Red Phosphorus Decorated and Doped TiO2 Nanofibers for Efficient Photocatalytic Hydrogen Evolution from Pure Water

In this study, we report a facile strategy to prepare a new class of red phosphorus (RP) decorated and doped TiO2 (A: anatase and B: TiO2(B)) nanofibers with boosted photocatalytic hydrogen evolution reaction (HER) performance from pure water. The optimized TiO2(B)/RP and TiO2(A)/RP heterostructure exhibits significantly enhanced photocatalytic performances, with HER rate reaching 11.4 and 5.3 μmol h−1, respectively. The optical absorption of TiO2 is significantly extended to the visible light region after decoration of RP. And more oxygen vacancies (VO) were introduced by phosphorus doping in the TiO2(B)/RP than TiO2(A)/RP. The theoretical calculations illustrate the formation of VO can decrease the ratio of effective masses of electron to hole, which gives rise to promoted photoinduced charge separation and transfer. These results suggest that the boosted photocatalytic HER performance should be mainly attributed to the synergetic enhancement in light harvesting and charge separation enabled by RP deposition and P5+ doping.


Introduction
With increasing interest in hydrogen as an environmentally acceptable, alternative energy carrier with high energy density, several emerging clean-energy technologies have been discovered to pursue sustainable and efficient hydrogen production, such as photocatalytic A C C E P T E D M A N U S C R I P T [1,2], electrocatalytic [3,4] and photoelectrocatalytic [5,6] water splitting into hydrogen.
Among them, photocatalytic hydrogen evolution reaction (HER) with semiconductors as photocatalysts has been considered as one of the most important pathways [7], since Fujishima-Honda initially developed TiO2 as a water splitting photocatalyst [8].
To date, TiO2 has been extensively investigated as an ideal model of the photocatalyst [9,10]. Nevertheless, its wide band gap limiting solar light harvesting and the fast charge recombination, as the main drawbacks, have lowered its photocatalytic efficiency to unsatisfied level. To improve the optical absorption properties, many approaches have been devoted to modifying TiO2, including high temperature reduction or hydrogenation [11][12][13], doping or co-doping with metal and/or non-metal elements [14][15][16][17], and surface sensitization with visible light active semiconductors and dyes [18,19]. Recently, elemental red phosphorus (RP) has emerged as a new class of photocatalyst owing to its narrow bandgap, low cost, nontoxicity, and earth abundance. [20][21][22][23][24][25] More importantly, its visiblelight absorption edge extends up to 700 nm, which is favorable to be as a sensitizer to couple with TiO2 to extend the visible light response. As another determinant for efficient photocatalytic activity, charge separation should be facilitated in TiO2, with some effective strategies well documented, such as fabrication of heterojunction and phase junction [26][27][28], surface passivation [29,30], and introduction of oxygen vacancies (VO) [31,32]. Among them, VO as a donor source which can bring an increased majority carrier concentration to improve the charge transfer ability. However, the excessive VO may become the charge recombination centers by trapping the photo-induced electrons [33]. Thus, the density of VO created in TiO2 should be optimized to balance their opposite roles as the electron donor and the trap sites.

A C C E P T E D M A N U S C R I P T
Given that photocatalytic reactions depend on a sequence of multiple steps, including optical light harvesting, charge separation and transportation, and surface reaction, it is highly desirable to design a coupling strategy that not only can enhance the optical absorption but also can promote the charge transfer in the photocatalysts [34]. In this work, we report a vaporization-deposition strategy to prepare a new class of TiO2 nanofiber/RP nanolayer core/shell heterostructure with boosted photocatalytic activity for HER from pure water. The optimized TiO2(B)/RP and TiO2(A)/RP composites (A: anatase and B: TiO2(B)), displayed significantly enhanced photocatalytic activities than that of bare TiO2, with the highest HER rates reaching 11.4 and 5.3 μmol h −1 , respectively. The great improvement in photocatalytic HER performances could be attributed to the synergy that the introduction of RP nanolayer acting as the visible light sensitizer can enhance light harvesting and the surface phosphorus (P 5+ ) doping induced VO can facilitate the migration of photogenerated electrons to the photocatalysts surface. This coupling strategy could be applied to the design and fabrication of other highly efficient photocatalysts for solar energy utilization by synergistically improving the light harvesting property and promoting the charge separation ability.

