CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201400065
Photoreduction of Iron(III) to Iron(0) Nanoparticles for Simultaneous Hydrogen Evolution in Aqueous Solution Chuan-Jun Wang,[a] Shuang Cao,[a] Biao Qin,[a] Chen Zhang,[b] Ting-Ting Li,[a] and WenFu Fu*[a, b] Crystalline Fe nanoparticles were obtained with fluorescein (Fl) as the photosensitizer in triethylamine (TEA) or triethanolamine (TEOA) aqueous solution with FeCl3 as the Fe precursor under bright visible-light light-emitting diode (LED) irradiation. Photoinduced electron transfer from excited state Fl* and Fl to Fe3+ produced the Fe nanoparticles, which served as the active catalyst for in situ photocatalytic hydrogen production with Fl and TEA or TEOA as the photosensitizer and electron donors, respectively, in the same system. Robust hydrogen production activities were observed under the Fe nanoparticle photore-
duction conditions in basic solution, and tens of milliliters of hydrogen were obtained over prolonged LED irradiation. If inorganic support materials such as NH2-MCM-41 or reduced graphene oxide were introduced, dispersed nanoparticles with different sizes and shapes were deposited on the supports, which led to variously enhanced hydrogen production activities. The relationships between the morphologies of the Fe/H2N-MCM41 or Fe/graphene composites generated in situ and the hydrogen production activities were investigated systematically.
Introduction Light-driven water splitting is regarded as a promising solarenergy utilization process for sustainable global energy supply.[1–6] Typically, hydrogen photogeneration systems consist of a multicomponent system with a light-harvesting material, a sacrificial electron donor, and a metal-based catalyst.[7–11] Recently, much effort has been directed toward the development of transition-metal complexes that function either as photosensitizers to absorb visible light and generate long-lived excited states efficiently or as catalysts with low overpotentials for hydrogen production from water.[11–18] Recent research has focused on the photogeneration of molecular hydrogen by the development of efficient and inexpensive systems that contain organic molecular sensitizers and earth-abundant metal-based complex catalysts, and inspiring and important results have been achieved.[19–22] However, most of the metal complexes operate in a mixed organic/water solution and their decomposition under irradiation is a drawback that hampers their recycla[a] Dr. C.-J. Wang,+ Dr. S. Cao,+ Dr. B. Qin,+ Dr. T.-T. Li, Prof. W.-F. Fu Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190 (P.R. China) Fax: (+ 86) 10-6255-4670 E-mail: [email protected] [b] C. Zhang, Prof. W.-F. Fu College of Chemistry and Engineering Yunnan Normal University Kunming 650092 (P.R. China) [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201400065.
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bility and limits their applications greatly. The value of these well-designed and presynthesized compounds would fall if they do not last under light irradiation or operate in pure water as real-life solar energy with its full spectrum of light generates considerable heat, which would further accelerate the decomposition of complexes. The development of catalysts formed in situ for hydrogen evolution is of significance as active catalysts self-assembled under photocatalytic conditions would not be subject to light-induced decomposition as much as presynthesized ones. Most recently, there have been reports of hydrogen evolution over catalysts formed in situ by the addition of metal salts and coordinating ligands in a photocatalytic system that contains a photosensitizer and an electron donor. Photocatalytic water reduction with a nickel thiolate catalyst self-assembled by nickel(II) acetate and 2-mercaptoethanol with erythrosin B and triethanolamine (TEOA) as a sensitizer and an electron donor, respectively, has been reported by Xu et al., and a quantum efficiency for hydrogen evolution of up to 24.5 % was obtained upon irradiation with 460 nm light.[23] Eisenberg et al. have also reported a new approach that uses CdSe quantum dots (QDs) capped with dihydrolipoic acid coupled to a NiII catalytic center self-assembled in water to form a heterogeneous photocatalytic system. This system exhibited undiminished stability upon irradiation at 520 nm for 110 h and had a turnover number (TON) of up to 600 000.[24] The use of NiII or CoII coordinated to a TEOA electron donor to form catalysts with graphene-C3N4 for photocatalytic hydrogen evolution has been reported by Sun and co-workers.[25] From another point of view, the exploitation of stable earth-abundant-metal nanoparticles for highly efficient hydrogen production in a multicomponent system with organic photosensitizers has been surprisingly less explored compared to that with Pt ChemSusChem 0000, 00, 1 – 11
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CHEMSUSCHEM FULL PAPERS nanoparticles, and only a few works report the presynthesis of Fe and Ni nanoparticles in a photocatalytic hydrogen generation system.[26–28] Usually, nanoparticles prepared by chemical reduction methods[29, 30] are afflicted by a sensitivity to oxygen, which would be detrimental to catalytic applications.[31] The practice of the introduction of a surfactant protective shell around the metal surfaces is not effective for catalytic reactions that take place on metal surfaces.[32] Therefore, the idea to use earth-abundant-metal nanoparticles generated in situ from photocatalytic systems that upon generation act instantly as catalysts for light-driven hydrogen evolution is intriguing. The selection of electron donors and photosensitizers that would generate excited species with sufficient driving force for the reduction of earth-abundant metal salts to metal nanoparticle catalysts for in situ photocatalytic hydrogen evolution is important. In 2013, Bernhard et al. reported the light-driven reduction of ZnII to Zn using an IrIII photosensitizer and triethylamine (TEA) as an electron donor in acetonitrile solution.[33] Recently, we have communicated the visible-light-driven reduction of NiII to Ni0 nanoparticles in a system that contains fluorescein (Fl), TEA or TEOA, and nickel chloride, and the subsequent photogeneration of hydrogen was achieved by the same system.[34] It was observed that the system was stable to produce hydrogen for days in the presence of the generated Ni nanoparticles with tens of milliliters of hydrogen produced, even under the relatively intense light-emitting diode (LED) light source exploited for the Ni2+ photoreduction process. Herein, we report a detailed investigation of the controlled photoreduction of FeIII to a highly active Fe0 nanoparticle catalyst in an aqueous solution that contains Fl, TEOA or TEA, and an FeIII salt under bright LED visible-light irradiation. Consequently, we achieved sustainable photogenerated hydrogen in this system with good catalytic stability and recyclability in a basic solution (Scheme 1). The photogenerated hydrogen efficiency improved upon the addition of NH2-MCM-41 or reduced graphene oxide (RGO) to the abovementioned systems compared to the system without support materials. The effects of the sacrificial electron donor on the photoreduction process and photocatalytic hydrogen evolution were studied systematically and are presented.
Results and Discussion The photocatalytic hydrogen production systems were assembled from FeCl3, Fl, and TEOA (pH 10.3) or TEA (pH 11.0) in an aqueous solution. The photocatalytic experiments were performed with ten vials of the samples irradiated under a special-
Scheme 1. Highly efficient photoreduction and hydrogen photogeneration system. (LED light source, l > 420 nm, 30 3 W)
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www.chemsuschem.org ly designed l > 420 nm LED (30 3 W). For studies that include support materials, NH2-MCM-41 or RGO was introduced into the systems to result in well-dispersed Fe0 nanoparticles on the surface or inside the pores of the support materials (NH2MCM-41 in TEOA) under light irradiation. Control experiments showed that no nanoparticles are generated in the absence of any of the following components: light, Fl, and TEOA or TEA, and no significant H2 production was observed (Figure S1). The photocatalytic hydrogen production activities in different electron donor (TEA and TEOA) solutions were investigated systematically and compared. If TEA was employed as the electron donor, the addition of FeCl3 to the TEA aqueous solution gives iron hydroxide precipitates readily. The amount of photogenerated hydrogen was measured as a function of pH over 8 h, and the results are shown in Figures S2–S3. Hydrogen evolution was almost linear over time, and the highest efficiency was achieved at pH 11. The effect of TEA concentration on the photocatalytic reaction was investigated to optimize the hydrogen production efficiency. A fast rate of hydrogen production was observed at higher TEA concentrations with fixed amounts of Fl and FeCl3, and the photocatalytic system exhibited a high hydrogen evolution efficiency (1.70 mL at 298 K) during the initial 10 h of operation (Figure 1 A). However, we did not observe an induction period of hydrogen generation in the present system because of the rapid formation of black precipitate on a short timescale. At low TEA concentrations, the system showed moderate photocatalytic activity but a prolonged lifetime. The system with a TEA concentration of 0.5 % produced 0.69 mL H2 in the first 10 h, which was only one third of the value achieved at 5 % TEA concentration (Figure 1 B). However, hydrogen production continued after the further addition of TEA at 18 and 36 h. This indicates that the photocatalytic process related to the oxidative quenching of the excited state of the Fl becomes more competitive at a lower TEA concentration, whereas reductive quenching dominates the catalytic reaction at a higher TEA concentration (5 %) to produce Fl and reduce the stability of the photosensitizer.[19, 35] The system ceased activity at around 25 h, which can be resumed by the addition of more Fl (Figure 1 D). Notably, the higher hydrogen production rates observed after the addition of more TEA at 18 h than at the beginning of the reaction shows the gradual accumulation of the Fe nanoparticle catalyst (Figure 1 B). We postulate that at the start, TEA was consumed largely by the photochemical reaction to reduce Fe(OH)3 to Fe nanoparticles (Fe-NPs). Therefore, only a portion of the TEA functioned as an electron donor for hydrogen production in the initial period of nanoparticle generation. To further show that the particles are the active catalyst, the effects of the FeCl3 concentration on the photocatalytic activity of the system were studied. The dependence of the hydrogen production efficiency on the FeCl3 concentration is shown in Figure 1 C. As expected, both the initial rate and the amount of hydrogen produced increased with FeCl3 concentration. At lower Fe3+ concentrations of 1.0 10 7 and 1.0 10 6 m, exceedingly high TONs of up to 31 000 and 6845, respectively, were achieved after 39 h irradiation in a single run. Although larger amounts of hydrogen ChemSusChem 0000, 00, 1 – 11
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Figure 1. Hydrogen production during the visible-light irradiation of an aqueous solution (30 mL) of 0.005 mm FeCl3 that contained Fl (2.0 mm) at pH 11 and A) TEA (5 % v/v) or B) TEA (0.1 and 0.5 % v/v). After irradiation for 18 and 36 h TEA (0.1 and 0.5 % v/v) was injected into the systems, respectively. C) Relationship between the amount of hydrogen production and the irradiation time for the photocatalytic system that contained TEA (5 % v/v) and Fl (2.0 mm) at various concentrations of FeCl3. D) Hydrogen production as a function of irradiation time for the system that contained 0.2 and 0.05 mm FeCl3 and TEA (5 % v/v). LED light: l > 420 nm, 30 3 W.
