Improved photoelectrode performance of chemical solution-derived Bi₂O₃ crystals via manipulation of crystal characterization

Abstract

Three-dimensional Bi₂O₃ crystals with various morphologies were successfully synthesized on F-doped tin oxide substrates with and without homoseed layers via chemical bath deposition (CBD) routes. The structural analysis reveals that control of the pH value of the reaction solution resulted in as-grown Bi₂O₃ crystals with nanosheet and plate morphologies. A lower pH value of the reaction solution engendered formation of a porous sheet-like morphology of Bi₂O₃; by contrast, a higher pH value of the reaction solution is favorable for formation of solid Bi₂O₃ plates on the substrates. Furthermore, a sputter coated Bi₂O₃ seed layer with dual α- and β-Bi₂O₃ phases plays an important role in the CBD-derived Bi₂O₃ crystallographic structures. The Bi₂O₃ crystals formed via CBD processes without a sputter coated Bi₂O₃ homoseed layer demonstrated a high purity in β-Bi₂O₃ phase; those grown with a homoseed layer exhibited a dual α/β phase. The photoactive performance results show that construction of an α/β-Bi₂O₃ homojunction in the CBD-derived Bi₂O₃ crystals substantially improved their photoactive performance. Comparatively, the porous Bi₂O₃ nanosheets with a dual α/β-Bi₂O₃ phase demonstrated the highest photoactive performance among various Bi₂O₃ crystals in this study. The superior photoactivity of the porous α/β-Bi₂O₃ nanosheets herein is attributed to their high light absorption capacity and photoinduced charge separation efficiency. The experimental results in this study provide a promising approach to design CBD-derived Bi₂O₃ crystals with desirable photoelectric conversion functions via facile morphology control and seed layer crystal engineering.

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Introduction

Bi₂O₃ is an environmental friendly material, and it has received much attention due to its unique electrical and optical properties, which leads to its extensive usage in various scientific devices.^1–3 Furthermore, it is a promising visible-light-driven photocatalyst with a narrow band gap (2.0–2.9 eV), and the valence band of bismuth oxide is made of a hybrid Bi 6s and O 2p orbit that decreases the symmetry of the band structure and leads to the corresponding dipoles which could enhance its photoactivity.⁴ Bi₂O₃ has different polymorphs;⁵ the monoclinic α and tetragonal β phases have been reported to exhibit superior photoactivity among various polymorphs.^2,6 Recently, several works on synthesis of α-Bi₂O₃ crystals have been reported.^7,8 By contrast, a relatively few works on synthesis of β-Bi₂O₃ crystals are proposed due to the difficulty of synthesizing the metastable β-phase.⁹ The β-Bi₂O₃ has a higher photoactivity than α-Bi₂O₃ because of its lower band gap energy and higher optical absorption in visible light region.¹⁰ Since the β-Bi₂O₃ phase represents a metastable high temperature modification and is known to transform readily to α-Bi₂O₃, controllable synthesis of defined β and α polymorphs and their stabilization at room temperature are complicated. Thus, the selective fabrication of β-Bi₂O₃ in a phase-pure form is still of great significance.

Bi₂O₃ crystals with various micro- and nanostructures, including nanosheet, nanorod, nanofiber, and sphere have been successfully synthesized via various chemical solution routes.^11–14 Among various morphologies, nanosheet semiconductor oxides have drawn substantial research interest due to their unique geometry with outstanding electronic properties and high specific surface area, which could provide large amount of reaction sites and enable rapid charges generation and transport.^15,16 Till now, various synthetic methods have been explored to prepare sheet- or plate-like β-Bi₂O₃ crystals with various surface features.^11,17 All the above sheet- or plate-like Bi₂O₃ products exhibited outstanding visible-light-driven light harvesting ability and photoactive performance. For practical scientific applications, the cost of energy, time and raw materials required for synthesizing Bi₂O₃ crystals processes should be considered. Therefore, the chemical bath deposition (CBD) because of its large area deposition ability, low cost process, and easy process parameter control, is a promising route to be used for developing Bi₂O₃ crystal growth.¹⁸ Notably, several works have confirmed the importance of homoseed layer for the crystal growth behavior of various chemical route derived oxide crystals. For example, hydrothermal grown c-axis oriented zinc oxide nanorods were prepared on the c-axis oriented ZnO homoseed layer.¹⁹ The WO₃ homoseed layer with desirable surface grain size shows important influence on the vertical alignment and aggregation in chemical route-derived one-dimensional WO₃ crystals.²⁰ Furthermore, homoseed layer of CuO has an important influence on the crystallinity and morphology of CuO crystals grown via CBD routes.²¹ In addition to the crystal growth behavior affected by the seed layer, the seed layer also dominates the as-grown crystals' characterization. However, systematical investigations of homoseed layer effect on CBD deposition of sheet- or plate-like Bi₂O₃ crystals and their resultant photoactive performance are not well proposed. In this study, a Bi₂O₃ homoseed layer was formed via sputtering deposition on substrates before the CBD Bi₂O₃ crystal growth. The present study introduces an important scientific aspect: the usage of a homoseed layer to improve the CBD process to change the growth and characteristics of two types of 3D Bi₂O₃ crystals has not been proposed so far. The effects of pre-formed Bi₂O₃ seed layer microstructures on the crystal features of the CBD-derived Bi₂O₃ crystals are demonstrated herein. Furthermore, the resultant crystal feature-dependent photoactive performance of various 3D Bi₂O₃ crystals with and without a homoseed layer is also discussed. The results herein are a good reference for design of chemical solution derived 3D Bi₂O₃ crystals with desirable photoactive performance with a homoseed layer engineering.

