Solar Water Splitting and Nitrogen Fixation with Layered Bismuth

Dec 23, 2016 - This Account details our endeavors that leverage layered bismuth oxyhalides as photocatalysts for efficient solar water splitting and n...
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Solar Water Splitting and Nitrogen Fixation with Layered Bismuth Oxyhalides Jie Li, Hao Li, Guangming Zhan, and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental & Applied Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China CONSPECTUS: Hydrogen and ammonia are the chemical molecules that are vital to Earth’s energy, environmental, and biological processes. Hydrogen with renewable, carbon-free, and high combustion-enthalpy hallmarks lays the foundation of next-generation energy source, while ammonia furnishes the building blocks of fertilizers and proteins to sustain the lives of plants and organisms. Such merits fascinate worldwide scientists in developing viable strategies to produce hydrogen and ammonia. Currently, at the forefronts of hydrogen and ammonia syntheses are solar water splitting and nitrogen fixation, because they go beyond the high temperature and pressure requirements of methane stream reforming and Haber−Bosch reaction, respectively, as the commercialized hydrogen and ammonia production routes, and inherit the natural photosynthesis virtues that are green and sustainable and operate at room temperature and atmospheric pressure. The key to propelling such photochemical reactions lies in searching photocatalysts that enable water splitting into hydrogen and nitrogen fixation to make ammonia efficiently. Although the past 40 years have witnessed significant breakthroughs using the most widely studied TiO2, SrTiO3, (Ga1−xZnx)(N1−xOx), CdS, and g-C3N4 for solar chemical synthesis, two crucial yet still unsolved issues challenge their further progress toward robust solar water splitting and nitrogen fixation, including the inefficient steering of electron transportation from the bulk to the surface and the difficulty of activating the NN triple bond of N2. This Account details our endeavors that leverage layered bismuth oxyhalides as photocatalysts for efficient solar water splitting and nitrogen fixation, with a focus on addressing the above two problems. We first demonstrate that the layered structures of bismuth oxyhalides can stimulate an internal electric field (IEF) that is capable of efficiently separating electrons and holes after their formation and of precisely channeling their migration from the bulk to the surface along the different directions, thus enabling more electrons to reach the surface for water splitting and nitrogen fixation. Simultaneously, their oxygen termination feature and the strain differences between interlayers and intralayers render the easy generation of surface oxygen vacancies (OVs) that afford Lewis-base and unsaturated-unsaturated sites for nitrogen activation. With these rationales as the guideline, we can obtain striking visible-light hydrogen- and ammonia-evolving rates without using any noble-metal cocatalysts. Then we show how to utilize IEF and OV based strategies to improve the solar water splitting and nitrogen fixation performances of bismuth oxyhalide photocatalysts. Finally, we highlight the challenges remaining in using bismuth oxyhalides for solar hydrogen and ammonia syntheses, and the prospect of further development of this research field. We believe that our mechanistic insights could serve as a blueprint for the design of more efficient solar water splitting and nitrogen fixation systems, and layered bismuth oxyhalides might open up new photocatalyst paradigm for such two solar chemical syntheses. and organisms.2 Hence, developing viable strategies to split water into hydrogen and reduce nitrogen to ammonia is at the forefront of chemistry research. Currently, the commercialized approaches for hydrogen and ammonia production are methane stream reforming and Haber−Bosch reaction, respectively.3,4 However, methane stream reforming would emit CO2 as the byproduct and Haber−Bosch reaction adopts H2 as feedstock, while both of their operations necessitate high temperature and pressure. These imperfections disharmonize chemistry’s goals of being green, sustainable, and low energy-consuming. In contrast,