Figure 2. Relationship between the amount of hydrogen production and irradiation time for the photocatalytic system that contained Fl (2.0 mm) in 30 mL aqueous solution and A) FeCl3 (0.1 mm) and TEA (5 % v/v) at pH 11 or TEOA (1.25 %) at pH 10.3; B) TEOA (1.25 %) and various concentrations of FeCl3 ; C) TEOA (1.25 %) and various concentration of FeCl3 with aliquots of TEOA (5 and 1.25 %) and Fl (2.0 mm) added at 36 and 38 h, respectively; D) FeCl3 (1.0 and 0.5 mm) and TEOA (1.25 %) at pH 10.3 with aliquots of TEOA (1.25 %) and Fl (2.0 mm) added at 36 and 38 h, respectively. LED light: l > 420 nm, 30 3 W.
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were produced at higher Fe3+ concentrations, the calculated TONs did not exhibit a linear proportional relationship.[19–22, 35] It was also observed that at higher concentrations of added Fe3+, the magnetic stirrer was covered with a layer of black particles, which resulted from the aggregation of the generated Fe-NPs. The photocatalytic hydrogen evolution was resumed upon the injection of an additional equivalent of Fl when the hydrogen production reached a plateau at 30 h (Figure 1 D). In addition, changes in the hydrogen production efficiency with different concentrations of Fl, as well as the use of other photosensitizer dyes, were also studied (Figures S4 and S5). Furthermore, comparison studies with Pt-catalyzed hydrogen evolution under optimized pH conditions were also investigated (Figures S6 and S7). If TEOA was used as the sacrificial electron donor, a slow initial rate of hydrogen production in the first few hours was observed with fixed amounts of Fl and FeCl3. The photogenerating hydrogen systems exhibit an induction period (Figure 2 and Figure S8), which differs from the systems that have a linear initial rate if TEA is used as the electron donor. The hydrogen production rate and amount are both enhanced with increasing amounts of added Fe3+ (Figure 2 B). Over 26 mL H2 was obtained at an Fe3+ concentration of 5.0 10 4 m over an irradiation time of 36 h. At an Fe3+ concentration of 10 7 m, a TON of approximately 14 000 was achieved in 1.25 % TEOA, although only 0.49 mL H2 was produced. When the hydrogen production reached a plateau at 36 h, the addition of further aliquots of TEOA (1.25 %) did not resume the photocatalytic activity (Figure 2 C). However, additional aliquots of Fl solution did resume ChemSusChem 0000, 00, 1 – 11
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the photocatalytic hydrogen evolution. A total amount of with an Fe3+ concentration of 5.0 10 4 m, the system with 32 mL H2 was obtained after 62 h of irradiation, at which point 1.0 mm Fe3+ had to undergo a longer period of photoreducthe system reached another plateau. tion to reduce all Fe(OH)3 to Fe, and, therefore, only some Fl Notably, unlike the TEA system, if Fe3+ is added to TEOA, no and TEOA were effective for the hydrogen production reactions. This resulted in a decreased hydrogen production effiformation of Fe(OH)3 flocculent precipitate was observed by ciency at a higher catalyst concentration of 1.0 mm at the bethe naked eye. Therefore, Fe3+ must have coordinated with ginning of the reaction (Figure 3 A). TEOA to form a complex as reported previously by Sun et al., However, a comparison of the hydrogen evolution profile which is an inefficient catalyst.[25] In the present system under between 25 and 35 h of irradiation showed that the system highly basic conditions with bright LED irradiation, there is an with 1.0 mm Fe3+ has a higher hydrogen production efficiency induction period with no significant amount of hydrogen produced in the first 2 h, and efficient hydrogen generation can than that of 0.5 mm Fe3+ (Figure 3 B). The picture taken after only be achieved if black nanoparticles appeared. It is reasona30 h of irradiation revealed that the flocculent Fe(OH)3 has ble to assume then that the presence of the induction period transformed into Fe-NPs in the 1.0 mm Fe3+ system (Figure S9). in the TEOA system is in some degree because of the coordiTherefore, the bifunctional system transformed into a mononation effect of Fe3+ with TEOA. Upon intense light irradiation, functional hydrogen production system, in which the higher Fe3+ undergoes dissociation gradually from an Fe3+ complex that contains a TEOA ligand and association with OH in the highly basic solution to form an Fe(OH)3 precatalyst, which then was photoreduced gradually to the Fe-NP catalyst to catalyze hydrogen evolution. This supposition is also supported by the longer induction period that was observed under a higher concentration of TEOA (5 %) (Figure 2 A and Figure S8). In the Figure 3. Hydrogen production profile over irradiation time with Fe3+ (5.0 10 4 or 1.0 10 3 m), Fl (2.0 mm), and TEA system, however, Fe(OH)3 is TEOA (1.25 %) in 30 mL aqueous solution A) over the first 10 h and B) from 25–35 h. For B), the tubes were dealready present to undergo the gassed after 25 h of irradiation. photoreduction process, therefore, a linear relationship with no induction period is observed in the first few hours. amount of Fe catalyst resulted in higher hydrogen production Additionally, if the concentration of added Fe3+ was inrates (Figure 3 B). As discussed above, the identification and characterization creased to 1.0 mm, the total hydrogen production efficiency of the black particles generated during irradiation is important was lower during the 35 h of irradiation than that of the to clarify the observed catalytic process. It has been reported system with an Fe3+ concentration of 0.5 mm (Figure 2 D). This previously that the Fl* and Fl generated in the photocatalytic means that the flocculent precipitate blocks the surface active process have reduction potentials of 1.7 and 1.3 V (vs. satusites of the nanoparticles. In the system with 1.0 mm Fe3+, only rated calomel electrode; SCE), respectively.[35] In the presence after a longer irradiation time was the Fe(OH)3 reduced entirely 0 to Fe nanoparticles, and the addition of Fl at 40 h resulted in of an excess of electron donors, the Fe(OH)3 precatalyst was higher hydrogen production rates than the system with likely to accept electrons from either Fl* or Fl to be reduced 0.5 mm Fe3+ as a result of the higher amount of Fe-NP catalyst. to metallic Fe-NPs.[33] The Fe-NPs were characterized by highA photographic comparison of the sediments formed showed resolution transmission electron microscopy (HRTEM), XRD, that after 10 h of irradiation the systems with an Fe3+ concenenergy-dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), and chemical methods. tration of 5.0 10 4 and 2.0 10 4 m precipitated fine black parThe black particles were collected in a sealed quartz tube ticles, which was characterized as Fe0 by XRD. However, floccuand degassed with N2 until no background oxygen was detectlent precipitate mingled with black Fe particles was observed in the system with an Fe3+ concentration of 1.0 mm after 10 h ed by GC. Degassed dilute HCl aqueous solution was injected into the tube, and the instant evolution of bubbles was obof irradiation (Figure S9). The flocculent precipitate was the served, which was proved to be H2 by GC. There was no clear Fe(OH)3 precatalyst that had not been reduced entirely to Fe in this time, which overwhelmed the active Fe-NP catalyst and color change if KSCN was added to the resulting solution, hampered the hydrogen production efficiency. Therefore, at which indicates the absence of Fe3+. The solution changed 3+ fixed Fl and TEOA concentrations, the system with 1.0 mm Fe from achromatic color to red upon standing under an air atmosphere overnight (Figure S10). was unable to reduce all the Fe(OH)3 precatalyst to Fe-NPs within the first 10 h of irradiation. 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TEM images show that the nanoparticles generated in situ in TEA solution consist of welldefined prismlike and cubic structures and that most of the particles had a size range of around 40–50 nm (Figure 4 A). In comparison, nanoparticles generated from TEOA solution had sizes of 10–20 nm (Figure 4 C). The higher surface area of the smaller nanoparticles generated is believed to be responsible for the overall higher hydrogen production activity in the TEOA system. The corresponding selected-area electron diffraction (SAED) pattern shows clear diffraction rings of a single crystal (inset in Figure 4 A and C).[36] The crystals were well dispersed and had a measured spacing of 0.205 nm, which is consistent with the (11 0) crystal lattice plane of a-Fe (Figure 4 B and D).[37] Importantly, HRTEM-EDX analysis on various regions also confirms that within detection limits Fe was the only element in these nanoparticles (Figure 4 E and Figure S11). The XRD pattern demonstrates an apparent sharp peak at 2 q = 44.98 (Figure 4 F), which is a clear indication of the presence of zero-valent Fe (aFe).[38] In addition, XPS measurements confirmed the formation of zero-valent Fe in the investigated system (Figure S12).