g-C3N4 heterojunction with enhanced visible light photocatalytic activity for hydrogen evolution

Journal of Colloid and Interface Science 526 (2018) 451–458

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Construction of novel Sr0.4H1.2Nb2O6
H2O/g-C3N4 heterojunction with enhanced visible light photocatalytic activity for hydrogen evolution Wanxia Ma a, Di Li b,⇑, Baowei Wen a, Xiaodong Ma a, Deli Jiang a,⇑, Min Chen a a b

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 31 March 2018 Revised 8 May 2018 Accepted 9 May 2018

Keywords: Heterojunctions Photocatalysis Hydrogen evolution Mechanism

a b s t r a c t Construction of heterojunction is an effective strategy to conquer the severe charge carrier recombination limitation of single component g-C3N4 photocatalyst. In the present work, novel heterojunctions composed of g-C3N4 nanosheets and Sr0.4H1.2Nb2O6
H2O nanooctahedrons were constructed via a simple hydrothermal method. The as-prepared Sr0.4H1.2Nb2O6
H2O/g-C3N4 (HSN/CN) heterojunction showed high photocatalytic activity in the water splitting reactions. Specially, it is found that the developed 20 wt%-HSN/CN heterojunction shows high water splitting activity with H2 evolution rate up to 469.4 lmol g1, which was much higher than that of bare CN. This enhanced photocatalytic activity for H2 evolution can be mainly attributed to the matched energy level and heterojunction structure which could improve the photo-generated charge carriers separation and transfer. This work implies that construction of heterojunctions with a wide band gap semiconductor is a feasible strategy for enhancement of photocatalytic activity of CN materials. Ó 2018 Published by Elsevier Inc.

1. Introduction Environmental pollution and the poor of green energy have been a topic global issues. Photocatalytic technology has shown great promising for renewable energy production in recent years [1–3]. Fabrication of highly efficient, low-cost, and stable photocatalytic materials is of great significance to the mitigation of ⇑ Corresponding authors. E-mail addresses: [email protected] (D. Li), [email protected] (D. Jiang). https://doi.org/10.1016/j.jcis.2018.05.019 0021-9797/Ó 2018 Published by Elsevier Inc.

pollution impact and renewable energy production [4–10]. Melon-based g-C3N4 (CN), a metal-free polymer semiconductor, has been proved to be a new alternative over the past years due to its outstanding electronic and photophysical properties and diverse applications in water splitting for hydrogen production [11–18], degradation of organic pollutants [19–21] and CO2 photoreduction [22–25]. However, the fast recombination of charge carriers and poor visible-light absorption restrict the photocatalytic activity of pristine CN [26,27]. To enhance the photocatalytic activity of CN, several feasible strategies, for instance, doping

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[28,29], modifying [30,31], construct heterojunction [32,33], have been developed. Especially, substantial researches have been focused on construction of CN-based heterojunction photocatalyst which can efficiently boost the separation of photo-generated electron-hole pairs [34–37]. In our previous work, we have successfully constructed the CN nanosheets-based heterojunction photocatalysts such as CaIn2S4/CN [21], BiOI/CN [33], and CN/KCa2Nb3O10 [36] and demonstrated that the as-prepared heterojunction showed the significantly enhanced photocatalytic activity mainly due to improved charge separation efficiency. To construct a heterojunction photocatalyst with high efficiency, another suitable semiconductor should be selected. Niobates, due to their unique structure and outstanding performance in photocatalytic reactions, have been widespread studied. As a typical nibobate photocatalyst, not only due to its good photocatalytic performance but also owing to its facial hydrothermal method instead of high temperature solid reaction, Sr0.4H1.2Nb2O6
H2O (HSN) has been developed as a promising niobate photocatalyst and has attracted more and more interesting in water splitting and degradation of organic pollutants [38,39]. Since the band-edge positions of HSN material match well with those of CN, we speculate a typical HSN/CN heterojunction can be constructed with efficient charge carriers separation. Based on the above considerations, we in this work developed a facile hydrothermal approach to synthesize novel HSN/CN heterojunction composed of HSN nanooctahedrons and CN nanosheets. The HSN/CN heterojunction exhibited the enhanced photocatalytic activity in the H2 evolution reactions as compared with the bare HSN and CN. The photocatalytic activity of optimal 20 wt%-HSN/ CN heterojunction is 1.8 times higher than those of bare CN nanosheets. The PL and the photoelectrochemical measurements indicate that the enhanced photocatalytic activity can be mainly due to the efficient charge separation thanks to the favorable interface combination and well-defined band alignment.

