Feng Guo*,Xiliu Huang,Zhihao Chen,Haoran SunLizhuang Chen,*

1 School of Energy and Power,Jiangsu University of Science and Technology,Zhenjiang 212003,China

2 School of Environmental and Chemical Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,China

Keywords:CoP nanoparticles CoO octahedrons Overall water splitting Catalyst Photochemistry Solar energy

ABSTRACT Designing an effective and stable composite photocatalyst is of significance for the further realization of practical applications.In this study,a series of CoP/CoO composites are successfully prepared by a straight one-step phosphating method.The reasonable design and controllable preparation of CoP/CoO composite make it exhibit improved photocatalytic performance for overall water splitting and excellent stability under visible light irradiation in comparison with pure CoO,which is derived from the CoP nanoparticles well dispersed on the (111) facets of CoO octahedrons,intimate interface between them and efficiently accelerated of photo-induced electrons from CoO to CoP.This study presents a simple method to design highly-effective composite photocatalysts for overall water splitting to meet the energy demand.

1.Introduction

Photocatalytic water splitting is regarded as a promising and sustainable strategy to alleviate the dual pressure of environmental pollution and energy crisis,which can directly use sunlight to split inexhaustible water resources into H2and O2in the absence of any other energy supply,showing significant potential for large-scale practical applications [1,2].To explore efficient photocatalysts with low cost,suitable band gaps and excellent durability still remains a huge challenge,which is of huge requirement for the future ecology and economy.Besides,this photocatalyst should also be equipped with the following three stringent prerequisites:(i) a large positive change in the Gibbs free energy (ΔGo=238 kJ·mol-1) or 1.23 eV owing to the ‘‘uphill reaction”of overall water splitting [3];(ii) the more negative bottom level of conduction band than the reduction potential of H+/H2energy level (0 Vvs.NHE)and the more positive top level of valence band than the oxidation potential of O2/H2O energy level(1.23 Vvs.NHE),satisfying the two half-reactions [4-6];(iii) the larger band gap (at least 2.0 eV) for overall water splitting in view of the existence of overpotentials for H2and O2production on semiconductor surfaces[7].

Recent years,Cobalt (II) Oxide (CoO) as a photocatalyst has drawn increasing attention since Baoet al.firstly revealed nanocrystalline CoO can achieve overall water splitting with a solar-to-hydrogen(STH)efficiency of about 5%by merits of its suitable band structure,high economic benefit and low productive cost[8].However,the complex synthetic method,rigorous synthetic condition,and an extremely short lifetime (only 1 h) severely restrict its practical application.In order to relieve the above problems,a series of strategies have been put forward to improve its stability and simplify its synthetic steps and requirements [9-11].For example,octahedral CoO with the exposed (111) facet has been successfully preparedviaa simple solvothermal method by our previous work,which can achieve overall water splitting with its moderate band gap (～2.5 eV),suitable conduction band(-0.1 eV) and valance band (2.4 eV) [12,13].Nevertheless,it exhibits very lower efficiency and unsatisfactory stability owing to the fast recombination and photothermal effect,damaging the (111)active facet of octahedral CoO.It is reported that utilization cocatalysts to modify semiconductors can improve remarkably its photocatalytic performance,which is ascribed to the effective inhibition recombination of photo-induced charges and lower the overpotential of hydrogen evolution[14-16].Therefore,in our previous reports,choosing the particular materials (carbon dots and MoS2) as cocatalysts to anchor onto the surface of octahedral CoO,preventing the CoO inactivation and improving its photocatalytic activity [17,18].Although great progress has been made to some extent,its photocatalytic activity still needs to be further improved from the point of view of application.

Transition metal phosphides,possessing metalloid characteristics and good electrical conductivity,have aroused comprehensive interest in electrocatalytic and photocatalytic field [19,20].Especially,CoP with the higher conductivity and lower overpotential in the transition metal phosphides has been proven to act as a co-catalyst to promote the photocatalytic H2evolution reactions,such as CoP/CdS [21],CoP/g-C3N4[22] and CoP/Zn0.5Cd0.5S [23],which stimulates us to utilize CoP as a co-catalyst modified octahedral CoO to boost its photocatalytic performance.Meanwhile,owing to the fact that the instability of octahedral CoO with the(111) surface facet is attributed to the alternation of Co2+cations and O2-anions from one plane to another,that is,the phase of CoO is prone to the Co3O4,thus it is a crucial issue to stabilize the facet of octahedral CoO.Fortunately,the combination of CoP and CoO can tend to form the O-P bond on the interface of CoO/CoP composite,which not only enhances the conductivity and electron transfer,but also effectively prevents the phase transition from CoO to Co3O4[24].Given the prominent advantages of CoP,it can be assumed that CoP will be a potential co-catalyst to modify CoO for the improved photocatalytic activity and stability.Nevertheless,as far as we know,no relevant report of CoP/CoO composite for photocatalytic water splitting has been reported.