Synthesis of TiO2/RP heterostructure composites
TiO2 nanofibers (NFs) were prepared using a hydrothermal method followed by a calcination treatment according to our previous reports [26,27]. Typically, the synthesized H2Ti3O7 NFs were calcined in a muffle furnace at 400 °C for 4 h to obtain TiO2(B) NFs, and calcined at 700 °C for 4 h to produce anatase TiO2(A) NFs. The commercial red phosphorus (RP, 99.999 %, Aladdin) was used after purification as follows: 1 g of RP was added into 60 mL of H2O, and hydrothermally treated at 200 °C for 12 h in an autoclave to remove the oxide layers.
In a typical process for the synthesis of TiO2/RP heterostructure composites, 200 mg of TiO2 NFs and an appropriate amount of RP powder (10 mg~60 mg) were well dispersed in 30 mL of distilled water by ultrasonic treatment for 30 min. The solution was then frozen using liquid nitrogen and freeze-dried to remove water. After that, the obtained pink powder was transferred into a quartz ampoule and sealed by an oxygen-hydrogen flame under a low vacuum condition (-0.09 MPa, high purity Ar filling before pumping) [21,22] The ampoules were heated in a furnace at 500~700 °C (at a ramping rate of 2 °C·min -1 ) for 4 h, then cooled down to 280 °C (at a ramping rate of 1 °C·min -1 ) and held at this temperature for 4 h. After slowly cooled to room temperature at 0.1 °C·min -1 to prevent the RP vapor transforming into white phosphorus, the as-prepared products were obtained by breaking the capsules and rinsing with CS2, ethanol and distilled water, respectively. The TiO2 NFs loaded with different contents (x mg, x = 10~60) of RP were labeled as TiO2(B)/RP(x) or TiO2(A)/RP(x).

Characterization
X-ray diffraction (XRD) was carried out with DX2700 operating at 40 kV and 30 mA equipped with Cu Kα radiation (λ = 1.5418 Å). The morphology and structure of the samples were investigated by an FEI Magellan 400 field emission scanning electron microscope (FESEM) and a JEOL JEM-2100F scanning transmission electron microscope (STEM) equipped with Cs probe corrector at 200 kV. X-ray photoelectron spectroscopy (XPS) was measured by ESCALAB 250XL electron spectrometer (Thermo Scientific Corporation) with monochromatic 150 W Al Ka radiation. All binding energies were calibrated with C 1s at 284.6 eV. Raman spectra were measured on a LabRAM HR800 evolution with the excitation wavelength of 532 nm. The diffuse reflectance A C C E P T E D M A N U S C R I P T spectra were recorded with Agilent Cary 5000 UV-Vis-NIR spectrophotometer using integrated sphere accessory. Electron paramagnetic resonance (EPR) experiments were performed on a Bruker EMX X-band spectrometer and microwave frequency = 9.40 GHz at 100 K in the dark. The synchrotron X-ray absorption spectroscopic (XAS) measurements were conducted at the National Synchrotron Radiation Research Center, Taiwan. The Ti K-edge and P K-edge were carried out at BL17C and BL16A, respectively, and Ti L-edge and O K-edge were performed at BL20A.