The synthesis of HSN/CN heterojunction was carried out as follows. The prepared CN (0.16 g) and HSN (0.04 g) were dissolved in 30 mL of deionized water respectively. Later, the HSN suspension was added into CN suspension dropwise under magnetic stirring for 2 h. The resultant suspension was transferred to the autoclave and stirred for 2 h, followed by the hydrothermal treatment at 140 °C for 12 h. Finally, the 20 wt%-HSN/CN (denoted as 20-HSN/ CN) heterojunction products were collected through centrifugation and washed with distilled water and ethanol for three times, and dried at 60 °C for overnight. To optimize the photocatalytic activity of HSN/CN heterojunctions, a range of samples with various HSN contents (10 wt%, 30 wt%, 40 wt%, and 50 wt%) with prepared and abbreviated as 10-HSN/CN, 30-HSN/CN, 40-HSN/CN, 50-HSN/ CN, respectively. 2.3. Characterization TEM and SEM images were measured from JEM-2100 transmission electron microscopy and JSM-6010 PLUS/LA scanning electron microscopy, respectively. Tecnai G2F30 S-TWIN (Philips) microscope operating at 200 kV was used for HRTEM and EDS analysis. The purity of the phase and crystal structure of the samples were examined through a XRD-6100 X-ray diffractometer at a scanning rate of 7° min1 with Cu-K radiation (k = 1.5406 Å). Infrared spectra were obtained on KBr pellets on Nicolet FTIR spectrophotometer (Nexus 470, Thermo Electron Corporation) in the range of 4000–500 cm1. An ESCA PHI500 X-ray photoelectron spectroscopy was used to record XPS spectra. The UV–vis diffuse reflection spectroscopy of samples was characterized by UV-2450 spectrophotometer (Shimadzu) and BaSO4 as the reference. The steady-state photoluminescence spectra and time-resolved photoluminescence spectra were measured by a Varian Cary Eclipse spectrometer. TriStar II 3020 automated gas sorption analyzer (Micromeritics Instrument Corporation USA) was carried out to analyze specific surface areas.

2. Experimental section 2.4. Evaluation of photocatalytic H2 production activity 2.1. Materials Niobium (IV) oxide (Nb2O5), potassium hydroxide (KOH), strontium nitrate (Sr(NO3)2), urea, ammonia aqueous solution (NH3
H2O (28%)), hydrochloric acid (HCl) and anhydrous alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd., China. All reagents were used directly without any other purification. 2.2. Synthesis of HSN/CN heterojunctions The CN nanosheets were synthesized using a previous reported method [40]. Typically, urea at 550 °C in an alumina crucible for 4 h in air at the rate of 2.3 K min1 and then cooled down naturally. The HSN nanooctahedrons were fabricated by means of a twostep approach. Firstly, 0.5 g of Nb2O5, 2.442 g of KOH and 35 mL of deionized water were added into 50 mL stainless-steel Teflonlined autoclave and heated at 180 °C for 48 h. Then, 2 M of HCl solution was dropped into the above solution to adjust the pH value to 4.0, and a mass of Nb2O5
nH2O white precipitates were then formed. Secondly, the as-obtained Nb2O5
nH2O was dispersed into 70 mL of deionized water, the pH values were adjusted to 9.0 by NH3
H2O (28%) aqueous solution under vigorous stirring. After that, 0.4244 g of Sr(NO3)2 was added , and the pH value was adjusted to 10.0. And then the mixture solution was sealed in a 100 mL stainless-steel Teflon-lined autoclave and heated at 200 °C for 2 days. The precipitate was collected by repeatedly washing with deionized water and ethanol for several times and then dried overnight.

