*Article* **Carbon Quantum Dots Accelerating Surface Charge Transfer of 3D PbBiO2I Microspheres with Enhanced Broad Spectrum Photocatalytic Activity—Development and Mechanism Insight**

**Ruyu Yan †, Xinyi Liu †, Haijie Zhang, Meng Ye, Zhenxing Wang, Jianjian Yi, Binxian Gu and Qingsong Hu \***

College of Environmental Science and Engineering, Yangzhou University, 196 West Huayang Road, Yangzhou 225127, China

**\*** Correspondence: huqs@yzu.edu.cn

† These authors contributed equally to this work.

**Abstract:** The development of a highly efficient, visible-light responsive catalyst for environment purification has been a long-standing exploit, with obstacles to overcome, including inefficient capture of near-infrared photons, undesirable recombination of photo-generated carriers, and insufficient accessible reaction sites. Hence, novel carbon quantum dots (CQDs) modified PbBiO2I photocatalyst were synthesized for the first time through an in-situ ionic liquid-induced method. The bridging function of 1-butyl-3-methylimidazolium iodide ([Bmim]I) guarantees the even dispersion of CQDs around PbBiO2I surface, for synchronically overcoming the above drawbacks and markedly promoting the degradation efficiency of organic contaminants: (i) CQDs decoration harness solar photons in the near-infrared region; (ii) particular delocalized conjugated construction of CQDs strength via the utilization of photo-induced carriers; (iii) π–π interactions increase the contact between catalyst and organic molecules. Benefiting from these distinguished features, the optimized CQDs/PbBiO2I nanocomposite displays significantly enhanced photocatalytic performance towards the elimination of rhodamine B and ciprofloxacin under visible/near-infrared light irradiation. The spin-trapping ESR analysis demonstrates that CQDs modification can boost the concentration of reactive oxygen species (O2 •−). Combined with radicals trapping tests, valence-band spectra, and Mott–Schottky results, a possible photocatalytic mechanism is proposed. This work establishes a significant milestone in constructing CQDs-modified, bismuth-based catalysts for solar energy conversion applications.

**Keywords:** PbBiO2I microspheres; CQDs; ionic liquid; charge separation; interface

#### **1. Introduction**

Semiconductor photocatalysis is deemed as a promising technique to purify water which is contaminated by various pollutants, such as dyes, antibiotics, endocrine disruptors, and so on [1–3]. Taking full advantage of solar energy has already been proven as a "green" strategy to settle environmental contamination and energy crunch [4–6]. For the sake of triggering an effective photocatalytic reaction and achieving a satisfying degradation efficiency, it is essential to boost effective interfacial contact between target organic molecules and reactive species, namely to involve more photo-generated carriers in surface catalysis. Additionally, several studies have shown that the photocatalytic efficiency can be improved via increasing the adsorption capability of photocatalysts towards contaminants [7–9].

Among various Bi-based semiconductor photocatalysts, BiOI displays some fascinating advantages, e.g., high chemical inertness, easy preparation, low toxicity, and broad visible-light absorption range [10]. Moreover, it displays some photocatalytic performance in the field of pollutants elimination, CO2 conversion, N2 reduction, and so forth [11–13]. Nevertheless, individual BiOI is subjected to its inherent weakness, such as the poor oxidation capacity caused by the high value of valence band and low charge separation efficiency. Moreover, during the contaminant degradation process, the contaminants' adsorption and

**Citation:** Yan, R.; Liu, X.; Zhang, H.; Ye, M.; Wang, Z.; Yi, J.; Gu, B.; Hu, Q. Carbon Quantum Dots Accelerating Surface Charge Transfer of 3D PbBiO2I Microspheres with Enhanced Broad Spectrum Photocatalytic Activity— Development and Mechanism Insight. *Materials* **2023**, *16*, 1111. https://doi.org/10.3390/ ma16031111

Academic Editor: Andrea Petrella

Received: 1 December 2022 Revised: 11 January 2023 Accepted: 21 January 2023 Published: 27 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

activation capacity over BiOI is still unsatisfactory [14,15]. Therefore, a variety of methods have been adopted to improve the photocatalytic performance of BiOI, e.g., elemental doping, defect regulation, exposed facet control, heterojunctions construction and bismuthrich strategy [16–19]. In addition, a part of Bi in the [Bi2O2] 2+ layer can be replaced by other main group elements (Pb, Ba, Sr, Ca, etc.) to generate [ABiO2] <sup>+</sup> [20–22], and I<sup>−</sup> and [ABiO2] <sup>+</sup> can be arranged alternately to form ABiO2I. The construction of bismuth-based bimetallic oxyiodide can maximize the photocatalytic performance of bismuth oxyiodide under broadband light irradiation. Considering that the radii of Pb2+ and Bi3+ are very close, the crystalline structure of PbBiO2I displays no obvious change by substituting Bi3+ with Pb2+ [23]. More importantly, PbBiO2I displays preferable band structure with a suitable narrow bandgap of 1.9 eV, which is beneficial to the degradation of contaminants. Therefore, PbBiO2I displays great potential in the field of environmental purification.

As a key member of the carbon-based nanomaterial family, 0D CQDs have drawn tremendous research attention [24,25]. Particular properties, e.g., high solubility, excellent electron conductivity, and up-conversion performance, endow them with broad applications [26–28]. Herein, CQDs have been extensively employed to modify photocatalysts to heighten their optical and electrochemical properties, and eventually promote their photocatalytic performance. In fact, the introduction of CQDs can boost the separation and transportation of photo-generated electron-hole pairs and enable more reactive oxygen species participating in the degradation of contaminants [29]. In spite of this, in previous references, the core character of CQDs during the photocatalytic reactions and pollutant degradation mechanism have not yet been studied at length. More importantly, considering that the diameter of CQDs is less than 10 nm, the uniform distribution of CQDs around catalyst surface requires further study.

In order to deeply analyze the aforementioned impending issues, CQDs modified PbBiO2I nanocomposite photocatalysts are obtained via an ionic liquid [Bmim]I assisted solvothermal method. In this approach, [Bmim]I can be employed as template and reaction source to control the growth of PbBiO2I crystals. In fact, the existence of coulomb force and hydrogen bond between [Bmim]I and CQDs is beneficial to in-situ anchoring more CQDs around PbBiO2I material [30]. Moreover, CQDs decoration can promote organic pollutants adsorption, boost interface charge separation and transportation, and ultimately enhance the photocatalytic degradation efficiency of rhodamine B (RhB) and ciprofloxacin (CIP) under visible/near-infrared light irradiation. Our research extends the knowledge into developing more CQDs-decorated, bismuth-based bimetallic catalysts with widespread applications in the area of wastewater treatment.

#### **2. Experimental Details**

#### *2.1. Sample Preparation*

CQDs powder was acquired on the basis of the previous reference and then managed by lyophilization [29]. Hence, 1.06 g citric acid monohydrate was dispersed into deionized water (11 mL), and ethylenediamine (340 μL) was injected and stirred for 1 h. This above clear solution was sealed in Teflon-lined autoclave (25 mL) and reacted at 200 ◦C for 5 h. After cooling down to room temperature, the brownish red solution was subjected to dialysis for 72 h to obtain the CQDs solution. In the end, CQDs powder was obtained after freeze-drying for 72 h.

The synthetic process of pure PbBiO2I and CQDs/PbBiO2I nanocomposite was as follows (Scheme 1) via ionothermal method: First of all, CQDs (× *g*), Bi(NO3)3·5H2O (0.24 g) and Pb(NO3)2 (0.16 g) are fully dispersible in ethylene glycol (15 mL) and defined as A. Then, ionic liquid [Bmim]I (0.13 g) was dispersed uniformly into ethylene glycol (5 mL) and defined as B. B was added into A bit by bit and stirred for 1 h. After that, the mixture was sealed in a Teflon-lined autoclave (25 mL) and reacted at 180 ◦C for 24 h. Subsequently, the sediment was collected via high-speed centrifugation, rinsed three times with deionized water and absolute ethanol, and dried at 80 ◦C for 12 h. The loading amount

of CQDs in CQDs/PbBiO2I nanocomposite was 1, 3, 5, and 8 wt.%, respectively. Individual PbBiO2I was also obtained without the introduction of CQDs.

**Scheme 1.** Schematic diagram for the formation of CQDs/PbBiO2I nanocomposite catalyst.

#### *2.2. Sample Characterization*

X-ray diffraction (XRD) was recorded on a D8 Advance diffractometer (Bruker, Germany) using monochromatic Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) spectrum was measured using an ESCALAB 250Xi XPS (Thermo Fisher, Waltham, MA, USA) with monochromatic Mg-Kα radiation as X-ray source for excitation. A laser Raman spectrometer (DXR, Thermo Fisher Scientific, Waltham, MA, USA) was employed to collect Raman spectra with a 532 nm laser as an excitation source. A specific surface and aperture analyzer (Quadrasorb EVO, Anton Paar, Ashland, VA, USA) was used to analyze the specific surface area and pore diameter of the catalysts by N2 adsorption-desorption isotherms analyzed at 77 K using the Brunauer–Emmett–Teller (BET) method and Barret–Joyner– Halenda (BJH) adsorption dV/dW pore volume distribution. The microstructures of the catalysts were investigated by Tecnai G2 F30 S-TWIN transmission electron microscopy (TEM, FEI, USA). UV-Vis diffuse reflection spectra was acquired on a UV-2450 spectrophotometer (Shimadzu, Japan). The photoluminescence (PL) spectra were obtained by a FLS980 fluorescence spectrometer (Edinburgh, UK). An electron spin resonance (ESR) spectrometer (JES-FA200, Bruker, Germany) was used to capture ESR signals of spin-trapped radicals using 5,5-dimethyl-1-pyrroline-*N*-oxide (DMPO, radical trapping reagent) in water and methanol solutions. The electrochemical data were obtained on a CHI 660B electrochemistry workstation (Chenhua, Shanghai) employing a conventional three-electrode cell (ITO slice, Pt wire and saturated Ag/AgCl).

