Ta3N5 for promoted photocatalytic hydrogen production under visible light irradiation

Journal of Catalysis 339 (2016) 77–83

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Magnesia interface nanolayer modification of Pt/Ta3N5 for promoted photocatalytic hydrogen production under visible light irradiation Shanshan Chen a, Yu Qi a,b, Qian Ding a,b, Zheng Li a,b, Junyan Cui a,c, Fuxiang Zhang a,⇑, Can Li a,⇑ a

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100049, China c Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun 130021, China b

a r t i c l e

i n f o

Article history: Received 20 January 2016 Revised 16 March 2016 Accepted 18 March 2016

Keywords: Photocatalysis Water splitting Ta3N5 Interface modification Hydrogen

a b s t r a c t Deposition of a co-catalyst is a general strategy for promoting the water splitting performance of semiconductor-based photocatalysts, but the interface barrier of the co-catalyst/semiconductor system often leads to unfavorable interfacial charge transfer and separation. In this work, the interface issue of the Pt/Ta3N5 proton reduction system was addressed via a magnesia interface nanolayer (MIN) modification strategy, and its effect on the structure and properties of both the Ta3N5 semiconductor and the Pt co-catalyst was investigated. UV–visible diffuse reflectance spectroscopy, field emission scanning electron microscopy, and high-resolution transmission electron microscopy characterizations indicate that the MIN can not only effectively passivate the Ta3N5 semiconductor, but also favor the deposition of Pt co-catalyst with small particle size and uniform dispersion, which can increase the catalytic active sites and enlarge the interfacial contact area between Ta3N5 and Pt. Time-resolved infrared spectroscopy further evidences that the promoted charge separation process is achieved by this magnesia interface engineering strategy. Based on our modification, the optimal H2 evolution rate on the Pt/MgO(in)–Ta3N5 photocatalyst reaches 22.4 lmol h 1, which is ca. 17 times that of pristine Pt/Ta3N5 photocatalyst. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction The photocatalytic production of hydrogen from water to convert solar energy to chemical energy is one of the most promising ways to solve the problems of energy crisis and environment pollution [1–4]. To achieve an efficient conversion process, a typical photocatalytic water splitting system is generally composed of a semiconductor acting as a light harvester and a co-catalyst that is well known to promote charge separation and catalyze the reactions by lowering the activation energy [5]. Some (oxy)nitrides have been regarded as suitable light harvesters for the water splitting reaction, due to their appropriate band edge positions, wide visible light absorption, and stability under light irradiation [6]. With suitable co-catalyst modification, several oxynitrides, such as (Ga1 xZnx)(N1 xOx), TaON, and LaMg1/3Ta2/3O2N, have been reported to achieve overall water splitting under visible light irradiation [7–9]. However, the corresponding photocatalytic performance is low and the reported candidate photocatalysts for visible-light-driven one-step overall water splitting reactions are still very limited. For the (oxy)nitrides, especially those with an ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Zhang), [email protected] (C. Li). http://dx.doi.org/10.1016/j.jcat.2016.03.024 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

absorption edge at 600 nm, photocatalytic H2 evolution rates are more or less two orders of magnitude lower than the O2 evolution rates in the presence of corresponding sacrificial reagents [10,11]. For example, Ta3N5 as a typical 600 nm-class semiconductor has been demonstrated to possess highly efficient photocatalytic and photoelectrocatalytic (PEC) water oxidation activities, while its photocatalytic proton reduction performance is extremely poor [10–13]. It is this low proton reduction ability that may be a major obstacle to achieving overall water splitting on them. Therefore, development of effective strategies to enhance the proton reduction ability is highly desirable. Various strategies have been reported to enhance the photocatalytic proton reduction ability of (oxy)nitrides, but most are focused mainly on preparation methods for reducing defect density, or the development of efficient reduction co-catalysts [14,15]. Since a semiconductor and a co-catalyst have different physical or chemical properties, a new interface barrier is generally formed, which may have a great influence on the charge transfer between them [16]. Recently, we have demonstrated that photogenerated charge separation as well as water oxidation performance can be greatly promoted by interface wettability modification [11]. Encouraged by this, the interface engineering strategy was further applied to the photocatalytic proton reduction

