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Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation
Accepted Manuscript Full Length Article Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation Kezhen Qi, Yubo Xie, Ruidan Wang, Shu-yuan Liu, Zhen Zhao PII: DOI: Reference:
S0169-4332(18)32733-8 https://doi.org/10.1016/j.apsusc.2018.10.037 APSUSC 40605
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
20 August 2018 30 September 2018 4 October 2018
Please cite this article as: K. Qi, Y. Xie, R. Wang, S-y. Liu, Z. Zhao, Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.10.037
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Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation Kezhen Qia,d,1, Yubo Xiea,1, Ruidan Wanga, Shu-yuan Liub*, Zhen Zhaoa,c* a
Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, China b Department of pharmacology, Shenyang Medical College, Shenyang 110034, China c State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing, 102249, China d Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin, 300071, China
*
E-mail: [email protected] (Shu-yuan Liu); [email protected] (Zhen Zhao)
1
These authors contributed equally to this work.
Abstract: This work reports the nickel-phosphorus (Ni-P) cocatalyst decorated graphitic C3N4 (g-C3N4/Ni-P) for photocatalytic water splitting to H2 generation. The g-C3N4/Ni-P composite is prepared through two steps, first step is thermal decomposition of urea to produce g-C3N4 and the second step is electroless plating to coast Ni-P on the g-C3N4 surface, resulting in the noticeable increase of the photocatalytic activity of g-C3N4. Among these as-prepared samples reported in this work, the highest photocatalytic activity is detected at 3 wt% (g-C3N4/Ni-P-3%), in which H2 production rate is 1051 μmol g-1h-1. However, higher than 3 wt% Pt modification the g-C3N4
generates only 841 μmol g-1h-1 of H2, and almost no
generation of H2 by pure g-C3N4 has been determined. Based on photoluminescence and photocurrent measurements, a photocatalytic mechanism for pure g-C3N4 and g-C3N4/Ni-P-3% has been suggested, that is, the loading of Ni-P NPs accelerates the separation of photogenerated e--h+ pairs and relaying photogenrated e- via Ni-P particles to react with H2O, and thus improves photocatalytic performance of g-C3N4 for water splitting into H2 generation. Keywords: Electroless plating, Ni-P, g-C3N4, photocatalysis, H2 generation
1. Introduction
Hydrogen energy is regarded as an ideal green energy, and the product of hydrogen combustion is water. So, hydrogen as fuel not only solves the crisis of fossil fuel shortage in the future, but also reduces the environmental pollution from fossil fuel consumption [1]. Since 1972, Fujishima et al reported the phenomenon of TiO2
photoelectrode water splitting [2], the photocatalytic H2 production has received much attention [3]. Semiconductors may catalyze the water splitting into H2 generation under sun shining, such as TiO2 [4, 5], CdS [6-8] CoP [9] and g-C3N4 [10-14]. Because being a nonmetal material, the g-C3N4 is very significant candidate to be investigated for its possible application in photocatalysis. Up to now, g-C3N4 has demonstrated high photocatalytic performance in organic pollutant degradation [15, 16], water splitting [17-20], and CO2 reduction [21-23], because of its suitable energy band position and its chemical stability. That is, g-C3N4 satisfies the thermodynamic requirements of the photocatalytic water splitting, the suitable location of valence band (VB) and conduction band (CB) for hydrogen and oxygen production, and the CB bottom (-1.3 V vs. NHE) meets the electrode potential (0 V vs. NHE) of reducing water to generate hydrogen [24, 25]. However, pure g-C3N4 does not meet the real application due to its low photocatalytic activity, caused by the low utilization efficiency of sunlight and the large recombination rate of photogenerated carriers. Many methods are used to improve the photocatalytic performance of g-C3N4, including of doping ion [26, 27], constructing heterojunction [28-34] loading noble metal [35, 36], coupling carbon material [37-41] etc. Among these methods, loading noble metals as cocatalysts (Ag, Au, Pd, Ru or Pt) on g-C3N4 is effectively to improve the photocatalytic H2 generation [42-45]. However, due to the noble metals are expensive and scarce, which restricts its large-scale application in the real life. Therefore, it is desirable to develop cocatalysts with high active and non noble-metal for the photocatalytic H2 production.
Transition metal phosphide, including FeP [46], CoP [47] and NiP [48, 49] have been used as an effective cocatalyst in photocatalysis process. However, synthesis of these metal phosphides, usually involve either using toxic organophosphorus compounds, white phosphorus, or the high calcination temperature [50, 51]. Especially, during the traditional hydrothermal method preparing the metal phosphides, a lot of H2 will be released and result in a high pressure in the vessel, which causes a big safety problem. It has been reported that among these phosphides, Ni-P has received much attention as an H2 evolution electrocatalyst, because of its appropriate band structure and high electronic conductivity [52]. However, up to now, very few studies have focused on Ni-P to be used in photocatalysis as a cocatalyst. In this work, g-C3N4 with different loading amount of Ni-P NPs was successfully prepared by two step including pyrolysis and electroless plating. Pure g-C3N4 is first prepared by thermal polymerization urea and the next electroless plating process makes the Ni-P NPs in situ growth on g-C3N4 surface. This synthesis method is under normal temperature and atmospheric pressure, importantly, the contact between Ni-P and g-C3N4 is tight and firm due to the in situ growth. Under simulated solar irradiation, the g-C3N4/Ni-P composite exhibits a higher photocatalytic activity of H2 production than that of g-C3N4 itself. The optimized Ni-P loading amount is also at 3 wt%, the photocatalytic H2 generation rate is 1051 μmol g-1h-1, which is higher than the 3 wt% Pt modified g-C3N4 (841 μmol g-1h-1). The reason behind is suggested to be that the relative position between the CB of C3N4 and Fermi energy of Ni-P NPs promotes the transfer of electrons to reaction site and accelerates the separation of the
e--h+ pairs. Of course, this work reports a simple method to synthesize g-C3N4/Ni-P with higher activity, which is hoped to take part in development of high efficient photocatalysts for hydrogen production.