[39] FTIR spectroscopy shows that the Fe-NPs formed in situ had relatively clean surfaces (Figure S13). From these data, we Figure 4. A) TEM (inset: SAED pattern) and B) HRTEM images of the nanoparticles photogenerated in situ in TEA aqueous solution under visible-light irradiation. C) TEM (inset: SAED pattern) and D) HRTEM images of the nanoconclude that Fe-NPs are formed particles photogenerated in situ in TEOA aqueous solution under visible-light irradiation. E) HRTEM-EDX and in the photocatalytic system F) XRD of the in situ generated nanoparticles. LED light: l > 420 nm, 30 3 W. under visible-light irradiation and function as an active metal at 42 h (Figure 5 A). In contrast with the systems that used catalyst for efficient hydrogen production. FeCl3 as the precatalyst in TEOA aqueous solution, if TEOA is We also performed photocatalytic measurements of hydrogen production by reusing the Fe-NP catalyst obtained after exploited as the electron donor, no induction period of generone photocatalytic run. The material was collected and purified ating hydrogen was observed (Figure 5 B). The system drives under an inert atmosphere, and highly efficient hydrogen prohydrogen evolution steadily for more than 40 h without the duction could still be achieved. If TEA is used as the sacrificial addition of any further components. The different photocataelectron donor, hydrogen production reached a plateau lytic stabilities of the systems with TEA and TEOA as sacrificial around 10 h, and the addition of more Fl at 15 h resumed the agents may be attributed mainly to the different mechanisms photocatalytic activity. The system lasted for more than 55 h if in the photocatalytic process. As discussed above (Figure 1 B another aliquot of both Fl (2.0 mm) and TEA (5 %) were added and D), a reductive quenching mechanism is the major process 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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that Fe-NPs acted as the catalyst during the photocatalytic reactions and that the chemical reaction between Fe-NPs and H2O contributed a practically negligible amount of hydrogen. Firstly, experiments proved that Fe(OH)2 and Fe(OH)3 precipitates can both dissolve in TEOA to form a clear Fe(TEOA)x solution (Figure S17 A–C). Secondly, two tubes that contained 30 mg FeFigure 5. Photocatalytic hydrogen production with Fe-NPs in 30 mL TEA or TEOA aqueous solution. LED light: NPs and 5 % TEOA aqueous solul > 420 nm, 30 3 W. A) Fe-NPs (2.5 mg), Fl (2.0 mm), TEA (5 %) and B) Fe-NPs (5.0 mg), Fl (2.0 mm), TEOA (1.25 %). tion were degassed and sealed. One of them was subjected to vigorous stirring and the other to intense sonication. (Figif 5 % TEA is used as the sacrificial agent. This produces unstaure S17 D) If the Fe-NPs react with H2O to produce H2 and ble Fl to cause the fast decomposition of Fl,[35] which results in the loss of photocatalytic activity of the system around Fe(OH)2, the Fe(OH)2 on the surface of the Fe-NPs could quickly 10 h; the activity can be resumed by further addition of Fl (Figdissolve in the excess TEOA solution especially under intense ure 5 A). In contrast, TEOA cannot quench the singlet excited sonication. This would expose the active surface of the Fe core state of Fl.[40] Our experiment showed that the generated Feand cause Fe to react with water to form Fe(OH)2 and to disNPs can quench the fluorescence of Fl effectively (Figure S14). solve in TEOA under sonication. It is reasonable to assume Therefore, we propose that the initial photocatalytic hydrogen then that if this reaction is fast enough, the amount of Fe-NPs production in TEOA goes through an oxidative quenching prowould diminish as it transforms gradually to Fe(OH)2 and dissolves completely in TEOA with the production of an equimocess, which produces less unstable Fl to result in prolonged system activity. Furthermore, the nanoparticles after 47 h of lar amount of H2. However, our results showed that less than photocatalytic reaction were isolated and characterized by 0.8 mL H2 was produced under intense sonication in 15 h and XRD, and still only the (11 0) peak of a-Fe was observed (Figno apparent diminishment of the black Fe-NPs was observed ure S15). This result confirms the integrity of the Fe-NPs during by the naked eye (Figure S17 E–F). In addition, the tube under the photocatalytic reaction and provides further evidence that vigorous stirring produced far less hydrogen (0.05 mL). This exFe-NPs are the real catalyst in our system. periment provides evidence that the reaction between Fe-NPs It is remarkable that the photocatalytic systems have such and H2O in basic TEOA solution is very slow, which supports prolonged lifetimes of more than 30 h for hydrogen evolution the theory that the efficient hydrogen production under visiunder relatively intense light irradiation conditions. There have ble-light irradiation was achieved by Fe-NPs as the catalyst. been reports that electromagnetic interactions between metal To understand the effects of nanoparticle size on the hydronanoparticles and fluorescent molecules can improve the phogen generation efficiency, hybrid hydrogen-generating catatostability of molecules greatly.[41] Therefore, UV/Vis spectrolysts were also obtained by the addition of NH2-MCM-41 or photometry was performed to monitor the changes in the abRGO to the abovementioned system. NH2-MCM-41 and RGO sorption of Fl during photolysis (Figure 6). In the absence of were prepared according to literature methods.[42, 43] It is well known that siliceous mesoporous materials composed of Si added FeCl3, Fl decomposes rapidly in TEA aqueous solution, and O atoms have accessible mesoporous frameworks and adwhich is because of the instability of the Fl formed through the reductive quenching process[35] (Figure 6 A and Figure S16). However, the photolysis system with FeCl3 showed only clear Fl decomposition in the first hour during which time FeCl3 was gradually photoreduced to Fe-NPs. After that, Fl exhibited a significantly enhanced photochemical stability in the presence of Fe-NPs with no dramatic decrease in absorption after 15 h of irradiation (Figure 6 B). Figure 6. UV/Vis absorption spectra measured at different irradiation times of Fl (6.7 10 5 m) and TEA (5 % v/v) in In addition, a delicate experi- H2O at pH 11 upon irradiation in the A) absence and B) presence of Fe3+ (6.7 10 6 m). LED light, l > 420 nm, ment was designed to prove 30 3 W. Aliquots of 30 mL of the solution at different times were diluted for UV/Vis measurements. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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justable porosities, which are beneficial for the incorporation and stabilization of nanoparticles. Previous studies have also demonstrated that RGO with its excellent electron-accepting and -transporting capability, could serve as a catalyst scaffold for in situ generated nanoparticles to be finely dispersed on its surface and as an electron relay mediator between the sensitizer and catalyst to increase the hydrogen production activity efficiently.[13] The addition of support materials led to variously enhanced photocatalytic hydrogen evolution activities in both TEA and TEOA systems (Figure 7). After the photocatalytic reaction, the composite materials were collected, and XRD measurements still showed only the (11 0) peak of a-Fe (Figure S18). For the TEOA system, the incorporation of 3.0 mg RGO or 4.0 mg NH2-MCM-41 into the photocatalytic hydrogen evolution system led to a clear increase in the hydrogen production efficiency (Figure 7 A). The hydrogen produced in the first 2 h for systems with RGO and NH2-MCM-41 was 3.4- and 2.5-fold more than that of the systems without support materials, respectively. The system that incorporated RGO showed a better efficiency than that with NH2-MCM-41 as the support in the first 4 h of irradiation. However, after 4 h, the system with NH2MCM-41 excelled in efficiency over that with the RGO support. After 12 h of irradiation, 8.4 and 13 mL H2 were produced for the systems with RGO and NH2-MCM-41, respectively, which is 2.2- and 3.3-fold more than that of the system without a support material (3.9 mL), respectively. Also, an induction period
was observed for photocatalytic hydrogen production if either of the supports was used, which is in accordance with the hydrogen production profiles of systems without support materials, and the induction period is also ascribed to the coordination effect of Fe3+ with TEOA in the initial hours of the photocatalytic reaction as discussed above (Figure 2). For the TEA system, the initial hydrogen evolution rate for the system that incorporates RGO increased by fourfold compared with the system without RGO and was twofold that of the system with NH2-MCM-41. In contrast to TEOA systems, photocatalytic hydrogen evolution with RGO was better than that with NH2-MCM-41 over 20 h of irradiation (Figure 7 B). The TEM images provide us with useful information about the formation of Fe/H2N-MCM-41 or Fe/graphene composite materials in different electron donor systems and they provide an explanation for the different photocatalytic hydrogen evolution behavior in the systems (Figures 8 and 9). If RGO was added to the system with either TEA or TEOA as the sacrificial electron donor, the system after irradiation showed no aggregation of nanoparticles on the magnetic stirrer. TEM images of the uniform distribution and in situ self-deposition of nanoparticles on RGO surfaces were obtained (Figure 8 B and C). RGO is also reported to have electron-transporting and -accepting abilities,[34] which would, therefore, accelerate electron-transfer processes from Fl to the well-dispersed nanoparticles and lead to an enhanced hydrogen evolution activity. For systems that incorporate NH2-MCM-41, the dispersion of nanoparticles was observed in systems with TEA and TEOA as sacrificial electron donors (Figure 9). In addition, in the TEOA solution, the morphology of the nanoparticles generated in situ was found to be modulated, and clusters of nanoparticles with a diameter around 50 nm were generated, in which each grain has a size of 10 nm. These nanoparticles were either attached to the surface or imbedded between the support materials. On close examination of NH2-MCM-41 after the photocatalytic reaction in the TEOA system, other nanoparticles with a much smaller size were formed and intercalated inside the slim channels (channel diameter = 3.5 nm; Figure 9 D). These nanoparticles inside the channels were believed to play a part in the improved performance of the system with NH2-MCM-41 Figure 7. Photocatalytic hydrogen production with and without 3.0 mg RGO or 4.0 mg NH2-MCM-41 with Fe3+ than that with RGO in the TEOA (1.0 10 4 m) and Fl (2.0 mm) in 30 mL A) TEOA (1.25 %) or B) TEA (5 %) aqueous solution. C) Photocatalytic hydrosystem. In the TEA system, the gen production with different support materials (3.0 mg RGO or 4.0 mg NH2-MCM-41) with Fl (2.0 mm), TEA (5 % addition of Fe3+ led to the inv/v) or TEOA (1.25 % v/v), and Fe3+ (1.0 10 4 m) in 30 mL aqueous solution upon light irradiation for 20 h. LED stant formation of Fe(OH)3 prelight, l > 420 nm, 30 3 W. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org would be of important practical value. In addition, higher TONs for the Fe system (6845) were obtained than for the Ni system (3300) with an equimolar quantity of metal precursors (1.0 10 6 m, 30 mL). The presence of an induction period in the Fe system (pH 10.3) and the effects of the increased amount of FeCl3 precursor (0.5–1.0 mm) have been discussed to provide a more in-depth insight into the photocatalytic processes. Furthermore, Fe/RGO and Fe/NH2MCM-41 composite materials of controlled Fe sizes and different morphologies (Fe/NH2-MCM-41) were formed in different electron donor solutions.
