DOI: 10.1002/cssc.201300561

Efficient Chemical and Visible-Light-Driven WaterOxidation using Nickel Complexes and Salts asPrecatalystsGui Chen, Lingjing Chen, Siu-Mui Ng, and Tai-Chu Lau*[a]

Introduction

In recent years there has been an intense effort to develop ar-tificial photosynthesis that can make use of solar energy toproduce renewable fuels.[1–9] The bottleneck in artificial photo-synthesis is water oxidation (2 H2O!O2 + 4 H+ + 4 e�), whichcan provide an unlimited source of protons and electrons forthe production of solar fuels such as H2, CO (CO2 + 2 H+ +

2 e�!CO + H2O), or methanol (CO2 + 6 H+ + 6 e�!CH3OH +

H2O).[10–15] As water oxidation is a complicated process thatusually involves a large overpotential, the development of anefficient water oxidation catalyst (WOC) is essential for this pro-cess to occur at a useful rate with a low overpotential. Howev-er, to be economically viable, the catalysts should be madefrom inexpensive, earth-abundant materials.[16, 17] So fara number of Co,[18–25] Mn,[26–30] Fe,[31–33] and Cu[34–36] WOCs havebeen reported. In contrast, Ni-based WOCs have received lessattention. NiOx has been used in anodic materials for O2 evolu-tion under alkaline conditions.[37] A NiOx–borate film electrode-posited from Ni2 + in borate buffer solution can also efficientlycatalyze electrochemical water oxidation at near-neutral pH.[38]

A NiOx film has also been used as a catalyst for photo-electro-chemical water oxidation that uses UV light.[39, 40] Water oxida-

tion catalyzed by NiFe2O4 that uses [Ru(bpy)3]2 + (bpy = 2,2’-bi-pyridine) and S2O8

2� has also been reported.[41] A pentanickelsilicotungstate complex was recently found to function asa molecular catalyst for visible-light-driven water oxidationwith a turnover number (TON) of 60.[42] Herein, we report thata series of simple Ni salts as well as Ni complexes that bearmultidentate N-donor ligands are highly efficient catalysts forchemical and visible-light-driven water oxidation with TONs>1200. We provide evidence that under oxidative conditionsthese Ni salts and complexes are converted to NiOx nanoparti-cles, which are the real catalyst for water oxidation.

Results and Discussion

Chemical water oxidation catalyzed by Ni complexes

As multidentate N-donor complexes of Ru,[43–47] Fe,[31–33]

Co,[18–25] and Cu[34–36] are known to be active catalysts/precata-lysts for water oxidation, we have prepared a series of Ni com-plexes that bear various hexa-, penta-, and tetradentate pyrid-yl/tertiary amine ligands [NiL]2 + (L = L1–L6, Scheme 1) to testtheir efficacy as WOCs.

We first investigated the catalytic activity of various Ni com-plexes towards chemical water oxidation at pH 7.0–8.5 byusing [Ru(bpy)3](ClO4)3 as the terminal oxidant (Table 1 and Fig-ure 1 a). [Ru(bpy)3]3 + is commonly used as an oxidant in thispH range because of its relative stability and high redox poten-tial (E0 = 1.21 V).[48] [NiL1]2 + , which contains a hexadentateligand, yielded only a trace amount of O2 upon addition of[Ru(bpy)3]3+ whereas the other Ni complexes ([NiL2]2 +–[NiL6]2 +)with either penta- or tetradentate ligands readily produced O2

Chemical and visible-light-driven water oxidation catalyzed bya number of Ni complexes and salts have been investigated atpH 7–9 in borate buffer. For chemical oxidation, [Ru(bpy)3]3 +

(bpy = 2,2’-bipyridine) was used as the oxidant, with turnovernumbers (TONs) >65 and a maximum turnover frequency(TOFmax) >0.9 s�1. Notably, simple Ni salts such as Ni(NO3)2 aremore active than Ni complexes that bear multidentate N-donor ligands. The Ni complexes and salts are also active cata-lysts for visible-light-driven water oxidation that uses[Ru(bpy)3]2+ as the photosensitizer and S2O8

2� as the sacrificialoxidant; a TON>1200 was obtained at pH 8.5 by using

Ni(NO3)2 as the catalyst. Dynamic light scattering measure-ments revealed the formation of nanoparticles in chemical andvisible-light-driven water oxidation by the Ni catalysts. Thesenanoparticles aggregated during water oxidation to form sub-micron particles that were isolated and shown to be partiallyreduced b-NiOOH by various techniques, which include SEM,energy-dispersive X-ray spectroscopy, X-ray photoelectronspectroscopy, XRD, and IR spectroscopy. These results suggestthat the Ni complexes and salts act as precatalysts that decom-pose under oxidative conditions to form an active nickel oxidecatalyst. The nature of this active oxide catalyst is discussed.

[a] Dr. G. Chen,+ L. Chen,+ Dr. S.-M. Ng, Prof. T.-C. LauDepartment of Biology and ChemistryCity University of Hong Kongand Institute of Molecular Functional MaterialsTat Chee Avenue, Kowloon Tong, Hong Kong (China)Fax: (+ 852) 34420522E-mail : bhtclau@cityu.edu.hk

[+] These two authors contributed equally to this paper.

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201300561.

