Biomass-derived active carbon (AC) modified TiO₂ photocatalyst for efficient photocatalytic reduction of chromium (VI) under visible light

2.1 Materials

The block copolymer Pluronic F-127 surfactant EO₁₀₆-PO₇₀EO₁₀₆(EO = -CH₂CH₂O-, PO = -CH₂ (CH₃) CHO-), molecular weight :12,600 g/mol, TBOT : Ti(OC (CH₃)₃)₄, acetic acid, hydrochloric acid, methanol and ethanol were obtained from Sigma-Aldrich and utilized as it is without any further processing.

2.2 Synthesis of mesoporous TiO₂ (m-TiO₂)

Mesoporous TiO₂ was successfully obtained by sol–gel technique. Briefly, fixed molar ratio of each involved species i.e. TiO₂/F127/C₂H₅OH/HCl/CH₃COOH (1:0.02:50:2.25:3.75) was taken in the starting solution. Typically, 1.6 g Pluronic F127, 2.3 mL CH₃COOH and 0.74 mL HCl were homogeneously mixed in 30 mL of ethanol for 30 min and then mixed with 3.5 mL TBOT by dropwise addition (Ismail et al., 2010). The obtained mixture was strongly stirred for 60 min and then put into a Petri dish. Evaporation of Ethanol at 40 °C in humidity chamber with relative humidity of 40% for 12 h followed by 24 h drying of the prepared sample at 65 °C. The product was calcined for 4 h at 450 °C in air maintaining heating rate to 1 °C/min and a cooling degree of 2 °C/min in order to obtain impurity free m-TiO₂.

2.3 Preparation of activated carbon (AC)

Sufficient amount of date seeds were taken and washed thoroughly with water followed by drying. Dried seeds were then crushed in an electrical grinder and sieved to obtain fine powder, followed by sonication in ethanol for 30 min and then evaporate the ethanol at 50 ⁰C for 24 h. At the final step, the powder was heat-treated to a pyrolysis temperature of 500 °C for 3 h, with a slow heating rate of 10 °C/min under the flow of nitrogen gas.

2.4 Preparation of AC/m-TiO₂ nanocomposites

AC/m-TiO₂ comprising of variable weight percentages of biomass derived AC (5.0, 10.0, 25.0 and 50.0) has been fabricated using ultrasonication methodology. Typically, calculated amount of AC as per wt% required for particular combination were added to fixed amount of m-TiO₂ in 100 mL of distilled water which then thoroughly ultra-sonicated for 30 min. The resultant suspensions were filtered and washed 4–5 times with distilled water. Resulting wet material put in an oven for overnight at 65 °C in order to get final dried product.

2.5 Material characterization

XRD analysis was accomplished on Bruker diffractometer having Cu Kα_1/2, λα₁ = 154.060 pm, λα₂ = 154.439 pm radiation (Model: AXS D4 Endeavour X). Exploration of surface morphological features were achieved on field emission scanning electron microscope (Model: JSM-7600F, Japan) and on Transmission electron microscope (Model: JEOL JEM-2100F-UHR) having well fitted with Gatan GIF 2001 energy filter with distinctive camera (1 k-CCD) operating at 200 kV to provide EEL spectral trends. All samples also passed through FTIR KBr dispersion mode in the range of 400 to 4000 cm⁻¹ in order to acquire FTIR spectra on Perkin Elmer FTIR spectrometer. The X-ray photoelectron spectroscopy (XPS) was performed on JEOL, JPS 9200: MgKα spectrometer having excitation radiation source (MgKα, pass energy = 50.0 eV (wide scan) and 30 eV (narrow scan), current = 20 mA, voltage = 10 kV). Fluorescence spectrophotometer (Model: F-7000-Hitachi) was utilized to monitor the photoluminescence (PL) response of each sample at an excitation wavelength 300 nm provided by xenon lamp. A diffuse reflectance has been attained in the range of 200 to 800 nm on UV–Vis spectrophotometer (Lambda 950 model: Perkin Elmer) for all samples. Acquired Kubelka-Munk function [F(R) E]² vs. absorbed light energy E, plot was exploited to calculate the band gap energy (E_g) applying below mentioned equation: $$\text{F}\left(\text{R}\right){\text{E}}^2={\left(\frac{{(1-R)}^2}{2R}\times h\upsilon \right)}^2$$