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system, and a more specific focus on the roles of the interface nanolayer is investigated in this work. Similarly, we tuned the surface wettability of Ta3N5 via in situ or ex situ magnesia modification, and the Pt co-catalyst was subsequently deposited. The asprepared samples were characterized by X-ray powder diffraction (XRD), UV–visible diffuse reflectance spectroscopy (UV–vis DRS), field emission scanning electron microscopy (FESEM), highresolution transmission electron microscopy (HRTEM), and timeresolved IR (TRIR) spectroscopy, etc., whose activities were tested under visible light irradiation (k P 420 nm) in the presence of methanol. It is shown that a coated magnesia nanolayer on the surface of Ta3N5 not only passivates Ta3N5 semiconductor, leading to a lower defect density, but also tunes the surface wettability from hydrophobicity to hydrophilicity, which contributes to decreased particle size and homogeneous dispersion of the Pt co-catalyst. With these positive factors, the photocatalytic H2 evolution rate of the MIN-modified sample is increased to 17 times that of the unmodified one.

2. Experimental 2.1. Preparation of magnesia-modified Ta3N5 samples Magnesia modification of Ta3N5 was carried out by an in situ or ex situ method, and the as-obtained samples are denoted as MgO (in)–Ta3N5 or MgO(ex)–Ta3N5, respectively [11]. For the in situ method, Ta2O5 powder (99.99%, High Purity Chemical) was impregnated into the MgSO4 (99.5%, Alfa Aesar) aqueous solution and then the as-obtained dried mixture was calcined in air at 1073 K for 2 h. The formed MgTa2O6/Ta2O5 precursor was nitrided under an ammonia flow (250 mL min 1) at 1223 K for 15 h, yielding a MgO(in)–Ta3N5 sample [17]. For the ex situ method, Ta3N5 was prepared by nitriding the Ta2O5 precursor at 1223 K for 15 h; then the as-synthesized Ta3N5 powder was impregnated in the MgSO4 aqueous solution, and the obtained dried mixture was finally calcined under an ammonia flow (250 mL min 1) at 1023 K for 1 h to form the MgO(ex)–Ta3N5 sample. The content of deposited magnesia is calculated by magnesium elemental analysis and fixed at 2 wt.%, if not otherwise stated. 2.2. Deposition of platinum co-catalyst Conventional impregnation and subsequent H2 reduction treatment were used to deposit the platinum co-catalyst for H2 evolution. Typically, 0.2 g pristine or modified Ta3N5 was immersed in a ca. 2 mL (NH4)2PtCl6 (98.8%, Alfa Aesar) aqueous solution and sonicated for ca. 5 min. After the solution was completely evaporated in a water bath at 353 K, the impregnated powder was reduced at 573 K for 2 h under a flow of 5 vol.% H2/Ar (200 mL min 1). The optimal deposition proportion of 2 wt.% is used, if not otherwise stated. 2.3. Preparation of Ta3N5-based films for CAs measurement Electrophoretic deposition was used to prepare Ta3N5 based films (2  3 cm2) on FTO substrate. Typically, 50 mL acetone solution (99.8%, Kermel Chemical) containing 50 mg Ta3N5-based powder and 20 mg iodine (99.5%, Dalian Inorganic Chemical) was dispersed by sonication for 20 min. The FTO electrode was immersed, paralleling the Pt electrode at a distance of ca. 5 cm. Here, 20 V and 1 A were applied for 3 min using a potentiostat (ITECH IT6834), and vacuum drying treatment was subsequently conducted at 333 K for 12 h before the contact angles (CAs) measurement.