1. INTRODUCTION In response to the imminent energy crisis and increasing pollution issues, hydrogen and ammonia gradually stand out as the paramount chemical molecules that are capable of guiding the evolution of our human society toward clean, sustainability and carbon-neutral direction. For hydrogen, it is green, sustainable, and carbon-footprint-free with combustion product of only water, and has a high specific enthalpy of combustion, being a promising next-generation energy-carrier.1 Compared to hydrogen, ammonia contains large fraction of hydrogen energy, but is much easier condensed into liquid for storage and transportation. In addition, ammonia is the key intermediate for Earth’s nitrogen cycle, as it can serve as the building block to synthesize fertilizers and proteins to maintain the lives of plants © 2016 American Chemical Society

Received: October 19, 2016 Published: December 23, 2016 112

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photoabsorption and accelerating the electron transfer on the photocatalyst surfaces. Layered bismuth oxyhalides have the potential to tackle the issues of inefficient bulk charge separation and faint N2 affinity, as they possess an internal electric field to efficiently steer bulk charge separation and their surface structures favor the generation of oxygen vacancies that manifest strong affinity with N2.19−25 In this Account, we mainly present our group’s progress using layered bismuth oxyhalides for solar water splitting and nitrogen fixation. The emphasis of this Account is to elucidate how we overcome the limitations of poor bulk charge separation and faint N2 adsorption to achieve superior solar hydrogen and ammonia generation activities by engineering internal electric field and oxygen vacancy of bismuth oxyhalides. We end this Account with some remarks that delineate the challenges and opportunities in this research field.

nature, only using solar light as the energy input, enables water splitting by chloroplast and nitrogen fixation by nitrogenase, both at room temperature and atmospheric pressure.5,6 Inspired by these natural photosyntheses, numerous researchers turned to solar water splitting and nitrogen fixation, which employ solar light and semiconductor photocatalyst, respectively, to split water into hydrogen and to fix nitrogen to make ammonia.7−13 The first step of these two artificial photosyntheses is photoexcitation, in which the excitons are formed with holes still staying within valence band and electrons being excited to jump onto the conduction band (Figure 1). Next, dissociation of the excitons occurs, and

2. LAYERED BISMUTH OXYHALIDES Bismuth oxyhalides belong to a group of V−VI−VII ternary compound semiconductors with the general formula of [BilOmXn], where X = Cl, Br, and I. They usually crystallize into a tetragonal matlockite (PbFCl-type) structure (space group P4/nmm).19−22 These crystal structures energetically and thermodynamically favor the formation of layered configurations comprised of many [BilOmXn] monolayers; each monolayer consists of [BilOm] and [Xn] layers. Bismuth and oxygen within [BilOm] layers are connected via strong covalent bonding, while both of the interactions between [BilOmXn] monolayers and between [BilOm] and [Xn] layers are van der Waals forces.22−25

Figure 1. Solar water splitting and nitrogen fixation based on semiconductor photocatalysis.

electrons escape the hole trapping and diffuse across both the bulk and surface of photocatalysts to reach the reactive sites. Finally, H2O (or N2) adsorbed onto these sites is catalyzed by the surviving electrons into H2 (or NH3). Therefore, the key for solar water splitting and nitrogen fixation lie in seeking an efficient photocatalyst. The past 40 years have witnessed significant advances using TiO2, g-C3N4, CdS, SrTiO3, and (Ga1−xZnx)(N1−xOx) for solar water splitting; yet so far their activities are still thwarted by their inefficient steering of the electron transportation from the bulk to the surface.14−18 Furthermore, these photocatalysts usually bear poor binding with N2, leading to limited successes in solar nitrogen fixation. Various protocols, including facet tailoring, surface functionalization, doping, plasmonic modification, and heterojunction construction,1,7,8,15,17 were used to improve photocatalytic efficiency, mainly via modulating the

2.1. Tunable Photophysical Absorption and Photochemical Stability

For bismuth oxyhalides BilOmXn, their valence band is mainly occupied by O 2p and X δp (δ = 3, 4 and 5, corresponding to X = Cl, Br, and I, respectively) states and their conduction band by Bi 6p states. These unique electronic structures make their photoabsorption sensitive to the compositions, such as Bi/O/X ratios and halogen species. We found that increasing the atomic numbers of X could extend the photoabsorption edge from the ultraviolet (BiOCl) region to shallow-yellow-light (BiOBr) and red-light (BiOI) regions (Figure 2a).26 Recently, Scanlon et al. theoretically clarified that this halogen-dependent photoabsorption originated from the interplay of orbital and relativistic effects.49 The absorption edges of BiOCl and BiOBr could be expanded into near-infrared region via