#### *2.3. Photocatalytic Degradation Test*

RhB and CIP were selected as the model contaminants. A 250 W xenon plus equipped with optical filter (λ > 400 nm or λ > 610 nm) was employed as the optical source. The reaction temperature was kept at 25 ◦C via a recirculating cooling water system. Hence, 0.03 g samples were added in RhB solution (100 mL, 20 mg/L) or CIP solution (100 mL, 10 mg/L) by ultrasonic dispersion. The mixed solution was stirred in the dark for 30 min. Subsequently, the optical source was switched on, and 4.5 mL reaction mixture was sampled at set intervals. The acquired mixture was centrifuged at 15,000 rpm for 4 min to acquire a clear solution. The concentrations of RhB and CIP were studied using a Shimadzu LC-20A HPLC system (Shimadzu, Japan), including an Agilent TC-C (18) column, two Varian Prostar 210 pumps and a ultraviolet detector. A mobile phase consisting of methanol and pure water in the ratio of 30:70 (*v*:*v*) was used at the flow rate of 0.8 mL min<sup>−</sup>1. The reaction solution (10 μL) was injected. To distinguish the major reactive species participating in photocatalytic reactions, various radical scavengers were introduced. Tert-butanol (tbutanol) can capture hydroxyl radical (•OH), nitrogen (N2) can inhibit the production of superoxide radical (O2 •−), while ammonium oxalate (AO) and triethanolamine (TEA) can capture holes (h+).

#### **3. Result and Discussion**

The crystal structures of the acquired materials were analyzed by the X-ray diffraction patterns (XRD). As depicted in Figure 1a, all the peaks can be indexed to tetragonal PbBiO2I (JCPDS NO. 38-1007) [31]. For single CQDs, a broad peak located at 25◦ can be observed. After the decoration of CQDs, the nanocomposite retains the same diffraction peaks of tetragonal PbBiO2I. Additionally, it is noteworthy that a small peak centered around 26.9◦ appears with the increased loading amount of CQDs. This phenomenon validates the successful introduction of CQDs. Raman spectra was conducted to further confirm the successful loading of CQDs. As shown in Figure 1b, the strong peaks (66.4 and 142.2 cm<sup>−</sup>1) are ascribed to the "lattice vibration" of PbBiO2I. Compared with single PbBiO2I, 5 wt.% CQDs/PbBiO2I displays two apparent peaks centered at 1389.1 and 1580.2 cm−1, which represent the disordered (D) and graphitic (G) bands of graphene. The peak intensity of G band is much higher than that of D band, which is in agreement with previous report [32]. There is background interference in the nanocomposite because of the strong fluorescence for CQDs. To explore the textural properties of the obtained samples, BET surface area measurement was carried out. In Figure 1c, the N2 adsorption/desorption isotherms can be categorized as type IV isotherm, indicating the characteristics of mesoporous [33]. This is in accord with the pore diameter distribution analysis (Figure S1). Moreover, the BET value of PbBiO2I and 5 wt.% CQDs/PbBiO2I is determined to be 10.20 and 31.77 m<sup>2</sup> g−1. As is well known to all, a higher surface area is beneficial to provide more exposed reactive sites and transport paths for reactants and products, probably resulting in the enhanced photocatalytic performance [34].

The morphology of CQDs and CQDs/PbBiO2I was studied by TEM analysis. As shown in Figure S2, the CQDs are monodisperse with a spherical shape diameter of 5 nm. Moreover, the lattice fringe spacing of 0.33 nm corresponds to the (002) plane of CQDs [35]. For CQDs/PbBiO2I nanocomposite, it displays flower-like spheres with a diameter of 2 μm assembled from nanosheets (Figure 1d,e). To further observe this microstructure, high-resolution TEM (HRTEM) measurement was carried out. As shown in Figure 1f, the lattice fringe spacing of 0.33 nm and 0.30 nm corresponds to the (002) plane of CQDs and (103) crystallographic plane of PbBiO2I. The results of TEM analysis demonstrate that CQDs/PbBiO2I nanocomposite catalyst have been constructed successfully.

The surface chemical states and compositions of the obtained catalysts were investigated by X-ray photoelectron (XPS) spectra (Figure 2). The survey XPS spectrum shows the co-existence of Pb, Bi, O, I, and C elements in the CQDs/PbBiO2I nanocomposite (Figure 2a). In Figure 2b, the peaks at roughly 142.5 and 137.6 eV are ascribed to the Pb 4f of Pb2+ [35]. In Figure 2c, the strong peaks located at 163.9 and 158.6 eV are assigned to the Bi 4f5/2 and Bi 4f7/2, manifesting the presence of trivalent bismuth [36]. What is more, after the decoration of CQDs, the binding energies of Pb 4f and Bi 4f shift to higher values when compared to those of PbBiO2I. The binding energy shift unequivocally proves the occurrence of electron re-distribution between PbBiO2I and CQDs. Similar phenomena have been reported in other references [29,37]. The binding energy of O 1s centered at 529.2 eV (Figure 2d) is assigned to the oxygen of PbBiO2I. The XPS peaks of I 3d centered

at 630.1 and 618.7 eV are ascribed to I 3d3/2 and I 3d5/2 of I<sup>−</sup> [31] (Figure 2e). Figure 2f displays the high-resolution XPS spectra of C 1s, in which three deconvoluted peaks can be ascribed to C-C (284.6 eV), C=C (286.5 eV) and C-N (288.1 eV), respectively [26,29]. The XPS results further prove the presence of PbBiO2I and CQDs in the CQDs/PbBiO2I nanocomposite.

**Figure 1.** (**a**) XRD patterns of the as-prepared samples; (**b**) Raman spectra and (**c**) Nitrogen adsorptiondesorption isotherms of PbBiO2I and 5 wt.% CQDs/PbBiO2I; (**d**,**e**) TEM and (**f**) HRTEM images of 5 wt.% CQDs/PbBiO2I.

**Figure 2.** XPS spectra of PbBiO2I and 5 wt.% CQDs/PbBiO2I: (**a**) survey, (**b**) Pb 4f, (**c**) Bi 4f, (**d**) O 1s, (**e**) I 3d, (**f**) C 1s.

The photoreactivity of the as-prepared catalysts was assessed by decomposition of RhB upon irradiation with visible light (λ > 400 nm). As we all know, RhB is broadly used as a coloration in textile and food processing industries and is also a popular water tracer fluorescent. It poses a threat to human beings and animals, and causes irritation of the eyes, skin, and respiratory passage. The carcinogenicity, reproductive and development

toxicity and chronic toxicity toward human beings and animals have been proven experimentally [38]. Therefore, it is crucial to reduce RhB concentrations to reach the national standards. Herein, CQDs/PbBiO2I nanocomposite are tentatively employed to remove RhB, and the importance of CQDs can also be confirmed from another angle. The RhB adsorption capacity over different samples is shown in Figure S3. The dark adsorption experiment results demonstrate that the adsorption capacity can be improved thanks to the π-π interactions between CQDs and RhB molecule [36]. In Figure 3a, the photolysis of RhB is almost negligible. For PbBiO2I, the degradation rate of RhB is only 29.7% within 30 min. After the introduction of CQDs, CQDs/PbBiO2I nanocomposites show higher photocatalytic efficiency than that of PbBiO2I, highlighting the key role of CQDs in environmental purification. Notably, 5 wt.% CQDs/PbBiO2I is obviously superior to other CQDs modified PbBiO2I samples. This can be explained as too many CQDs may generate an adverse shielding effect, hindering the PbBiO2I surface from absorbing visible-light photons and overlaying the reactive sites for photocatalysis via CQDs agglomeration [36,39]. Figure 3b shows the photocatalytic kinetics fit of RhB degradation on account of pseudo-first-order model (Langmuir-Hinshelwood model). It can be clearly seen that the degradation efficiency of RhB can be obviously enhanced after adding CQDs. Specifically, the degradation rate of 5 wt.% CQDs/PbBiO2I is approximately 2.27 times larger than that of PbBiO2I. To evaluate the catalyst's reusability, five consecutive cycles were conducted over 5 wt.% CQDs/PbBiO2I photocatalyst. In Figure S4a, the degradation rate remains 90% in the elimination of RhB, manifesting the preferable catalytic stability of the nanocomposite catalyst. Furthermore, XRD patterns further confirm the well-retained structure of the nanocomposite catalyst after photoirradiation reaction (Figure S4b).

**Figure 3.** Photocatalytic properties of the catalysts for the degradation of RhB (**a**) and CIP (**c**) under visible light irradiation (λ > 400 nm); reaction kinetics for the degradation of RhB (**b**) and CIP (**d**) under the same condition.

CIP is classified as belonging the second generation of fluoroquinolone antibiotics, being proven to damage the environment and display toxic effects in the surface water and groundwater [40]. This is the first report employing CQDs/PbBiO2I nanocomposite for CIP degradation, and the degradation plots are shown in Figure 3c. The degradation rate of CIP achieves 35.8% within 300 min while employing PbBiO2I as catalyst. Surprisingly, more than 73.9% of CIP can be eliminated over PbBiO2I loading with 5 wt.% CQDs. Further,

the calculating rate constant is 0.0042 min−1, which is much higher than that of PbBiO2I (0.0014 min<sup>−</sup>1) (Figure 3d). Additionally, a total organic carbon (TOC) experiment was carried out to study the mineralization of RhB and CIP over 5 wt.% CQDs/PbBiO2I (Figure S5). Under visible-light irradiation for 120 min, almost 76.2% of RhB is mineralized. For CIP, approximately 40.1% of CIP can be mineralized under illumination for 300 min. This implies that both RhB and CIP can be mineralized effectively over 5 wt.% CQDs/PbBiO2I under visible light irradiation.

