g-C3N4 for Highly Effective Oxygen

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Assembling Bi2MoO6/Ru/g-C3N4 for Highly Effective Oxygen Generation from Water Splitting under Visible-Light Irradiation Yuhang Wu, Meiting Song, Zhanli Chai, and Xiaojing Wang* Inner Mongolia Key Laboratory of Chemistry and Physics of Rare Earth Materials, College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, P. R. China

Downloaded via UNIV AUTONOMA DE COAHUILA on May 14, 2019 at 00:37:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Water oxidation is a kinetically challenging reaction for photocatalytic overall water splitting. Producing one molecule of O2 will consume four electrons, so it is an extremely difficult obstacle for researchers. Here, a Bi2MoO6/Ru/g-C3N4 composite was obtained by a gentle hydrothermal method, which could oxidize water into O2 highly efficiently. The optimal O2 production reached 328.34 μmol· g−1·h−1 under visible light irradiation. Moreover, the catalyst presented excellent stability, as shown by a still sustentative 91.4% photocatalytic activity and invariant textural structure after seven recycling tests. The ternary material had the smallest resistance, which indicated that it has a good photoelectron conductive tunnel, and a rapid transfer route is proposed through Bi2MoO6 → Ru → g-C3N4 → NaIO3 (electron acceptor). The massive holes (h+) with high oxidative potential are surely enriched due to the quick electron migration, being fit for a large promotion of the multiple-electron water oxidative proceedings. Therefore, the metallic Ru provided a powerful bridge for efficient transfer of the interface electrons which could be beneficial to spatial separation of photoexcited carriers without the loss of the high redox capacity. Finally, it is proposed that the Ru-assisted electron transport and constituent synergy in Bi2MoO6/Ru/g-C3N4 composite play crucial roles to its enhanced light utilization, efficient photoelectric conversion property, and high-producing oxygen capability.

1. INTRODUCTION Solar energy induced water splitting to generate H2 and O2 is a highly promising method for the issues of the energy crisis and environmental pollution.1−3 There are two ways to split water: one is the overall water splitting by only one kind of catalyst under the sun’s rays that is very difficult to reach, because the catalyst is required to have a proper band gap, strong redox capability, and high light utilization. Another relatively easy way is to combine two semireactions, namely, the oxidation to O2 and the reduction to H2 of water.4,5 H2 production by water splitting has been widely researched in recent years.6−8 However, O2 evolution is much more difficult kinetically and energetically, which will consume four electrons for the output of an oxygen molecule.9 Therefore, to develop outstanding water oxidation catalysts (WOCs) is of great importance for solar energy induced water splitting.10−12 For the past few decades, various homogeneous transitionmetal multi- or mononuclear complexes with organic ligands have proven to be effective WOCs such as Ru, Ir, Mn, and Co complexes.13−17 However, the drawbacks of these precious and transition metal complexes are the unstablility, high prices, and harsh reactive conditions for any practical applications. Therefore, in order to be economically viable, developing powdered semiconductor WOCs under mild reaction conditions will be expected, in order to give practical applications © XXXX American Chemical Society

of solar water splitting in terms of their cheap, environmentally friendly, and relatively nontoxic properties as well as their simplicity and stability. Various semiconductors such as SrTiO3:Rh, Bi4TaO8Cl, TiO2:Rh,Sb, TiO2:Cr,Sb, and Ir/ CoOx/Ta3N5 show noticeable WOCs activity.18−22 As WOCs, the materials should have overpotentials larger than H2O/O2 (1.23 eV vs NHE). Bi2MoO6 is a layered Aurivillius related oxide with the corner-sharing cell of MoO6 octahedra.23 It has reported that the effective connection of MoO6 octahedra could provide a tunnel for carriers transferring to promote the catalytic oxidative ability.24 Moreover, it has a suitable band gap and very positive valence band potential.25,26 Thus, Bi2MoO6 is considered to be a high performance visible-light-driven photocatalyst,27−29 and its photocatalytic activity has been established to be well improved by hybridizing other semiconductors with a suitable band potential. Many Bi2MoO6-based composites have been reported, such as graphene/Bi2MoO627 and carbon quantum dots/Bi2MoO6.28 Even so, it is still challenging to apply these composite materials for the water oxidation, since (1) oxygen production needs a sufficient overpotential and (2) holes will lose part of their oxidative capacities during the transfer Received: February 22, 2019

A

DOI: 10.1021/acs.inorgchem.9b00524 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry process in a composite semiconductor system.29 For instance, Shaobin Wang et al. found that 3D hierarchical microsphere rGO/Bi2MoO6 presented 18 μmol/L of O2 evolution after 3 h of 300 W xenon lamp irradiation while there is a slight decrease in activity after four-time recycling tests.30 Furthermore, the O2 producing process is far slower than the photogenerated carrier recombination. The oxygen production is second-order, which is far below the submillisecond-order rate of electron−hole combination.31,32 Therefore, the presence of an electron transporter is critical to boost the efficiencies for photocatalytic water oxidation by effective electron relaying. The 2D planar π-conjugated structure of gC3N4 is proven to have good electronic conductivity.33,34 At the same time, it has a high specific surface area and superior adsorption capacity. Combining the unique graphene-like gC3N4 with Bi2MoO6 is a promising option because of the better light harvesting and excellent photocatalytic activities. One has witnessed many efforts in obtaining significant degradation for various organic pollutions. 35 g-C3 N 4 / Bi2MoO6 was reported to exhibit superior photocatalytic activity toward the degradation of dyes (Rhodamine B and methyl blue) under visible light irradiation. Zhou et al. used a g-C3N4/Bi2MoO6 film as photocatalyst that degraded methylene blue (MB) in aqueous solution under visible light irradiation with a photocatalytic ability superior to the singlecomponent photocatalyst.36 Hou et al. used g-C3N4/Bi2MoO6 to decompose Rhodamine B with visible light irradiation, showing a photocatalytic activity higher than that of pure gC3N4 or Bi2MoO6.37 Nonetheless, g-C3N4/Bi2MoO6 is barely considered to be highly active for water oxidation because gC3N4 has a low O2 evolution ability due to its selfdecomposition by photoholes although it has been considered as a highly efficient photocatalyst for H2 generation.38−40 It should be noted that Ru complexes are distinguished catalysts in the aspect of water oxidation, despite their unstability under existing catalytic devices.41 Moreover, their stability can be improved by combining them with semiconductors. Numerous studies about the Ru complexes as the molecular photosensitizers for electron-transfer processes have been reported in the past few years in the field of photocatalysis, for instance, [Ru(bpy)3]2+/3+-type complexes.12−14 Inspired by the Ru complex for water oxidation, loading metallic Ru or its oxide RuO2 on g-C3N4 sheets is expected to be feasible to improve the stability of materials and give an easy transmission for photoexcited electrons. Meanwhile, the metallic Ru or RuO2 can be used as the active sites of water oxidation.40 In this work, a new efficient Z-scheme water oxidation photocatalyst Bi2MoO6/Ru/g-C3N4 was constructed. The excellent O2 evolution was achieved by linking the highly oxidative catalyst Bi2MoO6 and the high electron transfer gC3N4 with the metallic Ru’s aid. The prepared catalyst had been used to explore the photocatalytic performance of water for oxygen production under visible light irradiation. A welldesigned synergistic mechanism was proposed, including the Schottky barrier built in the interface between Ru and semiconductor and the unobstructed electron transfer channel formed across the interfacial domain, which could promote efficiently O2 generated activity by well separating the photogenerated carriers and the very positive overpotential in the oxidative terminal.