2.4. Structural characterizations The as-prepared samples were characterized by XRD (Rigaku D/Max-2500, Cu Ka radiation), UV–vis DRS (V-670 spectrophotometer, JASCO), FESEM (S-5500, Hitachi), HRTEM (JEM-2000EX, JEOL), and X-ray photoelectron spectroscopy (XPS, Thermo Esclab 250Xi, monochromatic Al Ka X-ray source). The C1s peak (284.6 eV) was referenced to normalize the measured binding energies for each sample. The Brunauer–Emmett–Teller (BET) surface area was measured at 77 K using a Micromeritics ASAP 2000 adsorption analyzer. A CA analyzer (DSA100, Kruss GmbH., Germany) was used to measure the CAs of water droplets for the prepared films in ambient atmosphere. 2.5. Photocatalytic proton reduction reaction The photocatalytic proton reduction reaction was carried out in a Pyrex top-irradiation-type reaction vessel connected to a closed gas-circulation system. A 150 mL aqueous solution containing 0.15 g photocatalyst was applied, and 20 vol.% methanol (99.5%, Bodi Chemical) was used as the sacrificial reagent. A quantity of 0.15 g La2O3 (99.95%, Sinopharm Chemical) was added to maintain the pH value of the reaction solution, which was measured at ca. 8.5. Prior to photoirradiation, the reaction suspension was evacuated to ensure that air was completely removed. A 300 W xenon lamp with an optical filter (Hoya, L-42; k P 420 nm) was used as a light source, and a flow of cooling water was used to maintain the reaction suspension at room temperature. Gas chromatography (Agilent; GC-7890A, MS-5A column, TCD, Ar carrier) was introduced to analyze the evolved gases. 3. Results and discussion 3.1. Photocatalytic hydrogen production performance The photocatalytic H2 evolution rates on the platinum-loaded samples were evaluated in the presence of methanol and La2O3 under visible light irradiation (k P 420 nm). The methanol is known as a hole scavenger, and the La2O3 is used as a pH buffer to maintain the pH value of the reaction aqueous solution, which was measured to be ca. 8.5 [18,19]. Without the magnesia modification, the platinum-deposited Ta3N5 shows a very low H2 evolution rate (1.3 lmol h 1), similar to most of the previous observations [10]. The photocatalytic H2 evolution rates are strongly dependent on the amount of magnesia modifier and deposited platinum co-catalyst, both of which are optimized to 2 wt.% (Fig. 1). Based on this condition, the H2 evolution rate of the 2 wt.% Pt/2 wt.% MgO(in)–Ta3N5 photocatalyst (22.4 lmol h 1) is ca. 17 times that of the 2 wt.% Pt/Ta3N5 sample (1.3 lmol h 1), demonstrating a remarkable promotion effect originating from the in situ magnesia modification. In comparison, the H2 evolution rate of the ex situ magnesia modified photocatalyst (2 wt.% Pt/2 wt. % MgO(ex)–Ta3N5) was measured to be 2.7 lmol h 1, a little higher than that of the 2 wt.% Pt/Ta3N5 photocatalyst. The activity results of three typical photocatalysts are summarized in Table 1, in which the H2 evolution rates are both promoted by in situ and ex situ magnesia modifications, and the former is much more effective than the latter. To examine the possible effect of the SO24 ion on the photocatalytic performance, the (NH4)2SO4 aqueous solution was used to replace the MgSO4 aqueous solution as the modification precursor with other conditions unchanged. It was found that the Ta3N5 samples with and without (NH4)2SO4 treatment exhibit similar H2 evolution rates (ca. 1.3 lmol h 1), ruling out the promotion effect of the SO24 ion on the activity. The time course of the H2 evolution using the 2 wt.% Pt/2 wt.% MgO(in)–Ta3N5 photocatalyst

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a

b

H2 evolution rate / µmol h-1

H2 evolution rate / µmol h-1

25

20

15

10

5

0

25

20

15

10

5

0

1.0

1.5

2.0

2.5

3.0

0

Pt / wt%

1

2

3

4

Mg / wt%

Fig. 1. Dependence of the photocatalytic H2 evolution rate on the content of platinum or magnesium for the (a) Pt/2 wt.% MgO(in)–Ta3N5 or (b) 2 wt.% Pt/MgO(in)–Ta3N5 photocatalyst, respectively. Reaction conditions: 0.15 g catalyst; 0.15 g La2O3; 150 mL aqueous 20 vol.% CH3OH solution; 300 W xenon lamp (k P 420 nm); 1 h reaction time.

