I. Introduction {#sec1}
===============

Titanium dioxide (TiO~2~) possesses outstanding photocatalytic activity,^[@ref1]^ which has been extensively used in wastewater purification,^[@ref2]−[@ref5]^ water splitting,^[@ref6]−[@ref8]^ volatile organic compound degradation,^[@ref9]^ CO~2~ reduction,^[@ref10],[@ref11]^ etc. Nevertheless, TiO~2~ can only be activated by ultraviolet light (UV, 5--9% in solar light) due to its large band gap (3.0--3.2 eV)^[@ref12]^ and fast recombination rate while most of the solar light is wasted.^[@ref13]^ To efficiently utilize the light energy, expanding the photoexcitation spectra of TiO~2~ to solar light is more desirable. A series of techniques have been developed to expand TiO~2~ to be active under solar light (or visible light) through grafting (polymers or nanoparticles),^[@ref14],[@ref15]^ doping,^[@ref16]−[@ref18]^ coating,^[@ref19]^ etc. Among of these methods, doping is extensively used to extend TiO~2~ to the visible light spectrum by narrowing the band gap, separating photoelectrons, and reducing the recombination rate of valance hole.^[@ref17],[@ref18]^

A variety of materials including transition metals and nonmetal elements (e.g., C,^[@ref20]−[@ref23]^ N,^[@ref24]^ S,^[@ref25]^ B,^[@ref26]^ I,^[@ref27]^ etc.) have been doped into TiO~2~ to achieve solar light activation.^[@ref28],[@ref29]^ However, doping with metal elements may cause poor thermal stability^[@ref30]^ and an increment of recombination centers due to aggregation and larger particle size.^[@ref30],[@ref31]^ The critically thin doping (10--20 nm) layer at the interface fails to act as both electron and hole traps under thermal treatment.^[@ref31]^ In comparison, doping with nonmetal elements might generate the impurity band gap^[@ref32]^ and create an overlap between the intrinsic band gap,^[@ref33]^ thereby expanding TiO~2~ to visible light activation. Asahi et al.^[@ref34]^ first reported that visible light excitation of TiO~2~could be achieved by N-doping. N enters the TiO~2~ lattice and replaces O, resulting in the formation of crystal defects, and the N 2p doping level overlaps with O 2p, which narrows the band gap, thereby switching TiO~2~ into visible light activation.^[@ref35],[@ref36]^ Since then, a variety of nonmetal-doped TiO~2~ photocatalysts have been developed. Sakthivel et al.^[@ref37]^ compared the visible light degradation efficiency of 4-chlorophenol, showing that C-doped TiO~2~ was 5 times more active than N-doped TiO~2~. Carbon is widely used as a doping agent due to its large electron storage capacity, capability of absorbing visible light in a wide range (400--800 nm), high efficiency in separating photogenerated carriers, stabilization on the anatase phase, and conductivity improvement.^[@ref38]−[@ref41]^ Besides, C enters the lattice and generates a C--Ti--O bond and forms a hybrid orbital above the valance band, and enhances visible light adsorption of TiO~2~.^[@ref42],[@ref43]^ Zainullina et al.^[@ref44]^ reported that doping with C resulted in visible excitation due to the formation of Ti 3d-C 2p-O 2p hybrid orbital states transition from the valance band to the impurity band. Dong and co-workers^[@ref45]^ mixed Ti(SO~4~)~2~ and C~12~H~22~O~11~ with water to synthesize C-doped mesoporous TiO~2~ with visible light photocatalytic activity via hydrothermal treatment. Colombo et al.^[@ref46]^ reported the fabrication of C-doped TiO~2~ nanoparticles with visible light activity by decreasing the recombination rate of electron/holes through sol--gel doping glucose into TiO~2~. Wang et al.^[@ref47]^ utilized benzoic acid as a carbon source for doping into platelike TiO~2~, resulting in a lattice expansion, thereby improving the visible light photocatalytic activity. Wu et al.^[@ref48]^ synthesized carbon-doped TiO~2~ nanoparticles through the hydrothermal reaction between titanium tetra-*n*-butoxide and ethanol obtaining visible light activity. Though carbon-doped TiO~2~ has been synthesized from sol--gel or hydrothermal reactions between organic carbon and tetrabutyl orthotitanate or titanium compounds, introducing the C atom into the TiO~2~ lattice to obtain uniformly doped TiO~2~ and stable catalysts still remains a challenge. Especially, doping carbon particles into the solid-state TiO~2~ powder is difficult.