Photocatalytic hydrogen evolution experiments
The photocatalytic hydrogen evolution experiments were carried out in a Pyrex top-irradiation reaction cell at atmospheric pressure. The reactant was pure water, and no sacrificial reagent or pH adjustment was used. 30 mg of the TiO2/RP sample was dispersed in pure water (150 mL), and H2PtCl6 solution equivalent to 3 wt% Pt was added into the reactor. After being purged with argon to remove dissolved air, the solution was irradiated by a 300 W Xe arc lamp (PLS-SXE 300C) with an AM 1.5G filter to simulate solar light. The power intensity at the central point of reactant was measured to be 500 mW·cm -2 by PM100D power meter with S302C thermal sensor (Thor Labs) [35]. And the irradiation area was controlled as about 20 cm 2 . The temperature of the reactant solution was held at 25 °C by a flow of cooling water. The amount of hydrogen evolution was measured in the air-tight continuous flow system which was connected with the online gas chromatograph (Shimadzu GC-2014C) equipped with a thermal conductivity detector. High purity argon gas (99.999%) was used as a carrier gas.

Density functional theory calculations
The Vienna Ab-initio Simulation Package (VASP) software on basis of the plane-wave method was utilized for density functional theory (DFT) calculations [36][37][38]. Our computations employed the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange- correlation function [39]. The Projector-Augmented-Wave (PAW) method was used to describe the interaction between the ionic core and valence electrons [40]. To localize the Ti 3d states, the wellknown Hubbard-like U correction GGA+U method has also been used [41]. Ueff = 4.5 eV for TiO2(B) and Ueff = 3.0 eV for TiO2(A) which are the same as the previous calculations (Table S1) [42]. The threshold for energy convergence was set to 10 -6 eV. The convergence of the force on each atom was set to 0.02 eV/Å during geometry optimization. The lattice parameters and atomic

Results and discussion
As shown in Fig. 1, the TiO2/RP core/shell heterostructure composites were fabricated via a vaporization-deposition strategy. Firstly, TiO2 NFs were synthesized by a hydrothermal method and a subsequent calcination treatment. Then, the as-prepared TiO2 NFs were mixed with different amounts of RP powders, which were sealed into a quartz ampoule under vacuum condition (Fig.   S1). During the calcination process at high temperature (500~700 °C), phosphorus gas was vaporized in the ampoule due to the vacuum environment, and some P element would be doped into the TiO2 NFs. After that, through a low-temperature procedure, the RP nanolayer would be deposited on the TiO2 NFs surface with the condensation of phosphorus vapor, to obtain the TiO2/RP core/shell heterostructure composites [24].
The phase composition and the crystal structure of the TiO2/RP composites were investigated by X-ray diffraction (XRD). As shown in  S2b,c). It was thus indicated that the addition of RP can inhibit the phase transformation. With the RP vaporization temperatures optimized to be 600 °C to obtain the TiO2(B)/RP sample with the highest photocatalytic HER activity (Fig. S3), the TiO2/RP composites with different RP contents calcined at 600 °C were characterized and discussed in more details in the following sections. As shown in Fig. 2, the obtained composites exhibited all diffraction peaks indexed to TiO2(B) (Fig.   2a) or anatase TiO2 (Fig. 2b). No typical diffraction peaks of RP were observed in the TiO2/RP composites due to the low amount and poor crystallization of RP. Whereas, when the weight of RP was increased to 80 mg, two small peaks at 2θ = 15.6° and 34.1° corresponding to Hittorf's phosphorus (monoclinic, JCPDS 44-0906) can be observed for TiO2(A)/RP (Fig. S2d).  