Internal Electric Field Enhancement by the I‐Rich Surface of Highly Crystallized BiOI Nanosheets for Boosted Photocatalytic Degradation of Phenol

Although the internal electric field (IEF) of bismuth oxyiodide (BiOI) is acknowledged as a potent driving force for efficient charge separation, enhancing the intensity of IEF remains a challenge. Herein, highly crystalline BiOI nanosheets with I‐rich surface are employed to intensify IEF and direct the charge migration. In comparison to I‐poor BiOI nanosheets, which possess Bi−O layer termination and I‐defects, the I‐rich BiOI demonstrates 62.5‐fold improvement in IEF intensity to its well‐developed high crystalline structure, and its IEF direction is reversed by the surface I‐rich layers. This intensified IEF of I‐rich BiOI induces numerous holes (h+) to migrate to the surface of primary exposed (001) facets and electrons (e−) to the lateral facets efficiently, resulting in efficient charge separation spatially. Additionally, the surface accumulates h+ and superoxide radicals and acts in synergy to enhance the photodegradation of phenol. The photocatalytic activity of the I‐rich BiOI is found to be approximately fivefold and threefold higher than that of I‐poor BiOI under full spectra and visible light, respectively. Herein, the manipulation of IEF through surface and bulk structure regulation of BiOI for efficient charge separation is discussed, expecting to rationally improve photocatalytic performances.


Introduction
Bismuth oxyiodine (BiOI) is a prominent semiconductor for visible light that has garnered attention as a promising candidate for photocatalysis, [1][2][3] particularly the 2D form.However, the limited bandgap (E g = 1.7 eV) and rapid charge recombination of pristine BiOI still impede its photocatalytic activity.[15] Due to the asymmetrical layered structure, the intrinsic built-in internal electric field (IEF) of BiOI is deemed a potent driving force to increase the efficiency of charge transfer and separation, [13,16,17] thereby improving the photocatalytic performance of BiOI.
Numerous studies have demonstrated that the improvement of the IEF can be achieved by regulating the phase from tetragonal BiOI to monoclinic Bi 4 O 5 I 2 or Bi 5 O 7 I, [18,19] and incorporating doping elements. [15,20]For instance, Zhu's groups have demonstrated that the IEF of Bi 4 O 5 I 2 was exhibited a 1.6-fold compared with that of BiOI, resulting in a 25-fold increase in the photodegradation reaction rate constant under visible light irradiation. [18]Expect phase regulation, considerable effort has been devoted to constructing BiOI-based heterostructures for interfacial internal electric field. [5,21]To illustrate, our previous research has shown that epitaxial growth of BiP 5 O 14 layers on BiOI nanosheets (BiOI/ BiP 5 O 14 ) resulted in a 77.3-fold increase in the interfacial internal electric field compared to pristine BiOI nanosheets.This improvement leads to an 8.9-fold enhancement in the photodegradation efficiency of phenol under full-spectrum irradiation. [22]lthough the internal electric field (IEF) of bismuth oxyiodide (BiOI) is acknowledged as a potent driving force for efficient charge separation, enhancing the intensity of IEF remains a challenge.Herein, highly crystalline BiOI nanosheets with I-rich surface are employed to intensify IEF and direct the charge migration.In comparison to I-poor BiOI nanosheets, which possess BiÀO layer termination and I-defects, the I-rich BiOI demonstrates 62.5-fold improvement in IEF intensity to its well-developed high crystalline structure, and its IEF direction is reversed by the surface I-rich layers.This intensified IEF of I-rich BiOI induces numerous holes (h þ ) to migrate to the surface of primary exposed (001) facets and electrons (e À ) to the lateral facets efficiently, resulting in efficient charge separation spatially.Additionally, the surface accumulates h þ and superoxide radicals and acts in synergy to enhance the photodegradation of phenol.The photocatalytic activity of the I-rich BiOI is found to be approximately fivefold and threefold higher than that of I-poor BiOI under full spectra and visible light, respectively.Herein, the manipulation of IEF through surface and bulk structure regulation of BiOI for efficient charge separation is discussed, expecting to rationally improve photocatalytic performances.