The photocatalytic hydrogen production experiments of the as-prepared samples were tested in a Pyrex flask with a closed gas circulation system under ambient temperature. In general, the reaction was carried out in mixed aqueous solution containing 120 mL of deionized water and 30 mL of methanol served as sacrificial agents (50 mg of photocatalyst) under a CEL-HXUV300 Xe lamp (CEAULIGHT, China) irradiate with a 420 nm cut-off optical filter. The power intensity was fixed as 128 mW cm2 by controlling the distance between the photocatalytic reactor and the light source. Through a photochemical reduction deposition, 1.0 wt% Pt cocatalyst was loaded on photocatalysts. Prior to irradiation, N2 gas was passed through the suspension for 30 min until dissolved oxygen was removed completely. The evolved hydrogen was sampled every half hour with a syringe and further quantified using a gas chromatograph (GC-14C, Shimadzu) equipped with high-purity nitrogen as a carrier gas. The apparent quantum efficiency (QE) was estimated by using the following equation.

QE ¼

number of evolved hydrogen molecules  2  100% number of incident photons

2.5. Photoelectrochemical measurement Photocurrent measurements were carried out in a standard three-electrode electrochemical system. Platinum wire and standard calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The working electrode

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was prepared by dispersed 4 mg of photocatalyst in the mixture of 1 mL of ethanol and 5 wt% Nafion solution. Then, 90 mL of ink was dropped onto the surface of a FTO (1  2 cm), a uniform catalyst film was obtained after dry in air for 30 min. 2 M Na2SO4 solution was used as the electrolyte. A 300 W Xe lamp with a 420 nm cutoff optical filter was applied as the light source for the measurement of photocurrent. A CHI 660E electrochemical workstation was used to record the light/dark short circuit photocurrent response at zero bias. Electrochemical impedance spectroscopy (EIS) was measured on a CHI660 B electrochemical analyzer in a 4 0.1 M KCl solution containing 5 mM [Fe(CN)3 6 /Fe(CN)]6 solution. The platinum electrode and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. The working electrodes were HSN, CN, and 20-HSN/CN, which were coated on glassy carbon electrode, respectively. The amplitude of applied sine wave potential in each case was 5 mV and frequency range was 0.01–100,000 Hz. 3. Results and discussion 3.1. Morphological and structural information The morphology and structure of the as-synthesized HSN, CN, and HSN/CN was firstly displays in Fig. 1. As shown in Fig. 1a–c, bare HSN sample is composed of discrete octahedron nanocrystals with lateral size of approximately 200 nm. The bare CN appears as layered nanosheet with wrinkles, which is the typical feature of CN synthesized from the urea (Fig. 1d). Fig. 1e displays that HSN was grown on the crumpled CN nanosheet, forming a well-defined HSN/CN heterojunction. High resolution transmission electron microscopy (HRTEM) image (Fig. 1f) of HSN/CN shows the discontinued crystal fringes, of which the 0.30 nm and 0.37 nm lattice

(a)

(b)

1 μm

100 nm

(d)

spacing can be attributed to the (2 2 2) and (2 2 0) lattice planes of HSN, respectively. The EDS mapping results (Fig. 1 g) evidence that the HSN octahedron nanocrystals was decorated on the CN nanosheets surface, further confirming the formation of HSN/CN heterojunction. The XRD patterns of the CN, HSN, HSN/CN heterojunctions are plotted in Fig. 2a. For HSN, the peaks at 14.52°, 28.00°, and 29.27° correspond to the (1 1 1), (3 1 1) and (2 2 2) crystal planes (JCPDS No. 77-1165), respectively, which are in agreement with previous reported results. The diffraction peak at 27.41° of CN can be well indexed to the standard interlayer stacking peak of aromatic systems [41]. For HSN/CN heterojunctions with different HSN contents, the XRD peaks are similar to pure HSN, which is possibly due to the low crystalline of the CN nanosheet as compared with HSN. Of note, the diffraction peaks intensity of at 28.00° ascribed to the HSN in HSN/CN heterojunctions gradually strengthened, which is attributed to the gradual increase of HSN content, further confirming that the HSN/CN heterojunction were successfully constructed. To better investigate the component of HSN/CN heterojunctions, FTIR spectra (Fig. 2b) were carried out. CN and HSN/CN exhibit multiple strong bands in the region of 1100–1700 cm1, which reflect the typical stretching modes of CN heterocycles. Specifically, the band at 1249 cm1 originates from the stretching vibration absorption peak of CANAC, and the band at 1332 cm1 corresponds to the stretching vibrations of the connected units of NA(C) [42]. The bands at 1457 cm1 caused by the amorphous sp3 CAN bond, and the 1572 and 1620 cm1 are attributed to the stretching vibration modes of heptazine-derived repeating units [43]. The peak at 607 cm1 is assigned to the characteristic internal NbAO stretching vibration in the HSN octahedron [32]. The absorption peaks appearing at 810 cm1 belongs to the character-

(c)

200 nm

(e)

(f) μ

0.5 μm

(g)

0.5 μm C-K

N-K

5 nm Nb-K

Sr-K

O-K

Fig. 1. (a) and (b) SEM images of HSN, (c)–(e) TEM images of HSN, CN, 20-HSN/CN, (f) HRTEM image of 20-HSN/CN, (g) HAADF-STEM image and the corresponding EDS elemental mapping images of the 20-HSN/CN heterojunction.

