Sn2+-Doped Double-Shelled TiO2 Hollow Nanospheres with Minimal

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Sn2+-Doped Double-Shelled TiO2 Hollow Nanospheres with Minimal Pt content for Significantly Enhanced Solar H2 Production Chao Zhang, Yuming Zhou, Jiehua Bao, Yiwei Zhang, Jiasheng Fang, Shuo Zhao, Wenxia Chen, and Xiaoli Sheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01122 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Sustainable Chemistry & Engineering

Sn2+-Doped Double-Shelled TiO2 Hollow Nanospheres with Minimal Pt content for Significantly Enhanced Solar H2 Production

Chao Zhang, Yuming Zhou*, Jiehua Bao, Yiwei Zhang, Jiasheng Fang, Shuo Zhao, Wenxia Chen, Xiaoli Sheng

School of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, China

Corresponding author: Yuming Zhou E-mail: [email protected] Address: Southeast University Avenue Jiangning District, Nanjing, Jiangsu Province, China Tel: +86 25 52090617; Fax: +86 25 52090617.

ABSTRACT H2 evolution by photocatalytic water splitting has attracted a lot of attention due to the global depletion of oil resources. Therefore, much effort is devoted to develop low cost highly active photocatalysts. Here, a facile strategy is proposed for the synthesis of low Pt content Sn2+-doped double-shelled Pt/TiO2 hollow nanocatalyst (DHS-PtSn2+) with excellent solar H2 production properties. After calcination in N2, 1 ACS Paragon Plus Environment

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DHS-PtSn2+ showed highest photocatalytic H2 production rate of 28502 µmol h−1 g−1, nearly three-fold higher than Sn4+-doped counterpart, thereby demonstrating better synergistic effect of Sn2+ than Sn4+ in H2 evolution. The influences of calcination atmosphere, Sn2+ content and Sn/Pt atomic ratio on H2 production have been investigated with a series of contrast experiments. Besides, the proposed Sn2+ doping strategy could also be applied in other light-sensitive materials (e.g. home-made TiO2 nanoparticles, commercial P25 and g-C3N4), suggesting its extensive applications in H2 production. Finally, based on the excellent synergistic effect of Sn2+ in H2 production, a possible photocatalytic mechanism was tentatively proposed.

KEYWORDS: H2, Sn2+, Pt, TiO2, C3N4, Photocatalyst

INTRODUCTION With global depletion of oil resources, solar fuels technology is considered to be the most promising route to generate renewable energy carriers, in particular, hydrogen.1-4 Currently, much effort has been devoted to generate H2 evolution from water splitting via cost-efficient photocatalytic system, which not only integrates solar light harvesting but also presents excellent durability. Among various photocatalytic materials, TiO2 has proved suitable candidate for photocatalytic water splitting because of their excellent chemical properties, such as biological inertness, controllable synthesis and long-term stability.5-8 Surface modification of TiO2 by Pt species is an efficient method to improve photocatalytic performance due to the 2 ACS Paragon Plus Environment

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enhanced photogenerated charge carriers separation.9-11 For example, Gong et. al. specially loaded Pt and MnOx onto the inner and outer surface of thin TiO2@In2O3 shells, achieving enhanced charger carriers separation.12, 13 The spatially separated Pt and MnOx could induce electrons and holes to migrate reversely and the In2O3 in TiO2 shell can separate charger carriers in bulk phase efficiently. But the high cost of Pt seriously limits its potential application in H2 production. One of the promising strategies to reduce cost is to develop low platinum photocatalyst by constructing special Pt-support interaction and selecting suitable catalyst support. Recently, Sn species have been commonly used in fabricating TiO2 based photocatalysts for H2 production.14-17 For instance, Long et. al. synthesized three kinds of Sn-modified TiO2 catalysts and found that the Ti4+-O-Sn4+ linkage formed at TiO2-SnO2 interface could accelerate the interfacial electron transfer between TiO2 and Sn moieties, thereby enhancing H2 production.18 Generally, the doped Sn4+ species serves as electron-sink function to improve photogenerated charge carriers separation and thus enhance H2 production performance. But most of the reported Sn doped catalytic systems are high in noble metal content (e.g. ≥0.5 wt%) and enriched with Sn4+. Until now, the effect of Sn2+ species in H2 production has not been investigated thoroughly. The incorporated Sn2+ species may exhibit much higher synergistic effect than Sn4+ in photocatalytic H2 production due to its unique reducibility. Very recently, our team reported a double-shelled TiO2 hollow sphere (DHS, a special nanosphere assembled with 2D TiO2 nanosheets).19 Besides, after doping Sn4+/Pt via in situ reduction method, DHS exhibited an improved solar H2 3 ACS Paragon Plus Environment