To dope element C into TiO~2~, several methods have been reported, e.g., doping carbon into Ti in an oxygen atmosphere,^[@ref49]^ TiC oxidation.^[@ref50],[@ref51]^ Shen et al.^[@ref49]^ oxidized TiC powder in an air atmosphere at different temperatures expanding TiO~2~ to visible light irradiation. Varnagiris et al.^[@ref50]^ synthesized carbon black-doped TiO~2~ (with the mixed-phase composition of anatase and rutile) through magnetron sputtering, where the carbon powder was placed on the Ti cathode and sputtered under DC current in an oxygen atmosphere. The sputtering time was varied, and different amounts of C were doped into TiO~2~. The highest degradation rate constant under visible light irradiation of methylene blue was 1.14 × 10^--3^ min^--1^ after being sputtered for 180 min. Nevertheless, the doping content of the carbon powder in TiO~2~ is difficult to precisely control. Besides, the effects of doping content of carbon powder on the phase behavior, crystal structure, and morphology are rarely reported.

In this work, C-doped TiO~2~ nanorods were synthesized through doping carbon black into hydrothermally synthesized solid-state TiO~2~ nanowires (NWs) via calcination. The effects of carbon content on the morphology, phase structure, crystal structure, and the photocatalytic property under both UV and solar light by the degradation of methylene blue (MB) were explored. Besides, the photoelectrochemical property of C-TiO~2~ was systematically studied to illustrate the solar light degradation mechanism. The photocatalytic activity of varied amounts of carbon black-doped TiO~2~ nanorods was compared to those of P25 and anatase TiO~2~ NWs, indicating that doping with carbon black resulted in much higher UV light and solar light photocatalytic activity via the degradation of MB. The generated free radicals and the dominant free radicals on MB degradation were also characterized. Results showed that C-doping into TiO~2~ leads to the lattice distortion and the formation of O--Ti--C bond and reduces the band gap, thereby extending TiO~2~ to the solar light activation region.

II. Results and Discussion {#sec2}
==========================

The alkali hydrothermal method^[@ref52]^ was utilized to synthesize TiO~2~ NWs and carbon black-doped TiO~2~ (C-TiO~2~) nanorods from commercial TiO~2~ (P25). A certain amount of P25 was well dispersed into 10 M NaOH and hydrothermally treated at 260 °C for 24 h in a Teflon autoclave ([Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}). The obtained intermediate was washed with 0.1 M HCl and DI water to remove Na^+^ and adjust the pH to around 7. Then, it was dried completely at 80 °C and heat-treated in an airtight tube furnace to obtain TiO~2~ NWs under 700 °C for 2 h. The as-prepared TiO~2~ NWs were mixed with a certain amount of carbon black and then heat-treated at 700 °C for 2 h in a N~2~-filled airtight tube furnace to obtain C-doped TiO~2~ nanorods ([Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}).

![Schematic Illustration of the Fabrication Process of C-Doped TiO~2~.](ao0c00504_0001){#fig1}

Since P25 is composed of 20% rutile and 80% anatase, the phase composition of carbon black-doped TiO~2~ nanorods was investigated via XRD. As shown in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a, only anatase was detected after doping with carbon black, and increasing the doping content of carbon black does not lead to peak shifting. The characteristic peaks of anatase were observed at 2θ values of 25.26, 36.88, 48.04, 53.80, 55.00, 62.62, 68.76, 70.32, and 75.04°, which correspond to (101), (004), (200), (105), (211), (204), (116), (200), and (215). The relatively strong intensity of these peaks indicates the good crystallinity of C-doped TiO~2~, and the increment of the doping content of C generated a negligible effect on the phase structure of TiO~2~. Further characterization was performed by calculating the crystallite size. The crystallite size of TiO~2~ nanowires---(0.2% C)-TiO~2~, (0.5% C)-TiO~2~ and (1.0% C)-TiO~2~---was calculated at 31.92 nm, 31.66 nm, 33.21 nm and 30.48 nm, respectively. This result illustrates the negligible effects of carbon content on the crystallite size of TiO~2~.