Experiments

Sample preparation

Various Bi₂O₃ crystals were synthesized by chemical bath deposition (CBD). Fluorine-tin-oxide (FTO) glass substrates were ultrasonically cleaned sequentially in distilled water, acetone and isopropyl alcohol. For preparation of CBD reaction solution, the 0.1 M bismuth nitrate was dissolved in deionized water to make 50 mL stock solution. A highly alkaline reaction solution was prepared by further adding 3 mL triethanolamine (TEA) as a complexing agent and 0.2 M NaOH. The FTO substrates were immersed vertically in a 200 mL beaker and the pH value of the resulting solution was adjusted to 10 and 13 with various addition contents of 0.1 M NaOH, and the subsequent CBD reaction of Bi₂O₃ crystals was carried out at 65 °C for 2 hours. The as-deposited samples were further annealed at 300 °C for 1 hour in ambient air to obtain crystalline Bi₂O₃ phase. For the preparation of seed layered CBD samples, the Bi₂O₃ homoseed layer was formed on the FTO substrates through post-annealing the sputtering deposited metallic Bi thin films at 400 °C in ambient air for 1 hour. The Bi metallic thin films were sputter deposited at room temperature with a pure Ar atmosphere. The working pressure during Bi thin-film growth was maintained at 2 Pa, and the sputtering power of the bismuth metallic target was fixed at 30 W. The distance between the FTO glass substrate and target is approximately 7 cm. Then the Bi₂O₃ seed layer coated FTO substrates were used to prepare the Bi₂O₃ crystals via CBD with the pH values of 10 and 13 as mentioned before. The sample codes of 10-Bi₂O₃, 13-Bi₂O₃, 10S-Bi₂O₃ and 13S-Bi₂O₃ represented Bi₂O₃ crystals formed at pH = 10 without a seed layer, pH = 13 without a seed layer, pH = 10 with a seed layer, and pH = 13 with a seed layer, respectively in this study.

The CBD route-derived Bi₂O₃ crystals herein are based on the following reaction steps:

In the nucleation process, bismuth nitrate reacts with the complexing (TEA) reagent by dissociating the bismuth ions as:

Bi(NO₃)₃ + N(CH₂–CH₂–OH)₃ ⇆ [BiN(CH₂–CH₂–OH)₃]³⁺ + 3NO₃ (1)

[BiN(CH₂–CH₂–OH)₃]³⁺ ⇆ Bi³⁺ + N(CH₂–CH₂–OH)₃ (2)

The pH value of the solution was adjusted to 10 or 13 with NaOH solution herein to form Bi(OH)₃:

NaOH ⇆ Na⁺ + OH⁻ (3)

Bi³⁺ + 3OH⁻ ⇆ Bi(OH)₃ (4)

After annealing in ambient air at the elevated temperature, the Bi(OH)₃ converts into bismuth oxide (Bi₂O₃) via the following reaction.

2Bi(OH)₃ ⇆ Bi₂O₃ + 3H₂O (5)

The thermal annealing treatment of the as-deposited film will cause oxygen to remove hydroxide ions and form the crystalline Bi₂O₃ crystals²²

Materials analysis

Sample crystal structures were investigated by X-ray diffraction (XRD; Bruker D2 PHASER) analysis using Cu Kα radiation (wavelength: 0.15406 nm) with a two theta scan range of 20–60° and scan rate of four degrees per min. The surface morphology of the thin film sample was characterized by scanning electron microscope at an accelerating voltage of 15 kV (SEM; Hitachi S-4800). A transmission electron microscope was used to study the detailed microstructure of the Bi₂O₃ sample at 200 KV (HRTEM; Philips Tecnai F20 G2). To prepare the TEM test piece, first scrape off the nanostructures grown on the substrate into the absolute alcohol solution, and then drip a few drops from the solution into the copper mesh. Finally place it in a 90 °C oven for more than 12 hours. The diffuse reflectance spectra of the Bi₂O₃ samples were recorded by using UV-vis spectrophotometer (Jasco V750) with a scanning speed of 17 nm s⁻¹ at the 300–700 nm range. The elemental binding states of the Bi₂O₃ crystals were explored by X-ray photoelectron spectroscopy (XPS ULVAC-PHI, PHI 5000 VersaProbe) using Mg Kα radiation. The electrochemical and photoelectrochemical properties of the samples were conducted by the potentiostat (SP150, BioLogic). The illumination excited from the 100 W Xe lamp combined with a 420 nm cut-off filter was used as the light source during the experiments. The Bi₂O₃ film grown on the 1 cm × 1 cm FTO glass was used as working electrode. Platinum and Ag/AgCl were used as counter and reference electrode respectively. The 0.5 M solution of Na₂SO₄ was used as electrolyte. The PEC measurement is carried out in a voltage of 1 V, and every 20 seconds is used as an interval for switching the lights. Electrochemical impedance spectroscopy (EIS) measurements were performed using the aforementioned potentiostat and the same electrode configuration. Furthermore, the Nyquist plots of various samples were measured at the open circuit potential with the frequency from 0.1 Hz to 150 kHz.

