Thickness-determined photocatalytic performance of bismuth tungstate nanosheets

Abstract

Bismuth tungstate (Bi₂WO₆) nanosheets with dominant exposed (010) facets and various thicknesses (H) and lateral sizes were hydrothermally synthesized via pH adjustment of precursor suspensions. As the pH increases from <1 to 8, the resultant nanosheets exhibit improved crystallinity and photoabsorption, decreased specific surface area, increased H, and decreased photoactivity in the degradation of rhodamine B (RhB), methylene blue (MB), and Eosin Y (EY) under visible light irradiation. The photoactivity of the Bi₂WO₆ sample obtained at pH < 1 is about 6, 100, and 25 times of that at pH 8 for RhB, MB, and EY degradation, respectively. The photoactivity enhancement is ascribed to reduction of the H. The photocatalytic efficiencies are inversely proportional to the reduction of H² when the nanosheets can be penetrated by incident light. This work reveals the structure–performance relationship of Bi₂WO₆ nanosheets and provides significant guidance for preparation of high efficient two-dimensional photocatalysts.

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1. Introduction

In recent decades, semiconductor photocatalysis has been valued as a promising technology for solving the energy and environmental crises by splitting water to generate H₂, reducing CO₂ to produce hydrocarbon fuels, degrading toxic contaminants and deactivating bacteria.^1–3 However, current photocatalysts haven't been able to satisfy the requirement of large-scale industrial applications due to their low photocatalytic performance.⁴

Understanding the structure–performance relationship of photocatalysts is the prerequisite for constructing photocatalysts with high performance.⁵ Photocatalytic performance of semiconductors is influenced by many factors, such as crystallinity,⁶ surficial defect,⁷ photoabsorption capability,⁸ specific surface area,⁹ elemental doping,¹⁰ exposed crystal facet,^11,12 and polarity of materials.¹³ As is now well known, good crystallinity, appropriate surficial defect and doped element amounts, strong photoabsorption ability and internal electric field, and high surface area and exposure percentage of high-photoactive facets of semiconductors are favorable for the enhancement of photoactivity.^6–13 Recently, we found that, for BiOBr nanosheets, nanosheet thickness is an important factor influencing their photocatalytic performance and there is an inversely proportional relationship between the photocatalytic efficiencies and the nanosheet thickness.^5,14 However, whether this experimental rule applies to the other two-dimensional (2D) semiconductor photocatalysts still needs to be studied.

Bismuth tungstate (Bi₂WO₆), a layered Aurivillius-type compound,¹⁵ is a stable and nontoxic visible light photocatalyst with a relatively small band gap of ∼2.6 eV, showing a good prospect in industrial application.^16–19 To further enhance its photoactivity, various methods have been developed, such as elemental doping,¹⁰ heterojunction or composite fabrication,^20,21 and construction of special morphologies.^16,22 Morphological adjustment is one of the most fundamental and studied ways. Maybe because of its layered structure, the Bi₂WO₆ synthesized by hydrothermal or solvothermal methods tends to form 2D nanosheets or nanosheet-based hierarchical nanostructures such as bipyramid,¹⁶ microspheres,^22–24 hollow tubes,²⁵ core/shell structures,²⁶ and nanorings.²¹ pH adjustment of reaction systems is an effective method to modulate the morphologies of the Bi₂WO₆.^27–33 Through changing pH of precursor suspensions, the crystallinity,³⁰ morphology,³¹ specific surface area,²⁸ and nanoparticle size²⁷ of the resultant Bi₂WO₆ samples can be substantially varied, which causes significant changes in their photocatalytic performance. Most results indicated that the Bi₂WO₆ samples prepared at pH ≥ 7 exhibit higher photoactivity than those synthesized at pH < 7,^28–30,32 but Zhang et al. and Tian et al. reported different results.^27,33 Toward these results, different mechanisms for the photoactivity improvement were put forward, such as high crystallinity, large specific surface area, and low nanoparticle or nanosheet size.^27–32 Among them, reduction of the nanosheet size is considered as an important reason for the photoactivity enhancement of the Bi₂WO₆ nanostructures. However, owing to the fact that the nanosheet size refers to both the nanosheet thickness and the lateral size (length or width),⁵ it is essential to understand whose change plays the main role for the photoactivity improvement.

