Environment-Friendly Nanoporous Titanium Dioxide with Enhanced Photocatalytic Activity

: Nanoporous TiO 2 anatase was environment-friendly prepared using co ﬀ ee husk extract (CHE) as bio-template instead of hazardous chemicals and solvents and ultrasonic waves. Ca ﬀ eine and ca ﬀ eic acid were found to be the main compounds in CHE to modify the morphology of TiO 2 . The properties of as-prepared titanium dioxide particles were determined by di ﬀ erent characterization techniques. The results demonstrate the formation of a meso / macro-porous channel consisting of small TiO 2 particles (8–10 nm). The as prepared green nanoparticles exhibited improved photocatalytic activity for the degradation of organic water pollutants with good recyclability. The enhancement in e ﬃ ciency of green nanoporous TiO 2 can be attributed to higher surface area and the presence of more active adsorption sites inside the pores. The current research provides for a low cost, safe, and eco-friendly way to produce e ﬃ cient photocatalysts for remediation of polluted water.


Introduction
Nowadays, water pollution has turned into a worldwide issue and the entire environment is aggravating as well as more diseases are caused worldwide due to water pollution [1,2]. Accordingly, water is treated in a number of traditional ways such as chemical precipitation, ion exchange, and electrochemical removal. But these procedures have huge drawbacks, as they lead to incomplete removal, require high-energy, and produce toxic materials [3][4][5][6]. Among various wastewater treatment methods, the photocatalytic process in aqueous suspension of semiconductor nanomaterials has received considerable attention in view of rapid and efficient destruction of pollutants [7][8][9][10][11]. Among various semiconductor nanomaterials, titanium dioxide (TiO 2 ) has been a widely investigated semiconductor as the most suitable material for environmental and energy applications due to its unique properties [12]. There are many ways for the preparation of TiO 2 such as the solvothermal method, hydrothermal method, sol-gel method, chemical vapor deposition, and electrodeposition. Nevertheless, these methods require high energy and hazardous materials and solvents [13][14][15][16]. Green synthesis of nanomaterials is an alternative route to physical and chemical methods that leads to environment-friendly products due to the use of non-toxic materials, and does not require high temperatures and pressure, and at a lower cost. Green synthesis may be by microorganisms, plant extracts, and plant biomass [17,18]. Therefore, using such a green and environmentally friendly method can also minimize the use of chemicals and toxic solvents. In order  Figure 2 shows the XRD patterns of chemically as well as green prepared TiO2 nanoparticles. XRD of both TiO2 samples shows the diffractions peaks at 25.28°, 37.93°, 48.37°, 53.89°, 55.29°, 62.73°, 69.00°, and 75.37° for anatase phase (JCPDS no.   [39]. Moreover, the intensities diffraction peak decreases and becomes broader for green TiO2 nanoparticles indicating smaller particle size [40]. The crystallite size of the as prepared TiO2 nanomaterials has been determined from the XRD data using the Scherrer equation to be 16 and 7.5 nm respectively, for chemically as well as green prepared TiO2 nanoparticles, respectively.