2. EXPERIMENTAL SECTION All the chemicals are analytically pure and were used directly after purchase. 2.1. Synthesis. The pure g-C3N4 was obtained through heating melamine with a speed of 5 K·min−1 up to 823 K for 4 h. The melamine was hydrothermally treated for 24 h at 453 K in advance by deionized water. The single Bi2MoO6 was synthesized by hydrothermal selfassembly method. First, Bi(NO3)3·5H2O (A) and NaMoO4·2H2O (B) with a mole ratio of 2:1 were dissolved into an equal volume of ethylene glycol solution, respectively. Then the A solution was added into the B solution drop by drop. After the mixture solution was stirred for 30 min, an equal volume of ethanol (99.7 wt %, analytical grade, Tianjin Fengchuan Chemical Reagent Technologies Co. Ltd.) was added. Last, the above mixture solution was transferred into a high pressure reactor with Teflon as liner (HPRT) and heated for 12 h at 433 K. The pale yellow power material was obtained through washing three times with deionized water. The binary Bi2MoO6/g-C3N4 was prepared by heating the different mass ratio of Bi2MoO6 and g-C3N4 at 433 K for 12 h with deionized water as solvent in HPRT. The pale yellow power Bi2MoO6/g-C3N4 was dried in an oven at 333 K for 12 h. A series of Bi2MoO6/g-C3N4 materials with different mass ratio (Bi2MoO6:g-C3N4 = 5/6, 5/5, 5/4, 5/3, 5/2 and 5/1) were prepared and used as Bi2MoO6/g-C3N4-x, where x is representing the mass ratio. The Ru/Bi2MoO6 and Ru/g-C3N4 were obtained by reduction of ruthenium(III) trichloride (RuCl3) with sodium borohydride (NaBH4) in solution. A RuCl3 solution (5.00 mmol·L−1) with a different volume was added to the Bi2MoO6 aqueous solution to form each Ru loading material. Then 0.01 mol·L−1 NaBH4 solution was dropwise into the above solution until RuCl3 was reduced to Ru0 under continuous stirring 5 h. The products were washed three times with distilled water and dried in an oven overnight (12 h) at 333 K to obtain Ru/Bi2MoO6 with different Ru loading contents (that is, 0.38, 1.14, 1.90, and 2.66 wt % according to the dosing ratio), used as Ru/ Bi2MoO6-x, where x represents the dosing mass ratio. For comparison, a Ru/g-C3N4 sample with a mass ratio of 1.9 wt % (Ru to g-C3N4) was prepared by a similar procedure, used as Ru/gC3N4 1.9 wt %. The Bi2MoO6/Ru/g-C3N4 was obtained by heating the same mass ratio of Ru/Bi2MoO6 and g-C3N4 at 433 K for 12 h with deionized water as solvent in HPRT. Then, the product was naturally cooled to the environment temperature and washed three times with deionized water. Finally, pale yellow power Bi2MoO6/Ru/g-C3N4 was obtained after drying in an oven overnight (12 h) at 333 K. The used characterization methods are provided in the Supporting Information (Experimental Section). 2.2. Photocatalytic Test. The photocatalytic activity was assessed by photocatalytic water oxidation using a vacuum closed circulation system as shown in Figure S1. The catalysts (0.05 g) were evenly dispersed in 100 mL of solution, and the pH was adjusted to be 10 with the help of 5 mol·L−1 NaOH solution. NaIO3 (0.396 g) was used as an electron trapping agent. The mixing solution was deoxidation by bubbling with N2 gas about 30 min in a glass reactor (200 mL). The quartz sheet served as light transmission window and the seal cover. The reaction was processed under visible light irradiation (wavelength >420 nm, 300 W xenon light source, beam diameter 5 cm, 30 mW· cm−2) at room temperature. Then the xenon light source was set at a 10 cm distance from the liquid level of the solution. The productivity of O2 was repeatedly measured at an interval of 1 h with the online gas chromatography connected to the closed reactor within continuous 7 h visible light irradiation. The generated O2 was separated by passing through a 2 m × 3 mm packed 5A column of molecular sieves with Ar carrier gas (high purity 99.999%) and measured by gas chromatography with a thermal conductivity detector (TCD). 2.3. Computational Details. The electronic structure, energy band structure, density of state, and work functions of Ru (100), Bi2MoO6 (100), Ru/Bi2MoO6, and Ru/g-C3N4 were calculated by B


Article

Inorganic Chemistry Scheme 1. Synthetic Process of Ternary Catalyst Bi2MoO6/Ru/g-C3N4

Figure 1. (a) XRD patterns of g-C3N4, Bi2MoO6, Bi2MoO6/g-C3N4 5/5, Ru/g-C3N4 1.90 wt %, Ru/Bi2MoO6 1.90 wt %, and Bi2MoO6/Ru/gC3N4. The vertical lines color of red and black represented the stand cards of Ru (JCPDS 03-065-7646) and Bi2MoO6 (JCPDS 01-072-1524). (b) XPS spectrum of Ru and C in Bi2MoO6/Ru/g-C3N4. density functional theory (DFT), which was performed with the CASTEP code of Materials Studio. The 3 × 1 × 1 supercells for the pure phase of Ru and Bi2MoO6 have been built. A perpendicular vacuum between neighboring slab images was 20 Å for the slab model. For the models of Ru/Bi2MoO6 and Ru/g-C3N4, the six-atom Ru cluster was established with a simple shape and the lengths of the x, y, and z axes are all 4 Å. Then the Ru cluster was placed on the surface of Bi2MoO6 (or g-C3N4) supercell, and the charge density difference, band structure, and density of state for g-C3N4, Ru/g-C3N4, Bi2MoO6, and Ru/Bi2MoO6 were calculated through the energy task. The Perdew−Burke−Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA) is used as the optimization functional. The electron wave function is expanded in plane waves up to a cutoff energy of 420 eV in the case of ultrafine potential. Calculations of the lattice parameters, as well as the atomic position relaxations, were carried out until the forces and total energy converged within 0.015 eV/Å and 10−5 eV, respectively. The work function was calculated by population analysis and obtained through the CASTEP AnalysisPotentials module.