2.6 Photocatalytic experiments

The efficiency of all newly prepared photocatalysts of pure TiO₂ and different weight percentages of AC doped TiO₂ (5 to 50 wt%) has been examined by photocatalytic reduction of Cr (VI) to Cr (III) i.e. (potassium dichromate K₂Cr₂O₇ solution in water) under visible light irradiation source. All photocatalytic examinations were conducted in a quartz photoreactor (150 mL) purchased from Lelesil systems (India) fitted well on magnetic stirrer. Photo-reactor has been provided with water-circulation to avoid any kind of overheating during treatment process. Proper supply of air has been maintained through the photoreactor aperture. Typically, for each reaction, 0.65 g/l of fabricated photocatalysts was transferred to 100 mL solution of potassium dichromate K₂Cr₂O₇ with a concentration 0.1 mM (29.5 ppm) along with 2.5 mM of citric acid acting as a reducing agent. The solution was then put in dark for 30 min to count the adsorption behavior of the photocatalyst. A 250 W visible light source has been utilized for irradiation purpose for each experiment. The initial pH of reaction solution was 3.50 whereas the pH at the end of reaction was found to be 3.58. At least two times centrifugation have been done for each withdrawn sample (5 mL) for appropriate removal of photocatalyst from K₂Cr₂O₇ followed by absorbance measurement. UV‐Vis absorbance spectra of Cr (VI) were measured at λ = 350 nm to evaluate the efficiency of designed photocatalysts using the below equation:

% photocatalytic activity = (1- C_t)/C_o × 100

Where C_o denotes the equilibrium concentration and C_t denotes the concentration at a particular irradiation time t. The photocatalytic reduction of Cr(VI) to Cr(III) was monitored from the concentration of peak appearing at λ = 350 nm at different time intervals (Faisal et al., 2019; Celebi et al., 2017). For every experiment, the rate constant (k) for photocatalytic reduction of Cr (VI) to Cr (III was obtained utilizing the accomplished primary slope from natural logarithm (ln) of absorbance versus exposure time plot by applying linear regression method i.e., first-order degradation kinetics.

2.7 Photo-electrochemical experiments

In order to examine the photo-electrochemical performance of newly designed photocatalyst, working electrodes were fabricated applying the below mentioned methodology. A glassy carbon electrode (GCE) with a surface area 0.071 cm² (diameter = 3.0 mm) (BAS Inc., Japan) concealed with a Teflon jacket was successively polished with 1.0 μm diamond paste and 0.05 μm alumina slurry, respectively. The sonicated shiny electrode surface was thoroughly washed with distilled water and ethanol, and finally dried in air. In order to modify the washed electrode, 10 mg of synthesized material was consistently mixed in 900 μL isopropanol and 100 μL Nafion. The as prepared suspension was then sonicated at ambient conditions for one hour, followed by vortex ca. 20 min in order to acquire uniform suspension. Out of this suspension, 4 μL was carefully employed in the form of uniform layered to the previously washed GC electrode and finally dried in an oven for 30 min at 60 °C to attain an active catalytic layer. The electrochemical analysis was carried out on three electrode-based electrochemical workstation (Zahner Zennium, Germany). Ag/AgCl electrode saturated with KCl was taken as a reference while a Pt wire as a counter electrode. For each experiments, 0.1 M Na₂SO₄ solution was utilized as an electrolyte. The EIS Nyquist plots were taken at 0.0 V applied potential and 10 mV signal amplitude, frequency ranging from 10^{− 2} to 10⁵ Hz. A 400 W (visible light) GE lamp was utilized as an irradiation source to inspect the photo-responsive performance of the modified electrodes.