![(a) XRD analysis of C-TiO~2~ nanorods at varied carbon doping contents. (b) Raman shift of TiO~2~ NWs and C-doped TiO~2~ nanorods at different doping contents.](ao0c00504_0002){#fig2}

[Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b shows the Raman spectra of TiO~2~ NWs and C-doped TiO~2~ nanorods showing that doping with C caused obvious peak shifting and widening ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b). Comparing with TiO~2~ NWs, significant peak shifting was observed at 135, 488, and 605 cm^--1^ after doping with carbon black. Furthermore, the peak shifting became more obvious with the increment of C-doping content ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b). In detail, at the peak shift of 135 cm^--1^, the peaks after C-doping shifted to the right at 137, 139, and 142 cm^--1^ corresponding to (0.2% C)-TiO~2~, (0.5% C)-TiO~2~, and (1.0% C)-TiO~2~ ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b inset (1)). However, the peaks shift to the left at 488 and 605 cm^--1^ ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b inset (2)) after C-doping. The 488 peak shifts to 487, 486, and 482 cm^--1^, while the peak of 605 cm^--1^ shifts to 604, 602, and 600 cm^--1^ after doping with 0.2, 0.5, and 1.0% C correspondingly ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b inset (2)). Moreover, the peak shifts at 135, 488, and 605 cm^--1^ became wider with the increment of the doping content of carbon black ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b insets). These results illustrate that doping with carbon black induced the generation of defects or impure states, and this phenomenon becomes more apparent with the increasing doping content of carbon black.^[@ref53],[@ref54]^ Furthermore, the intensity of these peaks is much stronger than that of TiO~2~ NWs ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b insets), indicating the good crystallization of TiO~2~ nanorods after doping with carbon black.^[@ref54]^

The morphology of as-prepared specimens was characterized by scanning electron microscopy (SEM) ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}). The TiO~2~ NWs synthesized by the hydrothermal reaction are relatively smooth with width and length of 200 nm and several millimeters, respectively ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}a,b). However, notable changes have been observed from the SEM that the long nanowires were turned into short nanorods after doping with carbon black ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}c--e). Furthermore, the surface of C-doped TiO~2~ nanorods became rough and dispersed with many nanoparticles ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}c--e). This phenomenon became more significant with the increment of C-doping content. The composition of these dispersed nanoparticles on (1.0% C)-TiO~2~ nanorods was analyzed by energy-dispersive X-ray spectroscopy (EDX), which shows that the main composition included C, Ti, and O ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}f). Moreover, the content of C in C-doped TiO~2~ increased with the increment of the doping load of carbon black ([Figure S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)). A continuous increase in the doping load of C (2.0 and 3.0 wt %) caused a significant reduction of the aspect ratio of TiO~2~ nanorods, and more particles were observed on the surface of TiO~2~ ([Figure S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)). The result shows the significant effects of carbon black on the morphology change after doping.

![Morphology of the as-prepared TiO~2~ NWs and C-doped TiO~2~ through SEM characterization: (a, b) TiO~2~ NWs, (c) (0.2% C)-TiO~2~, (d) (0.5% C)-TiO~2~, (e) (1.0% C)-TiO~2~, and (f) EDS analysis of (1.0% C)-TiO~2~.](ao0c00504_0003){#fig3}

To further investigate the surface structure of carbon black-doped TiO~2~ nanorods, transmission electron microscopy (TEM) was used to analyze the surface morphology and the results are compared to the original TiO~2~ nanorods, which shows that the original TiO~2~ is smooth and well crystallized ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a,b) while there are many nanoparticles dispersed on the surface ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c,d). Some of the carbon black nanoparticles coated onto TiO~2~ nanorods ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c, yellow square) form a thin layer (around 7 nm) at the interface, which can act as electron acceptors to separate electron and holes and protect Ti^3+^ from oxidation.^[@ref23]^ The high-resolution image of the selected square in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}c shows that there is a disconnected area ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}d) that might be formed by the doping of carbon black. Similarly, at the edge of TiO~2~ nanorods ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}e, selected area), the crystal faces were not well aligned, showing a slight distortion ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}f) due to the doped carbon black.