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with TONs and turnover frequencies (TOFs) that ranged from31–54 and 0.19–0.42 s�1, respectively, at pH 8.0 in boratebuffer. In the absence of a Ni catalyst, <5 % of O2 was detect-ed, which came from the background oxidation of water by[Ru(bpy)3]3+ .[49] Notably, simple Ni salts such as Ni(NO3)2 areeven more active, with a TON and TOF of 65 and 0.91 s�1, re-spectively. Other Ni salts such as NiCl2, NiSO4, and Ni(ClO4)2

gave similar results (Table S1).Chemical water oxidation by Ni2+

(aq) and [NiL6]2 + were stud-ied in more detail. The effects of pH were first investigated(Figures S1 and S2). Below pH 7.5, the yields of O2 were negli-gible. For both Ni2 +

(aq) and [NiL6]2+ , the TOF increased with in-creasing pH from 7.5–8.5, which is in accordance with the factthat water oxidation is thermodynamically more favorable athigher pH values. However, the TON first increased frompH 7.5 to 8.0 and then decreased as the pH value was furtherincreased to 8.5, which is probably because of the more rapiddecomposition of [Ru(bpy)3]3+ at a higher pH value.[49] The rateand yield of O2 increased with increasing [Ni2+] from 0.25–4.0 mm (Figure 2 a). The O2 yield also increased steadily with in-creasing [Ru(bpy)3

3+] concentration from 0.25–1.5 mm (Fig-ure 2 b). Similar concentration effects were also observed for[NiL6]2 + (Figures S3 and S4).

Scheme 1. Structures of L1–L6.

Table 1. Ni-catalyzed chemical water oxidation.[a]

Entry Catalyst O2 yield[b]

[%]TOF[c]

[s�1]TON[d]

1 – 2 (5) – –2 [NiL1](ClO4)2 3 (7) – 2 (4)3 [NiL2](ClO4)2 30 (35) 0.32 53 (56)4 [NiL3](ClO4)2 31 (36) 0.41 54 (58)5 [NiL4](ClO4)2 19 (24) 0.19 31 (37)6 [NiL5](ClO4)2 25 (28) 0.22 41 (43)7 [NiL6](ClO4)2 29 (34) 0.42 50 (54)8 Ni(NO3)2 37 (42) 0.91 65 (69)

[a] Conditions: [Ru(bpy)3](ClO4)3 (0.75 mm), catalyst (1.0 mm) in 8.25 mL of15 mm borate buffer (initial pH 8.0), time = 5 min, T = 23 8C. [b] Yield =

100 � (mol of O2)/(0.25�mol of [Ru(bpy)3](ClO4)3), obtained by Clark elec-trode. The numbers in parentheses were obtained by GC–TCD. Eachvalue is the average of at least three replicates. Error �5 %. [c] TOF ob-tained from steepest slope of TON versus time curve. [d] TON= (mol ofO2-blank)/(mol of catalyst).

Figure 1. a) Plots of O2 evolution versus time for Ni-catalyzed water oxida-tion by [Ru(bpy)3]3+ in 15 mm borate buffer (pH 8.0) at 23 8C. [Cata-lyst] = 1.0 mm, [Ru(bpy)3

3 +] = 0.75 mm. b) O2 evolution by Ni2+(aq), [NiL3]2 +

and [NiL6]2 + in three consecutive runs. The same amount of [Ru(bpy)3]3 +

was replenished at each down arrow.

Figure 2. Plots of O2 evolution versus time for Ni(NO3)2-catalyzed water oxi-dation by [Ru(bpy)3]3 + in 15 mm borate buffer (pH 8.0) at 23 8C. a) Effects of[Ni2 +] on O2 evolution. [Ru(bpy)3

3 +] = 0.75 mm. b) Effects of [Ru(bpy)33 +] on

O2 evolution. [Ni2+] = 1.0 mm.

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When a second portion of [Ru(bpy)3]3 + was added toNi2+

(aq), [NiL3]2 + , or [NiL6]2 + , the yields of O2 were decreased byaround 20 %, and the O2 yields were considerably lower in thethird run, which indicates the deactivation of the catalysts (Fig-ure 1 b).

Photocatalytic water oxidation by Ni com-plexes

The visible-light-driven water oxidation catalyzed by Ni2 +(aq)

and [NiL]2+ were also investigated by using [Ru(bpy)3](ClO4)2 asthe photosensitizer (PS) and Na2S2O8 as the sacrificial oxidantat pH 7.0–8.5 in borate buffer.[50, 51] Na2S2O8 has been used asa sacrificial oxidant in Co- and Ru-catalyzed water oxida-tion.[19, 52] The photochemical generation of the [Ru(bpy)3]3 + ox-idant in the presence of S2O8

2� is known to occur according toEquations (1) and (2). The overall reaction is shown in Equa-tion (3). O2 evolution was monitored by GC with thermal con-ductivity detection (TCD). The Ni complexes and salts that areactive catalysts in chemical water oxidation are also active inphotocatalytic water oxidation. Control experiments showedthat all three components, that is, the Ni catalyst, the PS, andthe sacrificial oxidant, are required for O2 evolution.

