withdrawn by the surface oxygen adsorbates [45]. In comparison with TiO2/Ti without the PDAP coating layer, PDAP behaves as a barrier layer avoiding direct exposure of the TiO2 surface to the electrolyte, thus suppressing the oxidation of surface Ti3þ/OV. Additionally, the heat treatment of PDAP…3DTiO2MSs/Ti results in the migration of the nonbonding electrons of the nitrogen atoms to Ti4þ, which ultimately enhance the generation of more Ti3þ in PDAP3DTiO2MSs/Ti. The mixed-cation composition in TiO2, which is associated with the modulation of its electronic properties, can boost the separation of photogenerated electrons. Moreover, the improved PEC water oxidation performance observed after the addition of PDAP to 3DTiO2MSs/Ti may also originate from the heterostructure formed between PDAP and

TiO2. CV measurements were conducted to evaluate the HOMO and LUMO positions of PDAP after calcination. Fig. 8(c) shows the oxidation onset potential was 0.01 V vs. Ag/Agþ and the reduction onset potential was 1.53 V vs. Ag/Agþ using Fc/Fcþ as an internal standard oxidation potential, which corresponds to the HOMO and LUMO levels of þ0.53 V vs. NHE and 0.99 V vs. NHE, respectively, according to Eq. (1). Therefore, the HOMO-LUMO band gap of PDAP was 1.52 eV. This result was in agreement with the UVevis DRS spectra. Since the CB level of TiO2 was at 0.30 V vs. NHE according to the literature [62,63], electron transfer in PDAP-3DTiO2MSs/Ti from the LUMO of PDAP to the CB of TiO2 is feasible. According to Anderson’s model [64], the intimate contact between PDAP and TiO2 in PDAP-3DTiO2MSs/Ti may be assigned to a type-II heterojunction, which can serve as an electron trap to efficiently capture the photo-induced electrons. We have proposed the mechanism of electron transfer observed in PDAP3DTiO2MSs/Ti (Scheme 1). The photoelectrons are driven from the LUMO of PDAP to the CB of TiO2 and then shuttled to the Ti substrate through the TiO2 backbone. Subsequently, the electrons are transferred to the Pt cathode to participate into the half-reaction of H2O reduction into H2. Correspondingly, the holes diffuse into the HOMO of PDAP to take part in the half-reaction of OH oxidation to O2 [65]. PL and EIS measurements were performed to verify the active role of the PDAP layer in enhancing the carrier transport kinetics and electronic conductivity. Fig. 8(d) shows a deep level (DL) emission around 430 nm was observed for all the samples using the HeeCd laser line of 325 nm as the excitation source [55]. The DL emission is commonly attributed to the recombination of photoexcited holes with electrons occupying the singly ionized oxygen vacancies in TiO2. A higher PL intensity shows a higher level of recombination of the photoinduced electronehole pairs. The PL intensity observed for PDAP-3DTiO2MSs/Ti was the lowest among the three samples studied, indicating its superior performance for impeding photo-induced charge recombination. The appropriate catalyst size and heterostructure of PDAP-3DTiO2MSs/Ti facilitates the efficient separation and transfer of photo-generated electrons and holes. The charge migration process was monitored using EIS. Fig. 8(e) presents the Nyquist plots obtained under dark and illumination conditions for the as-prepared electrodes. Generally, the diameter of the semicircle in the Nyquist plot obtained at high frequency is proportional to the electron transfer resistance and separation efficiency at the interface between the electrode and electrolyte [66,67]. PDAP-3DTiO2MSs/Ti displays much smaller arches than 3DTiO2MSs/Ti and PDAP…3DTiO2MSs/Ti both in the dark and under illumination, reflecting that the decoration of TiO2 with PDAP significantly enhanced electron mobility. In other words, PDAP

Table 2 e The EFB, ND, t and L values of the photoelectrodes studied. Photoelectrodes 3DTiO2MSs/Ti PDAP…3DTiO2MSs/Ti PDAP-3DTiO2MSs/Ti

k

EFB (V)

ND (cm3)

t (ms)

L (nm)

1.28 1.08 0.06

0.15 0.09 0.01

1.38  1019 1.63  1019 3.21  1020

132 162 817

823 912 2048

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Scheme 1 e A schematic illustration of the water splitting system and the photo-generated charge separation and migration process.

