Fei Chang a,* , Xiaomeng Wang a , Cheng Yang a , Sushi Li a , Jielin Wang b , Weiping Yang b ,  Fan Dong b , Xuefeng Hu c,**, Deng-guo Liu d,***, Yuan Kong e,****

ABSTRACT

Bi12GeO20-based composites Bi12GeO20-Bi2S3 (BGS) were successfully constructed through a facile ball-milling  method using sulfur powder for the first time. Systematical analyses verified the in-situ generation of n-p heterojunctions with surface oxygen vacancies (OVs). These composites showed reinforced photocatalytic removal  of NO at ppb level under visible light with high selectivity for NO2− /NO3− species, avoiding the generation of  toxic NO2 as far as possible. Especially, the best candidate BGS0.1 possessed 46% NO removal with 96%  selectivity for NO2− /NO3− species that were much higher than those by Bi12GeO20, mainly relevant to the  enhanced visible-light absorption, synergistic effect of heterojunctions containing surface OVs to promote charge  carriers segregation and reactive radicals formation, and suitable phase composition with proper band structures.  The effect of heterojunctions with surface OVs over band structures and reaction paths was demonstrated by  density functional theory (DFT) calculation. DRIFTS and FT-IR spectra were recorded to deduce NO conversion  routes. Eventually, a preliminary photocatalysis mechanism of these robust composites was conjectured in a Zscheme manner basing experimental and analytical results. This study might pave roads for in-situ construction of  sillenite-based composites with surface OVs by a mechanochemical approach with satisfactory photocatalytic NO  treatment under visible light.

1. Introduction

The formation of secondary organic aerosol (SOA) is closely associated with the high concentration of PM2.5 during the heavy haze  pollution period in typical cities of China [1], while nitrogen oxide (NOx,  mainly pointing to NO and NO2) is one kind of major precursors to  produce SOA and belongs to a kind of priority target pollutants [2]. In  addition, NOx is one kind of the main cause of environmental issues such  as acid rain, ozone formation, photochemical smog, and global warming. Besides, NOx is able to exert various detrimental effects on human  health [3]. Due to the obvious increase in ozone concentration in 2019,  the emission reduction of NOx has been listed in China’s 14th Five-Year Plan. In NOx from automotive power devices and industrials with  combustion units, NO is an essential component because of the large  proportion and ought to be effectively eliminated by proper treatment  strategies [4]. Since under mild conditions photocatalysis ensures  various contaminants removal by receiving incident light with suitable  energy and oxygen in the air as an oxidant [5,6], it is a suitable alternative technique for oxidation of NOx at part-per-billion (ppb) level at  ambient temperature [7]. Nevertheless, the oxidation of NO readily  causes the accumulation of NO2 that possesses riskier toxicity than NO,  requiring efficient systems with excellent photocatalytic NO removal  and high selectivity for nitrite or nitrate (NO2− /NO3− ) species as well  [8].

Recently, some studies centered on visible-light responsive sillenite  compounds such as Bi12TiO20, Bi12SiO20, and Bi12GeO20 have been reported with photocatalytic capabilities in aqueous solutions in most  cases instead of the gaseous atmosphere [9]. Bi12GeO20 has unique  physicochemical properties, a suitable bandgap (<2.9 eV), and a strong  oxidative capacity for photocatalytic degradation of assorted pollutants  [9–11]. Although it meets the fundamental requirements of a photocatalytic system, its further applications in environmental protection  and renovation are awfully limited due to the high recombination of  charge carriers and poor visible light absorption. For the sake of overcoming underlying barriers of bare Bi12GeO20 and improving photocatalytic efficiencies, appropriate modification strategies are urgently  desired. Some studies have focused on heterojunction creation with  suitable components [10,11], ions doping [12], and noble metal deposition [13]. However, as far as we know, there are no relevant reports  about Bi12GeO20 modifications for photocatalytic NO removal, offering  quite a wide space for further research.