production (6165 µmol g-1 h-1) due to the enhanced Pt/Sn interaction and special double-shelled structure.20 Herein, we report a robust, facile, and effective strategy to fabricate low Pt content Sn2+-doped double-shelled Pt/TiO2 hollow nanocatalyst (DHS-PtSn2+) with excellent solar H2 production capability. In this work, N2 calcined DHS-PtSn2+ (DHS-N2-PtSn2+) showed highest photocatalytic H2 production rate (28502 µmol h−1 g−1), approximately six-fold higher than pure double-shelled Pt/TiO2 hollow nanocatalyst (DHS-Pt). Furthermore, after calcination in air, DHS-PtSn2+ exhibited much lower H2 production rate (9943 µmol h−1 g−1) due to the oxidation of Sn2+ to Sn4+, thereby demonstrating the excellent synergistic effect of Sn2+ in H2 evolution. The Sn2+ doping strategy could also be applied in other titanium dioxide materials (e.g. home-made TiO2 nanoparticles and commercial P25) and visual light-sensitive materials (e.g. g-C3N4), indicating its great potential applications in H2 production.

EXPERIMENTAL SECTION Synthesis of DHS-PtSn2+ SiO2@TiO2 nanospheres were synthesized via simple sol-gel reactions.19 Then SiO2@TiO2 particles (0.2 g) were dispersed in a mixture of CTAB (1 ml, 5.5 mM), SnCl2 (3 ml, 5 mM), K2PtCl4 (1 mL, 4 mM) and deionized water (50 mL). Under vigorous stirring, a certain amount of ammonia solution was added to keep the solution at pH 11. Subsequently, the mixture was sealed in 100 mL teflon-lined autoclave, followed by hydrothermal treatment at 140 ℃ for 24 h in static state. The

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precipitate (DHS-PtSn2+) was isolated by centrifugation, washed with ethanol and dried at 60 ℃

under vacuum. Sn4+-doped double-shelled Pt/TiO2 hollow

nanocatalyst (DHS-PtSn4+) was synthesized with SnCl4 as stannum source via same experimental procedure as DHS-PtSn2+. Then DHS-PtSnx+ (X= 2 or 4) was calcined at 500 ℃ in Y atmosphere (Y= N2 or Air) for 6 h (DHS-Y-PtSnX+). The Sn/Pt atomic ratio in DHS-Y-PtSn2+ was adjusted via changing the dosage of SnCl2 solution during hydrothermal treatment process. The obtained DHS-N2-PtSn2+ samples with different Sn/Pt atomic ratio are denoted as DHS-N2-PtXSn2+, X is Sn/Pt atomic ratio. DHS-H2-PtSn2+ sample was synthesized by calcination treatment of DHS-Air-PtSn2+ in H2 atmosphere at 500 ℃ for 3 h.

Photocatalytic H2 production test for DHS-N2-PtSn2+ and HMTiO2-N2-PtSn2+ For photocatalytic hydrogen production, methanol was used as sacrificial agent. A total of 10 mg photocatalyst was added in 100 mL methanol/H2O (10% methanol) solution. Before solar light irradiation, the suspension was fully degassed to remove air. A 300 W Xenon lamp (PLS-SXE 300C (BF), Perfect-Light, Beijing) was used as solar light source. The amount of H2 was quantified by an online gas chromatograph (GC9890A/T, TCD detector, Ar carrier gas, 5A molecular sieve column). The photocatalytic H2 production test was performed at room temperature (27 ℃) for 4 h. The stability test of DHS-N2-PtSn2+ for H2 evolution was carried out by adding 10 mL methanol in the next cycle.

RESULTS AND DISCUSSION Structure and morphology 5 ACS Paragon Plus Environment


Figure 1. (a) Schematic illustration for the fabrication of DHS-PtSn2+. (b) TEM and (c) SEM images of DHS-PtSn2+. (d) EDX elemental mapping analysis of DHS-N2-PtSn2+. (e) XRD patterns of DHS-PtSn2+, DHS-N2-PtSn2+ and DHS-N2-3PtSn2+ (synthesized by tripling the dosage of PtCl42-).