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(b)

607

(a)

HSN

50-HSN/CN

50-HSN/CN

Transmittance (a.u.)

Intensity (a.u.)

40-HSN/CN 30-HSN/CN 20-HSN/CN 10-HSN/CN

HSN

40-HSN/CN 30-HSN/CN 20-HSN/CN 10-HSN/CN CN

10

20

30

40

50

60

70

80

4000

3500

3000

2500

2000

1500

810

1249 1332 1457 1572 1620

CN

1000

500

-1

2θ (degree)

Wavelength(cm )

Fig. 2. (a) XRD patterns and (b) the FTIR spectra of the as-prepared samples.

istic breathing mode of tri-s-triazine units [44]. The peak between 3500 and 3000 cm1 of HSN/CN heterojunctions correspond to secondary and primary amines [45]. These FTIR spectra results indicate that the CN structure was maintained in the HSN/CN heterojunctions. The surface chemical states and chemical composition of the asprepared samples were tested by the X-ray photoelectron spectroscopy (XPS) analysis and Fig. 3a is the survey spectra of HSN, CN, and 20-HSN/CN heterojunction sample. As observed in Fig. 3b, Nb 3d spectra showed typical peak pattern for HSN with peaks at 206.5, and 209.3 eV, corresponding to Nb 3d5/2, and Nb 3d3/2. There were two peaks in the Sr 3d region (Fig. 3c). The peak located at 132.7 eV corresponds to the Sr 3d5/2 and another one located at 134.5 eV is assigned to Sr 3d3/2. The O 1s spectrum (Fig. 3d) of HSN can be resolved into three peaks centered at 529.4, 530.6 and 532.5 eV, which represent adsorbed oxygen species, hydroxyl and carbonate species as well as adsorbed water,

O 1s

N 1s C 1s HSN

O1s

1000

800

600

400

200

(c) HSN

20-HSN/CN

0

211

210

Binding Energy (eV)

209

208

207

206

205

204

20-HSN/CN

136

135

Binding Energy (eV)

(d)

O 1S

(e)

C 1S

N-C=N

134

H 2O

538

536

534

532

530

Binding Energy (eV)

528

526

292

290

131

N 1S C-N=C π-excitation

CN

20-HSN/CN

540

132

(f) Intensity(a.u.)

Intensity(a.u.)

Intensity(a.u.)

O-/O2-

133

Binding Energy (eV)

C=C/C-C O2-

Sr 3d

Sr 3d5/2

Sr 3d3/2

HSN

Intensity(a.u.)

C 1s

20-HSN/CN

Nb 3d

Nb 3d3/2

N 1s

O 1s

CN

1200

Nb 3d5/2

Sr 3d

HSN

Intensity(a.u.)

(b)

Nb 3d

Intensity(a.u.)

(a)

respectively [46]. High-resolution C 1s XPS data (Fig. 3e) for pristine CN was fitted by two peaks at binding energies of 284.6 and 288 eV, which are assigned to graphitic carbon atoms and sp2hybridized carbon in the triazine rings (NAC@N), respectively [47]. For the N 1s fine XPS spectrum of CN (Fig. 3f), one peak at 398.4 eV is attributed to the CAN@C (sp2 hybridized nitrogen), peak at 399.2 eV represents NA(C)3 (tertiary nitrogen) and the other at 400.5 eV is ascribed to CANAH bond. A weak peak at 404.8 eV is assigned to the p-excitations [48,49]. Notably, the BE in the C 1s, N 1s, and Nb 3d over HSN/CN were shifted to the lowest binding energy (this shift is largest for x = 0.50 eV), indicating that the chemical state of photocatalysts might be changed. This result confirms that heterojunction successfully formed between CN nanosheets and HSN nano-octahedrons. Brunauer–Emmett–Teller (BET) specific surface areas were introduced and measured by N2 adsorption-desorption measurements at 77.4 K (Fig. 4). The BET surface areas of HSN, CN and

C-NH

N-(C)3

CN

20-HSN/CN

288

286

284

Binding Energy (eV)

282

404

402

400

Binding Energy (eV)

Fig. 3. (a) XPS survey spectra and high-resolution spectra of (b) Nb 3d, (c) Sr 3d, (d) O 1s, (e) C 1s, (f) N 1s.

