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Aug 18, 2017 - ABSTRACT: An asymmetric zinc porphyrin (ZnPy) derivative bearing one benzoic acid and three 3-pyridines as meso-position substituents ...

Research Article pubs.acs.org/journal/ascecg

Asymmetric Zinc Porphyrin Derivative-Sensitized Graphitic Carbon Nitride for Efficient Visible-Light-Driven H2 Production Jinming Wang, Ya Zheng, Tianyou Peng,* Jing Zhang, and Renjie Li* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P.R. China S Supporting Information *

ABSTRACT: An asymmetric zinc porphyrin (ZnPy) derivative bearing one benzoic acid and three 3-pyridines as meso-position substituents (zinc-5-(4-carboxyphenyl)-10,15,20-tri(3-pridyl)porphyrin, ZnMT3PyP) was used to sensitize graphitic carbon nitride (g-C3N4) for visible-light-driven photocatalytic H2 production. It was found that ZnMT3PyP exhibits more excellent photosensitization and stability on g-C3N4 than its counterpart bearing one benzoic acid and three phenyls (zinc-5(4-carboxyphenyl)-10,15,20-triphenylporphrin, ZnMTPP) under visible light (λ > 420 nm) irradiation even though they have very similar physicochemical properties such as optical absorption capacities and energy band structures. Especially, ZnMT3PyP-Pt/g-C3N4 gives an apparent quantum yield (AQY) up to 25.1% at λ = 420 nm light illumination, greater than that (11.6%) of ZnMTPP-Pt/g-C3N4. The differences in photosensitization and stability between ZnMT3PyP and ZnMTPP are mainly due to the substitution of 3-pyridine for the phenyls in ZnMTPP, which leads to the electron transfers between ZnMT3PyP and g-C3N4 faster than that between ZnMTPP and g-C3N4. The present results provide a new insight applying porphyrin derivatives to the photocatalytic H2 production and open up a new path for further improving the conversion efficiency of solar energy to hydrogen energy through molecular designing. KEYWORDS: Zinc porphyrin derivative, Pyridine substituent, Graphitic carbon nitride, Photocatalytic hydrogen production



INTRODUCTION

harvesting is highly desired in the area of light-to-chemical energy conversion.1,3,6,8 Nowadays, Ru-complexes are the most widely used sensitizers in the semiconductor photosensitization systems for photocatalytic H2 production owning to their great achievement in the field of DSSCs.9 Nevertheless, those Rucomplex sensitizers are facing certain problems such as noble metals, high cost, and inadequate visible absorption (mostly with wavelengths 420 nm) is used to obtain visible light irradiation. Before irradiation, fully dispersing the suspension in an ultrasonic bath for 5 min and thorough removal of air in the photoreactor are necessary. The H2 production rate under visible light

Scheme 1. Molecular Structures of ZnMT3PyP and ZnMTPP

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DOI: 10.1021/acssuschemeng.7b00700 ACS Sustainable Chem. Eng. 2017, 5, 7549−7556

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ACS Sustainable Chemistry & Engineering irradiation was analyzed with a gas chromatograph (GC, SP6890, TCD detector, 5 Å molecular sieve columns, and Ar carrier). Similarly, the H2 production rate under monochromatic light irradiation obtained from band-pass filter (λ = 420 ± 10 nm, etc.) was measured under the same photoreaction condition, and the incident photon number was determined by using a calibrated Si photodiode (SRC-1000-TC-QZ-N, Oriel). The apparent quantum yield (AQY) is calculated according to eq 1

AQY(%) =



2 × number of evolved H2 molecules × 100 number of incident photons

result implies there is effective interaction between dye molecules and g-C3N4, which is consistent with the above XPS analysis result. This interaction would be a benefit to the electron transfer from those ZnPy dye molecules to g-C3N4. The above conjecture on the electron transfer can be confirmed by the photoluminescence (PL) spectra (Figure 2)

(1)

