Carbon Nanotube-supported Cu3P as high-efficiency and low-cost

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Carbon Nanotube-supported Cu3P as high-efficiency and low-cost cocatalysts for exceptional semiconductor-free photocatalytic H2 Evolution Rongchen Shen, Jun Xie, Yingna Ding, Shu-yuan Liu, Andrzej Adamski, Xiaobo Chen, and Xin Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05185 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Sustainable Chemistry & Engineering

Carbon Nanotube-supported Cu3P as high-efficiency and low-cost cocatalysts for exceptional semiconductor-free photocatalytic H2 Evolution Rongchen Shen,a,b Jun Xie,a,b Yingna Ding,b Shu-yuan Liu,c Andrzej Adamski,d Xiaobo Chen,e* Xin Lia,b* a

College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants

Resource and Utilization, Ministry of Agriculture, South China Agricultural University, Guangzhou 510642, PR China b

College of Materials and Energy, South China Agricultural University, Guangzhou

510642, PR China c

Department of pharmacology, Shenyang Medical College, Shenyang, 110034, PR

China. d

Jagiellonian University, Faculty of Chemistry, Ingardena 3, PL 30-060 Cracow,

Poland. e

Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO, 64110, USA.

*Corresponding

author at: College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, P. R. China. Tel.: +86 20 85282633; fax: +86 20 85285596. E-mail address: [email protected] (X. Li), [email protected] (X. Chen).

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ABSTRACT Developing an inexpensive and high-efficiency hydrogen-production cocatalyst to replace the noble metal Pt remains a big challenge in the fields of sustainable photocatalytic hydrogen evolution. Herein, we report the exploration of a high-efficient binary noble metal free Cu3P-CNT H2-evolution cocatalyst by direct high-temperature phosphatizing of Cu(OH)2-CNT. Impressively, combining the advantages of noble metal free Cu3P and carbon nanotube (CNT), the binary Cu3P-CNT cocatalysts show high-efficient photocatalytic H2 evolution in Eosin Y(EY)-contained semiconductor-free photocatalytic systems. The maximum visible-light H2-generation rate for promising EY-Cu3P-CNT systems was 17.22 mmolg-1h-1. The highest apparent quantum efficiency (AQE) could reach 10.23% at 500 nm. More importantly, we found that the separation of photogenerated electrons and holes in the Eosin Y, the efficiency of electron transfer from EY to the active edge sites of Cu3P, and the electrocatalytic H2-evolution activity of Cu3P, could be simultaneously boosted via readily adding the conductive CNT, thus achieving the significantly improved photocatalytic H2 evolution. This work provides a simple and facile strategy to design highly efficient semiconductor-free photocatalytic proton-reduction systems using high-activity transition metal phosphides (TMPs) and inexpensive carbon nanomaterials. KEYWORDS: Photocatalytic Hydrogen Evolution, noble metal-free Cu3P Cocatalysts, Solar Fuel, Carbon nanotube (CNT), Dye sensitization.

Introduction Hydrogen, as an ideal replacement for fossil fuels, has long been recognized as one of the most promising, renewable and environmental fuels.1,2 The photocatalytic water splitting for hydrogen production, as a cost-effective and environmentally hydrogen-production way, has gained international interest in recent years.3,4 Previous work had fully confirmed that noble metal Pt could serve as an efficient cocatalyst for photocatalytic HER. However, its application was limited by its expensive price and scare abundance. Thus, it is of great urgency to


