Hybrid photocatalyst constructed using polymerized metal complexes and semiconductor powders for photocatalytic carbon dioxide reduction

Abstract

Hybrid photocatalysts prepared by the adsorption of metal complexes on semiconductors via anchoring groups are among notable photocatalysts for selective CO₂reduction using abundant electron donors.However，their photocatalytic activities are limited by their low adsorption amounts and strength.In this study， we demonstrate the reductive polymerization of vinyl groups in metal complexes triggered by excited electrons on semiconductors asanew immobilization method.This approach significantly enhanced the adsorption amount and durability，thus increasing the photocatalytic performance。

Hybrid photocatalystructed with metal complexes and semiconductors are notable photocatalysts for selective CO₂reduction using abundant electron donors.We herein report a new hybridization method using reductive polymerization of vinyl groups in metal complexes triggered by excited electrons on semiconductors.This approach significantly enhanced the adsorption amount and durability of metal complexes，thus increasinag the photoration。

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Global warming triggered by the enormous consumption of fossil fuels represents a significant environmental issue globally。^1个New scientific and industrial approaches for building a carbon-neutral society have been intensively studied and developed in recent years to tackle this issue.Notably，photocatalytic carbon dioxide（CO₂）可回收的可回收的chemical reaction as it can decrease atmospheric CO₂concentration while yielding valuable carbon compounds for industrial and human activities。^{2单击功能区上，3}Consequently，various types of effective（photo）catalysts have been reported for CO₂reduction，e.g.metal complexes，^4-8semiconductors，^9-13metal particles，^{14单击功能区上，15}and enzymes。^16–18oreover，hybridizing two of the aforementioned materials often exerts synergistic effects toward achieving remarkable photocatalytic activities.For instance，semiconductor/multinuclear metal complex-type hybrid photocatalysts，which are prepared by immobilizing a metal complex ona semiconductor particle（orelectrode）via anchoring groups in the ligand（Fig。1a），have attracted attention as effective photocatalysts for CO₂reduction that can utilize abundant electron donors，such as amines and alcohols。^19-29The semiconductor units in semiconductor particle/multinuclear metal complex-type hybrid photocatalysts photocatalytically oxidize the electron donors and reduce the adjacent metal-complexphotosensitizer in the excited state.The produced one-electron-reduced specof the photosensitizer in the excites states the bridging ligand（BL），绝缘绝缘性CO₂reduction on the catalyst unit.This electron relay process accompanied by the two-step excitation can be considered an artificialZ-scheme单击功能区上，which results in the high reducing and oxidizing powers of the photocatalysts。

However，the desorption of metal complexes from the semiconductor surfaces is afrequently observed phenomenon，accounting for the low durability of semiconductor/metal complex-type hybrid photocatalysts。^{21单击功能区上，23}This is mainly because the adsorption by the anchoring groups（e.g.phosphonic acid and carboxylic acid groups）is not sufficiently strong to maintain several hours of immobilization in reactions solutions under photoirradiation。In addition to the low durability，the immobilization by the anchoring groups limits the amounts of adsorbed metal complexes，lowering the visible-light absorptivity of the metal-complex units.Ishitani and coworkers，including two of the authors of this paper，recently developed efficient and high ly durable molecular photocathodes byimmobilizing metal complexes on a semiconductor electrode（Fig。1b）。^{30单击功能区上，31}To prepare these photocathodes，a Ru photosensitizer complex bearing phosphonic acid groups（P）as anchoring moieties and vinyl groups（V₂）as reactive points for electrochemical polymerization was first adsorbed onto the semiconductor electrode。电火花焊接法V₂，and the complexes were reduced using the electrode as aworking electrode and by repeatedly applying a negative potential₃）to trigger the vinyl polymerization of the metal complexes。Furthermore，the adsorption amount，visible-light absorptivity，and adsorption durability of the metal complexes were significantly improved by the multilayer polymerization，which increased the hydrophobicity and facilitated multipoint interaction，enhancing the photocatalytic activity of the photocathodes.In addition to the aforementioned system，several other relevant studies have demonstrated the polymerization of metal complexes on semiconductors asan effective preparation method for highly active molecular（photo）electrodes。^32–41