RESULTS AND DISCUSSION Pt/g-C3N4 exhibits an XRD pattern very similar to the pristine g-C3N4 (Figure S1), and there is no diffraction peak ascribable to Pt species. Nevertheless, the XPS spectra (Figure S2) confirm Pt species existing in the dye-sensitized Pt/g-C3N4. The survey spectra (Figure S2a) display obvious Pt 4f, Zn 2p, and N 1s peaks in addition to C 1s and O 1s, and the high resolution Pt 4f spectra (Figure S2b) present two symmetrical peaks with binding energies at ∼70.6 (Pt 4f7/2) and ∼74.0 (Pt 4f5/2) eV, suggesting metallic Pt existing in ZnMT3PyP- and ZnMTPP-sensitized Pt/g-C3N4.32 Furthermore, the N 1s spectrum of g-C3N4 can be deconvoluted into three peaks at 398.7, 400.0, and 401.3 eV, corresponding to sp2 hybridized aromatic N-bonded to C atoms (CN−C), tertiary N-bonded to C atoms ((C)3N), and N−H side groups,15,16 respectively. After loading with ZnPys, those N 1s peaks of g-C3N4 shift to lower binding energy, implying the interaction might exist between g-C3N4 and ZnPy dye molecules. Figure 1 depicts the diffuse reflectance absorption spectra (DRS) of g-C3N4, ZnMT3PyP-Pt/g-C3N4, and ZnMTPP-Pt/g-

Figure 2. Photoluminescence (PL) spectra (excited at 426 nm) of ZnMT3PyP and ZnMTPP in DMF solution without or with 0.67 g L−1 Pt/g-C3N4.

since it is an effective method to explore the electron transfer process in a dye-sensitized semiconductor system.33,34 Both ZnMT3PyP and ZnMTPP in DMF solution exhibit intense emission bands centered at ∼612 and ∼665 nm, indicating the strong charge recombination process of the excited dye molecules.31,35 A quenching effect can be seen by adding Pt/ g-C3N4 into those dye solutions, implying that the charge carriers can be efficiently separated due to the dye excited molecule electrons transferring to the g-C3N4.31,35 This phenomenon is due to the highly asymmetric structures of the present ZnPy dyes, which possess effective directionality of its electronic orbital in the excited state, and thus is favorable for the electron transfer from dye molecules to g-C3N4.30 The above results demonstrate that there are close interfacial connections between the dye molecules and a g-C3N4 surface, which then act as electron transfer paths to promote the charge carrier separation, thus causing an efficient photosensitization on g-C3N4. Furthermore, the PL quenching effect of a ZnMT3PyP solution by Pt/g-C3N4 is more obvious than that of a ZnMTPP solution, implying there is a more effective interfacial photogenerated charge transfer process between ZnMT3PyP and g-C3N4. This conjecture can be confirmed by the time-resolved fluorescence spectra (TRFS) shown in Figure 3, which is usually used to explore the photogenerated charge recombination rate.36,37 As can be seen, Pt/g-C3N4 can lead to the fluorescence quenching of ZnMT3PyP and ZnMTPP when observed at λex = 475 nm, which might be the result of charge transfer from the two ZnPy dyes to the Pt/g-C3N4 since g-C3N4 does not absorb 475 nm light.37 Once again, the fluorescence lifetime (τ) decrement of ZnMT3PyP with addition of Pt/gC3N4 is 41% (from 2.96 to 1.74 ns) more obvious than that (31%) of ZnMTPP when Pt/g-C3N4 is added. It also indicates the electron transfer between ZnMT3PyP and g-C3N4 is faster than that between ZnMTPP and g-C3N4. Electrochemical behaviors of ZnMT3PyP and ZnMTPP in DMF solution were studied by using cyclic voltammetry (CV),18 and the relative electrochemical data are summarized in Table 1, where the Eox value is estimated from the formula Eox (vs NHE) = Eox (vs Ag/AgNO3) − E1/2 (Fc+/Fc vs Ag/

Figure 1. (a) UV−vis absorption spectra of ZnMT3PyP and ZnMTPP DMF solution. (b) Diffuse reflectance absorption spectra (DRS) of gC3N4, ZnMT3PyP-Pt/g-C3N4, and ZnMTPP-Pt/g-C3N4.