Table 1 Comparison of photocatalytic H2 evolution rates of Pt/Ta3N5 photocatalyst with/without magnesia modification under visible light irradiation (k P 420 nm).

a b c

Entry

Photocatalysta

Surface area (m2 g

1 2 3

Pt/Ta3N5 Pt/MgO(ex)–Ta3N5 Pt/MgO(in)–Ta3N5

9 8 8

1

)

Contact angleb (°)

H2 evolution rate (lmol h

123 50 52

1.3 2.7 22.4

1 c

)

The content of both Pt and Mg is 2 wt.%. CAs were measured on the corresponding Ta3N5-based samples without Pt deposition. Reaction conditions: 0.15 g catalyst; 0.15 g La2O3; 150 mL aqueous 20 vol.% CH3OH solution; 300 W xenon lamp (k P 420 nm); 1 h reaction time.

Ta3N5

Intensity / a.u.

MgO(in)-Ta3N5

MgO(ex)-Ta3N5

Ta3N5-PDF#65-1247 10

20

30

40

50

60

2 theta / degree Fig. 2. XRD patterns of typical Ta3N5-based samples. The content of magnesium is 2 wt.%.

shows that there is no obvious deactivation in the tested period (see the Supporting Information, Fig. S1). In comparison, the turnover frequency (TOF) of the proton reduction reaction is calculated based on the amount of deposited platinum species as active sites of water reduction reaction (0.15 g photocatalyst is used for each reaction); it is 1.46 h 1 and 0.08 h 1 for the 2 wt.% Pt/2 wt.% MgO (in)–Ta3N5 and the 2 wt.% Pt/Ta3N5 sample, respectively. 3.2. Characterization of Ta3N5 with and without magnesia modifier In order to correlate the photocatalytic activity results with structural properties, we choose three typical samples for detailed characterizations. First of all, the influence of magnesia modifica-

tion on the structural properties of Ta3N5 semiconductor was examined. Compared with the pristine Ta3N5 sample, both in situ and ex situ magnesia (2 wt.%)-modified Ta3N5 samples show single crystal phases with similar crystallinity (Fig. 2). The shortage of diffraction peaks assigned to magnesium-related species should originate from its low content (2 wt.%). It needs to be pointed out that obvious XRD peaks assigned to MgO had been detected for the 5 wt.% MgO-modified sample [11]. Moreover, MgO will be gradually converted to Mg(OH)2, once it contacts the water molecules [20]. This is clearly confirmed by our HRTEM images of MgO(in)–Ta3N5 sample (Fig. S2), in which the magnesia nanolayer (ca. 2–5 nm) with a d value of 0.237 nm, in accordance with the crystal facet (1 0 1) of Mg(OH)2, is observed on the surface of Ta3N5. Expectedly, the magnesia nanolayer will become thick and nonuniform, with the amount of magnesia continuously increasing (Fig. S3). FESEM images of all typical samples show analogous morphology and particle size range of 0.4–1.0 lm (Fig. 3). BET results demonstrate that all of them have close surface areas (ca. 8 m2 g 1). The water wettability property was also characterized and CA values for Ta3N5, MgO(ex)–Ta3N5, and MgO(in)–Ta3N5 samples are 123°, 50° and 52°, respectively (Fig. S4). It indicated that the hydrophobic surface of Ta3N5 became hydrophilic after the ex situ or in situ magnesia modification. The specific data on surface areas and CA values of those typical samples are summarized in Table 1. UV–vis DRS of three typical samples exhibits the characteristic absorption of Ta3N5 with an absorption edge at ca. 600 nm (Fig. S5), but the absorption backgrounds are relatively decreased and increased for the in situ and ex situ magnesia-modified samples, respectively. The absorption background at a longer wavelength has been ascribed to the formation of reduced tantalum species (e.g., Ta4+ ions) [21]. The decreased defect density of the MgO (in)–Ta3N5 sample demonstrates that the magnesia nanolayer on