398

396

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469.4 lmol g1, which is about 1.8 times higher than that of bare CN. After calculation, the apparent quantum efficiency of 20HSN/CN photocatalyst is about 2.1%. However, the H2 production activity of the 50-HSN/CN heterojunction decreased, possibly owing to the fact that excess HSN in the heterojunction could possibly restrain the separation of photo-induced charges and decrease the visible light absorption capability, which could reduce the photocatalytic H2 production rate. The stability of the 20-HSN/ CN heterojunction photocatalyst was also studied. As shown in Fig. 5b, the 20-HSN/CN heterojunction was stable after react 12 h under irradiation with intermittent evacuation every 3 h, further indicating the good stability of this heterojunction photocatalyst. The composite photocatalyst can generate H2 after 9 h with no apparent decrease observed.

HSN

CN

20-HSN/CN

70 60 50 40 30

dV/dD Pore Volume (cm 3/g? m )

80

3

Quantity Adsorbed (cm /g STP)

90 0.007 0.006 0.005

CN 20-HSN/CN

0.004 0.003 0.002 0.001 0

20

2

4

6

8

10

12

14

16

Pore Diameter (nm)

10 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure(P/P0)

3.3. Optical and electronic properties

Fig. 4. Nitrogen gas adsorption–desorption isotherms of CN, HSN and 20-HSN/CN, and (the insert) the corresponding pore size distributions of CN and 20-HSN/CN.

20-HSN/CN samples were 2.86, 33.75, and 28.23 m2 g1, respectively, where the specific surface area of 20-HSN/CN heterojunction is slightly smaller than the bare CN. The Barrett– Joyner–Halenda (BJH) pore-size distribution curve shows that the CN and 20-HSN/CN were similar and rich in mesoporous with average size of around 4 nm. It can be found that the contribution of HSN to the specific surface area and pore-size distribution are negligible. 3.2. Photocatalytic activity and stability Fig. 5a shows the photocatalytic H2 evolution activities of the HSN/CN heterojunction photocatalysts, bare CN and HSN under visible-light irradiation (k > 420 nm). Control experiments have been performed to verify that H2 production is a light-catalyzed reaction as there was no appreciable H2 production under nonirradiation or in the absence of photocatalyst. Clearly, the bare HSN shows a negligible H2 production performance due to its inherent wide band gap nature, which cannot be irradiated under visible light. The bare CN nanosheet photocatalyst shows a moderate photocatalytic H2 evolution with a rate of 256.4 lmol g1. In contrast, when a certain amount of HSN nanooctahedrons were combined with CN nanosheets, the photocatalytic activity of HSN/CN heterojunction was increased. The optimal 20-HSN/CN photocatalyst exhibits a highly improved H2 evolution rate of

500

500

(b)

CN 10-HSN/CN 20-HSN/CN 30-HSN/CN 40-HSN/CN 50-HSN/CN HSN

300 200

400

-1

400

H2 production (μmolg )

-1

H 2 production (μmolg )

(a)