Bi2MoO6. For the binary (Bi2MoO6/g-C3N4 5/5, Ru/Bi2MoO6 1.90 wt %, Ru/g-C3N4 1.90 wt %) and ternary composite (Bi2MoO6/Ru/g-C3N4), no redundant peaks are observed, except for the separated components Bi2MoO6, g-C3N4, and Ru. In addition, for g-C3N4 supported samples, the expanded peak at 27.4° vests in the (0 0 2) plane and ascribes to superposition of conjugated aromatic systems (Figure S2). For the Ru-loaded samples, Ru/g-C3N4 1.90 wt % (Figure 1a), Ru/ Bi2MoO6 (Figure S3), and Bi2MoO6/Ru/g-C3N4 (Figure 1a), the characteristic peak at 44.02° of Ru (JCPDS 03-065-7646) could be found, indicated that Ru being successfully compounded together with Bi2MoO6 and g-C3N4. The X-ray photoelectron spectroscopy (XPS) was further measured to explore the elements valence state of Bi2MoO6/ Ru/g-C3N4 (Figure 1b and Figures S4 and S5). The survey spectrum shows all elements in the Bi2MoO6/Ru/g-C3N4 (Figure S4). The signals of 158.91 and 164.22 eV vest in the Bi 4f7/2 and Bi 4f5/2 of the Bi3+ 42 while the peaks at 232.20 and 235.35 eV are attributed to the Mo 3d5/2 and Mo 3d3/2 of the sample.29 The signals at 530.04, 531.31, and 532.28 eV vest in the lattice oxygen, hydroxyl oxygen, and adsorbed oxygen of the Bi2MoO6 surface (Figure S5, parts a, b, and c).43 The peak of N is fitted into three peaks (Figure S5d) where the binding energies at 398.46 eV vest in the sp2 of (CN−C)38 and the signals of 398.89 and 400.58 eV vest in g-C3N4 just as in the recited analysis.39 As shown in Figure 1b, the signals at 284.66 and 288.21 eV belong to the C 1s and the sp2 hybridization of (C-(N)3) for g-C3N4. At the same time, the weak peaks at 279.97 and 284.07 eV are assigned to Ru 3d5/2 and Ru 3d3/2 of the element Ru0.44 The information from XPS and XRD is the

3. RESULTS AND DISCUSSION 3.1. Morphologies and Structure of the Prepared Materials. The hollow spherical Bi2MoO6 was prepared by the hydrothermal self-assembly method. Then the catalyst Bi2MoO6/Ru/g-C3N4 was obtained by gentle hydrothermal synthesis method with g-C3N4 and Ru/Bi2MoO6 prepared in advance (Scheme 1). The prepared materials were explored by X-ray diffraction (XRD) in Figure 1a. There are two peaks at 2θ = 13° and 27.5° of g-C 3 N 4 (JCPDS 87-1526), corresponding to the (1 0 0) and (0 0 2) planes of g-C3N4, respectively. For the pure sample Bi2MoO6, all the signals correspond to the standard card JCPDS 01-072-1524 of C


Article

Inorganic Chemistry

microspheres, which are assembled by a large set of nanoplatelets (Figure 3b). When Bi2MoO6 is covered with gC3N4 (Figure 3c), the assembled materials mostly maintain the original appearance of the separated components with a close contact with each other whereas part of Bi2MoO6 decomposes into small pieces and distributes over the g-C3N4 surface. Interestingly for Bi2MoO6/Ru/g-C3N4, Ru was well attached to Bi2MoO6 and partially links with g-C3N4, which was marked with the magenta virtual coil in Figure 3d, confirming that most of the Ru particles preferentially adhered to Bi2MoO6. Similar results can be observed in the samples TEM and HRTEM (Figure 4). The ternary catalyst is distributed

strong evidence that the metallic Ru particles were successfully loaded to Bi2MoO6/g-C3N4 and formed the trinary catalyst Bi2MoO6/Ru/g-C3N4 according to the present preparation method. Furthermore, the light absorptive performances of the prepared catalysts were measured through UV−vis diffuse reflectance spectroscopy (Figure 2 and Figures S6 and S7). It

Figure 2. UV−vis diffuse reflectance spectroscopy of the prepared materials. Figure 4. TEM images (a) and HRTEM images (b) of Bi2MoO6/Ru/ g-C3N4.

was observed that the absorption edges of g-C3N4 and Bi2MoO6 were 485 and 467 nm, respectively. For Ru-loaded catalysts, the light absorption intensity of the catalysts increased with the increase of Ru content at 500 to 800 nm (Figure 2 and Figure S7), which indicated that the Ru-loaded samples have a good light response. Moreover, the ternary composite has the strongest absorption at the visible region than other samples. It indicated that the Bi2MoO6/Ru/g-C3N4 could well harvest visible light to produce photogenerated electrons and holes for the catalytic reaction. The morphologies of the as prepared catalysts were explored through the scanning electron microscope (SEM) and transmission electron microscopy (TEM). Figure 3a shows that g-C3N4 presents a typical irregular agglomerated piece structure. And Bi2MoO6 is in the form of petal-shaped hollow

uniformly in the field of view (Figure 4a), the small pieces of Bi2MoO6 exhibit microspheres and are scattered in crumbled multilayer sheets of g-C3N4 film. It is observed that the diameter of Ru particles is about 10 nm from the HRTEM (Figure 4b). The stripe width of 0.315 nm is attributed to the (1 3 1) plane of Bi2MoO6, and 0.234 nm is assigned to the (1 0 0) plane of Ru element (JCPDS 03-0657646) in the HRTEM images of Figure 4b. Thus, it is indicated the three-phase materials are in uniform contact, and Ru particles as a linkage bridge may play a key role for efficient transfer of the interface electrons. For further investigation of the elemental distribution in the ternary material, STEM-EDX chemical mapping was studied. Figure 5a is the area photograph of Bi2MoO6/Ru/g-C3N4, including as much as possible the three kinds of components. The weight percentages of all elements in Bi2MoO6/Ru/gC3N4 are 22.65, 35.25, 1.97, 27.45, 6.35, and 6.33% for C, N, Ru, Bi, Mo, and O (Figure 5, parts b and c), respectively, basically consistent with the theoretical value. In addition, the amount of Ru loading in Bi2MoO6/Ru/g-C3N4 sample was measured to be 1.46 wt % with inductively coupled plasma optical emission spectrum (ICP-OES), presenting consistency with both EDX and the initial dosing quantity. The elemental distributions of C and N have close consistent coordinates at the same area of the map, assigned to a collection of g-C3N4 (Figure 5, parts d and e). The elemental distribution profiles of Bi, Mo, and O present rough concordant patterns, belonging to a assembly of Bi2MoO6 (Figure 5, parts f−h). Meanwhile, it was observed that most of Ru particles distributed in nearly same locations with Bi2MoO6 (Figure 5i), inferring the strong interactive existence of Ru particles with Bi2MoO6 to inhibit Ru’s migration, consistent with the result of SEM (Figure 3d) and TEM (Figure 4d). 3.2. Activity Assessing of the Photocatalytic Reaction. Photocatalytic water oxidation tests were carried out