DHS-PtSn2+ was synthesized facilely by doping Sn2+ and Pt in DHS as illustrated in Figure 1a. The double-shelled structure of DHS-PtSn2+ was characterized by TEM. As shown in Figure 1b, DHS-PtSn2+ (ca. 500 nm in diameter) is assembled with numerous ultrathin nanosheets. The average distance between outer and inner shells is ca. 40 nm. The SEM image of DHS-PtSn2+ (Figure 1c) is consistent with above TEM analysis, proving the hierarchical structure of DHS-PtSn2+. Besides, EDX elemental mapping analysis was applied to investigate the elemental distributions of O, Ti, Sn and Pt in DHS-N2-PtSn2+. Representative results are shown in Figure 1d, O, Ti and Sn 6 ACS Paragon Plus Environment

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elements are found clearly in DHS-N2-PtSn2+ and no Pt element could be detected obviously due to the very low Pt content (ca. 0.07 wt%, Figure S1a). The crystalline phase of DHS-PtSn2+ was also investigated by XRD. From Figure 1e, because of the low crystallinity, DHS-PtSn2+ shows few weak diffraction peaks, which is roughly matching the reflection characteristic of anatase TiO2 (JCPDS no. 21-1272). After calcination in N2, all the peaks of anatase TiO2 are enhanced, while no diffraction peaks of Sn and Pt appear probably due to homogeneous distribution and low content. The typical anatase TiO2 diffraction peaks of DHS-N-PtSn2+ indicate that the doped Sn species don't affect the crystal structure of the catalysts. Besides, from Figure S1b, both of the Ti 2p spectrum of DHS-N2-PtSn2+ and DHS- N2-PtSn4+ exhibit two typical Ti 2p peaks at binding energies of 458.8 (Ti 2p3/2) and 464.6 eV (Ti 2p1/2), thereby demonstrating that the doped Sn species show no influence on the chemical status of Ti elements. When tripling the dosage of PtCl42- during the fabrication of DHS-N2-PtSn2+, the (111) crystal plane peak of Pt at 2θ = 39.8° could be observed, thereby demonstrating the successful introduction of Pt species.



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Figure 2. Comparison of solar-light H2 production rate over calcined DHS, DHS-Pt, DHS-PtSn4+, and DHS-PtSn2+ in different atmospheres.

Table 1. Comparison of H2 production over Sn or noble metal-based TiO2 photocatalysts H2 production rate Catalyst

Noble metal content (wt%)

Light power (µmol h−1 g−1)

DHS-N2-PtSn2+ (This work)

0.07

Solar light 300 W

28502

DHS/Sn4+/Pt20

0.24

Solar light 300 W

6165

1

Solar light 300 W

218.7

21

Sn3O4/TiO2/Pt

Sn and N doped TiO2

22

Pt1–Au2/TiO210 Au-embedded boron-doped TiO223 Au0.25/Pt0.75/TiO2 nanofibers24 4+

Sn -decorated Au/TiO2 Pt/TiO2

25

14

0

Solar light 300 W

2.81

Pt: 1, Au : 2

Solar light 300 W

1228

0.04 (at%)