![(a, b) TEM images of original TiO~2~ nanorod. TEM image of (0.5% C)-TiO~2~ nanorods: (c) carbon black coated onto TiO~2~ nanorods, and (d) carbon black forming a thin layer on the surface of TiO~2~ nanorod, and (e, f) the edge of TiO~2~ nanorod is not well aligned after doping with C.](ao0c00504_0004){#fig4}

The effects of C loading on the chemical composition of C-doped TiO~2~ nanorods were analyzed via X-ray photoelectron spectroscopy (XPS), which shows that a O--Ti--C bond was formed after doping with 0.2% carbon black and that the intensity of the O--Ti--C bond becomes stronger with the increment of the doping amount of C ([Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}a--c). The content of C=C and C=O decreased with doping more C, while the content of C--O increased with doping more C ([Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}a--c and [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}). The atomic percentages of O--Ti--C (283.0 eV), C=C (284.6 eV), C--O (286.1 eV), and C=O (288.4 eV) are 9.95, 44.45, 33.91, and 11.69% in 0.5% C-TiO~2~ ([Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}). In comparison, the intensity of O--Ti--C in (1.0% C)-TiO~2~ (10.72%) is higher than that of the O--Ti--C bond in (0.2% C)-TiO~2~ (5.76%). Interestingly, the intensities of the C--O bonds in (0.5% C)-TiO~2~ (33.91%) are the strongest ([Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}). It can be concluded that carbon black and N~2~ provided a reduction atmosphere, which enhances the doping of C into the TiO~2~ lattice. [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}d--f illustrates the effects of C dosage on the O 1s, showing that the content of the lattice O-ion state (O^2--^) at 529.5 eV and defects of O\* at 531.6 eV increase with the increment of doping loading of C (from 0.2 to 1.0%). Results showed that the generation of the oxygen defects and oxygen vacancies (O\*) lowers the energy band through doping C in TiO~2~ NWs. The intensity of the lattice O^--^ in (1.0% C)-TiO~2~ is much higher than that of (0.2% C)-TiO~2~, indicating that doping more C enhances the generation of lattice O^2--^. Lattice oxygen vacancies can be used as an effective transfer medium and electron acceptor, which can efficiently suppress the electrons and holes in the compound, promoting the catalytic process. The XPS results confirmed that carbon black doped into TiO~2~ nanorods via the C atom entering the lattice and the formation of the C--Ti--O bond at the interface of carbon black and TiO~2~ nanorods, which showed negligible effects of Ti 2p ([Figure S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)).

![Chemical states of C 1s and O 1s at varied carbon addition: (a)--(c) C 1s of (0.2% C)-TiO~2~ nanorods, (0.5% C)-TiO~2~ nanorods, and (1.0% C)-TiO~2~ nanorods, respectively. (d)--(f) O 1s of (0.2% C)-TiO~2~ nanorods, (0.5% C)-TiO~2~ nanorods, and (1.0% C)-TiO~2~ nanorods, respectively.](ao0c00504_0005){#fig5}

###### Atomic Percent (%) of Different C 1s Chemical States of As-Prepared Samples

  sample            (0.2% C)-TiO~2~   (0.5% C)-TiO~2~   (1.0% C)-TiO~2~
  ----------------- ----------------- ----------------- -----------------
  C 1s (C=C)        66.14             44.45             66.92
  C 1s (C--O)       15.82             33.91             16.24
  C 1s (C=O)        12.28             11.69             11.11
  C 1s (O--Ti--C)   5.76              9.95              10.72

The effects of C-doping content on the photocatalytic activity were characterized under UV and solar light via UV-DRS ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}). A relatively strong absorption peak in UV spectral range (380--400 nm) and certain visible light absorption in the visible light range (more than 400 nm) are observed in C-doped TiO~2~ nanorods ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}a). The absorption intensity of C-doped TiO~2~ nanorods increases with the increasing of the doping content of C under solar light. According to the turning point in these curves, a blueshift is observed, indicating the generation of electron--hole pairs and the stronger ability of photocatalytic oxidation in C-doped TiO~2~ nanorods. In comparison, the TiO~2~ NWs show a weak UV light adsorption peak and a lower solar light adsorption intensity ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}a). The band gap of TiO~2~ NWs and C-doped TiO~2~ nanorods decreased with increasing doping amount of C. The band energy ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}b) of doped samples was narrowed to 2.40--3.20 eV ([Table S1 and Figure S4](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)). This might be attributed to the doping of the C atom in the TiO~2~ lattice and the replacement of the O atoms, which narrow the band gap of TiO~2~.