C3N4, where UV−vis absorption spectra of ZnMT3PyP and ZnMTPP in DMF solution are also included. As can be seen from Figure 1a, ZnMT3PyP shows an absorption spectrum very similar to its counterpart (ZnMTPP) on the aspect of absorption band positions and shape. Both ZnMT3PyP and ZnMTPP have fairly strong single absorption peak (B-band) at ∼426 nm followed by less intensive Q-bands absorption at around 530−630 nm. Compared with the pristine g-C3N4 with an absorption edge at ∼450 nm (Figure 1b), both ZnMT3PyPand ZnMTPP-sensitized products show the optical absorption features of their respective solution with enhanced and broadened Q-bands absorptions that become more noticeable along with enhancing the dye-loaded amount (Figure S3). This 7551

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ACS Sustainable Chemistry & Engineering

Figure 3. Time-resolved fluorescence spectra (TRFS) of 30 μM dye solution without or with 2 mg Pt/g-C3N4. Excitation and detection wavelengths are 475 and 612 nm, respectively.

Figure 4. Normalized absorption and emission spectra of ZnMT3PyP and ZnMTPP solutions.

AgNO3) + E1/2 (Fc+/Fc vs NHE).30,31,38 The E* value is obtained according to the formula E* = Eox − E0−0, where the optical energy gap (E0−0) can be estimated by using the equation E0−0 = 1240/λint, and herein, λint is the intersection point of the normalized UV−vis absorption and fluorescence emission spectra (Figure 4). Among which, the calculated Eox and E* values of those ZnPy dyes correspond to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels,18,30 respectively. For comparison, Figure 5 listed the corresponding LUMO/ HOMO levels of those ZnPy dyes and the conduction band (CB)/valence band (VB) of g-C3N4. Although the calculated HOMO/LUMO levels are very similar to each other due to their similar molecular structures (Scheme 1), the LUMO levels (E*) of ZnMT3PyP and ZnMTPP are more negative than the g-C3N4’s CB (−1.12 V).15 It indicates that the photogenerated electron transfer from the excited ZnPy derivatives toward gC3N4 is feasible in thermodynamics, which is consistent with the above PL analysis results (Figure 2). Density function theory (DFT) calculation results, which were conducted on the ZnPy molecule at the B3LYP/ [LANL2DZ (for Zn)/6-31G* (for other elements)] level,39,40 also confirm the above suggestions on the charge transfer process from the ZnPy excited molecules to the g-C3N4’s CB. As can be seen Figure S4, those ZnPy dye’s HOMO−2 electrons mostly distributed around their Zn(II) ions, and those HOMO−1/HOMO electrons delocalized over the Py rings. It suggests that the metal-to-ligand charge transfer (MLCT) process could happen in those ZnPy molecules.30 Moreover, those LUMO+1/LUMO (π* orbitals) electrons delocalized across the Py rings and moved toward the carboxyl groups, while those LUMO+2 electrons mainly localized on the benzene rings and the carboxyl groups. These results indicate that the photoexcited electrons of these ZnPy dyes can effectively transfer from the Py ring to the carboxyl group, which is beneficial for the electron injection of the dye excited molecules to g-C3N4 and thus cause the visible-light-driven H2

Figure 5. Experimental values of MO energy levels of ZnMT3PyP and ZnMTPP, VB/CB levels of g-C3N4, and redox potentials of sacrificial reagents.