Inspired by these studies，we applied vinyl polymerization to the preparation of semiconductor particle/metal complex-type hybrid photocatalys.To trigger the polymerization on the semiconductor particles，visible light was irradiated in the presence of the reductants to reduce the metal complexes withV₂by the excited electrons on the semiconductors and/or photo-inducedZ-scheme electron transfer，which includes the excitations of the semiconductors and metal complexes（Fig。1c）。Employing this method，we prepared semiconductor particle/polymerized metal complex-type hybrid photocatalysts with enhanced photocatalytic activities。

Asa typical example in this study，we selected nanosheet-type polymeric carbon nitride（PCN）and Ru binuclear complex units as the semiconductor particle^{25单击功能区上，42}and CO₂photoreduction unit，^20-22respectively，which are reportedly among the most effective combinations for hybrid photocatalysts toward CO₂reduction.We prepared the hybrid photocatalysts comprising PCN particles and polymerized metal complexes（i.e。PCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）and，andPCN/P–Ru–poly–Ru–BL–Ru’（One Step））by two methods，as illustrated in Fig。2a and 2b单击功能区上，respectively.In both preparation methods，a Ru complex bearingPand，andV₂（P-Ru-V₂）as a starting point of the polymerization was first adsorbed onto the light yellow PCN powder（PCN/P–Ru-V₂，Fig。2a）by suspension in acetonitrile containing PCN andP-Ru-V₂打开；the maximum adsorption amount ofP-Ru-V₂was5.9µmol/g，which is similar to that of the Ru binuclear complex（P-Ru-BL-Ru'，6.1µmol/g）used to prepare the reported hybrid photocatalyst illustrated in Fig。2c（PCN/P-Ru-BL-Ru'）。

目的地，目的地PCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）单击功能区上，PCN/P–Ru-V₂水上回填网V₂and aBL（V₂–Ru–BL，Method1in Fig。2a）。To trigger the polymerization of the metal complexes withPCN/P–Ru-V₂单击功能区上，可观测的照明线路PCN/P–Ru-V₂整体式炉排控制装置V₂–Ru–BL.After10 min of irradiation，the resulting reddish orange solid（PCN/P–Ru–poly–Ru–BL）was obtained by filtration，并用过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载过载V₂–Ru–BLwas29µmol/g，as determined using the ultraviolet-visible（UV-Vis）absorption spectra of the filtrate（Supporting Information；the suitable irradiation time was determined from the results of reactions using[Ru（vbpy）₃（PF）_6个）₂（vbpy4-methyl-4'-vinyl-2,2'-bipyridine）asa model complex。The detail is discussed in supporting information）.In total，35Ⅹmol/g Ru tris（bipyridine）-type photosensitizer units were introduced intoPCN/P–Ru–poly–Ru–BL.The conduction band minimum of PCN was reported to be-2.1±0.1 V vs.Ag/AgNO₃单击功能区上，⁴³and the reduction wave ofP-Ru-V₂was observed in the cyclic voltammogram between-1.5 and-1.9 V；³¹在PCN probably proceeded to the excited and ground states ofP-Ru-V₂，resulting in relatively efficient polymerization.The introduction of the catalyst units（Ru'）toPCN/P–Ru–poly–Ru–BLto preparePCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）水上引出线连接PCN/P–Ru–poly–Ru–BLand[Ru（CO）]₂Cl₂]_nas a precursor complex。³¹Thereference hybrid photocatalyst，PCN/P–Ru–poly–Ru–BL–Ru’（One Step）单击功能区上，水上预应力发电站PCN/P–Ru-V₂和运行双轮压配合V₂and aRu'单位V₂–Ru–BL–Ru′打开；Method2in Fig。2b）。The adsorbed amount ofV₂–Ru–BL–Ru′was estimated to be24µmol/g.Consequently，the polymerization method deployed in this study increased the adsorption amounts by approximately four or five times。

The obtainedPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）and，andPCN/P–Ru–poly–Ru–BL–Ru’（One Step）samples were analyzed by UV-Vis diffuse reflectance spectroscopy（DRS）and Fourier-transform infrared（FT–IR）spectroscopy using the potassium bromide tablet method。In the DRS spectra，PCN exhibited a steep absorption edge around 450瀹nm，with a tail extending to 600瀹nm（Fig。3aand，andSupplementary Fig.S1）。After immobilizing the metal complexes，anew absorption band，which was mainly derived from the metal-to-ligand charge transfer（MLCT）absorption band of the Ru tris（bipyridine）units was observed asa shoulder peak around 460nm。⁴⁴The MLCT absorption bands were more evident using the polymerization method.The FT–IR spectra ofPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）and，andPCN/P-Ru-BL-Ru'revealed two characteristic absorption bands derived from the dicarbonyl complex units around2060and1990炻cm^-1（Fig。3b）。³¹The absorption bands ofPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）焊缝厚度PCN/P-Ru-BL-Ru'单击功能区上，reflecting the larger amounts of adsorbed complexes.Interestingly，no CO stretching band was observed in the FT–IR spectrum ofPCN/P–Ru–poly–Ru–BL–Ru’（One Step）.This result indicated that the carbonyl ligands were dissociated from the catalyst-complex units during polymerization to preparePCN/P–Ru–poly–Ru–BL–Ru’（One Step）单击功能区上，probably via photochemical decarbonylation reactions。⁴⁵

We also evaluated the adsorption durability of the metal complexes in the hybrid photocatalysts by suspending them in a mixture ofN单击功能区上，N-dimethylacetamide（DMA）and triethanolamine（TEOA），which is an effective mixed solvent for photocatalytic CO₂reduction。⁴⁶After suspending them overnight in the dark and filtering off the solid，the filtrate was analyzed by UV-Vis absorption spectroscopy.RegardingPCN/P-Ru-BL-Ru'，we detected evident MLCT absorption bands of the Ru tris（bipyridine）moieties（Supplementary Fig.S4C）。The absorbance at462 nm was49%of the simulated value when all of the Ru complexes were desorbed from PCN。This result indicated that half of the Ru complexes would be desorbed from the PCN surface in the reaction suspension for photocatalytic reactions.EmployingPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise），we observed a similar absorption spectrum despite the much higher amount of adsorbed Ru complexes（Supplementary Fig.S4A）。Theestimated desorption ratio was only5%.Additionally，PCN/P–Ru–poly–Ru–BL–Ru’（One Step）exhibited alow desorption ratio（14%，Supplementary Fig.S4B）。These results indicated that the adsorption durability was enhanced by the polymerization method。

Next，the photocatalytic activity of the hybrid photocatalysts was examined byirradiating the DMA–TEOA suspensions（4:1 \u003; v/v）containing a hybrid photocatalyst with visible light under a CO₂atmosphere for 6 \u003; h。In all cases，we detected CO，HCOOH，and a small amount of H₂after the reactions（Fig。4a）。ForPCN/P-Ru-BL-Ru'，1.6and2.1µmol CO and HCOOH，respectively，were produced；these values are similar to those of the reported system。²¹Conversely，PCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）produced approximately four or five times more products（i.e.6.7and11.1），respectively）.This increase in the photocatalytic activity was likely due to the large adsorption amount of the Ru complexes.Notably，PCN/P–Ru–poly–Ru–BL–Ru’（One Step）yieled similar or slightly lower amounts of the products compared withPCN/P-Ru-BL-Ru'（i.e.1.8and1.3µmol CO and HCOOH，respectively）.Despite the larger adsorption amount of the metal complexes，the lower catalytic activity ofPCN/P–Ru–poly–Ru–BL–Ru’（One Step）从左到右，从右到右Ru'，as aforementioned。