To investigate the mechanism of enhanced photocatalytic activity of HSN/CN heterojunction photocatalysts, a series of experiments have been carried out. The optical properties of HSN, CN, and their corresponding heterojunctions were analyzed via UV– vis diffuse reflection spectra (DRS). As shown in Fig. 6a, it could be observed that the absorption edge of the HSN is located at about 300 nm while the light absorption range of CN (450 nm) is wider than that of HSN. Since the HSN/CN heterojunction generated, the UV–vis diffuse reflection spectra of the heterojunction samples exhibit visible light responsible, and the absorption edges are close to CN’s, indicating their good light adsorption ability. The band gap energies of bare HSN and CN were determined by the Tauc equation (ahm = A (hm  Eg)n/2) from the DRS data. As shown in Fig. 6b, the band gaps of CN and HSN for indirect transition (n = 4) are determined to be 2.75 and 4.10 eV, respectively. These results are analogous with previously reported results. The electron trapping behavior was supported by photoluminescence (PL) emission spectra (see Fig. 6c). HSN/CN heterojunctions exhibit apparently decreased emission intensity with respect to pristine HSN, indicating the lower recombination efficiency of the photo-generated charge in the heterojunction. In addition, the time-resolved PL decay plots after pulsed laser excitation at k = 446 nm (Fig. 6d) were further used to analyze the electron trapping behavior. The decay traces were fitted using triexponential decay kinetics, and the specific parameters are listed in Table 1 (the average decay time (sA) is calculated by the followP P ing equation: s ¼ i¼1; 2 Ai s2i = i¼1; 2 Ai si ). The 20-HSN/CN has a longer average decay time of 6.42 ns than HSN (0.83 ns), and CN

100

300 200 100 0

0 0.0

0.5

1.0 1.5 2.0 Irradiation time (h)

2.5

3.0

0

2

4 6 8 Irradiation time (h)

10

12

Fig. 5. (a) Time courses of photocatalytic H2 evolution over different photocatalysts under visible light (k > 420 nm); (b) cycling runs for photocatalytic H2 evolution activity over the 20-HSN/CN heterojunction under the visible-light irradiation in 12 h.

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2.0

(a )

( b)

200

300

400

500

600

700

1/2

1.5

1/2

( αh ν) (eV)

Intensity(a.u.)

HSN 50-HSN/CN 40-HSN/CN 30-HSN/CN 20-HSN/CN 10-HSN/CN CN

CN HSN

1.0 0.5 0.0 2.0

800

2.5

3.0 3.5 hν (eV)

Wavelength (nm)

Intensiyt(a.u.)

CN 10-HSN/CN 20-HSN/CN 30-HSN/CN 40-HSN/CN 50-HSN/CN

CN 20-HSN/CN HSaN CPE

Z-imaginary / ohm

250 200

460 480 500 Wavelength(nm)

Rs Rct

Wd

150

520

CN 20-HSN/CN

60

40

20 150

135

200 250 Z-real / ohm

300

100

140

145

150

155

160

Time (ns)

( f)

80

4.5

20-HSN/CN CN HSN

130

540

Photocurrent( mA cm-2)

(e) 300

440

Z-imaginary / ohm

420

4.0

(d) Intensity (a.u.)

(c)

4.1 eV

2.7 5e V

light

20-HSN/CN CN HSN dark

50 100

200

300

400 500 600 Z-real / ohm

700

800

80

100 120 140 160 180 200 220

Time (sec)

Fig. 6. (a) UV–vis diffuse reflectance spectra of different samples, (b) plot of (ahv)1/2 vs. (hv) for the band gap energy of HSN and CN, (c) room temperature PL spectra of CN and HSN/CN heterojunctions under the excitation wavelength of 434 nm, (d) time-resolved fluorescence decay spectra, (e) Nyquist plots, and (f) transient photocurrent response of pure HSN, CN and 20-HSN/CN.

Table 1 Time-resolved fluorescence decay parameters of the HSN, CN and 20-HSN/CN.

HSN CN 20-HSN/CN

A1

s1

A2

s2

sA (average s)

123.62 845.59 550.14

0.83 3.88 0.10

123.62 224.06 767.32

0.83 0.45 6.49

0.83 3.78 6.42

(3.78 ns), implying that the constructed heterojunction can effectively prohibit the recombination of charge carriers. The electrochemical impedance spectra (EIS) analysis was conducted to study the interfacial carrier transport of the pristine CN, HSN and HSN/CN. As shown in Fig. 6e, remarkable decrease in semicircular Nyquist plots than CN are observed for 20-HSN/CN (inset), demonstrating that charge transfer in the interface of HSN and the electrolyte was increased due to the generation of heterojunction. To further assess the efficient separation of electron–hole pairs, we measured the transient photoresponse under

visible-light irradiation for four ON–OFF cycles in a chopping mode, and the photocurrent-time (I-t) curves are displayed in Fig. 6f. As expected, 20-HSN/CN exhibits the highest photocurrent intensity value, indicating the more efficient charge transfer in the heterojunction, which is consistent with EIS results. In addition, it can be observed that the photocurrent values have no clear attenuation, suggesting that the transient photocurrent of HSN, CN, and the 20-HSN/CN heterojunction are stable and the photoresponses are quite reversible and reproducibly. All of above results prove that the construction of HSN/CN heterojunction can obviously

W. Ma et al. / Journal of Colloid and Interface Science 526 (2018) 451–458

boosting the separation and transfer of photo-generated carriers and then enhance the photocatalytic activity. The possible mechanism of the photocatalytic property of HSN/ CN heterojunction is associated with their relative band edge positions [50]. The conduction band (CB) and valence band (VB) potentials of HSN and CN can be estimated by Mulliken electronegativity theory.

