Activating KlÃ¤ui-type Organometallic Precursors at Metal Oxide

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Letter

Activating Kläui-type Organometallic Precursors at Metal Oxide Surfaces for Enhanced Solar Water Oxidation Xiaokang Wan, Lu Wang, Chung Li Dong, Gabriel Menendez Rodriguez, Yu-Cheng Huang, Alceo Macchioni, and Shaohua Shen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00847 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Energy Letters

Activating Kläui-type Organometallic Precursors at Metal Oxide Surfaces for Enhanced Solar Water Oxidation Xiaokang Wan,#,1 Lu Wang,#,1 Chung-Li Dong,3 Gabriel Menendez Rodriguez,2 Yu-Cheng Huang,3 Alceo Macchioni,*,2 Shaohua Shen*,1 1

International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow

in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China 2

Department of Chemistry, Biology and Biotechnology, University of Perugia and CIRCC, Via

Elce di Sotto 8, 06123 Perugia, Italy 3

Department of Physics, Tamkang University, 151 Yingzhuan Road, New Taipei City, 25137,

Taiwan AUTHOR INFORMATION # Contributed equally Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract

Activating molecular catalysts at the surface of metal oxides can be a promising strategy to overcome

the

sluggish

interfacial

kinetics

and

enhance

the

efficiencies

for

photo(electro)chemical (PEC) water oxidation. However, the physical association between inorganic semiconductors for PEC process and organometallic molecular catalysts for surface catalytic reactions generally remains a challenging problem. In the present work, Kläui-Type organometallic precursor [Cp*Ir{P(O)(OH)2}3]Na has been first synthesized and subsequently successfully anchored onto BiVO4 nanopyramids grown on transparent conducting substrates through various procedures. Treating the resulting hybrid heteronanostructure with IO4- induces a strong synergism between iridium atoms and BiVO4 nanocrystals which exhibits a 5.5 times enhancement in photocurrent density at 1.23 V vs. reversible hydrogen electrode (RHE) for PEC water oxidation. This simple approach provides an effective alternative pathway for molecular catalysts anchoring on inorganic semiconductors for efficient renewable energy utilization.

TOC GRAPHICS


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Solar water splitting has been proposed as a promising solution to energy crisis as it generates molecular hydrogen and oxygen or electrons and protons for the artificial photosynthesis of various renewable fuels. However, the water oxidation half-reaction, which suffers from a complex four-electron process, remains a major challenge. Heterogeneous semiconductor photocatalysts and molecular catalysts are both practical solutions for efficient water oxidation catalysis.1-3 Molecular water oxidation catalysts (WOCs) has been developed for efficient water oxidation since the pioneer work by Meyer4 and much progress has been achieved.5-12 However, most WOCs are not photoactive and must work together with sacrificial oxidants. Heterogeneous photocatalysts such as TiO2, α-Fe2O3, BiVO4 can absorb and utilize solar illumination to split water into hydrogen and oxygen with excellent chemical stability.13-16 One of the major disadvantages of the semiconductor photocatalysts lies on the sluggish surface kinetics.17, 18 It has been demonstrated that anchoring molecular WOCs onto semiconductor photocatalysts is a promising strategy to combine the molecular and heterogeneous catalysts for water oxidation.1922

Much efforts have been devoted to developing various combinations of molecular complex

and semiconducting photocatalysts or photoelectrodes and fruitful results have been achieved.2328

However, the stable connection between molecular catalysts and semiconductors has been

scarcely investigated in depth. Kläui-type compounds (KTCs)29 [(C5R5) M{P(O)R’R’’}3]- (R=H, Me; M=Co, Rh; R’ and R’’=alkyl, aryl, or O-alkyl) has attracted our attention as an excellent precursor candidate for the fabrication of the hybrid catalysts.30 The three P(O)(OH)2 moieties in KTCs are particularly favorable for anchoring the metal center onto properly selected functional semiconductors and also can be easily generated by the hydrolysis of their KTCs precursors.31 In our previous work, the Kläui-type organometallic molecular catalyst has been immobilized onto rutile TiO2 and the resulting iridium-TiO2 hybrid materials exhibited remarkable TOF (turnover


