Van Der Waals gap-rich BiOCl atomic layers realizing efficient, pure-water CO2-to-CO photocatalysis

Photocatalytic CO2 reduction (PCR) is able to convert solar energy into chemicals, fuels, and feedstocks, but limited by the deficiencies of photocatalysts in steering photon-to-electron conversion and activating CO2, especially in pure water. Here we report an efficient, pure water CO2-to-CO conversion photocatalyzed by sub-3-nm-thick BiOCl nanosheets with van der Waals gaps (VDWGs) on the two-dimensional facets, a graphene-analog motif distinct from the majority of previously reported nanosheets usually bearing VDWGs on the lateral facets. Compared with bulk BiOCl, the VDWGs-rich atomic layers possess a weaker excitonic confinement power to decrease exciton binding energy from 137 to 36 meV, consequently yielding a 50-fold enhancement in the bulk charge separation efficiency. Moreover, the VDWGs facilitate the formation of VDWG-Bi-VO••-Bi defect, a highly active site to accelerate the CO2-to-CO transformation via the synchronous optimization of CO2 activation, *COOH splitting, and *CO desorption. The improvements in both exciton-to-electron and CO2-to-CO conversions result in a visible light PCR rate of 188.2 μmol g−1 h−1 in pure water without any co-catalysts, hole scavengers, or organic solvents. These results suggest that increasing VDWG exposure is a way for designing high-performance solar-fuel generation systems.


Supplementary Figures
Supplementary Figure 1. Schematic illustration of the structures of conventional 2D graphene-analogues and our developed 2D graphene-analogue. Conventional 2D graphene-analogues have a preferential exposure of (001)-faceted nanosheets with VDWGs on their lateral facets. This means that the exposure ratio of VDWGs in conventional 2D graphene-analogues is very low. Our developed 2D graphene-analogue has abundant VDWGs on the 2D basic plane. It is worth noting that, although exposing VDWGs on 2D basic planes has been enabled on BiOCl nanosheets with thinkness larger than 35 nm, we deliver the report that realizes the exposure of VDWGs on the dominant facet of an ultrathin nanosheet with thickness less than 3 nm.  XPS result shows that CBOC-VDWGs-O2 owns no carbon species. The TEM images indicate that, even using the syngas-synthesis-like reaction-driven, gas-phase exfoliation, CBOC-VDWGs-O2 cannot be exfoliated into ultrathin nanosheet.
These results demonstrate the crucial role that the carbon species play in the process of our new exfoliation strategy.  Bi3O4Cl-VDWGs-AL has the morphology of an ultrathin nanosheet. Clear VDWGs were observed on the two-dimensional facets. These results demonstrate that our syngas-synthesis-like reaction-driven, gas-phase exfoliation can be applicable to Bi3O4Cl.  BiOBr-VDWGs-AL has the morphology of an ultrathin nanosheet. Clear VDWGs were observed on the two-dimensional facets.
These results demonstrate that our syngas-synthesis-like reaction-driven, gas-phase exfoliation can be applicable to BiOBr.
Carbon-modified BiOBr was synthesized by solvothermal treatment of ethylene glycol (pH = 6.5) containing glucose, Bi(NO3)3•5H2O, and KBr at 160 °C for 18 hours. The carbon-modified BiOBr were then subjected to the syngas-synthesis-like reaction-driven, gas-phase exfoliation with the operating parameters with which BOC-VDWGs-AL was synthesized. The BiOI-VDWGs-AL has the morphology of an ultrathin nanosheet. Clear VDWGs were observed on the two-dimensional facets.
These results demonstrate that our syngas-synthesis-like reaction-driven, gas-phase exfoliation can be applicable to BiOI.
