The role of strontium on the enhancement of photocatalytic response of TiO₂ nanotubes – application in methylene blue and formic acid photodegradation under visible light and UV-A

Figures & data

Figure 1. XRD patterns of: (A) xwt% Sr-NT, x = {0, 0.2, 0.4, 0.6, 0.8 and 1.0} and (B) zoom of (101) peak.

Table 1. Textural and structural properties of x wt% Sr -NT catalysts, x = {0, 0.2, 0.4, 0.6, 0.8 and 1}.

Catalysts	SBET (m^2.g^-1)	Pore diameter (nm)	Eg (eV)	Crystallite sizes (nm)	EDAX Sr/Ti atomic ratio
NT	128	22.2	3.20	12.0	–
0.2 wt%Sr-NT	107	22.8	3.42	11.0	0.0067
0.4 wt%Sr-NT	95	22.1	3.71	11.2	–
0.6 wt%Sr-NT	117	21.8	3.46	11.2	–
0.8 wt%Sr-NT	106	21.7	3.41	11.1	0.020
1wt%Sr-NT	108	21.5	3.41	11.1	–

Figure 2. Raman spectra of xwt% Sr-NT with a. 0 wt% Sr, b.0.2 wt% Sr, c. 0.4 wt% Sr, d. 0.6 wt% Sr, e. 0.8 wt% Sr and f. 1.0 wt% Sr.

Figure 3. UV-visible spectra (reflectance) of xwt% Sr-NT: a. 0 wt% Sr, b.0.2 wt% Sr, c. 0.4 wt% Sr, d. 0.6 wt% Sr, e. 0.8 wt% Sr and f. 1.0 wt% Sr.

Figure 4. TEM (a) and HR-TEM (b) images of NT, and c. TEM of 0.8 wt% Sr-NT.

Figure 5. SEM image of (a) 0.8 wt% Sr-NT, and SEM mapping images of (b) O, (c) Sr and (d) Ti.

Figure 6. PL signal of a. Sr-free; b. 0.2 wt% Sr and c. 0.8 wt%Sr NT nanomaterials calcined at 400 °C.

Figure 7. XPS spectra of Sr-free and 0.2 wt% Sr and 0.8 wt%Sr-doped NT nanomaterials calcined at 400 °C: (a) full scan survey of all elements and high-revolution spectra of (b) O 1 s (c) Ti 2p_3/2, (d) Sr 3d_5/2.

Table 2. XPS results of different xwt% Sr-NT nanomaterials: surface state, binding energy, predominance, molar ratio O^2-/Ti⁴⁺.

Catalysts	Elements	Surface State	Binding Energy (eV)	Predominance %	Molar ratio O^2-/ Ti^4+
0 wt% Sr-NT	O	O^2-	529.14	100	2.15
	Ti	Ti^4+	458.18	95
		Ti^3	456.80	5
0.2 wt%SrNT	O	O^2-	529.14	56.8	2.22
		O-H	530.08	29.2
		Ov	530.7	14
	Ti	Ti^4+	458.52	90
		Ti^3+	456.68	10
	Sr	Sr^2+	134.17	100
0.8 wt%Sr-NT	O	O^2-	529.83	75	3.46
		Ov	530.72	25
	Ti	Ti^4+	458.56	86
		Ti^3+	456.78	14
	Sr	Sr^2+	134.35	100

Table 3. Rate constant values (k₁ and k₂) obtained using pseudo-first order and pseudo-second order models and MB photodegradation.

		Models
Catalysts	Wt% Sr	Pseudo-first order		Peudo-second order		MB degradation
		ki1 (min^-1)	R^2	ki2 (mmol^-1. L. min^-1)	R^2	$\frac{\left(C0-Ct\right) }{C0}*100$
Photolysis	0	0.0002	0.991	–	–	2%
0.2 Sr-NT	0.2	0.0016	0.998	1.05 10^-4	0.985	19%
0.4 Sr-NT	0.4	0.0019	0.992	1.41 10^-4	0.984	24%
0.6 Sr-NT	0.6	0.0028	0.991	2.20 10^-4	0.989	35%
0.8 Sr-NT	0.8	0.0056	0.999	5.57 10^-4	0.988	56%
1.0 Sr-NT	1.0	0.0053	0.999	4.98 10^-4	0.986	51%

Results are reported for xwt%Sr-NT nanocomposites with different Sr amounts. Photolysis experiment is used as reference.

Figure 8. Kinetic study of MB photodegradation: rate vs equilibrium concentration for a. 0 wt% Sr, b.0.2 wt% Sr, c. 0.4 wt% Sr, d. 0.6 wt% Sr, e. 0.8 wt% Sr and f. 1.0 wt% Sr.

Table 4. Rate constant (k_LH), Adsorption constant (K_LH) values and surface coverage θ determined using a Langmuir–Hinshelwood model for the Methylene Blue photodegradation.

		Langmuir Hinshelwood model
Catalysts	wt% Sr	kLH (mmol.L^-1min^-1)	KLH (L. mmol^-1)	R^2	θ
0.2 Sr-NT	0.2	0.0013	8.56	0.997	0.992
0.4 Sr-NT	0.4	0.0018	17.01	0.987	0.998
0.6 Sr-NT	0.6	0.0027	20.93	0.997	0.999
0.8 Sr-NT	0.8	0.0054	25.13	0.999	0.999
1.0 Sr-NT	1.0	0.0050	20.83	0.988	0.999

Results are reported for xwt%Sr-NT nanocomposites with different Sr amounts.

Figure 9. (a) Variation of C/C₀ in the photodegradation of formic acid (FA, 100 mg.L⁻¹) under UV-A light, (b) predominance of Ti species (Ti⁴⁺, Ti³⁺) and oxygen vacancies (O_v).

Figure 10. Experimental and theoretical impedance diagrams of the 0.2 wt% Sr-NT and 0.8 wt% Sr-NT (Inset the corresponding equivalent circuit).

Table 5. Impedance results obtained by fitting the experimental data to the equivalent electrical circuit shown in Figure 10.

Catalysts	R (k Ω)	A0 (10^-10 F cm^2 s^-1)	n	τ (μs)
0.2 wt%Sr-NT	1213	2.22	0.750	269
0.8 wt%Sr-NT	1117	48.80	0.870	5450