Bismuth sulfide (Bi2S3) is deemed as a promising material with  variable utilizations because of strong visible-light harvesting and a  large absorption coefficient [14]. Conspicuously, it is generally used as a  favorable semiconductor or photosensitive component for heterojunction constructions. Bi2S3 can be easily fabricated through solvothermal routes that need sealed environments with high temperature,  pressure, and long reaction time [15]. To avoid such, a facile manner  combined with simplicity and directness is requisite. On the other hand,  the choice of a sulfur source is careful and subtle for different purposes,  including sodium sulfide nonahydrate [16], carbon disulfide [17],  thiourea [18], and thioacetamide [19] as sulfurization reagents. As to  the powdery sulfur, most sulfurization procedures involve solvothermal  routes [20], and the ball-milling treatment has never been utilized to  produce Bi2S3 even though it would vastly pander to laboratory and  industrial demands. In addition, such treatment is considered as one of  the most economical, straightforward, and green techniques for material  processing and is regularly applied to achieve physical blend and  chemical modification between solid phases [21], which facilitates to  accomplish sulfurization transformations and crystallographic modifications simultaneously. Moreover, the solid-solid contact interface  basing on mechanochemical nature may induce generation of surface  OVs. Surface OVs can be regarded as shallow donors and serve as  adsorption and reaction sites, promoting transfer and separation of  charge carriers and further photocatalytic performance through boosting the generation of reactive oxygen species [17,22]. Thus, it impels us  to attempt sulfurization conversions with sulfur powder utilizing the  ball-milling process, by which Bi2S3-containing heterojunctions with  surface OVs ought to be simultaneously generated. To date, only one  research group exerted its efforts to pretreat WS2 samples with sulfur  powder [23], whose purpose was for physical mixing that was not  thorough enough to realize further sulfurization as we intended.

Herein, considering Bi12GeO20 and Bi2S3 as respectively an n-type  and p-type semiconductor, n-p heterojunction composites with sufficient surface OVs were fabricated through a facile ball-milling procedure by using Bi12GeO20 and powdery sulfur as precursors for the first  time. These composites were characterized by a series of analytical  techniques and subjected to photocatalytic removal of NO under visible  light. The relationship of microstructure and catalytic capacity was  correlated, and the enhancement of NO removal efficiency with avoidance of toxic NO2 generation of these surface OVs-contained heterojunctions was thoroughly discussed. Possible NO conversion paths were  proposed, and a relevant photocatalysis mechanism was evenly  conjectured.

2. Experimental section

2.1. Chemicals and reagents

Relevant chemicals and materials used are described in Supporting  Information for sake of brevity.

2.2. Catalysts preparation

The specific fabrication route of binary composites BGS consisted of  both hydrothermal and ball-milling steps schemed in Fig. S1. Bare  Bi12GeO20 was readily prepared by a traditional hydrothermal method  with finely tuned details [24]. Variable amounts of sulfur powder and  bare Bi12GeO20 were introduced into a reactor, where mechanochemical  reactions smoothly occurred to generate surface OVs-contained composites BGS with different phase compositions, driven by the strong  interaction of different phases and quite a small solubility product  constant (1 × 10− 97) of Bi2S3 [14].

According to stoichiometric ratios, 4.1571 g Bi(NO3)3⋅5H2O, 0.0750  g GeO2, and 0.3571 g hexadecyltrimethyl ammonium bromide (CTAB)  were successively added into a 50 mL aqueous solution containing 5 M  NaOH. The mixture was vigorously stirred for 0.5 h at room temperature  and then was transformed into a 100 mL stainless steel reactor lined with  Teflon. After heating at 180◦ for 12 h and cooling to room temperature,  the precipitate was filtered by centrifugation, washed with water and  ethanol three times, and dried at 60◦ for 12 h to supply bare Bi12GeO20.