Characterization of TiO2 Nanomaterials
The morphological features of chemically as well as green prepared TiO2 nanoparticles have been determined by SEM and TEM measurements. SEM images TiO2 sample prepared in presence and in absence of CHE are Figure 3a-c. It was found that, while agglomeration of TiO2 particles was observed for the TiO2 sample prepared without CHE (Figure 3a), the macro porous structure with pore size of range 1.3-1.6 μm was observed for the TiO2 sample prepared with CHE (Figure 3b,c). The diameter of the TiO2 particles was noticed to be smaller for green TiO2 nanoparticles (8-10 nm) ( Figure 4b) as compared to chemically prepared TiO2 (Figure 4a). The TEM results were found to be in good agreement with the XRD measurements.    [39]. Moreover, the intensities diffraction peak decreases and becomes broader for green TiO 2 nanoparticles indicating smaller particle size [40]. The crystallite size of the as prepared TiO 2 nanomaterials has been determined from the XRD data using the Scherrer equation to be 16 and 7.5 nm respectively, for chemically as well as green prepared TiO 2 nanoparticles, respectively.
The morphological features of chemically as well as green prepared TiO 2 nanoparticles have been determined by SEM and TEM measurements. SEM images TiO 2 sample prepared in presence and in absence of CHE are Figure 3a-c. It was found that, while agglomeration of TiO 2 particles was observed for the TiO 2 sample prepared without CHE (Figure 3a), the macro porous structure with pore size of range 1.3-1.6 µm was observed for the TiO 2 sample prepared with CHE (Figure 3b,c). The diameter of the TiO 2 particles was noticed to be smaller for green TiO 2 nanoparticles (8-10 nm) ( Figure 4b) as compared to chemically prepared TiO 2 (Figure 4a). The TEM results were found to be in good agreement with the XRD measurements.   [39]. Moreover, the intensities diffraction peak decreases and becomes broader for green TiO2 nanoparticles indicating smaller particle size [40]. The crystallite size of the as prepared TiO2 nanomaterials has been determined from the XRD data using the Scherrer equation to be 16 and 7.5 nm respectively, for chemically as well as green prepared TiO2 nanoparticles, respectively.
The morphological features of chemically as well as green prepared TiO2 nanoparticles have been determined by SEM and TEM measurements. SEM images TiO2 sample prepared in presence and in absence of CHE are Figure 3a-c. It was found that, while agglomeration of TiO2 particles was observed for the TiO2 sample prepared without CHE (Figure 3a), the macro porous structure with pore size of range 1.3-1.6 μm was observed for the TiO2 sample prepared with CHE (Figure 3b,c). The diameter of the TiO2 particles was noticed to be smaller for green TiO2 nanoparticles (8-10 nm) ( Figure 4b) as compared to chemically prepared TiO2 (Figure 4a). The TEM results were found to be in good agreement with the XRD measurements.      Figure 5a,b shows the Raman and FTIR spectra of the as synthesized nanomaterials. The Raman spectra of the TiO2 samples prepared with and without CHE show the characteristic peaks of pure anatase phase. The band at 390 cm −1 is assigned to B1g. The bands at 510 and 620 cm −1 are assigned to A1g and the band at 140 cm −1 is assigned to Eg, which are all related to the anatase phase [41]. As shown in the inset of Figure 7a, no apparent peaks of carbon have been observed, confirming that, carbon did not dope into the TiO2 lattice in both TiO2 samples. The FTIR spectrum of TiO2 nanomaterials prepared with and without CHE present similar FTIR spectra with peak around 450 cm −1 assigned to stretching vibration of Ti-O-Ti bonds. The broad band at 3428 cm −1 can be ascribed to the -OH stretching vibration of the adsorbed water molecules on TiO2 surface and the peak around 1630 cm −1 is attributed to the bending vibrations of -OH [42]. The FTIR results indicated no adsorption of the plant extract compounds on TiO2 surface after calcination at 773 K.     