Results and discussion

Fig. 1(a)–(d) show the XRD patterns of various Bi₂O₃ films. In addition to the Bragg reflections originated from the FTO substrate, Fig. 1(a) and (b) show Bragg reflections centered at approximately, 27.95°, 31.76°, 32.69°, 46.22°, 46.91°, 54.27°, 55.63° and 57.76° corresponding to the (210), (002), (220), (222), (400), (203), (213) and (402) of the tetragonal structured β-Bi₂O₃ (β-Bi₂O₃: JCPDS no. 027-0050). No other crystalline phases were detected, confirming the phase purity of the β-Bi₂O₃ product for the 10-Bi₂O₃ and 13-Bi₂O₃ films. Furthermore, a substantially high intensity of the (201) Bragg reflection appeared in Fig. 1(a) and (b), revealing a preferred (201)-oriented β-Bi₂O₃ crystals dominated the crystallographic feature of the CBD-derived 10-Bi₂O₃ and 13-Bi₂O₃ thin films herein. A similar (201)-oriented crystallographic feature has been observed in the spray pyrolysis technique and hydrothermal derived β-Bi₂O₃ crystals.^17,23 Fig. 1(c) and (d) display the XRD patterns of the 10S-Bi₂O₃ and 13S-Bi₂O₃ films, respectively. For a comparison, the XRD pattern of the Bi₂O₃ seed layer was demonstrated in Fig. 1(e). The visible Bragg reflections originated from α-Bi₂O₃ (120), α-Bi₂O₃ (200), β-Bi₂O₃ (201), β-Bi₂O₃ (222), β-Bi₂O₃ (203), and β-Bi₂O₃ (213) are identified in the Bi₂O₃ seed layer (α-Bi₂O₃: JCPDS no. 041-1449). The Bi₂O₃ seed layer exhibited a dual α/β mixed phase feature. In addition to the Bragg reflections of the originally identified β-Bi₂O₃ phase as observed in Fig. 1(a) and (b), several visible Bragg reflections originated from the crystallographic planes of the monoclinic α-Bi₂O₃ phase were also identified in Fig. 1(c) and (d). The XRD results demonstrate that the CBD-derived Bi₂O₃ crystals with a homoseed layer-assisted growth exhibited a mixed α/β mixed phase feature at the same given CBD growth conditions. The initially formed α-Bi₂O₃ phase in the seed layer might promote the α-Bi₂O₃ crystal formation during the CBD growth. The XRD results suggested that two crystal growth routes involved α-Bi₂O₃ and β-Bi₂O₃ crystal nucleation and growth respectively on the α-Bi₂O₃ and β-Bi₂O₃ phase regions in the seed layer might occur and competed during the given CBD growth conditions herein. Comparatively, the 13S-Bi₂O₃ film demonstrated a higher number of Bragg reflections originated from the α-Bi₂O₃ phase and the ratio of the integrated Bragg reflection intensity of the α-Bi₂O₃ over the all integrated Bragg reflection intensity (35.9%) is higher than that in the 10S-Bi₂O₃ film (12.2%). This reveals that a higher ratio of the α-Bi₂O₃ phase existed in the 13S-Bi₂O₃ film. Notably, a higher pH value in the CBD solution has been shown to be more favorable to the α-Bi₂O₃ growth environment as revealed in the earlier work by Chen et al.²⁴ The higher α-Bi₂O₃ phase content in the 13S-Bi₂O₃ film than that of the 10S-Bi₂O₃ film might also accounted for the integrated effect of dual phase seed layer together with the solution pH value during the phase evolution of the CBD-derived Bi₂O₃ films in this study.

Fig. 1 XRD patterns of various Bi₂O₃ films: (a) 10-Bi₂O₃. (b) 13-Bi₂O₃. (c) 10S-Bi₂O₃. (d) 13S-Bi₂O₃. (e) Sputtering deposited Bi₂O₃ seed layer.

Fig. 2 shows top- and cross-sectional views of SEM micrographs of various Bi₂O₃ films prepared by various process procedures. In Fig. 2(a), the as-synthesized 10-Bi₂O₃ film is composed of many nanosheets perpendicular to the substrate surface and crossing each other. These sheet-like structures have the thickness in the range of 40–70 nm and width in the range of 2.5–3.5 μm. The Bi₂O₃ nanosheets contained numerous tiny pores. Furthermore, the Bi₂O₃ nanosheets exhibited a rugged peripheral morphology, and the composition is loose from the cross-sectional view observation. By contrast, the 13-Bi₂O₃ film consisted of numerous plates with a high growth density and shape uniformity as observed in Fig. 2(b). The Bi₂O₃ plates were in a rectangular morphology and the surface of peripherals was smooth. Comparatively, the distribution density of the Bi₂O₃ plates over the area of the substrate is substantially higher than that of the 10-Bi₂O₃ film. From Fig. 2(b), the thickness and width of the plates can be evaluated to be approximately in the ranges of 500–700 nm and 2.5–3.5 μm, respectively. The SEM observations herein demonstrated that the 13-Bi₂O₃ plates are thicker than that of the 10-Bi₂O₃ nanosheets, revealing the higher pH reaction solution caused an increased Bi₂O₃ crystal thickness. This can be understood from earlier eqn (4) that the higher the pH value of the reaction solution, the more the bismuth ion reacts with the hydroxide ion; the thicker Bi(OH)₃ will be formed during the CBD. The initially formed thicker Bi(OH)₃ crystals will result in formation of the thicker Bi₂O₃ crystals after the postannealing procedure. Fig. 2(c) and (d) show the SEM images of the 10S-Bi₂O₃ and 13S-Bi₂O₃ films grown with the assistance of the homo-Bi₂O₃ seed layers, respectively. In Fig. 2(c), the Bi₂O₃ nanosheets grown on the homo-Bi₂O₃ seed layer showed similar crystal morphology as revealed in Fig. 2(a). Moreover, the size of the nanosheets herein is almost the same with those grown without a homo-Bi₂O₃ seed layer, but 10S-Bi₂O₃ nanosheets are more evenly and densely dispersed on the substrate. The Bi₂O₃ seed layer herein promoted the CBD-derived Bi(OH)₃ crystal growth to be more uniformly along the seed layer. By contrast, the 13S-Bi₂O₃ film exhibited a feathery crystal feature which differed from the 13-Bi₂O₃ film (Fig. 2(d)). The feathery crystals had a thickness ranged from 1.0–1.5 μm and their length was approximately 3.0–4.0 μm. These feathery crystals were densely contacted to each other; moreover, several feathery crystals are bundled and arranged in a specific crystal orientation. This might be associated with the crystallographic orientation of the homo-seed layer that affect the resultant CBD-derived crystal growth orientation herein. It has also been shown that the homo-WO₃ seed layer promotes growth density and orientation of CBD-derived H₂WO₄·nH₂O precursor crystals and therefore, highly oriented and dense WO₃ nanostructures are formed after postannealing procedures. Furthermore, a similar promotion of crystal distribution density and consistency of growth orientation from a homoseed layer has been shown in the CBD-derived Fe₂O₃ and TiO₂ oxide crystal growth. The improved growth density and consistency of growth orientation in CBD-derived FeOOH and Ti(OH)₄ precursor crystals respectively on the Fe₂O₃ and TiO₂ thin homoseed layers result in formation of dense and highly-oriented Fe₂O₃ and TiO₂ crystals after proper post annealing procedures.^25,26 Notably, from earlier XRD analyses, the formation of 13S-Bi₂O₃ film is related to the crystal structure of monoclinic α-Bi₂O₃ and tetragonal β-Bi₂O₃. A markedly higher phase content of α-Bi₂O₃ in the 13S-Bi₂O₃ film might explain the observed substantial morphology change of the Bi₂O₃ film derived from the pH = 13 reaction solution on the homoseed layer. However, this morphology change is not visibly observed for the Bi₂O₃ film formed from the pH = 10 reaction solution with the assistance of homoseed layer crystal growth. The relative low phase content of α-Bi₂O₃ in the 10S-Bi₂O₃ film might not substantially affect its crystal growth mode and the resultant morphology in this study.