Figure 3. SEM images of g-C3N4 (a), Bi2MoO6 (b), Bi2MoO6/g-C3N4 5/5 (c), and Bi2MoO6/Ru/g-C3N4 (d). The magenta marked area represents Ru particles. D


Article

Inorganic Chemistry

Figure 5. STEM-EDX chemical mapping of Bi2MoO6/Ru/g-C3N4 (a, b), the weight percentages of all elements in Bi2MoO6/Ru/g-C3N4 (c), and the separated elemental distributions of C, N, Bi, Mo, O, and Ru (d−i).

Figure 6. (a) Oxygen productivity of the as prepared catalysts with visible light irradiation. (b) Cycling experiment of Bi2MoO6/Ru/g-C3N4.

using NaIO3 as a sacrificial electron acceptor at room temperature to investigate the activities of the prepared materials. The catalysts were evenly dispersed in 100 mL solution under visible light irradiation. The productivity of O2 was measured by online gas chromatography connected to the closed reactor (Figure S1). The maximum O2 evolution amounts are 104.55 (Bi2MoO6), 12.91 (g-C3N4), 39.92 (Bi2MoO6/g-C3N4 5/5), 119.11 (Ru/Bi2MoO6 1.90 wt %), 20.19 (Ru/g-C3N4 1.90 wt %) and 328.34 μmol·g−1·h−1 (Bi2MoO6/Ru/g-C3N4) after visible light irradiation, respectively (Figure 6a). Obviously, the O2 production is poor for either the sole Bi2MoO6 or g-C3N4 and even the composites Bi2MoO6/g-C3N4 (Figure S8). However, oxygen evolution is slightly promoted by loading metallic Ru on both Bi2MoO6

and g-C3N4 (Figures S9 and S10). More importantly, after loading metallic Ru to the binary composite, the oxygen production of Bi 2 MoO 6 /Ru/g-C 3 N 4 could be quickly promoted (Figure 6a), 25.33 times as much as g-C3N4 and 3.14 times as much as single Bi2MoO6. Obviously, the excellent oxygen production efficiency of Bi2MoO6/Ru/g-C3N4 gives credit to the Ru-assisted electron transport and the composite effect of the separated component. It is inferred that the ternary Bi2MoO6/Ru/g-C3N4 can accelerate the separation of carriers in such Z-scheme structure, leading to the increased catalytic efficiency. It is well-known that water oxidation proceeds according to the following reaction formula: 2H2O + 4h+ → O2 + 4H+.45,46 Therefore, it is speculated that by consuming excess H+, it will speed up the photocatalytic water E


Article

Inorganic Chemistry

Figure 7. Photoluminescence spectra (a), the time-resolved fluorescence decay spectra (b), the transient photocurrent recorded (c), and the EIS Nyquist plots (d) of the as-prepared materials.

C3N4 is greatly reduced with a longest lifetime of fluorescence. It infers that the carriers’ recombination rate of ternary material is absolutely the lowest among these prepared samples. Furthermore, from transient photocurrent testing (Figure 7c), we have also observed the ternary material exhibits the highest transient photocurrent than any single and binary samples and has a tendency toward stabilization, evidencing there is a synergistic effect among the constitutions to inhibit the recombination of photoexcited carriers. It is also noted that g-C3N4 presents the lowest photocurrent density, while via loading Ru, the separation efficiency of the photogenerated carriers does really achieve high promotion. Furthermore, electrochemical impedance spectroscopy (EIS) is used to explore our samples conductive properties. Figure 7d shows the EIS Nyquist plots of as prepared materials under illumination. The arc radii of the samples loaded with Ru are quit smaller than those without Ru loadings. It indicates that the introduction of Ru helps to decrease resistance of the electrons migration, and greatly promote the separation efficiency of carriers. Meanwhile, it is more notable that Bi2MoO6/Ru/g-C3N4 has the smallest arc radius in all samples, implying the lowest interface resistance and the fastest interfacial electronic transfer. From the photoelectrochemical testing, one can deduce that the Ru loading and constituent synergy in Bi2MoO6/Ru/g-C3N4 composite play the crucial roles to its efficient photoelectric conversion property and excellent electron transport capability. To investigate sample redox, the polarization curves of the materials were tested in the dark (Figure S16). As we know, a positive current less than 1.4 V vs Ag/AgCl can favor oxygen evolution.31,32 The catalytic water oxidation voltage of

oxidation reaction rate. Therefore, the oxygen production of Bi2MoO6/Ru/g-C3N4 was investigated under different pH values (Figure S11). The oxygen production of the catalyst gradually increases with the increase of the pH value, and it reaches a maximum when the pH value is 10. It indicates that the alkaline environment helps to accelerate the rate of the oxygen generation by photocatalytic water oxidation. To explore the stability of Bi2MoO6/Ru/g-C3N4, the reaction was performed over seven recycling tests. As illustrated in Figure 6b, more than 91.4% of the photocatalytic activity of Bi2MoO6/Ru/g-C3N4 after seven runs could still be retained. The initial and used Bi2MoO6/Ru/g-C3N4 catalysts were examined by XRD (Figure S12) and SEM (Figure S13). No significant change in either crystal phase structure or the morphology of the catalysts is observed, confirming that the photocatalyst, Bi2MoO6/Ru/g-C3N4, is quite stable under reactive conditions. Furthermore, the O2 evolution activities from the references for several semiconductor catalysts and Rubased composite catalysts were listed in the Supporting Information, Table S1, for comparative purposes. Obviously, our sample Bi2MoO6/Ru/g-C3N4 presents an attractive performance as a promising water oxidative photocatalyst. 3.3. Investigation of the Photocatalytic Water Oxidation Mechanism. Photoluminescence spectra (PL) and the time-resolved fluorescence decay spectra were tested to characterize the recombination and transfer behaviors of photogenerated carriers of the prepared Bi2MoO6/Ru/g-C3N4 (Figure 7, parts a and b). The PL properties for Bi2MoO6/gC3N4 and Ru/Bi2MoO6 with the different composition ratio were also investigated and displayed in Figures S14 and S15, respectively. The fluorescence intensity of Bi2MoO6/Ru/gF


Article

Inorganic Chemistry

Figure 8. Charge density difference of Ru/g-C3N4 (a) and Ru/Bi2MoO6 (b) and the band structures of g-C3N4 and Ru/g-C3N4 (c) and Bi2MoO6 and Ru/Bi2MoO6 (d).