2. Experimental 2.1 Syntheses The g-C3N4 nanosheet is prepared via thermal polycondensation of urea. 15 g urea is placed into a ceramic crucible, then covered and heated up to 500 ℃, and the heating rate is 10 ℃ min-1 and then maintaining 500 ℃ for 5 hours in air. Then the product cools down to room temperature naturally, then is grounded to powder, and collected. Loading of Ni-P onto g-C3N4 surface is by electroless plating method. And the reaction process includes two main steps [53]: Ni2+ + H2PO2- + H2O → Ni + HPO32- + 3H+
(1)
3H2PO2- → H2PO3- + H2O + 2OH- + 2P
(2)
First 0.5g of g-C3N4 is put in 20 mL of water and ultrasonic treated for 5 min. The g-C3N4 is sensitized by the SnCl2/HCl solution, then activated by the PdCl2/HCl solution. Before electroless deposition, it is washed by distilled water. The coating bath contains Ni(NO3)2·6H2O (0.2 mol L-1), NH2CH2COOH (0.6 mol L-1), and NaH2PO2·H2O (0.8 mol L-1). The pH value of the coating bath is adjusted to 11 by controlling the amount of sodium hydroxide (NaOH). Electroless plating process is performed at 50 ℃ for 4 h. In this process, SnCl2/HCl solution is the plating solution and PdCl2/HCl solution is the activation solution, Sn2+ ions will reduce the Pd2+ to
Pd, and this Pd cluster on g-C3N4 surface will be the catalyst to promote the reaction for synthesis of Ni-P. Then, the product is collected after the product-solution is centrifuged and washed using ethanol and distilled water several times. Then the sample is dried in vacuum oven at 70 ℃ for 5 hours. By varying the amount of the coating bath solution, a series of ratio of [mNi-P:n(g-C3N4) = 1%, 2%, 3%, 4% and 5%.] samples are prepared and labeled as
g-C3N4/Ni-P-1%, g-C3N4/Ni-P-2%,
g-C3N4/Ni-P-3%, g-C3N4/Ni-P-4% and g-C3N4/Ni-P-5%, respectively.
2.2 Characterization The crystal phases are examined by X-ray diffraction (XRD) (Bruker D5005 X-ray diffractometer, Cu Kα, λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra are carried using a Nicolet Magna 560 spectrophotometer. X-ray photoelectron spectroscopy (XPS) is measured on a PHIQ uantum1600 XPS instrument. High resolution transmission electron microscopy (HRTEM) is taken by a JEOL JEM-2100F electron microscope. UV–vis absorbance spectra are collected on a Shimadzu
UV-3100
spectrophotometer,
using
BaSO4
as
reference.
The
photoluminescence (PL) spectra of g-C3N4 and g-C3N4/Ni-P samples are studied on a Varian Cary Eclipse spectrometer equipped with an excitation wavelength of 325 nm.
2.3 Photocatalytic activity The photocatalytic water splitting to produce hydrogen is performed in a 100 mL Pyrex flask with three openings, which are sealed by rubber plugs. The simulated
solar light source is provided by a 350 W Xe lamp, which is placed in 15 cm from the reaction solution. Put 10 mg photocatalyst into the reaction solution, which contained 10 mL triethanolamine (TEOA) and 70 mL water, by continuous magnetic stirring. In order to removed the air, the reaction solution is bubbled using N2 for 30 min, before the irradiation. The photocatalytic H2 generation is sampled by taking 0.4 mL gas at a time from the reaction system and analyzing by using a gas chromatograph (GC-14C, Shimadzu, Japan, TCD, N2 is used as the carrier gas.).
2.4 Photoelectrochemical property The photoelectrochemical performance is studied on a CHI 660D electrochemical work station with a standard three‐electrode system. Put g-C3N4 or g-C3N4/Ni-P on the ITO glass surface as the working electrode. A Pt wire and a calomel electrode are used as the counter electrode and reference electrode, respectively. The electrolyte is 0.1 mol/L Na2SO4 aqueous solution. 5 mg photocatalysts are mixed with 1 mL ethanol and then the mixture is coated on 2 cm × 4 cm ITO glass for using as an electrode. Electrochemical impedance spectroscopy (EIS) Nyquist plots are conducted at an open current potential with amplitude of 5 mV and the frequency range is from 105 to 1 Hz.
3. Results and Discussion 3.1. XRD patterns The crystal phase of as-prepared samples is examined and determined by XRD
measurements. The XRD patterns (Fig. 1) show that the pure g-C3N4 and the g-C3N4/Ni-P composites have two dominant peaks at 13.1◦ and 27.5◦, indexed to g-C3N4 (JCPDS87-1526). The peak at 27.5◦ is assigned to the typical (002) plane with planar distance of 0.33 nm corresponding to interlayer-stacking of aromatic segments. The peak located at 13.1◦ with distance of 0.675 nm is ascribed to the (100) plane [54]. After loading Ni-P NPs, the location of diffraction peaks has no obvious change, indicating that Ni-P has not affected the crystal structure of g-C3N4. And compared with the pure g-C3N4 nanosheets, the intensity of the diffraction peak at 27.5◦ becomes weaker with increasing content of loading Ni-P NPs. The diffraction peak related to Ni-P NPs is not found, because of the low Ni-P loading amount and the high dilution effect of Ni-P NPs on g-C3N4 surfaces [55].
Fig. 1 XRD patterns of pure g-C3N4 and g-C3N4/Ni-P samples.
3.2. FTIR analysis The FTIR spectra are similar between the pure g-C3N4 and the g-C3N4/Ni-P
composites (see Fig 2). The peak at 1639 cm-1 is assigned to the stretching vibration of C-N groups, and the peaks at 1249, 1319 and 1433 cm-1 assign to the stretching vibration of the aromatic C-N bonds [56]. The peak at 804 cm−1 corresponds to the breathing mode of triazine units [57]. The peak at 3255 cm-1 can be ascribed to N-H stretching vibration [58]. All these characteristic FTIR peaks suggested that the original g-C3N4 structure is not changed by loading Ni-P NPs. It suggests that the Ni-P NPs are just adsorbed on g-C3N4 surfaces, agreeing with the XRD result.