3.1 Structural investigation of AC/m-TiO₂ nanocomposites

Fig. 1 presented the XRD patterns of AC, m-TiO₂ and AC/m-TiO₂ nanocomposites with variable AC wt%. The pattern obtained for AC undoubtedly demonstrates its appearance in amorphous state. Obtained diffraction pattern for undoped TiO₂ confirms the anatase form of TiO₂ with peaks signifying the (1 0 1), (0 0 4), (2 0 0), (2 1 1) and (2 1 3) lattice planes suggesting that the titania phase nucleates during the course of heating and subsequently changes into nanocrystals upon calcination at 450 °C (Faisal et al., 2014; Faisal et al., 2014). The obtained peaks are distinct and sharp with no shift in position suggesting sufficient crystallinity and organized framework of the prepared TiO₂. We could not observe any secondary peaks related to any other phase in case of pure sample, indicating that formed TiO₂ has anatase phase only. As can be seen, after incorporation of AC into TiO₂, the intensity of main peak (1 0 1) at 2θ = 43.3° for all composite samples remains similar suggesting that the crystallinity of TiO₂ anatase phase remains preserved for all doped frameworks and doping of AC has no notable influence on the lattice organization of TiO₂. Additionally, the diffraction pattern showed no peak corroborating to AC in all AC/m-TiO₂ framework proposing that AC is either fully disseminated in bulk phase of m-TiO₂ or it is in an amorphous state. A slight lowering in peak intensity in case of 50% AC/m-TiO₂ has been observed which might be due to the formation of hybrid structure.

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Fig. 1

XRD patterns of AC, m-TiO₂ and AC /m-TiO₂ nanocomposites with variable AC wt%. (5.0–50.0%).

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The functional groups of pure as well as doped samples were determined by Fourier transform infrared spectroscopy (FTIR) as shown in Fig. 2. In case of pure TiO₂, a broad peak appeared in the range of 600–750 cm⁻¹ represents a typical vibration mode of titania (Faisal et al., 2018; Han et al., 2017). FTIR spectrum of biomass derived AC exhibits a highly distinctive absorption peak at 1610 cm⁻¹ suggesting stretching vibration mode of C = C bond whereas the absorption band at 1115 cm⁻¹ signifies the stretching vibration mode of C–O linkage (Zhang et al., 2018). The FTIR spectral pattern of all AC doped m-TiO₂ nanocomposites possessed the characteristic absorption peak of AC appearing at 1610 cm⁻¹ confirming the successful creation of AC/m-TiO₂ which marked the presence of AC in every nanocomposite sample. Furthermore, a small nudge at 1115 cm⁻¹ is also present in samples of higher concentration of AC (i.e. 25 or 50 wt%) confirming the success of AC/m-TiO₂ formation. As also revealed, the AC efficiently intermingled and created networking with m-TiO₂ with slight shifting which could be ascribed due to weak van der Waals pull between the AC and m-TiO₂ channels (Shahabuddin et al., 2016).

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Fig. 2

FTIR spectra measured for AC, m-TiO₂ and AC /m-TiO₂ nanocomposites with variable AC wt%. (5.0–50.0%).

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In order to explore the morphology, TEM investigation has been performed for pure TiO₂ and 10% AC/TiO₂ nanocomposites as shown in Fig. 3. The TEM analysis of pure TiO₂ shown in Fig. 3 (a) illustrated that pure TiO₂ comprises of almost regular shape nanoparticles grown in high density having little agglomeration. The size of these organized nanoparticles ranges from 8 to 20 nm. Fig. 3 (b), image of 10% AC/m-TiO₂ sample, demonstrates the successful creation of nanocomposite between AC and m-TiO₂. Obtained image is self-explanatory revealing that m-TiO₂ nanoparticles are in firm coordination or under the blanket of biomass derived AC nanosheets. High resolution TEM analysis of pure TiO₂ is shown in Fig. 3 (c), which exhibited the d spacing or the distance between two adjacent planes is about 0.15 nm which is in agreement with the (2 1 1) plane of anatase phase of TiO₂ (Yadav and Sinha, 2017). Fig. 3 (d) exhibits the SAED of 10% AC/m-TiO₂ nanocomposite. Obtained SAED image of the composite reflects highly ordered hollow centric rings signifying the decidedly crystalline characteristic of 10% AC/m-TiO₂ nanocomposite.

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Fig. 3

TEM images of (a) bare m-TiO₂ (b) 10% AC /m-TiO₂ (c) Higher magnification TEM of pure m-TiO₂ and (d) Selected area electron diffraction of 10% AC /m-TiO₂ photocatalysts.