Conclusions We have established a highly efficient noble-metal-free system by the direct incorporation of an Figure 8. TEM images of A) a bare RGO sheet, B) nanoparticles self-deposited on an RGO surface in the TEA photoFeIII salt as a precatalyst with an catalytic system, and C) nanoparticles self-deposited on an RGO surface in the TEOA photocatalytic system. organic photosensitizers. Under D) HRTEM image of one in situ generated nanoparticle on an RGO sheet in the TEOA photocatalytic system, in bright visible-light light-emitting which the crystal lattices of 0.205 nm correspond to the (11 0) lattice plane of a-Fe. diode irradiation, FeIII was reduced in situ to Fe0 nanoparticipitate, which could hardly penetrate into the channels of cles, which showed outstanding catalytic activity for hydrogen NH2-MCM-41, therefore, photocatalytic hydrogen evolution production in basic solution. The results of this study demonwas observed with no induction period. However, in the TEOA strate clearly that the initial rate of photogenerated hydrogen system, no ready formation of Fe(OH)3 was observed, but can be controlled by the nature of the sacrificial electron a complex generated by the reaction of Fe3+ with TEOA in the donor, triethylamine (TEA) or triethanolamine (TEOA). TEA assosolution was seen, which could permeate inside the channels ciates with Fe3+ to form an FeIII complex, whereas TEOA generunder stirring and was photoreduced to Fe inside the chanates Fe(OH)3 instantly. In contrast to systems without support nels. These additional nanoparticles inside NH2-MCM-41 give materials, Fe/NH2-MCM-41 or Fe/reduced graphene oxide cataan advantage over the RGO support in the TEOA system. A lysts display higher catalytic activities. Photocatalytic hydrogen comparison of the total amount of hydrogen produced during evolution in the TEOA system, which exhibits an induction 20 h of irradiation is shown in Figure 7 C, from which we can period before hydrogen generation, was better than that in see that photocatalytic hydrogen evolution with TEOA was the TEA system, in which larger Fe-NPs were formed. This work better than that with TEA over 20 h of irradiation. TEM images is expected to contribute to the production of highly active provide the explanation that Fe-NPs deposited on the support and efficient earth-abundant-metal catalysts by the new apmaterials were much smaller in the TEOA system than those in proach to harness solar energy, which is of both academic and the TEA system (Figures 8 and 9). applied interest. Compared to our Ni system reported previously, this Febased system presents some unique features and significance. Experimental Section Fe is a more earth-abundant and inexpensive metal. Although recently many Ni complexes have been developed and exploitAssembly of Fl-TEA/TEOA-Fe3+ hydrogen production sysed efficiently as catalysts for photocatalytic hydrogen evolutems and the photocatalytic activity measurement tion,[11, 19, 23–25, 35] most reported Fe-based catalysts are hydrogeIn a typical reaction, a quartz tube that contained FeCl3 (1.0 nase mimics,[3] which usually require stringent preparation con10 5 m) and an solution (30 mL) of TEA (5 % v/v) at pH 11 (adjusted ditions. Therefore, the development of an efficient noblewith 0.1 m HCl) or TEOA (1.25 % v/v; pH 10.3) was stirred under air. metal-free photocatalytic hydrogen production system with an Fl (20 mg, 2.0 mm) was added to the resulting solution with vigoFe-NP catalyst generated in situ from an Fe salt precursor rous stirring. For systems that incorporated support materials, RGO 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org Characterization of Fe-NPs The Fe-NPs that precipitated after irradiation were collected, washed several times with deoxygenated ethanol, centrifuged, and dried under an inert atmosphere. The dried nanoparticles were used directly for XRD and XPS measurements. For the TEM measurement, the nanoparticles were dispersed in deoxygenated ethanol solution. HRTEM and EDS measurements were performed by using a TEM (JEM 2100F) at an accelerating voltage of 200 kV.
Acknowledgements This work was financially supported by the Ministry of Science and Technology (2012DFH40090, 2013CB834804). We thank the Natural Science Foundation of China (NSFC Grant Nos. 21273257, 21267025, 21367026, and U1137606) and Yunnan Key Project (2010CC007) for financial Figure 9. TEM images of Fe/NH2-MCM-41 composite materials formed A) in the TEA photocatalytic system and B) in the TEOA photocatalytic system. C) TEM image of a nanocluster of Fe-NPs attached to the surface of NH2support. W.F.F. acknowledges the MCM-41 in the TEOA system. D) TEM image of much smaller nanoparticles formed in situ inside the pore channels China Scholarship Council for of the NH2-MCM-41 material (channel diameter = 3.5 nm) in the TEOA system. funding to be a senior visiting fellow at the University of Rochester (USA). We appreciate many fruitful discussions with Prof. (3.0 mg) or NH2-MCM-41 (4.0 mg) well dispersed in water were added directly to the photocatalytic system with stirring. The sysRichard Eisenberg. tems were deoxygenated with N2 for 30 min, and then the tube was sealed with a rubber cap. The 10 samples were subjected to irradiation in an apparatus that comprised an LED light source (30 3 W, l > 420 nm) and a magnetic stirrer. The generated hydrogen from the systems was measured at different time intervals by using a GC-14C system (Shimadzu), which was equipped with a 5 molecular sieve column (3 m 2 mm), a thermal conductivity detector, and N2 or Ar carrier gas. The amount of hydrogen was quantified by an external standard method. The TON was calculated based on the amount of added FeCl3. Electronic absorption spectra were recorded by using a HITACHI U3010 spectrophotometer. HRTEM was performed by using a JEM 2100F instrument operated at an accelerating voltage of 200 kV. Powder XRD patterns were collected by using a Bruker D8 Focus with CuKa radiation at (l = 1.54056 ). pH values were measured by using a Model pH S-3C meter (Mettler Toledo FE20, China). XPS data were obtained by using an ESCALa-b220i-XL electron spectrometer from VG Scientific using 300 W AlKa radiation. The binding energies were obtained with reference to the C 1s line at 284.8 eV. FTIR spectra were collected by using a Varian FTIR Excalibur 3100 plus with an average of 32 scans and a resolution of 4 cm 1 from 4000–500 cm 1.
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Keywords: electron transfer · hydrogen · iron · nanoparticles · photochemistry [1] A. J. Esswein, D. G. Nocera, Chem. Rev. 2007, 107, 4022 – 4047. [2] J. L. Dempsey, B. S. Brunschwig, J. R. Winkler, H. B. Gray, Acc. Chem. Res. 2009, 42, 1995 – 2004. [3] P. Du, R. Eisenberg, Energy Environ. Sci. 2012, 5, 6012 – 6120. [4] V. Artero, M. Fontecave, Chem. Soc. Rev. 2013, 42, 2338 – 2356. [5] A. Fihri, V. Artero, M. Razavet, C. Baffert, W. Leibl, M. Fontecave, Angew. Chem. Int. Ed. 2008, 47, 564 – 567; Angew. Chem. 2008, 120, 574 – 577. [6] B. Probst, A. Rodenberg, M. Guttentag, P. Hamm, R. Alberto, Inorg. Chem. 2010, 49, 6453 – 6460. [7] J. M. Lehn, J. P. Sauvage, Nouv. J. Chim. 1977, 1, 449 – 451. [8] A. Moradpour, E. Amouyal, P. Keller, H. B. Kagan, Nouv. J. Chim. 1978, 2, 547 – 549. [9] H. Vrubel, X. Hu, Angew. Chem. Int. Ed. 2012, 51, 12703 – 12706; Angew. Chem. 2012, 124, 12875 – 12878. [10] F. Lakadamyali, M. Kato, N. M. Muresan, E. Reisner, Angew. Chem. Int. Ed. 2012, 51, 9381 – 9384; Angew. Chem. 2012, 124, 9515 – 9518. [11] P. Zhang, M. Wang, Y. Na, X. Q. Li, Y. Jiang, L. C. Sun, Dalton Trans. 2010, 39, 1204 – 1206. [12] P. Du, J. Schneider, P. Jarosz, R. Eisenberg, J. Am. Chem. Soc. 2006, 128, 7726 – 7727. [13] H. Ozawa, M. A. Haga, K. Sakai, J. Am. Chem. Soc. 2006, 128, 4926 – 4927. [14] G. J. Zhang, X. Gan, Q. Q. Xu, Y. Chen, X. J. Zhao, B. Qin, X. J. Lv, S. W. Lai, W. F. Fu, C. M. Che, Dalton Trans. 2012, 41, 8421 – 8429. [15] P. Lei, M. Hedlund, R. Lomoth, H. Rensmo, O. Johansson, L. Hammarstrom, J. Am. Chem. Soc. 2008, 130, 26 – 27.
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CHEMSUSCHEM FULL PAPERS [16] B. F. DiSalle, S. Bernhard, J. Am. Chem. Soc. 2011, 133, 11819 – 11821. [17] P. D. Frischmann, K. Mahata, F. Wrthner, Chem. Soc. Rev. 2012, 41, 1847 – 1870. [18] S. Fukuzumi, Y. Yamada, T. Suenobu, K. Ohkubo, H. Kotani, Energy Environ. Sci. 2011, 4, 2754 – 2766. [19] Z. Han, L. Shen, W. W. Brennessel, P. L. Holland, R. Eisenberg, J. Am. Chem. Soc. 2013, 135, 14659 – 14669. [20] H. N. Kagalwala, E. Gottlieb, G. Li, T. Li, R. Jin, S. Bernhard, Inorg. Chem. 2013, 52, 9094 – 9101. [21] B. H. Solis, S. Hammes-Schiffer, J. Am. Chem. Soc. 2012, 134, 15253 – 15256. [22] X. Li, M. Wang, D. Zheng, K. Han, J. Dong, L. Sun, Energy Environ. Sci. 2012, 5, 8220 – 8224. [23] W. Zhang, J. Hong, J. Zheng, Z. Huang, J. S. Zhou, R. Xu, J. Am. Chem. Soc. 2011, 133, 20680 – 20683. [24] Z. Han, F. Qiu, R. Eisenberg, P. L. Holland, T. D. Krauss, Science 2012, 338, 1321 – 1324. [25] J. Dong, M. Wang, X. Li, L. Chen, Y. He, L. Sun, ChemSusChem 2012, 5, 2133 – 2138. [26] Y. Yamada, H. Tadokoro, S. Fukuzumi, RSC Adv. 2013, 3, 25677 – 25680. [27] H. Kotani, R. Hanazaki, K. Ohkubo, Y. Yamada, S. Fukuzumi, Chem. Eur. J. 2011, 17, 2777 – 2785. [28] Y. Yamada, T. Miyahigashi, H. Kotani, K. Ohkubo, S. Fukuzumi, Energy Environ. Sci. 2012, 5, 6111 – 6118. [29] F. Alonso, P. Riente, M. Yus, Tetrahedron 2008, 64, 1847 – 1852. [30] Y. Duan, J. Li, Mater. Chem. Phys. 2004, 87, 452 – 454. [31] P. Song, D. Wen, Z. X. Guo, T. Korakianitis, Phys. Chem. Chem. Phys. 2008, 10, 5057 – 5065.