Fig. 2 FTIR spectra of pure g-C3N4 and g-C3N4/Ni-P samples.
3.3 UV-vis analysis The optical adsorption property of the pure g-C3N4 and the g-C3N4/Ni-P is studied by UV-DRS measurement (Fig. 3). The light absorption edge of pure g-C3N4 is at 470 nm, corresponding to a band gap of ~2.7 eV, which agrees well with the previous literature [59]. It is known that Ni-P has a high visible light absorption, because its
band gap is ~1.0 eV, which corresponds to the absorption peak at ~1240 nm [60]. Compared with the pure g-C3N4, the color of g-C3N4/Ni-P changing from yellow to dark, depending on the amount of Ni-P coated. Although, g-C3N4/Ni-P still has a strong light absorption at 470 nm, the absorption intensity is enhanced at the visible light area, and the absorption edge does not shift when varying the Ni-P loading amount, too. This confirms that g-C3N4/Ni-P remains the intrinsic band gap of the g-C3N4.
Fig. 3 UV-Vis diffuse reflectance absorption spectra of pure g-C3N4 and g-C3N4/Ni-P samples.
3.4. XPS analysis The chemical states and bonding properties of the g-C3N4/Ni-P-3% are studied by XPS spectra. It is observed the elements C, N, O, Ni, P from the survey spectrum of
g-C3N4/Ni-P-3% (Fig. 4A). The peak at 531 eV is ascribed to O, which may be the water molecules adsorbed at the sample surface [61]. Two C 1s peaks locate at 284.7 eV and 288.2 eV (Fig. 4B). The peak at 284.7 eV is assigned to sp2- hybridized C atoms, and the 288.2 eV peak is ascribed to N-C=N2 groups [62]. Fig. 4C shows that the peaks of N 1s, locating at 398.8 eV, 399.9 eV and 401.2 eV, are assigned as sp2 bonded nitrogen C-N-C groups, sp3 tertiary nitrogen N-(C)3 and amino functional groups (C-N-H), respectively [54]. The Ni 2p spectrum (Fig. 4D) observe three peaks at 853.3, 856.3 and 862.1 eV for the Ni 2p1/2 energy level are ascribed to the Niδ+ state in Ni-P, the oxidized Ni species (Ni2+) and the satellite of the Ni 2p1/2, respectively [63]. Additionally, two peaks at 874.0 and 880.1 eV belong to the Ni 2p3/2 energy level, which are ascribed to the Ni2+ of oxidized Ni species and the satellite of Ni 2p3/2, respectively [64]. Fig. 4E shows the P 2p spectrum observes the two peaks located at 129.1 and 133.2 eV, which can be assigned as the Pδ− of Ni-P and the oxidized P species, respectively [65]. The oxidized Ni and P species are observed because the air contact [66]. It demonstrates that the dominated elemental states are Ni and P, besides small part of oxidation atates on the material surface. The above result shows that the Ni atoms in Ni-P have a positive charge (Niδ+) and the P atoms in Ni-P are negatively charged (Pδ-) [67], which indicates that the Ni and P atoms have as electronic interactions and form Ni-P alloy. All in all, the XPS signals of Ni 2p and P 2p are observed, which confirm that Ni-P successfully loading on the g-C3N4 surface. For check whether Pd affect on the surface composition, the XPS of Pd also check (Fig. 4F), it is difficult to see the Pd, it means that the Pd amount is
very small, even not test by XPS, which will not affect the photocatalytic activity.
Fig. 4 XPS spectra of the g-C3N4/Ni-P-3% sample: (A) survey XPS, high resolution of (B) C1s, (C) N1s, (D) Ni 2p, (E) P 2p, (F) Pd 3d.
3.3. TEM images The morphology and microstructure of the pure C3N4 and the g-C3N4/Ni-P-3% samples were studied by TEM measurements. The TEM image shows that the pure
g-C3N4 are nanosheet structures (Fig. 5A). Fig. 5B gives the TEM image of the g-C3N4/Ni-P-3%, it is clear that Ni-P NPs, appearing as the black dots, uniformly disperse on g-C3N4 surfaces. The size of Ni-P NPs is from 40 nm to 60 nm, which indicates that these Ni-P NPs are Ni-P clusters on g-C3N4 surfaces. From the HRTEM image (Fig. 5C), the clear lattice fringes of Ni-P NPs can be seen, and the lattice distance of Ni-P NPs is ~0.22 nm, belonging to the (111) plane of Ni-P [68]. From the TEM result concluded that Ni-P NPs with uniform size and well spread on g-C3N4 surfaces. Figs. 5D show the size distributions of the Ni-P NPs on g-C3N4 surfaces, after have been calculated. When the Ni-P NPs are deposited on the surface of g-C3N4, the size of Ni-P NPs is from 40 nm to 60 nm. These Ni-P NPs are uniformly adsorbed on the C3N4 surface.
Fig. 5 (A) TEM, (B, C) HR-TEM and (D) Size distribution of the g-C3N4/Ni-P-3% sample.
3.6 Photocatalytic activity The photocatalytic performance of g-C3N4/Ni-P is studied via photocatalytic H2 generation from water splitting under simulated solar light irradiation (Fig. 6). The generation rate of H2 is very low using pure g-C3N4 as the photocatalyst, almost no hydrogen production, because of the rapid recombination between the photogenerated e--h+ pairs. With increasing the Ni-P loading amount (1 wt% to 5 wt%) on g-C3N4, the generation rate of H2 shows enhancement, in particular, when the load of Ni-P reaches to 3 wt%, the photocatalytic activity of the g-C3N4/Ni-P-3% sample is the highest among these as-prepared photoocatalysts, reaching the hydrogen production rate of 937 μmol g-1 h-1, even higher that of g-C3N4/Pt-3% (637 μmol g-1 h-1). However, with increased loading of Ni-P amounts to 5 wt%, the rate of hydrogen production decrease to 665 μmol/g/h. The overloading of Ni-P, the photocatalytic activity is declined, which is due to the shielding effect, that is the excessive Ni-P NPs on the g-C3N4 surface block the light absorption [69]. Thus, the loading amount of Ni-P should be carefully selected in order to improve the photocatalytic activity of g-C3N4.