½RuðbpyÞ3�2þ þ S2O82� hv�! ½RuðbpyÞ3�3þ þ SO4C

� þ SO42� ð1Þ

½RuðbpyÞ3�2þ þ SO4C� ! ½RuðbpyÞ3�3þ þ SO4

2� ð2Þ

2 H2Oþ 2 Na2S2O8 ! O2 þ 4 Hþ þ 4 Naþ þ 4 SO42� ð3Þ

In the absence of a Ni catalyst, 3 % yield of O2 was detected(Table S2, entry 1), which came from the background oxidationof water by [Ru(bpy)3]3 + generated by light irradiation in thepresence of Na2S2O8.[49]

The effects of [Ru(bpy)32 +] and [Ni2 +] on O2 evolution were

examined. The amount of O2 increased with catalyst concentra-tion, but it reached saturation at [Ni2 +] = 0.6 mm (Figure 3 a).The amount of O2 first increased with [Ru(bpy)3

2+] , but thendecreased if [Ru(bpy)3

2+] was >0.12 mm (Figure 3 b). Similar ef-fects of [S2O8

2�] were also observed (Figure 3 c). These resultscan be attributed to a non-O2-producing decomposition reac-tion between S2O8

2� and [Ru(bpy)3]2 + , which limited the O2

yields to <50 % and became significant at high concentra-tions.[53, 54]

If [Co(NH3)5Cl]2+ was used as a sacrificial oxidant instead ofNa2S2O8, only a trace amount of O2 was detected (Table S2,entry 25). One possible reason is that the active Ni oxidantreacts more rapidly with [Co(NH3)5Cl]2+ or NH3 released afterits reduction than with water. It is also possible that the SO4C

�

radical, which is a very strong oxidant (E0 = 2.4 V),[48] may be in-volved in the oxidation of the Ni catalyst to generate theactive intermediate.

Figure 3. Plots of O2 evolution versus time for visible-light-driven (Xe lamp, 500 W, l = 457 nm) water oxidation by Ni(NO3)2 in the presence of [Ru(bpy)3](ClO4)2

and Na2S2O8 in 15 mm borate buffer at 23 8C. a) Effect of [Ni(NO3)2] . [Ru(bpy)3(ClO4)2] = 0.12 mm, [Na2S2O8] = 2.5 mm, pH 8.0. b) Effect of [Ru(bpy)3(ClO4)2] . [Ni-(NO3)2] = 0.6 mm, [Na2S2O8] = 2.5 mm, pH 8.0. c) Effect of [Na2S2O8]. [Ni(NO3)2] = 0.6 mm, [Ru(bpy)3(ClO4)2] = 0.12 mm, pH 8.0. d) Effect of pH. [Ni(NO3)2] = 0.6 mm,[Ru(bpy)3(ClO4)2] = 0.12 mm, [Na2S2O8] = 2.5 mm.

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The effects of pH were also investigated (Figure 3 d). TheTON increased with pH from 7.5–8.5 to reach a maximum TONof 1210 at pH 8.5 (Table S2, entry 21) and a quantum yield of33�2 %. Below pH 7.5, the yield of O2 was negligible.

At pH 8, the pH value of the solution was found to decreaseto 3–4 after O2 evolution stopped as a result of the release ofprotons according to Equation (3). If a more concentratedbuffer solution of 30 mm was used, the TON increased from830 to 930 (Table S2, entry 17). Further increase in [buffer] to50 mm only resulted in a slight increase of TON to 940(Table S2, entry 19). The addition of solid CaCO3 to 15 mm

borate buffer also caused the TON to increase from 830 to 950(Table S2, entry 26).

If the pH value of the final solution was adjusted back tothe initial pH of 8.0 with 1 m NaOH, only 0.4 mmol O2 (TON=

83) was produced upon further irradiation (Table S2, entry 28).The addition of more Na2S2O8 or [Ru(bpy)3](ClO4)2 after pH ad-justment also did not result in a significant increase in O2 evo-lution in the second run (Table S2, entries 29 and 30). These re-sults indicate that O2 evolution depends on a combination ofseveral factors, which include pH decrease, electron acceptorconsumption, and PS and catalyst deactivation.

The photocatalytic activities of various nickel amine/pyridylcomplexes towards water oxidation have also been investigat-ed. As in the case of chemical catalysis, the Ni complexes[NiL]2+ have a similar or slightly lower reactivity than Ni2+

(aq),except [NiL4]2+ , which is much less efficient, and [NiL1]2+ ,which is completely inactive (Table 2 and Figure S5).

18O-labeling experiments

Isotope-labeling experiments that used 18O-enriched water(46.4 %) were performed to determine the source of the O2

evolved in the photocatalysis. The O2 evolved in the headspaceof the reaction vessel was analyzed by GC–MS (Figure S6). Theratio of 16O16O/16O18O/18O18O was determined to be 30:45:25(after background correction), which is in good agreementwith the calculated ratio of 28:50:22 if all of the evolved O2

stemmed from water.

Detection and isolation of particles formed during water ox-idation

In catalytic water oxidation, metal complexes may functiononly as precatalysts that decompose under the oxidizing con-ditions to metal oxides that are the real catalysts.[23, 55, 56] To de-termine the nature of the active intermediate in water oxida-tion by Ni(NO3)2, dynamic light scattering (DLS) was used tomonitor any particles formed during the catalytic process.Under the conditions specified in Figure 4, particles ranging

from 100–600 nm were detected after photoirradiation for1 min (Figure 4 a). The particle size was found to increase withirradiation time to around 400–1300 nm after 10 min. However,the size became smaller (400–700 nm after 50 min) upon fur-ther irradiation (Figure 4 a). The decrease in the size of the par-ticles is probably because of partial dissolution of the particlesas the pH value of the solution decreased during water oxida-tion. Similar particle formation was also observed in photocata-lytic water oxidation for all of the Ni complexes except for[NiL1](ClO4)2 (Figures S7–S12).