However, the enhancement of IEF intensity in pristine BiOI has received little attention in the academic literature.Based on the theory of IEF, [23][24][25] the efficient route of improving the IEF should be altering the crystal structure of BiOI.Although altering the bulk structure of BiOI or other inorganic semiconductors for IEF modulation still remains a significant challenge, altering the local atomic termination of BiOI or other bismuth oxyhalides could be a feasible strategy.For instance, BiOBr with Br termination has been demonstrated to be more stable. [26]However, there is currently no published research on modifying the atomic termination of BiOI to achieve modulated IEF.
In this study, BiOI nanosheets were prepared with different surface atomic termination, including BiÀO layer termination and iodine-layers termination, through the manipulation of iodine source concentration while keeping the bismuth sources concentration constant.The IEF intensity and direction of these different BiOI nanosheets and corresponding charge separation efficiency were examined.Subsequently, the photocatalytic performances of various BiOI nanosheets were examined through the photodegradation of phenol solution.

I-Rich Termination of High Crystallized BiOI Nanosheets
The crystal structures of BiOI with different atomic terminations are presented in Figure 1.The general crystal structure of BiOI at an equal stoichiometric ratio of Bi and I (Bi:I) is shown in Figure 1a, in which the [Bi 2 O 2 ] 2þ layers and double iodine layers interacted through the Van der Waal force along the z-axis (001 direction).Significantly, the O-atom terminated surface is exposed, as consistent with previous studies. [27,28]While, the BiOI formed at a lower ratio of iodine are I-poor, creating iodine defects in the BiOI structure owing to lack of iodine atoms (Figure 1b).Correspondingly, the I-rich BiOI are formed at a relatively higher Bi:I value, in which exposed facets are terminated with massive amounts of iodine atoms due to the excessive amount of iodine (Figure 1c).
The BiOI nanosheets were prepared by increasing the concentration of iodine source from 0.5 to 5 mmol while fixing the concentration of the bismuth sources constant.That was, the molar ratio of Bi to I (Bi:I) was altered from 1:0.5 to 1:5 accordingly, leading to the production of I-poor BiOI, BiOI, and I-rich BiOI nanosheets, as shown in Figure 2a.The resulting BiOI showed sheet-like morphology, which remained consistent regardless of the variation of the Bi:I value (Figure 2b,c, S1a-d, S2a,c, Supporting Information).However, the size of the nanosheets increased gradually from 101.5 to 756.7 nm and even further to microscale in the supersaturated iodine solution (Figure 2b,c, S1, S2, Supporting Information).Furthermore, the I-poor BiOI (Bi:I = 1:0.5)and I-rich BiOI (Bi:I = 1:5) nanosheets were enclosed with (001) facet predominantly (Figure S2b-f, Supporting Information). [5,16]The I 3d XPS spectra of different BiOI nanosheets showed that the percentage of iodine of resulting BiOI was gradually increased with the concentration of iodine (Figure 2d).And the characteristic peaks of all the products were identical with the tetragonal BiOI (JCPDS No. 73-2062) and without shifting, regardless of varying the Bi:I value (Figure 2e,f ), implying that there was no expansion or shrinking in the bulk structure of generated BiOI.However, the intensity of the characteristic peak of BiOI increased gradually with the elevation of Bi:I value (Figure 2e), suggesting a well-developed high crystalline structure for I-rich BiOI.Moreover, the EPR and XPS spectra also confirmed that the I-rich BiOI nanosheets were well-developed (Figure S3, Supporting Information).