production activity in the present ZnPy derivative-sensitized systems as discussed below. Generally, the photocatalytic H2 production activity of a dyesensitized semiconductor can be influenced by the photocatalytic reaction condition such as dye/co-catalyst loading amount, sacrificial reagent, illumination light wavelength, and time.3,8,41 Since both ZnMT3PyP and ZnMTPP on Pt/g-C3N4 have dye-loaded amounts of ∼4.9 μmol g−1 in the present sensitization process as mentioned above, the Pt-loaded amount and photocatalyst dosage were optimized first by using 10 mM triethanolamine (TEOA) as the electron donor, and the relative results are depicted in Figure S5. As shown in Figure S5a, only few H2 evolved from the photoreaction system containing 10 mg of ZnMT3PyP-sensitized g-C3N4 (without Pt-loading) under visible light (λ > 420 nm) irradiation, and Pt loading can significantly enhance the H2 production activity. Among which, ZnMT3PyP-sensitized 1.5 wt % Pt/g-C3N4 reaches a maximum H2 production activity (342 μmol h−1), which then slightly decreases upon further enhancing the Ptloading amount. It can be because the co-catalyst Pt nanoparticles loaded on g-C3N4 are not only capable of forming Schottky barriers to promote the photogenerated

Table 1. Spectral and Electrochemical Data as well as Band Positions of ZnMT3PyP and ZnMTPP

a

Dye

λmax (nm)

log ε

Eox (V vs NHE)

E0−0a (eV)

E*b (V)

ZnMT3PyP ZnMTPP

426 426

5.57 5.77

0.90 0.91

2.05 2.05

−1.15 −1.14

Calculated with the equation E0−0 = 1240/λint. bCalculated with the equation E*= Eox - E0−0. 7552

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ACS Sustainable Chemistry & Engineering charge separation but can also debase the excess potential loss for H2 production, whereas excessive Pt loading (i.e., > 1.5 wt %) would lead to agglomeration and growth of Pt nanoparticles on g-C3N4 surface, which in turn weakens the co-catalyst functions and the dye-loaded amount, thus causing reduced photoactivity.41 The photocatalyst dosage effect on the photoactivity of ZnMT3PyP-sensitized 1.5 wt % Pt/g-C3N4 under λ > 420 nm light irradiation is exhibited in Figure S5b. The H2 production activity increases obviously with increasing the photocatalyst addition amount from 5 to 10 mg and then begins to decrease once the addition amount exceeds 15 mg. Therefore, the best photoactivity (373 μmol h−1) is obtained when the photocatalyst dosage is 15 mg. It can be rationalized when considering the availability of active sites on the photocatalyst surface, light absorption, and penetration depths of the suspension as mentioned in the previous report.42 In addition to TEOA, ascorbic acid (AA) and ethylenediaminetetraacetic acid disodium salt (EDTA), the commonly used electron donors, were also adopted into the present photoreaction system since a sacrificial reagent is also important influencing factor of the photoactivity. Although the addition of those electron donors can obviously enhance the photoactivity of ZnMT3PyP-Pt/g-C3N4 irradiated by λ > 420 nm light as compared to that without sacrificial reagent, their photoactivities for H2 production are markedly different as shown in Figure S6a. Although the overall charge transfers for H2 production are allowed thermodynamically since the ZnMT3PyP’s LUMO is more negative than the g-C3N4’s CB, and its HOMO is more positive than the redox potentials of those sacrificial reagents as shown in Figure 5,30,31,43−46 AA as sacrificial reagent exhibits better H2 production activity (437 μmol h−1) than TEOA (373 μmol h−1) or EDTA (149 μmol h−1). It suggests that the H2 production process of a dyesensitized system is determined by both the energy levels and the electron transfer kinetics, which are associated with the molecular interactions among dye molecules, g-C3N4 surface, and electron donors.30,43 Furthermore, it was reported that the adsorption/desorption capacities of dye molecules on a semiconductor surface are also highly sensitive to the pH value,30,43,44 and thus, a probable reason for the above different photoactivities might be related to the different acidity/basicity of those sacrificial reagent solutions.45 In this regard, it can be conjectured that acidic AA molecules might be possibly easier to combine with the dye molecules than the basic TEOA, thus causing more efficient dye regeneration. As for EDTA, it was reported that EDTA can hamper the adsorption of dye-bearing carboxylic acid onto a TiO2 surface due to its stronger coordination ability.46 Possibly, the adsorbed ZnMT3PyP molecules bearing carboxylic acid are desorbed from Pt/g-C3N4 during the photoreaction processes in the presence of EDTA, which would affect the adsorption/ desorption processes of dyes, causing the lowest photoactivity as shown in Figure S6a. The AA concentration effect on the photoactivity shown in Figure S6b indicates that the H2 production activity increases with enhancing AA concentration from 10 to 50 mM, and successive enhancing AA concentration leads to a slight decrease in the photoactivity. Therefore, the optimal photocatalytic reaction condition for the present ZnMT3PyP-Pt/g-C3N4 should be 15 mg of photocatalyst with 1.5 wt % Pt-loading dispersed in AA solution (50 mM). Figure 6 depicts the H 2 production activities over ZnMT3PyP- and ZnMTPP-sensitized Pt/g-C3N4 under the