The black particles formed after photocatalytic water oxida-tion were isolated and characterized by various techniques.The SEM image shows that the solid consists of aggregates ofsubmicron particles (Figure 4 b). Energy-dispersive X-ray spec-troscopy (EDX) reveals that the solid contains Ni and O (Fig-ure S14). In the X-ray photoelectron spectrum (XPS) of the par-ticles a Ni 2p3/2 peak appears at 855.5 eV (Figure 5 A), which issimilar to typical values observed for nickel oxyhydroxide (b-NiOOH: 855.5 eV;[57] g-NiOOH: 855.3 eV[58]). In addition, O 1s

Table 2. Ni-catalyzed photocatalytic water oxidation.[a]

Entry Catalyst O2 yield[b]

[%]TON[c]

1 – 5 –2 [NiL1](ClO4)2 5 –3 [NiL2](ClO4)2 44 8134 [NiL3](ClO4)2 46 8555 [NiL4](ClO4)2 18 2716 [NiL5](ClO4)2 42 7717 [NiL6](ClO4)2 38 6888 Ni(NO3)2 45 834

[a] Conditions: [Ru(bpy)3](ClO4)2 (0.12 mm), catalyst (0.6 mm), and Na2S2O8

(2.5 mm) in 30 mm borate buffer (pH 8.0), T = 23 8C. 500 W Xe lamp (l=

457 nm). [b] Yields = 100 � (mol of O2)/(0.5�mol of Na2S2O8). Each value isthe average of at least three replicates. Error �5 %. [c] TON = (mol of O2-blank)/(mol of catalyst).

Figure 4. a) Particle size distribution at various irradiation times (Xe lamp,500 W, l = 457 nm) determined by DLS measurements. [Ni(NO3)2] = 50 mm,[Ru(bpy)3(ClO4)2] = 0.12 mm, [Na2S2O8] = 2.5 mm, pH 8.0 (2 mL of 50 mm

borate buffer). b) SEM image of particles formed after water oxidation.

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peaks are observed at 529.1 and 530.5 eV, which are also simi-lar to typical values observed for nickel oxyhydroxide (b-NiOOH: 530.5 eV;[57] g-NiOOH: 528.7 and 530.6 eV[58]). PowderXRD measurements show that the solid is poorly crystalline asthe reflections are broad and weak (Figure 6). The diffraction

pattern is similar to that of Ni3O2(OH)4 (NiII/III oxide), which isa partially reduced form of b-NiOOH.[59]

The IR spectrum of the particles (Figure S15) shows strongbands at 3420 and 570 cm�1, which are assigned to the O�Hand Ni�O stretching modes, respectively.[60]

Isolated NiOx as water oxidation catalysts

The catalytic activity of the isolated NiOx towards chemical andvisible-light-driven water oxidation was found to be considera-bly lower than that of Ni2+

(aq). At pH 8, the TON for chemicalwater oxidation by NiOx is 17 (versus 67 for Ni2 +

(aq) ; Table S1,entry 6), whereas in visible-light-driven water oxidation theTON is 125 (versus 830 for Ni2+

(aq) ; Table S2, entry 31).

Mechanism of Ni-catalyzed water oxidation

We tried to investigate how nickel oxide was formed if the Nicomplexes that bear multidentate N-donor ligands were usedas catalysts. We first studied the stability of the Ni complexes(0.1 mm) in aqueous solutions by ESI-MS and UV/Vis spectro-photometry. For [NiL2]2 + , [NiL3]2 + , and [NiL6]2 + , substantialamounts of free N-donor ligands were observed in solutionwithin 30 min at pH 8 as revealed by the presence of peaks inthe MS as a result of [L2 + H+] (m/z = 390.2), [L3 + H+] (m/z =

299.2), and [L6 + H+] (m/z = 257.6), respectively (Figures S16–S24). However, these complexes are much more stable inCH3CN, as only trace amounts of the ligands were detected byMS. A comparison of the UV/Vis spectra of [NiL2]2+ , [NiL3]2+ ,and [NiL6]2+ in CH3CN and in borate buffer (pH 8) indicatesthat around 3, 6, and 14 %, respectively, of the complexes de-composed in borate buffer within 30 min. These results sug-gest that the observed catalytic activities of [NiL2]2+ , [NiL3]2+ ,and [NiL6]2 + arise at least partially from Ni2+

(aq) ions generatedin solution prior to oxidation. However, for [NiL1]2 + , [NiL4]2 + ,and [NiL5]2 + , the MS in borate buffer show only peaks thatarise from the complex ions, and no free ligands could be de-tected (Figures S25–S27). Consequently, with [NiL4]2 + and[NiL5]2 + , NiOx is formed directly from the oxidative degradationof the complexes in catalytic water oxidation.