EVB ¼ X   Ee þ 0:5Eg where X expressed as the electronegativity of the semiconductor (which is the geometric mean of constituent atoms). The XHSN and XCN are 6.42 and 4.715 eV, respectively. Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV versus NHE), and Eg is the band gap energy of the semiconductor. Based on the above data, the VB potentials of HSN and CN can be estimated to be 3.82 and 1.59 eV. According to ECB = EVB  Eg, the CB potentials of HSN and CN can be calculated out. The ECB of HSN and CN are 0.28 and 1.16 eV. The potentials of the CB and VB of the CN is found to be higher than HSN. Based on the above results, possible photocatalytic mechanisms of HSN/CN for the enhanced photocatalytic activity were speculated and illustrated in Fig. 7. When the HSN/CN heterojunction photocatalyst is irradiated by visible light, though the photogenerated electron-hole pairs will also be excited from the CN, the HSN could not response this light energy. Due to the Pt nanoparticles with high Fermi level acted as the cocatalyst, in the H2 production proceed, the generated electrons will transfer to the Pt nanoparticles and further reduce H+ to H2 [51–53]. Thus, the photocatalytic activity of HSN/CN heterojunction for H2 production was improved. Due to the role of specific surface area has been eliminated by the BET analysis, this enhanced photocatalytic activity for H2 evolution can be mainly attributed to the matched energy level and heterojunction structure which could improve the photo-generated charge carriers separation and transfer, as confirmed by the steady-state and time-resolved PL and photoelectrochemical analysis.