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exploit high-performance and earth-abundant cocatalysts toward HER.5-7 To date, many no-noble metal cocatalyst such as NiSx8-11 MoS212-18, Ni3C19,20, Ni(OH)x21-24 and WS225 had been studied. Particularly, transition metal phosphides (TMPs) have received lots of attention due to their merits of reversible binding and favorable dissociation of H2. Various earth-abundant TMPs, including NiPx26-29, CoPx30-32, CoNiPx33 and FeP34,35, have been shown to be the potential and appealing cocatalysts for photocatalytic HER applications.36 However, finding the really efficient and active TMPs-based photocatalytic hydrogen-production systems remains a great challenge. Noteworthily, integrating various nanostructured semiconducting photocatalysts (i.e, CdS,5,37-40 ZnxCd1-xS,41,42 g-C3N47,26-30,34, TiO231 and their composites43) and TMP cocatalysts (i.e., NiPx, CoPx, FePx and CoNiPx) have been extensively considered a promising and effective strategy toward improving the photocatalytic HER.44 For instance, in our previous work, we found that g-C3N4 could achieve a significant improvement in the photocatalytic activity by loading Co2P or Ni12P5 as a cocatalyst.27,36 Furthermore, Co2P and single Co1-P4 site have been found to act as a cocatalyst to boost the photocatalytic performance of g-C3N4.30 Cu3P as an effective electrocatalyst has been widely employed in electrocatalytic hydrogen evolution.45-47 In principle, Cu3P has been extensively employed as an efficient cocatalyst to modify semiconductor photocatalysts for photocatalytic HER.6,48,49 However, to the best of our knowledge, there has been no report on TMPs as the cocatalysts to construct the novel semiconductor-free hybrid systems for highly efficient photocatalytic H2 evolution. Compared with semiconductor-containing photocatalysts, semiconductor-free photocatalytic systems using the dye as light harvester could exhibit many disadvantages, such as easy fabrication, low cost, avoided light shading effects, large number of charge-transfer channels and interfaces, and the perfect focus on the design and optimization of active cocatalyst sites. As well known, the rapid shift of photogenerated charge is an important factor for efficient photocatalytic HER.50 Typically, in our previous work, the metallic nanocarbon materials with the excellent conductivity could serve as a conductive



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additives to effectively improve photocatalytic hydrogen-generation performance for the earth-abundant cocatalysts over the semiconductor photocatalysts.51 In general, the metallic carbon nanomaterials could accelerate the transfer of photogenerated electrons, thus resulting in an enhancement of the photocatalytic hydrogen-production

activity

and

stability

over

the

earth-abundant

cocatalysts.22,50,52-54 Inspired by these works, we aimed to improve the photocatalytic H2-evolution activity of TMP cocatalysts by loading a conductive CNT. In this way, CNT could induce the transfer of photogenerated charges from the excited dye to the surface of TMP due to its excellent conductivity. Thus result in an enhancement of the photocatalytic H2 evolution. Herein, strongly inspired by these meaningful works, we report for the first time on rationally integrating the noble metal-free Cu3P-CNT cocatalyst and Eosin Y dye in one semiconductor-free system for high-efficient photocatalytic H2 generation. The high-efficient binary noble metal free Cu3P-CNT H2-evolution cocatalyst was synthesized by high-temperature phosphatizing of Cu(OH)2-CNT. The Cu3P-CNT sensitized by Eosin Y (EY) displayed an excellent photocatalytic hydrogen-production performance. The optimal H2-generation rate of the photocatalysts in a 15% triethanolamine (TEOA) aqueous solution could reach 17.22 mmol g-1h-1. A high AQE of 10.23% over EY-Cu3P-CNT was achieved at 500 nm illumination. Experimental section Synthesis of Cu3P-CNT Cu3P were prepared by our previous work, first copper nitrate and NaOH were mixed by magnetic stirring for 0.5 h to obtain the Cu(OH)2. The Cu3P was synthesized by heating Cu(OH)2 and NaH2PO2 at 300 ℃ (3 ℃ min-1) for 1h. The Cu3P-CNT was prepared by adding the CNT into the copper nitrate solution and sonicating for 2 h. Then add the NaOH in to the previous slurry to obtain Cu(OH)2CNT. The Cu3P-CNT was synthesized by heating Cu(OH)2-CNT and NaH2PO2 at 300 ℃for 1h. The mass ratio of Cu(OH)2 to CNT was denoted as Cu3P-X%CNT. Characterizations


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XRD (Cu Kα, MSAL-XD2 diffractometer), TEM and HRTEM (JEM-2100HR 200 kV, Japan), photoluminescence (PL) spectra (LS 50B Perkin Elmer) were done according to our previous methods. 6,32 Photocatalytic and electrocatalytic reaction procedures The photocatalytic H2 production experiments were carried out in a 100 ml three-neck Pyrex flask. 0.05 g of Cu3P-CNT binary composites and 18 mg EY were sonicated in 80 ml 15 vol% TEOA aqueous solution. Before illuminating with a 350 W Xe lamp (PLS-SXE300, Beijing Perfect Light Technology Co., Ltd), the reactant solution was bubbled with N2 for 30 min. The produced hydrogen was determined using a GC-9500 chromatograph. The following equation 1 was used to calculate the AQE. < 𝝉 >=