![Diffuse reflectance spectra of TiO~2~ NWs and C-doped TiO~2~ nanorods between 200 and 600 nm (a) and the corresponding band energy (b).](ao0c00504_0006){#fig6}

The photocatalytic property of TiO~2~ NWs and C-doped TiO~2~ nanorods is evaluated via the degradation of MB in terms of ln(*C*~0~/*C*) and *K*~ap~ under UV light and solar light irradiation, respectively ([Figures [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"} and [S5](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)). The photocatalytic degradation efficiency of C-doped TiO~2~ nanorods increases notably with the increment of C-doping content under UV light, and the *K*~ap~ value reaches to as high as 0.1634 min^--1^ ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}a) and the MB solution turned transparent after degradation for 15 min ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}a, top right). The effect of C on enhancing the solar light photocatalytic performance of C-TiO~2~ was also investigated. The *K*~ap~ value of TiO~2~ NWs is 0.0112 cm^--1^ ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}b), and the MB solution remained blue after degradation of 120 min ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}b, bottom right). Compared to TiO~2~ NWs, the highest *K*~ap~ (0.0328 cm^--1^) was achieved after doping 1.0% C ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}b), showing the vital role of C in enhancing the solar light photocatalytic activity. Furthermore, MB was quickly decomposed within 30 min under solar light using (1.0% C-TiO~2~) ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}b, top right).

![Photocatalytic activities of TiO~2~ NWs and C-doped TiO~2~ nanorods under (a) UV light and (b) solar light.](ao0c00504_0007){#fig7}

To verify the active species participating in MB photodegradation, several charge-trapping agents were utilized to capture specific radical species. It is well known that photoinduced charges, i.e., photoinduced h^+^ and photoinduced e^--^, are generated through the photoexcitation of TiO~2~. The photoinduced e^--^ can react with the surface-absorbed O~2~ and transform into intermediate species of superoxide radical anions O~2~^--^, or recombine with photoinduced h^+^. The photoinduced h^+^ can participate in the degradation or react with H~2~O/OH^--^ producing active hydroxyl radicals OH^•^. In this work, AgNO~3~, *n*-butyalcohol, and (NH~4~)~2~C~2~O~4~ were used as e^--^, OH^--^, and h^+^ scavengers, respectively. (0.5% C)-TiO~2~ nanorods were used as a model to illustrate the generated active species and were irradiated under 365 nm UV irradiation for 60 min following the same procedure of C-TiO~2~ photodegradation of MB.

As shown in [Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"}a, the degradation rate (*D*%) of 0.03 mM MB was 94.5% (black), 95.9% with e^--^ trapping agent (AgNO~3~), 21.2% with OH^--^ trapping agent (*n*-butyalcohol), 15.8% with h^+^ trapping agent ((NH~4~)~2~C~2~O~4~), and 5.3% with OH^--^ + h^+^ trapping agents after UV radiation for 40 min. The photoinduced e^--^ was transformed to O~2~^--^ and then reacted with H~2~O generating H~2~O~2~, thereby producing OH^--^ with the addition of trapping agent *n*-butyalcohol OH^•^. However, the remaining h^+^ may still act as active species ([Figures [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"}a-(2), 21.2%). After adding (NH~4~)~2~C~2~O~4~ to trap h^+^, the generation of OH^--^ originated from photoinduced h^+^ was largely restricted, while the remaining OH^--^ or O~2~^--^ (produced by photoinduced e^--^) decomposed 15.8% MB, verifying that OH^--^ or O~2~^--^ may also act as active species. With both OH^--^ + h^+^ trapping agents added in the solution, a slight degradation (5.3%) was observed, verifying that the intermediate species of O~2~^--^ was not the main reaction species. The contribution rates of all species are listed in [Table S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf). [Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"}b shows the digital image of the degraded MB solution with varied scavengers at different degradation times (0, 10, 20, 30, 40, 50, 60 min). The results verified that the main free radicals in our study were OH^--^ and the photoinduced h^+^, which also play an important role in the photodecomposition of MB.