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frequency) and TON (turnover number) values in water oxidation with NaIO4.32 The novel strategy provides a useful guidance for combination of the molecular and heterogeneous catalysts. However, the system still needs the exploitation of a chemical sacrificial oxidant and operates in the absence of light. Herein, we report a combination of photoelectrocatalytic semiconductor oxide with a competent Kläui-Type WOC, [Cp*Ir{P(O)(OH)2}3]Na, and show that the molecular catalyst can be anchored onto BiVO4 with strong connection and enhanced activity without chemical sacrificial oxidants. [Cp*Ir{P(O)(OH)2}3]Na was synthesized by the hydrolysis of [Cp*Ir{P(O)(OMe)2}3]Na, prepared by a reported procedure (see details in Supporting Information).32, 33 Nanopyramidal BiVO4 photoanodes were fabricated according to previous work (see details in Supporting Information)34 and the special morphology was beneficial for catalyst deposition due to the advantageous surface area. Then the catalyst was dispersed in a water solution (1 mM), followed by contacting BiVO4 photoanodes for 12 h, 6 h and 24 h, respectively, and the final resulted composites were named as K1, K2 and K3 correspondingly. BiVO4 photoanode without deposition was named as K0. Moreover, K1 was tested in water oxidation with NaIO4 (10 mM) aqueous solution for two consecutive runs and the resulted sample was named as K1’.


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Figure 1. (a) Comparison of water oxidation results on catalyst attached BiVO4 photoanodes (12 h) in 10 mM NaIO4 aqueous solution as sacrificial oxidant. First run of BiVO4 with 12 h deposition (K1, magenta), the supernatant of first run of K1 (Sup, navy), the recovered photoelectrode (K1’, dark yellow) and subtraction of the supernatant from the first run (K1-Sup, dotted orange); (b) Experimental set-up for water oxidation tests. [Cp*Ir{P(O)(OH)2}3]Na has proved to be an excellent candidate to hybrid with a metal oxide for heterogeneous water oxidation.32 It has been demonstrated that the activation of the composite catalyst occurs through the initial oxidative dissociation of PO43-. Therefore, the oxidation activity of K1 was firstly tested with NaIO4 as chemical sacrificial oxidant and the results are shown in Figure 1a, the experimental device and the hybrid electrode are also shown in Figure 1b. K1 shows an almost quantitative production of O2 (yield 92 %, 0.28 µmol/min, magenta in Figure 1a) with NaIO4 during the first cycle. However, some leaching of the catalyst (ca. 2%, according to ICP) occurred after the first run, as confirmed by the fact that the supernatant was also active (0.18 µmol/min, navy in Figure 1a). Despite the concentration of iridium in the supernatant is rather low, production of O2 is fast and quantitative, likely due to the formation of


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extremely active Ir(OH)62-.35 The recovered photoelectrode K1’ was also active and showed a somewhat smaller catalytic with NaIO4 (0.10 µmol/min). According to ICP analysis (see Supporting Information), the catalyst leaching after the second run was only ~ 0.2 %. Interestingly, subtracting the O2 production of the supernatant from that of the first catalytic run leads to a trend (dotted orange in Figure 1a) almost coincident with that of the second cycle (dark yellow in Figure 1a). This indicates that leaching occurs at the early stage of the first cycle producing both homogeneous and heterogeneous active sites. Furthermore, the activity of the latter during the first cycle seems to be exactly equal to those of the second cycle, indicating that no further drop of performance is present. It is reasonable to assume that a strong connection between iridium atoms and the metal oxide has been established after the two catalytic runs in K1’, which might be beneficial for further catalytic oxidation. For comparison, the other two samples (K2, 6 h and K3, 24 h) were not treated in NaIO4 to induce the oxidative dissociation of PO43-.