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www.chemsuschem.org [32] F. Alonso, P. Riente, J. A. Sirvent, M. Yus, Appl. Catal. A 2010, 378, 42 – 51. [33] A. C. Brooks, K. Basore, S. Bernhard, Inorg. Chem. 2013, 52, 5794 – 5800. [34] C. Wang, S. Cao, W. F. Fu, Chem. Commun. 2013, 49, 11251 – 11253. [35] Z. Han, W. R. McNamara, M. S. Eum, P. L. Holland, R. Eisenberg, Angew. Chem. Int. Ed. 2012, 51, 1667 – 1670; Angew. Chem. 2012, 124, 1699 – 1702. [36] J. M. Yan, X. B. Zhang, S. Han, H. Shioyama, Q. Xu, Angew. Chem. Int. Ed. 2008, 47, 2287 – 2289; Angew. Chem. 2008, 120, 2319 – 2321. [37] S. Sasaki, K. Nakamura, Y. Hamabe, E. Kurahashi, T. Hiroi, Nature 2001, 410, 555 – 557. [38] J. T. Olegario, N. Yee, M. Miller, J. Sczepaniak, B. Manning, J. Nanopart. Res. 2010, 12, 2057 – 2068. [39] Y. P. Sun, X. Q. Li, J. Cao, W. X. Zhang, H. P. Wang, Adv. Colloid Interface Sci. 2006, 120, 47 – 56. [40] T. Lazarides, T. McCormick, P. Du, G. Luo, B. Lindley, R. Eisenberg, J. Am. Chem. Soc. 2009, 131, 9192 – 9194. [41] J. Enderlein, Appl. Phys. Lett. 2002, 80, 315 – 317. [42] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, J. Am. Chem. Soc. 1992, 114, 10834 – 10843. [43] S. Min, G. Lu, J. Phys. Chem. C 2011, 115, 13938 – 13945.
Received: January 17, 2014 Revised: April 24, 2014 Published online on && &&, 0000
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FULL PAPERS Shedding light on an iron catalyst: A bifunctional noble-metal-free system for the photoreduction of Fe3+ to Fe(0) nanoparticles in water and simultaneous highly efficient photocatalytic hydrogen evolution is established. The sizes of the Fe nanoparticles generated in situ are tuned by changing the sacrificial donor from triethylamine to triethanolamine and by the introduction of H2N-MCM-41 or graphene into the photocatalytic system.
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C.-J. Wang, S. Cao, B. Qin, C. Zhang, T.-T. Li, W.-F. Fu* && – && Photoreduction of Iron(III) to Iron(0) Nanoparticles for Simultaneous Hydrogen Evolution in Aqueous Solution
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Photoreduction of Iron(III) to Iron(0) Nanoparticles for Simultaneous Hydrogen Evolution in Aqueous Solution Chuan-Jun Wang,[a] Shuang Cao,[a] Biao Qin,[a] Chen Zhang,[b] Ting-Ting Li,[a] and WenFu Fu*[a, b] Crystalline Fe nanoparticles were obtained with fluorescein (Fl) as the photosensitizer in triethylamine (TEA) or triethanolamine (TEOA) aqueous solution with FeCl3 as the Fe precursor under bright visible-light light-emitting diode (LED) irradiation. Photoinduced electron transfer from excited state Fl* and Fl to Fe3+ produced the Fe nanoparticles, which served as the active catalyst for in situ photocatalytic hydrogen production with Fl and TEA or TEOA as the photosensitizer and electron donors, respectively, in the same system. Robust hydrogen production activities were observed under the Fe nanoparticle photore-
duction conditions in basic solution, and tens of milliliters of hydrogen were obtained over prolonged LED irradiation. If inorganic support materials such as NH2-MCM-41 or reduced graphene oxide were introduced, dispersed nanoparticles with different sizes and shapes were deposited on the supports, which led to variously enhanced hydrogen production activities. The relationships between the morphologies of the Fe/H2N-MCM41 or Fe/graphene composites generated in situ and the hydrogen production activities were investigated systematically.
Introduction Light-driven water splitting is regarded as a promising solarenergy utilization process for sustainable global energy supply.[1–6] Typically, hydrogen photogeneration systems consist of a multicomponent system with a light-harvesting material, a sacrificial electron donor, and a metal-based catalyst.[7–11] Recently, much effort has been directed toward the development of transition-metal complexes that function either as photosensitizers to absorb visible light and generate long-lived excited states efficiently or as catalysts with low overpotentials for hydrogen production from water.[11–18] Recent research has focused on the photogeneration of molecular hydrogen by the development of efficient and inexpensive systems that contain organic molecular sensitizers and earth-abundant metal-based complex catalysts, and inspiring and important results have been achieved.[19–22] However, most of the metal complexes operate in a mixed organic/water solution and their decomposition under irradiation is a drawback that hampers their recycla[a] Dr. C.-J. Wang,+ Dr. S. Cao,+ Dr. B. Qin,+ Dr. T.-T. Li, Prof. W.-F. Fu Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190 (P.R. China) Fax: (+ 86) 10-6255-4670 E-mail: [email protected] [b] C. Zhang, Prof. W.-F. Fu College of Chemistry and Engineering Yunnan Normal University Kunming 650092 (P.R. China) [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201400065.
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bility and limits their applications greatly. The value of these well-designed and presynthesized compounds would fall if they do not last under light irradiation or operate in pure water as real-life solar energy with its full spectrum of light generates considerable heat, which would further accelerate the decomposition of complexes. The development of catalysts formed in situ for hydrogen evolution is of significance as active catalysts self-assembled under photocatalytic conditions would not be subject to light-induced decomposition as much as presynthesized ones. Most recently, there have been reports of hydrogen evolution over catalysts formed in situ by the addition of metal salts and coordinating ligands in a photocatalytic system that contains a photosensitizer and an electron donor. Photocatalytic water reduction with a nickel thiolate catalyst self-assembled by nickel(II) acetate and 2-mercaptoethanol with erythrosin B and triethanolamine (TEOA) as a sensitizer and an electron donor, respectively, has been reported by Xu et al., and a quantum efficiency for hydrogen evolution of up to 24.5 % was obtained upon irradiation with 460 nm light.[23] Eisenberg et al. have also reported a new approach that uses CdSe quantum dots (QDs) capped with dihydrolipoic acid coupled to a NiII catalytic center self-assembled in water to form a heterogeneous photocatalytic system. This system exhibited undiminished stability upon irradiation at 520 nm for 110 h and had a turnover number (TON) of up to 600 000.[24] The use of NiII or CoII coordinated to a TEOA electron donor to form catalysts with graphene-C3N4 for photocatalytic hydrogen evolution has been reported by Sun and co-workers.[25] From another point of view, the exploitation of stable earth-abundant-metal nanoparticles for highly efficient hydrogen production in a multicomponent system with organic photosensitizers has been surprisingly less explored compared to that with Pt ChemSusChem 0000, 00, 1 – 11
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CHEMSUSCHEM FULL PAPERS nanoparticles, and only a few works report the presynthesis of Fe and Ni nanoparticles in a photocatalytic hydrogen generation system.[26–28] Usually, nanoparticles prepared by chemical reduction methods[29, 30] are afflicted by a sensitivity to oxygen, which would be detrimental to catalytic applications.[31] The practice of the introduction of a surfactant protective shell around the metal surfaces is not effective for catalytic reactions that take place on metal surfaces.[32] Therefore, the idea to use earth-abundant-metal nanoparticles generated in situ from photocatalytic systems that upon generation act instantly as catalysts for light-driven hydrogen evolution is intriguing. The selection of electron donors and photosensitizers that would generate excited species with sufficient driving force for the reduction of earth-abundant metal salts to metal nanoparticle catalysts for in situ photocatalytic hydrogen evolution is important. In 2013, Bernhard et al. reported the light-driven reduction of ZnII to Zn using an IrIII photosensitizer and triethylamine (TEA) as an electron donor in acetonitrile solution.[33] Recently, we have communicated the visible-light-driven reduction of NiII to Ni0 nanoparticles in a system that contains fluorescein (Fl), TEA or TEOA, and nickel chloride, and the subsequent photogeneration of hydrogen was achieved by the same system.[34] It was observed that the system was stable to produce hydrogen for days in the presence of the generated Ni nanoparticles with tens of milliliters of hydrogen produced, even under the relatively intense light-emitting diode (LED) light source exploited for the Ni2+ photoreduction process. Herein, we report a detailed investigation of the controlled photoreduction of FeIII to a highly active Fe0 nanoparticle catalyst in an aqueous solution that contains Fl, TEOA or TEA, and an FeIII salt under bright LED visible-light irradiation. Consequently, we achieved sustainable photogenerated hydrogen in this system with good catalytic stability and recyclability in a basic solution (Scheme 1). The photogenerated hydrogen efficiency improved upon the addition of NH2-MCM-41 or reduced graphene oxide (RGO) to the abovementioned systems compared to the system without support materials. The effects of the sacrificial electron donor on the photoreduction process and photocatalytic hydrogen evolution were studied systematically and are presented.