Solar light 300 W

2740

Au: 0.25, Pt: 0.75

Solar light 300W

2331

0.91

Solar light 300 W

ca. 14333

2

Solar light 350 W

16675

Au−Pt Alloyed TiO226

Au−Pt total content:1

Solar light 300 W

23666

Pt/TiO2 Nanofiber27

1

Pt1.0/Sn1.0/TiO2-S16

1

λmax=352 nm UV light 30 W λ≈365 nm 125 W


23850 26000

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The photocatalytic H2 production properties of various samples are measured and compared in Figure 2. No H2 was evolved over pure DHS. The H2 production rate of DHS-Pt and DHS-N2-PtSn4+ were 4830 and 9736 µmol h−1 g−1, respectively. It should be noted that both of DHS-Air-Pt and DHS-Air-PtSn4+ exhibited similar H2 production rate with DHS-N2-Pt and DHS-N2-PtSn4+ respectively, thereby suggesting that the oxidation of Pt during air calcination could not affect H2 production activity. In this work, DHS-N2-PtSn2+ showed highest photocatalytic H2 production rate of 28502 µmol h−1 g−1, nearly three-fold and six-fold higher than DHS-N2-PtSn4+ and DHS-N2-Pt, respectively. During solar water splitting process in the presence of DHS-N2-PtSn2+, clusters of rising H2 bubbles could be observed clearly (Supplemental movie). From the comparison of H2 production over Sn or noble metal-based TiO2 photocatalysts in Table 1, it can be found that DHS-N2-PtSn2+ performed much better than most catalysts. Besides, although some of the cited reports exhibited high H2 production rate also, these catalysts contained high noble metal content (ca. 1 wt%) or irritated by UV light.16, 26, 27 Therefore, considering both of H2 production rate and noble metal content, to some extent, the synthesized DHS-N2-PtSn2+ may exhibit high application potential in H2 production. The above results suggested that Sn2+ species may be the key factor to dramatically improve H2 production of Pt/TiO2 photocatalyst. In contrast, most of the Sn2+ species in DHS-PtSn2+ could be oxidized to Sn4+ by calcination in air. Accordingly, the H2 production rate of DHS-Air-PtSn2+ seriously decreased to 9943 µmol h−1 g−1, thereby suggesting that Sn2+ species may exhibit higher synergistic effect than Sn4+ in 9 ACS Paragon Plus Environment


photocatalytic H2 production.

Figure 3. The Sn 3d5/2 XPS spectra of DHS-PtSn2+, DHS-N2-PtSn2+, DHS-Air-PtSn2+ and DHS-H2-PtSn2+ (DHS-H2-PtSn2+ sample was synthesized by H2 reduction treatment of DHS-Air-PtSn2+).

XPS was utilized to analyze the valence states of Sn species. As shown in Figure 3, the Sn 3d5/2 signal at ca. 486 eV can be deconvoluted by Gaussian function into two parts at 485.8 and 487.1 eV, which were assigned to Sn2+ and Sn4+, respectively.28-30 The ratio of Sn2+ to the whole Sn species in DHS-PtSn2+ was estimated to be ca. 83.6 %. The appearance of small amount of Sn4+ could be ascribed to the oxidation of Sn2+ by PtCl42-.31-33 Besides, the Sn2+ content in DHS-N2-PtSn2+ was ca. 75 %, implying that most Sn2+ species could be protected from oxidation by 10 ACS Paragon Plus Environment

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calcination in N2. In contrast, the Sn2+ ratio in DHS-Air-PtSn2+ significantly decreased to 25.6 %, indicating that most Sn2+ were oxidized to Sn4+ during calcination in air. Therefore, the poor H2 production of DHS-Air-PtSn2+ could be ascribed to the oxidation of Sn2+. To further demonstrate the excellent synergistic effect of Sn2+ in H2 production, the as-obtained DHS-Air-PtSn2+ was calcined in H2 atmosphere (DHS-H2-PtSn2+) to reduce partial Sn4+ to Sn2+. Just as confirmed by XPS analysis of DHS-H2-PtSn2+ in Figure 3, the Sn2+ ratio increased to 38.5 % and, accordingly, the H2 production rate improved (15588 µmol h−1 g−1, Figure 2) dramatically.

Figure 4. (a) H2 production rate variation trend of DHS-N2-PtSn2+, DHS-Air-PtSn2+ and DHS-H2-PtSn2+ by changing Sn/Pt atomic ratio. (b) UV−vis absorption spectra of DHS-N2-PtXSn2+, X is Sn/Pt atomic ratio. (c) Nitrogen adsorption-desorption isotherms and (d) pore size distribution curves of DHS-N2, DHS-N2-Pt11Sn2+ and 11 ACS Paragon Plus Environment


DHS-N2-Pt27.5Sn2+.