Fig. 6. (A) Plots of photocatalytic hydrogen evolution amount simulated solar light
irradiation time for different photocatalysts; (B) comparison of the simulated solar light induced hydrogen evolution rate for different photocatalysts.
3.8. Photoelectrochemical performance The charge separation efficiency is studied by using the photoelectrochemical measurements. Fig. 7A shows that the photocurrent response of the as‐prepared pure g-C3N4 and g-C3N4/Ni-P-3% samples under the simulated sunlight irradiation, which shows stable reproducible photocurrent responses over five on-off cycles. The photocurrent starts when the light is turned on, the photocurrent is close to zero when the light is turned off. Obviously, the photocurrent density (~0.83 μA/cm2) of g-C3N4/Ni-P-3% is greatly stronger than the pure g-C3N4 (0.07 μA/cm2) which agreed well with its higher activity on photocatalytic water splitting. The higher photocurrent value indicates the higher separation efficiency of photogenerated e−–h+ pair generation in g-C3N4/Ni-P-3%, contribute to a higher rate of hydrogen generation. The electrochemical impedance spectroscopy (EIS) Nynquist plots also support this result, as shown in Fig. 7B. Usually, the smaller the radius of EIS Nyquist plots is, the higher separation efficiency of photogenerated e--h+ pairs is. Ni-P modification make the radius of EIS Nynquist plot of C3N4 smaller, indicating that loading Ni-P indeed reduces the recombination of charge carries and increase the separation efficiency of e--h+ pairs.
Fig. 7 (A) Photocurrent response and (B) Electrochemical impedance spectroscopy of the pure g-C3N4 and g-C3N4/Ni-P-3% samples.
3.6 PL spectra Photoluminescence measurement is one of useful method to analyze the recombination of the photogenerated carriers. PL spectra of the pure g-C3N4 and g-C3N4/Ni-P-3% excited by light of 280 nm wavelength, show a strong broad peak at about 460 nm (Fig. 8). And after Ni-P modification, the PL intensity of g-C3N4 decreases dramatically. The excess energy of the excited electron may be consumed via three routs: photoluminescence, e--h+ pair recombination, and thermal dissipation. However, because of the proper relative position of the energy bands between Ni-P and g-C3N4, the adsorbed Ni-P particle offers itself as a mediated storage for the excited electron, decreasing the probability of e--h+ pair recombination. So, the weaker peak intensity of PL means that Ni-P modification decreases the recombination rate of photogenerated e--h+ pairs [55]. More clearly, in the g-C3N4/Ni-P composites, the Ni-P NPs are strongly adsorbed on the g-C3N4 surface, so that the surface Ni-P NPs can capture these photogenerated electrons and enhance
the interfacial charge transfer form g-C3N4 to water molecule, resulting in the effective decrease of the recombination rate of photogenerated e--h+ pairs. This improvement of photogenerated carrier separation efficiency leads to the increase of eand h+ pairs joining in the g-C3N4/Ni-P photocatalytic process. Thus, the appropriate loading of Ni-P on g-C3N4 has a potential application for improving the photocatalytic water splitting into H2 production.
Fig. 8. Photoluminescence spectra of pure g-C3N4 and g-C3N4/Ni-P-3% samples excited at 375 nm.
3.4. Photocatalytic mechanism As shown in Fig. 9, a possible photocatalytic reaction mechanism for g-C3N4/Ni-P photocatalytic H2 generation is proposed. Under simulated sunlight, g-C3N4 is excited to generate charge carriers, the electrons are excited to CB and holes to VB. Due to the Ni-P cocatalyst has a large work function [70], it is like a electron pool, the photoexcited electrons easily transfer to Ni-P surface from CB of g-C3N4, and decompose the water. Meantime, the holes leaved at the VB of g-C3N4 rapidly are
captured and neutralized by the sacrificial agent of TEOA. Clearly, Ni-P co-catalyst accelerates separation of photoexcited charge carries and prolong the life time of electrons, thus enhanced the photocatalytic activity of g-C3N4. However, without the use of cocatalysts, in the pure g-C3N4, the excited e- and h+ will quickly recombine, resulting in poor photocatalytic H2 evolution. Also, the overloading of Ni-P is no good for improving the photocatalytic activity, because of the shielding effect, that is, the excessive Ni-P NPs on the g-C3N4 surface block the light absorption, and the efficiency sunlight utilization will be reduced.
Fig. 9 Schematic diagram for the photocatalytic H2 generation using g-C3N4/Ni-P as photocatalysts.
Conclusions In conclusion, g-C3N4/Ni-P with different loading amount of Ni-P was successfully prepared based on a two-step, pyrolysis and electroless, plating method, the g-C3N4/Ni-P composites show a high activity in photocatalytic H2 evolution. In this preparation method, Ni-P is in situ growth on the g-C3N4 surface, and the combination
of Ni-P catalyst and g-C3N4 is tight and firm. After loading Ni-P, the g-C3N4 shows an increased rate of hydrogen production. Among these prepared materials, the highest H2 generation rate is 1051 μmol g-1h-1 over g-C3N4/Ni-P-3%, which is higher than that of the same amount of Pt loading (841 μmol g-1h-1). The photocatalytic activity in H2 generation reaction of g-C3N4 is enhanced by loading Ni-P NPs, because the Ni-P NPs promote the transfer of electrons and accelerate the separation of the e--h+ pairs. This work report a simple method to synthesize high activity Ni-P/g-C3N4 photocatalyst, hoped for developing the high activity photocatalysts for hydrogen production.
Notes The authors declare no competing financial interest.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51602207, 91545117), 863 Program, (2015AA034603), the Doctoral Scientific Research Foundation of Liaoning Province (201601149, 20170520011), and Project of Education Office of Liaoning Province (LQN201712).
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Graphical abstract
TOC
Highlights
Highlight: (1) Electroless plating method is used to prepare g-C3N4/Ni-P composites. (2) g-C3N4/Ni-P shows high activity in photocatalytic H2 evolution. (3) g-C3N4/Ni-P heterojunctions significantly promote the charge separation.