Fig. 2 SEM top and cross-sectional views of various Bi₂O₃ films: (a) 10-Bi₂O₃. (b) 13-Bi₂O₃. (c) 10S-Bi₂O₃. (d) 13S-Bi₂O₃.

Fig. 3(a) shows a low-magnification TEM image of 10-Bi₂O₃ nanosheet. The nanosheet exhibited a rugged peripheral morphology. Tiny pores existed in the nanosheet. The nanosheet consisted of many nanoscaled Bi₂O₃ particles which can be easily distinguished from the grayscale contrast image of particles around the sample. Fig. 3(b) and (c) are high-resolution TEM (HRTEM) images taken from the outer regions of the nanosheet. The visible lattice fringes with multiple orientations are observed in the Bi₂O₃ nanosheet, which reveals its polycrystalline feature. The distinguishable interplanar spacings of approximately 0.31 nm and 0.27 nm are corresponded to the (201) and (220) planes of tetragonal β-Bi₂O₃, respectively. Fig. 3(d) displays the selected area electron diffraction (SAED) pattern in which the sharp and bright spots were arranged in concentric circles with various radii. The (201), (002), (220), (222), and (421) crystallographic planes were determined to correspond to the tetragonal β-Bi₂O₃ phase herein. The composition of the Bi₂O₃ nanosheet was obtained by energy dispersive spectroscopy (EDS) analysis (Fig. 3(e)); the EDS spectrum indicated that the ratio of Bi/O is approximately 0.71, and the C and Cu signals in the spectrum were originated from the TEM grid.

Fig. 3 TEM analyses of the 10-Bi₂O₃ film: (a) low-magnification image of 10-Bi₂O₃ nanosheet. (b and c) High-resolution TEM (HRTEM) images taken from the local regions of the sample in (a). (d) Selected area electron diffraction (SAED) pattern of the sample in (a). (e) Energy dispersive X-ray spectroscopy (EDS) spectrum of the sample in (a).

Fig. 4(a) presents a low-magnification TEM image of 10S-Bi₂O₃ nanosheet. Similar to Fig. 3(a), the nanostructure exhibits sheet morphology with an irregular periphery; moreover, abundant pores existed in the structure. Fig. 4(b) and (c) display HRTEM images taken from the outer regions of the nanosheet. The distinct, ordered lattice fringes with multiple orientations are observed in the nanosheet. The interplanar spacings of approximately 0.32 nm and 0.25 nm are corresponded to the β-Bi₂O₃ (201) and α-Bi₂O₃ (200), respectively. HRTEM images reveal well crystalline feature of the nanostructure. The composition of the Bi₂O₃ nanosheet was further evaluated by EDS spectrum (Fig. 4(d)); the Bi/O ratio of the nanostructure is approximately 0.71 in this study.

Fig. 4 TEM analyses of the 10S-Bi₂O₃ film: (a) low-magnification image of 10S-Bi₂O₃ nanosheet. (b and c) HRTEM images taken from the local regions of the sample in (a). (d) EDS spectrum of the sample in (a).

The low-magnification TEM images in Fig. 5(a) and (b) show that the morphology of the Bi₂O₃ plates formed at pH = 13 (13-Bi₂O₃) differed from that of the 10-Bi₂O₃ nanosheet. The periphery of 13-Bi₂O₃ plates was relatively flat and the plates were more rectangular shaped. The rectangular 13-Bi₂O₃ plate was made of Bi₂O₃ particles with a size of approximately 50 nm and can be observed in these TEM images. These Bi₂O₃ particles were firmly assembled to form the Bi₂O₃ plates, and the granular feature is still visibly distinguished on the surface. Fig. 5(c) and (d) are HRTEM images taken from the outer local regions in Fig. 5(a). The clearly lattice fringes with multiple orientations are also observed in the outer regions of the HR images where is the thinner regions of the Bi₂O₃ plate. The granular particle images can also be distinguished in these HR images. The interplanar spacings with 0.32 nm and 0.27 nm corresponded to the (201) and (220) planes of tetragonal β-Bi₂O₃, respectively herein. Fig. 5(e) reveals the SAED pattern taken from the selected Bi₂O₃ plate in Fig. 5(a). The pattern exhibited several distinct diffraction rings consisting of sharp and bright spots that originated from the tetragonal β-Bi₂O₃ (201), (220), (222), (400), and (203) crystallographic planes. A well crystalline quality of the Bi₂O₃ plate was formed in this study. The EDS spectrum taken from the Bi₂O₃ plate (Fig. 5(f)), revealing that the atomic ratio of Bi/O is 0.69.

Fig. 5 TEM analyses of the 13-Bi₂O₃ film: (a and b) low-magnification images of the 13-Bi₂O₃ plates. (c and d) HRTEM images taken from the local regions of the sample in (a). (e) SAED pattern of the sample in (a). (f) EDS spectrum of the sample in (a).

Fig. 6(a) and (b) show the low-magnification images of the 13S-Bi₂O₃ feathery crystals. The feathery crystals are in a cone shape with two different aspect ratios can be observed herein. The granular particle assembled surface feature of the 13-Bi₂O₃ plate was not easily observed in Fig. 6(a) and (b). Comparatively, the 13S-Bi₂O₃ feathery crystals exhibited dense and flat surface feature. Fig. 6(c) and (d) display HRTEM images taken from the outer region of the feathery crystal in Fig. 6(a). Notably, the lattice fringes are quite regularly arranged and are visibly distinguishable at the corner regions. The lattice spacings with 0.32 nm and 0.25 nm corresponded to the (201) plane of β-Bi₂O₃ and the (200) plane of α-Bi₂O₃, respectively. The result further confirmed that a phase junction between α-Bi₂O₃ and β-Bi₂O₃ was obtained in the 13S-Bi₂O₃ feather crystal.²⁷ Fig. 6(e) shows the SAED pattern of the 13S-Bi₂O₃ feathery crystal in Fig. 6(a). According to the SAED pattern, the (201), (002), (220), and (222) crystallographic planes were determined to correspond to the tetragonal β-Bi₂O₃ phase. Moreover, the (002), (111), (120), (200), (022) and (140) crystal planes corresponded to the monoclinic α-Bi₂O₃ phase were also identified in the SAED pattern. The SAED analysis revealed that the α- and β-Bi₂O₃ phases coexisted in the 13S-Bi₂O₃ film. The EDS spectrum in Fig. 6(f) supported the high composition purity of the as-synthesized, and the Bi/O atomic ratio is approximately 0.68. Finally, a TEM morphology comparison chart of various Bi₂O₃ crystals was summarized in Fig. 6(g), the effect of homoseed layer on the morphology evolution is especially profound in the CBD-derived Bi₂O₃ crystals synthesized at pH = 13 reaction solution.