Herein,we prepared CoP/CoO compositesviaa straightforward one-step phosphating method,which the CoP nanoparticles with different amount tightly anchor onto the (111) surface of octahedral CoO for photocatalytic overall water splitting from pure water under visible light irradiation.The H2/O2production rate of 3%CoP/CoO (43.4 μmol·g-1·h-1and 20.8 μmol·g-1·h-1) is almost 6.78 times higher than that of the pristine CoO (6.4 μmol·g-1·h-1and 3.3 μmol·g-1·h-1).And CoP/CoO composite possesses admirable stability.The reasonable design and simple synthetic method of composite is of significance not only for the further enhancement of the photocatalytic activity,but also for consolidating the stability of the CoO.

2.Experimental

2.1.Materials

Co(CH3COO)2·4H2O,ethanol,Co(NO3)2·6H2O,sodium citrate,NaOH,NaH2PO2were purchased from Sinopharm Chemical Reagent Co.,Ltd andn-octanol was purchased from Shanghai Macklin Biochemical Co.,Ltd.The reagents and solvents used were of analytical reagent grade and used without further purification.

2.2.Preparation of CoO photocatalyst

The CoO photocatalyst was prepared according to a reported solvothermal method [12].Typically,1.84 g Co(CH3COO)2·4H2O was added to a mixed solvent containing 64 mln-octanol and 16 ml ethanol.After vigorous stirring for 2 h,the formed mixture was heated at 220 °C for 4 h in an autoclave of 100 ml capacity.Finally,the obtained powders was collected,washed with ethanol for several times,and dried at 70 °C overnight.

2.3.Preparation of CoP nanoparticles

CoP nanoparticles were fabricated though a precipitationphosphorus method.Generally,Co(NO3)2·6H2O (200 mg) and sodium citrate (50 mg) were mixed in 100 ml aqueous solution,and then 3 ml NaOH solution(0.5 mol·L-1)was added with stirring was continued for 3.5 h,the precipitates were separated after standing for 3 h.The precipitate was centrifugated and dried in a vacuum oven at 60 °C for 8 h to obtain the Co(OH)2precursor.Afterwards,the mixture of Co(OH)2(50 mg) and NaH2PO2(250 mg) were ground uniformly in an agate.Subsequently,the mixed precursor was annealed at 300 °C for 2 h with a ramping rate of 1 °C·min-1in Ar atmosphere.Finally,the obtained black solid was washed with distilled water and ethanol for several times,and dried under vacuum condition at 60 °C.

2.4.Preparation of CoP/CoO composites

The strategy for the synthesis process of CoP/CoO composite photocatalyst is schematically demonstrated in Fig.1.The synthesis steps are as follows:at first,the mixture of different mass ratios of CoO and NaH2PO2were ground uniformly in an agate.Subsequently,the mixed precursor was annealed at 300 °C for 2 h with a ramping rate of 1 °C·min-1in Ar atmosphere.Finally,the obtained black solid was washed with distilled water and ethanol for several times,and dried under vacuum condition at 60 °C.The CoP/CoO photocatalysts with different mass ratios of CoP(0.5%,1%,3%,5% and 10%) were denoted as 0.5% CoP/CoO,1% CoP/CoO,3%CoP/CoO,5% CoP/CoO and 10% CoP/CoO,respectively.