Figure 5a,b shows the Raman and FTIR spectra of the as synthesized nanomaterials. The Raman spectra of the TiO2 samples prepared with and without CHE show the characteristic peaks of pure anatase phase. The band at 390 cm −1 is assigned to B1g. The bands at 510 and 620 cm −1 are assigned to A1g and the band at 140 cm −1 is assigned to Eg, which are all related to the anatase phase [41]. As shown in the inset of Figure 7a, no apparent peaks of carbon have been observed, confirming that, carbon did not dope into the TiO2 lattice in both TiO2 samples. The FTIR spectrum of TiO2 nanomaterials prepared with and without CHE present similar FTIR spectra with peak around 450 cm −1 assigned to stretching vibration of Ti-O-Ti bonds. The broad band at 3428 cm −1 can be ascribed to the -OH stretching vibration of the adsorbed water molecules on TiO2 surface and the peak around 1630 cm −1 is attributed to the bending vibrations of -OH [42]. The FTIR results indicated no adsorption of the plant extract compounds on TiO2 surface after calcination at 773 K.   Figure 5a,b shows the Raman and FTIR spectra of the as synthesized nanomaterials. The Raman spectra of the TiO 2 samples prepared with and without CHE show the characteristic peaks of pure anatase phase. The band at 390 cm −1 is assigned to B1g. The bands at 510 and 620 cm −1 are assigned to A1g and the band at 140 cm −1 is assigned to Eg, which are all related to the anatase phase [41]. As shown in the inset of Figure 7a, no apparent peaks of carbon have been observed, confirming that, carbon did not dope into the TiO 2 lattice in both TiO 2 samples. The FTIR spectrum of TiO 2 nanomaterials prepared with and without CHE present similar FTIR spectra with peak around 450 cm −1 assigned to stretching vibration of Ti-O-Ti bonds. The broad band at 3428 cm −1 can be ascribed to the -OH stretching vibration of the adsorbed water molecules on TiO 2 surface and the peak around 1630 cm −1 is attributed to the bending vibrations of -OH [42]. The FTIR results indicated no adsorption of the plant extract compounds on TiO 2 surface after calcination at 773 K. The UV-vis diffuse reflectance measurements ( Figure 6a) show that both TiO2 samples can only absorb UV light and reflect most light in the visible region. The band gap energy has been estimated using the Kubelka-Munk relation [43]. The band gap energy of nanomaterials prepared with and without CHS was estimated to be 3.21 eV and 3.16 eV, respectively. A slight increase in the band gap energy of the TiO2 sample prepared with CHE was observed which can be attributed to the smaller particle size [44]. Figure 6b shows the N2 adsorption-desorption measurements of the samples. The adsorptiondesorption isotherms of both samples can be assigned to typical H3 hysteresis loop. The BET surface The UV-vis diffuse reflectance measurements (Figure 6a) show that both TiO 2 samples can only absorb UV light and reflect most light in the visible region. The band gap energy has been estimated using the Kubelka-Munk relation [43]. The band gap energy of nanomaterials prepared with and without CHS was estimated to be 3.21 eV and 3.16 eV, respectively. A slight increase in the band gap energy of the TiO 2 sample prepared with CHE was observed which can be attributed to the Catalysts 2019, 9, 799 6 of 13 smaller particle size [44]. Figure 6b shows the N 2 adsorption-desorption measurements of the samples. The adsorption-desorption isotherms of both samples can be assigned to typical H3 hysteresis loop. The BET surface area of the samples revealed that TiO 2 prepared with CHE exhibited larger surface area of about 147.15 m 2 /g compared to TiO 2 nanoparticles prepared without CHE (53.96 m 2 /g) (Table 1), which can be readily attributed to the porous structure as well as the small particle size of the green TiO 2 particles prepared with CHE.  (Table 1), which can be readily attributed to the porous structure as well as the small particle size of the green TiO2 particles prepared with CHE.