Subsequently, the surface structures of I-rich BiOI and I-poor BiOI were verified by bonding with Ag þ .Theoretically, the I-rich BiOI with massive iodine ions termination, the Ag þ ions could bond with the I À to form AgI on the surface of BiOI even in the darkness.Conversely, the AgI particles cannot be generated on the surface of I-poor BiOI, which lacks surface iodine layer, thereby affirming the structure model of I-poor BiOI.This assumption is shown in Figure 2g, and the results of Ag þ deposition experiments are shown in Figure 2h-k and S2h-n, Supporting Information.The surfaces of the I-poor BiOI nanosheets (Bi:I = 1:0.5)were clear, and the AgI particles were barely bonded and deposited (Figure 2h,i, S2h-j, Supporting Information), affirming the BiOI with I-lacking and BiÀO termination.While, for the I-rich BiOI (Bi:I = 1:5), massive small particles were deposited on its predominant surface (Figure 2j, S2k-n).Furthermore, the lattice spacing of 0.377 and 0.228 nm, respectively ascribed to (002) and (110) atomic planes of AgI, were observed (Figure 2k, S2l, Supporting Information).These results confirmed that the resulting BiOI was I-rich BiOI terminated with surface I-layer, consisting of the proposed structure model (Figure 1c).Notably, the iodine layers can bond on the I-poor BiOI surface to form an I-rich BiOI surface through Van der Waals forces, as illustrated in Figure 2a and confirmed in Figure S4, S5, Supporting Information.As the concentration of iodine increased, the surface of produced BiOI nanosheets transformed from hydrophilic to hydrophobic (Figure S6, Supporting Information).The surface atomic termination and well-developed structure of BiOI have potential to modify its band structure.Here, the band structure of I-poor BiOI with BiÀO terminating and I-defects and I-rich BiOI with I-layer termination were calculated through density functional theory (Figure 3a,b, S7, Table S2, Supporting Information).As reported in previous studies, [13,29,30] the valence band (VB) of BiOI is mainly composed of the hybridized O 2p and I 5p states, and the Bi 6p state is the primary component of the conductive band (CB).Such features and hybridization were observed in I-poor BiOI and I-rich BiOI (Figure 3a,b).Generally, between the valence band maximum (VBM) and conductive band minimum (CBM), the zero energy (the Fermi energy level, E f ) of BiOI is set at the closer of VBM. [31]he calculated minimum bandgaps (E g ) for the I-poor BiOI with I-defects and I-rich BiOI with I-layers were 1.54 and 1.47 eV, respectively (Table S3, Supporting Information).Compared to BiOI without defects, the VBM of the I-poor BiOI with I-defects is down-shifted to a more positive level, and its E f level is closer to the CBM (Figure 3a).This down-shifted VBM and up-shifted E f level of BiOI suggested that the resulting BiOI could exhibit an n-type semiconductors feature, [18] potentially resulting in enriched electrons.In contrast, the VBM of I-rich BiOI with I-layers is up-shifted and closer to E f level (Figure 3b), indicating a p-type semiconductor characteristic, which may lead to enriched holes. [32]ccording to the DRS spectra of different BiOI (Figure 3c), the predominant absorption region of these BiOI nanosheets was similar in that the light response region of the resulting BiOI nanosheets was restricted within 200-650 nm.But, the light absorption intensity of BiOI decreased with the increasing concentration of iodine sources.Based on the DRS spectra, the bandgaps of different BiOI were estimated through the αhν = A(hν-E g ) n/2 (where α, h, ν, and A represent the absorption coefficient, Planck constant, and light frequency and proportionality, respectively.And n is equal to 4) (inset of Figure 3c). [27,28]hen, the E g of resulting BiOI was estimated to be around 1.80 eV.But the E g slightly and gradually decreased from 1.85 to 1.73 eV by increasing the concentration ratio of iodine sources (Bi:I from 1:0.5 to 1:5).Then, the flat band potentials of the BiOI nanosheets were estimated to be À0.94,À1.06, and À1.17 eV (vs Ag/AgCl at pH = 7), respectively.These values are equaled to be À0.70,À0.82, and À0.93 eV (vs the standard hydrogen electrode (NHE), pH = 0), respectively, which is equal to E f for an n-type semiconductor (Figure 3d). [18]The corresponding CB levels for different BiOI are À0.80,À0.92, and À1.03 eV. [33]ccordingly, the VB potentials are estimated to be 1.05, 1.0, 0.84, and 0.70 eV based on the equation of E CB = E VB ÀE G .The band structure illustrations are presented in Figure 3e.Raising the concentration ratio of iodine sources, the VB positions of the as-prepared BiOI were gradually up-shifted to a less positive potential, and the CB levels were up-shifted to a more negative level accordingly.This evolution of band structure of different BiOI may promote charge separation and photocatalytic oxidation ability.