Among the Ni complexes, [NiL1]2 + , which is coordinativelysaturated by six N-donor atoms, is a completely inactive cata-lyst for water oxidation. Similarly, the catalytic activity of Ni2 +

(aq) was almost completely quenched in the presence of5 molar equivalents of the strongly chelating ligand 2,2’:6’,2’’-terpyridine (terpy), which would convert Ni2 +

(aq) to coordina-tively saturated [Ni(terpy)2]2 + (Table S1, entry 7).[61] This sug-gests that an active Ni precatalyst should have at least a Ni�OH2 bond, which can undergo proton-coupled electron trans-fer to generate a highly valent nickel oxyl species, presumablyNiIII�OC. This may be followed by ligand degradation and thencondensation to give an active nickel oxide cluster.

A NiOx film electrodeposited from Ni2 +(aq)

[38] or NiII aminecomplexes[62, 63] in borate buffer solutions can efficiently cata-lyze electrochemical water oxidation. The structure of this NiOx

film is related to that of g-NiOOH, which consists of NiIII/IV ionsbridged by di-m-oxo and hydroxo groups to form sheets of

Figure 5. XPS spectra of particles formed after photocatalytic water oxida-tion by Ni(NO3)2. Binding energy of a) Ni 2p, b) O 1s. The binding energy ofeach element was corrected by the C 1s peak (284.6 eV).

Figure 6. XRD pattern of a) particles formed after water oxidation,b) Ni3O2(OH)4, c) b-NiOOH, and d) g-NiOOH.

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NiO6 octahedra.[64, 65] It is likely that the active catalysts generat-ed from Ni2+

(aq) or [NiL]2 + in chemical and light-driven wateroxidation have a similar structure. The active nano-sized g-NiOOH particles formed from the Ni precatalysts apparentlyaggregate into submicron particles as water oxidation pro-ceeds. The resulting NiII/III oxide submicron particles isolatedafter water oxidation are a much less active WOC, presumablybecause the oxidation of the NiII/III oxide to a g-NiOOH-typeactive species by [Ru(bpy)3]3 + is kinetically slow. The numberof active nickel oxo sites on the surface of the active inter-mediate available for O�O coupling or hydroxide attack path-ways is expected to be less for these larger particles. Thewater oxidation activity of electrodeposited CoOx cluster films,which have a similar structure to electrodeposited NiOx

films,[66, 67] was found to decrease with the increasing size ofthe clusters.[68]

A proposed catalytic cycle for visible-light-driven water oxi-dation by [Ru(bpy)3]2 +/Na2S2O8/Ni catalyst is shown inScheme 2.

Conclusions

Although NiOx films have been used as catalysts in electro-chemical and photo-electrochemical (UV light) water oxida-tion,[39, 40] we report here the first example of the use of NiOx

generated from Ni complex/salt precursors as highly efficientcatalysts for chemical and visible-light-driven water oxidation.The TONs (>1200) we obtained from these systems areamong the highest for earth-abundant-metal water oxidationcatalysts. This is in contrast to the recently reported pentanick-el silicotungstate complex, which functions as a molecularphotocatalyst but with a considerably lower TON of 60.[41]

Although Ni complexes can be electrochemically decom-posed to NiOx,

[62, 63] the observation of similar NiOx formationby chemical or photochemical means is not trivial. For exam-ple, in the electrocatalytic water oxidation with the cobaltpolyoxometalate [Co4(H2O)2(PW9O34)2]10�, CoOx is identified asthe dominant catalyst.[55] However, in chemical and photocata-lytic water oxidation, [Co4(H2O)2(PW9O34)2]10� appears to func-tion as a molecular catalyst with no evidence for the formationof CoOx.

[20, 21]

Experimental Section

Materials

[Ru(bpy)3](ClO4)2,[69] [Ru(bpy)3](ClO4)3,[70] [NiL1](ClO4)2,[71] [NiL2]-(ClO4)2,[72] [NiL3](ClO4)2,[72] [NiL4](ClO4)2,[73] [NiL5](ClO4)2,[74] and [NiL6]-(ClO4)2

[75] (see Scheme 1 for the structures of L1–L6) were prepared

according to literature methods. Ni(ClO4)2·6 H2O (Aldrich), Ni-(NO3)2·6 H2O (Strem, 99.999 %), NiCl2·6 H2O (Sigma–Aldrich, 97 %),NiSO4·7 H2O (Sigma–Aldrich, 99.0 %), and H2

18O (Medical Isotopes,98 at % 18O) were used as received. H3BO3 (Sigma, 99.5 %) andNa2B4O7·10 H2O (Sigma–Aldrich, 99.5 %) were recrystallized threetimes according to standard methods.[76] Borate buffer solutionswere prepared according to a literature procedure[77] using MilliQwater (18.2 MW cm resistivity), and the pH values were measuredby using a pH meter (Mettler Toledo).

Characterization of particles

DLS measurements were performed by using a Zetasizer Nano ZSinstrument (Malvern Instruments Ltd. , USA), which can detect par-ticle sizes ranging from 0.6–6000 nm. The light source was a HeNegas laser (4 mW, l= 632.8 nm). Data were obtained by using a scat-tering angle of 1758 at 23 8C. SEM and EDX were performed byusing a Philips XL30 environmental scanning electron microscope(ESEM) at an accelerating voltage of 10 and 25 kV, respectively. XPSspectra were obtained by using a Leybold Heraeus-ShengyangSKL-12 electron spectrometer equipped with a VG CLAM 4 MCDelectron energy analyzer, with AlKa as the excitation source. Thebinding energy of each element was corrected by the C 1s peak(284.6 eV) from residual carbon.