Figure 9. Plots of transformed Kubelka−Munk function vs the energy for g-C3N4 and Bi2MoO6 (a), VB-XPS spectra of the catalysts g-C3N4 and Bi2MoO6 (b), and the electrostatic potential of Bi2MoO6 (100) face (c) and Ru (100) face (d). G


Article

Inorganic Chemistry

Figure 10. •O2− (a) and •OH (b) radical active species tested by ESR with the visible light irradiating for 10 min over Bi2MoO6/g-C3N4 5/5 and Bi2MoO6/Ru/g-C3N4.

electron transfer are shown in Figure S18. When we combine Bi2MoO6 and g-C3N4 to build a p−n junction, electrons will flow from g-C3N4 to Bi2MoO6 (Figure S18a). As a result, the bands are totally moved up for Bi2MoO6 and moved down for g-C3N4 until the thermodynamic equilibrium being reached. The valence band potential of g-C3N4 is lower than the oxidation−reduction potential of •OH/OH− (2.40 eV vs NHE) and the conduction band potential of Bi2MoO6 is more positive than O2/•O2− (−0.33 eV vs NHE).47,48 Thus, this binary composite is expected to produce less O2, conformed to the experimental results (Figure 6a), which could be convinced by both weak signals of the •O2− and •OH species under visible light irradiation in ESR test (Figure S19, parts a and b). Furthermore, something completely different happens if we assemble Bi2MoO6 and g-C3N4 through a Ru bridge. In order to explore the band energy aligning in such complex system, the work functions of Bi2MoO6(100) and Ru(100) are calculated by the CASTEP Analysis-Potentials module and they are 6.160 and 5.203 eV, respectively (Figure 9, parts c and d). The work function of g-C3N4 was 4.025 eV as our previous report.35 As we all known, the greater the work function of the materials is, the stronger the binding electron capacity is, and the more difficult it is for the electron to get away from the material. Therefore, in terms of the assessment of the work function, it is concluded that Bi2MoO6 has the stronger binding electron ability. Meanwhile, the VB of Bi2MoO6 is involved in the existing elements including O, Bi, and Mo as observed in Figure S20. Furthermore, the Fermi energy (Ef) could be calculated from these work function values according to the following formula:

Bi2MoO6/Ru/g-C3N4 is tested at onset potential (current value: zero) to be about 0.9 V. Moreover, the current density is 1.0 mA·cm−2 for Bi2MoO6/Ru/g-C3N4, whereas in contrast, only a very minimum current (about 0.04 mA·cm−2) is obtained for Bi2MoO6/g-C3N4. Obviously, the current density is stronger and the overpotential is more negative for Ru loading samples than those without Ru loading. It clearly indicates that Ru component in the composites can be easier to promote multiple-electron water oxidative ability via an excessive driving potential. Furthermore, to theoretically investigate the recombination of photocatalytic e-h pair, the charge density difference, band structure, and density of state for g-C3N4, Ru/g-C3N4, Bi2MoO6, and Ru/Bi2MoO6 were calculated by DFT with the CASTEP code of Materials Studio. For both Ru/g-C3N4 and Ru/Bi2MoO6 (Figure 8a, b), obviously, the electronic densities of Ru atoms are decreased while those in substrates gC3N4 and Bi2MoO6 are increased. It indicates that the electrons of Ru atoms are easily transferred to substrate (gC3N4 or Bi2MoO6). Interestingly, for Ru/Bi2MoO6, the Ru atoms form a strong chemical bond with the O atom from substrate Bi2MoO6 whereas the case is obviously different for Ru/g-C3N4, confirming Ru preferentially adheres to the Bi2MoO6 surface with a strong interaction which is consistent with the results of SEM (Figure 3d). Furthermore, the energy band structures (EBS) and density of state (DOS) of g-C3N4, Ru/g-C3N4, Bi2MoO6, and Ru/Bi2MoO6 are obtained (Figure 8, parts c and d, and Figure S17). By analyzing EBS and DOS, it is obvious that with Ru addition, the band gaps of Ru/gC3N4 and Ru/Bi2MoO6 decrease, supporting the results of a wide solar harvesting performance from UV−vis diffuse reflectance spectroscopy. In addition, the energy states in VB for Ru/g-C3N4 and Ru/Bi2MoO6 are all significantly dense, which are expected to increase the mobility of photogenerated electrons and thus effectively reduce the recombination of e-h pairs. For further determining the transfer path of carriers, the band potentials of the single catalysts were measured by UV− vis diffuse reflectance spectroscopy and the VB-XPS spectrum (Figure 9). The band gaps (Eg) of g-C3N4 and Bi2MoO6 are 2.47 and 2.65 eV, respectively (Figure 9a). The valence band (VB) values of g-C3N4 and Bi2MoO6 are tested to be 1.51 and 2.43 eV (versus NHE) while the conduction band (CB) values of g-C3N4 and Bi2MoO6 are counted as −0.96 and −0.22 eV, respectively (Figure 9b). According to the alignment of their band levels, the illustrated pathways about the interface

Ef = Evacuum − Φ

Here Evacuum and Φ represent the energy of a vacuum stationary electron and work function, respectively. The Ef of Bi2MoO6, Ru, and g-C3N4 were obtained as 1.66, 0.703, and 0.475 eV vs NHE, respectively (Figure 9, parts c and d). Obviously, the Fermi level of Ru is higher than a p-type Bi2MoO6 and lower than a n-type g-C3N4 (Figure S18b). So the band potentials of Bi2MoO6 are bent downward when contacted with Ru, and the Schottky barrier is established between Ru and p-Bi2MoO6, which holds back photoexcited holes, but electrons can flow from Bi2MoO6 to Ru. Meanwhile, the band potentials of g-C3N4 are bending upward, and thus the electrons move into Ru is not allowed owning to the existence of the Schottky barrier. Conclusively, the electron could easily transfer from Bi2MoO6 via Ru to g-C3N4 which is H


Article

Inorganic Chemistry

Figure 11. Structure of the Z-scheme catalysts and the oxygen production mechanism for Bi2MoO6/Ru/g-C3N4.