火焰，火焰4bshows the time courses of the amount of reduction products.When usingPCN/P-Ru-BL-Ru'，the production of CO and HCOOH proceeded with relatively high efficiency but almost stopped within1炻h。空气质量控制₂还原（Φ）_CO + HCOOH）was0.04%。The photocatalysis byPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）预成形高度计（Φ）_CO + HCOOH=0.1%）and continued for6 h。为discuss the reason for the different photocatalytic activities，we measured the DRS of the photocatalysts after the reaction（Supplementary Fig.S5）。In the spectra ofPCN/P-Ru-BL-Ru'，the shoulder peaks attributed to the MLCT absorption bands around460Ⅹnm almost disappeared within1Ⅹh of the reaction（Supplementary Fig.S5B）。Conversely，employingPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise），the absorbance at the MLCT absorption bands decreased，although it was evidently detected even after irradiation for over6炻h（Supplementary Fig.S5A打开；the desorption ratios under photoirradiation were larger than those under the dark.Although the investigation of the reason is ongoing，the excitation and reduction of metal complexes and/or semiconductors might promote the desorption of metal-complex polymers[cf.Hanson et al。⁴⁷and Sahara et al。²³]。The production of bubbles of the gaseous products from the semiconductor surfaces[mainly H₂[might be another possible reason for promoting the desorption of polymer catalysts[cf.Sato et al。³²]），indicating that the adsorption durability of the metal complexes under irradiation was also enhanced by polymerization.The FT–IR spectrum changes suggested the decomposition of the catalyst units after irradiation for3邛h，probably leading to the decrease in the photocatalytic activity ofPCN/P–Ru–poly–Ru–BL–Ru’（Stepwise）（Supplementary Fig.S6）。We also revealed that the too-large adsorption amount of metal complexes led to lower photocatalytic activity（Supplementary Fig.S7）likely because the coverage of PCN surfaces by the metal-complex polymers suppressed the electron infection to PCN from TEOA。

As another example of hybrid photocatalysts prepared by the immobilization method deployed in this study，we also prepared hybrid photocatalysts bearingReunits（PCN/P–Ru–poly–Ru–BL–Re），which is an analog ofPCN/P–Ru–poly–Ru–BL–Ru’（One Step）（Supplementary Fig.S8）。The adsorption amount of the Ru–Re binuclear complexes in the polymerization step was28µmol/g，which is similar to the value obtained using Ru–Ru binuclear complexes.Photocatalytic CO₂减压装置PCN/P–Ru–poly–Ru–BL–Rerevealed selective CO formation.The amount of CO produced after irradiation for6炻hwas approximately two times that produced using the corresponding model photocatalysts prepared without polymerization（PCN/P-Ru-BL-Re单击功能区上，Supplementary Fig.S8）。These results indicated that the immobilization method deployed in this study can be applied to the preparation of various hybrid photocatalysts with different catalyst units.The polymerization methis study could also be applied to hybrididizingBiVO_4个/P–Ru–V₂and，andV₂–Ru–BL–Reto prepareBiVO_4个/P–Ru–poly–Ru–BL–Re（Supplementary Fig.S9），BiVO of even though the reducing power of_4个is not strong enough to reduce the ground state of the Ru units.This result suggests that the photochemical polymerization can be triggered via Z-scheme electron transfer.Unfortunately，however，BiVO_4个/P–Ru–poly–Ru–BL–Redid not show photocatalytic activity for CO₂reduction.The investigation of the application scope of the preparation methis study and detailed photocatalytic activities of hybrid photocatalysts are undergoing。

In conclusion，we applied the vinyl polymerization of metal complexes triggered by excited electrons from the semiconductor to the immobilization of metal complexplexes on semiconductor particles to prepare hybrid photocatalys.The obtained hybrid photocatalystoalystoxhibited enhanced adsorptionamounts，vivivitery，sorpy， and adsorption durability of the metal complexes.Particularly，employing the stepwise preparation method，i.e.introducing the catalyst units after the photochemical polymerization to avoid decarbonylation， the photocatalytic activity increased significantly.Our findings can be applied to the design of anew series of hybrid photocatalysts for CO₂reduction。

Acknowledgments

We thank Dr.Shigeo Satokawa（Seikei University）for the DRS measurement and Dr.Noritaka Sakakibara and Mr.Mitsuhiko Shizuno（Tokyo Institute of Technology）for the preparation of PCN。

Supplementary data

Supplementary material可操作，可操作Chemistry Lettersonline。

Funding

（JSPS）Grants-in-Aid for Scientific Research（KAKENHi，Grant numbers JP22K14715，JP22H05148），and a grant from Seikei University。

参考，参考

Author notes