Enhanced IEF and Spatial Charge Separation of I-Rich BiOI Nanosheets
The IEF intensity of BiOI was investigated in light of the surface I-rich of highly crystallized BiOI, and the examination revealed that the surface I-rich structure modulates the IEF intensity.The IEF intensity of BiOI nanosheets was calculated based on the surface potential and zeta potential. [18,22]The surface potentials were measured through the Kelvin probe force microscopy (KPFM), and the average surface potential of BiOI obtained at different Bi:I value (1:0.5, 1:1, and 1:5) were found to be À1.71,À11.94, and À10.98 mV, respectively (Figure 4a-f ).Simultaneously, the zeta potential of coordinated BiOI gradually increased from neutral to a much negative state (Figure 4g).As a result, the IEF intensity of these BiOI nanosheets was raised with the increasing concentration of iodine.Notably, the IEF intensity of I-rich BiOI nanosheets (Bi:I = 1:5) was 62.5 folds higher than that of I-poor BiOI nanosheets (Bi:I = 1:0.5)(Figure 4h).The well-developed structure of I-rich BiOI was responsible for such a significant enhancement in IEF intensity, [34] which resulted in the spatial transfer and separation of charge carriers.However, the IEF intensity of BiOI nanosheets did not change significantly after adding NaH 2 PO 4 , possibly because of the inhibited epitaxial growth on the hydrophobic surface of I-rich BiOI (Figure S8, Supporting Information).
The I-poor BiOI (Bi:I = 1:0.5)with weak IEF drives the electrons (e À ) from bulk to the surface predominantly, showing weak and negative SPV signal mainly (inset of Figure 5a).Regarding BiOI (Bi:I = 1:1), the SPV signal of BiOI was slightly increased due to its slightly increased IEF intensity (inset of Figure 5a).However, the SPV signal of I-rich BiOI (Bi:I = 1:2) was reversed to predominant positive (Figure 5a, inset of Figure 5a), indicating the accumulation of separated holes.Furthermore, the positive SPV signal intensity of I-rich BiOI nanosheets (Bi:I = 1:5) was the highest, implying that the massive accumulated holes (h þ ) on the surface as driven by the strong IEF.Then, the surface potentials of the I-rich BiOI nanosheets (Bi:I = 1:5) were further examined under light irradiation (Figure S9, Supporting Information, Figure 5b-d).Compared with its surface potential in the dark, the surface potential of the I-rich BiOI nanosheets was raised, indicating the h þ accumulated on the surface (Figure 5d), [35] which is consistent with the SPV spectra.Here, the other surface charges accumulation of I-poor BiOI and I-rich BiOI should attribute to the different directions of IEF.Based on previous reports and the above results, [36] the direction of weak IEF for the I-poor BiOI or BiOI with BiÀO termination should point from I-layers to the BiÀO layers, driving the photogenerated h þ from bulk to the lateral facets, while e À to predominant exposed (001) crystal facets (Figure 5e).Conversely, the photogenerated e À of I-rich BiOI was driven from bulk to the lateral facets, and h þ transferred to the predominant exposed (001) facets under the reversed direction of enhanced IEF (Figure 5e).