The photocatalytic performance of single PbBiO2I and 5 wt.% CQDs/PbBiO2I was further studied under near-infrared photoirradiation (λ > 610 nm). As shown in Figure 4a, only 13.5% RhB can be degraded by PbBiO2I after 120 min near-infrared photoirradiation. Accompanied with the decoration of CQDs, the photocatalytic efficiency is significantly improved and 53.3% RhB is degraded over 5 wt.% CQDs/PbBiO2I under the same condition. The corresponding rate constant for 5 wt.% CQDs/PbBiO2I is 5.36 times higher than pure PbBiO2I (Figure 4b). The ratio of rate constant for 5 wt.% CQDs/PbBiO2I to individual PbBiO2I under near-infrared photoirradiation is analogous to the ratio under visible light irradiation, which indicate the analogical activation pattern. In comparison to visible light condition, the ratio of rate constant for 5 wt.% CQDs/PbBiO2I to individual PbBiO2I under near-infrared light condition is higher, indicating that CQDs can transport photo-generated electrons more efficiently under near-infrared photoirradiation [29]. Given that the dye-sensitization effect involved in the degradation of RhB, the degradation of colorless CIP was carried out under the same condition, pure PbBiO2I only degrades 12.9% CIP after 300 min irradiation. After the modification of CQDs, 5 wt.% CQDs/PbBiO2I can degrade 25.7% CIP after 300 min irradiation (Figure 4c). The reaction rate constant of 5 wt.% CQDs/PbBiO2I is 2.0 times that of PbBiO2I (Figure 4d). The above experimental data demonstrate the key roles of CQDs during a photocatalytic reaction process [29,41].

**Figure 4.** Photocatalytic properties of the catalysts for the degradation of RhB (**a**) and CIP (**c**) under near-infrared light irradiation (λ > 610 nm); reaction kinetics for the degradation of RhB (**b**) and CIP (**d**) under the same condition.

The optical characteristics of the obtained samples across the UV-Vis region are recorded by diffuse reflectance spectra (DRS). In Figure 5a, pristine PbBiO2I displays intrinsic bandgap absorption from 200 nm to 570 nm. This can be attributed to their intrinsic band-to-band transition [36]. After the modification of CQDs, the optical absorption is substantially extended to the near-infrared region. As a result, it may boost the generation of photo-induced carriers thanks to the enhanced light-harvesting capability. Consequently, more reactive species can be involved in the photocatalytic reaction. The Eg values of PbBiO2I and CQDs/PbBiO2I nanocomposite can be obtained employing the Kubelka–Munk function [8]:

$$\alpha \mathbf{h} \mathbf{v} = \mathbf{A} \left( \mathbf{h} \mathbf{v} - \mathbf{E\_g} \right)^{\mathbf{n}/2}$$

where α, h, ν, A, and Eg represent the absorption coefficient, Planck constant, light frequency, a constant, and band gap energy, respectively. As depicted in Figure 5b, the Eg values of PbBiO2I, 1 wt.% CQDs/PbBiO2I, 3 wt.% CQDs/PbBiO2I, 5 wt.% CQDs/PbBiO2I, and 8 wt.% CQDs/PbBiO2I are calculated to be 1.92, 1.83, 1.77, 1.72, and 1.67 eV, respectively.

**Figure 5.** (**a**) UV-vis DRS of the acquired samples; (**b**) Bandgap of PbBiO2I and CQDs/PbBiO2I nanocomposite; (**c**) PL spectra, (**d**) transient photocurrent spectra, (**e**) EIS spectra of PbBiO2I and 5 wt.% CQDs/PbBiO2I; (**f**) XPS Valence-band spectra of PbBiO2I.

To clarify the charge separation and transfer kinetics over the obtained catalysts, photoluminescence (PL) and photoelectrochemical measurements are performed. Figure 5c displays that both PbBiO2I and 5 wt.% CQDs/PbBiO2I possess an obvious emission peak centered at 440 nm. More importantly, 5 wt.% CQDs/PbBiO2I shows an apparent quenching in comparison to PbBiO2I. This characterization result indicates an improving separation probability of photo-generated carriers, which is beneficial for visible/near-infrared light-driven catalytic reactions [42]. The charge separation/transfer process is further monitored by transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements. Figure 5d exhibits the transient photocurrent responses of the two samples under chopped light irradiation. It can be found that the photocurrent density is in the order PbBiO2I < 5 wt.% CQDs/PbBiO2I, which coincide with the trend of photocatalytic performance. The photocurrent results demonstrate that the charge separation efficiency of 5 wt.% CQDs/PbBiO2I is superior to that of PbBiO2I [43]. The results of transient photocurrent are further confirmed by EIS. In Figure 5e, the semicircle arc of 5 wt.% CQDs/PbBiO2I in the Nyquist plot is smaller than that of PbBiO2I, reflecting the lower interfacial charge transfer resistance [44]. Taking the above PL and photoelectrochemical results into account, CQDs modification will boost charge migration and separation, which is favorable for the generation of reactive species.

Apart from the light absorption efficiency and spatial separation efficiency of photoinduced charge carriers, the energy band structure also plays a critical role in determining photocatalytic efficiency. The total density of states of valence band (VB) that can be ob-

tained based on the valence-band XPS spectra with Fermi level (*E*f) of semiconductors is 0 eV (Figure 5f). The VB value of PbBiO2I is measured to be 1.38 eV, and the positive slopes of Mott–Schottky curves show that PbBiO2I is defined as a n-type semiconductor (Figure S6). According to the extrapolation of *X* intercept in the Mott–Schottky plots, the flat band potential of PbBiO2I is measured to be −0.62 V vs. NHE (pH = 7). With regarding to the n-type semiconductors, the Fermi level is close to the flat band potential [45]. As a result, the VB value of PbBiO2I is 0.76 V vs. NHE. On the basis of the *E*g, the CB minimum of PbBiO2I occur at approximately −1.16 V vs. NHE.

The ESR (electron spin resonance) technique and free radicals capturing tests are conducted to ascertain the major active species involved in the degradation process [46,47]. The results of ESR analysis are presented in Figure 6a,b. In the darkness, no characteristic signals can be observed for DMPO-O2 •− and DMPO-•OH from the two catalysts, and no reactive species can be trapped. Nevertheless, typical characteristic signal peaks of DMPO-O2 •− are observed under photoirradiation, and the DMPO-O2 •− signals of 5 wt.% CQDs/PbBiO2I are obviously higher than that of PbBiO2I. Furthermore, DMPO-•OH cannot be trapped from the two catalysts upon light illumination. ESR analysis demonstrate that superoxide radical (O2 •−) can be generated upon light illumination, and the modification of CQDs is beneficial for the generation of reactive oxygen species. To further verify the presence of these active species, free radical quenching tests are carried out in the presence of 5 wt.% CQDs/PbBiO2I (Figure 6c). After the addition of AO and TEA, notable inhibition of photocatalytic activity can be observed, indicating that direct hole oxidation plays a critical role during visible/near-infrared light-driven catalytic reactions. Moreover, after N2 is pumped into the reaction solution, the degradation efficiency declines greatly [48]. This can be explained as a large amount of O2 •− is being generated and acting in a key role during the degradation process. The analysis of radical capture tests are in accordance with ESR results.

**Figure 6.** (**a**) DMPO- O2 •− and (**b**) DMPO-•OH of PbBiO2I and 5 wt.% CQDs/PbBiO2I; (**c**) trapping experiments of active species during the degradation of RhB; (**d**) photocatalytic mechanism of CQDs/PbBiO2I nanocomposite in photocatalytic degradation.

Considering the above experimental results, the separation and transformation paths of photo-generated carriers involved in visible/near-infrared light-driven degradation of organic contaminants are presented in Figure 6d. Under broadband light irradiation, electrons in the VB of PbBiO2I can be motivated and then migrated to the CB. Because of the narrow band gap width, the recombination of photo-induced electrons and holes occurs in very little time. After the modification of CQDs, the mobility of photo-generated electrons can be enhanced thanks to the electron acceptor property of CQDs. Consequently, more powerful oxidants participate in photocatalytic reactions. Even so, the holes on the VB cannot thermodynamically oxidize H2O (H2O/•OH 2.34 V vs. NHE) or OH<sup>−</sup> (•OH/OH<sup>−</sup> 1.99 V vs. NHE) to generate •OH [37,49]. Therefore, the main reactive species, including O2 •− and holes generated under visible/near-infrared light illumination, engage in organic contaminant elimination together, synergistically boosting the enhancement of photocatalytic performance over the CQDs/PbBiO2I nanocomposite.

#### **4. Conclusions**

In conclusion, CQDs modified PbBiO2I microspheres have been fabricated via a simple ionothermal method. (i) In this prepared procedure, ionic liquid serves as a high performance template, reactant, and dispersant, which is beneficial for the uniform distribution of CQDs around PbBiO2I microspheres; (ii) the photocatalytic activity for removing organic contaminants is greatly enhanced; and (iii) CQDs modification can successfully reduce charge carrier recombination and accelerate the transformation of photoinduced hole-electron pairs to the catalyst surface. Therefore, more reactive oxygen species can be generated and involved in the elimination of RhB and CIP. This work may offer some insights for constructing a carbon-based material-modified, bismuth-based catalyst, which will be widely welcomed in environmental purification and energy conversion.