Fig. 6 TEM analyses of the 13S-Bi₂O₃ film: (a and b) low-magnification images of the 13S-Bi₂O₃ plates. (c and d) HRTEM images taken from the local regions of the sample in (a). (e) SAED pattern of the sample in (a). (f) EDS spectrum of the sample in (a). (g) A summarized TEM morphology chart of various Bi₂O₃ crystals.

Fig. 7 shows typical XPS survey spectra of various Bi₂O₃ films. In these XPS survey spectra, the main binding feature peaks can be assigned to originate from Bi, and O, proving the purity of the prepared Bi₂O₃ sample. Notably, the existence of carbon signal in XPS survey spectra is associated with the exposure of samples to ambient air, engendering surface carbon contamination. Fig. 8 shows the high-resolution XPS spectra of Bi 4f taken from various Bi₂O₃ films. The Bi 4f_7/2 peak at 157.1 eV and Bi 4f_5/2 at 162.4 eV could be observed with a separation of 5.3 eV, which was a characteristic of Bi³⁺ in Bi₂O₃ crystals according to the previous report.⁴ No distinct shoulder peaks appeared in Bi 4f spectra, revealing bismuth was only in +3 oxidation state, and no +5 and metallic states are present in the as-synthesized Bi₂O₃ films.²⁸ Fig. 9 presents the high-resolution O 1s spectra. The asymmetric XPS spectra of the O 1s could be deconvoluted into two subpeaks, which were oxygen in the lattice (Bi–O) at approximately 528.1 eV (peak I) and surface-absorbed oxygen at 529.5 eV (peak II).⁴ Comparatively, Fig. 9(a) and (c) demonstrate the size of the subpeak at the higher binding energy is higher than that of Fig. 9(b) and (d). This revealed that the surface-absorbed oxygen content of the 10-Bi₂O₃ and 10S-Bi₂O₃ films is larger than that of the 13-Bi₂O₃ and 13S-Bi₂O₃ films. An increased pH value of the CBD reaction solution to grow Bi₂O₃ films decreased the surface-absorbed oxygen size of the Bi₂O₃ thin films accordingly. The relative surface-absorbed oxygen content in the crystalline Bi₂O₃ films was further evaluated according to the intensity area ratio of Gaussian deconvolution subpeaks: (peak II)/(peak I + peak II). The relative surface-absorbed oxygen contents of the 10-Bi₂O₃, 13-Bi₂O₃, 10S-Bi₂O₃, and 13S-Bi₂O₃ films are approximately 31.6, 19.8, 33.4, and 23.1%, respectively. We inferred that this diversity may be caused by the different crystal morphology of various Bi₂O₃ films; more loose and porous structure for the Bi₂O₃ films synthesized at a lower pH value of the CBD reaction solution might accounted for the observed results.

Fig. 7 XPS survey spectra of various Bi₂O₃ films: (a) 10-Bi₂O₃. (b) 13-Bi₂O₃. (c) 10S-Bi₂O₃. (d) 13S-Bi₂O₃.

Fig. 8 High-resolution XPS spectra in the Bi 4f region of various Bi₂O₃ films: (a) 10-Bi₂O₃. (b) 13-Bi₂O₃. (c) 10S-Bi₂O₃. (d) 13S-Bi₂O₃.

Fig. 9 High-resolution XPS spectra in the O 1s region of various Bi₂O₃ films: (a) 10-Bi₂O₃. (b) 13-Bi₂O₃. (c) 10S-Bi₂O₃. (d) 13S-Bi₂O₃.

Fig. 10(a) shows the optical absorption properties of various Bi₂O₃ films. For the 10-Bi₂O₃ and 13-Bi₂O₃ films, a sharp drop of the absorption edge was observed, and the onset was at approximately 537 nm, which is consistent with the intrinsic band-gap absorption of tetragonal β-Bi₂O₃.⁶ The result shows that β-Bi₂O₃ films have good absorption in visible light region. The 10-Bi₂O₃ and 13-Bi₂O₃ films have the similar absorption range of light wavelength, but the difference in light absorption intensity is very obvious. The 10-Bi₂O₃ film seems to have significant light utilization efficiency, which might be more effective in photoactive applications. By contrast, 10S-Bi₂O₃ and 13S-Bi₂O₃ films have an absorption edge (about 526 nm and 512 nm, respectively) slightly lower than that of the 10-Bi₂O₃ and 13-Bi₂O₃ films, respectively. The absorption edges of the 10S-Bi₂O₃ and 13S-Bi₂O₃ thin films were between that of pure α-Bi₂O₃ and β-Bi₂O₃ phases, indicating that the existence of dual α/β-Bi₂O₃ phase in the samples.²⁹ Notably, no much difference in the light absorption ability was found for the 10S-Bi₂O₃ and 10-Bi₂O₃ thin films; this might be attributed to the fact that the content of α-Bi₂O₃ crystallite in the 10S-Bi₂O₃ film is low, and β-Bi₂O₃ phase still dominated the light absorption property of the 10S-Bi₂O₃ film. By contrast, the α-Bi₂O₃ phase content in the 13S-Bi₂O₃ film was markedly increased to affect the overall light absorption characterization of the 13S-Bi₂O₃ film. This induced the marked blue-shift of absorption edge of the 13S-Bi₂O₃ film and resulted in its absorption edge closer to that of pure α-Bi₂O₃ phase in comparison with that of the 10S-Bi₂O₃. The optical absorption result of various Bi₂O₃ films is consistent with the XRD results. The bandgap energy (Eg) was evaluated using the Kubelka–Munk function equation³⁰ and the results are further shown in Fig. 10(b). The bandgap energy of the 10-Bi₂O₃ and 13-Bi₂O₃ thin films was 2.5 and 2.43 eV, respectively. These bandgap energies are consistent with the reported values of pure β-Bi₂O₃ phase.³¹ Furthermore, the evaluated bandgap energies of the 10S-Bi₂O₃ (2.53 eV) and 13S-Bi₂O₃ (2.64 eV) films are between that of reported values of pure α-Bi₂O₃ and β-Bi₂O₃,³² which is in agreement with the aforementioned discussions. Notably, Urbach energy corresponds to the width of localized states, and it is used to characterize the disorder degree in crystalline material systems. Fig. 10(c) shows that the absorption coefficient near the absorption edge has an exponential behavior and obeys the empirical Urbach rule³³ given by