Carbon-modified BiOI was synthesized by hydrothermal treatment of the mixed solution (pH = 6.5) of water and ethanol containing glucose, Bi(NO3)3•5H2O, and KI at 120 °C for 12 hours. The carbon-modified BiOI were then subjected to the syngas-synthesis-like reaction-driven, gas-phase exfoliation with the operating parameters with which BOC-VDWGs-AL was MoS2-VDWGs-AL has the morphology of an ultrathin nanosheet. Clear VDWGs were observed on the two-dimensional facets.
These results demonstrate that our syngas-synthesis-like reaction-driven, gas-phase exfoliation can be applicable to MoS2.
Carbon-modified MoS2 was synthesized by hydrothermal treatment of the mixed solution of glucose, (NH4)6Mo7O24· 4H2O, and thiourea at 220 °C for 36 hours. The carbon-modified MoS2 were then subjected to the syngas-synthesis-like reaction-driven, gas-phase exfoliation with the operating parameters with which BOC-VDWGs-AL was synthesized.
Length ( Meanwhile, the VBs of BOC−VDWGs−76, CBOC−VDWGs and BOC−VDWGs−AL were measured by ultraviolet photoelectron spectra (UPS) because the correlation between the photoelectrode potential and the absolute potential of electrons has already been established (Pure Appl. Chem. 1986, 58, 955-966;J. Phys. Chem. B 2003, 107, 1798-1803. UPS was carried out in an ultrahigh vacuum (UHV) apparatus of a PHI5000 VersaProbe III (Scanning ESCA Microprobe) electron energy spectrometer with a base pressure of < 5 × 10 −2 mbar. The sample was cleaned several times by argon−ion sputtering (1000 eV, 60 min) to remove the surface contaminants. UPS was measured using He I excitation ( to N2 and air (O2 + N2) to bubbled CO2.
In order to check the effect of adventitious carbon on CO2 reduction products, we monitored the GC signals variation of detectable gases in our photoreactor during the whole reaction process because the permeation of trace gas into the photoreactor would result in the sharp variation of GC signals. The whole operating process consisted of three sections: (i) Vacuum treatment (from B to H): Before the vacuum treatment (B), the peaks of air (O2 and N2) emerged simultaneously and no peak of CO2 could be found, whose volume fraction in air is less than 0.03% (Supplementary Figure   26b). The ratio of N2 and O2 in air is 3.51:1, corresponding to their theoretical ratio in air (Supplementary Figure 26c). After the vacuum treatment for 5 min (C), we noticed that the peaks intensity of O2 and N2 decreased obviously while the ratio of O2 to N2 almost remains unchanged. With the prolonged vacuum treatment time (from D to G), the peaks intensity of O2 and N2 decreased persistently while the ratio of O2 to N2 is still close to their theoretical ratio in air. No obvious GC signals of O2 and N2 could be observed with the further increase of vacuum time (H), suggesting that the residual air in the photoreactor was almost completely removed.
On-line apparatus for PCR experiment a c b S29 (ii) Holding vacuum (from I to J): In order to check the air impermeability of our measurement setup for a long time, we hold the vacuum for another two hours (I and J). Excitingly, the peaks intensity of O2 and N2 still kept unchanged and the ratio of O2 to N2 is constant, manifesting the superior air impermeability of our measurement setup.
(iii) Bubbling CO2 (from K to L): With the bubbling of high-purity CO2 gas through the solution for 60 min, a prominent peak of CO2 emerged at 15.93 min (K). Although the peaks of O2 and N2 increased slightly, the ratio of air (O2 and N2) to CO2 is less than 0.004:1, suggesting that the bubbling of high-purity CO2 gas could not bring air into the photoreactor. After holding the pressure of high-purity CO2 gas in the photoreactor for another 180 min (L), the peaks intensity of detectable gases (O2, N2 and CO2) and ratio of air (O2 and N2) to CO2 still remained unchanged, excluding the possibility that exotic gases permeated into the measurement setup.      BiOCl-AL-1 was synthesized by ultrasonication (driven by an ultrusonic cell disrupter system) of BiOCl for 12 hours.