Chemical water oxidation

To a solution of Ni catalyst (0–4.0 mm) in borate buffer under Arwas added a freshly prepared aqueous solution of [Ru(bpy)3](ClO4)3

(250 mL). The O2 evolved was monitored either by using a standardYSI Clark-type electrode or by GC–TCD. For Clark-type electrodemeasurements, the gas-permeable membrane of the electrode wasreplaced prior to each measurement to ensure the highest sensitiv-ity. The electrode, secured in a Teflon tube, was inserted intoa gas-tight water-jacketed glass vessel kept at 23 8C in a waterbath. The signal was recorded at 1 s intervals and calibrated usingair-saturated aqueous solutions ([O2] = 244 mm, T = 25 8C). The maxi-mum turnover frequency (TOFmax) was determined from the steep-est slope of the O2 evolution curve.

For GC analysis, the gas in the headspace of the reaction vial wassampled with a syringe (100 mL) and injected into the GC equippedwith a 5 � molecular sieve column and a TCD.

Visible-light-driven water oxidation

A solution of Ni complex (0–50.0 mm) in borate buffer that con-tained Na2S2O8 (0–5.0 mm) and [Ru(bpy)3](ClO4)2 (0–0.24 mm) ina 10 mL round-bottomed flask (total volume 14.7 mL) sealed witha rubber septum was degassed with Ar for 15 min and then irradi-ated with visible light (500 W Xe lamp, l= 457 nm) at RT. At regu-lar intervals, the gas in the headspace was sampled with a syringe(100 mL) and analyzed by GC–TCD equipped with a 5 � molecularsieve column. After O2 evolution ceased, the solution was degassedwith Ar for 15 min and then pure O2 gas was added by usinga graduated gas-tight syringe to prepare the calibration curve. Thequantum yield for O2 evolution was determined by ferrioxalate ac-tinometry.[78]

Scheme 2. Photocatalytic water oxidation catalyzed by Ni catalysts.

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Acknowledgements

The work described in this paper was supported by the HongKong University Grants Committee Area of Excellence Scheme(AoE/P-03-08), the Shenzhen Science and Technology ResearchGrant (JCYJ20120613115247045), and the Shenzhen Research In-stitute, City University of Hong Kong. The photochemical equip-ment used in this project was also supported by a Hong KongUGC Special Equipment Grant (SEG_CityU02). The XRD and XPSused in this work were provided by the Institute of Advanced Ma-terials of the Hong Kong Baptist University and the MaterialsCharacterisation and Preparation Facility of the Hong Kong Uni-versity of Science and Technology, respectively.

Keywords: nanoparticles · nickel · oxidation · photocatalysis ·water splitting

[1] D. K. Desai in Artificial Photosynthesis : From Basic Biology to IndustrialApplication (Eds. : A. F. Collings, C. Critchley), Wiley-VCH, Weinheim,2005, pp. 291 – 300.

[2] J. H. Alstrum-Acevedo, M. K. Brennaman, T. J. Meyer, Inorg. Chem. 2005,44, 6802 – 6827.

[3] T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G.Nocera, Chem. Rev. 2010, 110, 6474 – 6502.

[4] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729 –15735.

[5] R. Eisenberg, H. B. Gray, Inorg. Chem. 2008, 47, 1697 – 1699.[6] H. B. Gray, Nat. Chem. 2009, 1, 7.[7] S. W. Gersten, G. J. Samuels, T. J. Meyer, J. Am. Chem. Soc. 1982, 104,

4029 – 4030.[8] J. J. Concepcion, M. K. Tsai, J. T. Muckerman, T. J. Meyer, J. Am. Chem.

Soc. 2010, 132, 1545 – 1557.[9] L. Hammarstrçm, S. Hammes-Schiffer, Acc. Chem. Res. 2009, 42, 1859 –

1860.[10] J. J. Concepcion, J. W. Jurss, M. R. Norris, Z. F. Chen, J. L. Templeton, T. J.

Meyer, Inorg. Chem. 2010, 49, 1277 – 1279.[11] X. Sala, I. Romero, M. Rodr�guez, L. Escriche, A. Llobet, Angew. Chem.

2009, 121, 2882 – 2893; Angew. Chem. Int. Ed. 2009, 48, 2842 – 2852.[12] V. Artero, M. Charvarot-Kerlidou, M. Fontecave, Angew. Chem. 2011, 123,

7376 – 7405; Angew. Chem. Int. Ed. 2011, 50, 7238 – 7266.[13] L. Duan, L. Tong, Y. Xu, L. Sun, Energy Environ. Sci. 2011, 4, 3296 – 3313.[14] A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253 – 278.[15] S. W. Kohl, L. Weiner, L. Schwartsburd, L. Konstantinovski, L. J. W.

Shimon, Y. Ben-David, M. A. Iron, D. Milstein, Science 2009, 324, 74 – 77.[16] H. Yamazaki, A. Shouji, M. Kajita, M. Yagi, Coord. Chem. Rev. 2010, 254,

2483 – 2491.[17] P. Du, R. Eisenberg, Energy Environ. Sci. 2012, 5, 6012 – 6021.[18] M. W. Kanan, D. G. Nocera, Science 2008, 321, 1072 – 1075.[19] D. Shevchenko, M. F. Anderlund, A. Thapper, S. Styring, Energy Environ.

Sci. 2011, 4, 1284 – 1287.[20] Q. Yin, J. M. Tan, C. Besson, Y. V. Geletii, D. G. Musaev, A. E. Kuznetsov, Z.