where α₀ is a constant and ΔE is the Urbach energy. A large Urbach energy would have greater tendency to convert weak bonds into defects. This is evident from the experimental observations of the correlation between the defect density and Urbach energy in the ZnO and TiO₂.^16,34 The evaluated Urbach energies of the 10-Bi₂O₃, 13-Bi₂O₃, 10S-Bi₂O₃ and 13S-Bi₂O₃ films are approximately 0.2078, 0.1254, 0.2263 and 0.1723 eV, respectively. It is found that the Urbach energy of the Bi₂O₃ films formed at pH = 10 is generally greater than the Bi₂O₃ films formed at pH = 13. This is attributable to the fact that the Bi₂O₃ films formed at the reaction solution with a lower pH value resulted in the intermediate stage formed Bi(OH)₃ precursor crystals with a porous and loose structure. This may increase the possibility of initial crystal defects in the prepared Bi₂O₃ thin films after the postannealing procedure in this study.

Fig. 10 (a) UV-vis absorption spectra of various Bi₂O₃ films. (b) Evaluation of band gap of various Bi₂O₃ films. (c) Urbach energy plots of various Bi₂O₃ films.

The morphology effect on electrochemically active surface area (ECSA) size of porous (10-Bi₂O₃ and 10S-Bi₂O₃) and solid (13-Bi₂O₃ and 13S-Bi₂O₃) Bi₂O₃ films are further investigated; the relevant results provide overall electro-catalytic activity of the Bi₂O₃ electrode materials.³⁵ The ECSA of the 10-Bi₂O₃, 13-Bi₂O₃,10S-Bi₂O₃, and 13S-Bi₂O₃ films was compared through the values of electrochemical double-layer capacitance (C_dl) because the C_dl is positively proportional to ECSA of the sample.³⁶ Fig. 11(a)–(d) show cyclic voltammetry (CV) curves at non-faradaic potential regions (−0.2–0.2 V vs. NHE) with different scan rates for various Bi₂O₃ films. All the Bi₂O₃ films showed the rectangular CV curves and peak separation took place with scanning speed from 0.1 to 0.5 V s⁻¹. Notably, the 10-Bi₂O₃ and 10S-Bi₂O₃ films exhibited the higher current density at the constant potential than did the 13-Bi₂O₃ and 13S-Bi₂O₃ films. The double-layer charging current (Δj) is equal to j_a − j_c, in which j_a is anodic current and j_c is cathodic current at the middle potential (0 V) against the CV scan rate. The correlation between the double-layer charging current and electrochemical double-layer capacitance follows Δj = vC_dl, in which v is the scan rate.³⁷ A plot of Δj as a function of v yields a straight line with a slope equal to C_dl value of the Bi₂O₃ film (Fig. 11(e)). According to Fig. 11(e), the C_dl values of 10-Bi₂O₃ and 10S-Bi₂O₃ thin films are closed to be 0.024 mF cm⁻² and 0.027 mF cm⁻², respectively. This is associated with the similar morphology of the 10-Bi₂O₃ and 10S-Bi₂O₃ films through earlier SEM observations. By contrast, the 13-Bi₂O₃ and 13S-Bi₂O₃ films exhibited smaller C_dl values of approximately 0.008 mF cm⁻² and 0.013 mF cm⁻², respectively. The C_dl value of the Bi₂O₃ films formed at pH = 10 is almost 2–3 times higher than that of the Bi₂O₃ films formed at pH = 13 herein. Thus, the ECSA of the four Bi₂O₃ films follows the trend 10S-Bi₂O₃ > 10-Bi₂O₃ > 13S-Bi₂O₃ > 13-Bi₂O₃. The higher ECSA of the 10S-Bi₂O₃ and 10-Bi₂O₃ films herein revealed more surface active sites exposed during electrochemical measurements and maintained a sufficient electrochemical reaction between thin-film electrode and electrolyte ions. This thus might deliver improved photo-/electrocatalytic performance of the 10-Bi₂O₃ and 10S-Bi₂O₃ films in comparison with that of the 13-Bi₂O₃ and 13S-Bi₂O₃ films.³⁸ The morphology effect on electrochemical active of various as-synthesized Bi₂O₃ films with and without a porous structure is visibly demonstrated herein.

Fig. 11 Cyclic voltammetry curves of various Bi₂O₃ thin-film electrodes measured at the non-faradaic region of −0.20–0.20 V vs. NHE with different scan rates, varying from 0.1–0.5 V s⁻¹: (a) 10-Bi₂O₃. (b) 10S-Bi₂O₃. (c) 13-Bi₂O₃. (d) 13S-Bi₂O₃. (e) Scan rate dependent current density curves of various Bi₂O₃ thin-film electrodes.