Figure 2. Optical absorption spectra (a), alkalization-induced phase transformations (b), and band alignments (c) of layered bismuth oxyhalides. 113

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Figure 3. (a) Charge density contour plot and (b) electrostatic potential of Bi12O17Cl2 monolayers. (c, g) Side-view atomic-resolution aberrationcorrected HAADF-STEM images and (d−f, h−j) corresponding EELS elemental maps of Bi12O17Cl2 monolayers photodeposited with Pt and MnOx. Figures (a)−(j) reproduced with permission from ref 39. Copyright 2016 Nature Publishing Group. (k) IEF-mediated bulk-charge separation in carbon-doped Bi3O4Cl. Figure (k) reproduced with permission from ref 38. Copyright 2016 Wiley.

engineering of their surface oxygen vacancy.27−29 For BilOmCln and BilOmBrn, enhancing the ratios of Bi to X enables the aggrandizement of their optical absorption, and strengthens the hybridization of conduction band Bi 6p state with O 2p and X δp states to favor the electron migration to evade the hole trapping (Figure 2b, c).30−33 Besides photoabsorption, photostability is also tunable for bismuth oxyhalides. Their stability depends on the composition, preparation condition, structural dimension, and reaction environment.19,48 Although their stoichiometric forms (BiOCl, BiOBr, BiOI) are susceptible to photocorrosion, increasing Bi/ X ratio can largely improve the stability.30−32,34,37−39 Furthermore, bismuth oxyhalides are more stable in powdersuspended photocatalysis systems than in photoelectrochemical systems.50 The tunable photoabsorption and photostability render bismuth oxyhalides promising for solar water splitting and nitrogen fixation.

nanosheets was found to be proportional to their {001} facet exposure amounts. Motivated by this progress, we further increased Bi3O4Cl IEF magnitude by 126-fold via homogeneous carbon doping to substitute for Cl.38 Such giant IEF enhancement was arisen from the carbon doping induced enlargement of electrostatic potential differences between [Bi3O4] and [Cl] layers. Through femtosecond-resolved transient absorption spectroscopy to decode the electron transfer dynamics and Pt and MnOx photodeposition to directly image the destinations of electrons and holes,39 we clarified that, IEF is able to (1) rapidly dissociate the excitons to efficiently separate electrons and holes (Figure 3a, b), (2) drive the separated electron and hole, respectively, to [BilOm] and [Xn] layers (Figure 3c−j), and (3) localize these electrons and holes within [BilOm] and [Xn] layers to preclude their recombination during their transportation from the bulk to the surface (Figure 3k). By means of these functions, IEF enables highly directional charge separation, atomic-level charge transportation, and thus, more electrons migrate from the bulk to the surface where they take part in H2 and NH3 formation. All these knowledge lays the foundation of our next works on the full utilization of IEF for establishing highly efficient solar water splitting and nitrogen fixation by boosting bulk charge separation and transfer. As expected, IEF was also applied to improve the photocatalytic activity of other photocatalysts such as BiOIO3.36,44

2.2. Internal Electric Field

The characteristic of bismuth oxyhalides that enthralls photocatalysis researchers most, is the internal electric field (IEF) with the extraordinary capability to enable efficient charge separation and transfer from the bulk to the surface.34−36 Although many works including theoretical ones previously have predicted such importance of IEF, the fundamental issues, including the direction, the magnitude measurement, and the mechanism of how IEF improves charge separation and transfer, are not addressed, until the emergence of our group’s recent advances on engineering the IEF of bismuth oxyhalides. We first synthesized {001} and {010} faceted BiOCl nanosheets by pH modulation in 2012 and, by exploring the origin of their photoactivity difference, realized that the {001} facet exposure might benefit the use of IEF in a better manner to promote the charge separation and transportation.37 Following this finding that gave us a correlation of IEF with facet exposure, we tailored the layer numbers of Bi3O4Cl to tune the exposure percentages of its {001} facet by liquid exfoliation and epitaxial growth.30 We, for the first time, measured the magnitude variation of IEF by Kanata’s calculation model, with which the IEF magnitude of Bi3O4Cl