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All newly created nanocomposites were also examined through diffuse reflectance UV–Vis spectroscopic analysis in order to explore the various optical features of the synthesized materials. The obtained trends are shown in Fig. 4. The well-known fact and observation from UV–Vis spectroscopic analysis is that the optical properties of photocatalyst are correlated to its optical bandgap (Eg) energy. For that reason, the Kubelka-Munk (K–M) function F(R) was determined and plotted to acquire the band gap energy values of different nanocomposites by dropping the tangent lines on Kubelka-Munk (K–M) function vs photon energy curve. Fig. 4 (inset) exhibits the plot following the Kubelka–Munk vs. energy (light) absorbed for 10% AC/m-TiO₂ for the evaluation of band gap. The E_g for m-TiO₂ is 3.14 eV, perfectly matches with an earlier report (Faisal et al., 2018). Additionally, the values attained for different nanocomposites with 5, 10, 25 and 50 wt% of AC are estimated to be 3.1, 2.89, 2.88, and 2.88 eV, respectively. Almost all hybrid nanostructures showed lowering in band gap i.e. displaying band gap in the visible region of absorption spectra. The lowering of band gap or absorption in visible region could be highly beneficial during the course of reaction when photo-induced charge transfer occurs.

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Fig. 4

UV–visible diffuse reflectance spectra of for pure m-TiO₂ and AC /m-TiO₂ nanocomposites with variable AC wt%. (5.0–50.0%) and the plots of transferred Kubelka–Munk vs. energy of light absorbed for m-TiO₂ and AC /m-TiO₂ nanocomposites with variable AC wt%. (5.0–50.0%) photocatalysts (Inset).

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XPS analysis was performed in order to acquire the surface oxidation states of AC doped m-TiO₂ nanocomposites. Fig. 5 (a) presents the wide scan XPS spectra of 10 %AC/m-TiO₂ photocatalyst, confirming the presence of Ti, C and O elements at their respective binding energy values. Fig. 5 (b) displays the high resolution XPS spectrum of the Ti2p showing two prominent peaks at 458.60 and 464.20 eV which can be assigned to Ti2p doublets, i.e. Ti 2p_3/2 and Ti 2p_1/2, corresponding to Ti⁴⁺, confirming the main valence state of + 4 for Ti in AC doped m-TiO₂ (Jia et al., 2018; Xie et al., 2018). Fig. 5 (c) demonstrates the spectrum of C1s with spectral peak at 284.43 eV which represents carbon atom in C-C/C-H moieties (Sivaranjini et al., 2018). Other three deconvulated spectral peaks centered at 286.19, 288.4 and 292.10 eV signifying the presence of carbon atoms in C-O-C, C = O and O-C = O arrangements (Sivaranjini et al., 2018; Jayaraman et al., 2014). XPS spectrum of oxygen O1s is shown in Fig. 5 (d) where two deconvulated peaks appeared at binding energy values 529.80 eV and 531.44 eV. The peak marked its presence at 529.80 eV can be ascribed due to the presence of O²⁻ ion in lattice region whereas the peak appeared at 531.40 eV is accredited to O²⁻ ion present in oxygen deficient areas (Lee et al., 2012).

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Fig. 5

Wide-scan (survey spectrum) and narrow-high resolution scan XPS spectra of 10% AC /m-TiO₂ nanocomposite photocatalyst. The high resolution scan spectra were recorded for Ti2p C1s and O1s.

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The textural properties were further measured for both TiO₂ and AC doped TiO₂ nanocomposite using N₂ adsorption–desorption isotherm as displayed in Fig. 6. As could be revealed, the undoped and the AC doped TiO₂ samples exhibited a typical type IV isotherm with hysteresis profiles observed at higher P/P0 range, suggesting the formation of mesoporous materials with larger pore sizes. The calculated BET surface area values for both samples are respectively 112.33 and 92.07 m²/g for undoped and AC doped TiO₂. The decrease in BET value for the AC doped TiO₂ sample is likely due to the presence of the AC which might block the access of N₂ molecules into the original structure of the TiO₂. The total pore volume for the undoped TiO₂ was 0.357504 cc/g while the value obtained for the AC doped TiO₂ was 0.200770 cc/g. The pore size distribution curves are shown in the inset of Fig. 6. It can be noticed that the average pore size of 19 nm was detected in case of m-TiO₂, whereas the mean pore size of the AC doped m-TiO₂ sample located at the range less than 10 nm, indicating again that the original structure of m-TiO₂ is maintained after the AC doping.

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Fig. 6

N₂ sorption isotherm with pore size distribution shown in inset for bare m-TiO₂ and10% AC/m-TiO₂.