To investigate the effect of Sn/Pt atomic ratio on H2 production, a series of contrasting Sn-dependent experiments were carried out whilst keeping all other reaction conditions the same. As shown in Figure 4a, with increasing Sn/Pt atomic ratio from 0 to 11, the H2 production rate of DHS-N2-PtSn2+ improved from 4830 to 28502 µmol h−1 g−1 dramatically due to the excellent synergistic effect of Sn2+. When Sn/Pt atomic ratio was further increased to 31.2, the H2 production rate of DHS-N2-PtSn2+ decreased to 9381 µmol h−1 g−1 gradually. The poor photocatalytic performance of DHS-N2-PtXSn2+ (X＞11) can be ascribed to the reduced light absorbance ability. Just as illustrated in Figure 4b, when Sn/Pt atomic ratio increased from 11 to 27.5, the UV absorbance intensity of DHS-N2-PtXSn2+ (X＞11) in UV region became weak. Furthermore, the pore structure parameters of DHS-N2-PtSn2+ with different Sn/Pt atomic ratio were investigated by N2 adsorption-desorption measurement (Figure 4c and d). The surface area of DHS-N2, DHS-N2-Pt11Sn2+ and DHS-N2-Pt27.5Sn2+ were calculated to be 417.6, 394.7 and 373.4 m2 g−1, respectively. Additionally, when increasing Sn/Pt atomic ratio, the corresponding BJH pore size distribution shifted to the large size. Meanwhile, compared with DHS-N2 and DHS-N2-Pt11Sn2+, no mesoporous structure (ca. 3 nm in diameter) could be found in DHS-N2-Pt27.5Sn2+. Therefore, Sn/Pt atomic ratio can affect the surface area and pore size distribution of DHS-N2-PtSn2+. The poor H2 production of DHS-N2-PtXSn2+ (X＞11) could also be attributed to the decreased porosity and surface area. Besides, 12 ACS Paragon Plus Environment

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as marked in Figure 4a, all DHS-Air-PtSn2+ samples showed much lower H2 production rate than DHS-N2-PtSn2+. Then, after H2 calcination treatment, the photocatalytic performance of DHS-Air-PtSn2+ increased obviously due to the reduction of Sn4+ to Sn2+.

Figure 5. (a) The effect of PtCl42- dosage on H2 production of DHS-N2-PtSn2+, the inset is Sn 3d5/2 XPS spectra of DHS-N2-3PtSn2+. (b) PL emission spectra of calcined DHS-N2, DHS-Pt27.5Sn2+, DHS-Pt6.9Sn2+ and DHS-Pt11Sn2+ in N2 or air at an excitation wavelength of 320 nm. (c) Photocurrent response under solar light irradiation and (d) electrochemical impedance spectroscopy plots of DHS-N2-Pt, DHS-N2-PtSn4+ and DHS-N2-PtSn2+.

The influence of PtCl42- dosage on H2 production of DHS-N2-PtSn2+ was 13 ACS Paragon Plus Environment


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investigated. As illustrated in Figure 5a, when increasing PtCl42- dosage from 0 to 1 mL, the H2 production rate of DHS-N2-PtSn2+ improved significantly from 0 to 28502 µmol h−1 g−1 due to the electron-sink function of Pt.34 As PtCl42- dosage further increased to 3 mL, the H2 production rate of DHS-N2-PtSn2+ anomalously decreased to 18642 µmol h−1 g−1. The reduced H2 production rate could be ascribed to the oxidation of Sn2+ by enough PtCl42-.31, 33 Just as demonstrated in the inset of Figure 5a, the Sn2+ ratio of DHS-N2-3PtSn2+ decreased to 23 %. When further increasing PtCl42dosage, the H2 production rate of DHS-N2-PtSn2+ began to improve. Until quintupling PtCl42- dosage, the H2 production rate was close to 28502 µmol h−1 g−1. Thus it may be accessible to reduce noble metal consumption by doping Sn2+ species in water spitting photocatalyst. PL emission spectra were used to study the effect of Sn2+ on photoexcited electron−hole pair separation. From Figure 5b, the emission intensity of all N2 calcined DHS-PtXSn2+ is much lower than air calcined counterparts, thereby indicating

the

excellent

DHS-N2-PtXSn2+.35,

36

photogenerated

charge

carriers

separation

of

Photoelectrochemical measurements were performed in a

typical three-electrode cell to further investigate the performance of Sn2+ or Sn4+ doped photocatalysts. As shown in Figure 5c, DHS-N2-PtSn2+ exhibited the highest intensity among all samples, which strongly illustrating the promoted mobility of photoexcited charge carriers by doping Sn2+. Figure 5d presented the electrochemical impedance spectra (EIS) in dark. DHS-N2-PtSn2+ showed a smaller EIS semicircular than DHS-N2-Pt and DHS-N2-PtSn4+ due to the improved interfacial charge transfer rate. The photocurrent response and EIS result were in line with above H2 production 14 ACS Paragon Plus Environment