2.3. Surface Oxygen Vacancy

Another compelling feature of layered bismuth oxyhalides for solar chemical synthesis lies in the facile creation of oxygen vacancies (OVs) on their surfaces.23,27−29 OVs can accommodate the electron-accumulating Lewis-base and catalytically reactive unsaturated-unsaturated sites to guide the chemical reactions, which demands high thermodynamic activation energy on the perfect crystal surface, to proceed through other pathways requiring the low energy barrier. Compared with other groups’ attention to only exploiting OVs to widen the light absorption range and reinforce the charge transfer of bismuth oxyhalides, our group has devoted extensive efforts to divulging the generation origin, modulation route, and surface 114

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Figure 4. Surface OVs formation on layered bismuth oxyhalides.

3. SOLAR WATER SPLITTING WITH LAYERED BISMUTH OXYHALIDES Solar water splitting into hydrogen is recognized as the Holy Grail of the chemistry field.5,7 It uses solar light as the only energy input, semiconductor materials as the photocatalysts, and water as the proton source, with O2 as the only byproduct. Through solar water splitting, energy is transduced from solar energy into the chemical bonds of H2, opening up the dreamscape of superseding fossil fuels by carbon-free and sustainable hydrogen energy. During the past 40 years, although significant breakthroughs on solar water splitting have been achieved by TiO2, SrTiO3, (Ga1−xZnx)(N1−xOx), CdS, and g-C3N4, their further progresses are still impeded by their unsatisfactory bulk charge separation and transportation.14−18 In this section, we present our advances using the layered bismuth oxyhalide photocatalysts for solar water splitting, with a focus on elaborating how we improve the IEF to boost the bulk charge separation and transfer to establish highly efficient systems toward photocatalytic water oxidation and hydrogen evolution.

structure of OVs, with a focus on demystifying how OVs catalyze chemical reactions. We created OVs on bismuth oxyhalides mainly by UV irradiation and nonaqueous solvothermal synthesis.27−29 Using OV-rich BiOCl nanosheets, respectively, exposed by {001} and {010} facets, we found that these OVs descended from their layered structures with each [Bi2O2] layer sandwiched two adjacent [Cl] layers.28 In such layered structures, Bi and O within [Bi2O2] layers are linked via strong covalent bonding, while both of the interactions between [Cl−Bi−O2−Bi−Cl] monolayers and between [Bi2O2] and [Cl] layers are van der Waals forces. The strain differences between weak out-of-plane interactions and strong in-plane bonding make the atoms exposed on the surface easily flee from mother crystals when they are subjected to the exotic thermal and light energy (Figure 4). We have theoretically revealed that (001) and (010) faces, usually as the dominant facets of bismuth oxyhalides, were both terminated by O atoms. Therefore, OVs are facilely generated, either by UV irradiation to dislodge the O atoms, or by nonaqueous solvothermal treatment to drag away the O atoms via the strong interaction between the solvent ligand and the O atoms. Following this rule, Ye et al. increased the BiOCl photoreactivity for RhB decomposition via prolonged UV irradiation to augment OV amounts.23 Further, using OV-rich BiOCl nanosheets, we observed intriguing facet-dependent O2 activation behaviors. The OVs on {001} facet favored the O2 adsorption via an end-on configuration, where O2 is inclined to interact with the two nearest sublayer Bi atoms to extract one electron from the redistributed surface charges to yield ·O2−. Whereas for {010} facet, O2 would bind three Bi atoms near the OV sites to deliver a bridge-on coordination that facilitate the simultaneous extracting of two electrons to adsorbed O2 for the ·O22− formation. These mechanistic insights into surface OV chemistry of bismuth oxyhalide pave the way for our following works toward building up robust solar water splitting and nitrogen fixation systems by leveraging OV to ameliorate the surface chemical reactivity.