Figure 2. (a) PEC water oxidation photocurrents of bare BiVO4 and hybrid photoanodes in 0.5 M Na2SO4 aqueous solution under AM 1.5G illumination; (b) Tested and simulated results of the electrochemical impedance spectroscopy for bare BiVO4 and the catalyst attached photoanodes; (c) Incident photon to current conversion efficiency (IPCE) plots of the photoanodes measured at


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1.23 V vs. RHE in 0.5 M Na2SO4 solution. Bare BiVO4 without modification (K0, cyan), BiVO4 with 12 h deposition and further oxidation (K1’, black), BiVO4 with 6 h deposition (K2, red) and BiVO4 with 24 h deposition (K3, blue). To investigate the photoelectrochemical (PEC) water oxidation performances, the composite electrodes were tested in a 0.5 M Na2SO4 aqueous solution under a three-electrode system under AM 1.5 G illumination. The bare BiVO4 K0 shows a stable but low photocurrent density with value of 0.1 mA/cm2 at 1.23 V vs. RHE (Figure 2a). All the composite photoanodes show obvious enhancement on photocurrent densities especially at lower applied potential, which can be ascribed to the successful surface catalysis effect to overcome the surface sluggish water oxidation kinetics on BiVO4. The Faradaic yields of the photoanodes are all estimated to be around 85% (Figure S1), indicating that the current flows derived mostly from water oxidation and not (or only marginally) from the oxidation of the iridium catalysts. Although K1’ shows decreased oxidation efficiency with NaIO4 compared to K1 due to the combination of molecular and heterogenized active sites in the latter, its photocurrent is impressively enhanced compared to that of bare BiVO4. The photocurrent density at 1.23 V vs. RHE reaches ~0.65 mA/cm2, which is 5.5 times higher than that of bare BiVO4. Meanwhile, K2 and K3 also show a slightly increased photocurrent and K3 with longer contacting time (24 h) shows a better performance compared to K2 (6 h). It can be inferred that with longer deposition time, more catalyst will be attached onto the photoanode surface. However, this attachment is very likely unstable and might leach during further tests or treatments because of the loose contact under oxidative conditions, which can be inferred from the NaIO4 oxidation test and ICP analysis. As a consequence, with a much longer deposition time of catalyst, K3 shows only a limited enhancement for PEC performance. On the other hand, the impressive photocurrent for K1’ can be ascribed to the


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strong attachment of the catalyst on semiconductor surface. K1’ with the medium deposition time shows the best activity, indicating oxidation treatment while not the deposition time might be the dominant factor on activity and electrochemical property. Although the deposition time of K1’ is not very long, the effective catalyst anchored at the oxide surface with strong connection is the most after the oxidative process, which contributes to the best PEC performance. More detailed characterizations and analysis will be provided and discussed below to support the conclusions. Electrochemical impedance spectroscopy (EIS) measurements were employed to evaluate the catalyst attachment and surface catalysis effect on the photoanodes. An equivalent circuit consisting of two RQ elements in series was chosen to simulate the electrode and interface behaviors. The calculated data from the equivalent circuit fit the original impedance data well (Figure 2b, Table S1). In the equivalent circuit, RS represents the solution resistance; Q is the constant phase element (CPE) and the two RQ elements (RCT, QH and RSC, QSC) in series account for the semiconductor and surface processes respectively. The electrodes show much smaller radii of the semicircles after organometallic catalysts loading and K1’ has the smallest resistance for surface water oxidation. The tendency of the EIS results agree well with the PEC tests, indicating that loading of the catalysts efficiently decrease the charge transfer resistance and improve the surface reaction kinetics for water oxidation. Incident photon to current conversion efficiency (IPCE) measurements were performed to investigate the efficiencies of the catalyst attached photoanodes at various incident wavelengths. All the samples show substantial visible light responses in almost the same regions and the onset wavelength is around 500 nm (Figure 2c), which are in accordance with the UV-vis spectra. For bare BiVO4 K0, the IPCE value reaches 1.21% at 450 nm, while for K1’ the value increases to


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4.74%, which is nearly 4 time of the unmodified sample. The IPCE data agree well with the photocurrent results and verify again the catalysis effect of the organometallic attached in different routines onto semiconductor oxide photoanodes.