Photocatalytic Performance
Photocatalytic performance of the as prepared nanomaterials has been evaluated for the degradation MB dye. The change in MB concentration was determined from UV-vis absorbance measurements. The absorbance of MB with λ max 668 nm was found to decrease gradually with irradiation (Figure 7a,b).

Photocatalytic Performance
Photocatalytic performance of the as prepared nanomaterials has been evaluated for the degradation MB dye. The change in MB concentration was determined from UV-vis absorbance measurements. The absorbance of MB with λmax 668 nm was found to decrease gradually with irradiation (Figure 7a, b). The photocatalytic degradation efficiency for MB degradation is shown in Figure 7c. Slow degradation was shown when TiO2 (ethanol) nanomaterial is used as a catalyst, while more efficient degradation was obtained in presence of TiO2 (CHE).
Since phenol frequently exists in industry effluents and is difficult to be degraded due to the stability of the benzene ring, the photocatalytic degradation of phenol was of interest. The degradation of phenol was monitored using the UV-vis absorbance measurement in the wavelength The photocatalytic degradation efficiency for MB degradation is shown in Figure 7c. Slow degradation was shown when TiO 2 (ethanol) nanomaterial is used as a catalyst, while more efficient degradation was obtained in presence of TiO 2 (CHE).
Since phenol frequently exists in industry effluents and is difficult to be degraded due to the stability of the benzene ring, the photocatalytic degradation of phenol was of interest. The degradation of phenol was monitored using the UV-vis absorbance measurement in the wavelength range (200-400 nm) (Figure 8a,b) with two characteristic bands at 210 nm and 269 nm. Upon irradiation in the presence of TiO 2 nanoparticles, a gradual decrease in intensity of phenol absorbance was observed, confirming the decrease in phenol concentration. Figure 8c shows the photocatalytic degradation efficiency for phenol degradation using various TiO 2 nanomaterials. Noticeable photocatalytic improvement has been achieved when TiO 2 (CHE) was used as the catalyst.  (Figure 8a,b) with two characteristic bands at 210 nm and 269 nm. Upon irradiation in the presence of TiO2 nanoparticles, a gradual decrease in intensity of phenol absorbance was observed, confirming the decrease in phenol concentration. Figure 8c shows the photocatalytic degradation efficiency for phenol degradation using various TiO2 nanomaterials. Noticeable photocatalytic improvement has been achieved when TiO2 (CHE) was used as the catalyst. The kinetics of MB and phenol degradation have been also investigated by plotting -ln (C/C0) with time. Linear correlation for MB and phenol degradation was obtained for all samples which fit well with the pseudo first-order model (Figure 7d and Figure 8d, respectively) and the apparent rate constant k' (min −1 ) has been estimated and the data have been gathered in Table 2. It was found that, the observed rate constant of photocatalytic degradation of MB in the presence of TiO2 (CHE) is around 2 times higher than that in the presence of TiO2 (ethanol). In addition, the photocatalytic degradation of phenol in the presence of TiO2 (CHE) is three times higher than that when used TiO2 (ethanol) ( Table 2). The kinetics of MB and phenol degradation have been also investigated by plotting -ln (C/C 0 ) with time. Linear correlation for MB and phenol degradation was obtained for all samples which fit well with the pseudo first-order model (Figures 7d and 8d, respectively) and the apparent rate constant k' (min −1 ) has been estimated and the data have been gathered in Table 2. It was found that, the observed rate constant of photocatalytic degradation of MB in the presence of TiO 2 (CHE) is around 2 times higher than that in the presence of TiO 2 (ethanol). In addition, the photocatalytic degradation of phenol in the presence of TiO 2 (CHE) is three times higher than that when used TiO 2 (ethanol) ( Table 2). The improvement of the photocatalytic performance of TiO 2 (CHE) can be attributed to increasing the surface area as well as the active adsorption sites inside the pores, leading to increased accessibility for the organic pollutant and therefore enhancing photocatalytic performance.
The recyclability of the green TiO 2 has been studied for 3 cycles of photocatalytic degradation of MB (20 mg L −1 ). Ninety-eight percent of MB was degraded even after the 3rd cycle, indicating the stability and recyclability of green nanoporous TiO 2 photocatalyst (Figure 9). Nanomaterial Reactant k ' (min −1 ) R 2 The improvement of the photocatalytic performance of TiO2 (CHE) can be attributed to increasing the surface area as well as the active adsorption sites inside the pores, leading to increased accessibility for the organic pollutant and therefore enhancing photocatalytic performance.
The recyclability of the green TiO2 has been studied for 3 cycles of photocatalytic degradation of MB (20 mg L −1 ). Ninety-eight percent of MB was degraded even after the 3rd cycle, indicating the stability and recyclability of green nanoporous TiO2 photocatalyst (Figure 9).

Preparation of Coffee Husk Extract
Coffee husk of Arabian coffee obtained from the western province, Saudi Arabia were washed several times with distilled water to remove the dust particles on their surface then dried in air. The dried husk was grinded to obtain fine powder. Twenty (20) g of coffee husk was boiled with 400 mL of distilled water for 30 min, then allowed to cool down slightly and finally filtered to obtain a clear brown solution of coffee husk extract (CHE) (Figure 10). It can be kept in a dark bottle in the fridge for further use.