S4b) [27]. For the TiO2/RP composites, the main peaks centered at 352, 399 and 463 cm −1 came from elemental red phosphorus could be observed. Moreover, the intensity of these three peaks increased gradually with the increasing contents of RP added during the vaporization-deposition process. These analysis in XRD and Raman spectra could confirm the successful decoration of RP onto the TiO2 NFs during the vaporization-deposition process. 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 Intensity (a.u.) 2 Theta (degree) The morphologies of as-prepared TiO2/RP samples were observed by field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM). It is clear that all the samples displayed the one-dimensional (1-D) fibril morphology, and the deposition of RP would not bring obvious morphology change (Fig. S5,S6). STEM images (Fig. 3a,d,h,k) further confirm the 1-D fibril structure for all the samples. For the pristine TiO2 NFs, the lattice fringes which were parallel or vertical to the NFs axis, with interplanar distance of 0.58 nm (Fig.   3b,c) and 0.35 nm (Fig. 3i,j), respectively, could be assigned to the (200) and (101) plane of well crystallized TiO2(B) and anatase TiO2, with mesoporous structure clearly observed. After the vaporization-deposition process, a uniform amorphous RP nanolayer was deposited on the surface of TiO2(B) and TiO2(A) NFs, with thickness estimated to be 6~7 nm for TiO2(B)/RP(40) (Fig. 3e,f and TiO2(A)/RP(30) (Fig. 3l,m). The corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping images (Fig. 3g,n) clearly show the uniform distribution of Ti, O, and P elements, confirming the successful deposition of RP nanolayer onto the TiO2 NFs. It could be further observed that a disordered layer with a thickness of ca. 1.5 nm was formed at the interface of TiO2 NFs and RP nanolayer for both TiO2(B)/RP and TiO2(A)/RP, which indicated that defects were introduced into the surface region of TiO2 NFs during the vaporization-deposition process [11,31]. X-ray photoelectron spectroscopy (XPS) was used to better understand the physicochemical interaction between TiO2 core and RP shell. The XPS survey spectra revealed the presence of Ti 2p, O 1s and P 2p in the TiO2/RP composites (Fig. S7), indicating that phosphorus has been introduced in the composites successfully. As shown in Fig. 4a, the pristine TiO2(B) and TiO2(A) revealed Ti 2p3/2 peaks at 458.7 eV and 458.2 eV and Ti 2p1/2 peaks at 464.4 eV and 463.9 eV, respectively, with a spin-orbit splitting of 5.7 eV, indicating the Ti 4+ chemical state [11]. In comparison, these Ti 2p peaks in TiO2/RP composites were all positively shifted to higher binding energy. As observed in the O 1s spectra of the TiO2(B) and TiO2(A) (Fig. 4b), the peaks at 529.  [45][46][47][48]. As shown in Fig. 4b, the OOH peaks of the TiO2/RP composites exhibited enhanced intensity as compared to the pristine TiO2 NFs (Table S2). Given the general fact that OOH concentrations correlates to the formation of VO, TiO2/RP composites should have more VO created after the vaporizationdeposition process as compared to the pristine TiO2 NFs [11,31,49]. The P 2p spectra of RP show two peaks at 129.7 eV and 130.6 eV (Fig. 4c), which could be attributed to the spin-orbit doublets of P 2p3/2 and P 2p1/2, respectively, indicating the P 0 chemical state [20][21][22]. In comparison to  [46][47][48]50]. Electron paramagnetic resonance (EPR) spectra were further performed to elucidate the formation of VO in the surface region of TiO2 NFs in the TiO2/RP composites, as induced by the introduction of P 5+ at Ti 4+ sites. As shown in Fig. 4d, the TiO2/RP samples exhibited EPR signals at g = 2.003, suggesting that surface-free electrons were trapped at VO created in TiO2 NFs. Further comparison in the intensity of the EPR signals revealed that the concentration of VO generated in TiO2(B)/RP was much higher than in TiO2(A)/RP [11,31]. XPS and EPR results provided clear evidence solidifying the strong chemical interaction between the TiO2 core and RP shell, which further confirmed the presence of elemental RP and the doping of P 5+ in the TiO2/RP core/shell heterostructure.  