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Fig. 3. FESEM images of typical samples: (a) Ta3N5, (b) MgO(ex)–Ta3N5, (c) MgO(in)–Ta3N5. The content of magnesium is 2 wt.%.

the surface of Ta3N5, acting as a surface passivation layer, can effectively inhibit the formation of reduced tantalum species. In contrast, since the ex situ magnesia modification treatment underwent an additional nitridation process in the flow of ammonia with a strong reducing ability, the defect density of MgO(ex)– Ta3N5 slightly increased even for the existence of a surface passivation effect of the partial magnesia modifier. XPS characterization was further carried out for the passivation effect of magnesia modifier on the structure of Ta3N5-based samples. As shown in Fig. S6, binding energies of Ta4p, N1s, and O1s are shifted to lower regions after the in situ magnesia modification. Considering that the electronegativity of magnesium atoms is smaller than that of tantalum atoms, and the ion sizes of Mg2+ (86 pm) and Ta5+ (78 pm) are close, it is reasonable to deduce that the partial magnesia is doped onto the surface region of the MgO (in)–Ta3N5 sample, which would be responsible for remarkably decreased defect density, leading to a prominent surface passivation effect (Fig. S5). Based on the above results, the short summary can be made that MIN modification can not only efficiently regulate the surface wettability of Ta3N5 from hydrophobic to hydrophilic, but also passivate the surface of Ta3N5 with a lower defect density. Beyond those aspects, the Ta3N5 semiconductor undergoes minor differences in crystal structure and morphology after in situ or ex situ magnesia modification. 3.3. Influence of MIN modification on the platinum co-catalyst Besides the basic structural characterizations of Ta3N5-based semiconductors with and without magnesia modification, extended characterizations of the influence of MIN on the particle size and the dispersion of the deposited platinum co-catalyst were carried out. Fig. S7 and Fig. 4a1–c1 give the representative FESEM images of typical samples before and after platinum deposition. The surface of the pristine Ta3N5 sample is smooth (Fig. S7), and additional bright nanoparticles can be clearly observed for all the platinum-deposited samples (Fig. 4a1–c1). However, the particle size and the size distribution of the deposited platinum species are different for the samples with and without magnesia modification treatment. Comparatively, the particle size of deposited platinum species on the surface of pristine Ta3N5 is much greater,

and the size distribution is broader (Fig. 4a2–c2). The average particle sizes of over 200 nanoparticles are ca. 20.9, 3.0, and 2.7 nm for the Pt/Ta3N5, Pt/MgO(ex)–Ta3N5, and Pt/MgO(in)–Ta3N5 samples, respectively. This means that magnesia modification promotes uniform dispersion and narrows the particle size distribution of deposited platinum species. The difference in particle size and size distribution on the modified and unmodified surfaces of Ta3N5 can be further confirmed by HRTEM images given in Fig. 5. Interestingly, the deposited platinum on the in situ modified sample is exactly located on the surface of the magnesia nanolayer instead of that of Ta3N5 (Fig. 5c and d). The differences in particle size and size distribution of deposited platinum species can be understood by the fact that the hydrophilic surface of the modified samples is more favorable for a uniform dispersion of the solute (in this work, it is (NH4)2PtCl6) from an aqueous solution, compared with the hydrophobic surface of pristine Ta3N5. A similar phenomenon was reported in previous literature [22]. Except for the particle size and the dispersion of the deposited platinum co-catalyst, the chemical state was also checked by XPS characterization, with results given in Fig. 6. Similarly, all of the Pt4f peaks can be fitted to two pairs of peaks assigned to Pt and PtO species, with the binding energies of the Pt4f7/2 peaks located at 70.5 and 71.7 eV for the Pt/Ta3N5 sample, respectively (C1s at 284.6 eV was used as reference) [23,24]. In comparison, the binding energies of the Pt4f peaks for the in situ or ex situ modified sample are obviously shifted to higher values, which may be mainly attributed to the smaller particle sizes of platinum species [25]. In addition, the molar ratios of PtO/Pt in the magnesiamodified samples are higher than that of the unmodified sample (Pt/Ta3N5: 0.35; Pt/MgO(ex)–Ta3N5: 0.49; Pt/MgO(in)–Ta3N5: 0.44), which should be attributed to the fact that the deposited platinum species with a decreased particle size exposes much more surface platinum atoms that are easily partially oxidized to PtO. 3.4. Discussion Deposition of the co-catalyst on the surface of the semiconductor is an extremely important strategy to simultaneously promote the charge separation process and accelerate the catalytic conversion kinetics in the field of solar-to-chemical energy conversion