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3.2 Evaluation of photocatalytic activity

Photocatalytic reduction of K₂Cr₂O₇/reduction of Cr (VI) to Cr (III)

All designed nanocomposites have been utilized for the photocatalytic reduction of K₂Cr₂O₇ solution under visible light at room temperature. The reduction process can easily be identified by changes taken place in Cr (VI) spectra with irradiation time. The Cr (VI) i.e. K₂Cr₂O₇ solution exhibits two distinct peaks marked their presence at ∼ 256 and 350 nm (Faisal et al., 2019). The photocatalytic reduction of Cr (VI) was scrutinized by monitoring of the absorption band appearing at 350 nm at regular time intervals. First investigation revealed that when Cr (VI) kept in dark for 30 min, there was no observable reduction of Cr (VI) solution in the presence of newly designed photocatalysts. Also, direct exposure of Cr (VI) solution to visible light source in absence of photocatalysts (blank test) produced no effect or almost negligible percentage of reduction. However, all newly developed nanocomposites i.e. pure m-TiO₂ and different wt% (5.0–50%) of AC doped m-TiO₂ when treated along with 2.5 mmol/L citric acid under visible light source were found to be quite influencing with effective reduction of Cr (VI) solution to various percentages. Amongst various fabricated photocatalysts, the 10% AC/m-TiO₂ was the utmost with highest degree of photocatalytic reduction of Cr (VI) solution and this combination was found to be the most promising among all synthesized products.

Fig. 7(a) collects the results obtained when 10% AC/m-TiO₂ photocatalyst used for Cr (VI) photocatalytic reduction under visible light source. The 10% AC/m-TiO₂ was proven to be highly capable candidate among all, by destructing the prominent absorption band appearing at 350 nm to the ground level with totally flattened after 25 min exposure to visible light irradiation. This highly destructive performance of 10% AC/m-TiO₂ photocatalyst in just 25 min of exposure to visible light confirmed the potential of this encouraging photocatalytic material.

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Fig. 7

(a) Change in the absorption spectra during the photo‐reduction of Cr(VI) at different time intervals in presence of 10% AC/m-TiO₂ photocatalysts (b) Change in concentration vs. illumination time of Cr(VI) solution in presence of m-TiO₂ and AC/m-TiO₂ nanocomposites with variable AC wt%. (5.0–50.0%) (c) Comparison of degradation rate for the photo‐reduction of Cr (VI).

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Fig. 7(b) exhibits the reduction efficiency of various newly synthesized photocatalysts Achieved results clearly demonstrated that Cr (VI) solution showed no photoreduction tendency when exposed to visible light source in absence of photocatalyst but results were quite encouraging once the solution has been treated in presence of various newly synthesized photocatalysts. Mesoporous titania (m-TiO₂) was proven to be quite satisfactory, by declining the initial absorption band to 72.0%, i.e. a reduction efficiency ∼ 72.0% after 25 min of exposure to visible light irradiation. Under the same testing parameters, the AC doped samples of m-TiO₂ displayed quite elevated performances in comparison to undoped m-TiO₂ towards the accomplishment of reduction process of Cr (VI). Above increment in reduction skills of AC doped samples might be the result of perfect designing of heterojunctions among AC and m-TiO₂. The outcomes of combined nanostructures are highly motivating and explaining the importance of hetero-structures creation in the field of photocatalysis. Principally, among different newly created AC doped m-TiO₂ hybrid structures, the 10% AC/m-TiO₂ was proven to be supreme candidate with maximum conversion of Cr (VI) target analyte, exhibiting the reduction efficiency of 94.7% in just 25 min irradiation.

The newly designed photocatalysts were additionally scrutinized for the acquisition of highly important parameter i.e. rate constant (k) as shown in Fig. 7(c), which directly gives the photodegradation rate value of specific photocatalytic reaction. The rate constant (k) increases linearly from 5.4 × 10^-2 min⁻¹ , 12.10 × 10^-2 min⁻¹ to 13.15 × 10^-2 min⁻¹ as the doping percentage of AC increases from 0 to 10.0 wt% and further increment in AC content (25 to 50 wt%) results in drop in rate constant value. As the AC content increases from 25 to 50 wt%, the active sites of m-TiO₂ were not properly exposed to light source or the active site blockage occurred resulting in lesser production of photogenerated charge carriers i.e. electron and hole pairs and ultimately affected the photocatalytic performance or leading to poor photoreduction by these higher AC content based photocatalysts. The 10% AC/m-TiO₂ sample showed the highest rate constant (k) which is ∼ 2.44 times higher than that of undoped m-TiO₂.