Figure 3. (a) SEM, TEM images and energy-dispersive elemental mapping of the hybrid BiVO4 photoanode; (b) UV-Vis spectra of the catalyst attached photoanodes; (c) V L-edge and O Kedge XANES spectra of the catalyst attached photoanodes and reference IrO2. Bare BiVO4 without modification (K0, cyan), BiVO4 with 12 h deposition and further oxidation (K1’, black), BiVO4 with 6 h deposition (K2, red) and BiVO4 with 24 h deposition (K3, blue). In order to further understand the role of attached catalysts in the PEC performance, physical characterizations were performed. First of all, the surface morphology of the composite electrodes was investigated with SEM and TEM (Figure 3a). BiVO4 shows a nanopyramid arrays morphology and the molecular catalysts anchored can hardly be observed due to the trace amount of loading. From the TEM mapping result, iridium is distributed homogeneously on BiVO4 nanopyramid, indicating the effective attachment of the catalysts. As to the optical


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absorption, all the samples show almost the same UV-Vis plots and the absorption edges all locate at ~500 nm (Figure 3b). This is also due to the trace amount of loading and the influence of optical factor on the PEC performance can be excluded. Now that the surface morphology and optical absorption remain unchanged, we can attribute the enhancement on PEC performance only to the surface cocatalysis effect. More detailed characterization on the molecular catalyst and the connection between molecular catalyst and semiconductor oxide were performed. Fourier transform infrared spectroscopy (FT-IR) was performed to further examine the catalysts anchored at the photoanode surface. From Figure S2, we can get information about the functional groups of the attached catalysts. The peaks at ~3440 cm-1 and 1631 cm-1 are ascribed to the hydroxy group. This hydroxy might come from both the composite catalysts and the surface adsorbed water in air. The surface coordination of hydroxyl group should be confirmed together with XPS and X-ray absorption analysis. The Cp* group shows the C=C bond peak at ~1520 cm1

. However, this weak signal vanishes in the spectrum of K1’. The peaks located at 1020, 943,

770 cm-1 in K2 and K3 are ascribed to PO43- functional group in the organometallic catalysts. However, negligible signal can be found at the positions for K1’, indicating that the phosphorous component was removed during the oxidation test and a possible new attachment form might be established. The surface coordination of the bismuth vanadate and attached catalysts were further investigated with X-ray absorption near-edge structure (XANES) spectra, which is sensitive and effective to examine the local coordination structure for certain elements. Bi L3-edge is presented in Figure S3 and all the spectra show the identical spectral profile and absorption peak intensity, indicating that the structure is not affected by the deposition time and the oxidation treatment, and the latter only occurs on the surface. The concentration of P and Ir are too low (< 0.2%) to be


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detected by fluorescence yield mode using ion gas chamber. However, considering that the conduction band of BiVO4 is mainly determined by the V 3d, O 2p orbitals (lower energy region), and Bi 6p orbital (higher energy region), V L2,3- and O K-edge were then measured. V L2,3-edges and O K-edge provide the information of hybridization of V 3d-O 2p orbitals and Bi 6p-O 2p orbitals. If there is any modification of electronic structures in the surface of BiVO4 caused by the molecular catalyst, the hybridized states may vary. Therefore, the V L2,3-edges and O K-edge XANES spectra are presented in Figure 3c. Almost no spectral difference can be observed in the V L2,3-edges and O K-edge made by fluorescence yield (FY), indicating that the crystal structure of bismuth vanadate remains unaffected by the catalyst deposition and oxidation treatment. On the contrary, spectral variations are revealed in the V L2,3-edges and O K-edge collected by total electron yield (TEY), which is more surface sensitive than FY detection mode. The spectral deviations between FY and TEY strongly suggest the local atomic and electronic structures on the surface is modified by deposited catalyst and oxidation treatment. Spectrum of V L2,3-edges consists of two main regions. L3-edge (around 517.5 eV) is originated from excitation from 2p3/2 to 3d states and L2-edge (around 524.3 eV) is resulted from the excitation from 2p1/2 to 3d states. V L3-edge further splits into two peaks which are associated with t2g and eg states under the tetrahedral crystal field. Normally, L2-edge is a replica of L3-edge, but the core-hole lifetime broadens the spectral feature and so no obvious eg state could be revealed. Notably, the L3-edge is shifted to higher energy for BiVO4 deposited with catalysts compared with bare BiVO4, indicating the partially oxidation during the catalyst deposition, as clearly seen in the inset of Figure 3c (left panel). At the same time, the intensity of L3 peak of deposited BiVO4 is lower than that of bare BiVO4, implying the redistribution of electrons in V 3d orbitals after the catalyst deposition which could be attributable to the formation of P-O-V bond. Oxygen