BiOCl-AL-2 was synthesized by hydrothermal treatment of the mixed solution of mannitol, Bi(NO3)3•5H2O and hexadecyltrimethylammonium chloride at 160 °C for 6 hours.    and triethanolamine (as hole scavenger). The theoretical positron lifetimes in our cases were firstly calculated based on the atomic superposition (ATSUP) method [J.
Phys . F 1983, 13, 333;J. Phys.: Condens. Matter 2007, 19, 176222]. Considering that the non-self-consistent superposition of free atom electron density and Coulomb potential in the absence of the positron, the electron density and the positron crystalline Coulomb potential were constructed. The electron-positron enhancement factor was used in the positron lifetime calculations, which was described within the generalized gradient approximation by Barbiellini et al [Phys. Rev. B 1996, 53, 16201]. Once the collected PAS data fitted into two or four positron lifetime components, an incorrect program prompt would pop up.

Supplementary Methods
Computational Details. The theoretical calculations were performed using CASTEP package in which the plane-wave pseudopotential approach and ultrasoft pseudopotentials was employed for all the atoms with corresponding accuracy set as medium. The generalized gradient approximation (GGA) with the Perdew-Burkle-Ernzerhof (PBE) exchange-correlation function was employed. A (2×2) BiOCl (010) supercell with a slab thickness of four atoms (the Bi, O and Cl are roughly considered as locating in the same atomic layer) and a vacuum thickness being larger than 10 Å was used. All the models were first fully relaxed via geometry optimization and then applied for the energy calculation. All the transitional state (TS) search were conducted via a complete LST/QST protocol (the max number of QST step was set at 20) with 10 fragments from the initial state to the final state. The adsorption energies of adsorbates were defined as Ead(m) = Em-s -Es − Em, where m represents molecular adsorbate and s represents the surface of BiOCl (111).

Measurement of bulk charge separation efficiency (ηbulk). The indium doped tin oxide (ITO, China Southern Glass
Co., Ltd., Shenzhen, China) substrates were first ultrasonically cleaned in distilled water, absolute ethanol, and isopropanol for 15 min sequentially. Both edges of the conducting glass substrates were then covered with adhesive tape. Typically, the aqueous slurries of the samples were spread on an ITO glass substrate with a glass rod, using adhesive tapes as spaces. The suspension was prepared by grinding 20 mg of samples, 40 μL of PEDOT-PSS (Sigma-Aldrich, 1.3-1.7%) aqueous solution, and 200 μL of water. The resulting film was dried in air and annealed at 150 °C for 10 min, yielding an electrode with catalyst loading amount of ca. 0.254 mg cm −2 . The photocurrents were measured by an electrochemical analyzer (CHI660D, Shanghai, China) in a standard three-electrode system with the samples as the working electrodes, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. A 300 W Xe arc lamp equipped with a 400 nm cutoff filter (λ ≥ 400 nm) was utilized as a light source. We measured the ηbulk of BOC-VDWGs-AL, CBOC-VDWGs, and BOC-VDWGs-76 as follows.
Briefly, the measured photocurrent densities (J) obey the following equation of J = Jabs × ηbulk × ηsurface, where Jabs is the current S73 density converted from the absorbed photons when assuming that the absorbed photons were completely converted into electrons, ηbulk is the efficiency of e-h separation in the bulk of photocatalyst, and ηsurface is the efficiency of e-h separation on the surface of photocatalyst. When adding 1 M Na2SO3 as hole scavenger to the above system of photocurrent measurement, we assume that Na2SO3 can completely hinder the e-h recombination on the surface of photocatalyst without affecting e-h separation in the bulk of photocatalyst. In this case ηsurface is assumed to be 1, so ηbulk can be determined via ηbulk = Jsulfite/Jabs, where Jsulfite is the photocurrent density in the presence of Na2SO3.