3.1. Water Oxidation

In an overall water splitting system powered by semiconductor photocatalysts, holes in the valence band first oxidize H2O to yield O2 via 4h+ + 2H2O → O2 + 4H+, and then the released H+ is reduced to H2 by the electrons in the conduction band via 4e− + 4H+ → 2H2.5,40 The key to water splitting lie in the first step of hole-mediated water oxidation, because it consumes holes to suppress the electron−hole recombination and helps protons to be extracted from water for H2 evolution.3 However, water oxidation into O2 is a process that involves the multihole/multi-H2O transfer; this makes its operation available only at prohibitively high overpotential. Therefore, enabling a robust solar water oxidation is the first yet the most important step for achieving high-performance solar overall water splitting. Our group’s recent advance in this research direction is to construct a efficient photocatalysis system that could directly split pure water into oxygen without any cocatalysts and electron scavengers, but only via giant enhancement of IEF.38 115

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Figure 5. IEF intensity variation (assuming BOC’s intensity to be “1”) (a), bulk charge separation efficiency (ηbulk) (b), and visible-light photocatalytic oxygen evolution (c) of carbon-doped Bi3O4Cl. Bi3O4Cl doped with 0.92%, 1.86%, and 3.16% carbon concentration were called BOCC1, BOC-C2, and BOC-C3, respectively. (d) Photocatalytic water splitting mechanism with carbon-doped Bi3O4Cl. Reproduced with permission from ref 38. Copyright 2016 Wiley.

Figure 6. Solar water splitting into hydrogen over carbon-doped BiOCl. Pristine and carbon-doped BiOCl with {001} or {010} facet exposure were called BOC-001, BOC-010, BOC-001HC, and BOC-010HC, respectively. Reproduced with permission from ref 42. Copyright 2015 Wiley.

photocatalytic O2-evolving rate of 0.9 mmol h−1 g−1 (Figure 5c, d). This O2 evolution activity from pure water, is still challenging for most of the existing semiconductor photocatslysts.15,17,41 Unfortunately, this system is unable to yield hydrogen, because the conduction band potential of carbondoped Bi3O4Cl is more positive than the hydrogen-evolving one. Despite this flaw, this system may serve as a sufficient proton-offering source to facilitate the construction of solar overall water splitting systems or of solar nitrogen fixation systems.

Using a general carbon doping strategy that we developed previously, we homogeneously incorporated carbon dopants into Bi3O4Cl to substitute for Cl. This doping could double the electrostatic potential differences between [Bi3O4] and [Cl] layers to increase the IEF by 126 folds (Figure 5a). Such giant IEF enhancement made more electrons and holes separated and conferred stronger confinement capability during these charge transfer from the bulk to the surface, thereby leading to a bulk charge separation efficiency (ηbulk) of up to 80%, a milestone for semiconductor photocatalyst (Figure 5b). Owing to such an ultrahigh ηbulk, more holes could be extracted from the bulk to the surface for water oxidation, thus allowing a 116

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Figure 7. (a) TEM image, (b) AFM image, (c) HAADF-STEM image, (d−h) corresponding EELS elemental maps, (j) crystal structure, (m) visiblelight photocatalytic hydrogen evolution, (n) hydrogen-evolving quantum yields, and (o) water splitting mechanism of Janus Bi12O17Cl2/MoS2 bilayer junctions (called BOC-MS) assembled by Bi12O17Cl2 monolayers (called 1L-BOC) and MoS2 monolayers (called 1L-MS). (j−l) Charge-flow processes within BOC-MS. (i) 3D topographic color-coded intensity map derived from image (c). Reproduced with permission from ref 39. Copyright 2016 Nature Publishing Group.