V vs. NHE -2 -1

e- e-

-1.16 eV

O2

H2 e-

-0.28 eV

1.59 eV

•O2–

ee-

e-

ePt

0 HSN 1

e-

H+

CN 2.75eV h+

h+

HSN 4.1 eV

h+

2 CH3OH CO2+H2O

3

3.82eV

h+

h+

h+

4 Fig. 7. Proposed mechanism of charge transfer in the HSN/CN heterojunctions.

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4. Conclusion To summarize, a novel HSN/CN heterojunction was successfully synthesized by a facile hydrothermal method. The HSN/CN heterojunction exhibited superior photocatalytic activity towards H2 evolution under visible light irradiation. Specially, the developed 20 wt%-HSN/CN heterojunction showed high H2 evolution rate up to 469.4 lmol g1, which was 1.8 times higher than that of bare CN. The enhanced photocatalytic performance could be attributed to the heterojunction structure which could significantly inhibit the recombination of photo-generated electron-hole pairs. It can be expected the as-prepared HSN/CN heterojunction can be used as a promising photocatalyst in the renewable energy production. Acknowledgement This work was supported by the financial supports of National Nature Science Foundation of China (Nos. 21606111, 21406091, and 21576121), Natural Science Foundation of Jiangsu Province (BK20150482 and BK20140530), China Postdoctoral Science Foundation (2015M570409), and Research Foundation for Talented Scholars of Jiangsu University (15JDG054). References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] C. Liu, B.C. Colón, M. Ziesack, P.A. Silver, D.G. Nocera, Science 352 (2016) 1210– 1213. [3] H.L. Wang, L.S. Zhang, Z.G. Chen, J.Q. Hu, S. Li, Z.H. Wang, J.S. Liu, X.C. Wang, Chem. Soc. Rev. 43 (2014) 5234–5244. [4] C.C. Chen, X.Z. Li, W.H. Ma, J.C. Zhao, J. Phys. Chem. B 106 (2002) 318–324. [5] Z.X. Gan, X.L. Wu, M. Meng, X.B. Zhu, L. Yang, P.K. Chu, ACS Nano 8 (2014) 9304–9310. [6] X.D. Ma, W.X. Ma, D.L. Jiang, D. Li, S.C. Meng, M. Chen, J. Colloid Interface Sci. 506 (2017) 93–101. [7] J.W. Sun, Y.S. Fu, P. Xiong, X.Q. Sun, B.H. Xu, X. Wang, RSC Adv. 3 (2013) 22490– 22497. [8] D. Lang, T.T. Shen, Q.J. Xiang, ChemCatChem 7 (2015) 943–951. [9] L.J. Zhang, R. Zheng, S. Li, B.K. Liu, D.J. Wang, L.L. Wang, T.F. Xie, ACS Appl. Mater. Interfaces 6 (2014) 13406–13412. [10] S.D. Guan, X.L. Fu, Y. Zhang, Z.J. Peng, Chem. Sci. 9 (2018) 1574–1585. [11] X.C. Wang, K. Maeda, X.F. Chen, K. Takanabe, K. Domen, Y.D. Hou, X.Z. Fu, M. Antonietti, J. Am. Chem. Soc. 131 (2009) 1680–1681. [12] X.F. Yang, H. Tang, J.S. Xu, M. Antonietti, M. Shalom, ChemSusChem 8 (2015) 1350–1358. [13] J.S. Zhang, Y. Chen, X.C. Wang, Energy Environ. Sci. 8 (2015) 3092–3108. [14] J.W. Sun, B.V. Schmidt, X. Wang, M. Shalom, ACS Appl. Mater. Interfaces 9 (2017) 2029–2034. [15] J. Liu, Y. Liu, N.Y. Liu, Y.Z. Han, X. Zhang, H. Huang, Y. Lifshitz, S.T. Lee, J. Zhong, Z. Kang, Science 347 (2015) 970–974. [16] L.N. Li, M. Shalom, Y.B. Zhao, J. Barrio, M. Antonietti, J. Mater. Chem. A 5 (2017) 18502–18508. [17] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [18] K.L. He, J. Xie, M.L. Li, X. Li, Appl. Surf. Sci. 430 (2018) 208–217. [19] J.S. Xu, T.J. Brenner, Z.P. Chen, D. Neher, M. Antonietti, M. Shalom, ACS Appl. Mater. Interfaces 6 (2014) 16481–16486. [20] G.H. Dong, L.Z. Zhang, J. Mater. Chem. 22 (2012) 1160–1166. [21] D.L. Jiang, J. Li, C.S. Xing, Z.Y. Zhang, S.C. Meng, M. Chen, ACS Appl. Mater. Interfaces 7 (2015) 19234–19242. [22] J.L. Lin, Z.M. Pan, X.C. Wang, ACS Sustain. Chem. Eng. 2 (2014) 353–358. [23] P. Niu, Y.Q. Yang, J.C. Yu, G. Liu, H.M. Cheng, Chem. Commun. 50 (2014) 10837– 10840. [24] H.F. Shi, G.Q. Chen, C.L. Zhang, Z.G. Zou, ACS Catal. 4 (2016) 3637–3643. [25] J.S. Xu, T.J. Brenner, L. Chabanne, D. Neher, M. Antonietti, M. Shalom, J. Am. Chem. Soc. 136 (2014) 13486–13489. [26] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2008) 76–80. [27] Y. Wang, X.C. Wang, M. Antonietti, Angew. Chem. Int. Edit. 51 (2012) 68–89. [28] G. Liu, P. Niu, C.H. Sun, S.C. Smith, Z.G. Chen, G.Q. Lu, H.M. Cheng, J. Am. Chem. Soc. 132 (2010) 11642–11648. [29] Y. Kofuji, S. Ohkita, Y. Shiraishi, H. Sakamoto, S. Tanaka, S. Ichikawa, T. Hirai, ACS Catal. 6 (2016) 7021–7029. [30] J.J. Tian, L.X. Zhang, X.Q. Fan, Y.J. Zhou, M. Wang, R.L. Cheng, M.L. Li, X.T. Kan, X. X. Jin, Z.H. Liu, J. Mater. Chem. A 4 (2016) 13814–13821. [31] S.Y. Wang, X.L. Yang, H.J. Hou, X. Ding, S.H. Li, F. Deng, Y.G. Xiang, H. Chen, Catal. Sci. Technol. 7 (2016) 418–426. [32] H.Y. Li, S.Y. Gan, H.Y. Wang, D.X. Han, L. Niu, Adv. Mater. 27 (2015) 6906–6913.

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