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Fig. 4. (a1–c1) FESEM images and (a2–c2) histograms of particle-size distributions of typical samples: (a) Pt/Ta3N5, (b) Pt/MgO(ex)–Ta3N5, (c) Pt/MgO(in)–Ta3N5. The content of both platinum and magnesium is 2 wt.%. A.S. stands for ‘‘average size.”

[4,5], but an interface barrier between a semiconductor and a cocatalyst is commonly created, which greatly affects the interfacial charge transfer [16]. To address this, the introduction of a passivation interface nanolayer between them has been known to work well in the PEC system [26,27]. However, little attention has been paid to the powder-based photocatalytic system, which can be understood as a micro-PEC reactor free of bias. In this work, we focused on the platinum-deposited Ta3N5 (Pt/Ta3N5) photocatalyst with a special MIN modification for the hydrogen evolution reaction. Systematic characterization results show that the MIN not only effectively passivates the surface of the Ta3N5 semiconductor, leading to a decreased defect density, but also promotes uniform dispersion and narrows the particle size distribution of deposited platinum species, with average particle size being reduced from ca. 21 nm to 3 nm (Fig. 4). This is mainly ascribed to the remarkably improved surface hydrophilicity, which is more favorable for a homogenous dispersion of (NH4)2PtCl6 solute (platinum precursor) from an aqueous solution than with a hydrophobic surface. It has been demonstrated that the photocatalytic proton reduction rate is highly dependent on the particle size of the Pt co-catalyst, and smaller ones can provide more catalytic conversion sites and/or proper energetic position of the LUMO for the hydrogen evolution reaction [28,29]. In addition, the homogeneous dispersion of the Pt co-catalyst with a reduced particle size can also create more interfacial contact area for charge transfer between the Ta3N5 semiconductor and the Pt co-catalyst. TRIR spectra (Fig. 7) do give positive support that the recombination of photogenerated

carriers has been greatly inhibited by the MIN modification, even though the photoexcited electrons need to be tunneled through the inert magnesia nanolayer [26,30]. However, it needs to be pointed out that the deposited platinum can not only provide active sites and decrease the activation energy for the H2 evolution reaction [5], but also absorb incident photons to reduce the light absorption efficiency of the Ta3N5 semiconductor, which will lower the overall H2 evolution rate when an excess amount of platinum is used. Balanced by those factors, an optimal content of deposited platinum (2 wt.%) was observed in Fig. 1a. As for the case of magnesia, the magnesia modification can remarkably improve the dispersion of deposited platinum co-catalyst with a small particle size. However, the magnesia modifier is an insulator and unfavorable for direct electron transfer, so the photogenerated electrons must tunnel through the insulator to the platinum co-catalyst for the proton reduction reaction. The continuously increased amount of magnesia will make the coated magnesia nanolayer become thick and nonuniform (Fig. S3), leading to decreased electron tunneling efficiency. As an integral, there is an optimized content of magnesia (2 wt.%) shown in Fig. 1b. Different lifetimes of carriers between the ex situ and in situ magnesia-modified samples may mainly originate from their distinct defect density, as a result of their similarity in crystallization (Fig. 2), morphology (Fig. 3), surface wettability (Fig. S4), surface area (Table 1), and particle size and dispersion of platinum species (Fig. 4). The different defect densities of ex situ and in situ magnesia-modified samples should be responsible for their