2.5.Characterization

The morphology of photocatalysts was characterized using scanning electron microscopy (SEM,FEI,Hitachi-S3400N) and transmission electron microscopy(TEM,FEI,Joel/JEM 2100 model).The crystal structure was inspected by a powder X-ray diffractometer (XRD,Rigaku smart Lab) with Cu Kα1 irradiation(λ=0.15406 nm).UV-vis diffuse reflectance spectra (DRS) were obtained on a Shimadzu UV-2450 spectrophotometer,and BaSO4was used as a reflectance standard.X-ray photoelectron spectroscopy (XPS) patterns were measured on a PHI 5000 VersaProbe high-performance electronic spectrometer with MgKα X-rays as the excitation source at 300 W,and all the binding energy values passed C 1s=284.6 eV as reference.Photoluminescence(PL)spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer (Horiba Jobin Yvon) at room temperature,and the wavelength of the excitation light was 325 nm.Brunauer Emmet Teller (BET) surface area was measured using Micrometrics ASAP-2020-HD88 adsorption apparatus with nitrogen adsorption for all samples at 77 K.

2.6.Photocatalytic overall water splitting

The photocatalytic overall water splitting reaction was performed in an online photocatalytic hydrogen production system under an irradiation of 300 W Xe lamp with the UV cut-off filter(λ ＞420 nm).In a typical experiment,50 mg of photocatalyst was dispersed in 100 ml ultrapure water,and magnetically stirred under vacuum.The temperature of solution was maintained around 5°C through a circulating condensation-device.The photocatalytic hydrogen and oxygen evolution rate was obtained by calculating the peak areas of chromatography (GC-7920 gas chromatograph,TCD detector,N2carrier,5A molecular sieve column).In addition,apparent quantum efficiency (AQE) measurements at the wavelengths of 420 nm band-pass filter by using the same steps as in the photocatalytic water splitting experiment.The AQE was calculated by the following formula (1):

Fig.1. Schematic illustration of the preparation process of CoP/CoO composite photocatalyst.

2.7.Photoelectrochemical measurements

Photocurrent response and electrochemical impedance spectroscopy (EIS) measurements of photocatalysts were performed on an electrochemical workstation (CHI 660D,Shanghai Chen Hua Instrument Company,China) by using a working electrode,a platinum foil counter electrode and an Ag/AgCl reference electrode battery in 0.1 mol·L-1Na2SO4solution.A working electrode was prepared on indium tin oxide (ITO) glass,and was ultrasonically cleaned with acetone,ethanol and deionized water for 30 min,respectively.5 mg sample and 30 μl of naphthol were dispersed in 1 ml of deionized water and dissolved by ultrasound to form a homogeneous catalyst colloid.Then,30 μl of colloid was deposited on ITO glass(1 cm×1 cm as effective area),and the uncoated part of the electrode was isolated with epoxy resin.Electrochemical impedance spectra were carried out using a 5 mV sinusoidal AC voltage in the frequency range from 0.01-105Hz,which irradiated with incident visible light (＞420 nm) for photocurrent responses measurements.

3.Results and Discussion

The crystal structure of the as-synthesized CoO,CoP and 3%CoP/CoO composite powders was investigated by XRD patterns(Fig.2).For pristine CoO (blue line),all the diffraction peaks can be exactly matched the face-centered cubic phase CoO (JCPDS No.71-1178) [25].Pure CoP (black line) shows six pronounced diffraction peaks at 31.7° (011),36.5° (111),46.3° (112),48.4°(211),52.3° (103),and 56.6° (301),which are in good agreement with reflections of CoP (JCPDS No.29-0497) [26].While the peak at 40.9° is attributed to (201) reflections of Co2P (JCPDS No.32-0306),which is consistent with previous reports in the literature[27].In the case of 3% CoP/CoO composites,the existence of twophase diffraction peaks of CoP and CoO can be observed in the XRD pattern.Notably,the diffraction peak of CoP is weaker than that of CoO due to the low quantity of CoP.

The morphology of as-synthesized products were explored by SEM,as demonstrated in Fig.3.From Fig.3(a),it could be clearly observed that CoO exhibited regular octahedron structures,and the surface of CoO particles is very smooth with the size of about 300-800 nm[Fig.3(b)].The pure CoP sample presented the overall morphology of the agglomerated nanoparticles as shown in Fig.S1.Noticeably,after the phosphating process,the morphology of the CoO octahedron hardly changed as shown in Fig.3(c).An enlarged image of 3% CoP/CoO composite is illustrated in Fig.3(d),which can be intuitively observed that the surfaces of the octahedron become rough,and the CoP nanoparticles are uniformly dispersed on the surfaces of regular octahedral CoO.The EDS spectrum[Fig.3(e)] displays that the main elements of the 3% CoP/CoO composite are composed of Co,O and P.