Fig. 12(a) shows transient photoresponse curves of various Bi₂O₃ films. The Bi₂O₃ films exhibited perfect switching behavior under the chopping visible light irradiation. Notably, all the Bi₂O₃ photoelectrodes showed clear spike-like transient response. The possible cause of this spike transient feature is associated with the recombination of the photoinduced electrons and holes at the surface states of the oxides.^39,40 The maximum steady-state photocurrent density value achieved by the Bi₂O₃ photoelectrodes under irradiation follows the order: 10S-Bi₂O₃ > 10-Bi₂O₃ > 13S-Bi₂O₃ > 13-Bi₂O₃. The photocurrent densities of the 10-Bi₂O₃ and 13-Bi₂O₃ photoelectrodes with a pure β-Bi₂O₃ phase are 0.0039 mA cm⁻² and 0.0012 mA cm⁻², respectively. Comparatively, 10S-Bi₂O₃ and 13S-Bi₂O₃ photoelectrodes with a dual α/β-Bi₂O₃ phase showed markedly enhanced photocurrent density under irradiation with respect to that of the 10-Bi₂O₃ and 13-Bi₂O₃ photoelectrodes, respectively. The maximum steady-state photocurrent densities of the 10S-Bi₂O₃ and 13S-Bi₂O₃ photoelectrodes are approximately 0.0078 mA cm⁻² and 0.0034 mA cm⁻², respectively; these photocurrent densities are around 2 and 3 times higher than that of the 10-Bi₂O₃ and 13-Bi₂O₃ photoelectrodes, respectively. Notably, the 10S-Bi₂O₃ photoelectrode achieved the highest photocurrent density in this study. The significant increase in photocurrent density of the Bi₂O₃ photoelectrode under irradiation indicates the improved separation ability of photogenerated electron (e⁻)–hole (h⁺) pairs in semiconductor oxides.^2,3 The transient photocurrent density vs. time curves results herein demonstrated that the 10S-Bi₂O₃ photoelectrode exhibited the superior photoinduced electron–hole separation efficiency among various Bi₂O₃ photoelectrodes. Notably, the photocurrent density of the Bi₂O₃ photoelectrodes with a dual α/β-Bi₂O₃ phase is significantly higher than that of the Bi₂O₃ photoelectrodes with a pure β-Bi₂O₃ phase under the similar synthesis solution pH value but with and without seed layer assisted growth, indicating that the α/β homojunction is effective to separate the photoinduced electron–hole pairs.^2,41 This is related to the proper type II energy band arrangement between α-Bi₂O₃ and β-Bi₂O₃, which markedly enhances photoinduced charge separation efficiency in the polymorphic Bi₂O₃ film.² It is worth noting that 10-Bi₂O₃ photoelectrode has better photoresponse performance than did 13S-Bi₂O₃ thin-film photoelectrode. One possible reason can be explained by the porous structure of the 10-Bi₂O₃ film observed earlier. The Bi₂O₃ films with a porous structure could lead to a larger specific surface area which contributed to more favorable adsorption of OH⁻ over the surfaces and lead to an improved light harvesting ability of the porous film compared with the solid one.⁴² The aforementioned both reasons can explain the higher photoactive performance of the 10-Bi₂O₃ photoelectrode than that of the 13S-Bi₂O₃ photoelectrode. Fig. 12(b) shows the Nyquist plots of various Bi₂O₃ photoelectrodes under irradiation. Small semi-circular radii are typically attributed to photoelectrodes that exhibit small charge-transfer resistance; moreover, due to the small charge transfer resistance, there will be a higher separation efficiency of photogenerated electrons and holes in material systems.^2,16 From Fig. 12(b), the semi-circular radii of the Bi₂O₃ photoelectrodes with a seed layer assisted crystal growth were smaller than those of the Bi₂O₃ photoelectrodes prepared under the same process condition but without a Bi₂O₃ seed layer assisted growth, indicating that the charge-transfer resistance of the Bi₂O₃ films was substantially improved via a seed layer assisted crystal growth during CBD. The earlier mentioned coexistence of α/β-Bi₂O₃ phases in the 10S-Bi₂O₃ and 13S-Bi₂O₃ photoelectrodes might explain their lower internal charge transfer resistance than that of the 10-Bi₂O₃ and 13-Bi₂O₃ photoelectrodes, respectively. The relatively lower internal charge transfer resistance can accelerate electron transfer and inhibit photoinduced electron/hole recombination.⁴³ Comparatively, the Nyquist plot radius of the 10S-Bi₂O₃ photoelectrode was the smallest among various Bi₂O₃ photoelectrodes. The multiple factors associated with a thin layer morphology, high porosity, good light-capturing ability, and coexistence of α/β-Bi₂O₃ phases might accounted for the observed results herein. Fig. 12(c) exhibits the possible equivalent circuits for a quantitative analysis of interfacial charge transfer ability of various Bi₂O₃ photoelectrodes. As the illustrations shown, the solution resistance Rs depends on the concentration and conductivity of the electrolyte.⁴⁴ The C is an ordinary double layer capacitance for the electrode surface.⁴⁵ R_ct represents the electron transfer resistance, and it can be estimated through the fitting of arc radii of the Nyquist curves. R_ct (α) is a charge transfer resistance across the formed α-Bi₂O₃ electrolyte interface, and C (α) is an ordinary double layer capacitance for the formed α-Bi₂O₃ surface.⁴⁶ In the current work, the separately evaluated R_ct values of the 10-Bi₂O₃, 13-Bi₂O₃, 10S-Bi₂O₃, and 13S-Bi₂O₃ are approximately 1000, 6369, 618 and 1196 ohm. Overall, the PEC and EIS results herein behaved that the Bi₂O₃ films with a dual α/β-Bi₂O₃ phase (10S-Bi₂O₃ and 13S-Bi₂O₃) exhibited superior charge separation and transportation efficiencies, in turn reduced the interfacial resistance in comparison with that of their counterparts (10-Bi₂O₃ and 13-Bi₂O₃).

Fig. 12 (a) Transient photocurrent density versus time curves of various Bi₂O₃ photoelectrodes under chopped illumination at 1 V. (b) Nyquist plots of various Bi₂O₃ photoelectrodes under visible light irradiation. (c) The possible equivalent circuit used for R_ct evaluation of various Bi₂O₃ photoelectrodes.