In this work, four kinds of Bi₂WO₆ nanosheets with different crystallinities, thicknesses, lateral sizes, specific surface areas, and photoabsorption performances were hydrothermally prepared via adjusting pH of precursor suspensions. With decreasing pH, the resultant samples exhibit a prominent enhancement of photocatalytic efficiencies in degradation of rhodamine B (RhB), methylene blue (MB), and Eosin Y (EY) under visible light irradiation. The photoactivity enhancement is ascribed to the reduction of the nanosheet thickness and the increase of the specific surface area, rather than the variations of the lateral sizes, crystallinity, and photoabsorption performance of the nanosheets. This study provides a deeper insight into the structure–performance relationship of the Bi₂WO₆ nanosheets, which supplies guidance for preparation of 2D high-performance photocatalysts.

2. Experimental

2.1. Materials

Bi(NO₃)₃·5H₂O, NaWO₄·2H₂O, NaOH, RhB, MB, and EY were purchased from Aladdin (Shanghai, P. R. China). Water with a resistivity of 18.2 MΩ cm at 25 °C, obtained from a Hitech-Kflow water purification system (Shanghai, P. R. China), was used in this study.

2.2. Preparation of Bi₂WO₆ nanosheets

20 mmol of Bi(NO₃)₃·5H₂O and 10 mmol of NaWO₄·2H₂O were dissolved in 150 mL of 1 M HNO₃ solution and 150 mL of water, respectively. Then, the NaWO₄ solution was dripped into the Bi(NO₃)₃ solution under stirring, after which the mixed solution with the initial pH of <1 was divided into four parts. pH values of three parts were adjusted to 4.0, 6.0, and 8.0, respectively, with 6 M NaOH solution. After stirring for 10 min, the suspensions were transferred to 80 mL polytetrafluoroethylene-lined autoclaves and heated at 180 °C for 24 h. Afterwards, the autoclaves cooled naturally to the room temperature. The Bi₂WO₆ samples were obtained after filtered, washed three times with water, and dried at 60 °C for 24 h. The samples synthesized at pH < 1, 4, 6, and 8 were denoted as W1, W4, W6, and W8, respectively.

2.3. Characterization

Powder X-ray diffraction (XRD) was performed on a D8 ADVANCE diffractometer (Bruker, Germany), with Cu Kα radiation (λ = 1.54184 Å). X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific Escalab 250Xi spectrometer (UK) with Al Kα radiation. The C 1s peak at 284.6 eV was used to calibrate peak positions. Morphology observations were performed on an S-4800 field emission-scanning electron microscope (SEM, Hitachi, Japan) and a JEM-2100F transmission electron microscope (TEM, Jeol, Japan). Selected area electron diffraction (SAED) pattern was obtained from the TEM. UV-Vis diffuse reflectance spectra were obtained using a U-4100 spectrophotometer (Hitachi, Japan), with a BaSO₄ reference. Photoluminescence (PL) spectra were measured using an F-7000 spectrophotometer (Hitachi, Japan) with excitation wavelength of 300 nm, and excitation and emission slit widths of 10 and 5 nm, respectively. Volumetric N₂ adsorption–desorption isotherms at liquid nitrogen temperature (−196 °C) were measured by a Quantachrome NOVA2000E instrument (USA). Samples were degassed at 150 °C for 3 h under vacuum before measurements. Photocurrent density was tested on an electrochemical workstation (Gamry Reference 600, USA) with a standard three-electrode cell. Ag/AgCl and Pt wire were used as the reference and counter electrodes, respectively. The electrolyte solution was 0.5 M Na₂SO₄. The applied bias voltage was −0.3 eV. The working electrodes were prepared by coating the sample slurries (obtained by grinding the mixture of 0.02 g of Bi₂WO₆, 40 μL of PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), 1.3%, Sigma) and 200 μL of H₂O onto a clean indium doped tin oxide (ITO) glass surface, followed by vacuum drying for 3 h. A 300 W xenon lamp was used as the light source. The zeta potentials of Bi₂WO₆ suspension were measured using a Beckman Coulter Delsa Nano C particle analyzer (USA).