tail-like absorption band in 600~900 nm, as compared to TiO2(A)/RP, which might be due to the higher concentration of VO created in TiO2(B)/RP [12]. Correspondingly, it was obvious that these samples showed a gradient evolution in color from white to brown with the increase of RP contents (Fig. 5c,d), also demonstrating their efficient optical absorption in visible light region. In comparison to the physical mixtures of TiO2 NFs and RP as the reference, the as-prepared TiO2/RP composites displayed much stronger optical absorption in the range of 360-600 nm (Fig. S8) with colors deeper and darker (Fig. 5c,d and Fig. S9). These results again evidenced the chemical interaction between TiO2 and RP in the TiO2/RP composites with RP shell coated on and P 5+ /VO introduced into TiO2 NFs by a vaporization-deposition method. To further evaluate the photocatalytic activity of the obtained composites, photocatalytic HER in pure water without any sacrificial reagents was also performed at ambient pressure and temperature . As shown in Fig. 6a,b and Fig. S10, the pure TiO2(B) showed no HER performance, while TiO2(A) and RP exhibited very weak HER activity (0.5 and 2.1 μmol h −1 ) under simulated solar light illumination (Fig. S11a). Both TiO2(B)/RP and TiO2(A)/RP displayed a volcano-type curve of H2 evolution rates depending on the RP contents, with the highest HER rates reaching 11.4 and 5.3 μmol h −1 , respectively, for TiO2(B)/RP (40) and TiO2(A)/RP(30) with optimized RP contents. It is noteworthy that the TiO2(B)/RP(40) composite stands at the highest level of TiO2-based photocatalysts for hydrogen production via pure water splitting (see Table S3). Similar to the trend of photocatalytic HER properties depending on RP contents, the transient photocurrent density was enhanced and Nyquist plots arc radius was decreased after the decoration of RP, which indicate the faster charge carrier transfer and separation in TiO2/RP composites than that of TiO2 (Fig. S12) [34]. Based on the analytic discussion on these above characterization results, such drastic improvement in photocatalytic activity can be attributed to the fact that the RP sensitization nanolayer on the TiO2 NFs could harvest more visible light, and the surface P 5+ doping induced VO could enhance charge transportation and inhibit the recombination process. Interestingly, instead of O2 evolution by water oxidation, hydrogen peroxide (H2O2) was produced by photogenerated holes via a two-electron process of water oxidation (Fig. S12) [51][52][53], which would benefit gas separation for hydrogen production via pure water splitting. As shown in Fig. 6c, the photocatalytic HER activities were great dependence on the wavelengths under different LEDs light irradiation ( Fig. S11b) for both TiO2(B)/RP (40) and TiO2(A)/RP (30) composites, confirming that the extended optical absorption by RP decoration was greatly responsive to enhanced HER activities, as compared to the pure TiO2 NFs. Furthermore, TiO2/RP composites displayed good photocatalytic stability under a 12 h continuous light irradiation (Fig. 6d). It is interesting that, even though the photocatalytic activity of TiO2(A) was higher than TiO2(B), the TiO2(A)/RP composites showed poor activities than the TiO2(B)/RP composites. To achieve a better understanding of the intrinsic characters (e.g., the RP sensitization, the introduced P 5+ dopants and VO) contributing to the improved photocatalytic activity, X-ray absorption structure spectroscopy (XAS) and density functional theory (DFT) calculations were further performed. it is suggested the decoration of RP does not affect severely the crystal structure of TiO2 NFs (either TiO2(B) or TiO2(A)), which is in agreement with the XRD characterization (Fig. 2) [54]. Anyhow, further investigation on the pre-peak region (about 4969-4977 eV) mainly originated from Ti 3d states would be of importance to look into the fine structures. After background subtraction, the pre-peak area was shown in Fig. 7c and decomposed into four components, A1, A2, A3, and A4 [55].