To understand the origin of the enhanced PEC activity in the α/β-Bi₂O₃ composite photoelectrodes, the Mott–Schottky plots of various Bi₂O₃ films are measured (Fig. 13(a)–(d)). The Mott–Schottky plots of various Bi₂O₃ films with a positive slope indicated the n-type characteristic with electron conduction.⁴⁷ The Mott–Schottky plot according to the slope in the quasi-linear region of Fig. 13(a) indicated that the flat band potential of 10-Bi₂O₃ is around 0.48 eV (vs. normal hydrogen electrode, NHE). The flat-band potential of 13-Bi₂O₃ is estimated at 0.49 eV (Fig. 13(b)). In general, the conduction band (CB) potentials are more negative by about −0.1 eV than the flat potentials for n-type semiconductors.^48,49 The CB potentials of 10-Bi₂O₃ and 13-Bi₂O₃ are evaluated to be approximately 0.38 eV and 0.39 eV, respectively. Moreover, according to the earlier bandgap energy results of the Bi₂O₃ films, the VB potentials of 10-Bi₂O₃ and 13-Bi₂O₃ could be calculated as 2.88 eV and 2.82 eV (vs. NHE) according to the equation E_CB = E_VB − E_g, respectively, which also match well with the values in the literature.^50,51 Notably, 10S-Bi₂O₃ and 13S-Bi₂O₃ are polymorphic structures (dual α/β-Bi₂O₃ phases), the position of the conduction band cannot be known from the current Mott–Schottky plots, so they are not further discussed. The flat band potentials of 10S-Bi₂O₃ and 13S-Bi₂O₃ in Fig. 13(c) and (d) are between the reported reference values of the α-Bi₂O₃ and β-Bi₂O₃ phases,⁴¹ supporting a α-Bi₂O₃/β-Bi₂O₃ junction existed in the 10S-Bi₂O₃ and 13S-Bi₂O₃ thin films. Furthermore, the carrier density of the oxide semiconductor is inversely proportional to the slope of the straight-line portion in the Mott–Schottky plot according to the proposed relationship of 1/C² versus the applied potential.⁵² The tangent slope size in Fig. 13(a)–(d) shows the order: 10S-Bi₂O₃ < 10-Bi₂O₃ < 13S-Bi₂O₃ < 13-Bi₂O₃. It is evidently that an increased charge carrier density was found in the 10S-Bi₂O₃ and 13S-Bi₂O₃ films than that of their counterparts without a seed layer assisted growth. This result might indicate that the formation of α-β homojunction in the Bi₂O₃ film could efficiently create a sufficient space charge layer to enhance the charge carrier transportation in comparison with that of a pure β-Bi₂O₃ thin film. Fig. 13(e) shows possible energy band diagram of the 10-Bi₂O₃ and 13-Bi₂O₃ films with a pure β-Bi₂O₃ phase according to the earlier analysis results. By contrast, for the 10S-Bi₂O₃ and 13S-Bi₂O₃ thin films in which dual α/β-Bi₂O₃ phases existed in the films, their homojunction energy band diagrams were constructed with the assistance of α-Bi₂O₃ energy band diagram from literatures.²⁹ A possible schematic for visible-light-driven electron–hole separation and transport at the homojunction interface of both 10S-Bi₂O₃ and 13S-Bi₂O₃ is shown in Fig. 13(e). Since the CB edge potential of α-Bi₂O₃ is more negative than that of β-Bi₂O₃, the photoinduced electrons on the CB of the α-Bi₂O₃ will transfer easily to the CB of the β-Bi₂O₃ via the well-developed interface. Similarly, the photoinduced holes on the VB of the β-Bi₂O₃ move to the VB of the α-Bi₂O₃ due to the large difference in VB edge potentials retarding the recombination of the photoinduced electron–hole pairs in the α/β-Bi₂O₃ heterojunction of the 10S-Bi₂O₃ and 13S-Bi₂O₃ films. Thus, the formation of the α-Bi₂O₃ phase in the β-Bi₂O₃ matrixed film via a seed layer assisted growth could function as traps to capture the photoinduced holes and the homojunction of α/β-Bi₂O₃ could act as an active center for hindering the rapid recombination of photoinduced electron–hole pairs. For the reasons mentioned above, it is known that 10S-Bi₂O₃ and 13S-Bi₂O₃ will enhance the charge separation and thus the enhanced PEC and EIS performance in comparison with their counterparts of the 10-Bi₂O₃ and 13-Bi₂O₃, respectively.

Fig. 13 Mott–Schottky plots of various Bi₂O₃ photoelectrodes: (a) 10-Bi₂O₃ (b) 13-Bi₂O₃. (c) 10S-Bi₂O₃. (d) 13S-Bi₂O₃. (e) Possible photogenerated electron–hole pathways between two polymorphic Bi₂O₃ semiconductors (α-Bi₂O₃ and β-Bi₂O₃) in this study.

Conclusions

In summary, the porous β-Bi₂O₃ nanosheets and solid β-Bi₂O₃ plates were successfully synthesized by CBD routes with reaction solution of pH = 10 and pH = 13, respectively. These β-Bi₂O₃ crystals were vertically aligned on the substrates with a high crystallinity. The use of sputter coated α/β-Bi₂O₃ homoseed layer during CBD crystal growth shows an important effect on the resultant crystallographic structures of the CBD-derived Bi₂O₃ under the similar processes conditions. The sputter coated α/β-Bi₂O₃ homoseed layer induced formation of α-Bi₂O₃ phase in the β-Bi₂O₃ matrix during the CBD crystal growth. The PEC and EIS results show that the photoactive performance of the CBD-derived Bi₂O₃ crystals is enhanced via a homoseed layer crystal growth engineering. The proper type II band alignment between α-Bi₂O₃ and β-Bi₂O₃ phase improved the efficiency of photoinduced charge separation in the seed layer assisted grown Bi₂O₃ crystals. Moreover, the porous structure of the Bi₂O₃ nanosheets formed at a lower pH value of reaction solution improved their specific surface area size and light-trapping performance; therefore, the porous α/β-Bi₂O₃ nanosheets demonstrated the highest photoactive performance in this study. The porous and dual phase Bi₂O₃ nanosheets synthesized herein may have the potential for applications in photoactive devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by Ministry of Science and Technology of Taiwan. Grant No. MOST 108-2221-E-019-034-MY3.

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