Binary composites BGS were fabricated through a facile ball-milling  procedure. Specifically, the mixture of Bi12GeO20 and sulfur powder in a  certain ratio was introduced into an omnidirectional planetary ball mill  (DECO-PBM-AD-0-4L, Changsha Deke Instrument Equipment Co., Ltd).  Three different sizes of alumina balls (ф = 3, 5, and 9 nm with numbers  as 15, 90, and 290) were mixed in the tank with 5 mL ethanol as a  dispersant. The whole process was performed at a speed of 500 rpm for  2 h. As-achieved powder was collected, centrifuged, and washed with  water and ethanol three times for each. Finally, the product was dried at  room temperature for 12 h and grounded into powders. Prior to the ballmilling process, 2 g Bi12GeO20 and desirable amounts of sulfur 0.001,  0.002, 0.004, 0.008, and 0.016 g were mixed thoroughly to provide BGS  compounds and denoted as BGSX, where X referred to different sulfur/  Bi12GeO20 mass ratios of 0.05, 0.1, 0.2, 0.4, and 0.8 wt%. Ball-milled  Bi12GeO20 was nominated as BGO. Besides, Bi2S3 was prepared by an  anion exchange route with a stoichiometric addition of CH3CSNH2 and  Bi(NO3)3⋅5H2O, and a well-washed black precipitate was collected,  dried, and ball-milled, labeled as BS.

2.3. Characterization and analyses

Apparatus and methods for analyzing physiochemical properties are  collected in Supplementary materials for pithiness.  2.4. Photocatalytic removal of NO under visible light

The evaluation of photocatalytic capability of samples was conducted via NO removal experiments at ppb level in a continuous flow  system under visible light. The setup consisted of a gas supply, irradiation source, photocatalytic reactor, and analytical appliance. Catalytic  reactions proceeded in a customized reactor of polycarbonate plastic  with a nonopaque quartz glass cover, whose volume was approximately  2.0 L (28 cm × 12 cm × 6 cm). A 500 W Xe lamp (CEL-LAX500, Aulight,Beijing) with a 420–780 nm cutoff glass filter vertically located 40 cm  above the reactor was employed as the light source. 0.4 g catalyst was  uniformly dispersed in 40 mL ethanol by the agitator and ultrasonic for  15 min separately. The suspension was evenly divided and coated onto  two glass dishes whose diameter was 10 cm. After pretreatment at 60 ◦C  until the entire evaporation of ethanol, naturally cooled dishes covered  with samples were put at the center of the reactor. Concerning the target  pollutant, NO (diluted by N2) from a gas cylinder and air generated by an  air generator were amply mixed by a mass flow controller in a sealed  tank to dilute NO at a concentration of 580 ppb with a flow rate of gas  1.8 L min− 1 and relative humidity 50 ± 3%. After attaining adsorption desorption equilibria on catalysts surface, the lamp was turned on to  initiate photocatalytic processes. The real-time concentration variations  of NO, NO2, and NOx were continuously measured using a NOx analyzer  (Thermo Scientific, 42i).

2.5. Identification of reactive species and NO2− /NO3− generation

To quest a probable photocatalytic NO removal mechanism over  different samples systems, effects of active species on photocatalytic  outcome were tested by dosing different scavengers. In an experimental  procedure identical to photocatalytic performance above-mentioned,  0.1 g potassium iodide (KI), potassium dichromate (K2Cr2O7), tertbutanol (TBA), and 0.1 mM p-benzoquinone (PBQ) were used as scavengers to trap holes (h+), electrons (e− ), hydroxyl radicals (⋅OH), and  superoxide radicals (⋅O2− ), respectively. In addition, the generation of  NO2− /NO3− species was checked using FT-IR spectra before and after  five runs of catalytic reactions of composite BGS0.1.

2.6. In-situ DRIFTS studies on species evolution

In-situ DRIFTS experiments were performed by employing the Bruker  Tensor II FTIR spectrometer. It was formed with an in-situ Harrick  diffuse-reflectance cell and an HVC high-temperature reaction chamber.  The gas source was made up of He, O2, and NO (target pollution gas).  The chamber was sealed with two ZnSe and one quartz window, and an  MVL-210 Xe lamp was served as the visible light source. The purge gas  flow was bled into the cell at 100 mL min− 1 to remove impurities before  measurement. After the background spectrum was recorded, asprepared samples were exposed to mixture gas with 50 mL min− 1 NO  and 50 mL min− 1 O2. The resultant data were recorded as a function of  the DRIFTS scanning time to present adsorption and photocatalytic  oxidation processes over samples.