As RP was decorated onto TiO2 NFs, the peak A2 of TiO2/RP was greater than that of bare TiO2 NFs, indicating that P 5+ doping induced the formation of VO in TiO2 NFs and thus increased the five-fold coordinated geometry or disordered local atomic structure. A more significant increase in peak A2 intensity was observed in TiO2(B)/RP (40) in comparison with TiO2(A)/RP (30), meaning that more VO was created in TiO2(B)/RP (40), agreeing well with optical absorption spectra as well as EPR spectra. To further verify the induction of VO in TiO2 NFs by P 5+ doping, the Fouriertransform of extend X-ray absorption fine structures (EXAFS) k 3  data at Ti K-edge was also investigated (Fig. 7d)  DFT calculations were further performed to reveal the contribution of VO to the efficient charge transfer process and the improved photocatalytic activity, by elucidating the electronic structure and effective carrier masses. According to the TEM experimental investigation, the (001) and (100) planes were the predominantly exposed crystal facets for TiO2(B) and TiO2(A) [26], respectively, which were then built for the comparative DFT calculations. Fig. 8a,b show the schematics of the P 5+ doping induced VO in TiO2(B) and TiO2(A) supercells, with one P 5+ dopant replacing one Ti atom which induced one VO. The P 5+ doping reduced the VO formation energy by 1.60 eV for indicated that the P-induced formation of VO in TiO2(B) was more favorable than in TiO2(A). As shown in Fig. 8c, in comparison to the total density of states (TDOS) of pristine TiO2, impurity states created by VO can be seen in P-doped TiO2, which were responsible for the observed talelike absorption in Fig. 5. The effective masses of electrons and holes can be used to assess the transfer rate of charge carriers. It has been well accepted that a lower charge carrier effective mass corresponds to a higher charge carrier mobility, and the large difference between effective masses of electron and hole suggests a low recombination rate of the electron-hole pairs [43,44]. Therefore, the relative effective masses of holes and electrons in pristine and P-doped TiO2 NFs were further computed, by the quadratic fits of the band structures, to examine how the P doping induced VO affected the mobility and separation of charge carriers. The carrier effective masses (m*) were calculated and shown in Table S4. The value of the effective mass of electrons (me*) was smaller than that of the corresponding value of holes (mh*) for each sample and thus electron had higher mobility than hole in TiO2. The values of me*/mh* were then determined to be 0.918, 0.544, 0.894 and 0.612 for TiO2(B), P doped TiO2(B), TiO2(A) and P doped TiO2(A), respectively. It is clear that the P 5+ doping induced VO can decrease the ratio of electron to hole effective mass (me*/mh*) significantly, indicating the reduced recombination rates of the charge carriers in P doped TiO2 NFs.
Moreover, in comparison to P doped TiO2(A), P doped TiO2(B) possessed the smaller me*/mh*, implying the more efficient charge carrier separation. These DFT calculations revealed that the formation of VO was easier in TiO2(B) by P 5+ doping than in TiO2(A), as experimentally supported by the EPR and XAS results indicating that more VO was generated in TiO2(B)/RP than  Based on the above experimental and theoretical results, the photocatalytic H2 production mechanism was proposed in Fig. 9. Coating RP sensitization nanolayer on the TiO2 NFs surface can enhance optical absorption in the visible light region, and the doping of P 5+ in TiO2 crystal lattices would introduce VO in the surface region of TiO2 NFs and thus facilitate the charge transfer process. Thus, more electrons can be photoexcited and then efficiently transferred to the photocatalyst surface active sites, which synergistically boosts photocatalysis efficiency of the TiO2/RP composites.

Conclusion
In summary, a novel vaporization-deposition strategy was developed to simultaneously coat red phosphorous (RP) on and dope P 5+ into the surface of TiO2 nanofibers. It was demonstrated that the obtained TiO2/RP composites showed much increased photocatalytic activity for hydrogen production from pure water. By optimizing the RP amounts used during the vaporization-deposition process, the TiO2/RP composites showed photocatalytic hydrogen evolution rates as high as 11.4