improved the electronic conductivity of TiO2, thereby boosting photoexcited charge separation during photocatalysis. The lifetime of the injected electrons (t) in the photoelectrode can be obtained using Eq. (6) [66,68]. Based on the Bode-phase spectra shown in Fig. 8(f), the t values listed in Table 2 follow an increasing trend of 3DTiO2MSs/Ti < PDAP… 3DTiO2MSs/Ti < PDAP-3DTiO2MSs/Ti. PDAP-3DTiO2MSs/Ti shows an even higher t value of 817 ms when compared to 162 and 132 ms obtained for PDAP…3DTiO2MSs/Ti and 3DTiO2MSs/ Ti, respectively. t ¼ 1=2pf

(6)

where f denotes the minimum inverse frequency. Based on t, the carrier diffusion length L given by Eqs. (7) and (8) was calculated to be 823 nm for 3DTiO2MSs/Ti. Since the electronic conductivity of 3DTiO2MSs/Ti was enhanced by PDAP modification. The L for PDAP…3DTiO2MSs/Ti and PDAP-3DTiO2MSs/Ti should be greater than 912 and 2048 nm, respectively [34]. Generally, if the catalyst size is greater than the L value, the produced charges will undergo recombination within the bulk of the photocatalyst [69]. Conversely, the photogenerated charges can transfer to the catalyst surface to participate into reaction. In the present work, the catalyst size is ~2 mm for all three photoanodes. Thus, only the L value of PDAP-3DTiO2MSs/Ti is greater than the catalyst size. Therefore, PDAP-3DTiO2MSs/Ti required less bias to effectively separate the photogenerated electrons and holes. L ¼ ðDtÞ1=2 D¼

kTm e

the incident light on the photoanodes was turned off and only have a transient increase when the light was turned on again. This variation was in agreement with the LSV results. Clearly, PDAP-3DTiO2MSs/Ti exhibits a maximum photocurrent of 1.56 mA cm2 at a potential of 1.23 V vs. RHE, which is 2.89and 7.80-fold greater than those obtained for PDAP…3DTiO2MSs/Ti (0.54 mA cm2) and 3DTiO2MSs/Ti (0.20 mA cm2), respectively. In addition, the PDAP/ITO exhibited a photocurrent of 0.06 mA cm2, which is approximately 3 times greater than the photocurrent density of ITO (0.02 mA cm2). This result confirmed that the separation of photo-generated electrons and holes could occur in PDAP calcined at 450  C under Ar atmosphere. Combining the above-mentioned findings, the combined function of enhanced light harvesting and accelerated separation of photocarriers due to the existence of more Ti3þ/OV and the heterojunction formed between PDAP and TiO2 contribute to the higher ABPE of PDAP-3DTiO2MSs/Ti when compared with PDAP…3DTiO2MSs/Ti and 3DTiO2MSs/Ti.

(7)

(8)

where m is the carrier mobility, which is assumed to be 0.2 cm2 V1 s1 for TiO2 [34]. The transient photocurrent response of the various photoanodes was studied under chopped light conditions to further probe the excitation, transfer and separation of the charge carriers. As shown in Fig. 9, the photocurrent values obtained for the electrodes decreased to nearly zero as soon as

Fig. 9 e Photocurrent density under chopped AM 1.5G illumination at 1.23 V vs. RHE of photoelectrodes.

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Conclusions [6]

In conclusion, a novel PDAP-3DTiO2MSs/Ti photoanode has been successfully prepared under hydrothermal conditions using Ti plates in an aqueous solution containing HF and isopropanol, the electrodeposition of PDAP and calcination under a flow of Ar. The heterostructured material behaves as a favorable photoanode for PEC water oxidation in terms of its activity and stability. Under AM 1.5G one sun illumination, PDAP-3DTiO2MSs/Ti gave an impressive 0.85% PEC water splitting efficiency. Furthermore, the initial photocurrent density of 1.56 mA cm2 at 1.23 V vs. RHE can be retained (93%) during 4 h of the continuous stability test. Thanks to the mixed valence of Ti3þ and Ti4þ in 3DTiO2MSs, the additional light absorption of PDAP and the type II heterostructure form between PDAP and TiO2, their joint action improved exciton generation and the separation of photocarriers, resulting in high PEC activity and good stability. We believe that this work with further improvement will inspire the application of our PEC system in the field of solar hydrogen production.

Acknowledgements This work was supported by the Zhejiang Provincial Ten Thousand Talent Program (Grant 2018R52013), the Zhejiang Provincial Natural Science Foundation of China (Grants LZ18B070001 and LGF18E080017) and the National Science and Technology Major Project for Water Pollution Control and Treatment (2017ZX07201004).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.211.

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