**Supplementary Materials:** The following Supplementary Materials can be downloaded at: https://www.mdpi.com/article/10.3390/ma16031111/s1. Figure S1: The pore size distribution curves of PbBiO2I and 5 wt.% CQDs/PbBiO2I; Figure S2: HR-TEM image of CQDs; Figure S3: The adsorption equilibrium of RhB over various catalysts in the darkness; Figure S4: Cycling runs for RhB degradation over 5 wt.% CQDs/PbBiO2I under visible light irradiation (λ > 400 nm) (a), XRD patterns of 5 wt.% CQDs/PbBiO2I before and after five cycles (b); Figure S5: The decrease of TOC during the photodegradation of RhB (a) and CIP (b) over 5 wt.% CQDs/PbBiO2I; Figure S6: Mott-Schottky plots of PbBiO2I.

**Author Contributions:** Data curation, investigation, writing—original draft, R.Y.; data curation, investigation, writing—original draft, X.L.; data curation, investigation, H.Z.; data curation, investigation, M.Y.; writing—review & editing, supervision, Z.W.; data curation, investigation, J.Y.; writing—review & editing, resources, B.G.; conceptualization, writing—review & editing, funding acquisition, Q.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the China Postdoctoral Science Foundation (NO. 2021M691389), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (NO. 22KJB610026), the State Key Laboratory of Pollution Control and Resource Reuse (Project NO. PCRRF20019), and the Science and Technology Innovation Fund Project of Yangzhou University Students (NO. 202211117020Z).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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## *Article* **TiO2 Nanotubes Decorated with Mo2C for Enhanced Photoelectrochemical Water-Splitting Properties**

**Siti Nurul Falaein Moridon 1, Khuzaimah Arifin 1,\*, Mohamad Azuwa Mohamed 1,2, Lorna Jeffery Minggu 1, Rozan Mohamad Yunus <sup>1</sup> and Mohammad B. Kassim 1,2**


**Abstract:** The presence of Ti3+ in the structure of TiO2 nanotube arrays (NTs) has been shown to enhance the photoelectrochemical (PEC) water-splitting performance of these NTs, leading to improved results compared to pristine anatase TiO2 NTs. To further improve the properties related to PEC performance, we successfully produced TiO2 NTs using a two-step electrochemical anodization technique, followed by annealing at a temperature of 450 ◦C. Subsequently, Mo2C was decorated onto the NTs by dip coating them with precursors at varying concentrations and times. The presence of anatase TiO2 and Ti3O5 phases within the TiO2 NTs was confirmed through X-ray diffraction (XRD) analysis. The TiO2 NTs that were decorated with Mo2C demonstrated a photocurrent density of approximately 1.4 mA cm<sup>−</sup>2, a value that is approximately five times greater than the photocurrent density exhibited by the bare TiO2 NTs, which was approximately 0.21 mA cm−2. The observed increase in photocurrent density can be ascribed to the incorporation of Mo2C as a cocatalyst, which significantly enhances the photocatalytic characteristics of the TiO2 NTs. The successful deposition of Mo2C onto the TiO2 NTs was further corroborated by the characterization techniques utilized. The utilization of field emission scanning electron microscopy (FESEM) allowed for the observation of Mo2C particles on the surface of TiO2 NTs. To validate the composition and optical characteristics of the decorated NTs, X-ray photoelectron spectroscopy (XPS) and UV absorbance analysis were performed. This study introduces a potentially effective method for developing efficient photoelectrodes based on TiO2 for environmentally sustainable hydrogen production through the use of photoelectrochemical water-splitting devices. The utilization of Mo2C as a cocatalyst on TiO2 NTs presents opportunities for the advancement of effective and environmentally friendly photoelectrochemical (PEC) systems.

**Keywords:** titanium dioxide; anodization; self-doping; cocatalyst; Mo2C

#### **1. Introduction**

The extreme reliance on fossil fuels for energy generation since the industrial revolution has triggered a global energy crisis and various other environmental problems [1,2]. Therefore, reducing energy dependence on fossil fuels through the provision of clean and renewable energy sources is urgent. One alternative is to use clean and green hydrogen (H2) directly produced from water molecules using solar light energy, known as the photoelectrochemical (PEC) process. H2 can be used as fuel in an electrochemical fuel cell device to produce electricity, with pure water as the only byproduct.

In the PEC process, semiconductor materials are employed as photoelectrodes [2]. To date, numerous semiconductor materials, such as TiO2 [3–5], Co3O4 [6,7], WO3 [8], Cu2O [9], and Fe2O3 [10], have been investigated as photoelectrode materials. Among them, TiO2 has garnered considerable attention due to its photoactivity, low cost, excellent chemical stability, and abundance in nature [11]. However, TiO2 can only be stimulated by

**Citation:** Moridon, S.N.F.; Arifin, K.; Mohamed, M.A.; Minggu, L.J.; Mohamad Yunus, R.; Kassim, M.B. TiO2 Nanotubes Decorated with Mo2C for Enhanced Photoelectrochemical Water-Splitting Properties. *Materials* **2023**, *16*, 6261. https://doi.org/ 10.3390/ma16186261

Academic Editors: Xingwang Zhu and Tongming Su

Received: 9 June 2023 Revised: 6 July 2023 Accepted: 7 July 2023 Published: 18 September 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

UV light, which accounts for only 3–5% of solar energy radiation, because of its large band gap, further causing quick recombination of photoinduced electron-hole pairs as well as inefficient charge carrier separation [11]. Therefore, various methods have been used to improve the PEC performance and photocatalytic activity of TiO2, including morphology modification [12–14], synthesis of composite heterojunctions with other materials [15], ion doping [16,17], facet engineering [18], and cocatalyst addition [19].

In terms of morphology, TiO2 nanotube arrays (NTs) have attracted attention due to their high specific surface area, excellent adsorption capacity, and good structural properties for electron transport. Several methods have been employed to fabricate TiO2 NT photoelectrodes, and electrochemical anodization is considered one of the most promising methods for fabricating a highly ordered NT structure [20]. Although the electrochemical anodization method is considered a promising fabrication method for producing ordered TiO2 NTs with defect engineering and doping, the PEC performance of fabricated TiO2 NTs still does not reach satisfactory levels due to their restricted light harvesting and the high resistance at the interface between the nanotubes and the substrate. Therefore, a synergistic approach combining various strategies, such as the formation of heterojunctions with other semiconductor materials or the addition of cocatalyst materials, could prove excellent for obtaining TiO2 NTs with efficient light harvesting and charge separation for high PEC watersplitting performance. Currently, 2D MXenes have been investigated as promising catalysts or cocatalysts in many applications. The nomenclature "MXene" has been used to represent a group of compounds that includes transition metal carbides, nitrides, and carbonitrides. The nomenclature "MXene" is derived from its chemical composition, wherein the symbol "M" represents a transition metal, "X" signifies carbon and/or nitrogen, and the suffix "ene" refers to its two-dimensional structural arrangement [21]. Among the reported MXene materials, dimolybdenum carbide (Mo2C) has been reported to show excellent electrocatalytic performance in the hydrogen evolution reaction (HER) [21]. It has an electronic density of states comparable to that of Pt and excellent electrical conductivity. In addition to being used as a catalyst for the HER, Mo2C has also been investigated for photocatalytic water splitting for hydrogen generation [22]. Shen et al. reported that the Mo2C/CdS nanocomposite produced a photocurrent density 7.83 times higher than that of pure CdS [23]. Furthermore, Yue et al. reported that dandelion-like TiO2 nanoparticles with 1% Mo2C were able to produce H2 with a production rate of 39.4 mmol h−<sup>1</sup> g−1, which is 25 times that obtained with pristine TiO2 [22].

Although Mo2C has been shown to be a good cocatalyst for TiO2, the photocatalytic performance of TiO2 NTs on Ti foil substrates with Mo2C has yet to be reported. This study presents the effectiveness of TiO2 NTs with Mo2C incorporated as a cocatalyst for PEC water-splitting applications. Here, TiO2 NTs were prepared by two-step electrochemical anodization, and Mo2C was inserted by dip coating the NTs into Mo2C precursors of various concentrations for various dipping times. Our findings indicated that the combined effect of multiple PEC improvement strategies could offer a versatile and systematic way to overcome the intrinsic and extrinsic limitations of TiO2 NTs for PEC water-splitting applications.

#### **2. Materials and Methods**

#### *2.1. Materials*

Titanium foil (0.127 nm thickness, obtained from Sigma Aldrich, St. Louis, MI, USA), molybdenum carbide (Mo2C) (99%, obtained from Sigma Aldrich), Pt mesh (99%, obtained from Sigma Aldrich), ethylene glycol (analytical grade, obtained from Merck, Darmstadt, Germany), ammonium fluoride NH4F (analytical grade, obtained from Merck, Germany), distilled water, ethanol (analytical grade, obtained from QReC, Kuala Lumpur, Malaysia) and sodium sulfate (Na2SO4) were used. All chemicals were used as received from the manufacturer without additional purification.