![(a) Photocatalytic activities of (0.5%C)-TiO~2~ under UV light for 40 min with AgNO~3~, *n*-butyalcohol, (NH~4~)~2~C~2~O~4~, and *n*-butylalcohol + (NH~4~)~2~C~2~O~4~ as charge-trapping agents and (b) the corresponding degradation process in (a).](ao0c00504_0008){#fig8}

The photocurrent of the samples under UV irradiation was characterized by performing six on--off cycles^[@ref55]^ with a duration of 120 s ([Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}a). The average stable photocurrent of nanowire series samples was close to each other and much lower than that of P25. P25 exhibited the highest photocurrent (8.0 μA/cm^2^) in the first 20 s, which then dropped and remained at 3.4 μA/cm^2^ with increasing operation time, while the photocurrents of TiO~2~ NWs and varied C-doped TiO~2~ were 6.0 μA/cm^2^ (TiO~2~ NWs), 7.2 μA/cm^2^ (0.2% C), 7.2 μA/cm^2^ (0.5% C), and 6.4 μA/cm^2^ (1.0% C), which then decreased and remained stable at 0.67, 0.3, 0.8, and 0.9 μA/cm^2^, respectively. This result illustrates that the instantaneous photocurrents in the first 20 s of TiO~2~ NWs and C-TiO~2~ were close to that of P25, while the long-term photocurrent of C-TiO~2~ dropped sharply ([Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}a). The long-term photocurrent of C-TiO~2~ was restricted by the doping of carbon black, implying the low transmission speed of photocurrent in the long-term operation.

![Photoelectric properties of P25, TiO~2~ NWs, and C-doped TiO~2~ nanorods: (a) photocurrent spectra, (b) EIS Nyquist spectra, and (c) PL spectra.](ao0c00504_0009){#fig9}

Electrochemical impedance spectroscopy (EIS) is one of the most powerful tools for studying the electrochemical processes occurring at the electrode/electrolyte interface (medium-frequency region, MFR, 10^4^--10^2^).^[@ref56]^ The radius of AC impedance represents the resistance of carrier migration (1/2*R*~ct~) and reveals the transportation of charge carriers. Generally, a larger AC impedance radius stands for the higher resistance of the charge carrier migration. The EIS Nyquist spectra in [Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}b show that the catalysts of C-doped TiO~2~ had a lower *R*~ct~ than those of P25 and TiO~2~ NWs, indicating the rapid electron transport of electron--hole pairs through the electrode reaction. According to the EDX and XPS results, the generated defects and the coated carbon black nanoparticle (oxycarbides, C--O bond) might contribute to the decrease of the charge transfer resistance. Although the as-prepared TiO~2~ NWs showed a lower generation of electron--hole pairs ([Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}a), C-doping reduced the resistance of carrier migration and enhanced the transport of electron--hole pairs.

Photoluminescence (PL) spectra were measured to approximately characterize the recombination rate of photoinduced charges; a higher PL intensity normally indicates a higher carrier recombination. As shown in [Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}c, the high intensity in P25 verified the high recombination rate of photoinduced carriers, which was in accordance with the results of photocurrent in [Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}a. The intensity was largely reduced in TiO~2~ NWs and C-TiO~2~ (0.2, 0.5, 1.0%). The lower PL intensity of C-doped samples revealed an efficient separation of charge carriers. Though doping with carbon black led to the reduction of photocurrent in the long-term operation, C-doping reduced the electron--hole recombination and enhanced the carrier migration.