Preparation of Coffee Husk Extract
Coffee husk of Arabian coffee obtained from the western province, Saudi Arabia were washed several times with distilled water to remove the dust particles on their surface then dried in air. The dried husk was grinded to obtain fine powder. Twenty (20) g of coffee husk was boiled with 400 mL of distilled water for 30 min, then allowed to cool down slightly and finally filtered to obtain a clear brown solution of coffee husk extract (CHE) (Figure 10). It can be kept in a dark bottle in the fridge for further use. The improvement of the photocatalytic performance of TiO2 (CHE) can be attributed to increasing the surface area as well as the active adsorption sites inside the pores, leading to increased accessibility for the organic pollutant and therefore enhancing photocatalytic performance.
The recyclability of the green TiO2 has been studied for 3 cycles of photocatalytic degradation of MB (20 mg L −1 ). Ninety-eight percent of MB was degraded even after the 3rd cycle, indicating the stability and recyclability of green nanoporous TiO2 photocatalyst (Figure 9).

Preparation of Coffee Husk Extract
Coffee husk of Arabian coffee obtained from the western province, Saudi Arabia were washed several times with distilled water to remove the dust particles on their surface then dried in air. The dried husk was grinded to obtain fine powder. Twenty (20) g of coffee husk was boiled with 400 mL of distilled water for 30 min, then allowed to cool down slightly and finally filtered to obtain a clear brown solution of coffee husk extract (CHE) (Figure 10). It can be kept in a dark bottle in the fridge for further use.

Synthesis of TiO 2 Nanoparticles Using CHE
Two hundred (200) mL of the CHE solution was placed in a conical flask. Five (5) mL of titanium tetraisopropoxide (TTIP) was drop-wise added to the CHE solution under ultrasonic waves. The obtained suspension was stirred overnight. After that, solid TiO 2 nanoparticles were separated and washed 3 times by centrifuge. Then, dried in the oven at 70 • C and finally calcined at 500 • C for 1 h (Figure 11). For comparison, TiO 2 nanoparticles were prepared in absence of CHE, in typical synthesis, 5 mL of (TTIP) in 15 mL ethanol was drop-wise added to ethanol/water solution (1:1) under continuous stirring for overnight. The TiO 2 nanoparticles were separated and washed 3 times by centrifuge and denoted as (TiO 2 ethanol).
Two hundred (200) mL of the CHE solution was placed in a conical flask. Five (5) mL of titanium tetraisopropoxide (TTIP) was drop-wise added to the CHE solution under ultrasonic waves. The obtained suspension was stirred overnight. After that, solid TiO2 nanoparticles were separated and washed 3 times by centrifuge. Then, dried in the oven at 70 °C and finally calcined at 500 °C for 1 h (Figure 11). For comparison, TiO2 nanoparticles were prepared in absence of CHE, in typical synthesis, 5 mL of (TTIP) in 15 mL ethanol was drop-wise added to ethanol/water solution (1:1) under continuous stirring for overnight. The TiO2 nanoparticles were separated and washed 3 times by centrifuge and denoted as (TiO2 ethanol). Figure 11. Schematic representation of preparation of porous TiO2 nanoparticles.