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has higher electronegativity and thus as P-O-V bond is formed, oxygen attracts the electron from P and some charges redistribute to V site. Interestingly, further tetrahedral distortion in K1’ is evidenced by further enlarged energy separation of eg and t2g states. This distortion could be ascribed to the strong connection Ir-O-V after removal of PO43-. The strong interaction between catalyst and the BiVO4 surface is also evidenced by the fact that the intensity of L3 peak further declines, meaning more charges redistributed to V 3d states. O K-edge provides complementary information to V L-edge. As shown in Figure 3c (right panel), two features are assigned as hybridized O 2p-V 3d(eg) and O 2p-V 3d(t2g) states. Small bump at energy about 534.8 eV is owing to the O 2p-Bi 6p states, which is absent at the surface of bare BiVO4. However, this feature is revealed after catalysts deposited, implying the surface electronic structure is modified. Thus, there is a change in the oxygen coordination structure of the molecular catalyst attached photoanodes. Compared with bare BiVO4, the catalyst attached photoanodes show the most intense peak at ~531 eV and the least intense peak at ~538 eV, which indicates that more carboxyl-type oxygen-containing functional groups and less ether-type functional groups are present in catalyst attached photoanodes.36 This provides evidence for the strong surface coordination of iridium catalysts at the BiVO4 surface. An intense peak is revealed in K1’ which indicates that K1’ has a significant hybridization possibly owing to strong connection Ir-O-V. The strong hybridization may provide easy conduction path and is consistent with the PEC performance. The peaks at 537.5 eV and 540.4 eV for the photoanodes can be attributed to the combination of the signals of molecular catalysts and bare BiVO4 as marked by two arrows in Figure 3c (right panel), which indicates good connection between iridium catalysts and BiVO4. Furthermore, the peaks of the catalyst attached photoanodes shift to lower photo energy compared with that of bare BiVO4, suggesting the partially oxidation during the catalyst


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deposition,37 which is consistent with the result from V L-edge. The effect of oxidation on the photoanodes will be discussed in detail in the XPS analysis.

Figure 4. XPS spectra of (a) Ir 4f; (b) P 2p; (c) C 1s and (d) O 1s of the catalyst attached photoanodes. BiVO4 with 12 h deposition and further oxidation (K1’, black), BiVO4 with 6 h deposition (K2, red) and BiVO4 with 24 h deposition (K3, blue). The X-ray photoelectron spectroscopy (XPS) characterization provides useful information on the elements distribution and valence states of the molecular catalyst on BiVO4 (Figure 4, S4). All the samples show XPS signals of Ir 4f at binding energy of 62.4 eV and 65.4 eV, which confirms the existence of catalyst attached at the surface. Moreover, for K1’, the 4f7/2 peak shows another smaller component at higher binding energy of 63.1 eV. The feature peak at 62.4 eV can be attributed to Ir4+, as in IrO2, while the higher energy feature peak would indicate the presence of some iridium with higher oxidized valence.38 With the higher oxidation state, the iridium center might coordinate with more ligands, which might be totally different from Ir4+. It can be inferred


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that the higher oxidation state of Ir originated from the oxidation treatment and iridium atoms might form a totally different connection with the bismuth vanadate surface. Furthermore, XPS for phosphorus also show some results in accordance with FT-IR. K2 and K3 both show obvious P 2p signals at 132.7 eV while no signal can be found for K1’. The difference for the catalysts anchored at the surface inspire us that the extra oxidation test treatment for K1’ leads to a totally different catalyst attachment mode. For K2 and K3, catalysts were deposited onto the photoanodes in a physical and original way, which was not quite stable and no strong connection was established. This can be inferred from the leaching of the catalyst during the oxidation tests. However, after the oxidative process, PO43- in the catalyst was removed to establish a stronger connection between Ir and the semiconductor (Ir–O–M). This explains why K1’ showed the best PEC performance and how the best catalyst attachment was achieved. The XPS C 1s spectra shows no difference in all the samples and the three feature peaks are located at 284.8 eV, 285.6 eV and 288.3 eV. The O 1s XPS provides useful information for the different oxygen species in the samples. Three main feature peaks are located at ~529.6 eV, 530.6 eV, 532.1 eV, which can be attributed to lattice oxygen O2-, vacancy oxygen and surface hydroxyl groups, respectively.39 After oxidation treatment, K1’ shows a slighter higher binding energy of 529.8 eV for lattice oxygen and a higher proportion of the peak for surface hydroxyl groups. This result confirms the influence of oxidation treatment and indicates that more surface hydroxyl groups could be coordinated with iridium center.