Figure 6. Comparisons of H2 production rates (a) or AQY values (b) over ZnMT3PyP- and ZnMTPP-sensitized Pt/g-C3N4 under λ > 420 or λ = 420 nm light irradiations. Conditions: 15 mg of catalyst, 10 mL of 50 mM AA solution, 1.5 wt % Pt-loading, and 4.9 μmol g−1 dyeloading.

above optimal photocatalytic reaction condition. ZnMT3PyPPt/g-C3N4 exhibits better photoactivity (437 μmol g−1) than ZnMTPP-Pt/g-C3N4 (293 μmol g−1) under λ > 420 nm light irradiation, while pristine Pt/g-C3 N 4 only gives an H 2 production activity of 112 μmol h−1. Also, ZnMT3PyP-Pt/gC3N4 shows a higher AQY value (25.1%) than that (11.6%) of ZnMTPP-Pt/g-C3N4 at λ = 420 nm light illumination. These results mean the 3-pyridine substitutions for the phenyls in ZnMTPP lead to ZnMT3PyP showing more effective photosensitization for H2 production than ZnMTPP on Pt/g-C3N4. Since the above investigations indicate that ZnMT3PyP and ZnMTPP have very similar physicochemical properties such as optical absorption capacities and energy band structures as exhibited in Table 1 and Figure 5, it can be concluded that the different photosensitization between ZnMT3PyP and ZnMTPP on g-C3N4 are not closely associated with their analogous molecule orbitals and light absorption capability but may be due to the 3-pyridine substitutions for the phenyls in ZnMTPP. Generally, more effective and fast electron transfer should be the primary causes for the better photocatalytic activity of ZnMT3PyP-Pt/g-C3N4 than ZnMTPP-Pt/g-C3N4. This conjecture is supported by the transient photocurrent curves of the dye-sensitized g-C3N4 suspension irradiated by λ > 420 nm light (Figure 7). ZnMT3PyP-Pt/g-C3N4 shows higher photocurrent than ZnMTPP-Pt/g-C3N4, indicating the electron

Figure 7. Transient photocurrent curves of Pt/g-C3N4, ZnMT3PyP -Pt/g-C3N4, and ZnMTPP-Pt/g-C3N4 suspension systems containing 1.0 M Na2SO4 solution under visible light irradiation. 7553

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ACS Sustainable Chemistry & Engineering transfer between ZnMT3PyP and g-C3N4 is more efficient than that between ZnMTPP and g-C3N4, which is consistent with the above analysis results of photocatalytic activity and PL spectra. Simultaneously, the photogenerated holes of those ZnPy molecules can be consumed by the acidic sacrificial reagent (AA) in the present photoreaction system. Possibly, those N atoms of the three 3-pyridines in ZnMT3PyP are a benefit to the proton-coupling process with AA molecules and then to more efficient dye regeneration as compared to ZnMTPP without 3-pyridine. In other words, the possibility of forming H-bonds between the ZnMT3PyP and AA molecules leads to better photoactivity for H2 production as compared to ZnMTPP bearing three phenyls. This conjecture can be confirmed to some extent by the DFT calculation results as shown in Table S1, whereby the calculated electronic densities of N atoms in ZnMT3PyP are about −0.38e, while the corresponding electronic densities of C atoms in ZnMTPP are close to 0. It implies that the formation of H-bonds between AA molecules and N atoms of ZnMT3PyP is possible, which would cause a more efficient regeneration of ZnMT3PyP as compared with ZnMTPP. Moreover, the above TRFS results also indicate that the electron transfers between ZnMT3PyP and g-C3N4 are obviously faster than those between ZnMTPP and g-C3N4. Therefore, it can be concluded that the 3-pyridine substitutions for the three phenyls in ZnMTPP and the effective electron transfer from ZnMT3PyP to g-C3N4 should be the main reasons for the better photoactivity of ZnMT3PyPPt/g-C3N4 than ZnMTPP-Pt/g-C3N4. Furthermore, ZnMT3PyP-Pt/g-C3N4 also exhibits much better stability for H2 production than ZnMTPP-Pt/g-C3N4 as shown in Figure 8. The photoactivity of ZnMTPP-Pt/g-