The photocatalytic activity of samples was evaluated by degrading the RhB, MB, and EY dyes at the room temperature, on a XPA-7 photocatalytic reaction apparatus (Xujiang Electromechanical Plant, Nanjing, P.R. China).^5,34 A 500 W xenon lamp equipped with an ultraviolet cutoff filter (λ ≥ 420 nm) was used as the visible light source. 0.02 g of photocatalyst was added to 50 mL of dye (10 mg L⁻¹) solution. Prior to irradiation, the suspension was stirred in dark for 1, 2, and 2 h for MB, RhB, and EY, respectively, to ensure adsorption equilibrium. After a given irradiation time, ∼4 mL of the dispersion was taken out, filtered through a 0.45 μm polyether sulfone syringe filter, and analyzed by a Hewlett-Packard 8453 UV-Vis spectrophotometer (USA), at the wavelength of 554 nm for the RhB, 664 nm for the MB, and 515 nm for the EY. The ratio of remaining dye concentration to its initial concentration (C/C₀) was obtained by calculating the ratio of corresponding absorbances.

3. Results and discussion

3.1. Structure and morphology

Fig. 1 shows XRD patterns of the Bi₂WO₆ nanosheets. All the samples show intense and clear diffraction peaks corresponding to pure orthorhombic phase of the Bi₂WO₆ (JCPDS file No. 73-1126). From W1 to W8 (or with increasing pH), the diffraction peak intensity gets higher, suggesting crystallinity of the samples turns better, which is in accordance with reported results.^27,29,30

Fig. 1 XRD patterns of Bi₂WO₆ nanosheets.

To probe chemical microenvironments of surface elements in the Bi₂WO₆ nanosheets, XPS spectra of the samples were measured (Fig. 2). Survey spectra (Fig. 2a) show that all the samples contain W, O, Bi, and adventitious C elements.³⁵ Bi 4f, W 4f, and O 1s core level spectra (Fig. 2b–d) indicate that the samples W1, W4, and W6 possess similar peak binding energies (E_B), as shown by dashed lines in the figure. Peaks at E_B of 164.7 and 159.4 eV (Fig. 2b) correspond to Bi 4f_5/2 and 4f_7/2 of Bi³⁺,²⁰ respectively; those at 37.8 and 35.6 eV (Fig. 2c) correspond to W 4f_5/2 and 4f_7/2 of W⁶⁺,²⁰ respectively; and the peaks at 532.4, 530.8, and 530.1 eV (Fig. 2d) correspond to O 1s of O²⁻ in surficial adsorption oxygen species (–OH or H₂O), O–Bi, and O–W bonds,¹⁰ respectively, in Bi₂WO₆. In addition, for the W8, new peaks at 165.4 and 160.1 eV in the core level spectrum of Bi 4f (Fig. 2b) and those at 38.3 and 36.3 eV in the core level spectrum of W 4f (Fig. 2c) are probably due to the doped ions Bi⁴⁺ and/or Bi⁵⁺ replacing W⁶⁺.^10,36 The doped ions lead to the formation of Bi⁴⁺/Bi⁵⁺–O (Fig. 2b) and W–O–Bi⁴⁺/Bi⁵⁺ (Fig. 2c) bonds. From Fig. 2b, the molar ratio of Bi⁴⁺/Bi⁵⁺ to Bi³⁺ in the W8 is figured out to be ∼0.135 according to related peak areas, which is close to the highest doped amount reported in the literature.¹⁰ The substitution of Bi⁴⁺/Bi⁵⁺ for W⁶⁺ may cause breakage of W–O–W bonds because of less valence electron number of the Bi, and thus may lead to production of some oxygen vacancies. The formation of oxygen vacancies increases local surface electron density around Bi³⁺ of the W8,^16,37 which makes the peak E_B of Bi 4f of Bi³⁺ in Bi³⁺–O bonds and W 4f of W⁶⁺ in W–O–W bonds decrease by 0.3 and 0.4 eV (Fig. 2b and c), respectively,^38–40 in comparison with W1, W4 and W6. However, the peak E_B of O 1s of O²⁻ in O–Bi bonds of the W8 (Fig. 2d) increases by ∼0.2 eV, which we think is caused by the doped Bi⁴⁺/Bi⁵⁺.

Fig. 2 (a) XPS survey spectra and high-resolution XPS spectra of (b) Bi 4f, (c) W 4f and (d) O 1s for Bi₂WO₆ nanosheets.