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a

b Ta3N5 d = 0.362 nm

Ta3N5

Pt

Pt d = 0.227 nm Pt d = 0.227 nm

10 nm

c

5 nm

d Mg(OH)2 Pt d = 0.196 nm

Pt Ta3N5

Ta3N5 d = 0.256 nm 2 nm

10 nm

Fig. 5. HRTEM images of (a, b) 2 wt.% Pt/Ta3N5 and (c, d) 2 wt.% Pt/2 wt.% MgO(in)–Ta3N5 samples.

70.5 eV

Pt4f

1.0

4f7/2

Intensity / a.u.

(a) 71.2 eV 72.6 eV

4f5/2

4f7/2

(b)

Normalized unit

71.7 eV

4f5/2

0.8 0.6

(a) 0.4

(b) (c)

0.2 0.0

4f5/2

-0.2

4f7/2

0.0

(c) 80

78

76

74

72

70

68

66

Binding energy / eV

0.2

0.4

0.6

0.8

1.0

Time declay / ms Fig. 7. Decay of transient absorption of the representative Ta3N5-based photocatalysts under vacuum: (a) Pt/MgO(in)–Ta3N5, (b) Pt/MgO(ex)–Ta3N5, and (c) Pt/ Ta3N5. The pulse laser at 355 nm (1 Hz, 3 mJ/pulse) was used to excite the samples for the IR tests. In this figure, the content of both Mg and Pt is 2 wt.%.

Fig. 6. Pt4f XPS spectra of typical samples: (a) 2 wt.% Pt/Ta3N5, (b) 2 wt.% Pt/2 wt.% MgO(ex)–Ta3N5, and (c) 2 wt.% Pt/2 wt.% MgO(in)–Ta3N5.

remarkable difference in H2 evolution rate. As a result, the lower defect density of the Ta3N5 semiconductor, the smaller particle size of Pt co-catalyst, and better interfacial charge separation integrally contributes to the remarkably enhanced H2 evolution rate for the Pt/MgO(in)–Ta3N5 sample. 4. Conclusions The influence of MIN modification on Pt/Ta3N5 photocatalyst has been investigated in this work. It is found that MIN modifica-

tion not only passivates the surface of the Ta3N5 semiconductor to cause decreased defect density, but also provides a hydrophilic surface for a more homogenous dispersion of (NH4)2PtCl6 (platinum precursor) from aqueous solution, leading to decreased particle size and uniform dispersion of the Pt co-catalyst. These factors integrally promote efficiencies of charge separation and photocatalytic conversion. This work provides a further illustration of the importance of interfacial hydrophilic–hydrophobic compatibility between a semiconductor and a co-catalyst for promoting the charge separation process as well as the photocatalytic H2-evolving performance. The interface engineering strategy based

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on wettability compatibility is expected to apply to other powderbased photocatalytic systems for enhanced solar energy conversion efficiency. Acknowledgments This work was supported by the Basic Research Program of China (973 Program: 2014CB239403) and the Natural Science Foundation of China (Nos. 21522306, 21373210). Fuxiang Zhang thanks the ‘‘Hundred Talents Program” of the CAS for support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2016.03.024. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

A. Fujishima, K. Honda, Nature 238 (1972) 37. A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253. T. Hisatomi, J. Kubota, K. Domen, Chem. Soc. Rev. 43 (2014) 7520. Y. Ma, X.L. Wang, Y.S. Jia, X.B. Chen, H.X. Han, C. Li, Chem. Rev. 114 (2014) 9987. J.H. Yang, D.E. Wang, H.X. Han, C. Li, Acc. Chem. Res. 46 (2013) 1900. K. Maeda, K. Domen, J. Phys. Chem. C 111 (2007) 7851. K. Maeda, K. Teramura, D.L. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 440 (2006) 295. K. Maeda, D.L. Lu, K. Domen, Chem. Eur. J. 19 (2013) 4986. C.S. Pan, T. Takata, M. Nakabayashi, T. Matsumoto, N. Shibata, Y. Ikuhara, K. Domen, Angew. Chem., Int. Ed. 54 (2015) 2955. L. Yuliati, J.H. Yang, X.C. Wang, K. Maeda, T. Takata, M. Antonietti, K. Domen, J. Mater. Chem. 20 (2010) 4295.