Figure 9. Time course of H2 production over ZnMT3PyP-Pt/g-C3N4 suspension system. Conditions: 15 mg of photocatalyst, 10 mL of 50 mM AA solution, 1.5 wt % Pt, 4.9 μmol g−1 dye loading, λ > 420 nm light irradiation.

h−1 in the first run of 10 h photoreaction, which then slightly decreases to 379.5/354.2 μmol h−1 in the second and third runs of 10 h photoreaction. The percentages of the mean H2 generation of the second and third runs as compared with the first run are estimated to be 94.8% and 88.5%. It indicates that ZnMT3PyP-Pt/g-C 3 N 4 has good stability for H 2 production during the long-term photoreactions. Usually, dye desorption is unavoidable in a dye-sensitized semiconductor aqueous suspension under long-term stirring and irradiation. In addition, certain species resulting from the sacrificial agent decomposition may adsorb on a semiconductor surface during the irradiation process, which is also unfavorable for the H2 production reaction. Once again, the better stability of ZnMT3PyP-Pt/g-C3N4 than ZnMTPP-Pt/g-C3N4 might be ascribed to the existence of N atoms in 3-pyridines, which is beneficial for combining with AA molecules and thus easier to get electrons from AA to regeneration. Namely, the fairly good photostability of the present ZnMT3PyP-Pt/g-C3N4 can be attributed to the efficient dye regeneration stemming from the H-bond forming between the ZnMT3PyP and AA molecules. To confirm this issue, the spectral changes of ZnMT3PyP and ZnMTPP solutions before and after 5 h light irradiation are studied, in which the photocatalyst separated from the aqueous suspension before or after irradiation was redispersed in 0.1 M KOH DMF/water (1:1) solution with ultrasonication, and then, the supernatant is collected through centrifugation. As shown in Figure S7a, the visible absorption intensity of the ZnMT3PyP solution desorbed from ZnMT3PyP-Pt/gC3N4 after irradiation is very similar to that desorbed from ZnMT3PyP-Pt/g-C3N4 without irradiation, indicating very limited ZnMT3PyP molecules desorbed from Pt/g-C3N4. On the contrary, the visible absorption intensity of the ZnMTPP solution desorbed from ZnMTPP-Pt/g-C3N4 after irradiation is much less than that desorbed from ZnMTPP-Pt/g-C3N4 without irradiation (Figure S7b), implying some ZnMTPP molecules were desorbed from Pt/g-C3N4 during the photoreaction process. This result indicates that the loss of ZnMT3PyP is lower than that ZnMTPP, which might be responsible for the better average photoactivity and stability than ZnMTPP. Anyway, the dye solutions desorbed from ZnMT3PyP-Pt/g-C3N4 (or ZnMTPP-Pt/g-C3N4) before and after irradiations exhibit identical UV−vis absorption spectra (Figure S7), implying the intact skeleton structures of those ZnPy derivatives during the photoreaction process. Furthermore, ZnMT3PyP-Pt/g-C3N4 before and after irradiation

Figure 8. Photocatalytic H2 production rate over ZnMT3PyP-Pt/gC3N4, ZnMTPP-Pt/g-C3N4, and Pt/g-C3N4 suspension systems during the initial 5 h light irradiation. Conditions: 15 mg of photocatalyst, 10 mL of 50 mM AA solution, 1.5 wt % Pt loading, λ > 420 nm light irradiation.