#### *2.2. Fabrication of TiO2 NTs and TiO2 NTs Decorated with Mo2C*

The TiO2 NTs were fabricated using a multiple anodization technique [24]. First, cleaned Ti foil (1.5 cm × 1.5 cm) was used as the anode, and Pt foil was used as the counter electrode connected to a power supply at a voltage of 50 V for one hour. Ethylene glycol containing 0.3 vol. % NH4F and 2 vol. % distilled water was used as the electrolyte. The anodized film was then sonicated in a mixture of ethanol and distilled water (1:1) for 5 min to clean dirt away from the openings of the grown nanotubes. Subsequently, the Ti foil underwent a second anodization process for 30 min in the same electrolyte at the same voltage. Then, ethanol and distilled water were used to flush the samples. The anodized samples were then annealed at 450 ◦C for 3 h at a ramping rate of 2 ◦C/min. The best TiO2 NTs that achieved the highest photocurrent were then dip coated in an ethanol/distilled water mixture containing highly dispersed Mo2C at four different concentrations of 5 g/L, 10 g/L, 15 g/L and 20 g/L, and the obtained samples were labeled S-1, S-2, S-3 and S-4, respectively.

#### *2.3. Characterization*

X-ray diffraction (XRD) patterns were acquired via a Bruker D-8 Advance (Ettlingen, Germany, Equipment sourced by Bruker Malaysia), and X-ray photoelectron spectroscopy (XPS) was performed using a Kratos/Shimadzu instrument (model: Axis Ultra DLD) (Milton Keynes, UK, Equipment sourced by Shimadzu Malaysia) to determine the chemical phases present in the crystalline substances. The XRD patterns were analyzed using X'Pert HighScore software (Version 2.2b). To investigate the topographic nature of the surface, field emission scanning electron microscopy (FESEM) was carried out using a Zeiss Merlin Compact microscope (Oberkochen, Germany, Equipment sourced by Zeiss Malaysia). The optical properties were analyzed using a Perkin Elmer ultraviolet/visible/near-infrared spectrophotometer (UV–VIS-NIR) (model: Lambda 950) (Waltham, MA, USA, Equipment sourced by Perkin Elmer Malaysia).

#### *2.4. PEC Property Measurements*

An Ametek Versastat 4 was used to carry out the PEC analysis. An exposed area of 1 cm2 was employed for testing the thin films that served as the working electrode in a PEC cell. The counter electrode consisted of a platinum wire; the reference electrode was a Ag/AgCl electrode. The counter electrode measured the potential difference between the two electrodes. In these experiments, 0.5 M Na2SO4 (pH 6.7) was used as the electrolyte. The current density on the thin film surfaces was measured in the dark and under solar AM 1.5 illumination using a xenon lamp (Oriel with an intensity of 100 mW cm<sup>−</sup>2). Linear sweep voltammetry (LSV) was conducted from 0 to +1.0 V versus Ag/AgCl in 0.5 M Na2SO4 at a scan rate of 5 mV s−1. To obtain a deeper understanding of the charge transport behavior shown by the synthesized photoanodes, Mott-Schottky analysis was performed at 1 kHz. This allowed for calculation of the charge carrier densities, as well as the conduction band (CB). The electrochemical impedance spectra (EIS) Nyquist plots were constructed by utilizing 10 mV sinusoidal perturbations at a frequency of 100 kHz.

#### **3. Results and Discussion**

#### *3.1. Physical Characterization of TiO2 NTs*

To thoroughly investigate the growth of TiO2 NTs, a detailed analysis was conducted comprising analysis of the morphology, crystal phase, crystallinity, and optical properties. Figure 1 shows the FESEM results that capture the microstructure of the TiO2 NT sample.

Based on the FESEM images, the TiO2 NT sample clearly exhibited non-interconnected single tubes (Figure 1a). The diameter of the TiO2 NTs was ~151–160 nm. Figure 1b displays cross-sectional views of TiO2 NTs. The length of the TiO2 NTs was ~3.4–3.8 μm.

Next, XRD analysis was carried out to identify the phases and determine the chemical composition. Figure 2a shows the XRD patterns of the Ti foil substrate and TiO2 NT sample.

**Figure 1.** FESEM images of the (**a**) surface morphology and (**b**) cross-section of TiO2 NTs.

**Figure 2.** (**a**) XRD patterns and (**b**) XPS survey spectrum of TiO2 NTs; (**c**) Ti 2p and (**d**) O 1s spectra of TiO2 NTs.

The spectra show Ti alpha diffraction peaks representing the Ti foil at 35.01◦, 38.27◦, 40.10◦, 52.99◦, 62.89◦, 70.66◦ and 76.18◦, corresponding to the (0 1 0), (0 0 2), (0 1 1), (0 1 2), (0 1 3), (1 1 0) and (1 1 2) planes, respectively. However, when the sample was anodized, anatase peaks appeared at 25.28◦, 47.83◦, 53.09◦ and 55.02◦, corresponding to the (0 1 1), (020), (0 1 5) and (1 2 1) planes, respectively. The intensity of the Ti alpha peaks was reduced because the surface of the titanium substrate was oxidized during the anodization process, which resulted in the formation of a layer of anatase titanium oxide. The XRD patterns found in this study are similar to those found in Quiroz et al., (2015), which contain a tri-titanium pentoxide (Ti3O5) phase [25]. Based on the XRD library patterns, the Ti alpha peaks overlapped with the Ti3O5 peaks at 38.27◦, 52.99◦ and 70.66◦ and overlapped with some anatase peaks at 25.28◦ and 47.83◦.

Figure 2b shows the results of the XPS analysis conducted to verify the presence of Ti3O5 in the TiO2 NT samples. The peaks at ~464 eV and 458 eV correspond to Ti 2p, and the Ti 2p peaks of Ti3+ were observed at binding energies of 463.1 eV (Ti3+ 2p3/2) and 459.1 eV (Ti4+ 2p1/2). The Ti 2p peaks of Ti4+ appeared at 458.5 eV (Ti4+ 2p3/2) and 464.2 eV (Ti4+ 2p1/2). The XPS spectra of TiO2 NTs revealed a modest shift in position and a change in the size of the original peaks of TiO2 NTs from those in a previous study after self-doping with Ti3+. The observed peak shift indicates that the self-doping of Ti with Ti3+ affected its electronic state. As a consequence of this process, some of the Ti4+ ions in the lattice are believed to have been replaced by Ti3+ ions. Furthermore, the decrease in the Ti4+ peaks suggests that there was less TiO2 present in the sample. The creation of oxygen vacancies in the surface layer during the multistep anodization procedure can be deduced to be responsible for the diminishing area of the Ti4+ species peak [26]. Furthermore, the peaks at binding energies of 529.9 eV and 530.6 eV were ascribed to lattice oxygen for TiO2 NTs. The O 1s spectra provided more evidence demonstrating that more oxygen defects were present in the TiO2 NTs [27]. The percentage of atomic oxygen vacancies for TiO2 NTs was 6.25%. The XPS results agreed with the XRD results, suggesting that more Ti3+ was produced during the anodization process.

Figure 3a shows the UV–Vis absorption spectra over the wavelength range from 250 to 800 nm, showing that the TiO2 NTs had higher absorption in the UV range. Next, the band gap of the sample was calculated using Kubelka–Munk theory, and the value for the TiO2 NTs was 3.15 eV [3].

**Figure 3.** (**a**) Absorption spectra and (**b**) band gap determination by Kubelka–Munk plot analysis.

#### *3.2. Physical Characterization of Mo2C as a Cocatalyst Decorated on TiO2 NTs*

Mo2C was added to TiO2 NTs using a dip coating technique. Dip coating is a simple, dependable, and robust process that can be used to cover almost any substrate material by immersing it in a solution and then removing it to drip dry to form a conformal coating.

FESEM analysis was performed to study the effect of different concentrations of the Mo2C precursor on the morphology of TiO2 NTs. Figure 4(a1–d3) shows micrographs of TiO2 NTs for different precursor Mo2C concentrations.

As shown in Figure 4, increasing the concentration resulted in an increasing deposition amount. Figure 4(a1,b1) illustrate that the distribution of Mo2C on the surface of the TiO2 NTs was not uniform. Figure 4(c1) shows that Mo2C was well distributed inside the TiO2 NT tubes at a concentration of 15 g/L, which was crucial for the photoelectrode activity. Increasing the concentration to 20 g/L resulted in large nanoclusters of Mo2C blocking most TiO2 NTs (Figure 4(d1)). Next, Figure 4(a2–d2) illustrates cross-sectional views of the decorated Mo2C on the TiO2 NTs. The length of the TiO2 NTs increased as the concentration of Mo2C increased; this finding may support the idea that Mo2C is distributed on the upper openings of the TiO2 NTs. In addition, an image of the cross-section of sample S-3 can be seen in the inset of Figure 4(c2); this image demonstrates that Mo2C decorated the outside wall of the tubes. Energy-dispersive X-ray spectroscopy (EDX) mapping and cross-section analysis were performed to determine the distribution of Mo2C in sample S-3 with a concentration of 15 g/L. The findings are shown in Figure 4(a3–d3), suggesting that Mo2C was equally dispersed over the TiO2 NT surface and in the interstices. This indicates that Mo2C was efficiently distributed across the sample, resulting in a uniform distribution.

The XRD patterns of TiO2 NTs after deposition of Mo2C are presented in Figure 5.

**Figure 4.** FESEM images of the surface and cross-section, as well as EDX mapping of samples at different concentrations (**a1**–**a3**) S-1, (**b1**–**b3**) S-2, (**c1**–**c3**) S-3 and (**d1**–**d3**) S-4.