83

[11] S.S. Chen, S. Shen, G.J. Liu, Y. Qi, F.X. Zhang, C. Li, Angew. Chem., Int. Ed. 54 (2015) 3047. [12] G. Hitoki, A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, K. Domen, Chem. Lett. 31 (2002) 736. [13] G.J. Liu, J.Y. Shi, F.X. Zhang, Z. Chen, J.F. Han, C.M. Ding, S.S. Chen, Z.L. Wang, H. X. Han, C. Li, Angew. Chem., Int. Ed. 53 (2014) 7295. [14] K. Maeda, H. Terashima, K. Kase, M. Higashi, M. Tabata, K. Domen, Bull. Chem. Soc. Jpn. 81 (2008) 927. [15] M. Hara, J. Nunoshige, T. Takata, J.N. Kondo, K. Domen, Chem. Commun. 24 (2003) 3000. [16] Z. Zhang, J.T. Yates Jr., Chem. Rev. 112 (2012) 5520. [17] S.S. Chen, Y. Qi, T. Hisatomi, Q. Ding, T. Asai, Z. Li, S.S.K. Ma, F.X. Zhang, K. Domen, C. Li, Angew. Chem., Int. Ed. 54 (2015) 8498. [18] S.S. Chen, J.X. Yang, C.M. Ding, R.G. Li, S.Q. Jin, D.E. Wang, H.X. Han, F.X. Zhang, C. Li, J. Mater. Chem. A 1 (2013) 5651. [19] S.S. Chen, Y. Qi, G.J. Liu, J.X. Yang, F.X. Zhang, C. Li, Chem. Commun. 50 (2014) 14415. [20] J.T. Newberg, D.E. Starr, S. Yamamoto, S. Kaya, T. Kendelewicz, E.R. Mysak, S. Porsgaard, M.B. Salmeron, G.E. Brown Jr., A. Nilsson, H. Bluhm, Surf. Sci. 605 (2011) 89. [21] Y.C. Lee, H. Teng, C.C. Hu, S.Y. Hu, Electrochem. Solid-State Lett. 11 (2008) P1. [22] T.W. Kim, K.S. Choi, Science 343 (2014) 990. [23] V.K. Kaushik, Z. Phys. Chem. 173 (1991) 105. [24] B.S. Huang, F.Y. Chang, M.Y. Wey, Int. J. Hydrogen Energy 35 (2010) 7699. [25] R.J. Isaifan, S. Ntais, E.A. Baranova, Appl. Catal., A: Gen. 464–465 (2013) 87. [26] Y.W. Chen, J.D. Prange, S. Dühnen, Y. Park, M. Gunji, C.E.D. Chidsey, P.C. Mclntyre, Nat. Mater. 10 (2011) 539. [27] F.L. Formal, N. Tétreault, M. Cornuz, T. Moehl, M. Grätzel, K. Sivula, Chem. Sci. 2 (2011) 737. [28] F.F. Schweinberger, M.J. Berr, M. Döblinger, C. Wolff, K.E. Sanwald, A.S. Crampton, C.J. Ridge, F. Jäckel, J. Feldmann, M. Tschurl, U. Heiz, J. Am. Chem. Soc. 135 (2013) 13262. [29] Á. Kmetykó, K. Mogyorósi, V. Gerse, Z. Kónya, P. Pusztai, A. Dombi, K. Hernádi, Materials 7 (2014) 7022. [30] L. Huang, X.L. Wang, J.H. Yang, G. Liu, J.F. Han, C. Li, J. Phys. Chem. C 117 (2013) 11584.