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properties, further certifying the better synergistic effect of Sn2+ than Sn4+. In addition, the double-shelled structural advantage and excellent catalytic stability of DHS-N2-PtSn2+ were demonstrated by constructing Sn2+-doped single-shelled Pt/TiO2 catalyst (Figure S2a) and four recycle photocatalytic experiments (Figure S2b), respectively. Furthermore, DHS-N2-PtSn2+ also exhibited an excellent photocatlytic activity toward the degradation of Rhodamine B under UV light irradiation with a rate constant of 0.6 min-1, which is much faster than that of DHS-Air-PtSn2+ (0.32 min-1, Figure S3).

Figure 6. (a) SEM image of Sn2+ doped home-made TiO2 nanoparticles (HMTiO2-N2-PtSn2+). (b) Comparison of solar-light H2 production rate over HMTiO2-N2-Pt, HMTiO2-N2-PtSn2+ and HMTiO2-N2-PtSn4+.

The proposed Sn2+ doping strategy can be applied in other titanium dioxide materials (e.g. home-made TiO2 nanoparticles and commercial P25), thereby indicating its extensive applications. From Figure S4, it could be observed that the 15 ACS Paragon Plus Environment


home-made TiO2 nanoparticles (HMTiO2) exhibited irregular blocky structure. After hydrothermal treatment, numerous nanosheets structure in situ grew on HMTiO2 surface (Figure 6a). When Sn2+ was introduced in HMTiO2-N2-Pt, the photocatalytic H2 production rate increased from ca. 4592 to 26940 µmol h−1 g−1, nearly three-fold higher than Sn4+-doped counterpart (HMTiO2-N2-PtSn4+, Figure 6b). Additionally, the synthesized P25-N2-PtSn2+ nanocatalyst (Figure S5) via Sn2+ doping strategy also exhibited high photocatalytic H2 production rate of 25780 µmol h−1 g−1. Besides, the simple Sn2+ doping strategy is easy to scale up. As much as 5 g of P25-N2-PtSn2+ can be obtained in one reaction (Figure S6). Therefore, it can be concluded that the proposed Sn2+ doping strategy has great potential applications in TiO2 based photocatalyst for H2 production.

Figure 7. (a) SEM image of g-C3N4. (b) TEM image of g-C3N4-PtSn2+, the inset is high-resolution TEM image of Pt. In order to specially study the H2 production properties of DHS-N2-PtSn2+ under visible light irradiation, a cut-off optical filter of λ > 420 nm was used to remove UV light in solar light. It was found that DHS-N2-PtSn2+ showed no photocatalytic H2 production. It is easy to understand that TiO2 cannot be photoexcited by visible light. 16 ACS Paragon Plus Environment

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As confirmed by UV−vis absorption spectra in Figure 4b, all DHS-N2-PtXSn2+ samples showed no clear absorbance in visible region. To study the H2 production properties of Sn2+ under visible light, an elaborate experiment was designed to dope Sn2+ in visual light-sensitive materials (e.g. g-C3N4). The g-C3N4 power was synthesized by one step polymerization of urea in air.37 From Figure 7a, the obtained pure g-C3N4 is cotton-like particles aggregated by irregular nanosheets. After doping Pt and Sn2+ in g-C3N4 via hydrothermal treatment, many Pt clusters appeared on g-C3N4 surface (Figure 7b). The inset showed a lattice fringe measured with a spacing of 0.226 nm, corresponding to the (111) atomic plane of fcc Pt (JCPDS no. 04-0802), thereby demonstrating the successful synthesis of g-C3N4-PtSn2+.