𝒕𝒉𝒆 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒉𝒚𝒅𝒓𝒐𝒈𝒆𝒏 𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒆 𝒕𝒉𝒆 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒊𝒏𝒄𝒊𝒅𝒆𝒏𝒕 𝒑𝒉𝒐𝒕𝒐𝒔

∗ 𝟏𝟎𝟎%

(1)

The electrocatalytic H2 production reactions were also same as our previous work.5,6,32,52 Results and discussion The structures and compositions of photocatalysts Figure 1 displays the XRD patterns of prepared samples. None difference could be found between pure Cu3P and Cu3P-3%CNT. Three primary peaks at 35.4°, 44.1° and 45.6° are attributed to the (102), (103) and (110) plane diffraction for hexagonal Cu3P (PDF#25-0302). Furthermore, the intense peak at around 24° was attributed to amorphous carbon22. Obviously, three main diffraction peaks do not show a significant change, which indicates the crystal structure of hexagonal Cu3P still remains untouched after loading CNT.



Figure 1. The XRD patterns of CNT, Cu3P and Cu3P-CNT The structure and morphology of the as prepared Cu3P-CNT were further investigated by TEM and HRTEM. As displayed in Figure 2A and B, CNT is comprised of nanotubes with diameters range from 30 to 50 nm. For the HRTEM images (Figure 2C and D), the hexagonal Cu3P exhibits an interlayer distance of 0.204 nm and 0.252 nm, corresponding to the (110) and (102) plane of hexagonal Cu3P. Notably, there is not lattice fringe corresponding to CNT could be found in the HRTEM images due to the amorphous structures of CNT.53 The TEM and HRTEM results clearly show that CNT were deposited on the surface of the Cu3P. Additionally, elemental maps (Figure 3A-D) displayed a uniform spatial distribution of C, Cu and P species. The profiles revealed the coexistence of Cu3P and CNT.


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Figure 2. (A) and (B) TEM images of Cu3P-3%CNT sample; (C) and (D) HRTEM of Cu3P-3%CNT sample.

Figure 3. (A) FESME and (B-D) the corresponding elemental mapping of C, Cu and P elements Cu3P-3%CNT sample.



Figure 4. (A)XPS survey spectra, (B)-(D) high-resolution of XPS spectra of C 1s, Cu 2p and P 2p region for Cu3P-3%CNT composites.

Figure 5. high-resolution of XPS spectra of (A) Cu 2p and (B) P 2p region for Cu3P and Cu3P-3%CNT composites.

To future understand the chemical bonds of the Cu3P-CNT, X-ray photoelectron spectra (XPS) of Cu3P-CNT were analyzed. As shown in Figure 4A. The binary Cu3PCNT mainly consisted of P, C, O and Cu, which consists with the elemental mapping results. Notably, as observed in Figure 4B, one peak of the C=C bonds at around 284.4 eV, which was often used to represent the standard reference carbon. Furthermore, for


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high resolution Cu 2p spectra (Figure 4C), two peaks of Cu 2p3/2 and Cu 2p1/2 at around 932.6 eV as well as 952.8 eV, which are attributed to Cu3P. The peaks at 935.1 eV (Cu 2p3/2), 940.9 eV and 943.1 eV (Cu1+ satellite) as well as 954.8 eV (Cu2+ 2p1/2) are corresponding to oxidized copper Cu(II) due to the surface oxidation

probably.47,55

Additionally, as showed in Figure 4D, the characteristic peaks at 129.2 eV, 130.1 eV and 133.4 eV can be supposed to the phosphide species (P 2p3/2 and P2p1/2) and oxidized species (P2O5 or PO43-), respectively. Furthermore, in Cu3P, the Cu 2p3/2 (932.6 eV)

is

positively shifted from that of metallic Cu(932.3 eV), while the P 2p3/2 (129.2 eV) has a lower binding energy than red P (130.0 eV), indicating that the Cu possess positive charge but the P possess negative charge.45 As showed in Figure 5A, compared to the Cu 2p3/2 binding energy in the pure Cu3P, the binding energy of Cu 2p3/2 in the Cu3P3%CNT was positive to 932.6 eV. Furthermore, the binding energy of P 2p3/2 and P2p1/2 in Cu3P-3%CNT also showed a positive shift compared to the P 2p3/2 and P2p1/2 in Cu3P (Figure 5B). These results suggest that there is a strong interaction between Cu3P and CNT, which was from the co-electron cloud formed between Cu3P and CNT.56 The XRD, TEM, HRTEM, EDS mapping and XPS results suggest that the CNT were successfully loaded on the surface of Cu3P and CNT have little effect on the crystal structure of the Cu3P. The activities and stabilities of photocatalysts The HER occurs on the surface of catalysts via the Volmer reaction.57 C + e- + H2O→C-Hads + OH-