2.7. Theoretical calculations

Computational details were mentioned in Supplementary materials  for pithiness.

3. Results and discussion

3.1. Morphological characterization and analyses

XRD patterns of ball-milled samples are employed to analyze phase  composition and crystallinity, as depicted in Fig. 1a. The pattern of BGO  possesses good crystallinity, and intensive diffraction peaks agree well  with the pure Bi12GeO20 phase (JCPDS No. 77–0861) [24]. BS was  prepared through anion exchange instead of a ball-milling protocol, and  relevant discussion is in Supplementary materials. Diffraction peaks of  BS in Fig. S2 are consistent with the pure Bi2S3 phase (JCPDS No.  17–0320) [25]. Characteristic XRD patterns of BGS composites exhibit  no obvious signs of BS, possibly ascribing to the trace amount (sulfur/Bi12GeO20 mass ratios below 1 wt%) and even distribution of Bi2S3  nanocrystals. Furthermore, the high purity can be confirmed by undetectable peaks of other substances. Similar results are acquired using  Raman spectra of relevant samples in Fig. S3, where feature signals of BS  at 104 and 965 cm− 1 are unobservable in composites BGS0.1 and  BGS0.8. As shown in FT-IR spectra in Fig. S4, two peaks at around 1375  and 1073 cm− 1 belonging to Bi–S vibration bonds are recognized in  composite BGS0.1, implying the successful formation of Bi2S3 [13,25].  The existence of BS in composites ought to be further certified by other  techniques.

Morphological and microstructural observations of BGO and composite BGS0.1 are presented by recording SEM, TEM, and EDS patterns  in Fig. 1. In Fig. 1b–c, owing to intensely mechanochemical treatment,  samples BGO and BGS0.1 possess similarly combined morphologies,  including partially broken irregular polyhedron particles with uneven  fragments, which ought to be crashed regular hexagonal-shaped particles after one-step hydrothermal synthesis [13]. However, by careful  comparison, BGO appears as angular features with relatively clear edges  that become blurred and rounded off in composite BGS0.1 since edges  and some coarse surfaces facilitate the generation of BS and creation of  reactive sites with the presence of considerable surface OVs. By means of  TEM images, similar morphologies of BGO and BGS0.1 are verified in  Fig. 1d–e, where BS particles in a trace amount are hardly recognizable.  Uniform crystal lattices with the interplanar spacing of 0.275 nm are  attributed to (3 2 1) crystallographic planes of BGO in Fig. 1f. Evidently,  extra crystal lattices with an interplanar spacing of 0.215 nm are also  observed in Fig. 1g, assigning to (2 4 1) crystallographic planes of BS  nanocrystals, respectively. The HRTEM image directly affirms the  coexistence of both components and the formation of heterojunction  structures at interface that supply efficient bridges to migrate and  separate charge carriers [25]. To further identify the formation of BS,  EDS elemental mapping was scanned, and relevant patterns are shown  in Fig. 1i–m from the selected area of SEM image in Fig. 1h, ascertaining  the presence and uniform distribution of relevant elements in composite  BGS0.1, especially the element S that is also discovered in EDS spectrum  in Fig. S5.

Specific surface areas, average pore size, and pore volumes of samples BGO and BGS0.1 were analyzed and collected in Table S1. A trend of  slight increase is observed in specific surface area of composite BGS0.1  in comparison to BGO, possibly attributing to a slight modification of  textural structure in vulcanized BGO, which may not be a significant  effect over photocatalytic performance variation. Fig. S6 displays N2  adsorption-desorption isotherms and pore volumes of BGO and BGS0.1.  According to the IUPAC classification, the N2 adsorption-desorption  isotherms of BGO and composite BGS0.1 are assigned to typical type  IV patterns with H3-type hysteresis loops [25,26]. Both isotherms with  resemblances indicate that microstructures of BGO and BGS0.1 are  similar, whose hysteresis loops display adsorption capability at the  relative pressure P/P0 ranging from 0.6 to 1.0, implying  mesopore-dominant structures [27]. There is an obvious increase of  average pore size and pore volume in BGS0.1, suggesting that surface  deficiency can be amplified during extrusion and sulfurization modifications in Fig. S6b.