Figure 8. XPS spectra of (a) C 1s, (b) N 1s, (c) Sn 3d5/2 and (d) Pt 4f of g-C3N4-PtSn2+. XPS analysis was employed to determine the detailed chemical status of g-C3N4-PtSn2+. As shown in Figure 8, the C 1s spectrum can be fitted by Gaussian 17 ACS Paragon Plus Environment


curves with dominant components centered at 284.8, 285.8 and 288.3 eV, corresponding to C−C, C−N−C and C−(N)3, respectively.38, 39 The N 1s spectrum could be deconvoluted into three peaks, ascribable to C−N−C at 398.8 eV, N−(C)3 at 399.6 eV, and N−H at 401 eV.34 Besides, from Figure 8c, the Sn2+ ratio in g-C3N4-PtSn2+ is ca. 68.1 %, indicating that most Sn species in g-C3N4-PtSn2+ are divalent. The Pt 4f spectrum (Figure 8d) consists of two peaks with binding energies of 72.4 and 75.8 eV for Pt 4f7/2 and 4f5/2, respectively. Furthermore, it should be noted that the two peaks of Pt 4f present a 3.4 eV gap, which is slightly higher than pure Pt 4f by 0.2 eV.34 The negative shift of Pt 4f could be attributed to the enhanced interaction between Pt and Sn species.40

Figure 9. (a) UV−vis absorption spectra and (b) PL emission spectra of g-C3N4, g-C3N4-Pt, g-C3N4-PtSn4+ and g-C3N4-PtSn2+. (c) Visual-light H2 production rate over g-C3N4-Pt, g-C3N4-PtSn4+ and g-C3N4-PtSn2+. (d) Correlation between visual-light H2 18 ACS Paragon Plus Environment

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production rate and Sn/Pt atomic ratio of g-C3N4-PtSn2+.

To study the optical properties of g-C3N4, g-C3N4-Pt, g-C3N4-PtSn2+ and g-C3N4-PtSn4+, UV−vis diffuse reflectance spectroscopy was measured. As shown in Figure 9a, pure g-C3N4 exhibited an absorption band lower than about 450 nm, indicating low visible light utilization. Compared with g-C3N4, the light absorption of g-C3N4/Pt, g-C3N4-PtSn4+ and g-C3N4-PtSn2+ in visible-light region (450 to 500 nm) was enhanced significantly. Additionally, photoluminescence spectra (PL) of the samples were illustrated in Figure 9b. All the samples have a strong intrinsic emission band with a peak at 430 nm due to the recombination of photoexcited electron−hole pair.41 Whereas, compared with the other samples, g-C3N4-PtSn2+ exhibited a much weaker emission profile, suggesting the inhibited recombination of photogenerated charge carriers and enhanced photocatalytic H2 production. Just as confirmed by photocatalytic water splitting under visible light irradiation in Figure 9c, g-C3N4/Pt and g-C3N4-PtSn4+ showed low H2 production rate of ca. 253 and 902 µmol h−1 g−1, respectively. The slightly increased H2 production of g-C3N4-PtSn4+ is due to the electron-sink function of SnO2. Motivated by schottky barrier, the photogenerated electrons could migrate from g-C3N4 to SnO2 and Pt sequentially, accelerating photogenerated charge carriers separation process.16, 42 In contrast, when introducing Sn2+ in the ternary heterostructure, the H2 production rate of g-C3N4-PtSn2+ was remarkably improved to 2491 µmol h−1 g−1, which is ca. 2.7-fold higher than g-C3N4-PtSn4+, proving the excellent synergistic effect of Sn2+ in H2 production. Furthermore, it should be noted that the samples of g-C3N4-Pt, g-C3N4-PtSn4+ and 19 ACS Paragon Plus Environment


g-C3N4-PtSn2+ were synthesized without calcination treatment, thereby completely ruling out the potential influence of oxidized Pt on H2 production. The effect of Sn/Pt atomic ratio of g-C3N4-PtSn2+ on visible-light H2 production had been studied by changing the dosage of SnCl2 (Figure 9d). When adjusting Sn/Pt atomic ratio to 21.8 (Figure S7), g-C3N4-PtSn2+ showed the highest H2 production rate of 3185 µmol h−1 g− 1