With above hypothesis of charge transfer direction, the photodeposition tests of Au and MnO x were carried out (Figure S10, Supporting Information, Figure 5f-i). [37]For the I-poor BiOI (Bi:I = 1:0.5), the Au particles were primarily deposited on the predominant (001) facets through electron reduction (Figure 5f-g).Clear lattice spacing fringes of 0.233 nm assigned to the (111) planes of Au were observed on the surface of such BiOI nanosheets (Figure 5h).Furthermore, the elemental mapping also confirmed the presence of Au (Figure 5i).Reversely, the Au particles were deposited at the lateral edge of I-rich BiOI nanosheets (Bi:I = 1:5) (Figure 5j-n, S10a,b, Supporting Information).Furthermore, the clear lattice fringes of 0.233 nm ascribed to (111) planes of Au on the edge of I-rich BiOI (Figure 5l, S10c, Supporting Information) were observed.The elements mapping and EDS spectrum of this I-rich BiOI nanosheets verified the presence of Au particles on the lateral edge of BiOI (Figure 5m,n, S10d, Supporting Information).In comparison, MnO x particles deposited on the top surface of such BiOI nanosheets via the oxidation ability of h þ were observed (Figure S10e-h, Supporting Information).These photodeposition tests confirmed the correction of the presented hypothesis and the transfer directions of the photogenerated e À and h þ in BiOI with different structures.In addition, the efficacy of charge separation in these BiOI nanosheets, driven by the IEF, was also investigated through various techniques, including electrochemical impedance spectroscopy (EIS), photoelectrochemical characterizations of photocurrent, photoluminescence (PL), and time-resolved transient PL spectroscopy (Figure 6).The smallest EIS radius of I-rich BiOI (Bi:I = 1:5) implied the lowest interfacial barriers for charge transfer in this BiOI (Figure 6a). [38]Compared with other BiOI nanosheets, this I-rich BiOI (Bi:I = 1:5) showed the highest photocurrent response owing to the highest charge separation efficiency (Figure 6b).Accordingly, the I-rich BiOI (Bi:I = 1:5) also exhibited the lowest PL intensity and longer lifetime of charge carriers, suggesting the efficient suppression of charge recombination (Figure 6c,d). [22,39]All these results demonstrated the I-rich BiOI with enhanced IEF could promote higher separation efficiency of charge carriers.

I-Rich BiOI Nanosheets Boosted the Photocatalytic Degradation of Phenol
The performance of BiOI nanosheets in photocatalysis was examined by studying the degradation of phenol under the irradiation of full spectra and visible light (Figure 7 and S11a, Supporting Information).The I-rich BiOI nanosheets (Bi:I = 1:2 and 1:5) exhibited superior photocatalytic activity under the full spectra or visible light irradiation (Figure 7a,c).Furthermore, the reaction rate constant (k) of I-rich BiOI (Bi:I = 1:2 or 1:5) was approximately fivefold and threefold higher than that of I-poor BiOI (Bi:I = 1:0.5)under the irradiation of full spectra light and visible light, respectively (Figure 7b,d, S11a, Supporting Information).According to the performance recycling, the I-rich BiOI (Bi:I = 1:5) exhibited excellent stability (Figure 7e).Additionally, the I-rich BiOI (Bi:I = 1:5) achieved 45.7% total organic carbon (TOC) removal efficiency within 4 h, which was approximately 4.5 times higher than that of iodine-poor BiOI (Bi:I = 1:0.5)(Figure 7f ).Furthermore, compared to other related works on BiOI (Table S4, Supporting Information), [2,3,16,29,40] this I-rich BiOI exhibited superior photocatalytic performances.Notably, the photocatalytic performances of I-rich BiOI were not as high as that of IEF intensity, possibly due to the hydrophobic surface feature that restricted the surface photocatalytic reaction.Additionally, the photocatalytic performance of BiOI nanosheets produced at higher a concentration of iodine (Bi:I = 1:10 and 1:16) was inhibited compared to iodine-rich BiOI nanosheets (Bi:I = 1:5), probably due to the bulk size and broader size distribution of the generated BiOI nanosheets (Figure S12, Supporting Information).