Morphologies of the Bi₂WO₆ samples were observed by SEM and TEM. All the samples reveal irregular nanosheet structures (Fig. 3a and b, S1 and S2 in ESI†). Average lateral sizes (L) and thicknesses (H) of the nanosheets (Fig. S3 and S4, ESI†) are shown in Table 1. From W1 to W8, the L and H increase from 96 to 417 nm and from 16 to 93 nm, respectively. Fig. 3c shows the high-resolution TEM image of the W1. The crystal fringes with spacing of 0.275 nm correspond to the (006) facets. SAED pattern in Fig. 3d shows clear ordered diffraction spots, indicating the nanosheet has a single crystal structure. The (006), (200), and (206) facets are all perpendicular to (010) facets, demonstrating that the nanosheets expose the (010) facets at top and bottom surfaces and the (001) and (100) facets at side surfaces (Fig. 3e). The exposure percentages of the (010) facets (P₀₁₀) were figured out by the equation P₀₁₀ = 1/(2H/L + 1), provided that the nanosheets are regularly polygon. As shown in Table 1, the P₀₁₀ exhibits a reduced trend with increasing pH of the precursor suspensions. The (010) facets of the Bi₂WO₆ are terminated by high-density oxygen atoms (Fig. 3e). Adsorption of H⁺ on the facets can significantly decrease the surficial energy.⁴¹ Thus, at low pH, the H⁺ tends to adsorb on the (010) facets, which prohibits crystal growth along the y axis and increases exposure percentage of the (010) facets.

Fig. 3 (a) SEM, (b) TEM, (c) HRTEM, and (d) SAED images of W1, and (e) schematic illustration and crystal structure of Bi₂WO₆. Octahedrons in the figure mean one W atom coordinate with six O atoms to form WO₆.

Table 1 Physicochemical properties of Bi₂WO₆ nanosheets

Sample	L ^a (nm)	H ^b (nm)	P 010 ^c (%)	S BET ^d (m^2 g^−1)	Mesopore size (nm)	Γ m ^e (mg g^−1)			Γ s ^f (mg m^−2)
						RhB	MB	EY	RhB	MB	EY
a Lateral size of nanosheets from SEM images. b Thickness of nanosheets from SEM images. c Exposure percentage of (010) facets. d BET specific surface area. e Adsorption capacities per gram of samples. f Adsorption capacities per m^2 of samples.
W1	96	16	75	40.3	4.3	9.8	16.1	9.2	0.24	0.40	0.23
W4	122	25	71	15.0	12.8	5.5	8.6	4.7	0.37	0.57	0.31
W6	112	34	62	11.4	6.7	4.7	7.0	4.1	0.41	0.61	0.36
W8	417	93	69	5.9	5.7	1.5	2.0	1.7	0.25	0.34	0.29

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3.2. UV-Vis diffuse reflectance spectra and N₂ sorption isotherms

The UV-Vis diffuse reflectance spectra of the Bi₂WO₆ samples (Fig. 4) show that the photoabsorption of the Bi₂WO₆ samples exhibit an enhanced trend at λ < 600 nm from the W1 to the W8. Band gaps (E_g) of the samples were estimated via the equation ahν = K(hν − E_g)^n/2,¹⁴ where K, a, and ν are the proportionality constants, the absorbance, and the incident light frequency, respectively. n is a constant depending on the transition characteristics in a semiconductor (n = 1 for a directly allowed transition and n = 4 for an indirectly allowed transition), n = 1 was confirmed here.¹⁴ As shown in the inset in Fig. 4, all the samples possess the similar E_g (∼2.60 eV).

Fig. 4 UV-Vis diffuse reflectance spectra and ahν vs. hν curves of Bi₂WO₆ nanosheets.

N₂ adsorption–desorption isotherms were measured to determine the specific surface areas and pore structures of the samples. All of the samples exhibit a type II isotherm with a type H3 hysteresis loop (Fig. 5a), which features the slit-shaped mesopores.^8,14,42 The Brunauer–Emmett–Teller (BET) specific surface areas (S_BET) of the samples are 40.3–5.9 m² g⁻¹ and decrease from W1 to W8 (Table 1). The Barrett–Joyner–Halenda (BJH) method is used to calculate the pore size distributions of the samples. The pore distribution curves (Fig. 5b) show that the four samples contain two types of pores with sizes of <2 and 4–13 nm, respectively (Table 1). The pores with a size of <2 nm are probably formed by stack of the nanosheets,⁵ and the pores with a size of 4–13 nm probably arise from the aggregation of the stacked nanosheet structures (Fig. S5, ESI†).

Fig. 5 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution curves of Bi₂WO₆ nanosheets.