(2)

followed by an electrochemical desorption via the Heyrovsky reaction58 H2O + e- + C-Hads→C+ OH + H2

(3)

Where C is the cocatalyst and Hads is H-adsorbed state. The as-prepared Cu3PCNT binary cocatalysts were carried out for hydrogen evolution in 15% TEOA and EY solution under visible light. The results showed that CNT play an important role in enhancing the photocatalytic hydrogen evolution activity. Figure 6A shows the comparison of the photocatalytic hydrogen generation performance over Cu3PCNT and pure Cu3P. The corresponding average H2-evolution rates were calculated



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and shown in Figure 6B. It could be found that an appropriate amount of CNT could significantly improve the photocatalytic hydrogen generation over the Cu3P cocatalyst. For Cu3P cocatalyst alone, a hydrogen evolution rate of 7.44 mmolh1g-1

can be achieved. When loading an appropriate amount of CNT as the additives,

Cu3P-3%CNT shows a hydrogen-generation rate of 17.22 mmolh-1g-1, which was 2.5-folder enhanced compared to the pure Cu3P. Notably, the binary Cu3P-CNT cocatalyst displayed an increase of photocatalytic performance when the loading contents of CNT are increased from 0.5 to 3 wt%, giving an average hydrogen evolution rate of 8.99 to 17.22 mmolh-1g-1. On the contrary, an obvious decrease of photocatalytic hydrogen-evolution performance was observed when the content of CNT is higher 3%. This result could be explained by follows: excessive amount of CNT may lead to decrease the surface-active sites of the Cu3P.22,50,54,59 More importantly,

compared

with

previously-reported

system,

although

the

photocatalytic performance over this EY-Cu3P-CNT is lower than that of THPP/Pt system (19.5mmolh-1g-1)60, its performance is better than the previously reported photocatalytic system for TPPH/RGO/Pt system (11.2 mmolh-1g-1)61, ZnTCPPMoS2/TiO2 system (0.102 mmolh-1g-1)62 and [ZnTMPyP]4+−MoS2/RGO system (2.56 mmolh-1g-1)63. These results suggested that the Cu3P-CNT could be served as a potential cocatalysts for HER. The stability of the photocatalyst was an important criterion. The stability of the EY-Cu3P-CNT photocatalyst was measured by repeating experiments were shown in Figure 7. The stability performance of Cu3P-3%CNT was tested over four cycles. In first cycle, the amount of hydrogen evolution was 34.7 mmol. After renewing the TEOA and EY, the hydrogen production performance was measured in the next cycle. The H2-evolution performance has fallen by about 20 percent after four cycle. This phenomenon is caused by the separation of the Cu3P from CNT during the photocatalytic process. Additionally, the AQE of EY-Cu3P-CNT system have also measured under 500nm. The highest AQE could reach as high as 10.23%.


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Figure 6. (A) Time profiles of photocatalytic hydrogen generation and (B) the average hydrogen-generation rate of (a) Cu3P (b) Cu3P-0.5%CNT, (c) Cu3P1%CNT, (d) Cu3P-3%CNT, (e) Cu3P-5%CNT and (f) Cu3P-10%CNT.

Figure 7. Repeated cycles of hydrogen production with Cu3P-3%CNT The mechanism investigation



Figure 8. (A) Steady state PL spectra and (B) impedance spectroscopy for different composites. Here, the loading of CNT could improve the migration and transfer of the photogenerated electron from EY to Cu3P cocatalyst. To confirm this, the photoluminescence (PL) spectra had been measured. It can be found that CNT loading could improve photocarrier separation compared to the pure Cu3P. From Figure 8A, the EY solution showed a typical emission peak at around 540 nm. With the incorporation of CNT into the Cu3P, the fluorescence emission of Cu3P was decreased obviously. It has been further confirmed that CNT could effectively improve the photogenerated-electron transfer in Cu3P-CNT binary photocatalyst. The role of CNT in the separation of hole-electron pairs could be further confirmed by using electrochemical impedance spectroscopy (EIS). As showed in Figure 8B, the diameter of the EIS spectrum for Cu3P-3%CNT was smaller than that of other samples. This result suggests that the lower electron-transfer resistance and recombination rate of charges in Cu3P-CNT-EY due to the introduction of CNT51. This EIS result is consistent with the result of the PL emission spectroscopy.