XPS spectra were recorded to analyze the surface composition and  elemental valence states of relevant samples. As depicted in Fig. 2a, fullscan XPS spectra of samples BGO, BGS0.1, and BGS0.8 exhibit main  elements Bi, Ge, and O, indicating similar composition of main components. In Fig. 2b, a weak signal at 225.1 eV in BGS0.8 instead of BGO  and BGS0.1 is attributed to S2− species [17], revealing a gradual generation of BS phase on BGO surface. Bi 4f XPS spectrum of BGO in Fig. 2c  show two peaks located at 158.5 and 163.8 eV, which are assigned to Bi  4f7/2 and Bi 4f5/2 of Bi3+ orbitals in lattices [8,28]. As displayed in  Fig. 2d, signals centered at around 25.5 and 28.5 eV can be ascribed to Bi  5d and Ge 3d orbitals in BGO [13]. O 1s profile of BGO in Fig. 2e is  asymmetric and can be separated into three peaks positioned at 529.4,  530.7, and 531.7 eV, belonging to Ge–O bonds, Bi–O bonds, and  adsorbed oxygen-containing species or O atoms close to OVs [8]. As  observed in Fig. 2c–e, signal positions of Bi, Ge, and O in lattices are  gradually shifted up-field from BGO to BGS0.1 and further BGS0.8,  verifying that the present chemical interaction between both phases

promotes electrons transfer from BGO to BS through heterojunction  interfaces [29]. To identify whether OVs exist or not, EPR spectra of  BGO, BGS0.1, and BGS0.4 were measured and shown in Fig. 2f. Clearly,  in the dark, an obvious paramagnetic signal is detectable in BGO with a g  value of 2.003, indicating the involvement of bulky OVs because of its  intrinsic structure [30] and surface OVs created during ball-milling  processes. In addition, such signal is gradually increased from BGS0.1  to BGS0.4 with the increase of sulfur power content, revealing the  gradual creation of surface OVs on BGO adjacent to sulfur-substituted  regions because of structural disorder. Moreover, these OVs signals of  relevant samples can be slightly intensified as soon as exposed to irradiation for 10 min in Fig. S7, revealing possibilities of adsorption of  oxygen-containing species and further generation of reactive free radicals [22].

UV–Vis DRS spectra of as-prepared samples were recorded to analyze  light responsive-capacity and band structures. As seen in Fig. 3a, BGO in  light yellow can absorb ultraviolet and partial visible light. The relevant  absorption edge around 440 nm is obviously blue-shifted in comparison  to that of the unball-milled sample [13]. In addition, BS in black exhibits  a strong light absorption over the whole visible-light region because of  the narrow bandgap and large optical absorption coefficient [25]. With  the increase of sulfur addition, the visible-light absorption of composites  BGS gradually reinforces, implying that progressive generation of BS  phase with surface OVs induces an obvious variation of the optical  property, which is in good accordance with color change in the inset of  Fig. 3a. Bandgap energy (Eg) of a semiconductor is generally derived  from the equation αhv = A(hv – Eg) n/2 [31], and value n is associated  with whether the transition is direct (n = 1) or indirect (n = 4). On the  basis of prevenient reports [18], both BGO and BS adopt a direct transition manner. As a result, values Eg of BGO and BS are estimated at around 2.90 and 1.30 eV in Fig. 3b. Moreover, values Eg of as-prepared  composites are determined and collected in Fig. S8. As analyzed, the  decoration of BGO with BS and surface OVs induces enhanced visible  light harvesting with red-shifted adsorption edges, facilitating the generation of sufficient charge carriers and further enhancement of photocatalytic performance.

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