**Figure 5.** XRD patterns of S-1, S-2, S-3 and S-4.

The diffraction peaks at 25.1◦, 37.8◦, 48.0◦, 52.9◦ and 62.3◦ in the pattern of bare TiO2 NTs were identified as corresponding to the planes of anatase TiO2 and Ti3O5 phases (JCPDS nos. 98-009-4632 and 98-007-1775). Upon deposition of Mo2C nanoparticles, the patterns displayed additional peaks at 27.2◦, 37.2◦, 38.3◦, 41.1◦ and 68.8◦, which correspond to the standard diffraction peaks of Mo2C (JCPDS no. 98-006-1705). These findings are consistent with the FESEM results, in which increasing the concentration of Mo2C leads to increases in the amount of deposited Mo2C and the intensity of the peaks. The observed

peaks indicate the successful deposition of Mo2C nanoparticles onto the TiO2 NT surface, which can potentially enhance the PEC properties of the material.

The UV–Vis absorption spectra of Mo2C/TiO2 NTs with various concentrations are presented in Figure 6a. The TiO2 NTs loaded with Mo2C nanoparticles exhibited a broader absorption in the visible light region (450 nm to 800 nm) compared to pure TiO2 NTs. Among the samples, S-3 showed the highest absorption and thus had the highest PEC activity. The band gap of the samples is displayed in Figure 6b, revealing that the Mo2Cloaded TiO2 NTs had a lower band gap than the pure TiO2 NTs.

**Figure 6.** (**a**) UV–Vis absorption spectra and (**b**) Kubelka–Munk plots for band gap determination of S-1, S-2, S-3 and S-4.

The band gap of S-3 was determined to be ~2.80 eV, the smallest among the samples. Previous reports suggest that higher absorption in the visible region corresponds to better PEC water-splitting activity. Fine-tuning the band gap and band locations is necessary when creating visible light-responsive photocatalysts for hydrogen production.

#### *3.3. PEC Properties of TiO2 NTs and Mo2C as a Cocatalyst Decorated on TiO2 NTs*

Mo2C has garnered interest in the field of PEC applications due to its exceptional stability in challenging environments and remarkable electrical conductivity, making it a promising cocatalyst for such purposes. Mo2C applied onto TiO2 NTs has been observed to function as an electron transfer mediator, thereby facilitating the separation of photogenerated charge carriers and resulting in an improved overall PEC performance of TiO2 NTs. The hybridization of TiO2 NTs with Mo2C has been found to exhibit a synergistic effect, whereby the distinctive characteristics of each material are combined to overcome the constraints of TiO2 NTs. This discussion explores the PEC characteristics of TiO2 NTs and TiO2 NTs that have been decorated with Mo2C as a cocatalyst.

The correlation between the TiO2 NTs with Mo2C as a cocatalyst and the PEC behavior of TiO2 NTs was investigated by chronoamperometric measurements under light chopping, and the test was carried out in 0.5 M Na2SO4 at a bias potential of 0.7 V vs. Ag/AgCl in the presence and absence of illumination (light-off and light-on). The concentration of Mo2C varied, with values of 5 g/L (S-1), 10 g/L (S-2), 15 g/L (S-3) and 20 g/L (S-4), as shown in Figure 7a.

All samples demonstrated a satisfactory photocurrent density as well as a good level of stability after 900 s. The photocurrent density of the TiO2 NT sample was determined to be 0.21 mA cm−2, and this value of photocurrent increased approximately one-fold when compared to the value of pure TiO2 NT due to the presence of oxygen vacancy defects, as reported in previous work [28–30]. Meanwhile, the photocurrent densities produced by samples S-1, S-2 and S-4 were similar to that of bare TiO2 NTs. The significant photocurrent density produced by sample S-3 had a value of ~1.4 mA cm−2, nearly five times higher than that of bare TiO2 NTs.

**Figure 7.** (**a**) Photocurrent density and stability under light chopping of TiO2 NTs and S1–S4; (**b**) EIS spectra of the TiO2 NT and S1–S4 samples; (**c**) N-type Mott–Schottky plots of TiO2 NTs and S1–S4; (**d**) ABPE % of TiO2 NTs and S-3.

EIS is a trustworthy method for investigating the charge transfer and recombination rate at semiconductor electrolyte interfaces, where "Zre" is the real portion and "Zim" is the imaginary part. Due to the relationship between the arc of the circle and the charge transfer resistance, the Nyquist plots provide sufficient information on the charge transfer. In the Nyquist plot, a smaller arc indicates greater charge carrier separation and higher charge transfer efficiency (conductivity) [30]. Figure 7b shows that sample S-3 has the smallest semicircle radius. This indicates that photogenerated electron-hole pairs are more effectively separated and that electrons may more easily cross the valence band in response to a relatively low-energy excitation. As a consequence, the charge transfer in S-3 is enhanced, proving the presence of a large separation between the holes and electrons.

The Mott–Schottky (M–S) curves of all photoelectrode samples are shown in Figure 7c. The flat band potential (*E*fb) was estimated by projecting the linear section of the plots onto the potential axis. In addition, the donor density (*N*D) was calculated using the slope of the M–S curves and Equation (1) obtained from previous work [25,30]. The *N*<sup>D</sup> and *E*fb values determined are reported in Table 1, and the *E*fb values of sample S-3 are less negative, which indicates an upward shift of the Fermi level [30,31].

$$N\_{\rm D} = \left(\frac{2}{e\varepsilon\varepsilon\_o A^2}\right) \left[\frac{d\left(\frac{1}{C2}\right)}{dE}\right]^{-1} \tag{1}$$

where *d*( <sup>1</sup> *<sup>C</sup>*<sup>2</sup> ) *dE* is the slope of the tangent line in the Mott–Schottky plot, *e* is the electron charge, *ε* is the dielectric constant of the TiO2 film, *ε*<sup>0</sup> is the vacuum permittivity, and *A* is the surface area of the TiO2 NT thin film electrode.

**Table 1.** Flat band potential (*E*fb) and donor density (*N*D) of TiO2 NTs and S-3.


The efficiency with which a PEC cell converts light into electricity is quantified by the applied bias photon-to-current efficiency (ABPE). To assess how well a PEC cell converts solar energy into a usable form, the ABPE test is crucial. This method is useful for comparing the efficiency of various materials in converting light into electricity and for determining the efficacy of individual materials. To further improve the PEC cell design, the ABPE may be utilized to investigate how an applied bias affects the cell output. In conclusion, the ABPE is a useful metric for assessing PEC cell performance, yielding crucial data for improving future solar energy conversion technologies. Figure 7d shows the ABPE measurements of TiO2 NTs and S-3. The ABPE value was calculated using Equation (2) [32,33]:

$$\text{ABPE } \%= \left[ I\_{\text{P}} \left( E\_{\text{0 rev}} - E\_{\text{app}} \right) / I\_{\text{0}} \right] \times \text{(100)}\tag{2}$$

where *J*<sup>p</sup> is the photocurrent density (mA/cm2), *I*<sup>0</sup> is the illumination intensity (mW/cm2), *E*<sup>0</sup> rev is the standard reversible potential for water splitting (1.23 V), and *E*app is the applied potential. The highest ABPE value for TiO2 NTs is 0.19% at 0.8 V, while that of S-3 is 0.89% at 0.2 V. Next, the solar-to-hydrogen efficiency (STH) was also calculated for PEC water splitting with a visible light source of irradiance 100 mW cm−<sup>2</sup> using Equation (3) [32,33]:

$$STH(\%) = f\_{\rm P} \frac{1.23 - V\_{\rm APP}}{P} \times 100\tag{3}$$

where *J*<sup>p</sup> is the photocurrent density (mA/cm2), *V*app is the applied potential, and *P* is the intensity of the light source. The maximum STH % for TiO2 NTs is 0.05%, while that for Mo2C/TiO2 NTs (S-3) is 0.32%, as shown in Table 2.

**Table 2.** Measured parameters of the PEC cell.


The increased PEC properties of TiO2 NTs decorated with Mo2C are further explained by the proposed mechanism shown in Figure 8.

In this work, oxygen vacancies produce localized electronic states inside the energy gap, which correspond to Ti3+ species, which are in the mid-band gap. These restricted states may serve as traps or recombination sites for electrons and holes created by photons. The Mo2C deposited onto the TiO2 photoanode may serve as a cocatalyst to accelerate the HER and oxygen evolution reaction (OER), leading to a higher efficiency in PEC water splitting. Moreover, the Mo2C cocatalyst may minimize the energy barrier for charge transfer and avoid charge recombination, resulting in an increased photocurrent and better stability.

**Figure 8.** Proposed mechanism of Mo2C as a cocatalyst in TiO2 NTs self-doped by Ti3+.

Upon irradiation with solar light, the TiO2 electrons may be excited into the CB during the process, and electrons move from the TiO2 photoanode to the cathode, resulting in the reduction of protons to hydrogen at the cathode. Mo2C works as a cocatalyst, boosting the oxidation of water molecules by facilitating the flow of holes from the TiO2 photoanode surface to the water molecules. The Mo2C catalyst helps reduce the energy barrier for the OER, which leads to a decrease in the overpotential needed to drive the reaction. This may lead to an increase in the rate of the reaction and consequently a greater efficiency of the entire water-splitting process.

The method for achieving very uniform Mo2C nanoparticles distributed on both the inside and outside vertically aligned TiO2 NTs via the dip coating deposition process has great promise. These novel interactions of Mo2C/TiO2 NTs dramatically enhance both the light absorption and the PEC activity under visible light illumination by 5-fold compared to those of pure TiO2 NTs. These characterization results are consistent with an improved Mo2C/TiO2 NT performance, although the result is not as incredible as that reported by previous researchers on the photocatalytic effect of Mo2C for pristine powder TiO2 [34]. There is still much room to improve the performance, such as by optimizing the length and tube diameter of TiO2 NTs so that they are suitable for Mo2C diffusion, as well as the Mo2C deposition methods.