Characterization
XRD measurements were performed using Shimadzu's-XRD Powder diffractometer (xrd-700, Maxima, Shimadzu, Kyoto, Japan) with a Cu radiation source, at a scanning speed of 2 min −1 , 40 kV tube voltage, and 30 mA tube current. FTIR measurements were recorded on IR AFFINITY-1 Shimadzu (Shimadzu, Kyoto, Japan) by Fourier transform and KBr (Potassium Bromide) pellets were used to obtain the absorption spectra in infrared region for the CHE as well as nanomaterials. Raman spectroscopy was conducted on iHR320 with CCD detector, HORIBA at 632 nm excitation wavelength, DXR Raman Microscope, DXR 532 nm Laser. SEM images were taken using Hitachi S-4700 (Hitachi, Žatec, Czech Republic) using the following parameters: acceleration voltage = 20 kV, working distance about 10 mm and spot size 3. The powder specimens were mounted onto a metallic stub with a double-sided adhesive tape. Images were captured at different magnifications. For transmission electron microscopy (TEM), the powder was dispersed in ethanol, sonicated for 5 min and deposited onto TEM grid having carbon support film. The grids were dried before mounting into the TEM. A TEM, FEI, Morgagni, Czec Republic at 80 kV was used to record the images. The UV-vis diffuse reflectance measurements were performed on Shimadzu UV4100 spectrometer (Shimadzu, Kyoto, Japan) and UV-vis absorption measurements were recorded on UV-vis Spectrophotometer-UVD-3200 (LABOMED, Los Angeles, CA, USA).

Photocatalytic Activity
The photocatalytic performance of the green as well as chemically synthesized TiO2 nanoparticles has been evaluated for photocatalytic degradation of methylene blue (MB) as model Figure 11. Schematic representation of preparation of porous TiO 2 nanoparticles.

Characterization
XRD measurements were performed using Shimadzu's-XRD Powder diffractometer (xrd-700, Maxima, Shimadzu, Kyoto, Japan) with a Cu radiation source, at a scanning speed of 2 min −1 , 40 kV tube voltage, and 30 mA tube current. FTIR measurements were recorded on IR AFFINITY-1 Shimadzu (Shimadzu, Kyoto, Japan) by Fourier transform and KBr (Potassium Bromide) pellets were used to obtain the absorption spectra in infrared region for the CHE as well as nanomaterials. Raman spectroscopy was conducted on iHR320 with CCD detector, HORIBA at 632 nm excitation wavelength, DXR Raman Microscope, DXR 532 nm Laser. SEM images were taken using Hitachi S-4700 (Hitachi, Žatec, Czech Republic) using the following parameters: acceleration voltage = 20 kV, working distance about 10 mm and spot size 3. The powder specimens were mounted onto a metallic stub with a double-sided adhesive tape. Images were captured at different magnifications. For transmission electron microscopy (TEM), the powder was dispersed in ethanol, sonicated for 5 min and deposited onto TEM grid having carbon support film. The grids were dried before mounting into the TEM. A TEM, FEI, Morgagni, Czec Republic at 80 kV was used to record the images. The UV-vis diffuse reflectance measurements were performed on Shimadzu UV4100 spectrometer (Shimadzu, Kyoto, Japan) and UV-vis absorption measurements were recorded on UV-vis Spectrophotometer-UVD-3200 (LABOMED, Los Angeles, CA, USA).

Photocatalytic Activity
The photocatalytic performance of the green as well as chemically synthesized TiO 2 nanoparticles has been evaluated for photocatalytic degradation of methylene blue (MB) as model pollutant under solar light irradiation. The photocatalytic activity was further evaluated for the photocatalytic mineralization of phenol under solar light irradiation. The photocatalytic experiments were carried out using solar simulator (SLB-300A, 300 W). The amount of the degraded organic pollutant (MB or phenol) was evaluated from UV-vis spectroscopic measurement before and during illumination.

Conclusions
Nanoporous TiO 2 has been synthesized using plant waste extract (CHE). The properties of produced green nanomaterials were well determined using diverse characterization techniques. SEM and TEM analysis revealed that the produced green nanomaterials have macroporous/mesoporous structures composed of small-sized TiO 2 particles and narrow particle distribution. The as synthesized green porous TiO 2 nanomaterials showed enhanced photocatalytic performance under UV and solar light irradiation for degradation of organic dyes and phenol. The enhanced photocatalytic activity of the green TiO 2 can be attributed to higher surface area and the presence of more active adsorption sites inside the pores leading to increased accessibility for the organic pollutant and therefore enhancing photocatalytic activity. The results demonstrate the efficiency of CHE as bio-template to produce porous TiO 2 nanoparticles with enhanced photocatalytic activity.