S0169-4332(18)32733-8 https://doi.org/10.1016/j.apsusc.2018.10.037 APSUSC 40605
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
20 August 2018 30 September 2018 4 October 2018
Please cite this article as: K. Qi, Y. Xie, R. Wang, S-y. Liu, Z. Zhao, Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.10.037
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Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation Kezhen Qia,d,1, Yubo Xiea,1, Ruidan Wanga, Shu-yuan Liub*, Zhen Zhaoa,c* a
Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, China b Department of pharmacology, Shenyang Medical College, Shenyang 110034, China c State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing, 102249, China d Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin, 300071, China
*
E-mail: [email protected] (Shu-yuan Liu); [email protected] (Zhen Zhao)
1
These authors contributed equally to this work.
Abstract: This work reports the nickel-phosphorus (Ni-P) cocatalyst decorated graphitic C3N4 (g-C3N4/Ni-P) for photocatalytic water splitting to H2 generation. The g-C3N4/Ni-P composite is prepared through two steps, first step is thermal decomposition of urea to produce g-C3N4 and the second step is electroless plating to coast Ni-P on the g-C3N4 surface, resulting in the noticeable increase of the photocatalytic activity of g-C3N4. Among these as-prepared samples reported in this work, the highest photocatalytic activity is detected at 3 wt% (g-C3N4/Ni-P-3%), in which H2 production rate is 1051 μmol g-1h-1. However, higher than 3 wt% Pt modification the g-C3N4
generates only 841 μmol g-1h-1 of H2, and almost no
generation of H2 by pure g-C3N4 has been determined. Based on photoluminescence and photocurrent measurements, a photocatalytic mechanism for pure g-C3N4 and g-C3N4/Ni-P-3% has been suggested, that is, the loading of Ni-P NPs accelerates the separation of photogenerated e--h+ pairs and relaying photogenrated e- via Ni-P particles to react with H2O, and thus improves photocatalytic performance of g-C3N4 for water splitting into H2 generation. Keywords: Electroless plating, Ni-P, g-C3N4, photocatalysis, H2 generation
1. Introduction
Hydrogen energy is regarded as an ideal green energy, and the product of hydrogen combustion is water. So, hydrogen as fuel not only solves the crisis of fossil fuel shortage in the future, but also reduces the environmental pollution from fossil fuel consumption [1]. Since 1972, Fujishima et al reported the phenomenon of TiO2
photoelectrode water splitting [2], the photocatalytic H2 production has received much attention [3]. Semiconductors may catalyze the water splitting into H2 generation under sun shining, such as TiO2 [4, 5], CdS [6-8] CoP [9] and g-C3N4 [10-14]. Because being a nonmetal material, the g-C3N4 is very significant candidate to be investigated for its possible application in photocatalysis. Up to now, g-C3N4 has demonstrated high photocatalytic performance in organic pollutant degradation [15, 16], water splitting [17-20], and CO2 reduction [21-23], because of its suitable energy band position and its chemical stability. That is, g-C3N4 satisfies the thermodynamic requirements of the photocatalytic water splitting, the suitable location of valence band (VB) and conduction band (CB) for hydrogen and oxygen production, and the CB bottom (-1.3 V vs. NHE) meets the electrode potential (0 V vs. NHE) of reducing water to generate hydrogen [24, 25]. However, pure g-C3N4 does not meet the real application due to its low photocatalytic activity, caused by the low utilization efficiency of sunlight and the large recombination rate of photogenerated carriers. Many methods are used to improve the photocatalytic performance of g-C3N4, including of doping ion [26, 27], constructing heterojunction [28-34] loading noble metal [35, 36], coupling carbon material [37-41] etc. Among these methods, loading noble metals as cocatalysts (Ag, Au, Pd, Ru or Pt) on g-C3N4 is effectively to improve the photocatalytic H2 generation [42-45]. However, due to the noble metals are expensive and scarce, which restricts its large-scale application in the real life. Therefore, it is desirable to develop cocatalysts with high active and non noble-metal for the photocatalytic H2 production.
Transition metal phosphide, including FeP [46], CoP [47] and NiP [48, 49] have been used as an effective cocatalyst in photocatalysis process. However, synthesis of these metal phosphides, usually involve either using toxic organophosphorus compounds, white phosphorus, or the high calcination temperature [50, 51]. Especially, during the traditional hydrothermal method preparing the metal phosphides, a lot of H2 will be released and result in a high pressure in the vessel, which causes a big safety problem. It has been reported that among these phosphides, Ni-P has received much attention as an H2 evolution electrocatalyst, because of its appropriate band structure and high electronic conductivity [52]. However, up to now, very few studies have focused on Ni-P to be used in photocatalysis as a cocatalyst. In this work, g-C3N4 with different loading amount of Ni-P NPs was successfully prepared by two step including pyrolysis and electroless plating. Pure g-C3N4 is first prepared by thermal polymerization urea and the next electroless plating process makes the Ni-P NPs in situ growth on g-C3N4 surface. This synthesis method is under normal temperature and atmospheric pressure, importantly, the contact between Ni-P and g-C3N4 is tight and firm due to the in situ growth. Under simulated solar irradiation, the g-C3N4/Ni-P composite exhibits a higher photocatalytic activity of H2 production than that of g-C3N4 itself. The optimized Ni-P loading amount is also at 3 wt%, the photocatalytic H2 generation rate is 1051 μmol g-1h-1, which is higher than the 3 wt% Pt modified g-C3N4 (841 μmol g-1h-1). The reason behind is suggested to be that the relative position between the CB of C3N4 and Fermi energy of Ni-P NPs promotes the transfer of electrons to reaction site and accelerates the separation of the
e--h+ pairs. Of course, this work reports a simple method to synthesize g-C3N4/Ni-P with higher activity, which is hoped to take part in development of high efficient photocatalysts for hydrogen production.