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Scheme 1. Proposed mechanism for activation of surface anchoring organometallic catalyst on BiVO4 photoanode (X = hydroxyl groups or water). Based on the characterization results, we propose the possible mechanism for the anchoring and transformation of molecular organometallic catalysts on the semiconductor photoanode as shown in Scheme 1. [Cp*Ir{P(O)(OH)2}3]Na is firstly anchored onto oxide photoanodes by contacting the photoanodes with the catalyst in water solution. The connection between catalyst and oxide, being strong enough to resist to washing with water, reasonably consists of a combination of P– O–H…O–M hydrogen bonding, coordinative P=O…M and P–O–M bonds. Nevertheless, this connection is not enough stable under oxidative conditions, leading to catalyst transformation and leaching. Particularly, the oxidative tests with NaIO4 causes the oxidative degradation of Cp* and the expulsions of H3PO4 molecules (Scheme 1).32 Coordination sites liberated by the removal of Cp* and P(O)(OH)2 ligands are reasonably occupied by anionic species (hydroxyl groups most probably) or water molecules40 (X in Scheme 1). Only when the last oxidative transformation of Ir–PIII(O)(POH)2 into H3PVO4 occurs, and Ir–O–M bonds form, a strong connection between the catalyst and oxide establishes (Scheme 1). The removal of the PO43component was verified by XPS elemental analysis and FT-IR spectra. The established chemical bonding is quite stable so that the catalyst scarcely leaches and the PEC performance shows a remarkable enhancement. Apparently, photoelectrochemical oxidation is not enough aggressive to cause significant oxidative transformation of the molecular precursor into the active species, as indicated by the lower catalytic activity of K2 and K3 with respect to that of K1’. It is important to outline that at least one H3PO4 molecule has to be expelled from the organometallic precursor in order to generate a coordinative vacancy where a water molecule can bind and undergo the oxidative


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process, ultimately leading to O2. Because PEC methodology is much less aggressive toward catalysts than using a sacrificial oxidant, it can be envisioned that anchoring a catalytic precursor already possessing a coordination vacancy onto BiVO4 should provide high activity, adjustable by the nature of ancillary ligands; at the same time, molecule anchored species should remain intact or undergo a much smaller oxidative transformation. Studies in this direction are in progress in our laboratories. In conclusion, an organometallic water oxidation catalyst with P(O)(OH)2 functional group has been examined to be an excellent precursor for the fabrication of hybrid photoanodes for PEC water oxidation. The modified iridium-BiVO4 hybrid photoanode with further oxidative transformation shows a remarkable 5.5 times enhancement of photocurrent due to the catalysis effect to overcome the surface sluggish kinetics. The hybrid strategy proves to be effective to establish strong connection between the iridium atoms and the metal oxide. This demonstration provides a beneficial guidance for the combination of homogenous molecular catalysts and metal oxide semiconductors materials for renewable energy utilization. Further work on various combinations will be carried on soon for better efficiency.


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ASSOCIATED CONTENT Supporting Information. Brief statement of experimental details, inductively coupled plasmaoptical emission spectroscopy (ICP-OES) results, characterization methods, oxygen evolution measurements, and XPS survey spectra. AUTHOR INFORMATION * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 51672210) and SABIC, PRIN 2015 (20154X9ATP_004), University of Perugia and MIUR (AMIS, “Dipartimenti di Eccellenza - 2018-2022” program). S. Shen was supported by the National Program for Support of Top-notch Young Professionals and the “Fundamental Research Funds for the Central Universities”.


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Activating KlÃ¤ui-type Organometallic Precursors at Metal Oxide

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