Results and Discussion The photocatalytic hydrogen production systems were assembled from FeCl3, Fl, and TEOA (pH 10.3) or TEA (pH 11.0) in an aqueous solution. The photocatalytic experiments were performed with ten vials of the samples irradiated under a special-
Scheme 1. Highly efficient photoreduction and hydrogen photogeneration system. (LED light source, l > 420 nm, 30 3 W)
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www.chemsuschem.org ly designed l > 420 nm LED (30 3 W). For studies that include support materials, NH2-MCM-41 or RGO was introduced into the systems to result in well-dispersed Fe0 nanoparticles on the surface or inside the pores of the support materials (NH2MCM-41 in TEOA) under light irradiation. Control experiments showed that no nanoparticles are generated in the absence of any of the following components: light, Fl, and TEOA or TEA, and no significant H2 production was observed (Figure S1). The photocatalytic hydrogen production activities in different electron donor (TEA and TEOA) solutions were investigated systematically and compared. If TEA was employed as the electron donor, the addition of FeCl3 to the TEA aqueous solution gives iron hydroxide precipitates readily. The amount of photogenerated hydrogen was measured as a function of pH over 8 h, and the results are shown in Figures S2–S3. Hydrogen evolution was almost linear over time, and the highest efficiency was achieved at pH 11. The effect of TEA concentration on the photocatalytic reaction was investigated to optimize the hydrogen production efficiency. A fast rate of hydrogen production was observed at higher TEA concentrations with fixed amounts of Fl and FeCl3, and the photocatalytic system exhibited a high hydrogen evolution efficiency (1.70 mL at 298 K) during the initial 10 h of operation (Figure 1 A). However, we did not observe an induction period of hydrogen generation in the present system because of the rapid formation of black precipitate on a short timescale. At low TEA concentrations, the system showed moderate photocatalytic activity but a prolonged lifetime. The system with a TEA concentration of 0.5 % produced 0.69 mL H2 in the first 10 h, which was only one third of the value achieved at 5 % TEA concentration (Figure 1 B). However, hydrogen production continued after the further addition of TEA at 18 and 36 h. This indicates that the photocatalytic process related to the oxidative quenching of the excited state of the Fl becomes more competitive at a lower TEA concentration, whereas reductive quenching dominates the catalytic reaction at a higher TEA concentration (5 %) to produce Fl and reduce the stability of the photosensitizer.[19, 35] The system ceased activity at around 25 h, which can be resumed by the addition of more Fl (Figure 1 D). Notably, the higher hydrogen production rates observed after the addition of more TEA at 18 h than at the beginning of the reaction shows the gradual accumulation of the Fe nanoparticle catalyst (Figure 1 B). We postulate that at the start, TEA was consumed largely by the photochemical reaction to reduce Fe(OH)3 to Fe nanoparticles (Fe-NPs). Therefore, only a portion of the TEA functioned as an electron donor for hydrogen production in the initial period of nanoparticle generation. To further show that the particles are the active catalyst, the effects of the FeCl3 concentration on the photocatalytic activity of the system were studied. The dependence of the hydrogen production efficiency on the FeCl3 concentration is shown in Figure 1 C. As expected, both the initial rate and the amount of hydrogen produced increased with FeCl3 concentration. At lower Fe3+ concentrations of 1.0 10 7 and 1.0 10 6 m, exceedingly high TONs of up to 31 000 and 6845, respectively, were achieved after 39 h irradiation in a single run. Although larger amounts of hydrogen ChemSusChem 0000, 00, 1 – 11
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Figure 1. Hydrogen production during the visible-light irradiation of an aqueous solution (30 mL) of 0.005 mm FeCl3 that contained Fl (2.0 mm) at pH 11 and A) TEA (5 % v/v) or B) TEA (0.1 and 0.5 % v/v). After irradiation for 18 and 36 h TEA (0.1 and 0.5 % v/v) was injected into the systems, respectively. C) Relationship between the amount of hydrogen production and the irradiation time for the photocatalytic system that contained TEA (5 % v/v) and Fl (2.0 mm) at various concentrations of FeCl3. D) Hydrogen production as a function of irradiation time for the system that contained 0.2 and 0.05 mm FeCl3 and TEA (5 % v/v). LED light: l > 420 nm, 30 3 W.
Figure 2. Relationship between the amount of hydrogen production and irradiation time for the photocatalytic system that contained Fl (2.0 mm) in 30 mL aqueous solution and A) FeCl3 (0.1 mm) and TEA (5 % v/v) at pH 11 or TEOA (1.25 %) at pH 10.3; B) TEOA (1.25 %) and various concentrations of FeCl3 ; C) TEOA (1.25 %) and various concentration of FeCl3 with aliquots of TEOA (5 and 1.25 %) and Fl (2.0 mm) added at 36 and 38 h, respectively; D) FeCl3 (1.0 and 0.5 mm) and TEOA (1.25 %) at pH 10.3 with aliquots of TEOA (1.25 %) and Fl (2.0 mm) added at 36 and 38 h, respectively. LED light: l > 420 nm, 30 3 W.
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were produced at higher Fe3+ concentrations, the calculated TONs did not exhibit a linear proportional relationship.[19–22, 35] It was also observed that at higher concentrations of added Fe3+, the magnetic stirrer was covered with a layer of black particles, which resulted from the aggregation of the generated Fe-NPs. The photocatalytic hydrogen evolution was resumed upon the injection of an additional equivalent of Fl when the hydrogen production reached a plateau at 30 h (Figure 1 D). In addition, changes in the hydrogen production efficiency with different concentrations of Fl, as well as the use of other photosensitizer dyes, were also studied (Figures S4 and S5). Furthermore, comparison studies with Pt-catalyzed hydrogen evolution under optimized pH conditions were also investigated (Figures S6 and S7). If TEOA was used as the sacrificial electron donor, a slow initial rate of hydrogen production in the first few hours was observed with fixed amounts of Fl and FeCl3. The photogenerating hydrogen systems exhibit an induction period (Figure 2 and Figure S8), which differs from the systems that have a linear initial rate if TEA is used as the electron donor. The hydrogen production rate and amount are both enhanced with increasing amounts of added Fe3+ (Figure 2 B). Over 26 mL H2 was obtained at an Fe3+ concentration of 5.0 10 4 m over an irradiation time of 36 h. At an Fe3+ concentration of 10 7 m, a TON of approximately 14 000 was achieved in 1.25 % TEOA, although only 0.49 mL H2 was produced. When the hydrogen production reached a plateau at 36 h, the addition of further aliquots of TEOA (1.25 %) did not resume the photocatalytic activity (Figure 2 C). However, additional aliquots of Fl solution did resume ChemSusChem 0000, 00, 1 – 11
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the photocatalytic hydrogen evolution. A total amount of with an Fe3+ concentration of 5.0 10 4 m, the system with 32 mL H2 was obtained after 62 h of irradiation, at which point 1.0 mm Fe3+ had to undergo a longer period of photoreducthe system reached another plateau. tion to reduce all Fe(OH)3 to Fe, and, therefore, only some Fl Notably, unlike the TEA system, if Fe3+ is added to TEOA, no and TEOA were effective for the hydrogen production reactions. This resulted in a decreased hydrogen production effiformation of Fe(OH)3 flocculent precipitate was observed by ciency at a higher catalyst concentration of 1.0 mm at the bethe naked eye. Therefore, Fe3+ must have coordinated with ginning of the reaction (Figure 3 A). TEOA to form a complex as reported previously by Sun et al., However, a comparison of the hydrogen evolution profile which is an inefficient catalyst.[25] In the present system under between 25 and 35 h of irradiation showed that the system highly basic conditions with bright LED irradiation, there is an with 1.0 mm Fe3+ has a higher hydrogen production efficiency induction period with no significant amount of hydrogen produced in the first 2 h, and efficient hydrogen generation can than that of 0.5 mm Fe3+ (Figure 3 B). The picture taken after only be achieved if black nanoparticles appeared. It is reasona30 h of irradiation revealed that the flocculent Fe(OH)3 has ble to assume then that the presence of the induction period transformed into Fe-NPs in the 1.0 mm Fe3+ system (Figure S9). in the TEOA system is in some degree because of the coordiTherefore, the bifunctional system transformed into a mononation effect of Fe3+ with TEOA. Upon intense light irradiation, functional hydrogen production system, in which the higher Fe3+ undergoes dissociation gradually from an Fe3+ complex that contains a TEOA ligand and association with OH in the highly basic solution to form an Fe(OH)3 precatalyst, which then was photoreduced gradually to the Fe-NP catalyst to catalyze hydrogen evolution. This supposition is also supported by the longer induction period that was observed under a higher concentration of TEOA (5 %) (Figure 2 A and Figure S8). In the Figure 3. Hydrogen production profile over irradiation time with Fe3+ (5.0 10 4 or 1.0 10 3 m), Fl (2.0 mm), and TEA system, however, Fe(OH)3 is TEOA (1.25 %) in 30 mL aqueous solution A) over the first 10 h and B) from 25–35 h. For B), the tubes were dealready present to undergo the gassed after 25 h of irradiation. photoreduction process, therefore, a linear relationship with no induction period is observed in the first few hours. amount of Fe catalyst resulted in higher hydrogen production Additionally, if the concentration of added Fe3+ was inrates (Figure 3 B). As discussed above, the identification and characterization creased to 1.0 mm, the total hydrogen production efficiency of the black particles generated during irradiation is important was lower during the 35 h of irradiation than that of the to clarify the observed catalytic process. It has been reported system with an Fe3+ concentration of 0.5 mm (Figure 2 D). This previously that the Fl* and Fl generated in the photocatalytic means that the flocculent precipitate blocks the surface active process have reduction potentials of 1.7 and 1.3 V (vs. satusites of the nanoparticles. In the system with 1.0 mm Fe3+, only rated calomel electrode; SCE), respectively.[35] In the presence after a longer irradiation time was the Fe(OH)3 reduced entirely 0 to Fe nanoparticles, and the addition of Fl at 40 h resulted in of an excess of electron donors, the Fe(OH)3 precatalyst was higher hydrogen production rates than the system with likely to accept electrons from either Fl* or Fl to be reduced 0.5 mm Fe3+ as a result of the higher amount of Fe-NP catalyst. to metallic Fe-NPs.