2. Experimental 2.1 Syntheses The g-C3N4 nanosheet is prepared via thermal polycondensation of urea. 15 g urea is placed into a ceramic crucible, then covered and heated up to 500 ℃, and the heating rate is 10 ℃ min-1 and then maintaining 500 ℃ for 5 hours in air. Then the product cools down to room temperature naturally, then is grounded to powder, and collected. Loading of Ni-P onto g-C3N4 surface is by electroless plating method. And the reaction process includes two main steps [53]: Ni2+ + H2PO2- + H2O → Ni + HPO32- + 3H+
(1)
3H2PO2- → H2PO3- + H2O + 2OH- + 2P
(2)
First 0.5g of g-C3N4 is put in 20 mL of water and ultrasonic treated for 5 min. The g-C3N4 is sensitized by the SnCl2/HCl solution, then activated by the PdCl2/HCl solution. Before electroless deposition, it is washed by distilled water. The coating bath contains Ni(NO3)2·6H2O (0.2 mol L-1), NH2CH2COOH (0.6 mol L-1), and NaH2PO2·H2O (0.8 mol L-1). The pH value of the coating bath is adjusted to 11 by controlling the amount of sodium hydroxide (NaOH). Electroless plating process is performed at 50 ℃ for 4 h. In this process, SnCl2/HCl solution is the plating solution and PdCl2/HCl solution is the activation solution, Sn2+ ions will reduce the Pd2+ to
Pd, and this Pd cluster on g-C3N4 surface will be the catalyst to promote the reaction for synthesis of Ni-P. Then, the product is collected after the product-solution is centrifuged and washed using ethanol and distilled water several times. Then the sample is dried in vacuum oven at 70 ℃ for 5 hours. By varying the amount of the coating bath solution, a series of ratio of [mNi-P:n(g-C3N4) = 1%, 2%, 3%, 4% and 5%.] samples are prepared and labeled as
g-C3N4/Ni-P-1%, g-C3N4/Ni-P-2%,
g-C3N4/Ni-P-3%, g-C3N4/Ni-P-4% and g-C3N4/Ni-P-5%, respectively.
2.2 Characterization The crystal phases are examined by X-ray diffraction (XRD) (Bruker D5005 X-ray diffractometer, Cu Kα, λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra are carried using a Nicolet Magna 560 spectrophotometer. X-ray photoelectron spectroscopy (XPS) is measured on a PHIQ uantum1600 XPS instrument. High resolution transmission electron microscopy (HRTEM) is taken by a JEOL JEM-2100F electron microscope. UV–vis absorbance spectra are collected on a Shimadzu
UV-3100
spectrophotometer,
using
BaSO4
as
reference.
The
photoluminescence (PL) spectra of g-C3N4 and g-C3N4/Ni-P samples are studied on a Varian Cary Eclipse spectrometer equipped with an excitation wavelength of 325 nm.
2.3 Photocatalytic activity The photocatalytic water splitting to produce hydrogen is performed in a 100 mL Pyrex flask with three openings, which are sealed by rubber plugs. The simulated
solar light source is provided by a 350 W Xe lamp, which is placed in 15 cm from the reaction solution. Put 10 mg photocatalyst into the reaction solution, which contained 10 mL triethanolamine (TEOA) and 70 mL water, by continuous magnetic stirring. In order to removed the air, the reaction solution is bubbled using N2 for 30 min, before the irradiation. The photocatalytic H2 generation is sampled by taking 0.4 mL gas at a time from the reaction system and analyzing by using a gas chromatograph (GC-14C, Shimadzu, Japan, TCD, N2 is used as the carrier gas.).
2.4 Photoelectrochemical property The photoelectrochemical performance is studied on a CHI 660D electrochemical work station with a standard three‐electrode system. Put g-C3N4 or g-C3N4/Ni-P on the ITO glass surface as the working electrode. A Pt wire and a calomel electrode are used as the counter electrode and reference electrode, respectively. The electrolyte is 0.1 mol/L Na2SO4 aqueous solution. 5 mg photocatalysts are mixed with 1 mL ethanol and then the mixture is coated on 2 cm × 4 cm ITO glass for using as an electrode. Electrochemical impedance spectroscopy (EIS) Nyquist plots are conducted at an open current potential with amplitude of 5 mV and the frequency range is from 105 to 1 Hz.
3. Results and Discussion 3.1. XRD patterns The crystal phase of as-prepared samples is examined and determined by XRD
measurements. The XRD patterns (Fig. 1) show that the pure g-C3N4 and the g-C3N4/Ni-P composites have two dominant peaks at 13.1◦ and 27.5◦, indexed to g-C3N4 (JCPDS87-1526). The peak at 27.5◦ is assigned to the typical (002) plane with planar distance of 0.33 nm corresponding to interlayer-stacking of aromatic segments. The peak located at 13.1◦ with distance of 0.675 nm is ascribed to the (100) plane [54]. After loading Ni-P NPs, the location of diffraction peaks has no obvious change, indicating that Ni-P has not affected the crystal structure of g-C3N4. And compared with the pure g-C3N4 nanosheets, the intensity of the diffraction peak at 27.5◦ becomes weaker with increasing content of loading Ni-P NPs. The diffraction peak related to Ni-P NPs is not found, because of the low Ni-P loading amount and the high dilution effect of Ni-P NPs on g-C3N4 surfaces [55].
Fig. 1 XRD patterns of pure g-C3N4 and g-C3N4/Ni-P samples.
3.2. FTIR analysis The FTIR spectra are similar between the pure g-C3N4 and the g-C3N4/Ni-P
composites (see Fig 2). The peak at 1639 cm-1 is assigned to the stretching vibration of C-N groups, and the peaks at 1249, 1319 and 1433 cm-1 assign to the stretching vibration of the aromatic C-N bonds [56]. The peak at 804 cm−1 corresponds to the breathing mode of triazine units [57]. The peak at 3255 cm-1 can be ascribed to N-H stretching vibration [58]. All these characteristic FTIR peaks suggested that the original g-C3N4 structure is not changed by loading Ni-P NPs. It suggests that the Ni-P NPs are just adsorbed on g-C3N4 surfaces, agreeing with the XRD result.
Fig. 2 FTIR spectra of pure g-C3N4 and g-C3N4/Ni-P samples.