During the photocatalytic process, the reactive species of these BiOI (Bi:I = 1:0.5 and 1:5) were examined (Figure 8 and S11b, Supporting Information).In comparison to the photocatalytic performances of the I-poor BiOI (Bi:I = 1:0.5) in the absence of scavengers, O 2 À was found to be the predominant reactive species (Figure 8a).For I-rich BiOI (Bi:I = 1:5), the h þ and O 2 À were identified as the primary reactive species (Figure 8b).The negligible impact of •OH on the photocatalytic performance of I-rich BiOI was attributed to its less positive VB level (0.7 eV), which hindered •OH production (E 0 (OH À /•OH) = 1.99 eV, NHE). [41]urthermore, the significant DMPO-•O 2 À signal and the scarce Hence, this study elucidated the photocatalytic process for I-poor BiOI with BiÀO termination or I-defects and I-rich BiOI with I-rich layers termination (Figure 8e).In the case of I-rich BiOI, the abundant iodine termination changed the direction of IEF from the surface I-layers to the Bi 2 O 2 2þ layers.The well-developed structure of I-rich BiOI enhanced IEF and facilitated the efficient and spatial separation of charge carriers.Specifically, the h þ was transferred to the top primary exposure (001) facets of I-rich BiOI nanosheets, while e À was favorable to migrate toward the lateral surface of nanosheets.Consequently, the •O 2 À stemmed from e À (E 0 (O 2 /•O 2 À = À0.046eV, NHE) and h þ played a synergetic effect on the photodegradation of phenol during the photocatalytic process. [42]In contrast, the I-poor BiOI with BiÀO termination and I-defects have IEF direction from inner I-layers to surface Bi 2 O 2 2þ layers.Moreover, the weak IEF-driven inefficient separation of e À toward the primary exposed facet and h þ toward the lateral surface.As a result, the inadequate e À for •O 2 À and h þ for •OH serve as reactive species for the photodegradation of phenol.

Conclusion
This study investigated the manipulation of the intensity of IEF in BiOI nanosheets by altering their surface atomic termination and crystallinity.The results confirmed that the surface I-rich structure of BiOI enhances IEF.Specifically, the IEF of I-rich

Figure 1 .
Figure 1.Crystal structure of BiOI with different terminated atoms, a) fine-structured BiOI with BiÀO termination, b) I-poor BiOI with BiÀO termination, and c) I-rich BiOI with surface IÀ layer termination.

Figure 2 .
Figure 2. a) Schematic illustration of producing different BiOI and the transition between I-poor BiOI and I-rich BiOI.TEM images of b) I-poor BiOI, c) I-rich BiOI.d) I 3d XPS spectra, e) XRD patterns, and f ) corresponding partial enlarged XRD region of 28-33°.g) Scheme of surface structure validation of I-poor BiOI and I-rich BiOI.h) TEM and i) lateral HRTEM of I-poor BiOI (Bi:I = 1:0.5)after depositing with Ag þ .j) Lateral TEM, k) corresponding HRTEM images of I-rich BiOI (Bi:I = 1:5) after depositing with Ag þ .

Figure 3 .
Figure 3. Calculated band structures and DOS of BiOI with various atomic termination, a,b) BiÀO and I-defects.c) DRS spectra of different BiOI (inset with plots of (αhv) 1/2 versus hv), d) Mott-Schottky plots of various BiOI nanosheets, e) the band structure illustration of BiOI obtained by altering the ratio of Bi:I.

Figure 5 .
Figure 5. a) SPV spectra of different BiOI (inset with partially enlarged SPV spectra).The surface potential of I-rich BiOI nanosheets (Bi:I = 1:5) b) in the dark, and c) under light irradiation, d) corresponding comparison of surface potential.e) Schematic illustration of charge transfer in I-poor BiOI and I-rich BiOI.Photodepositing Au particles on I-poor BiOI nanosheets (Bi:I = 1:0.5),f ) SEM, g) TEM, and h) HRTEM, i) bright field TEM images, and corresponding elements mapping.Photodepositing Au particles on I-rich BiOI nanosheets (Bi:I = 1:5), j) TEM, k) partial enlarged TEM, l) HRTEM, m) bright field TEM, and n) partial enlarged bright field TEM images and corresponding elements mapping.