III. Conclusions {#sec3}
================

In summary, solar light-activated TiO~2~ nanorods were fabricated by doping carbon black into solid-state TiO~2~ NWs under thermal heating treatment. The effects of doping content on the surface morphology, crystal structure, photocatalytic activity under both UV and solar light, and photoelectrochemical property were systematically studied. Results showed that doping with C caused peak shifting and widening, and this phenomenon became more apparent with increasing doping loading of C. The doping of C also caused the transfer of TiO~2~ NWs into short nanorods, and the aspect ratio decreased with the increment of the doping content of C. The chemical composition of TiO~2~ after doping with C showed that C entered the crystal lattice and alternated O to generate O--Ti--C and oxygen vacancies. The photocatalytic activities of doped TiO~2~ were investigated under full-spectra solar light and UV light by degrading methylene blue. Comparing with P25 and TiO~2~ NWs, UV and solar light photocatalytic activity can be enhanced through C-doping, and the degradation efficiency of MB under solar light reached as high as 0.0328 min^--1^ in (1.0% C)-TiO~2~. Charge scavenger tests indicated that the major active species were OH^--^ and photogenerated holes during the photodegradation of methylene blue. The EIS Nyquist spectra confirmed a faster surface charge charier transport, and the PL spectra demonstrated an efficient charge carrier separation in C-doped TiO~2~ catalysts, which contributed to the enhancement of UV and solar light photocatalysis.

IV. Experimental Section {#sec4}
========================

IV.I. Chemicals {#sec4.1}
---------------

Commercial P25 (Evonik, composites of rutile and anatase), NaOH (Sigma-Aldrich), N~2~ (99.9%), carbon black (99.95%, Sigma-Aldrich), deionized water (DI water, supplied by the lab), HCl (20 wt %, Chemistore University of Waterloo), and methylene blue (MB, Sigma-Aldrich).

IV.II. Synthesis of Materials {#sec4.2}
-----------------------------

### IV.II.I. Synthesis of TiO~2~ Nanowires (TiO~2~ NWs) {#sec4.2.1}

Commercial P25 was employed as the initial raw material to prepare TiO~2~ nanowires, which were synthesized in an alkali hydrothermal solution. Typically, 2.0 g of P25 powder was completely dispersed in a 10 M NaOH solution, followed by magnetic stirring for 1 h. Then, it was put into a Teflon autoclave and reacted under 260 °C for 24 h. The precursor was collected by centrifugation after the complete reaction and washed with DI water and 0.1 M HCl solution for complete ion exchange. After that, the as-prepared product was placed in an oven to dry completely at 80 °C. The dried powder was then put in a ceramic crucible and thermally treated in a muffle furnace under 700 °C for 2 h to obtain the anatase TiO~2~ nanowires, which were denoted as TiO~2~ NWs.

### IV.II.II. Synthesis of Carbon Black-Doped TiO~2~ (C-TiO~2~) Nanorods {#sec4.2.2}

The as-prepared TiO~2~ NWs were used as a titanium source and carbon black (99.95%, Sigma-Aldrich) was used as a C source (the weight ratios between carbon black and TiO~2~ NWs are 0, 0.2, 0.5, and 1.0%). The obtained specimens were labeled as TiO~2~ NWs, (0.2% C)-TiO~2~, (0.5% C)-TiO~2~, and (1.0% C)-TiO~2~. In a typical experiment, carbon black nanoparticles and the as-prepared TiO~2~ NWs were mixed homogeneously in anhydrous ethanol with magnetic stirring. After drying at 80 °C for 12 h, the mixture was transferred into an airtight tube furnace and thermally treated at 700 °C for 2 h under a nitrogen (N~2~, 99.9%) atmosphere.

IV.III. Characterization {#sec4.3}
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The phase composition and crystal behavior of TiO~2~ NWs and C-TiO~2~ nanorods were characterized by X-ray diffraction (XRD, Bruker D8 FOCUS) and Raman spectroscopy (Raman, equipped with a He--Ne ion laser at 633 nm, Renishaw, U.K.). The microstructure of the synthesized samples was characterized by field emission scanning electron microscopy (SEM, LEO-Ultra, Gemini, Germany). The surface chemical composition of these specimens was characterized using X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific ESCALAB 250, Al Kα radiation, 5 × 10^--9^ mbar of chamber vacuum, 0.2 mA of emission current) equipped with a hemispherical analyzer (150 nm in radius). The surface composition of these as-prepared specimens was calibrated according to the standard C 1s at a fixed binding energy of 284.6 eV before fitting into the point peak software (CasaXPS). The optical property and band gap energy of the prepared samples were measured by an ultraviolet--visible diffuse reflection spectroscope (UV-DRS, UV-2501PC, Shimadzu, Japan, 200--800 nm) with solid BaTiO~3~ as a reference. The photoelectric properties of the specimens were characterized by an Autolab Electrochemical workstation (Nova2.1.1 software, Metrohm, China). EIS was measured by a three-electrode system, glassy carbon electrode was selected as a counter electrode, saturated calomel electrode as a reference electrode, stainless steel mesh (10 μm) as the base of the working electrode, a 1.0 M KOH solution as an electrolyte, and the light source was a 300 W xenon lamp (PLS--SXE300/300UV, PerfectLight, Beijing).^[@ref57],[@ref58]^ During the test of AC impedance, the frequency range and amplitude were set at 100--10 MHz and 0.01 V, respectively. The PL spectra were measured by a steady/transient fluorescence spectrometer (FLS 980-STM, Edinburgh, U.K.), and the excitation wavelength was set at 250 nm.