#### **4. Conclusions**

The implementation of Mo2C as a cocatalyst led to a significant enhancement in the photocurrent density of TiO2 NTs. The photocurrent density of the modified TiO2 NTs was observed to be significantly enhanced by a factor of five when compared to the unmodified TiO2 NTs. The implications of these findings are of great importance for the further development of environmentally sustainable PEC water-splitting technologies, specifically in the domain of hydrogen production. The successful decoration of TiO2 NTs with Mo2C was confirmed through XRD and FESEM analysis. Moreover, the incorporation of Mo2C has been observed to significantly decrease the band gap and enhance the light absorption capabilities of TiO2 NTs. Significantly, it was determined that the most favorable concentration of Mo2C was 15 g/L (S-3), exhibiting the highest photoelectrochemical efficiency. In general, this study provides insights into the possible utilization of Mo2C-decorated TiO2 NTs, specifically the Ti3O5 phase, for enhancing the effectiveness and efficacy of photoelectrochemical systems. The utilization of Mo2C as a cocatalyst in PEC water-splitting applications is highly promising due to several factors. These include the achievement of enhanced photocurrent density, the confirmation of Mo2C's presence through X-ray diffraction (XRD) analysis, the reduction of the band gap, and the determination of an optimal concentration.

**Author Contributions:** Conceptualization, K.A.; methodology, K.A. and S.N.F.M.; validation, S.N.F.M.; formal analysis, S.N.F.M.; investigation, S.N.F.M.; writing—original draft preparation, S.N.F.M.; writing—review and editing, K.A., L.J.M., R.M.Y., M.A.M. and M.B.K.; supervision, K.A.; project administration, K.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Ministry of Education, Malaysia, through FRGS/1/2019/ STG01/UKM/03/2.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The authors do not have permission to share data.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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**Xiaodong Xu 1, Wangping Wu 1,\* and Qinqin Wang 2,\***


**Abstract:** To improve the efficiency of polycrystalline silicon solar cells, process optimization is a key technology in the photovoltaic industry. Despite the efficiency of this technique to be reproducible, economic, and simple, it presents a major inconvenience to have a heavily doped region near the surface which induces a high minority carrier recombination. To limit this effect, an optimization of diffused phosphorous profiles is required. A "low-high-low" temperature step of the POCl3 diffusion process was developed to improve the efficiency of industrial-type polycrystalline silicon solar cells. The low surface concentration of phosphorus doping of 4.54 <sup>×</sup> 1020 atoms/cm3 and junction depth of 0.31 μm at a dopant concentration of *N* = 1017 atoms/cm3 were obtained. The open-circuit voltage and fill factor of solar cells increased up to 1 mV and 0.30%, compared with the online low-temperature diffusion process, respectively. The efficiency of solar cells and the power of PV cells were increased by 0.1% and 1 W, respectively. This POCl3 diffusion process effectively improved the overall efficiency of industrial-type polycrystalline silicon solar cells in this solar field.

**Keywords:** polycrystalline silicon; solar cells; low-high-low; phosphorus diffusion

#### **1. Introduction**

Carbon-neutral development strategies have a significant impact on the Earth's environment, and silicon (Si) solar cells have attracted much attention as a means to use solar energy to convert sunlight into electricity [1,2]. However, a compromise between cost reduction and efficiency improvement must be reached [3]. The PN junction is one of the key technologies in crystalline Si solar cells, which affects photoelectric conversion efficiency. Therefore, the PN junction has excellent performance and stable uniformity. To improve the photoelectric conversion efficiency, the high sheet resistance of over 90 Ω/sq, which has low surface doping concentration and a shallow junction process, was accepted [4]. High open-circuit voltage (*V*oc) and short-circuit current (*J*sc) values were obtained by this process. Tube furnace diffusion using phosphorus oxychloride (POCl3) as a dopant precursor is the dominant emitter formation technology for p-type Si solar cells [5]. The majority of the PV industry currently uses POCl3 diffusion to remove metal impurities, including iron [6,7]. The solar cell emitters are obtained by P (phosphorus) diffusion in p-type Si inside of a diffusion tube furnace under controlled conditions of temperature, pressure, and gas flow to form an emitter layer [3].

The photovoltaic (PV) industry has used a quartz diffusion tube furnace to form an emitter layer of the POCl3 source. However, there are three flaws in this process [8]: (1) the high sheet resistance, (2) the difficulty in controlling the uniformity of high sheet resistance, and (3) the shallow junction. Many manufacturers tried to reduce the heavy inactive phosphorus concentration and the thickness of the dead zone through an additional step in the industrial process: i.e., chemical etching of the PSG layer after the phosphorus diffusion [9–11]. This solution increased the duration of the industrial process and it is expensive. A one-step diffusion process was a common method in which a single

**Citation:** Xu, X.; Wu, W.; Wang, Q. Efficiency Improvement of Industrial Silicon Solar Cells by the POCl3 Diffusion Process. *Materials* **2023**, *16*, 1824. https://doi.org/10.3390/ ma16051824

Academic Editor: Cristobal Voz

Received: 31 December 2022 Revised: 18 February 2023 Accepted: 21 February 2023 Published: 23 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

temperature and continuous flow of dopant gas were used to deposit phosphor silicate glass (PSG) and to drive dopants to the desired depth [12]. This is a fast process but it tends to create an excessively doped emitter that deteriorates electrical performance [13]. POCl3 diffusion could be performed in a two-step process: a PSG deposition step, followed by a drive-in step at variable temperature. During the process, POCl3 gas is allowed in the PSG layer, and subsequently, dopants are moved deeply from the PSG layer to the Si substrate in the drive-in step [14]. Wolf et al. [5] presented the status and perspective of emitter formation by the POCl3-diffusion process and discussed the diluted source and in-situ post-oxidation technological options for advanced tube furnace POCl3-diffusion processes. Cui et al. [15] studied POCl3-based diffusion optimization for the formation of homogeneous emitters and the correlation with metal contact in p-type polycrystalline Si solar cells and found that the sheet resistance is high and that the P surface concentration and emitter saturation current density (*J*oe) are low. Cho et al. [16] compared POCl3 diffusion and P ion-implantation induced gettering in solar cells and found that the increase in P implantation dose improved the gettering efficiency by increasing bulk lifetime and decreasing iron concentration, but the process remained inferior to POCl3 diffusion. POCl3 diffused cast quasi-mono cells showed 0.4% higher efficiency due to their higher bulk lifetimes compared to P-implanted emitters. Ghembaza et al. [17] studied the optimization of P emitter formation from POCl3 diffusion for p-type Si solar cells and showed that the emitter standard sheet resistances of ~60 Ω/sq and wafer uniformity <3% were obtained from the low-pressure tube furnace. Li et al. [18] investigated POCl3 diffusion for the emitter layer formation of industrial Si solar cells and presented the impact of processing parameters on emitter layer formation and electrical performance.

According to the above review, P diffusion can be performed in a single step by controlling a parameter, such as temperature or time [4], or a two-step process, such as ion implantation. In this work, a "low-high-low" (LHL) diffusion process, low-high-low temperature, and three-step diffusion were used to diffuse P elements with different POCl3 flows. The ECV profile, open-circuit voltage (*V*oc), fill factor (FF), and overall efficiency of solar cells of this process were studied and simultaneously compared with the baseline using the online conventional process.

#### **2. Experimental**

P diffusion emitters were prepared on 156 × 156 mm 0.5–3 Ω·cm p-type mc-Si wafers with a thickness of ~180 μm in the quartz furnace tube. The distance between the wafers was about 2.35 mm. These wafers were vertically inserted into the quartz boat and then placed in the furnace. There were 1000 pieces per batch. There were two diffusion processes. One was the online process, and another was the LHL diffusion process. Figure 1 shows the schematic of the online and LHL diffusion processes.

Table 1 displays the process parameters of low-temperature online diffusion, namely the BKM (Best Known Method) diffusion process and the LHL diffusion process for solar cells. For the low-temperature online diffusion process, the temperature was kept at 810 ◦C, and the flow of POCl3, O2, and N2 gas was 1600 mL/min, 800 mL/min, and 30,000 mL/min, respectively. For the LHL diffusion process, the first step is a low temperature and high POCl3 flow diffusion. The low temperature was controlled at about 810 ◦C, and the flows of POCl3, O2, and N2 gas were set at 1900 mL/min, 800 mL/min, and 30,000 mL/min, respectively. Next, the high temperature was controlled at 825 ◦C and the flows of POCl3, O2. and N2 gas were fixed at 2100 mL/min, 800 mL/min, and 30,000 mL/min, respectively. Finally, a low temperature and low POCl3 flow diffusion process was used. The diffusion temperature was the same as in the first step. The flows of POCl3, O2, and N2 gas were 1600 mL/min, 800 mL/min, and 30,000 mL/min, respectively. The three-step variabletemperature diffusion LHL process is useful in the gettering process [19].

**Figure 1.** Schematic of BKM and LHL diffusion process.

**Table 1.** Process parameters of BKM and LHL diffusion processes for p-type mc-Si solar cells.


Three batches of samples for each process were manufactured in order to get the average values. The sheet resistance was measured using four-point probe equipment, and the P diffusion profiles of selected samples were determined using electrochemical capacitance voltage profiling (ECV-profiling, WEP CVP21). The microstructure and morphology of the textured structure and front metalized areas were observed by scanning electronic microscopy (SEM, Quanta FEG 250, FEI). The electrical properties of solar cells were characterized by a Berger cell tester.