boosted by both inner interface electric field and the Schottky barrier. Obviously, the CB electrons of g-C3N4 and the VB holes of Bi2MoO6 are not transported, and thus the strong redox properties are retained simultaneously. It is evidenced by the strong signals of DMPO−•O2− and DMPO−•OH of ternary catalyst Bi2MoO6/Ru/g-C3N4 under the 10 min visible light irradiation from EPR testing (Figure 10 and Figure S19c,d). In this case, a Ru sandwiched p−n junction was built in the ternary catalyst, and the metallic Ru could provide a powerful bridge for efficient transfer of the interface electrons. Therefore, the catalyst Bi2MoO6/Ru/g-C3N4 benefits, to lower the recombination rate of carriers as well as maintain a high redox capability. On the basis of the above discussion, the photocatalytic water oxidation mechanism is depicted in Figure 11. With visible light irradiating, either Bi2MoO6 or g-C3N4 can be readily excited to form carries own to its narrow band gap. The VB value of Bi2MoO6 is effective for the O2 generation due to the more positive potential than that of H2O/O2 (1.23 eV vs NHE),42 as shown by a better O2 yield of 104.55 μmol·g−1·h−1 (Figure 6). However, the recycle activity is quickly deceased due to the slower oxidative proceeding compared with the quick recombination. For a sole g-C3N4, it has poor water oxidation ability as indicated by weak O2 production (12.91 μmol·g−1·h−1), which is the result of the lower positive potential compared with that of H2O/O2. For Bi2MoO6/gC3N4 (Figure 11a), the electrons could be collected on CB of Bi2MoO6; however, the holes will move to VB of g-C3N4 via a p−n junction contact, which greatly enhanced the electron and hole separation efficiencies. Unfortunately, as the electrons/ holes move toward the more positive/negative energy bands, the electronic reduction and the hole oxidizing ability are greatly weakened. Therefore, a bad O2 evolution (39.92 μmol· g−1·h−1) of Bi2MoO6/g-C3N4 is observed. Naturally, the O2 evolution is not promoted by Ru loading in sole Bi2MoO6 due to the fact that recombination could not be inhibited. Meanwhile Ru/g-C3N4 is also unable to split water to produce O2 because the valence potential (h+) of g-C3N4 could not be shifted to positive and to beneficial water oxidation via Ru loading although the carriers’ separation can indeed be promoted. As is very interesting for ternary catalyst (Figure 11b), the Ef of Ru (0.475 eV) is between the CB of p-type Bi2MoO6 and VB of n-type g-C3N4. Thence, this helps to quickly make the photogenerated electrons from CB of Bi2MoO6 flow into metallic Ru through a Schottky barrier49 and then combine with photogenerated holes from g-C3N4, leaving the holes in VB of Bi2MoO6 with stronger oxidative ability for further water oxidation reaction. It also was noticed

that at the same reaction conditions (Figure S21), only without NaIO3, 1 μmol of O2 product was obtained after 1 h visible light irradiation, which supported a rapidly transferred route through the Z-scheme (Bi2MoO6 → Ru → g-C3N4) due to the good conductive behavior of g-C3N4. It is indisputable that the recombination of photocarriers is effectively inhibited and the multiple-electron water oxidative proceeding O2 is largely promoted due to the preferential transfer and controllable retention of photoexcited carriers in each redox terminals.

4. CONCLUSIONS In summary, we have successfully constructed a ternary Bi2MoO6/Ru/g-C3N4 with excellent activity for O2 evolution through water splitting under visible light irradiation via assembling metallic Ru into the p−n junction of Bi2MoO6/gC3N4. As is expected, Bi2MoO6/Ru/g-C3N4 shows a durable excellent O2 productivity with a value of 2298.38 μmol·g−1 after 7 h of visible light irradiation and seven repeated uses, absolutely higher than that of the separated single components and binary composites. In such a ternary system, the loading Ru particles could form a Schottky barrier with both p-type Bi2MoO6 and n-type g-C3N4, which has been aligned with a p− n junction due to the matched band levels. United with a good conductive behavior of g-C3N4, a rapid transfer route of the photoelectrons through Bi2MoO6 → Ru → g-C3N4 → NaIO3 is formed. In such a particular structure, the preferential transfer and controllable retention of photoexcited carriers in each redox terminals is well achieved. Thus, the electron−hole pairs are efficiently separated without the loss of the high redox capacity, as well as largely promoting the multiple-electron water oxidative reaction. It is proposed that the Ru loading and constituent synergy in Bi2MoO6/Ru/g-C3N4 composite play the crucial roles to its enhanced visible light utilization, efficient photoelectric conversion, excellent photogenerated carrier separation, and electron transport capability.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00524. Details of catalyst characterization, experiment device diagram, XRD patterns, XPS survey spectrum and the enlarger images of the separated elements, UV−vis diffuse reflectance spectra, time courses of O2 evolution, SEM images, photoluminescence spectra, linear sweep voltammetry (LSV) curves, the total density of state I