Fig.2. XRD patterns of CoP,CoO and 3% CoP/CoO composite with the JCPDS card numbers:71-1178 (CoO),32-306 (Co2P) and 29-417 (CoP).

The morphology of the as-prepared samples was further studied by TEM.As shown in Fig.4(a),pristine CoO exhibited a regular octahedral morphology with a smooth surface,which is agreement with our previous report[10].As demonstrated in Fig.4(b)and(c),the surface of the 3%CoP/CoO composite became rough,indicating that the ultra-fine CoP nanoparticles were successfully anchored on the surfaces of CoO regular octahedrons.From the HR-TEM[Fig.4(d)],it could be easily observed that the lattice fringes with spacing of 0.25 nm and 0.19 nm were obtained which can be attributed to the exposed crystal planes of the obtained CoO was the (111) planes and CoP was the (112) planes,respectively[10,28].Furthermore,the HAADF-STEM image [Fig.4(e)] and the corresponding elemental mapping images were displayed in Fig.4(f)-(h).The results display that Co,O and P elements were distributed in 3% CoP/CoO,which provides a sufficient evidence for the successful formation of CoP/CoO composite photocatalyst.

Fig.3. SEM images of (a,b) CoO,and (c,d) 3% CoP/CoO.(e) EDS pattern of 3% CoP/CoO.

The chemical composition and valence states of the elements over CoO and 3% CoP/CoO were further investigated by XPS and demonstrated in Fig.5.The existence of three elements of Co,O and P can be determined in CoP/CoO composite from the comparison of survey scan in Fig.5(a).For the high-resolution Co 2p spectrum [Fig.5(b),green line)],two typical characteristic peaks were obtained at 779.7 eV and 795.2 eV with weaker peaks (786.5 and 802.6 eV),corresponding to the Co 2p3/2and Co 2p1/2peaks and satellite features,respectively,which proves that Co exists as Co2+in the pristine CoO [8].The peaks at 529.4 eV and 530.8 eV in the O 1s XPS spectrum [Fig.5(c),green line] of CoO correspond to the Co-O bond and hydroxide (OH) in pure CoO phase,respectively[29].Compared with pure CoO,positive shifts of Co 2p peaks(shift to780.1 and 795.4 eV) and O 1s peaks (shift to 529.6 and 530.9 eV) are observed for the 3% CoP/CoO [red lines of Fig.5(b)and(c)],implying that the local charge density of CoO has changed after modification with CoP.In the P 2p spectrum of 3% CoP/CoO[Fig.5(d),orange line],the peak at 129.1 eV belongs to P-Co in CoP,while the peak at 133.8 eV is the oxidized P due to the P-O bonds [19,24].Moreover,the P 2p signal peak was not detected in pure CoO sample [Fig.4(d),green line],which is in sharp contrast to the obvious P 2p characteristic peaks presented in the 3%CoP/CoO composite,further verifying the successful formation of CoP/CoO composite by this simple method of direct phosphating CoO.

In order to investigate the absorption capacity,a comparison of the UV-Vis diffuse reflectance spectra (DRS) of CoO and CoP/CoO composites were recorded in Fig.6.As can be seen,pure CoO exhibits obvious light absorption from ultraviolet to visible light region,and its band gap absorption edge is estimated to be about 490 nm.With the introduction of CoP nanoparticles after phosphating,compared with pure CoO,the absorption edge of binary CoP/CoO composites is effectively extended beyond 500 nm,implying that the composites can absorb more visible light,which in turn helps to enhance the photocatalytic activity.This phenomenon is mainly derived from the intrinsic absorption of black CoP nanoparticles[21].Simultaneously,it can be changed from brown to black according to the color change of the samples (insets of Fig.6),and it is further verified that loading CoP nanoparticles on the surface of CoO octahedrons can improve their visible light harvesting range.

Fig.4. TEM images of (a) CoO and (b,c) 3% CoP/CoO.(d) HR-TEM,(e) HAADF-STEM images and (f-h) the corresponding elemental mapping images of 3% CoP/CoO.

Fig.5. (a) XPS survey spectra of CoO and 3% CoP/CoO.(b) High-resolution XPS spectra of (b) Co 2p,(c) P 2p and (d) O 1s of pristine CoO and 3% CoP/CoO.

Fig.6. UV-vis diffuse reflectance spectra of as-prepared samples.