Precise band gap engineering or tailoring with flawless synchronization between band gap and formed heterojunctions are highly recommended for the development of novel photocatalytic framework to overcome the problem of recombination process of electron and hole pairs produced during the course of reaction. Rate constant (k) values clearly revealed that the 10% AC/m-TiO₂ showed the highest reduction efficiency which might be due to the effective reduction in band gap. Reduction in band gap led to easier and frequent generation of electron and hole pairs under visible light with significant increase in visible light-harvesting, ultimately enhances the performance of designed photocatalyst. Second reason for elevated performance of the 10% AC/m-TiO₂ might be due to the successful creation of heterojunction between AC and m-TiO₂ decidedly key factor in perfect charge separation which in turn led to decline the recombination of photogenerated charge carrier’s resulting in extraordinary performance. Our attained photocatalytic reduction trends are in perfect line to our PL analysis. This suggests that the effect of impeccable synchronization between AC and m-TiO₂ offers influential outstanding performance of the 10% AC/m-TiO₂ towards the conversion of Cr (VI) solution.

It is well recognized that the photocatalytic response of any photocatalyst is markedly dependent on the adsorbing potential of that framework along with superior surficial catalytic skills, significant light harnessing in addition to proficiency in separation of charged species i.e. electrons and holes produced during the process. Mainly, deprived response of separation of photo‐generated charge carriers directs faster recombination rate of electrons and holes and is a main drawback that restricts the appropriate functioning of designed photocatalyst. In current investigation, the biomass derived AC nanosheets intermingled with the TiO₂, as confirmed by FTIR, TEM, EDS and XPS exploration, could provide a sink for electrons produced during the photocatalytic treatment, leading to separation of electrons and holes pairs which in turn boosted the performance of the newly designed material (Hiroshi et al., 2003; Dong et al., 2011). However, the photocatalytic activity using pure TiO₂ as a photocatalyst was low under identical experimental conditions. Significant enhancement in the photo-activity has been displayed by all AC combination with TiO₂ signifying the real value of hybrid structures in photocatalysis. Previous reports have explained that carbon doped TiO₂ possessed sufficient capability and found to be highly effective in comparison to undoped TiO₂ during the decontamination of aqueous organic pollutants (Hiroshi et al., 2003; Dong et al., 2011). Similarly, in present study, the biomass derived AC led to an effective increment in charge separation efficiency and also directs band gap lowering for extended utilization of visible light.

A probable photocatalytic reduction mechanism of Cr (VI) solution in presence of AC doped TiO₂ under visible light irradiation has been proposed and expressed as follows (Faisal et al., 2019):

Exposure of AC/TiO₂ nanostructures to visible light irradiation results in generation of e^- and h⁺ pairs with movement of photogenerated electron to conduction band part of designed framework generating hole in valence band. The biomass derived AC present on the surface of TiO₂ would serve as sink or reservoir for trapping the photogenerated electrons leading to an effective separation of e⁻ and h⁺. These electrons readily combined with Cr (VI) to form reduced Cr (III) as shown in Eq. (2). Simultaneously, the photogenerated h⁺ in valence band would interact with H₂O molecules to form highly reactive OH· radicals as shown in Eq. (3). Feasible back interaction of Cr(III) with photogenerated holes and hydroxyl radicals (OH^·) radicals could produce Cr(VI) as depicted in Eq. (4). The presence of organic molecule like citric acid (CA) stimulates the photoreduction response of Cr(VI) transformation to Cr(III) in two ways; (a) citric acid efficiently consumed the holes, dropping the chances of back re‐oxidation of Cr (III) to Cr(VI) as presented in Eq. (4), and (b) effectively retarded the recombination process of charge carriers by swiftly consuming the photo‐generated holes. The above process undoubtedly interpreting that the oxidation of citric acid in presence of light is a sacrificial reaction counteracting the unfavorable re‐oxidation process as shown in Eq. (4). Photocatalytic testing carried out in absence of citric acid led to low photo‐reduction efficiency for the same photocatalytic conversion process. The minimal citric acid concentration required to attain valuable results was 2.5 mmol/L, below this level of citric acid, separation of e⁻ and h⁺ pairs was not effective resulting in decline in photo‐reduction efficiency. Thus, the whole undergone procedure comprises of photo conversion of Cr(VI) to Cr(III) by exploiting well organized newly fabricated nanostructures under visible light in synchronization with the oxidation of citric acid to CO₂. The overall pooled reaction can be explained by below Eq. (5) (Yang et al., 2010):