Luo, K. I. Hardcastle, C. L. Hill, Science 2010, 328, 342 – 345.[21] Z. Huang, Z. Luo, Y. V. Geletii, J. W. Vickers, Q. Yin, D. Wu, Y. Hou, Y. Ding,

J. Song, D. G. Musaev, C. L. Hill, T. Lian, J. Am. Chem. Soc. 2011, 133,2068 – 2071.

[22] S. Tanaka, M. Annaka, K. Sakai, Chem. Commun. 2012, 48, 1653 – 1655.[23] D. Hong, J. Jung, J. Park, Y. Yamada, T. Suenobu, Y. M. Lee, W. Nam, S.

Fukuzumi, Energy Environ. Sci. 2012, 5, 7606 – 7616.[24] F. Song, Y. Ding, B. Ma, C. Wang, Q. Wang, X. Du, S. Fu, J. Song, Energy

Environ. Sci. 2013, 6, 1170 – 1184.[25] C. F. Leung, S. M. Ng, C. C. Ko, W. L. Man, J. Wu, L. Chen, T. C. Lau, Energy

Environ. Sci. 2012, 5, 7903 – 7907.[26] A. K. Poulsen, A. Rompel, C. J. McKenzie, Angew. Chem. 2005, 117,

7076 – 7080; Angew. Chem. Int. Ed. 2005, 44, 6916 – 6920.[27] G. C. Dismukes, R. Brimblecombe, G. A. N. Felton, R. S. Pryadun, J. E.

Sheats, L. Spiccia, G. F. Swiegers, Acc. Chem. Res. 2009, 42, 1935 – 1943.

[28] G. F. Swiegers, J. Huang, R. Brimblecombe, J. Chen, G. C. Dismukes, U. T.Mueller-Westerhoff, L. Spiccia, G. G. Wallace, Chem. Eur. J. 2009, 15,4746 – 4759.

[29] R. Brimblecombe, A. Koo, G. C. Dismukes, G. F. Swiegers, L. Spiccia, J.Am. Chem. Soc. 2010, 132, 2892 – 2894.

[30] E. A. Karlsson, B. L. Lee, T. �kermark, E. V. Johnston, M. D. K�rk�s, J. Sun,�. Hansson, J. E. B�ckvall, B. �kermark, Angew. Chem. 2011, 123, 11919 –11922; Angew. Chem. Int. Ed. 2011, 50, 11715 – 11718.

[31] W. C. Ellis, N. D. McDaniel, S. Bernhard, T. J. Collins, J. Am. Chem. Soc.2010, 132, 10990 – 10991.

[32] J. L. Fillol, Z. Codol�, I. Garcia-Bosch, L. Gmez, J. J. Pla, M. Costas, Nat.Chem. 2011, 3, 807 – 813.

[33] G. Chen, L. Chen, S. M. Ng, W. L. Man, T. C. Lau, Angew. Chem. 2013, 125,1833 – 1835; Angew. Chem. Int. Ed. 2013, 52, 1789 – 1791.

[34] S. M. Barnett, K. I. Goldberg, J. M. Mayer, Nat. Chem. 2012, 4, 498 – 502.[35] M. T. Zhang, Z. Chen, P. Kang, T. J. Meyer, J. Am. Chem. Soc. 2013, 135,

2048 – 2051.[36] Z. Chen, T. J. Meyer, Angew. Chem. 2013, 125, 728 – 731; Angew. Chem.

Int. Ed. 2013, 52, 700 – 703.[37] K. Kinoshita, Electrochemical Oxygen Technology, Wiley-Interscience, New

York, 1992, Chap. 2.[38] M. Dinca, Y. Surendranath, D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A.

2010, 107, 10337 – 10341.[39] K. Sun, N. Park, Z. Sun, J. Zhou, J. Wang, X. Pang, S. Shen, S. Y. Noh, Y.

Jing, S. Jin, P. K. L. Yua, D. Wang, Energy Environ. Sci. 2012, 5, 7872 –7877.

[40] T. K. Townsend, N. D. Browning, F. E. Osterloh, Energy Environ. Sci. 2012,5, 9543 – 9550.

[41] D. Hong, Y. Yamada, T. Nagatomi, Y. Takai, S. Fukuzumi, J. Am. Chem.Soc. 2012, 134, 19572 – 19575.

[42] G. Zhu, E. N. Glass, C. Zhao, H. Lv, J. W. Vickers, Y. V. Geletii, D. G.Musaev, J. Song, C. L. Hill, Dalton Trans. 2012, 41, 13043 – 13049.

[43] Z. F. Chen, J. J. Concepcion, X. Q. Hu, W. T. Yang, P. G. Hoertz, T. J. Meyer,Proc. Natl. Acad. Sci. USA 2010, 107, 7225 – 7229.

[44] H. W. Tseng, R. Zong, J. T. Muckerman, R. Thummel, Inorg. Chem. 2008,47, 11763 – 11773.

[45] S. Romain, L. Vigara, A. Llobet, Acc. Chem. Res. 2009, 42, 1944 – 1953.[46] D. J. Wasylenko, C. Ganesamoorthy, M. A. Henderson, B. D. Koivisto,

H. D. Osthoff, C. P. Berlinguette, J. Am. Chem. Soc. 2010, 132, 16094 –16106.

[47] L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet, L. Sun,Nat. Chem. 2012, 4, 418 – 423.