[33] The Fe-NPs were characterized by highA photographic comparison of the sediments formed showed resolution transmission electron microscopy (HRTEM), XRD, that after 10 h of irradiation the systems with an Fe3+ concenenergy-dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), and chemical methods. tration of 5.0 10 4 and 2.0 10 4 m precipitated fine black parThe black particles were collected in a sealed quartz tube ticles, which was characterized as Fe0 by XRD. However, floccuand degassed with N2 until no background oxygen was detectlent precipitate mingled with black Fe particles was observed in the system with an Fe3+ concentration of 1.0 mm after 10 h ed by GC. Degassed dilute HCl aqueous solution was injected into the tube, and the instant evolution of bubbles was obof irradiation (Figure S9). The flocculent precipitate was the served, which was proved to be H2 by GC. There was no clear Fe(OH)3 precatalyst that had not been reduced entirely to Fe in this time, which overwhelmed the active Fe-NP catalyst and color change if KSCN was added to the resulting solution, hampered the hydrogen production efficiency. Therefore, at which indicates the absence of Fe3+. The solution changed 3+ fixed Fl and TEOA concentrations, the system with 1.0 mm Fe from achromatic color to red upon standing under an air atmosphere overnight (Figure S10). was unable to reduce all the Fe(OH)3 precatalyst to Fe-NPs within the first 10 h of irradiation. Compared to the system 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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TEM images show that the nanoparticles generated in situ in TEA solution consist of welldefined prismlike and cubic structures and that most of the particles had a size range of around 40–50 nm (Figure 4 A). In comparison, nanoparticles generated from TEOA solution had sizes of 10–20 nm (Figure 4 C). The higher surface area of the smaller nanoparticles generated is believed to be responsible for the overall higher hydrogen production activity in the TEOA system. The corresponding selected-area electron diffraction (SAED) pattern shows clear diffraction rings of a single crystal (inset in Figure 4 A and C).[36] The crystals were well dispersed and had a measured spacing of 0.205 nm, which is consistent with the (11 0) crystal lattice plane of a-Fe (Figure 4 B and D).[37] Importantly, HRTEM-EDX analysis on various regions also confirms that within detection limits Fe was the only element in these nanoparticles (Figure 4 E and Figure S11). The XRD pattern demonstrates an apparent sharp peak at 2 q = 44.98 (Figure 4 F), which is a clear indication of the presence of zero-valent Fe (aFe).[38] In addition, XPS measurements confirmed the formation of zero-valent Fe in the investigated system (Figure S12).[39] FTIR spectroscopy shows that the Fe-NPs formed in situ had relatively clean surfaces (Figure S13). From these data, we Figure 4. A) TEM (inset: SAED pattern) and B) HRTEM images of the nanoparticles photogenerated in situ in TEA aqueous solution under visible-light irradiation. C) TEM (inset: SAED pattern) and D) HRTEM images of the nanoconclude that Fe-NPs are formed particles photogenerated in situ in TEOA aqueous solution under visible-light irradiation. E) HRTEM-EDX and in the photocatalytic system F) XRD of the in situ generated nanoparticles. LED light: l > 420 nm, 30 3 W. under visible-light irradiation and function as an active metal at 42 h (Figure 5 A). In contrast with the systems that used catalyst for efficient hydrogen production. FeCl3 as the precatalyst in TEOA aqueous solution, if TEOA is We also performed photocatalytic measurements of hydrogen production by reusing the Fe-NP catalyst obtained after exploited as the electron donor, no induction period of generone photocatalytic run. The material was collected and purified ating hydrogen was observed (Figure 5 B). The system drives under an inert atmosphere, and highly efficient hydrogen prohydrogen evolution steadily for more than 40 h without the duction could still be achieved. If TEA is used as the sacrificial addition of any further components. The different photocataelectron donor, hydrogen production reached a plateau lytic stabilities of the systems with TEA and TEOA as sacrificial around 10 h, and the addition of more Fl at 15 h resumed the agents may be attributed mainly to the different mechanisms photocatalytic activity. The system lasted for more than 55 h if in the photocatalytic process. As discussed above (Figure 1 B another aliquot of both Fl (2.0 mm) and TEA (5 %) were added and D), a reductive quenching mechanism is the major process 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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that Fe-NPs acted as the catalyst during the photocatalytic reactions and that the chemical reaction between Fe-NPs and H2O contributed a practically negligible amount of hydrogen. Firstly, experiments proved that Fe(OH)2 and Fe(OH)3 precipitates can both dissolve in TEOA to form a clear Fe(TEOA)x solution (Figure S17 A–C). Secondly, two tubes that contained 30 mg FeFigure 5. Photocatalytic hydrogen production with Fe-NPs in 30 mL TEA or TEOA aqueous solution. LED light: NPs and 5 % TEOA aqueous solul > 420 nm, 30 3 W. A) Fe-NPs (2.5 mg), Fl (2.0 mm), TEA (5 %) and B) Fe-NPs (5.0 mg), Fl (2.0 mm), TEOA (1.25 %). tion were degassed and sealed. One of them was subjected to vigorous stirring and the other to intense sonication. (Figif 5 % TEA is used as the sacrificial agent. This produces unstaure S17 D) If the Fe-NPs react with H2O to produce H2 and ble Fl to cause the fast decomposition of Fl,[35] which results in the loss of photocatalytic activity of the system around Fe(OH)2, the Fe(OH)2 on the surface of the Fe-NPs could quickly 10 h; the activity can be resumed by further addition of Fl (Figdissolve in the excess TEOA solution especially under intense ure 5 A). In contrast, TEOA cannot quench the singlet excited sonication. This would expose the active surface of the Fe core state of Fl.[40] Our experiment showed that the generated Feand cause Fe to react with water to form Fe(OH)2 and to disNPs can quench the fluorescence of Fl effectively (Figure S14). solve in TEOA under sonication. It is reasonable to assume Therefore, we propose that the initial photocatalytic hydrogen then that if this reaction is fast enough, the amount of Fe-NPs production in TEOA goes through an oxidative quenching prowould diminish as it transforms gradually to Fe(OH)2 and dissolves completely in TEOA with the production of an equimocess, which produces less unstable Fl to result in prolonged system activity. Furthermore, the nanoparticles after 47 h of lar amount of H2. However, our results showed that less than photocatalytic reaction were isolated and characterized by 0.8 mL H2 was produced under intense sonication in 15 h and XRD, and still only the (11 0) peak of a-Fe was observed (Figno apparent diminishment of the black Fe-NPs was observed ure S15). This result confirms the integrity of the Fe-NPs during by the naked eye (Figure S17 E–F). In addition, the tube under the photocatalytic reaction and provides further evidence that vigorous stirring produced far less hydrogen (0.05 mL). This exFe-NPs are the real catalyst in our system. periment provides evidence that the reaction between Fe-NPs It is remarkable that the photocatalytic systems have such and H2O in basic TEOA solution is very slow, which supports prolonged lifetimes of more than 30 h for hydrogen evolution the theory that the efficient hydrogen production under visiunder relatively intense light irradiation conditions. There have ble-light irradiation was achieved by Fe-NPs as the catalyst. been reports that electromagnetic interactions between metal To understand the effects of nanoparticle size on the hydronanoparticles and fluorescent molecules can improve the phogen generation efficiency, hybrid hydrogen-generating catatostability of molecules greatly.[41] Therefore, UV/Vis spectrolysts were also obtained by the addition of NH2-MCM-41 or photometry was performed to monitor the changes in the abRGO to the abovementioned system. NH2-MCM-41 and RGO sorption of Fl during photolysis (Figure 6). In the absence of were prepared according to literature methods.[42, 43] It is well known that siliceous mesoporous materials composed of Si added FeCl3, Fl decomposes rapidly in TEA aqueous solution, and O atoms have accessible mesoporous frameworks and adwhich is because of the instability of the Fl formed through the reductive quenching process[35] (Figure 6 A and Figure S16). However, the photolysis system with FeCl3 showed only clear Fl decomposition in the first hour during which time FeCl3 was gradually photoreduced to Fe-NPs. After that, Fl exhibited a significantly enhanced photochemical stability in the presence of Fe-NPs with no dramatic decrease in absorption after 15 h of irradiation (Figure 6 B). Figure 6. UV/Vis absorption spectra measured at different irradiation times of Fl (6.7 10 5 m) and TEA (5 % v/v) in In addition, a delicate experi- H2O at pH 11 upon irradiation in the A) absence and B) presence of Fe3+ (6.7 10 6 m). LED light, l > 420 nm, ment was designed to prove 30 3 W. Aliquots of 30 mL of the solution at different times were diluted for UV/Vis measurements. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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justable porosities, which are beneficial for the incorporation and stabilization of nanoparticles. Previous studies have also demonstrated that RGO with its excellent electron-accepting and -transporting capability, could serve as a catalyst scaffold for in situ generated nanoparticles to be finely dispersed on its surface and as an electron relay mediator between the sensitizer and catalyst to increase the hydrogen production activity efficiently.[13] The addition of support materials led to variously enhanced photocatalytic hydrogen evolution activities in both TEA and TEOA systems (Figure 7). After the photocatalytic reaction, the composite materials were collected, and XRD measurements still showed only the (11 0) peak of a-Fe (Figure S18). For the TEOA system, the incorporation of 3.0 mg RGO or 4.0 mg NH2-MCM-41 into the photocatalytic hydrogen evolution system led to a clear increase in the hydrogen production efficiency (Figure 7 A). The hydrogen produced in the first 2 h for systems with RGO and NH2-MCM-41 was 3.4- and 2.5-fold more than that of the systems without support materials, respectively. The system that incorporated RGO showed a better efficiency than that with NH2-MCM-41 as the support in the first 4 h of irradiation. However, after 4 h, the system with NH2MCM-41 excelled in efficiency over that with the RGO support. After 12 h of irradiation, 8.4 and 13 mL H2 were produced for the systems with RGO and NH2-MCM-41, respectively, which is 2.2- and 3.3-fold more than that of the system without a support material (3.9 mL), respectively. Also, an induction period