Considering the above discussion, the proposed reduction steps are exemplified in Scheme 1. To further scrutinize the photocatalytic potential or capability of newly designed 10% AC/TiO₂ photocatalyst, it has been further exploited for the demolition of highly organized and stable framework of methylene blue (MB) dye molecule under the same visible light source. Fig. 8(a) displays the demolition trend or decrease in concentration of MB dye by changing in absorption spectra under visible light source. Two highly recognized bands with prominent humps appearing at λ = 291 nm and λ = 663 nm very expeditiously shattered in just 25 min in presence of 10% AC/TiO₂ photocatalyst under visible light. Highly encouraging performance by the 10% AC/TiO₂ further clear all ambiguity towards the capability and potential in photocatalytic treatment processes. Previous reports suggested that the flattening of MB structural organization taken place either by (1) oxidative route for the destruction dye molecule or (2) by a two-electron reduction approach transforming the color form of MB to colorless form (leuco-MB) (Zhang and Yu, 2003; Faisal et al., 2020). During the course of whole treatment it was hardly to detect any characteristic peak at λ = 256 nm signifying the leuco-MB, indisputably confirming the removal of MB proceeds via an oxidative approach. The above findings fully support the highly skills of 10% AC/TiO₂ photocatalyst and its bright future in photocatalysis.

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Scheme 1

Schematic representation of photocatalytic reduction mechanism of Cr(VI) over AC/TiO₂ photocatalyst; CA = Citric Acid.

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Fig. 8

(a) Absorbance vs. wavelength as a function of illumination time for the photocatalytic degradation of methylene blue (0.022 mM solution) with 10% AC /m-TiO₂ photocatalysts (b) Room temperature PL spectra at an excitation wavelength 300 pure m-TiO₂ and AC /m-TiO₂ nanocomposites with variable AC wt%. (5.0–50.0%).

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The PL examination is a significant approach to get better understanding of photocatalytic excitation mechanism involved. Thus, all newly designed photocatalysts were explored for PL investigation conducted at an excitation wavelength of 300 nm and their respective trends are shown in Fig. 8(b). Obtained PL spectral trends revealed that pure TiO₂ photocatalyst showed the maximum PL intensity while other composite structures with AC showed lower PL intensity spectra. Such lowering in PL spectral intensity denotes an efficient charge separation in case of the composite structures (Ismail et al., 2018; Faisal et al., 2018; Helal et al., 2017). Among all investigated samples, the PL intensity of 10% AC/TiO₂ photocatalyst was found to be the lowest suggesting the highest separation efficiency of charges which directly related to its superior performance. Acquired PL trends are perfectly in accordance with our photocatalytic results.

In any photocatalytic reaction, the role of different active species generated during the course of reaction is extremely important to be examined. Therefore, various trapping experiments have been performed under different scavengers to understand the role played by each active moiety. Typically, for each experiment 0.022 mM of MB dye solution with each scavenger having 0.01 M concentration was taken along with 0.65 g/l of 10% AC/TiO₂ photocatalyst. Hole (h⁺) scavenger i.e. potassium iodide (KI), superoxide radical anions (O₂^−•) scavenger i.e. ascorbic acid (AA), hydroxyl radical (^•OH) trapper isopropyl alcohol (IPA) were utilized during the course of photocatalytic examination (Peng et al., 2017). Obtained results shown in Fig. 9(a) suggested that addition of isopropyl alcohol (IPA) results in the highest retardation in degradation efficiency whereas KI has minimal effect proposing that hydroxyl radicals (^•OH) produced the photocatalytic reaction are the main dynamic key players liable for the high level performance of 10% AC/TiO₂ photocatalyst. On the other hand, addition of agents like KI and AA have little or nominal influence, indicating that h⁺ and O₂^−• have supportive role during the treatment of target analyte.

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Fig. 9

(a) Reactive species trapping experiments of the photodegradation of MB under visible light irradiation using 10% AC /m-TiO₂ nanocomposite photocatalyst (b) Repeated cycles up to 5 times of the photocatalytic degradation of MB with 10% AC /m-TiO₂ nanocomposite photocatalyst (c) Repeated cycles up to 5 times of the photo-reduction of Cr (VI) with 10% AC /m-TiO₂ nanocomposite photocatalyst.