Article

Inorganic Chemistry

(12) Karkas, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial photosynthesis: molecular systems for catalytic water oxidation. Chem. Rev. 2014, 114, 11863−2001. (13) Kärkäs, M. D.; Åkermark, T.; Chen, H.; Sun, J.; Åkermark, B. A tailor-made molecular ruthenium catalyst for the oxidation of water and its deactivation through poisoning by carbon monoxide. Angew. Chem., Int. Ed. 2013, 52, 4189−4193. (14) Thompson, D. W.; Ito, A.; Meyer, T. J. [Ru(bpy)3]2+ and other remarkable metal-to-ligand charge transfer (MLCT) excited states. Pure Appl. Chem. 2013, 85, 1257−1305. (15) Volpe, A.; Sartorel, A.; Tubaro, C.; Meneghini, L.; Di Valentin, M.; Graiff, C.; Bonchio, M. N-heterocyclic dicarbene iridium(III) catalysts enabling water oxidation under visible light irradiation. Eur. J. Inorg. Chem. 2014, 2014, 665−675. (16) DePasquale, J.; Nieto, I.; Reuther, L. E.; Herbst-Gervasoni, C. J.; Paul, J. J.; Mochalin, V.; Zeller, M.; Thomas, C. M.; Addison, A. W.; Papish, E. T. Iridium dihydroxybipyridine complexes show that ligand deprotonation dramatically speeds rates of catalytic water oxidation. Inorg. Chem. 2013, 52, 9175−9183. (17) McAlpin, J. G.; Surendranath, Y.; Dinca, M.; Stich, T. A.; Stoian, S. A.; Casey, W. H.; Nocera, D. G.; Britt, R. D. EPR evidence for Co(IV) species produced during water oxidation at neutral pH. J. Am. Chem. Soc. 2010, 132, 6882−6883. (18) Okunaka, S.; Tokudome, H.; Abe, R. Z-scheme water splitting into H2 and O2 under visible light over photocatalyst panels consisting of Rh-doped SrTiO3 and BiVO4 fine particles. Chem. Lett. 2016, 45, 57−59. (19) Nakada, A.; Saeki, A.; Higashi, M.; Kageyama, H.; Abe, R. Abe.Two-step synthesis of Sillén−Aurivillius type oxychlorides to enhance their photocatalytic activity for visible-light-induced water splitting. J. Mater. Chem. A 2018, 6, 10909−10917. (20) Oropeza, F. E.; Egdell, R. G. Control of valence states in Rhdoped TiO2 by Sb co-doping: A study by high resolution X-ray photoemission spectroscopy. Chem. Phys. Lett. 2011, 515, 249−53. (21) Calatayud, J. M.; Balbuena, J.; Cruz-Yusta, M.; Martín, F.; Pardo, P.; Sánchez, L.; Alarcón, J. Mesocrystalline Cr and Sb-codoped anatase visible-light-driven photocatalyst. Ceram. Int. 2018, 44, 8232− 8241. (22) Ma, S. S. K.; Maeda, K.; Hisatomi, T.; Tabata, M.; Kudo, A.; Domen, K. A redox-mediator-free solar-driven Z-scheme watersplitting system consisting of modified Ta3N5 as an oxygen-evolution photocatalyst. Chem. - Eur. J. 2013, 19, 7480−74866. (23) Zhang, L.; Xu, T.; Zhao, X.; Zhu, Y. Controllable synthesis of Bi2MoO6 and effect of morphology and variation in local structure on photocatalytic activities. Appl. Catal., B 2010, 98, 138−146. (24) Zhou, F.; Shi, R.; Zhu, Y. Significant enhancement of the visible photocatalytic degradation performances of γ-Bi2MoO6 nanoplate by graphene hybridization. J. Mol. Catal. A: Chem. 2011, 340, 77−82. (25) Wang, Z.; Huo, Y.; Zhang, J.; Lu, C.; Dai, K.; Liang, C.; Zhu, G. Facile preparation of two-dimensional Bi2MoO6@Ag2MoO4 coreshell composite with enhanced visible light photocatalytic activity. J. Alloys Compd. 2017, 729, 100−108. (26) Wu, Y. H.; Song, M. T.; Wang, Q. J.; Wang, T.; Wang, X. A highly selective conversion of toxic nitrobenzene to nontoxic aminobenzene by Cu2O/Bi/Bi2MoO6. Dalton Trans. 2018, 47, 8794−8800. (27) Wang, P. F.; Ao, Y. H.; Wang, C.; Hou, J.; Qian, J. A one-pot method for the preparation of graphene-Bi2MoO6 hybrid photocatalysts that are responsive to visible-light and have excellent photocatalytic activity in the degradation of organic pollutants. Carbon 2012, 50, 5256−5264. (28) Di, J.; Xia, J. X.; Ji, M. X.; Li, H. P.; Xu, H.; Li, H. M.; Chen, R. The synergistic role of carbon quantum dots for the improved photocatalytic performance of Bi2MoO6. Nanoscale 2015, 7, 11433− 11443. (29) Hou, J.; Cheng, H.; Takeda, O.; Zhu, H. Unique 3D heterojunction photoanode design to harness charge transfer for efficient and stable photoelectrochemical water splitting. Energy Environ. Sci. 2015, 8, 1348−1357.

(TDOS) and projected density of state (PDOS), schematic diagram, EPR testing, and the activity contrast with the values reported in the literature (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(X.W.) E-mail: [email protected]. Fax: +86-4714994406. Telephone:+86-13948315232. ORCID

Yuhang Wu: 0000-0002-4339-0753 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work is financially supported by the National Natural Science Foundation of China (Grant Numbers 21777078 and 21567017) and the Project of Research and Development of the Applied Technology for Inner Mongolia (201702112).

(1) Ding, E.; Li, A.; Liu, H.; Liu, W.; Chen, F.; Li, T.; Wang, B. Facile synthesis of ultrathin two-dimensional nanosheets-constructed MCo2O4 (M = Ni, Cu, Zn) nanotubes for efficient photocatalytic oxygen evolution. Nanoscale 2018, 10, 3871−3876. (2) Reza Gholipour, M.; Dinh, C. T.; Béland, F.; Do, T. O. Nanocomposite heterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting. Nanoscale 2015, 7, 8187− 8208. (3) Melvin, A. A.; Illath, K.; Das, T.; Raja, T.; Bhattacharyya, S.; Gopinath, C. S. M-Au/TiO2 (M = Ag, Pd, and Pt) nanophotocatalyst for overall solar water splitting: role of interfaces. Nanoscale 2015, 7, 13477−13488. (4) Maeda, K.; Higashi, M.; Lu, D.; Abe, R.; Domen, K. Efficient nonsacrificial water splitting through two-step photoexcitation by visible light using a modified oxynitride as a hydrogen evolution photocatalyst. J. Am. Chem. Soc. 2010, 132, 5858−5868. (5) Xie, Y. P.; Wang, G. S. Visible light responsive porous Lanthanum-doped Ag3PO4 photocatalyst with high photocatalytic water oxidation activity. J. Colloid Interface Sci. 2014, 430, 1−5. (6) Yu, F. T.; Cui, S. C.; Li, X.; Peng, Y. Y.; Yu, Y.; Yun, K.; Zhang, S. C.; Li, J.; Liu, J. G.; Hua, J. L. Effect of anchoring groups on Nannulated perylene-based sensitizers for dye-sensitized solar cells and photocatalytic H2 evolution. Dyes Pigm. 2017, 139, 7−18. (7) Ma, G.; Chen, S.; Kuang, Y.; Akiyama, S.; Hisatomi, T.; Nakabayashi, M.; Shibata, N.; Katayama, M.; Minegishi, T.; Domen, K. Visible light-driven z-scheme water splitting using oxysulfide H2 evolution photocatalysts. J. Phys. Chem. Lett. 2016, 7, 3892−3896. (8) Mao, Z.; Chen, J.; Yang, Y.; Wang, D.; Bie, L.; Fahlman, B. D. novel g-C3N4/CoO nanocomposites with significantly enhanced visible-light photocatalytic activity for H2 evolution. ACS Appl. Mater. Interfaces 2017, 9, 12427−12435. (9) Chen, Y.; Guo, L. Highly efficient visible-light-driven photocatalytic hydrogen production from water using Cd0.5Zn0.5S/TNTs (titanate nanotubes) nanocomposites without noble metals. J. Mater. Chem. 2012, 22, 7507−7514. (10) Wang, D.; Li, R.; Zhu, J.; Shi, J.; Han, J.; Zong, X.; Li, C. Photocatalytic water oxidation on BiVO4 with the electrocatalyst as an oxidation cocatalyst: essential relations between electrocatalyst and photocatalyst. J. Phys. Chem. C 2012, 116, 5082−5089. (11) Yi, Z. G.; Ye, J. H.; Kikugawa, N.; Kako, T.; Ouyang, S. X.; Stuart-Williams, H.; Yang, H.; Cao, J. Y.; Luo, W. J.; Li, Z. S.; Liu, Y.; Withers, R. L. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nat. Mater. 2010, 9, 559−564. J