3.2. Hydrogen Evolution

with an asymmetric structure that could make full use of IEF to channel holes and electrons, respectively, to [Cl2] and [Bi12O17−x] end-faces.39 The stable existence of Bi12O17Cl2 monolayer was theoretically supported by Li’s work and they also predicted that single-layered bismuth oxyhalides with suitable band structure would be stable for photocatalytic water splitting.48 Then, we utilized the chemical reactivity of OVs, to assemble metallic MoS2 monolayers as the hydrogen-evolving catalyst, selectively onto [Bi12O17‑x] end-faces to precisely extract the electrons separated by IEF (Figure 7a−j). Simultaneously, OVs could mediate the chemical coordination of unsaturated Bi atoms near the OVs sites with the S atoms terminated on the MoS2 surface to form Bi−S bonds as the highways for the interfacial electron transfer from [Bi12O17‑x] end-faces to MoS2 monolayers (Figure 7k, l). Such a Janus bilayer design enables atomic-level control over charge separation, transportation and consumption, thus rationalizing a spectacular visible-light photocatalytic hydrogen-evolving rate of 33 mmol h−1 g−1 and a ultrahigh quantum yield of around 36% at 420 nm (Figure 7m−o). In terms of photocatalytic hydrogen evolution activity, this system is superior to all of MoS2-based, monolayer-based and bismuth oxyhalide-based systems, representing a quantum jump in precisely steering charge flow for solar water splitting.

Although the solar overall water splitting is regarded as an ideal approach for hydrogen production, this reaction is prohibitively uphill in thermodynamics and, so far, can be realized only with few photocatalysts. A promising alternative with the relatively lower thermodynamical barrier is to use photocatalytic reductive half-reaction for hydrogen evolution, which consists of semiconductor photocatalyst converting solar light into electrons and holes, sacrificial electron donor prohibiting the electron−hole recombination, and noble-metal cocatalyst trapping and thus utilizing electrons to catalyze the proton reduction.7 However, some obstacles that plague the further progress of such a system still remain, including the disability of semiconductor photocatalysts to precisely steer the bulk charge separation and transfer, the increased cost with the use of noble-metal as cocatalyst, and the limited capability of sacrificial electron donor to only prevent the electron−hole recombination on the photocatalyst surface. Our initial effort on tackling these issues is to use carbondoped BiOCl nanosheets as the photocatalyst, NiOx as the noble-metal free cocatalyst, and triethanolamine (TEOA) as the hole scavenger.42 By carbon doping to increase the optical absorption and enhance the IEF to boost the bulk charge separation, this system could yield a maximum H2-evolving rate of 0.418 mmol h−1 g−1 under visible light (Figure 6), superior to other bismuth oxyhalide-based systems.43−46 However, in this system, carbon doping only could improve the IEF to a moderate level, the electrons and the holes separated by IEF would rejoin at the sites between the two neighboring carbondoped BiOCl monolayers, and the random distribution of NiOx on carbon-doped BiOCl disfavored the electron extracting. In view of these shortcomings, we, via chemical liquid exfoliation, gained OV-rich Bi12O17Cl2 monolayer nanosheets

4. SOLAR NITROGEN FIXATION WITH LAYERED BISMUTH OXYHALIDES The groundbreaking study on solar nitrogen fixation date back to 1977, when Schrauzer et al. employed TiO2 to fix nitrogen under UV light.13 However, since then, the development of solar nitrogen fixation is not in a satisfactory pace. This is because, in contrast to solar water splitting, solar nitrogen fixation is more challenging; the potential for N2 reduction is 117

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Figure 8. Adsorption model (a) and charge density difference (b) of N2 on the OV of BiOBr {001} facet. (c) N2 activation energy barrier required for pure and OVs-rich BiOBr. (d) Visible light nitrogen fixation over pure (BOB-001-H) and OVs-rich (BOB-001-OV) {001} faceted BiOBr. (e) Multicycle N2 fixation with BOB-001-OV. (f) Electron transfer processes during OVs-mediated nitrogen fixation. Reproduced with permission from ref 27. Copyright 2015 American Chemical Society.