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Future prospects and applicability of any newly designed photocatalytic framework fully depend on chief concerning factors which are stability and reproducibility. Therefore, the 10% AC/TiO₂ photocatalyst has been continually exploited for five consecutive photocatalytic reactions for the removal of MB and for the photoreduction of Cr (VI) as shown in Fig. 9 (b) & (c). Obtained results were highly encouraging and motivating where excellent stability with negligible drop in photocatalytic efficiency of 10% AC/TiO₂ photocatalyst. This highly acceptable performance of the newly designed photocatalyst opens a new gateway towards its utilization in various environmental concerning issues. Slight drop in photocatalytic efficiency might be due to the loss of photocatalytic material during washing and separation process of photocatalyst for further utilization.

The electrochemically attained impedance spectra (EIS) of bare TiO₂ and 10% AC/TiO₂ nanocomposite layered electrodes were investigated under visible-light source as shown in Fig. 10. The degree or length of the arcs obtained through Nyquist plot is the representative of effectiveness of charge separation or swift movement of electron through the electrode surface. The arc size pattern in obtained plot clearly reveals that bare TiO₂ (dark) arc > bare TiO₂(light) arc and 10% AC/TiO₂ (dark) arc > 10% AC/TiO₂ (light) arc where 10% AC/TiO₂ nanocomposite under visible-light exposure exhibited the shortest arc, clearly revealing the importance of structural engineering or modification towards the easier flow of electron and notably decrease in charge transfer resistance. Trends obtained in Nyquist plots unambiguously proven the drop in resistance generated during charge transfer in case of 10% AC/TiO₂ nanocomposite, leading to outstanding results by this newly created photocatalyst. Obtained trends of EIS study are perfect in line to our PL and photocatalytic results.

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Fig. 10

EIS Nyquist plots of pure TiO₂ and 10% AC /m-TiO₂ coated glassy carbon electrodes in 0.1 M Na₂SO₄ aqueous solution at 0.0 V vs. Ag/AgCl (KCl sat.) and 10 mV signal amplitude in the frequency range from 10^{− 2} to 10⁵ Hz with and without light irradiation.

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The EDS results of 10% AC/TiO₂ photocatalyst after the photoreduction anlaysis are shown in Fig. 11 (a-f). The treated or used photocatalysts i.e. 10% AC/TiO₂ is shown in Fig. 11 (a) whereas Fig. 11 (b) exhibited the very well-developed heterojunction among C, O and Ti. The appearance of chromium in 10% AC/TiO₂ sample after the photoreduction reaction clearly confirmed the adsorbed Cr(III) on AC based TiO₂ photocatalyst. In order to explore the distribution of various components involved in nanocomposites formation, the EDS elemental mapping has also been carried out on reused 10% AC/TiO₂ nanocomposite. The EDS elemental mapping analysis as shown in Fig. 11 (c–f) revealed excellent distribution of C, O and Ti elements, which confirmed the proper homogeneity of the newly designed 10% AC/TiO₂ nanocomposite photocatalyst along with the adsorbed Cr (III) on AC based TiO₂ photocatalyst.

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Fig. 11

After photoreduction process: FESEM images of (a) 10% AC /m-TiO₂ (b) Energy-Dispersive X-ray analysis of 10% AC/m-TiO₂ and EDS elemental mapping (c) C (black) (d) O (blue) (e) Ti (orange) (f) Cr (green) of 10% AC /m-TiO₂ nanocomposite photocatalyst.

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To further confirm the structural recyclability of the current developed photocatalyst, the TEM and HR-TEM images were taken for the 10% AC/TiO₂ nanocomposite photocatalyst after several repeated cycles of the photoreduction of Cr(VI). Fig. 12 (a) and (b) shows the corresponding TEM and HR-TEM images, where one can notice no morphological or structural changes of the reused photocatalyst compared to the original, untreated photocatalyst (please compare TEM images of Fig. 3), confirming the excellent structural recyclability of the 10% AC/TiO₂ nanocomposite photocatalyst.

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Fig. 12

After photoreduction process: TEM (a) and HR-TEM (b) images of 10% AC/m-TiO₂ nanocomposite photocatalyst.

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