Article

Inorganic Chemistry (30) Ke, J.; Duan, X. G.; Luo, S.; Zhang, H. Y.; Sun, H. Q.; Liu, J.; Tade, M.; Wang, S. B. UV-assisted construction of 3D hierarchical rGO/Bi2MoO6 composites for enhanced photocatalytic water oxidation. Chem. Eng. J. 2017, 313, 1447−1453. (31) Li, J. X.; Ye, C.; Li, X. B.; Li, Z. J.; Gao, X. W.; Chen, B.; Tung, C. H.; Wu, L. Z. A Redox shuttle accelerates O2 evolution of photocatalysts formed in situ under visible light. Adv. Mater. 2017, 29, 1606009. (32) Chen, S.; Shen, S.; Liu, G.; Qi, Y.; Zhang, F.; Li, C. Interface engineering of a CoO(x)/Ta3N5 photocatalyst for unprecedented water oxidation performance under visible-light-irradiation. Angew. Chem., Int. Ed. 2015, 54, 3047−3051. (33) Song, M. T.; Wu, Y. H.; Wang, X. J.; Liu, M. Q.; Su, Y. G. Switching charge transfer process of carbon nitride and bismuth vanadate by anchoring silver nanoparticle toward cocatalyst free water reduction. J. Colloid Interface Sci. 2018, 529, 375−384. (34) Han, C.; Ge, L.; Chen, C.; Li, Y.; Xiao, X.; Zhang, Y.; Guo, L. Novel visible light induced Co3O4-g-C3N4 heterojunction photocatalysts for efficient degradation of methyl orange. Appl. Catal., B 2014, 147, 546−553. (35) Yan, T.; Yan, Q.; Wang, X. D.; Liu, H. Y.; Li, M. M.; Lu, S. X.; Xu, W. G.; Sun, M. Facile fabrication of heterostructured g-C3N4/ Bi2MoO6 microspheres with highly efficient activity under visible light irradiation. Dalton Trans. 2015, 44, 1601−1611. (36) Liu, Y. J.; Zhou, F.; Zhan, S.; Yang, Y. F.; Yin, Y. F. Significantly enhanced performance of g-C3N4/Bi2MoO6 films for photocatalytic degradation of pollutants under visible-light irradiation. Chem. Res. Chin. Univ. 2016, 32 (2), 284−290. (37) Li, H. P.; Liu, J. Y.; Hou, W. G.; Du, N.; Zhang, R. J.; Tao, X. T. Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity. Appl. Catal., B 2014, 160−161, 89−97. (38) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation. Langmuir 2010, 26, 3894−3901. (39) Ji, H.; Chang, F.; Hu, X.; Qin, W.; Shen, J. Photocatalytic degradation of 2,4,6-trichlorophenol over g-C3N4 under visible light irradiation. Chem. Eng. J. 2013, 218, 183−190. (40) Bhowmik, T.; Kundu, M. K.; Barman, S. Growth of onedimensional RuO2 nanowires on g-carbon nitride: an active and stable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions at all pH values. ACS Appl. Mater. Interfaces 2016, 8, 28678−28688. (41) Paoli, E. A.; Masini, F.; Frydendal, R.; Deiana, D.; Malacrida, P.; Hansen, T. W.; Chorkendorff, I.; Stephens, I. E. L. Fine-tuning the activity of oxygen evolution catalysts: The effect of oxidation pretreatment on size-selected Ru nanoparticles. Catal. Today 2016, 262, 57−64. (42) Yue, D.; Chen, D.; Wang, Z.; Ding, H.; Zong, R.; Zhu, Y. Enhancement of visible photocatalytic performances of a Bi2MoO6BiOCl nanocomposite with plate-on-plate heterojunction structure. Phys. Chem. Chem. Phys. 2014, 16, 26314−26321. (43) Sui, C.; Niu, X.; Wang, Z.; Yuan, F.; Zhu, Y. Activity and deactivation of Ru supported on La1.6Sr0.4NiO4 perovskite-like catalysts prepared by different methods for decomposition of N2O. Catal. Sci. Technol. 2016, 6, 8505−8515. (44) Ma, Z.; Zhao, S.; Pei, X.; Xiong, X.; Hu, B. New insights into the support morphology-dependent ammonia synthesis activity of Ru/CeO2 catalysts. Catal. Sci. Technol. 2017, 7, 191−199. (45) Ou, M.; Wan, S.; Zhong, Q.; Zhang, S.; Song, Y.; Guo, L.; Cai, W.; Xu, Y. Hierarchical Z-scheme photocatalyst of g-C3N4@Ag/ BiVO4 (040) with enhanced visible-light-induced photocatalytic oxidation performance. Appl. Catal., B 2018, 221, 97−107. (46) Li, R.; Han, H.; Zhang, F.; Wang, D.; Li, C. Highly efficient photocatalysts constructed by rational assembly of dual-cocatalysts separately on different facets of BiVO4. Energy Environ. Sci. 2014, 7, 1369−1376. (47) Yu, H.; Huang, B.; Wang, H.; Yuan, X.; Jiang, L.; Wu, Z.; Zhang, J.; Zeng, G. Facile construction of novel direct solid-state Z-

scheme AgI/BiOBr photocatalysts for highly effective removal of ciprofloxacin under visible light exposure: Mineralization efficiency and mechanisms. J. Colloid Interface Sci. 2018, 522, 82−94. (48) Cao, M.; Wang, P.; Ao, Y.; Wang, C.; Hou, J.; Qian, J. Preparation of graphene oxide-loaded Ag3PO4@AgCl and its photocatalytic degradation of methylene blue and O2 evolution activity under visible light irradiation. Int. J. Hydrogen Energy 2015, 40, 1016−1025. (49) Si, H. Y.; Mao, C. J.; Zhou, J. Y.; Rong, X. F.; Deng, Q. X.; Chen, S. L.; Zhao, J. J.; Sun, X. G.; Shen, Y. M.; Feng, W. J.; Gao, P.; Zhang, J. Z-scheme Ag3PO4/graphdiyne/g-C3N4 composites: enhanced photocatalytic O2 generation benefiting from dual roles of graphdiyne. Carbon 2018, 132, 598−605.

K


g-C3N4 for Highly Effective Oxygen

Recommend Documents