3.3. Photocatalytic activity

The photodegradation efficiencies of the RhB, MB, and EY on various Bi₂WO₆ samples were measured under visible light irradiation. All the suspensions reach the adsorption–desorption equilibrium before light illumination (Fig. S6, ESI†). Adsorption capacities (Γ_m, mg g⁻¹) of the RhB, MB, and EY on the photocatalysts are 1.5–9.8, 2.0–16.1, and 1.7–9.2 mg g⁻¹, respectively, and decrease gradually from W1 to W8. To determine the effect of the S_BET on the Γ_m of dyes, the Γ_m normalized by the S_BET⁴³ (Γ_s, mg m⁻²) were calculated (Table 1). The Γ_s values of the four Bi₂WO₆ samples for each dye are almost equal, being 0.33 ± 0.09, 0.47 ± 0.14, and 0.30 ± 0.07 mg m⁻² for the RhB, MB, and EY, respectively. This demonstrates that the difference in the Γ_m of the Bi₂WO₆ samples results mainly from their S_BET differences.

Photodegradations of the dyes on the Bi₂WO₆ samples are shown in Fig. 6. The photodegradation efficiencies of these dyes gradually increase from W8 to W1. For example, the W1 can degrade ∼84% of the RhB in 280 min, ∼62% of the MB in 22 h, and ∼97% of the EY in 6 h, while the W8 degrades only ∼28, 37, and 45%, respectively, at the same time intervals (Fig. 6a–c). To more clearly compare the photocatalytic efficiencies of the Bi₂WO₆ samples, the photodegradation data were fitted to a pseudo-first-order kinetic model, −ln(C/C₀) = kt, where k and t are the rate constant and the time, respectively.⁸ All the data can be well described by this model (Fig. S7, ESI†). As shown in Fig. 6d–f, the k values of the RhB, MB, and EY increase about 4.6, 98.6, and 25 times, respectively, from W8 to W1.

Fig. 6 Photodegradation of (a) RhB, (b) MB, and (c) EY on Bi₂WO₆ nanosheets under visible light irradiation, and related pseudo-first-order kinetic rate constant (k) and S_BET-normalized k (k′) for (d) RhB, (e) MB, and (f) EY. The k and k′ in (e) and (f) are data excluding self-degradation of dyes (Fig. S7, ESI†).

3.4. Thickness-dependence of photoactivity

From W8 to W1, the crystallinity gets worse (Fig. 1) and the photoabsorption becomes weaker (Fig. 4), which are not favorable for the photoactivity enhancement, indicating that neither of them is the factor leading to the improvement of photocatalytic performance observed in this work. The doped amount of Bi in the W8 (Fig. 2) favors the photoactivity improvement according to Ding and coauthors' work,¹⁰ so the Bi doping is not the reason for the low photocatalytic performance of the W8. The MB can't be photodegraded through the photosensitization process, suggesting the photosensitization effect is negligible.⁴⁴

In view of the gradual increase in the photoactivity of the Bi₂WO₆ samples with increasing S_BET, the S_BET is probably an important factor causing the photoactivity enhancement. To exclude the effect of the S_BET, the S_BET-normalized k (k′) values were estimated (Fig. 6d–f). Though having greatest k values, the W1 possesses the least k′ value for the RhB and less k′ value than the W4 for the MB, indicating that the S_BET indeed has a great effect on the photoactivity enhancement. However, an obvious increase of the k′ from W8 to W4 for the RhB and MB and from W8 to W1 for the EY can be observed, which is consistent with the changes of their k values (Fig. 6), suggesting that some other factors are also responsive for the photoactivity increase. The continuous increase of the k′ for the EY from W4 to W1 suggests that the influence of the S_BET on the EY photodegradation is much weaker than that on the RhB and MB photodegradations. This is because the W1, W4, W6, and W8 with zeta potentials of −1.59, −0.14, −1.85, and −0.98 mV, respectively (Fig. S8, ESI†), are liable to adsorb the cationic dyes RhB and MB and promote their degradation, which makes the influence of the S_BET on the EY (an anionic dye) photodegradation relatively weak.

As is well known, the recombination rate of photogenerated charge carriers (RPC) substantially influences the photocatalytic performance of semiconductors.^45,46 Photoluminescence (PL) spectroscopy and photoelectrochemical methods are usually used to indirectly characterize the RPC.⁴⁷ As shown in Fig. 7, the photocurrent density gradually increases from W8 to W1, indicating the reduced RPC. In addition, PL intensities of the samples show a decreasing trend from W8 to W1 (Fig. S9, ESI†), also demonstrating the reduction of RPC.⁴⁷ Thus, the increase of photocatalytic performance from W8 to W1 is attributed to the reduction of RPC.