Fig.7. H2/O2 evolutions from pure water with (a) CoO and (b) 3% CoP/CoO as photocatalysts.(c) H2/O2 evolutions of 3% CoP/CoO with different contents of CoP.(d) The stability of photocatalytic water splitting over 3% CoP/CoO within three hours’ irradiation.The photocatalytic reactions were performed with 50 mg of photocatalyst dispersed in 100 ml of ultrapure water under visible-light irradiation (λ ＞400 nm).

Table 1 Photocatalytic overall water splitting rates and corresponding AQE values of recent reported CoO-based photocatalysts

Photocatalytic overall water splitting reaction of the prepared photocatalysts was tested in the absence of applying a bias potential or any sacrificial agent under visible light irradiation(λ ＞400 nm).It is worth noting that gas cannot be produced without light or catalyst when other conditions remain unchanged,which explains that the H2/O2produced in this study can be considered as photocatalytic water splitting reaction [Fig.7(a) and(b)].Moreover,it can be clearly found that the molar ratios of H2/O2from pure water by pristine CoO and 3%CoP/CoO are almost equal to the theoretical value of 2:1 for the overall water splitting.Significantly,H2/O2production rates of 3%CoP/CoO(43.4 μmol·g-1-·h-1and 20.8 μmol·g-1·h-1) is almost 7 times higher than that of the pristine CoO (6.4 μmol·g-1·h-1and 3.3 μmol·g-1·h-1).Moreover,the photocatalytic performance of CoP/CoO with different CoP content was also investigated,as presented in Fig.7(c).Where photocatalytic activity of CoP/CoO composites is strongly related to the loading amount of CoP.Among them,the CoP/CoO composite with a mass content of 3% CoP (3% CoP/CoO) shows the highest photocatalytic performance for the overall water splitting.With the further increase the mass content of CoP (more than 3%),the H2/O2evolution of CoP/CoO photocatalyst gradually decreased.This phenomenon is mainly ascribed to two reasons as follows:(i) the surplus CoP nanoparticles could cover the reactive sites on the (111) crystal planes of octahedral CoO [30];(ii) the excessive CoP species could seriously hinder the absorption of incident light of CoO octahedrons[30].The apparent quantum efficiency(AQE)of the 3% CoP/CoO composite photocatalyst was also measured and the AQE is calculated to be as high as 1.66% at 420 nm.Furthermore,Table 1 enumerates a comparative investigation of photocatalytic hydrogen production rates and AQE values by representative reported CoO-based photocatalysts,implying that 3% CoP/CoO photocatalyst possesses excellent photocatalytic activity for overall water splitting under visible light without adding any cocatalyst or sacrificial agent.Furthermore,the photocatalytic stability of as-prepared CoP/CoO photocatalyst in a long-term reaction was evaluated by cycle experiments during with quartic visible light irradiation,as shown in Fig.7(d).After four cycles,the amount of H2and O2decreased slightly,which could be attributed to the loosened interface connection between co-catalysts and basal photocatalysts [34].In order to further study whether the structure of the sample changed after the reaction,the XRD pattern and TEM image of 3% CoP/CoO before and after the reaction were conducted and exhibited in Figs.S2 and S3.The results reveal that the XRD peak intensity and position and morphology of asprepared composite after the photocatalysis have not changed significantly,indicating that the CoP/CoO composite photocatalyst is relatively stable.