3.3 UV-vis analysis The optical adsorption property of the pure g-C3N4 and the g-C3N4/Ni-P is studied by UV-DRS measurement (Fig. 3). The light absorption edge of pure g-C3N4 is at 470 nm, corresponding to a band gap of ~2.7 eV, which agrees well with the previous literature [59]. It is known that Ni-P has a high visible light absorption, because its
band gap is ~1.0 eV, which corresponds to the absorption peak at ~1240 nm [60]. Compared with the pure g-C3N4, the color of g-C3N4/Ni-P changing from yellow to dark, depending on the amount of Ni-P coated. Although, g-C3N4/Ni-P still has a strong light absorption at 470 nm, the absorption intensity is enhanced at the visible light area, and the absorption edge does not shift when varying the Ni-P loading amount, too. This confirms that g-C3N4/Ni-P remains the intrinsic band gap of the g-C3N4.
Fig. 3 UV-Vis diffuse reflectance absorption spectra of pure g-C3N4 and g-C3N4/Ni-P samples.
3.4. XPS analysis The chemical states and bonding properties of the g-C3N4/Ni-P-3% are studied by XPS spectra. It is observed the elements C, N, O, Ni, P from the survey spectrum of
g-C3N4/Ni-P-3% (Fig. 4A). The peak at 531 eV is ascribed to O, which may be the water molecules adsorbed at the sample surface [61]. Two C 1s peaks locate at 284.7 eV and 288.2 eV (Fig. 4B). The peak at 284.7 eV is assigned to sp2- hybridized C atoms, and the 288.2 eV peak is ascribed to N-C=N2 groups [62]. Fig. 4C shows that the peaks of N 1s, locating at 398.8 eV, 399.9 eV and 401.2 eV, are assigned as sp2 bonded nitrogen C-N-C groups, sp3 tertiary nitrogen N-(C)3 and amino functional groups (C-N-H), respectively [54]. The Ni 2p spectrum (Fig. 4D) observe three peaks at 853.3, 856.3 and 862.1 eV for the Ni 2p1/2 energy level are ascribed to the Niδ+ state in Ni-P, the oxidized Ni species (Ni2+) and the satellite of the Ni 2p1/2, respectively [63]. Additionally, two peaks at 874.0 and 880.1 eV belong to the Ni 2p3/2 energy level, which are ascribed to the Ni2+ of oxidized Ni species and the satellite of Ni 2p3/2, respectively [64]. Fig. 4E shows the P 2p spectrum observes the two peaks located at 129.1 and 133.2 eV, which can be assigned as the Pδ− of Ni-P and the oxidized P species, respectively [65]. The oxidized Ni and P species are observed because the air contact [66]. It demonstrates that the dominated elemental states are Ni and P, besides small part of oxidation atates on the material surface. The above result shows that the Ni atoms in Ni-P have a positive charge (Niδ+) and the P atoms in Ni-P are negatively charged (Pδ-) [67], which indicates that the Ni and P atoms have as electronic interactions and form Ni-P alloy. All in all, the XPS signals of Ni 2p and P 2p are observed, which confirm that Ni-P successfully loading on the g-C3N4 surface. For check whether Pd affect on the surface composition, the XPS of Pd also check (Fig. 4F), it is difficult to see the Pd, it means that the Pd amount is
very small, even not test by XPS, which will not affect the photocatalytic activity.
Fig. 4 XPS spectra of the g-C3N4/Ni-P-3% sample: (A) survey XPS, high resolution of (B) C1s, (C) N1s, (D) Ni 2p, (E) P 2p, (F) Pd 3d.
3.3. TEM images The morphology and microstructure of the pure C3N4 and the g-C3N4/Ni-P-3% samples were studied by TEM measurements. The TEM image shows that the pure
g-C3N4 are nanosheet structures (Fig. 5A). Fig. 5B gives the TEM image of the g-C3N4/Ni-P-3%, it is clear that Ni-P NPs, appearing as the black dots, uniformly disperse on g-C3N4 surfaces. The size of Ni-P NPs is from 40 nm to 60 nm, which indicates that these Ni-P NPs are Ni-P clusters on g-C3N4 surfaces. From the HRTEM image (Fig. 5C), the clear lattice fringes of Ni-P NPs can be seen, and the lattice distance of Ni-P NPs is ~0.22 nm, belonging to the (111) plane of Ni-P [68]. From the TEM result concluded that Ni-P NPs with uniform size and well spread on g-C3N4 surfaces. Figs. 5D show the size distributions of the Ni-P NPs on g-C3N4 surfaces, after have been calculated. When the Ni-P NPs are deposited on the surface of g-C3N4, the size of Ni-P NPs is from 40 nm to 60 nm. These Ni-P NPs are uniformly adsorbed on the C3N4 surface.
Fig. 5 (A) TEM, (B, C) HR-TEM and (D) Size distribution of the g-C3N4/Ni-P-3% sample.
3.6 Photocatalytic activity The photocatalytic performance of g-C3N4/Ni-P is studied via photocatalytic H2 generation from water splitting under simulated solar light irradiation (Fig. 6). The generation rate of H2 is very low using pure g-C3N4 as the photocatalyst, almost no hydrogen production, because of the rapid recombination between the photogenerated e--h+ pairs. With increasing the Ni-P loading amount (1 wt% to 5 wt%) on g-C3N4, the generation rate of H2 shows enhancement, in particular, when the load of Ni-P reaches to 3 wt%, the photocatalytic activity of the g-C3N4/Ni-P-3% sample is the highest among these as-prepared photoocatalysts, reaching the hydrogen production rate of 937 μmol g-1 h-1, even higher that of g-C3N4/Pt-3% (637 μmol g-1 h-1). However, with increased loading of Ni-P amounts to 5 wt%, the rate of hydrogen production decrease to 665 μmol/g/h. The overloading of Ni-P, the photocatalytic activity is declined, which is due to the shielding effect, that is the excessive Ni-P NPs on the g-C3N4 surface block the light absorption [69]. Thus, the loading amount of Ni-P should be carefully selected in order to improve the photocatalytic activity of g-C3N4.