Fig. 7 Photocurrent density (j) of Bi₂WO₆ nanosheets.

Reduction of the nanoparticle size profits the reduction of the RPC.²⁷ For randomly generated charge carriers, the average diffusion time (t_D) from the interior to the surface of colloidal semiconductor particles follows the equation t_D = r²/(π²D), where r and D are the particle radius and the diffusion coefficient of the carriers, respectively.⁴⁸ If the r decreases, the photogenerated charge carriers can more easily transfer to the surface of grains to participate the interfacial reactions, leading to the decrease of the RPC.²⁷ However, in our previous work,^5,14 we found that, for the 2D BiOBr nanosheets, the photoactivity is not related to the lateral size of the nanosheets, but is significantly influenced by the thickness. Thereby, the relationships between the k′ and H and between the k′ and L for the Bi₂WO₆ nanosheets were studied.

As shown in Fig. 8a, the k′ increases gradually with the reduction of the H from W8 to W4, indicating the H is a significant factor causing the photoactivity increase of the Bi₂WO₆ nanosheets. This is because the reduction of the H effectively decreases transfer distances of photogenerated charge carriers from the inside to the surface of the Bi₂WO₆ nanosheets (Fig. 3e). The decrease of the k′ of RhB and MB for the W1 is because of the effect of the S_BET as discussed above. Differently, there is no regular change of the k′ with increasing L (Fig. S10, ESI†).

Fig. 8 (a) Variations of specific surface area (S_BET) normalized pseudo-first-order rate constant (k′) with nanosheet thickness (H), (b) pseudo-first-order rate constant (k) with S_BET/H, and (c) k with H² for various substrates.

Given that both the S_BET and the H have great effects on the photoactivity of the Bi₂WO₆ nanosheets, the relationship between the “shape factor”, S_BET/H, suggested in our previous work⁴⁹ and the k is shown in Fig. 8b. The k increases monotonously with the increment of the S_BET/H, manifesting that the “shape factor” is a reasonable parameter to reflect the photocatalytic performance of the 2D photocatalysts.

Actually, the S_BET depends on the L and H if the nanosheets were completely dispersed in aqueous medium. Provided top and bottom surfaces of these nanosheets are regular polygons, the S_BET can be calculated from the equation: S_BET = 2/ρ(1/H + 2/L), where ρ is the density of Bi₂WO₆ nanosheets (S1, ESI†). Because the H is much less than the L for the Bi₂WO₆ nanosheets, the variation of (1/H + 2/L) from W8 to W1 mainly depends on that of the 1/H (Fig. S11, ESI†). Thus, the 2/L is negligible and the S_BET is proximately proportionate to the 1/H. In this case, the effect of the S_BET/H on the photoactivity can be proximately considered as the influence of H⁻² or H². The variation of the k with the H² for the Bi₂WO₆ nanosheets is shown in Fig. 8c. The k linearly decreases by and large with the increase of the H² for W1–W6. Notably, the data for the W8 deviate from this regularity. This is possibly because some (∼42%) W8 nanosheets have an H value of >100 nm (Fig. S4†) which is larger than the penetration depth of visible light (∼100 nm^50–52). When the H of nanosheets is higher than the light penetration depth, the H-dependence of their photocatalytic efficiency is not obvious. Another possible reason is the existence of Bi doping for the W8 (Fig. 2) that favors the enhancement of photocatalytic efficiency.¹⁰

In association with our previous results that, for the BiOBr nanosheets with different H and similar S_BET, the k linearly increases with the reduction of the H,^5,14 it may be concluded that for the nanosheets with different H and different S_BET whose variation is caused by the change of the H, the k increases linearly with the reduction of the H².

4. Conclusions

Four Bi₂WO₆ nanosheet samples with different thicknesses (16–93 nm), lateral sizes (96–417 nm), and specific surface areas (5.9–40.3 m² g⁻¹) but with a similar energy bands of ∼2.60 eV were synthesized hydrothermally via the pH adjustment of precursor dispersions. As the pH decreases from 8 to <1, photocatalytic efficiencies of the samples in degradation of the RhB, MB, and EY increase by about 5, 99, and 25 times, respectively. The photoactivity enhancement is mainly ascribed to the reduction of the H. The pseudo-first-order kinetic rate constant increases with the reduction of the H². This study supplies important guidance for structure–performance relationship research of 2D photocatalysts.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (No. 21573133 and 21273135) and the Fundamental Research Funds of Shandong University in China (No. 12320075614004).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03268k

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