The N2adsorption-desorption isotherms of pristine CoO and 3%CoP/CoO composite were measured to investigate the porosity and specific surface area.As depicted in Fig.8(a),the typical type IV isotherm with H3-type hysteresis loop based on the IUPAC classification,demonstrated the presence of micropores in the CoP/CoO composite [19].The specific surface area of the 3% CoP/CoO composite (2.34 m2·g-1) is significantly larger than that of pure CoO(0.67 m2·g-1).It can conclude that the CoP nanoparticles were modified on the octahedral CoO after phosphating,which increased the corresponding surface area and then provided more active sites to promote the photocatalytic activity.PL spectroscopy is a common method to study the separation and recombination of electron-hole pairs produced by photocatalysts [35,36].The lower the PL intensity,the lower the recombination efficiency of the photoelectron-hole pairs,which determines the higher photocatalytic performance [30,37,38].The PL spectra of CoO and 3% CoP/CoO photocatalysts were tested at an excitation wavelength of 325 nm,as demonstrate in Fig.8(b).Obviously,3% CoP/CoO composite shows significantly weaker PL emission peaks around 470 nm compared to bare CoO,which indicates that the introduction of CoP nanoparticles on the surface of CoO octahedrons can effectively decelerate the recombination rate activity of photoinduced electron-hole pairs.Fig.8(c) shows the photocurrent response curves of CoO and 3% CoP/CoO samples under intermittent visible-light irradiation.As can be seen,3%CoP/CoO displayed a higher photocurrent response than CoO,which indicates that the photo-induced charge separation and transfer processes are more effective in composites.Moreover,in order to reveal the charge separation efficiency of the prepared photocatalyst,electrochemical impedance spectroscopy (EIS) measurement was also performed as depicted in Fig.8(d).It is well known that a smaller arc radius means more efficient separation of photogenerated electron-hole pairs [39-43].It can be clearly seen that the arc radius of the 3%CoP/CoO electrode is smaller than CoO,indicating that the 3% CoP/CoO composite enables the photogenerated electron-hole pairs to effectively separate and migrate,which is consistent with the photocurrent response results.

In light of the above characteristic analysis and experimental results,a possible photocatalytic mechanism for the overall water splitting of the CoP/CoO composite system can be proposed,as shown in Fig.9.The band gap energy (Eg,eV) is determined based on the Tauc formula (2) [44]:

where α,h,ν andAis absorption coefficient,Planck constant,light frequency and a constant,respectively.According to the equation,theEgof octahedral CoO was estimated to be 2.50 eV [Fig.S4(a)].Fig.S4(b) exhibits the XPS valance spectrum of CoO,which can be clearly seen that the VB maximum is 2.27 eV.Due to the contact potential difference between the material and analyzer,the VB position of CoO is calculated to be 2.13 Vversusnormal hydrogen electrode (NHE) at pH 7 using the formula (3):

whereENHErepresents the potential of normal hydrogen electrode(eV),Φ is the electron work function of the analyzer of 4.30 eV andEVLis potential of vacuum level (eV).In addition,the corresponding CB value of the CoO sample can be calculated by the Eq.(4) [45]:

Fig.8. (a) Nitrogen adsorption isotherms and measured parameters,(b) PL spectra,(c) photocurrent response and (d) EIS spectra of CoO and 3% CoP/CoO.

Fig.9. Proposed mechanism for visible-light photocatalytic overall water splitting over CoP/CoO composite photocatalyst.

Thus,theECBvalue of CoO is calculated to be-0.37 eV.In view of the measured band positions above,the possible mechanism of photocatalytic H2and O2evolutions by pure water over CoP/CoO composite photocatalyst is described as below.Under visible light irradiation,CoO can be excited to generate electrons and holes.The photoinduced electrons will be captured and transferred from the CB of CoO to CoP,which will be used to react with water to generate H2,while the holes will remain on the VB of CoO.Then,the holes on the surface of CoO will react with H2O to generate O2,which derives from the more positive VB of CoO (2.13 eV) than O2/H2O (1.23 eV).Due to the introduction of CoP nanoparticles,the recombination rate of photogenerated carriers over CoO octahedrons is inhibited,thus CoP/CoO photocatalysts exhibit remarkably photocatalytic H2and O2evolution activity in visible-lightdriven overall water splitting.

4.Conclusions

In summary,the efficient and stable CoP/CoO composite photocatalysts were constructedviaa one-step phosphating strategy for photocatalytic overall water splitting under visible light irradiation.Experimental results reveal that the as-prepared CoP/CoO composites exhibit not only the outstanding photocatalytic activity,but also the excellent stability.This is ascribed that CoP nanoparticles were averagely dispersed on the surface of CoO octahedrons to form intimate interfaces,which not only can be used as co-catalysts to enhance the separation of the electron-hole channels,but also can alleviative CoO photo-corrosion deriving from the strong P-O bond between CoO and CoP.Therefore,it is affirmed that the one-step synthesis of CoP/CoO composites may be an advisable choice for develop highly efficient photocatalyst for overall water splitting.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No.21906072 and 21671084),the Natural Science Foundation of Jiangsu Province (BK20190982),Jiangsu 333 talents project funding(BRA2018342),Jiangsu provincial government scholarship for overseas studies,the Doctoral Scientific Research Foundation of Jiangsu University of Science and Technology (China) (1142931803).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.030.