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Figure 9. Polarization curves of different composites. The HER polarization curve is fit for analyzing the underlying photocatalytic H2-evolution mechanism. As displayed in Figure 9, the HER polarization curves of Cu3P, CNT and CNT-Cu3P composite photocatalysts. The cathodic current ranging from −0.4 to −0.8V vs RHE is dominantly attributed to the electrocatalytic H2 evolution. Obviously, electrocatalytic hydrogen-production overpotentials of the CNT-Cu3P sample exhibits much lower than Cu3P and CNT. A much lower overpotential at same condition for Cu3P-CNT suggests that CNT with high electrical conductivity and poor electrocatalytic HER performances could effectively separate the photogenerated holes and electrons as well as improve the hydrogen-generation kinetics during photocatalytic reaction. Therefore, Cu3PCNT exhibit a higher photocatalytic hydrogen-production activity in photocatalytic HER.



Scheme 1. The possible photocatalytic H2 production mechanism. Based on compression with previous work, as showed in Figure 9A, the work function of singlet state energy of excited EY (-2.97 eV vs. AVS.)64 is higher than the CB of Cu3P (-3.8 eV vs. AVS)48. As Cu3P contact with EY, electrons move from excited EY to Cu3P is thermodynamically favorable. More important, The CB of Cu3P is higher than the energy level for hydrogen production from water. (-4.50 eV vs. AVS).65 Thus, the reaction process of photocatalytic hydrogen generation in ternary EY-Cu3P-CNT systems could be described in Scheme 1. During the light absorption, EY were excited at a state of 1*EY form. Then the 1*EY transform to triplet excited states 3*EY quickly by an intercrossing transition. Next, the 3*EY were reduced to EY•− by TEOA66. EY was combined with the CNT with the noncovalent π−π interactions. The electrons were input from the EY•− to the CNT.56,67 Lastly, electrons were transferred to the surface of


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Cu3P. Interestingly, the loading of CNT on the surface of Cu3P could trap more electrons from photoexcited EY. Based on previous report, p atoms on the one hand can trap the protons, and on the other hand provide high performance for the separation of hydrogen.68 Wang and his coworkers indicated that P atoms on the P terminated surface of (001)-MoP could combine hydrogen at low coverage whilst desorbed hydrogen at high coverage.69 Based on the above conclusions, we forecast the hydrogen-evolution active sites of Cu3P catalyst is P atoms on the P terminated. Furthermore, the boosted electron transform from the surface of the CNT to the surface of the Cu3P could drive the more efficient photocatalytic hydrogen generation. These results demonstrate that dye-sensitization of Cu3P-CNT cocatalyst provides a new idea to construct nanocarbonbased photocatalysts for photocatalytic HER. Conclusions In this work, the Cu3P-CNT cocatalyst had been successful fabricated by a one-step phosphatizing Cu(OH)2-CNT composite. The as-prepared cocatalyst shows a excellent photocatalytic hydrogen evolution activity in a EY sensitized photocatalytic system. The maximum hydrogen-generation rate of the photocatalysts could reach 17.22 mmol g-1h-1 under visible light. The highest apparent quantum efficiency (AQE) could reach 10.23% at 500 nm. The CNT could act as an admirable electron acceptor and transporter for enhancing the charge separation, thus resulting in a great improvement in photocatalytic HER performance. we believed that the dye-sensitization low-cost cocatalyst and nanocarbon materials strategy will provide a fresh idea to construct a superefficient non nobol metal cocatalyst for photocatalytic water splitting HER. Acknowledgements X. Li would like to thank National Natural Science Foundation of China (51672089), Special funding on Applied Science and technology in Guangdong (2017B020238005) and the State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology) (2015-KF7) for their support. X. Chen appreciates the financial support from the U.S.



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The Carbon Nanotube-supported Cu3P as high-efficiency and low-cost cocatalysts could significantly boost the photocatalytic H2 evolution in Eosin Y(EY)-contained semiconductor-free systems.


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Carbon Nanotube-supported Cu3P as high-efficiency and low-cost

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