. Additionally, g-C3N4-PtSn2+ presented an excellent recyclability and, after four

recycling runs, it showed no obvious decrease in photocatalytic activity (Figure S8). Moreover, compared with the unused sample (Figure S9a), the recycled g-C3N4-PtSn2+ features more scattered nanosheets without aggregation (Figure S9b). From the above observations, a possible mechanism was proposed to explain the excellent photocatalytic enhancement of Sn2+ for hydrogen evolution. Firstly, Pt active sites were introduced in the catalytic system via the in situ reduction of Sn2+. Thus, the Pt active sites tended to locate beside Sn species regularly, thereby resulting in high Sn/Pt interface (Figure S10). The doped Sn species could be utilized fully to serve as electron-sink function. In contrast, with regard to Sn4+, PtCl42- was randomly distributed in the catalytic system and the Pt2+ ion could be reduced to elemental Pt by photoexcited electrons during subsequent photocatalytic reaction.34, 35 The randomly distributed Pt active sites can hardly contact with Sn species adequately, thereby resulting in poor catalytic performance (Figure S11). Furthermore, as shown in Figure 8d, the reduced Pt active sites exhibited an enhanced interaction with Sn species, accelerating the migration of photoexcited electrons from Sn to Pt. Lastly, the excellent synergistic effect of Sn2+ in H2 production may also relate to its special 20 ACS Paragon Plus Environment

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reducibility. Take g-C3N4-PtSn2+ for example (Figure S12a), g-C3N4 could be photoexcited to produce conduction band (CB) electrons (e-) and valence band (VB) holes (h+) under visible-light irradiation. Then, motivated by schottky barrier, the photogenerated e- in g-C3N4 transferred to SnO because of the slight lower CB of g-C3N4 (-1.09 eV) than that of SnO (-0.91 eV), while the VB h+ in g-C3N4 reacted with sacrificial reagent (methanol) in aqueous solution.34, 42, 43 Furthermore, due to the electron-sink function of Pt, the accumulated photogenerated e- in the CB of SnO continued to migrate to Pt (work function φ is 5.65 eV).34 Similarly, in g-C3N4-PtSn4+, the photogenerated e- in g-C3N4 could migrate to SnO2 and Pt in sequence (Figure S12b). However, the Sn2+ species exhibit high reducing property and tend to release electrons to increase valence state. Thus, in g-C3N4-PtSn2+, the gathered photogenerated electrons in the CB of SnO may be more likely to migrate to Pt due to the reducibility of Sn2+ (Figure S12c), thereby further enhancing charge carriers separation. CONCLUSIONS In summary, a low platinum photocatalyst of DHS-N2-PtSn2+ with excellent solar photocatalytic H2 production properties has been fabricated successfully. In this work, DHS-N2-PtSn2+ showed highest H2 production rate of 28502 µmol h−1 g−1, which was nearly three-fold and six-fold higher than DHS-N2-PtSn4+ and DHS-N2-Pt, respectively. Besides, the H2 production of DHS-PtSn2+ can be influenced apparently by calcination atmosphere, Sn2+ content and Sn/Pt atomic ratio. Furthermore, the Sn2+ doping strategy could also be applied in other titanium dioxide materials (e.g. 21 ACS Paragon Plus Environment


home-made TiO2 NPs and commercial P25) and visual light-sensitive materials (e.g. g-C3N4), thereby demonstrating its extensive applications in H2 production. The significantly improved H2 production of DHS-N2-PtSn2+ and g-C3N4-PtSn2+ can be ascribed to the regularly distributed Pt active sites, enhanced Pt/Sn interaction and unique reducibility of Sn2+.

ASSOCIATED CONTENT Supporting Information Chemicals and materials, Detailed experimental procedures, Characterization, SEM images, EDX data, XPS data, Stability test results, TEM images, catalytic performance measurements and Schematic illustration. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Yuming Zhou. E-mail: [email protected]. Tel: +86 25 52090617. Fax: +86 25 52090617. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT The authors are grateful to the financial supports of the National Natural Science 22 ACS Paragon Plus Environment

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Foundation of China (Grant No. 21676056, 21376051 and 51673040), Graduate student scientific research innovation program of Jiangsu Province (KYCX17_0134), Scientific Research Foundation of Graduate School of Southeast University (YBJJ1732), Postgraduate Research ＆ Practice Innovation Program of Jiangsu Province (KYCX17_0134), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100), Fundamental Research Funds for the Central Universities (2242018k30008, 3207047402, 3207046409) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (1107047002). ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (JNHB-006), Qing Lan Project of Jiangsu Province (1107040167).

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For table of content use only

Synopsis The synthesized Sn2+-doped double-shelled Pt/TiO2 hollow nanocatalyst showed a high photocatalytic H2 production rate of 28502 µmol h−1 g−1.



Graphic abstract

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Sn2+-Doped Double-Shelled TiO2 Hollow Nanospheres with Minimal

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