was observed for photocatalytic hydrogen production if either of the supports was used, which is in accordance with the hydrogen production profiles of systems without support materials, and the induction period is also ascribed to the coordination effect of Fe3+ with TEOA in the initial hours of the photocatalytic reaction as discussed above (Figure 2). For the TEA system, the initial hydrogen evolution rate for the system that incorporates RGO increased by fourfold compared with the system without RGO and was twofold that of the system with NH2-MCM-41. In contrast to TEOA systems, photocatalytic hydrogen evolution with RGO was better than that with NH2-MCM-41 over 20 h of irradiation (Figure 7 B). The TEM images provide us with useful information about the formation of Fe/H2N-MCM-41 or Fe/graphene composite materials in different electron donor systems and they provide an explanation for the different photocatalytic hydrogen evolution behavior in the systems (Figures 8 and 9). If RGO was added to the system with either TEA or TEOA as the sacrificial electron donor, the system after irradiation showed no aggregation of nanoparticles on the magnetic stirrer. TEM images of the uniform distribution and in situ self-deposition of nanoparticles on RGO surfaces were obtained (Figure 8 B and C). RGO is also reported to have electron-transporting and -accepting abilities,[34] which would, therefore, accelerate electron-transfer processes from Fl to the well-dispersed nanoparticles and lead to an enhanced hydrogen evolution activity. For systems that incorporate NH2-MCM-41, the dispersion of nanoparticles was observed in systems with TEA and TEOA as sacrificial electron donors (Figure 9). In addition, in the TEOA solution, the morphology of the nanoparticles generated in situ was found to be modulated, and clusters of nanoparticles with a diameter around 50 nm were generated, in which each grain has a size of 10 nm. These nanoparticles were either attached to the surface or imbedded between the support materials. On close examination of NH2-MCM-41 after the photocatalytic reaction in the TEOA system, other nanoparticles with a much smaller size were formed and intercalated inside the slim channels (channel diameter = 3.5 nm; Figure 9 D). These nanoparticles inside the channels were believed to play a part in the improved performance of the system with NH2-MCM-41 Figure 7. Photocatalytic hydrogen production with and without 3.0 mg RGO or 4.0 mg NH2-MCM-41 with Fe3+ than that with RGO in the TEOA (1.0 10 4 m) and Fl (2.0 mm) in 30 mL A) TEOA (1.25 %) or B) TEA (5 %) aqueous solution. C) Photocatalytic hydrosystem. In the TEA system, the gen production with different support materials (3.0 mg RGO or 4.0 mg NH2-MCM-41) with Fl (2.0 mm), TEA (5 % addition of Fe3+ led to the inv/v) or TEOA (1.25 % v/v), and Fe3+ (1.0 10 4 m) in 30 mL aqueous solution upon light irradiation for 20 h. LED stant formation of Fe(OH)3 prelight, l > 420 nm, 30 3 W. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org would be of important practical value. In addition, higher TONs for the Fe system (6845) were obtained than for the Ni system (3300) with an equimolar quantity of metal precursors (1.0 10 6 m, 30 mL). The presence of an induction period in the Fe system (pH 10.3) and the effects of the increased amount of FeCl3 precursor (0.5–1.0 mm) have been discussed to provide a more in-depth insight into the photocatalytic processes. Furthermore, Fe/RGO and Fe/NH2MCM-41 composite materials of controlled Fe sizes and different morphologies (Fe/NH2-MCM-41) were formed in different electron donor solutions.
Conclusions We have established a highly efficient noble-metal-free system by the direct incorporation of an Figure 8. TEM images of A) a bare RGO sheet, B) nanoparticles self-deposited on an RGO surface in the TEA photoFeIII salt as a precatalyst with an catalytic system, and C) nanoparticles self-deposited on an RGO surface in the TEOA photocatalytic system. organic photosensitizers. Under D) HRTEM image of one in situ generated nanoparticle on an RGO sheet in the TEOA photocatalytic system, in bright visible-light light-emitting which the crystal lattices of 0.205 nm correspond to the (11 0) lattice plane of a-Fe. diode irradiation, FeIII was reduced in situ to Fe0 nanoparticipitate, which could hardly penetrate into the channels of cles, which showed outstanding catalytic activity for hydrogen NH2-MCM-41, therefore, photocatalytic hydrogen evolution production in basic solution. The results of this study demonwas observed with no induction period. However, in the TEOA strate clearly that the initial rate of photogenerated hydrogen system, no ready formation of Fe(OH)3 was observed, but can be controlled by the nature of the sacrificial electron a complex generated by the reaction of Fe3+ with TEOA in the donor, triethylamine (TEA) or triethanolamine (TEOA). TEA assosolution was seen, which could permeate inside the channels ciates with Fe3+ to form an FeIII complex, whereas TEOA generunder stirring and was photoreduced to Fe inside the chanates Fe(OH)3 instantly. In contrast to systems without support nels. These additional nanoparticles inside NH2-MCM-41 give materials, Fe/NH2-MCM-41 or Fe/reduced graphene oxide cataan advantage over the RGO support in the TEOA system. A lysts display higher catalytic activities. Photocatalytic hydrogen comparison of the total amount of hydrogen produced during evolution in the TEOA system, which exhibits an induction 20 h of irradiation is shown in Figure 7 C, from which we can period before hydrogen generation, was better than that in see that photocatalytic hydrogen evolution with TEOA was the TEA system, in which larger Fe-NPs were formed. This work better than that with TEA over 20 h of irradiation. TEM images is expected to contribute to the production of highly active provide the explanation that Fe-NPs deposited on the support and efficient earth-abundant-metal catalysts by the new apmaterials were much smaller in the TEOA system than those in proach to harness solar energy, which is of both academic and the TEA system (Figures 8 and 9). applied interest. Compared to our Ni system reported previously, this Febased system presents some unique features and significance. Experimental Section Fe is a more earth-abundant and inexpensive metal. Although recently many Ni complexes have been developed and exploitAssembly of Fl-TEA/TEOA-Fe3+ hydrogen production sysed efficiently as catalysts for photocatalytic hydrogen evolutems and the photocatalytic activity measurement tion,[11, 19, 23–25, 35] most reported Fe-based catalysts are hydrogeIn a typical reaction, a quartz tube that contained FeCl3 (1.0 nase mimics,[3] which usually require stringent preparation con10 5 m) and an solution (30 mL) of TEA (5 % v/v) at pH 11 (adjusted ditions. Therefore, the development of an efficient noblewith 0.1 m HCl) or TEOA (1.25 % v/v; pH 10.3) was stirred under air. metal-free photocatalytic hydrogen production system with an Fl (20 mg, 2.0 mm) was added to the resulting solution with vigoFe-NP catalyst generated in situ from an Fe salt precursor rous stirring. For systems that incorporated support materials, RGO 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org Characterization of Fe-NPs The Fe-NPs that precipitated after irradiation were collected, washed several times with deoxygenated ethanol, centrifuged, and dried under an inert atmosphere. The dried nanoparticles were used directly for XRD and XPS measurements. For the TEM measurement, the nanoparticles were dispersed in deoxygenated ethanol solution. HRTEM and EDS measurements were performed by using a TEM (JEM 2100F) at an accelerating voltage of 200 kV.
Acknowledgements This work was financially supported by the Ministry of Science and Technology (2012DFH40090, 2013CB834804). We thank the Natural Science Foundation of China (NSFC Grant Nos. 21273257, 21267025, 21367026, and U1137606) and Yunnan Key Project (2010CC007) for financial Figure 9. TEM images of Fe/NH2-MCM-41 composite materials formed A) in the TEA photocatalytic system and B) in the TEOA photocatalytic system. C) TEM image of a nanocluster of Fe-NPs attached to the surface of NH2support. W.F.F. acknowledges the MCM-41 in the TEOA system. D) TEM image of much smaller nanoparticles formed in situ inside the pore channels China Scholarship Council for of the NH2-MCM-41 material (channel diameter = 3.5 nm) in the TEOA system. funding to be a senior visiting fellow at the University of Rochester (USA). We appreciate many fruitful discussions with Prof. (3.0 mg) or NH2-MCM-41 (4.0 mg) well dispersed in water were added directly to the photocatalytic system with stirring. The sysRichard Eisenberg. tems were deoxygenated with N2 for 30 min, and then the tube was sealed with a rubber cap. The 10 samples were subjected to irradiation in an apparatus that comprised an LED light source (30 3 W, l > 420 nm) and a magnetic stirrer. The generated hydrogen from the systems was measured at different time intervals by using a GC-14C system (Shimadzu), which was equipped with a 5 molecular sieve column (3 m 2 mm), a thermal conductivity detector, and N2 or Ar carrier gas. The amount of hydrogen was quantified by an external standard method. The TON was calculated based on the amount of added FeCl3. Electronic absorption spectra were recorded by using a HITACHI U3010 spectrophotometer. HRTEM was performed by using a JEM 2100F instrument operated at an accelerating voltage of 200 kV. Powder XRD patterns were collected by using a Bruker D8 Focus with CuKa radiation at (l = 1.54056 ). pH values were measured by using a Model pH S-3C meter (Mettler Toledo FE20, China). XPS data were obtained by using an ESCALa-b220i-XL electron spectrometer from VG Scientific using 300 W AlKa radiation. The binding energies were obtained with reference to the C 1s line at 284.8 eV. FTIR spectra were collected by using a Varian FTIR Excalibur 3100 plus with an average of 32 scans and a resolution of 4 cm 1 from 4000–500 cm 1.
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Received: January 17, 2014 Revised: April 24, 2014 Published online on && &&, 0000
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FULL PAPERS Shedding light on an iron catalyst: A bifunctional noble-metal-free system for the photoreduction of Fe3+ to Fe(0) nanoparticles in water and simultaneous highly efficient photocatalytic hydrogen evolution is established. The sizes of the Fe nanoparticles generated in situ are tuned by changing the sacrificial donor from triethylamine to triethanolamine and by the introduction of H2N-MCM-41 or graphene into the photocatalytic system.
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C.-J. Wang, S. Cao, B. Qin, C. Zhang, T.-T. Li, W.-F. Fu* && – && Photoreduction of Iron(III) to Iron(0) Nanoparticles for Simultaneous Hydrogen Evolution in Aqueous Solution
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