Fig. 6. (A) Plots of photocatalytic hydrogen evolution amount simulated solar light
irradiation time for different photocatalysts; (B) comparison of the simulated solar light induced hydrogen evolution rate for different photocatalysts.
3.8. Photoelectrochemical performance The charge separation efficiency is studied by using the photoelectrochemical measurements. Fig. 7A shows that the photocurrent response of the as‐prepared pure g-C3N4 and g-C3N4/Ni-P-3% samples under the simulated sunlight irradiation, which shows stable reproducible photocurrent responses over five on-off cycles. The photocurrent starts when the light is turned on, the photocurrent is close to zero when the light is turned off. Obviously, the photocurrent density (~0.83 μA/cm2) of g-C3N4/Ni-P-3% is greatly stronger than the pure g-C3N4 (0.07 μA/cm2) which agreed well with its higher activity on photocatalytic water splitting. The higher photocurrent value indicates the higher separation efficiency of photogenerated e−–h+ pair generation in g-C3N4/Ni-P-3%, contribute to a higher rate of hydrogen generation. The electrochemical impedance spectroscopy (EIS) Nynquist plots also support this result, as shown in Fig. 7B. Usually, the smaller the radius of EIS Nyquist plots is, the higher separation efficiency of photogenerated e--h+ pairs is. Ni-P modification make the radius of EIS Nynquist plot of C3N4 smaller, indicating that loading Ni-P indeed reduces the recombination of charge carries and increase the separation efficiency of e--h+ pairs.
Fig. 7 (A) Photocurrent response and (B) Electrochemical impedance spectroscopy of the pure g-C3N4 and g-C3N4/Ni-P-3% samples.
3.6 PL spectra Photoluminescence measurement is one of useful method to analyze the recombination of the photogenerated carriers. PL spectra of the pure g-C3N4 and g-C3N4/Ni-P-3% excited by light of 280 nm wavelength, show a strong broad peak at about 460 nm (Fig. 8). And after Ni-P modification, the PL intensity of g-C3N4 decreases dramatically. The excess energy of the excited electron may be consumed via three routs: photoluminescence, e--h+ pair recombination, and thermal dissipation. However, because of the proper relative position of the energy bands between Ni-P and g-C3N4, the adsorbed Ni-P particle offers itself as a mediated storage for the excited electron, decreasing the probability of e--h+ pair recombination. So, the weaker peak intensity of PL means that Ni-P modification decreases the recombination rate of photogenerated e--h+ pairs [55]. More clearly, in the g-C3N4/Ni-P composites, the Ni-P NPs are strongly adsorbed on the g-C3N4 surface, so that the surface Ni-P NPs can capture these photogenerated electrons and enhance
the interfacial charge transfer form g-C3N4 to water molecule, resulting in the effective decrease of the recombination rate of photogenerated e--h+ pairs. This improvement of photogenerated carrier separation efficiency leads to the increase of eand h+ pairs joining in the g-C3N4/Ni-P photocatalytic process. Thus, the appropriate loading of Ni-P on g-C3N4 has a potential application for improving the photocatalytic water splitting into H2 production.
Fig. 8. Photoluminescence spectra of pure g-C3N4 and g-C3N4/Ni-P-3% samples excited at 375 nm.
3.4. Photocatalytic mechanism As shown in Fig. 9, a possible photocatalytic reaction mechanism for g-C3N4/Ni-P photocatalytic H2 generation is proposed. Under simulated sunlight, g-C3N4 is excited to generate charge carriers, the electrons are excited to CB and holes to VB. Due to the Ni-P cocatalyst has a large work function [70], it is like a electron pool, the photoexcited electrons easily transfer to Ni-P surface from CB of g-C3N4, and decompose the water. Meantime, the holes leaved at the VB of g-C3N4 rapidly are
captured and neutralized by the sacrificial agent of TEOA. Clearly, Ni-P co-catalyst accelerates separation of photoexcited charge carries and prolong the life time of electrons, thus enhanced the photocatalytic activity of g-C3N4. However, without the use of cocatalysts, in the pure g-C3N4, the excited e- and h+ will quickly recombine, resulting in poor photocatalytic H2 evolution. Also, the overloading of Ni-P is no good for improving the photocatalytic activity, because of the shielding effect, that is, the excessive Ni-P NPs on the g-C3N4 surface block the light absorption, and the efficiency sunlight utilization will be reduced.
Fig. 9 Schematic diagram for the photocatalytic H2 generation using g-C3N4/Ni-P as photocatalysts.
Conclusions In conclusion, g-C3N4/Ni-P with different loading amount of Ni-P was successfully prepared based on a two-step, pyrolysis and electroless, plating method, the g-C3N4/Ni-P composites show a high activity in photocatalytic H2 evolution. In this preparation method, Ni-P is in situ growth on the g-C3N4 surface, and the combination
of Ni-P catalyst and g-C3N4 is tight and firm. After loading Ni-P, the g-C3N4 shows an increased rate of hydrogen production. Among these prepared materials, the highest H2 generation rate is 1051 μmol g-1h-1 over g-C3N4/Ni-P-3%, which is higher than that of the same amount of Pt loading (841 μmol g-1h-1). The photocatalytic activity in H2 generation reaction of g-C3N4 is enhanced by loading Ni-P NPs, because the Ni-P NPs promote the transfer of electrons and accelerate the separation of the e--h+ pairs. This work report a simple method to synthesize high activity Ni-P/g-C3N4 photocatalyst, hoped for developing the high activity photocatalysts for hydrogen production.
Notes The authors declare no competing financial interest.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51602207, 91545117), 863 Program, (2015AA034603), the Doctoral Scientific Research Foundation of Liaoning Province (201601149, 20170520011), and Project of Education Office of Liaoning Province (LQN201712).
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Graphical abstract
TOC
Highlights
Highlight: (1) Electroless plating method is used to prepare g-C3N4/Ni-P composites. (2) g-C3N4/Ni-P shows high activity in photocatalytic H2 evolution. (3) g-C3N4/Ni-P heterojunctions significantly promote the charge separation.