C3N4 exhibits a continuously decreasing trend during the initial 5 h light irradiation, and the photoactivity retention ratio at the fifth hour irradiation is as low as ∼41% as compared to that at the first hour irradiation, whereas ZnMT3PyP-Pt/g-C3N4 exhibits much better photoactivity and stability than that of ZnMTPP-Pt/g-C3N4 with a photoactivity retention ratio of ∼92% at the fifth hour irradiation. Moreover, ZnMT3PyP-Pt/gC3N4 exhibits excellent stability for H2 production even during the 30 h irradiation shown in Figure 9, whereby fresh AA solution was replaced periodically in each run. ZnMT3PyP-Pt/ g-C3N4 can produce a mean H2 generation rate of 400.2 μmol 7554

DOI: 10.1021/acssuschemeng.7b00700 ACS Sustainable Chem. Eng. 2017, 5, 7549−7556

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ACS Sustainable Chemistry & Engineering shows very similar XRD patterns (Figure S1), also indicating the relative good stability in composition and structure of the dye-sensitized Pt/g-C3N4. The above results indicate that ZnMT3PyP molecules would have much better regeneration capacity than ZnMTPP on Pt/gC3N4, which can be due to the additional N atoms in ZnMT3PyP that can efficiently combined with AA by forming H-bonds as mentioned above. The present results are fundamentally important for exploring the roles that molecular structure plays in the photocatalytic performance of dyesensitized semiconductors and demonstrate a significant advance to more efficient H2 production through optimizing the molecular structure and light absorption property of ZnPy derivatives.

Author Contributions

CONCLUSIONS In summary, an asymmetric zinc porphyrin (ZnPy) derivative bearing one benzoic acid and three 3-pyridines as meso-position substituents (zinc-5-(4-carboxyphenyl)-10,15,20-tri (3-pridyl)porphyrin, ZnMT3PyP) was used to sensitize g-C3N4 for visible-light-driven H2 production. For comparison, its counterpart bearing one benzoic acid and three phenyls (zinc-5-(4carboxyphenyl)-10,15,20-triphenylporphrin, ZnMTPP) was also used as dye of g-C3N4. The related experimental results suggest that the 3-pyridine substitutions for the three phenyls in ZnMTPP cause ZnMT3PyP to show significantly improved photocatalytic activity and stability on Pt/g-C3N4 under visible light irradiation even though they have similar physicochemical properties such as optical absorption ability and energy band structures. For instance, ZnMT3PyP-Pt/g-C3N4 exhibits much better photoactivity (437 μmol g−1) than ZnMTPP-Pt/g-C3N4 (293 μmol g−1) under λ > 420 nm light irradiation. These differences in photosensitization and stability between ZnMT3PyP and ZnMTPP on g-C3N4 are mainly ascribed to the 3-pyridine substitutions for the three phenyls in ZnMTPP, which leads to the electron transfers between ZnMT3PyP and g-C3N4 being faster than those between ZnMTPP and g-C3N4. The present results provide a new insight for applying porphyrin derivatives to photocatalytic H2 production and open up a new path for further improving the conversion efficiency of solar energy to hydrogen energy through molecular designing. Further work will focus on the effects of N atom positions in pyridines of this kind of ZnPy derivative for photoactivity and stability for H2 production.

REFERENCES

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21573166, 21271146, 20973128, 20871096), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (BK20151247), China.







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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00700. XRD patterns and XPS spectra, effects of photoreaction conditions, and optical properties of the photocatalyst after irradiation. (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] Fax: +86-27-68752237 (T. Peng). *E-mail: [email protected] Fax: +86-27-68752237 (R. Li). ORCID

Tianyou Peng: 0000-0002-2527-7634 Jing Zhang: 0000-0003-4754-1659 7555

DOI: 10.1021/acssuschemeng.7b00700 ACS Sustainable Chem. Eng. 2017, 5, 7549−7556

Research Article

ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.7b00700 ACS Sustainable Chem. Eng. 2017, 5, 7549−7556

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