Figure 9. Free energy diagrams of OV-mediated N2 fixation on the (001) and (010) surfaces of BiOCl. Reproduced with permission from ref 47. Copyright 2016 Royal Society of Chemistry.

scientists to impose a renewed focus on solar nitrogen fixation. Although some remarkable achievements have been made by using diamond, plasmonic Au/TiO2, chalcogels, and nitrogenase/CdS for photocatalytic ammonia synthesis,9−12 the

more negative, while the dissociation enthalpy of NN triple bond of N2 is as high as 962 kJ mol−1.2 Until recent years, prodigious breakthroughs in solar chemical synthesis including solar water splitting and photoredox organic synthesis, enthuse 118

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activities have interested many scientists from chemistry, physics, material, and engineering communities, to use bismuth oxyhalides and beyond, for photocatalytic hydrogen and ammonia evolution and beyond. The design concepts and catalysis mechanisms delivered by bismuth oxyhalides might serve as the general guideline to inspire new ideas to further the development of solar water splitting and nitrogen fixation. Despite these advances and effects, bismuth oxyhalides still face challenges before their continuous contribution to solar water splitting and nitrogen fixation. First, there is a long desire to achieve efficient photocatalytic hydrogen evolution without use of any cocatalysts, by OV tuning or other defect engineering or beyond, some strategies that can improve surface proton-catalyzing activity. Second, solar overall water splitting using bismuth oxyhalides is still a challenge. Third, using some in situ characterizations, such as extended X-ray absorption fine structure spectroscopy, aberration-corrected HAADF-STEM, and atomic pair distribution functions, to monitor the catalyst structure variation and the reaction intermediates is highly required. Fourth, when bismuth oxyhalides are used for photoelectrocatalysis, methods including introducing a passivation layer and coating carbonbased nanomaterials should be adopted to resist photocorrosion. Fifth, taking advantage of relativistic effects may lead to new performance breakthrough for water splitting and nitrogen fixation with bismuth oxyhalides. Sixth, from a longterm perspective for industrial application, it is necessary to engineer bismuth oxyhalides, possibly by synergistically tuning photochemical stability, relativistic effects, and collaboration between internal and external electric fields, for solar natural water splitting and ammonia synthesis with atmospheric N2.

poor N2 activation ability is still the Achilles heel for solar nitrogen fixation. In this section, we showcase our progress using the layered bismuth oxyhalide for solar ammonia synthesis, and mechanistically unveil how we leverage OVs to boost ammonia-evolving photoreactivity. 4.1. NN Bond Activation

Our recent investigation on molecule oxygen activation with BiOCl OVs offers us the inspiration to activate the NN triple bond in N2 by OV chemistry.28 By employing our previously developed solvothermal synthesis, we created OVs on the BiOBr {001} facets.27 We theoretically revealed that OVs of BiOBr {001} facets could activate N2 by prolongating the N N triple bond from 1.078 Å in the original N2 to 1.133 Å in the adsorbed N2 onto OVs via an end-on configuration (Figure 8a, b), and this coordination model could be verified by the peak at 265 °C in the temperature-programmed desorption spectrum of N2. This activation was found to be arisen from the Lewisbase and unsaturated-unsaturated sites, both bestowed by BiOBr {001} facet OVs. By means of this activation, electrons from the conduction band of photoexcited BiOBr could be facilely injected into the π antibonding orbitals of N2 adsorbed onto the OVs (Figure 8f). As a result, N2 reduction to ammonia catalyzed by BiOBr OVs required ultralow energy barrier and could be initiated with a rate of 104.2 mmol h−1 g−1 under visible light without cocatalyst and organic hole scavenger (Figure 8c−e), presenting a quantum leap in solar nitrogen fixation. 4.2. Proton-Assisted Electron Transfer