IV.IV. Photocatalytic Performance Characterization {#sec4.4}
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The photocatalytic property of TiO~2~ NWs and C-TiO~2~ nanorods was evaluated by the degradation of methylene blue (MB, Sigma-Aldrich) solution under UV light and solar light. Before adding TiO~2~ and C-doped TiO~2~ into the methylene blue solution, a photolysis experiment was carried out to verify if methylene blue can be degraded by solar light or UV light. The result showed that methylene blue cannot be degraded by UV and solar light ([Figure S6](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)). Then, TiO~2~ photocatalysts were added into the methylene blue solution. In a typical test, 20 mg of TiO~2~ NWs and C-TiO~2~ nanorods were dispersed into 50 mL of a 0.03 mM MB solution, respectively. Adsorption equilibrium was obtained before illumination under magnetic stirring for 1 h in a dark environment. After 1 h adsorption in the dark environment, the adsorption reached the equilibrium state because the adsorption lines overlapped at varied adsorption time ([Figure S7](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf)). Photocatalytic degradation was carried out under the irradiation of a UV lamp (Philips, the Netherlands, 400 W, 365 nm) or a xenon lamp (Newport, 4.4 W/m^2^,150 W). In a typical degradation experiment, 5 mL aliquots were taken out (the left solution was vigorously stirred to maintain the homogeneous dispersion of the nanorods) using a pipette at specific time points of 0, 10, 20, 30, 40, 50, 60, 80, 100, and 120 min to characterize the degradation rate of MB by UV--Vis spectroscopy (UV-2501PC, Shimadzu, Japan) between 800 and 200 nm.

The photoactivity of C-TiO~2~ nanorods was quantitatively evaluated by calculating the apparent reaction rate constant (*K*~ap~) via the pseudo-first-order kinetic reactionwhere *c*~0~ and *c* are the original and final concentrations of the MB solution, respectively, and *t* stands for the relative irradiation time. The generated active species were characterized using different types of scavengers. In this work, AgNO~3~, *n*-butyalcohol, and (NH~4~)~2~C~2~O~4~ were used as e^--^ scavenger, OH^--^ scavenger, and h^+^ scavenger, respectively, during the degradation of 0.03 mM MB under UV light irradiation.

The Supporting Information is available free of charge at [https://pubs.acs.org/doi/10.1021/acsomega.0c00504](https://pubs.acs.org/doi/10.1021/acsomega.0c00504?goto=supporting-info).Adsorption experiment of C-doped TiO~2~ on methylene blue in the dark environment after 1 h adsorption; EDX analysis of C-doped TiO~2~; surface morphology of carbon black; surface morphology and EDX analysis of 2.0 wt % C and 3.0 wt % C-doped TiO~2~ nanorods; band gap energy lists of the as-prepared samples; measurement of band energy of the as-prepared samples; removal efficiency of *C*/*C*~0~ of the as-prepared samples under UV and solar light sources; and contribution rate of each species ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00504/suppl_file/ao0c00504_si_001.pdf))

Supplementary Material
======================

###### 

ao0c00504_si_001.pdf

The authors declare no competing financial interest.

This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Canadian Water Network Innovative Technologies for Water Treatment Program, and Scientific. This work was financed by Grant-in-aid for scientific research from the National Natural Science Foundation of China (grant no. 21808131) and Technological Innovation Programs of Higher Education Institutions in Shanxi (2019L0240).