#### **3. Results and Discussion**

We designed the LHL diffusion process with low-high-low temperature and a threestep diffusion with different POCl3 flows. The specific schematic diagrams are shown in Figure 2. Figure 2a shows the schematic diagram of the conventional primary diffusion process. Firstly, pre-oxidation is carried out with oxygen at a low temperature of 700–800 ◦C to generate silicon oxides on the surface of Si wafers, which is helpful to the distribution of POCl3 diffusion. Then, POCl3 is deposited at a low temperature of 800 ◦C, and finally at a high temperature of 850 ◦C, in order to redistribute the P element. Figure 2b presents the schematic diagram of the LHL diffusion process. LHL diffusion is characterized by three sets of P doping and three sets of redistribution. Variation in temperature is the simplest way to control the phosphorous diffusion profile. As the temperature increases, doping increases, and the formed junctions are deeper. This behavior is explained by the variation of coefficient diffusion and limited solubility with temperature. For this reason, the temperature parameter needed to achieve the necessary exact junction depth has proven to be rather delicate. With a long drive-in time, the junction is deeper. PSG deposited during the pre-deposition step acts as an infinite phosphorus source. All these results confirm that the phosphorus profile is highly affected by the tube furnace conditions. Clearly, time and temperature must be considered carefully. In this work, firstly, the preoxidation is carried out with oxygen at a low temperature of 800 ◦C. Then, the first step of low-temperature POCl3 deposition is carried out, and the impurities are redistributed at variable temperatures. Subsequently, the second step of high-temperature POCl3 deposition

is carried out, which is distributed with high-temperature P impurities. Finally, the third step is to deposit POCl3 at a high temperature to cool down, and the impurities are redistributed. LHL diffusion process adopts low-high-low temperature and three-step diffusion with different POCl3 flows. In the first step, the low temperature and high POCl3 flow are the best to control the tail concentration of ECV curves. In the low-concentration tail, P diffuses into Si wafers primarily via interaction with Si self-interstitials [20,21]. In the second step, the high temperature and high POCl3 flow control the kink of the slope. For high P concentration, a conversion from an interstitially to a slow vacancymediated process occurs, giving rise to anomalous P diffusion profiles [22]. In the third step, the low temperature and low POCl3 flow can control the surface concentration of P doping. This method has the objective to decrease inactive phosphorus through an LHL step. Graphically this implies the reduction of the plateau width, which appears on the top of diffusion profiles near the high-phosphorus concentration zone. The low surface concentration of P doping could be beneficial to the *V*oc and *J*sc values of solar cells. However, it influences series resistance and FF values. Therefore, it is important to weigh the benefits against the risks.

**Figure 2.** Schematic diagrams of the PN junction of solar cells obtained from (**a**) BKM and (**b**) LHL diffusion processes.

Figure 3 shows the sheet resistance box plots of solar cells and the ECV profiles of P doping for solar cells. The sheet resistance of solar cells was obtained (see Figure 3a). It can be observed that solar cells produced from LHL and BKM diffusion processes had the same sheet resistance of about 90 Ω/sq. However, the sheet resistance of solar cells from the LHL process was much more uniform than that of the cells from the BKM diffusion process. The results indicated that the LHL process could be beneficial for the *FF* and the series resistance of solar cells [17]. Figure 3b presents the P doping profile of solar cells produced by LHL and BKM diffusion processes. The solar cells obtained from the LHL diffusion process had a lower surface concentration of P doping, approximately 4.54 × <sup>10</sup><sup>20</sup> fewer atoms/cm<sup>3</sup> than those produced from the BKM diffusion process, which produced about 6.08 × <sup>10</sup><sup>20</sup> atoms/cm<sup>3</sup> at the junction depth of about 0.02 <sup>μ</sup>m. For LHL and BKM diffusion processes, the solar cells had the same junction depth of around 0.3 μm at a dopant concentration of *N* = 1017 atoms/cm3. During emitter formation and at high phosphorus concentrations, precipitates were formed on the silicon surface and promoted the existence of electrically inactive phosphorus which formed a dead layer at the silicon surface. This behavior is characterized by a kinked shape in the experimental profiles. This kink has a great impact on solar cell performance since it results in low collection efficiency near the front surface.

**Figure 3.** Sheet resistance box-plots of solar cells obtained from BKM and LHL diffusion processes (**a**), and the ECV profiles of P doping (**b**).

In the module of PV solar cell marketing, only the *V*oc and *FF* values have more advantages on the power. In this study, the advantage of the LHL diffusion process could be beneficial to the increase in *V*oc and *FF* values. Table 2 displays the gap of electrical characteristics of solar cells obtained by LHL and BKM diffusion processes. The solar cells obtained from the LHL diffusion process have an increase in median *V*oc value of about 1 mV, compared with the BKM diffusion process. This increase might be due to the low surface concentration of P doping (see Figure 4b). At the same time, the median *FF* value is increased by 0.30%, which can be contributed to the strong impurity absorption effect of Si wafers and the decrease in inactive phosphorus in the LHL diffusion process.

**Table 2.** The gap in electrical characteristics for solar cells obtained by LHL and BKM diffusion processes.


Note: *V*oc: open circuit voltage; *J*sc: short circuit current; *R*ser: series resistance; *FF*: fill factor; *E*ff: efficiency.

**Figure 4.** Box plots of the electrical characteristics of solar cells obtained from BKM and LHL diffusion processes (**a**) *V*oc, (**b**) *J*sc, (**c**) *FF,* and (**d**) *E*ff.

Figure 4 shows the box plots of electrical characteristics of solar cells produced from BKM and LHL diffusion processes. The median *V*oc values of the solar cells produced by LHL and BKM processes are 630 ± 1 mV and 629 ± 1 mV, respectively (see Figure 4a). For LHL and BKM diffusion processes, the median *Jsc* values of the solar cells are the same, about 35.55 ± 0.25 mA (see Figure 4b). The median FF values of the solar cells obtained by LHL and BKM diffusion processes are 78.9 ± 0.1% and 78.6 ± 0.1%, respectively (see Figure 4c). The median *Eff* values of the cells produced by LHL and BKM diffusion processes are 17.65 ± 0.15% and 17.55 ± 0.15%, respectively (Figure 4d). The *Eff* value of the solar cells from the LHL diffusion process is increased by 0.08–0.10%, which was mainly attributed to the increase in *V*oc value of 1 mV and the *FF* value of 0.30%. These results show the convergence of the *V*oc, *FF* and *Eff* values of the solar cells obtained from the LHL diffusion process is better than the parameters of cells produced from the BKM diffusion process, which was attributed to the distribution and diffusion of impurities. At the same time, the effective doping concentration is effectively controlled, the generation of the dead layer is reduced, and recombination is reduced, resulting in the increase of the *V*oc value. Furthermore, due to the LHL diffusion process with three-time temperature changes, it is more conducive to the precipitation of harmful impurities into the PSG layer, which improves the service life of the bulk, thus resulting in the improvement of *FF* values. The increase in Voc and FF can be explained by active phosphorus atoms and the optimized contact-formation process. Table S1 summarizes the performance of solar cells doped P by different diffusion processes (see Supplementary Material).

Figure 5 presents the typical top-view SEM micrographs of the front side of solar cells. It shows that the front busbar is of uniform height: about 12.4 μm (see Figure 5a), and the

height of the front finger is also smooth: about 24.3 μm (see Figure 5b). However, the Ag paste does not uniformly corrode the P doping layer. In Figure 5c, the corroded depth is approximately 0.12 μm, so the effect of the surface concentration at the depth of 0.15 μm on the contact resistance (*ρ*c) could be emphasized, which is important for the FF values. Figure 6 shows the contact resistivity of solar cells from two diffusion processes. The *<sup>ρ</sup>*<sup>c</sup> values of the cells from LHK and BKM processes were 18.86 mΩ·cm2 and 19.09 mΩ·cm2. The LHL diffusion process has no additional costs. It would be a benefit for the large-scale industrial production of the P doping process of PV solar cells.

**Figure 5.** SEM images of (**a**) front busbar, (**b**) front finger, and (**c**) Ag-Si alloy.

**Figure 6.** Contact resistivity of solar cells from LHL and BKM diffusion processes.

#### **4. Conclusions**

The non-uniformity of worm-shaped structures increases the difficulty of P diffusion. In addition, the phosphorus profile is highly affected by the tube furnace conditions. (Time and temperature must be considered carefully). A diffusion process featuring low-high-low temperature and three steps was used to diffuse P elements for solar cells with different POCl3 flows in every step. This allows for systematic manipulation of doping profiles, especially for manipulation of the surface-active concentration of P doping, control of the doping depth, and reduction in the dead layer at the silicon surface, respectively. The solar cells with a low surface concentration of P doping of 4.54 × 1020 atom/cm3 and junction depth of 0.31 μm at a dopant concentration of *N* = 10<sup>17</sup> atoms/cm<sup>3</sup> were obtained. The

open-circuit voltage and FF values of solar cells increased up to 1 mV and 0.30%, compared with the online low-temperature diffusion process respectively, which can be contributed to the low surface concentration of P doping (decreasing inactive phosphorus) and the strong impurity absorption effect of Si wafers obtained from the low-high-low temperature diffusion process. The efficiency and the power of solar cells were increased by 0.1% and 1 W, respectively. The LHL diffusion process has no additional costs. It would be beneficial for the large-scale industrial production of the P doping process of PV solar cells.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma16051824/s1.

**Author Contributions:** X.X.: Data curation, Formal analysis, Investigation, Methodology; W.W.: Writing—original draft, Writing—review& editing, Supervision; Q.W.: Writing—review& editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been partially supported by Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology (SKLPST 202201).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Conflicts of Interest:** We declare that we do not have any commercial or associative interests that represent a conflict of interest in connection with the work submitted.

#### **References**


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