Although solar nitrogen fixation by BiOBr {001} facet OVs is efficient, it remains unclear why nitrogen reduction into ammonia catalyzed by its OVs demands low energy barrier. To solve this problem, we further used OV-rich BiOCl exposed by {001} and {010} facets as a platform to gain the mechanistic insights into the ammonia-evolving process. 47 By the theoretical simulation and in situ Fourier transform infrared (FTIR) spectra, we found that OVs-catalyzed N2 conversion underwent several low-energy reaction steps of successive proton-coupled electron transfer, and the detailed reaction pathway depended on the surface structures of OVs. OVs on BiOCl {001} facets favored the adsorption of N2 via an end-on coordination model that energetically resulted in the formation of NH3 via an asymmetric distal pathway: N2 → N-NH3 → N + NH3 → 2NH3 (Figure 9a). While for the BiOCl {010} facet, N2 adsorbed on its OVs via a side-on bridging configuration to prefer the N2H4 generation following a symmetric trajectory: N2 → N2H3 → N2H4 (Figure 9b). These mechanistic insights widen our understanding of solar ammonia synthesis process, and are instructive to establishing more efficient solar nitrogen fixation systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lizhi Zhang: 0000-0002-6842-9167 Notes

The authors declare no competing financial interest. Biographies Jie Li gained his Ph.D. degree from Central China Normal University (CCNU) in 2016 under the supervision of Professor Lizhi Zhang. Currently, he is a postodoctoral fellow at University of Toronto with Professor Edward H. Sargent. His research interests focus on engineering the layered-structure chemistries of two-dimensional nanomaterials for solar energy conversion and storage. Hao Li is currently pursuing his Ph.D. degree in physical chemistry at CCNU under the supervision of Professor Lizhi Zhang. His interests focus on photocatalytic nitrogen fixation using bismuth oxyhalides.

5. CONCLUSION AND OUTLOOK Solar water splitting and nitrogen fixation are important technologies for solar energy conversion and storage, and have gradually been at the forefronts of chemistry research. Among the promising semiconductor photocatalysts for solar hydrogen and ammonia syntheses are layered bismuth oxyhalides. Their layered structures confer two most fascinating characteristics of IEF and OV. Modulations of IEF and OV allow bismuth oxyhalide photocatalysts to overcome the limitations faced by other photocatalysts, thereby enabling efficient and durable systems for solar water splitting and nitrogen fixation. These supernormal attributes and photo-

Guangming Zhan is currently pursuing his Ph.D. degree in physical chemistry at CCNU under the supervision of Professor Lizhi Zhang. His interests focus on photocatalytic water splitting and nitrogen fixation using bismuth oxyhalides. Lizhi Zhang received his Ph.D. degree in environmental science from Chinese University of Hong Kong, China in 2003. After his postdoctoral research in department of chemistry, Chinese University of Hong Kong, he joined in college of chemistry, Central China Normal University as a full professor of chemistry since 2005. He worked at Max-Planck Institute of Colloid and Interface as an 119

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Accounts of Chemical Research

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Alexander von Humboldt Scholar from 2006 to 2007. His research interests include developing layered bismuth oxyhalides for solar water splitting and nitrogen fixation and utilizing iron cycle for pollution control and environmental remediation. He won National Natural Science Funds for Distinguished Young Scholars of China in 2014, and was appointed as the Changjiang Scholar Distinguished Professor of the Ministry of Education of China in 2015.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Funds for Distinguished Young Scholars (Grant 21425728), National Basic Research Program of China (973 Program) (Grant 2013CB632402), National Key Research and Development Program of China (Grant 2016YFA0203002), and National Natural Science Foundation of China (Grant 51472100).

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ABBREVIATIONS IEF, internal electric field; OV, oxygen vacancy; HAADFSTEM, high-angular annular dark field scanning transmission electron microscopy; EELS, energy loss spectroscopy

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