[48] A. R. Parent, R. H. Crabtree, G. W. Brudvig, Chem. Soc. Rev. 2013, 42,2247 – 2252.

[49] P. K. Ghosh, B. S. Brunschwig, M. Chou, C. Creutz, N. Sutin, J. Am. Chem.Soc. 1984, 106, 4772 – 4783.

[50] H. S. White, W. G. Becker, A. J. Bard, J. Phys. Chem. 1984, 88, 1840 – 1846.[51] Y. V. Geletii, Z. Huang, Y. Hou, D. G. Musaev, T. Lian, C. L. Hill, J. Am.

Chem. Soc. 2009, 131, 7522 – 7523.[52] F. Li, Y. Jiang, B. Zhang, F. Huang, Y. Gao, L. Sun, Angew. Chem. 2012,

124, 2467 – 2470; Angew. Chem. Int. Ed. 2012, 51, 2417 – 2420.[53] A. B. Tossi, H. Gçrner, J. Photochem. Photobiol. B 1993, 17, 115 – 125.[54] K. Henbest, P. Douglas, M. S. Garley, A. Mills, J. Photochem. Photobiol. A

1994, 80, 299 – 305.[55] J. J. Stracke, R. G. Finke, J. Am. Chem. Soc. 2011, 133, 14872 – 14875.[56] D. Hong, M. Murakami, Y. Yamada, S. Fukuzumi, Energy Environ. Sci.

2012, 5, 5708 – 5716.[57] L. M. Moroney, R. S. Smart, M. W. Roberts, J. Chem. Soc. , Faraday Trans.

1 1983, 79, 1769 – 1778.[58] A. N. Mansour, C. A. Melendres, Surf. Sci. Spectra 1994, 3, 271 – 278.[59] A. F. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 2nd ed. , Wiley-

Interscience, New York, 1966, Section 29-G-5.[60] A. Surca, B. Orel, B. Pihlar, P. Bukovec, J. Electroanal. Chem. 1996, 408,

83 – 100.[61] U. S. Schubert, C. Eschbaumer, P. Andres, H. Hofmeier, C. H. Weidl, E.

Herdtweck, E. Dulkeith, A. Morteani, N. E. Hecker, J. Feldmann, Synth.Met. 2001, 121, 1249 – 1252.

[62] A. Singh, S. L. Y. Chang, R. K. Hocking, U. Bach, L. Spiccia, Catal. Sci. Tech-nol. 2013, 3, 1725 – 1732.

[63] A. Singh, S. L. Y. Chang, R. K. Hocking, U. Bach, L. Spiccia, Energy Environ.Sci. 2013, 6, 579 – 586.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 127 – 134 133

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

[64] M. Risch, K. Klingan, J. Heidkamp, D. Ehrenberg, P. Chernev, I. Zaharieva,H. Dau, Chem. Commun. 2011, 47, 11912 – 11914.

[65] D. K. Bediako, B. Lassalle-Kaiser, Y. Surendranath, J. Yano, V. K. Yachandra,D. G. Nocera, J. Am. Chem. Soc. 2012, 134, 6801 – 6809.

[66] M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca, V. K. Yachandra, D. G.Nocera, J. Am. Chem. Soc. 2010, 132, 13692 – 13701.

[67] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser, ChemCat-Chem 2010, 2, 724 – 761.

[68] M. Risch, K. Klingan, F. Ringleb, P. Chernev, I. Zaharieva, A. Fischer, H.Dau, ChemSusChem 2012, 5, 542 – 549.

[69] R. A. Palmer, T. S. Piper, Inorg. Chem. 1966, 5, 864 – 877.[70] V. Y. Shafirovich, N. K. Khannanov, A. E. Shilov, J. Inorg. Biochem. 1981,

15, 113 – 129.[71] T. Pandiyan, V. M. Consuelo-Estrada, R. Moreno-Esparza, L. Ruiz-Ram�rez,

Inorg. Chim. Acta 2003, 343, 79 – 89.[72] S. V. Kryatov, B. S. Mohanraj, V. V. Tarasov, O. P. Kryatova, E. V. Rybak-Aki-

mova, R. J. Staples, A. Y. Nazarenko, Inorg. Chem. 2002, 41, 923 – 930.

[73] H. J. Guadalupe Hernndez, T. Pandiyan, S. Bernes, Inorg. Chim. Acta2006, 359, 1 – 12.

[74] C. M. Che, S. T. Mak, W. O. Lee, K. W. Fung, T. C. W. Mak, J. Chem. Soc.Dalton Trans. 1988, 2153 – 2159.

[75] E. K. Barefield, F. Wanger, Inorg. Chem. 1973, 12, 2435 – 2439.[76] W. L. F. Armarego, D. D. Perrin, Purification of Laboratory Chemicals,

6th ed. , Pergamon, New York, 2009, pp. 453 and pp. 492.[77] J. Mendham, R. C. Denney, J. D. Barnes, M. J. K. Thomas, Vogel’s Textbook

of Quantitative Chemical Analysis, 6th ed. , Pearson Education Ltd. ,Harlow, 2000, pp. 28 – 31.

[78] S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry,2nd ed. , Marcel Dekker, New York, 1993, pp. 602 – 616.

Received: June 10, 2013Revised: August 11, 2013Published online on October 23, 2013

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